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A study of rubidium, strontium and strontium isotopes in some mafic and sulphide minerals Maxwell, Robert 1976

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A STUDY OF RUBIDIUM, STRONTIUM AND STRONTIUM ISOTOPES IN SOME MAFIC AND SULPHIDE MINERALS by ROBERT JAMES MAXWELL B.Sc. Queen's University, 1974 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Geology) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1976 © Robert James Maxwell, 1976 In p r e s e n t i n g t h i s t h e s i s in p a r t i , a l f u l f i l m e n t o f the requ i remen t s f o r an advanced deg ree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t he L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depa r tment The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook Place Vancouver, Canada V6T 1W5 - i i -ABSTRACT Galena from Pine Point, N.W.T. contains l e s s than 0.3 ppm Sr and 87 86 l e s s than 0.005 ppm Rb with a Rb /Sr r a t i o l e s s than 0.2. Sphalerite from the same l o c a l i t y contains 0.070 - 0.002 ppm Sr and 0.025 - 0.003 87 86 -J~ ppm Rb with a Rb /Sr r a t i o of 1.05 - 0.3. Despite large a n a l y t i c a l u ncertainties i n measuring such small q u a n t i t i e s of Rb and Sr, i t appears s p h a l e r i t e has a su i t a b l e Rb/Sr r a t i o f o r age determination. C a l c i t e , Presqu'ile-type dolomite and sparry dolomite associated with Pine Point 87 86 mi n e r a l i z a t i o n have d i s t i n c t Sr /Sr r a t i o s of 0.7153, 0.7080 and 0.7096 r e s p e c t i v e l y . A two point isochron defined by s p h a l e r i t e and sparry dolomite i n d i c a t e s an age of 165 60 m.y. and an i n i t i a l r a t i o of 0.7095 - 0.0001. Sphalerite from the B l u e b e l l Mine, B.C. contains 0.012 - 0.003 ppm Rb. Phrrhotite and galena from the same l o c a l i t y contain smaller quan-t i t i e s of Rb (0.0022 - 0.0002 ppm and 0.0003 - 0.0001 ppm r e s p e c t i v e l y ) . No r e l i a b l e Sr analyses are a v a i l a b l e due to the extremely small amount of Sr present. The r e l a t i v e l y large Rb concentration of sph a l e r i t e supports the conclusion drawn from analyses of Pine Point s p h a l e r i t e that the mineral i s s u i t a b l e , f o r age determination. O l i v i n e and orthopyroxene within l h e r z o l i t e nodules from Boss Mountain 87 and Jacques Lake, B.C. contain 1-2 ppm Sr and 0.1 - 0.01 ppm Rb with Rb /Si r a t i o s close to 0.1. Associated clinopyroxene contains from 22-42 ppm Sr, 87 86 le s s than 0.3 ppm Rb, with Rb /Sr r a t i o s close to 0.01. Two nodules from Jacques Lake show apparent isochron r e l a t i o n s h i p s f o r the c o n s t i t u -ent o l i v i n e , orthopyroxene and clinopyroxene giving ages of 2.0 - 0.1 b.y. and 4.7 - 2.4 b.y. with i n i t i a l r a t i o s of 0.7024 - 0.0001 and 0.7020 -0.0004 re s p e c t i v e l y . Rb-Sr data f o r the Boss Mountain nodule does not define an isochron, although Sr i s o t o p i c d i s e q u i l i b r i u m between phases i s present. It i s not yet clear whether the observed Sr i s o t o p i c d i s e q u i l i -87 brium i s due to i n s i t u decay of Rb or to s e l e c t i v e contamination of o l i v i n e and orthopyroxene p r i o r to t h e i r emplacement i n the crust. - i i i -TABLE OF CONTENTS Page ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENT S CHAPTER I INTRODUCTION 1 CHAPTER II EXPERIMENTAL PROCEDURES 4 I I - l Introduction 4 II-2 Sample Preparation 5 II-3 Chemical Procedures 8 Reagents 8 Weighing P r e c i s i o n '. 9 Sample D i s s o l u t i o n 10 Ion Exchange 11 II-4 Isotope D i l u t i o n Analysis of Rubidium 14 Spike 14 Isotopic Composition 14 Concentration 14 Rubidium Mass Spectrometry 16 Rb Blanks 18 Data Reduction 19 Method 19 Example 19 II-5 Isotope D i l u t i o n Analysis of Strontium 23 89 Sr Tracer 23 Sr Spike 25 Isotopic Composition 25 Concentration / 26 Sr Blanks 26 i i i i i v i i i x - i v -TABLE OF CONTENTS (CONT'D) Page II-5 I s o t o p e D i l u t i o n A n a l y s i s o f S t r o n t i u m ( c o n t ' d ) S t r o n t i u m Mass S p e c t r o m e t r y 28 P r o c e d u r e 28 S i g n a l A m p l i f i c a t i o n 31 M e a s u r e d S r 8 7 / S r 8 6 R a t i o s o f N . B . S . S t a n d a r d SRM-988 32 Rb C o r r e c t i o n 33 D a t a R e d u c t i o n 34 II-6 Rb and S r A n a l y s e s o f U . S . G . S . S t a n d a r d PCC-1 38 II-7 Decay C o n s t a n t and C a l c u l a t i o n o f Ages 39 I I - 8 P r e v i o u s L o w - L e v e l Rb and S r A n a l y s e s 39 CHAPTER I I I THE A P P L I C A T I O N OF THE R B / S R DATING TECHNIQUE TO THE PINE POINT L E A D - Z I N C  DEPOSIT I I I - l I n t r o d u c t i o n 41 I I I-2 G e o l o g i c S e t t i n g 41 I I I-3 P a r a g e n e s i s 43 T e x t u r e s 43 F l u i d I n c l u s i o n s 47 S t a b l e I s o t o p e s 47 I I I-4 Age o f M i n e r a l i z a t i o n 49 I I I-5 T h e o r i e s o f O r i g i n o f t h e Ore 56 I I I-6 Sample D e s c r i p t i o n 57 I I I-7 A n a l y t i c a l R e s u l t s 59 S r 8 7 / S r 8 6 Measurements 59 Rb and S r A n a l y s e s 62 I s o c h r o n R e l a t i o n s h i p s 63 I I I-8 I n t e r p r e t a t i o n o f R e s u l t s 67 O r i g i n o f C a l c i t e 67 I n i t i a l R a t i o o f t h e M i n e r a l i z i n g S o l u t i o n s 68 Age o f M i n e r a l i z a t i o n 69 I I I-9 C o n c l u s i o n s 70 - v -TABLE OF CONTENTS (CONT'D) Page CHAPTER IV THE A P P L I C A T I O N OF THE R B / S R DATING TECHNIQUE 71 TO THE BLUEBELL L E A D - Z I N C DEPOSIT I V - I I n t r o d u c t i o n 71 IV-2 G e o l o g i c S e t t i n g o f t h e B l u e b e l l M i n e 71 IV-3 C o n d i t i o n s o f M i n e r a l i z a t i o n 72 TV-4 Age o f M i n e r a l i z a t i o n 73 IV-5 P e t r o g r a p h y o f Samples 73 IV-6 Sample P r e p a r a t i o n 74 IV-7 R e s u l t s 75 IV-8 D i s c u s s i o n o f R e s u l t s 75 S r 8 7 / S r 8 6 R a t i o s 75 Sr C o n c e n t r a t i o n o f S u l p h i d e Samples 80 Rb A n a l y s e s 80 I V - 9 C o n c l u s i o n s 82 CHAPTER V THE A P P L I C A T I O N OF THE R B / S R DATING TECHNIQUE TO SOME ULTRAMAFIC NODULES V - l I n t r o d u c t i o n 83 V-2 B o s s M o u n t a i n l h e r z o l i t e N o d u l e s 83 V-3 J a c q u e s L a k e N o d u l e s 84 V-4 P r e v i o u s Work 85 V-5 Samples 91 V-6 D i s c u s s i o n o f A n a l y t i c a l R e s u l t s 91 S r 8 7 / S r 8 6 R a t i o s 93 Rb and Sr A n a l y s e s 95 I s o c h r o n R e l a t i o n s h i p s 99 V-7 I n t e r p r e t a t i o n o f R e s u l t s 100 O r i g i n o f t h e N o d u l e s 100 M i n e r a l I s o t o p i c D i s e q u i l i b r i u m 104 The S r 8 7 / S r 8 6 R a t i o o f t h e M a n t l e 107 The Age o f t h e M a n t l e B e n e a t h B . C . 112 V-8 C o n c l u s i o n s 112 - v i -TABLE OF CONTENTS (CONT'D) CHAPTER VI BIBLIOGRAPHY APPENDIX 1 APPENDIX 2 APPENDIX 3 APPENDIX 4 APPENDIX 5 SUMMARY AND CONCLUSIONS - v i i -LIST OF TABLES Table Page 1 Balance check: Replicate weighings of a t e f l o n 9 washer 2 Concentration of Rb standard 15 3 C a l i b r a t i o n of Rb spike 15 4 Rb blank measurements 18 5 Amount of Rb i n 20 ml. a l i q u o t s of reagent 18 6 Rb concentration c a l c u l a t i o n s 21 89 7 Measured i s o t o p i c composition of the Sr tracer 23 8 Isotopic composition of N.B.S. Sr spike SRM-988 25 9 Sr blank measurements 27 10 Sr content of 20 ml. ali q u o t s of reagent 27 8 7 86 11 Isotopic measurements of the Sr /Sr r a t i o of 31 N.B.S. standard SRM-987 at various i n t e n s i t i e s 87 86 12 Sr /Sr r a t i o s of the N.B.S. standard SRM-987 32 13 Previous Rb and Sr analyses of U.S.G.S. Standard 38 PCC-1 14 Rb and Sr analyses of U.S.G.S. Standard PCC-1 39 15 Some previously reported Rb and Sr blanks 40 16 Isotopic composition of lead from Pine Point 54 17 Mineral separates from sample PP-1 58 18 Mineral separates from sample PP-4 59 8 7 86 19 Rb and Sr concentrations and Sr /Sr r a t i o s 60 of mineral separates from sample PP-1 8 7 86 20 Rb and Sr concentrations and Sr /Sr r a t i o s of 61 mineral separates from sample PP-4 87 86 21 Sr /Sr r a t i o s of c a l c i t e from sample PP-2 62 87 86 22 Preferred Sr /Sr r a t i o s and Rb and Sr concen- 64 tr a t i o n s of mineral separates from sample PP-1 87 86 23 Preferred Sr /Sr r a t i o s and Rb and Sr concen- 65 tra t i o n s of mineral separates from sample PP-1 87 86 24 Rb and Sr concentrations and Sr /Sr r a t i o s of 66 mineral separates from sample RJ-1 87 86 25 Rb and Sr concentrations and Sr /Sr r a t i o s of 67 mineral separates from sample RJ-2 - v i i i -LIST OF TABLES (CONT'D) Table Page 26 Number of blockg^of data c o l l e c t e d on sulphide samples 79 to which the Sr tracer was not added. 27 Replicate Rb analyses of s p h a l e r i t e , p y r r h o t i t e and galena 81 from the B l u e b e l l Mine. 8 7 86 28 Sr /Sr whole rock r a t i o s of ultramafic nodules and 86 t h e i r host b a s a l t s . 87 86 29 Ages and Sr /Sr i n i t i a l r a t i o s of ultramafic nodules. 88 87 86 30 Rb and Sr concentrations and Sr /Sr r a t i o s of mineral 92 separates from Boss Mountain and Jacques Lake l h e r z o l i t e nodules 87 86 31 Comparison of the Sr /Sr r a t i o measured on samples with and samples without addition of the Sr t r a c e r . of samples BM.JL-10 and JL-39, 87 86 Ages and Sr /Sr i n i t i a l ral with .reported strontium-standards and decay constants. 94 32 Replicate Rb and Sr nalyses mineral sep rates from 97 Jacques Lake ultramafic nodules. 8 7 86 33 Preferred Rb and Sr concentrations and Sr /Sr r a t i o s 100 34  and Sr /Sr i n i t i a l r a t i o s of ultramafic nodules 111 - i x -LIST OF FIGURES Figure Page 1 C a l i b r a t i o n of ion exchange columns showing the 12 aliquots i n which 8 elements are c o l l e c t e d . 2 T y p i c a l strontium mass spectrometer run showing 30 the v a r i a t i o n i n beam i n t e n s i t y with sample current. 3b Plot of P b 2 0 7 / P b 2 0 4 vs. P b 2 0 6 / P b 2 ° 4 of Pine Point 51 galena leads (table 15). 3b Plot of P b 2 0 8 / P b 2 0 4 v s . P b 2 0 6 / P b 2 0 4 of Pine Point 52 galena leads (table 15). 4 Growth curve of Stacey and Kramers(1975).The Pine 53 Point leads are plotted for reference. 5a,5b Growth curves of Doe and Zartmann(1976). Curve 53 a.represents the i s o t o p i c growth of lead within a mantle or lower c r u s t a l environment.Curve b represents the i s o t o p i c composition of leads within an orogenic zone. Curve c represents the i s o t o p i c growth of leads within accontinental environment.The Pine Point leads are p l o t t e d for reference. 87 86 87 86 6 Sr /Sr vs. Rb • /Sr of sphalerite,galena and 66 dolomite from Pine Point. 7 S r 8 7 / S r 8 ^ r a t i o s of. sulphide samples shewing the 78 e f f e c t s of ad d i t i o n o f " c a r r i e r free"SR tra c e r . 87 86 87 86 8 Sr /Sr r a t i o s of ultramafic nodules vs. Sr /Sr 87 of t h e i r b a l a s t i c host rocks. The plotted l i n e has a slope of 1, intercepts of 0,and represents e q u i l -i b r a t i o n between the nodules and t h e i r hosts. 87 86 87 86 9 Sr /Sr vs. Rb /Sr of mineral separates from a 101 Boss Mountain l h e r z o l i t e nodule. 87 86 87 86 10 Sr /Sr vs. Rb /Sr of mineral separates from 102 Jacques Lake l h e r z o l i t e nodule JL-10. 87 86 87 86 11 Sr /Sr vs. Rb /Sr of mineral separates from 103 Jacques Lake l h e r z o l i t e nodule JL-39. 87 86 12 I n i t i a l Sr /Sr r a t i o s vs. age of ultramafic 109 nodules from world-wide l o c a l i t i e s . - x -ACKNOWLEDGEMENTS I w o u l d l i k e to t h a n k many i n d i v i d u a l s w i t h o u t whose h e l p t h i s work c o u l d n o t have been c o m p l e t e d . F i r s t and f o r e m o s t o f t h e s e i s D r . R i c h a r d L e e A r m s t r o n g who g e n e r o u s l y gave c a r e f u l g u i d a n c e and was a c o n t i n u a l s o u r c e o f i n s p i r a t i o n t h r o u g h o u t t h e c o u r s e o f t h e w o r k . I w o u l d a l s o l i k e t o t h a n k K r i s t a S c o t t who , w i t h e x c e p t i o n a l p a t i e n c e , p r o v i d e d i n v a l u a b l e a s s i s t a n c e i n t h e l a b o r a t o r y . P a t Shore k i n d l y p r o c e s s e d and r a n t h e Pb s ample s and w i l l i n g l y d i s c u s s e d and h e l p e d r e s o l v e many o f t h e a n a l y t i c a l p r o b l e m s . D r . B i l l S l a w s o n , D r . B a r r y R y a n and D r . A l a s t a i r S i n c l a i r c a r e f u l l y r e a d and c o r r e c t e d t h e m a n u s c r i p t . I am a l s o g r a t e f u l f o r h e l p and u s e f u l d i s c u s s i o n s w i t h o t h e r f a c u l t y and s t u d e n t s i n c l u d i n g D r . G a r y M e d f o r d , D r . Mary B a r n e s , D r . P e t e r P e t o , T i m A h e r n , and N a t h a n G r e e n . Samples were o b t a i n e d t h r o u g h t h e c o u r t e s y o f D r . A l a s t a i r S i n c l a i r , D r . G a r y M e d f o r d , I a n Duncan and J o e N a g e l . F i n a n c i a l s u p p o r t was p r o -v i d e d by t h e N a t i o n a l R e s e a r c h C o u n c i l o f Canada o p e r a t i n g g r a n t A 8 8 4 1 . -1-CHAPTER I INTRODUCTION Since the recognition that cogenetic mineral separates or rock 87 86 87 86 samples could be dated by t h e i r Rb /Sr and Sr /Sr i s o t o p i c r a t i o s (Compston and J e f f e r y , 1959; Nicolaysen, 1961), the method has enjoyed wide a p p l i c a t i o n . In the past few years the Rb/Sr method has been applied to geological materials containing l e s s than 2 p.p.m. Rb and Sr. This has been made possible by several technological advances including routine use of t e f l o n labware,de-velopment of a rapid simple method f or preparation of ultrapure reagents (Mattinson, 1971), and the development of extremely sen-s i t i v e , high p r e c i s i o n mass spectrometers (Wasserburg et al,1969) . This thesis i s an attempt to assess the s u i t a b i l i t y of two sets of mineral assemblages, which contain low concentrations of Rb and Sr, for age determination by the Rb/Sr method: the sulphides galena, s p h a l e r i t e , and py r r h o t i t e ; and the mafic minerals o l i v i n e , c l i n o -pyroxene (Cr diopside), and orthopyroxene (augite). -2-Economic geologists perpetually debate an epigenetic versus syngenetic o r i g i n of conformable ore deposits. The main reason for the continuation of these controversies i s that minerals s u i t a b l e for age determination by standard Rb/Sr, K/Ar, or U/Pb techniques r a r e l y p r e c i p i t a t e from the ore-forming solutions. It may be possible, however, by precise analysis of the small 87 86 quantities of Rb and Sr and the Sr /Sr r a t i o s within the sulphide minerals to date the ore deposits d i r e c t l y . If t h i s i s true, the method may put an end to many problems regarding the origin...of s t r a t i f o r m ore deposits. P r i o r to t h i s study no r e l i a b l e analyses of the Rb and Sr concentrations of sulphide minerals were a v a i l a b l e . E arly spectro-g r a p h ^ analyses of these elements i n sulphides were aimed at de-termining regional v a r i a t i o n s i n trace elements and indicated i n -determinate or nominal (0 - 20 p.p.m.) concentrations (e.g./El Shazly, Webb et a l , 1957). These studies are of no value i n assessing the s u i t a b i l i t y of these minerals f o r dating. Reeseman (1968) attempted to date massive sulphides by the Rb/Sr method, analyzing intimate mixtures of chalcopyrite, p y r i t e , p y r r h o t i t e and minor s p h a l e r i t e . His a n a l y t i c a l r e s u l t s show a range of Rb concentrations from 0.05 to 0.9 p.p.m. and a range of Sr concen-87 86 tr a t i o n s from 0.13 to 1.09 p.p.m. The corresponding Rb /Sr r a t i o s range from 0.1 to 2.8. Reeseman concluded that some sulphide 87 86 minerals may have a s u i t a b i l i t y high Rb /Sr r a t i o f or age determin-at i o n by the Rb/Sr method. It i s possible that the sulphide samples -3-analyzed by Reeseman suffered from contamination by t h e i r host rocks. As part of t h i s work, highly p u r i f i e d s p h a l e r i t e , p y r r h o t i t e , and galena separates were analyzed f o r Rb, Sr, and.Sr i s o t o p i c composition. Chapters III and IV of t h i s thesis present these a n a l y t i c a l r e s u l t s and an assessment of t h e i r g e o l o g i c a l s i g n i f i c a n c e . L h e r z o l i t e nodules, which contain orthopyroxene, clinopyroxene and o l i v i n e occur i n a l k a l i c b asalts. Such nodules are interpreted as pieces of the upper mantle (Kutolin, 1970). The ages of these nodules, provided they have not been reset during transport to the sur-face, may provide valuable information on mantle events. Previous workers (e.g. Stueber and Ikramuddin, 1974) have shown that o l i v i n e and orthopyroxene contain small q u a n t i t i e s of Rb (0.01 - 1 p.p.m.) and Sr (0.01 - 3 p.p.m.), but r e l a t i v e l y high ( 0.1) Rb/Sr r a t i o s . Associated clinopyroxene usually has r e l a t i v e l y high concentrations Q -J Q £ of Sr (10 - 100 p.p.m.) and very low ( 0.01) Rb /Sr r a t i o s . This s i t u a t i o n suggests that separates of associated o l i v i n e , orthopyroxene and clinopyroxene may be su i t a b l e f o r Rb/Sr dating. A few studies (Basu and Murthy, 1976'; Dasch and Green, 1975; Stueber and Ikramuddin, 1974) have reported age determinations f o r l h e r z o l i t e nodules by t h i s method. Chapter V of t h i s thesis i s an assessment of the geo l o g i c a l 8 7 86 si g n i f i c a n c e of Rb and Sr analyses and Sr /Sr r a t i o s of o l i v i n e , orthopyroxene and clinopyroxene mineral separates from l h e r z o l i t e nodules c o l l e c t e d from two l o c a l i t i e s i n B r i t i s h Columbia. _4-CHAPTER II EXPERIMENTAL PROCEDURES I I - l Introduction This Chapter describes i n d e t a i l the experimental procedures used during the course of t h i s work. Many samples have been analyzed for rubidium and strontium by the isotope d i l u t i o n method. Some of these samples contain extremely low concentrations of Rb and Sr, so extreme care must be taken during sample preparation. The f i r s t sections of t h i s Chapter describe these preparation techniques. Subsequently a d e s c r i p t i o n of the isotope d i l u t i o n procedure i s given. I I - 2 Sample Preparation -5-In order to insure that strontium isotope r a t i o s of mineral separates containing l e s s than 1 p.p.m. Sr are meaningful, i t i s necessary that the separates be absolutely pure and that foreign contaminants, i n c l u s i o n s , and gangue be completely removed. This i s of p a r t i c u l a r concern i n the case of sulphides that are assoc-iated with carbonate minerals. The gangue carbonate contains approximately 1000 times more Sr than the sulphides, so that a small contamination of the separate may cause large errors i n the measured S r 8 7 / S r 8 ^ r a t i o and Sr concentration. Many workers (Compston and Lovering, 1969; Erlank, 1969; Alsopp et a l . , G r i f f i n and Murthy, 1968; Hutchinson and Dawson, 1970) have shown that the i n t e r s t i t i a l material i n ultramafic rocks contains high concentrations of rubidium and/or strontium. This material must be completely removed from the mineral separates i n order to obtain rubidium and strontium concentrations free of a contamination component. The following steps, are followed i n the preparation of sulphide mineral separates: 1) The hand specimen i s broken with a hammer on an aluminum p l a t e . About 200 grams of the sulphide to be separated i s picked with tweezers from the broken rock. 2) The sulphide grains are crushed i n a diamond mortar to 20-40 mesh s i z e . 3) The 20-40 mesh grains are hand-picked under a binocular micro-scope. Mixed grains and i r r e g u l a r l y fractured grains are discarded. -6-4) The hand-picked grains are leached i n Quartz HC1 f o r about 5 minutes, washed i n Quartz H^ O and placed i n an u l t r a s o n i c bath fo r f i f t e e n minutes. The grains are then washed i n specpure acetone, dried and re-sieved. Material f i n e r than 60 mesh i s discarded;. 5) The leached grains are again hand-picked under the binocular microscope. Grains which have been excessively leached are d i s -carded. 6) The material i s ground i n an agate mortar to 100-325 mesh s i z e and stored. Ultramafic nodules from Boss Mountain contain -euhedral o l i v i n e and orthopyroxene c r y s t a l s that are l o o s e l y consolidated and can be e a s i l y separated with f i n e tweezers. The following procedure i s used fo r the preparation of o l i v i n e and orthopyroxene separates: 1) The grains are plucked with f i n e tweezers d i r e c t l y from the ex-posed f r e s h surface of the nodule. 2) These grains are leached for 5 minutes with 6N Quartz HC1, then repeatedly washed with d i s t i l l e d H^O. The grains are placed i n the u l t r a s o n i c bath for 15 minutes, then washed with d i s t i l l e d H^O, specpure acetone and dried. 3) These grains, which are up to 2 mm. i n diameter are hand-picked under a binocular microscope. As the grains are glassy and.trans-parent due to leaching, and are of large s i z e , mixed grains and grains with inclusions are e a s i l y i d e n t i f i e d and discarded. (1) A d e s c r i p t i o n of reagent grades such as "Quartz" and " U l t r a " i s given i n Section II-3. -7-4) The grains are crushed i n a clean polished mortar, then ground i n an agate mortar to 100-325 mesh s i z e and stored. Clinopyroxene within the ultramafic nodules from Boss Mountain occurs i n t e r s t i t i a l l y . As the grains are too small to separate by hand, the following procedure i s used: 1) A fine-grained mixture of material i s c o l l e c t e d during the separ-a t i o n of o l i v i n e and orthopyroxene from the nodules. This.material contains s p i n e l , orthopyroxene, o l i v i n e , clinopyroxene and i n t e r -s t i t i a l material. Without further grinding, the material i s sieved to 20-60 mesh. 2) This 20-60 mesh s i z e f r a c t i o n i s passed through.a Franz magnetic separator at the following s e t t i n g s : a forward t i l t of 25°; a side t i l t of 25°; and a current of 0.7 amps. The "nonmagnetic" f r a c t i o n i s repeatedly passed through the separator at the same settings u n t i l an e s s e n t i a l l y pure clinopyroxene f r a c t i o n i s ob-tained. 3) The clinopyroxene i s hand-picked under a.binocular microscope, then leached and ground.according to the same procedure used f o r o l i v i n e and orthopyroxene. Mineral separates of o l i v i n e , orthopyroxene and clinopyroxene from the Jacques Lake l h e r z o l i t e nodules previously prepared by L i t t l e -john (1972) were obtained. These separates were leached and-hand-picked according to the same procedure used f o r o l i v i n e and orthopyroxene from the Boss Mountain nodule. - 8 -II-3 Chemical Procedures  Reagents In order to reduce contamination of the samples during t h e i r d i s s o l u t i o n most reagents were d i s t i l l e d p r i o r to use. Three cate-gories of d i s t i l l e d reagents were used. The p r e f i x " D i s t i l l e d " r e f e r s to d i s t i l l a t i o n i n a Barnstead F l b o r o s i l i c a t e glass s t i l l . The p r e f i x "Quartz" r e f e r s to a subsequent d i s t i l l a t i o n i n a s i l i c a -glass s t i l l i n order to further reduce impurities. " U l t r a " reagents were prepared using a two-bottle subsoiling t e f l o n s t i l l of the type described by Mattinson (1971). The following reagents were used i n the preparation of sulphide and ultramafic mineral separates: 1) Quartz HC1 of 6.2 N concentration, prepared by d i l u t i n g Mallinckrodt reagent grade HC1 with an appropriate quantity of d i s t i l l e d EL^ O, then d i s t i l l i n g the 6.2 N (constant b o i l i n g ) acid i n the s i l i c a glass s t i l l . 2) Quartz H^O, prepared by r e - d i s t i l l i n g d i s t i l l e d H^ O i n the s i l i c a glass s t i l l . 3) Quartz HC1 of 2.5 N concentration, prepared by mixing an appro-p r i a t e quantity of 6.2 N quartz HC1 with quartz H^O. 4) U l t r a H2O, prepared by r e - d i s t i l l i n g quartz ILjO at subboiling temperatures i n the two-bottle t e f l o n s t i l l . 5) U l t r a HC1 of 6.2 N concentration, prepared by r e - d i s t i l l i n g quartz 6.2 N HC1 at subboiling temperatures i n the two-bottle t e f l o n s t i l l . -9-6) U l t r a HC1 of 2.5 N concentration, prepared by d i l u t i n g an appro-p r i a t e quantity of 6.2 N U l t r a HC1 with U l t r a H 20. 7) U l t r a HF, prepared by d i s t i l l i n g Mallinckrodt reagent grade HF at subboiling temperatures i n the two-bottle t e f l o n s t i l l . 8) HCIO^, J.F. Smith v y c o r - d i s t i l l e d HCIO^, as received. Weighing P r e c i s i o n During the course of t h i s work a l l weighing was done using a Mettler H2Q balance, which weighs to 0.00001 gms. A l l weights were calculated by d i f f e r e n c e . Replicate measurements of a l l weights were made. In order to determine the p r e c i s i o n of the weighings, a t e f l o n washer was weighed several times i n January and February.1976. The r e s u l t s are shown i n Table 1. Table 1: Balance check: Replicate weighings of a t e f l o n washer Date Jan. 21 1:30 p.m. Jan. 21 3:00 p.m. Jan. 22 Jan. 23 Jan. 28 Feb. 5 Average Weight of Washer (gms) 4.79139 4.79121 4.79135 4.79153 4.79152 4.79173 4.79145 ± 0.00018 (la) Table 1 indicates that the weight of the washer increased during the experiment, probably due to handling. The p l a s t i p a c syringes were also handled during the spike dispensing procedure however. The measured weight of the t e f l o n washer varied between 4.79121 gms and 4.79173 gms, -10-a d i f f e r e n c e of 0.00052 gms. It i s l i k e l y that the p r e c i s i o n of weight measurements of the samples and spikes i s much better, as the weighing was done by d i f f e r e n c e , handling of the spike dispensing syringes was minimal, and the weighings were done during a short i n t e r v a l of time. It i s believed the p r e c i s i o n of a l l weighings i n t h i s work i s ± 0.0001 gms ( l a ) . Sample D i s s o l u t i o n The.chemical procedure for -dissolution of. the ultramafic separates was modified s l i g h t l y from the standard U.B.C. procedure (Appendix 1). The following procedure was used. 1) From 0.1 to 0.3 grams of sample i s weighed by d i f f e r e n c e on a f r e s h piece of aluminum f o i l and placed i n a t e f l o n beaker. 2) Rb^^ and S r ^ spikes are added using a P l a s t i p a c disposable syringe. The quantity of spike added i s determined by weighing the syringe p r i o r to and a f t e r dispensing the spike a l i q u o t . 3) About 20 mis. of U l t r a HF and a few mis. of HCIO^ are added to the sample. The t e f l o n beakers are covered with t e f l o n l i d s and placed on a warm hot plate i n a laminar flow hood f o r 12 hours. 4) A f t e r d i g e s t i o n , the samples are evaporated to dryness i n a laminar flow hood. A few mis. of U l t r a HF i s then added and the samples taken to dryness. To convert the p r e c i p i t a t e d material to c h l o r i d e s , about 10 mis. of U l t r a 2.5 N EC1 i s added and the samples taken to dryness. 5) The p r e c i p i t a t e d c h l o r i d e s are taken up i n 2.5 N Quartz HC1, placed i n a centrifuge tube, and centrifuged for 5 minutes i n order to separate any i n s o l u b l e m a t e r i a l . -11-6) T h e - ' i o n exchange co lumns a r e p r e p a r e d as d e s c r i b e d i n A p p e n d i x 1'.;' 7) The sample s o l u t i o n i s t h e n c a r e f u l l y p o u r e d o n t o t h e i o n exchange r e s i n f o r s e p a r a t i o n o f Rb and S r a c c o r d i n g t o t h e p r o c e d u r e s d e s c r i b e d i n A p p e n d i x 1. The c h e m i c a l p r o c e d u r e f o l l o w e d f o r d i s s o l u t i o n o f t h e s u l p h i d e s ample s was s i m i l a r . The f o l l o w i n g p r o c e d u r e was f o l l o w e d : 1) A p p r o x i m a t e l y 0.5 grams o f sample i s w e i g h e d by d i f f e r e n c e on a c l e a n p i e c e o f a luminum f o i l , and p l a c e d i n a t e f l o n b e a k e r . Rb and Sr s p i k e s a r e added t o t h e sample as p r e v i o u s l y d e s c r i b e d . 2) The s a m p l e s a r e m o i s t e n e d w i t h U l t r a H ^ O . A b o u t 20 m i s . o f 6.2 N U l t r a HC1 i s t h e n a d d e d . The s ample s a r e c o v e r e d w i t h t e f l o n l i d s and p l a c e d on a h o t p l a t e a t a s e t t i n g o f 4 (=100° C . ) f o r 24 - 48 h o u r s . 3) A f t e r d i g e s t i o n t h e t e f l o n l i d s a r e removed and t h e s ample s e v a p o r a t e d t o d r y n e s s . The p r e c i p i t a t e i s m o i s t e n e d w i t h U l t r a ' . H2O, t h e n 20 m i s . o f 2.5 N U l t r a HC1 i s added and t h e s a m p l e s a r e e v a p o r a t e d t o d r y n e s s . The p r e c i p i t a t e i s t a k e n up i n 2.5 N Q u a r t z HC1, c e n t r i f u g e d and p l a c e d on t h e i o n exchange r e s i n . 89 D u r i n g t h e c h e m i c a l d i s s o l u t i o n o f some s a m p l e s , a S r t r a c e r was added i n o r d e r t o e s t i m a t e t h e y i e l d f rom t h e i o n exchange c o l u m n s . T h i s t r a c e r i s d i s c u s s e d i n some d e t a i l i n S e c t i o n I I-5. I o n Exchange - ~ A c a l i b r a t i o n o f t h e i o n exchange co lumns w i t h r e s p e c t t o 8 e l e m e n t s was c a r r i e d o u t t o d e t e r m i n e p o s s i b l e o v e r l a p s w i t h S r e l u t i o n d u r i n g r o u t i n e sample p r o c e s s i n g . The r e s u l t s a r e shown i n F i g u r e 1. - 1 2 -Figure .1: Calibration of ion exchange columns showing the aliquots in which 8 elements are collected. -13-Figure 1 indicates that none of the elements examined overlap with Sr i n the 90 - 100 ml. a l i q u o t . Despite the large separation of Sr and Mg i t was found that o l i v i n e and orthopyroxene samples frequently produced a noisy Sr beam i n the mass spectrometer. Other workers (e.g., Brueckner, 1974) have a t t r i b u t e d t h i s to Mg on the filament. Consequently, a l l o l i v i n e and orthopyroxene samples were passed through the columns twice, r e s u l t i n g i n a reduction of s i g n a l noise. Presumably a small quantity of Mg t a i l s into the Sr a l i q u o t . The metals present i n sulphide minerals a l l pass very quickly through the columns, and so apparently do not i n t e r f e r e with the Sr mass spectrometry. In order to obtain a s a t i s f a c t o r y Sr s i g n a l from some galena samples, i t was necessary to combine 3 separate 0.5 gram unspiked samples. To prevent interference with the Sr s i g n a l due to t a i l i n g of heavy metals into the Sr aliquot of these e x t r a o r d i n a r i l y large samples the following procedure was followed: 1) The 0.5 gram samples are dissolved separately and passed through the ion exchange columns. 2) Each 90 - 100 ml. Sr. aliq u o t i s c o l l e c t e d . The three aliquots are mixed, evaporated to dryness, and then re-dissolved i n a few mis. of 2.5 N Quartz HC1. 3) This mixed Sr s o l u t i o n i s passed through an ion exchange a second time and the 90 - 100 ml. aliq u o t c o l l e c t e d for mass spectrometric a n a l y s i s . Calcium and chromium overlap with the Rb 55 - 65 ml. a l i q u o t . The large quantity of calcium present i n c a l c i t e and clinopyroxene samples causes a large b u i l d up of C a C l 9 on the tantalum filament during drying -14-of the RbCl. I t was found that a s a t i s f a c t o r y beam could be obtained by using a small drop of sample and heating the filament slowly to melt the CaC^. The CaCl^ presumably oxidizes upon melting, leaving a t h i n f i l m of residue which does not s e r i o u s l y i n t e r f e r e with the Rb v o l a t i z a t i o n and i o n i z a t i o n during mass spectrometry. II-4 Isotope D i l u t i o n Analysis of Rubidium Spike 87 Rb spike of uncertain past h i s t o r y was used i n the isotope d i -l u t i o n analysis of rubidium. 85 87 Isotopic Composition. The Rb /Rb r a t i o of the spike was given as 00(2008 on the b o t t l e which was found i n the Geophysics b u i l d i n g . The 85 87 "i-measured Rb /Rb r a t i o was 0.00804 - 0.0002. This measured r a t i o 87 8S + * + gives Rb and Rb i s o t o p i c abundances of 0.9,9:2:11 - 0.000:2. and 0.0079 -0.0002. Concentration. The concentration of the rubidium spike given on i t s container was 0.03253 moles/gram. Presumably t h i s concentration was determined g r a v i m e t r i c a l l y when the spike was prepared or by c a l i -b ration with Rb standard s o l u t i o n . To check the concentration of the spike, a rubidium standard was prepared using rubidium chloride (N.B.S. standard 984). The concentration of t h i s standard was deter-mined g r a v i m e t r i c a l l y . Replicate determinations of the spike concen-t r a t i o n were made by mixing weighed amounts of the standard and spike solutions, determining the i s o t o p i c composition of the mixture on the -15-mass spectrometer, and then determining the composition of the spike by inverse isotope dilution analysis. A detailed description of the standard preparation and spike calibration procedure i s given in Appendix 2. Table 2 gives the concentration of the standard. Table 3 gives the results of re-plicate spike calibrations. Table 2: " Concentration of Rb standard i • • . , „, „.. . , r T T „ concentration of standard, sample weight moles of RbCl weight of H„0 . , o v * & " 2 ymoles Rb/gram p.p.m. Rb 0.01057 ± (8.740 ± 0.041) 998.3 ± .4 gms. (8.755 ± .041 4.474 ± 0.00005 x i o ~ 5 x 10"2 . 0.035 Table 3: Calibration of Rb spike weight Rb standard weight spike /„,85, 87, (Rb /Rb ) meas. Rb p.p.m. Rb ymoles/gram 0.12000 0.0001 0.04177 0.0001 1.77 + 0.03 2.86 ± 0.07 0.03291 + 0.0008 0.02646 0.0001 0.11058-0.0001 0.389 ± 0.007 2.95 +'0.08 0.03395 0.0009 average values 2.91 ± 0.05 0.03349 ± 0.0006 -16-Rubldium Mass Spectrometry Rubidium produces an extremely intense ion beam during mass spectro-metric measurements. A small quantity of rubidium produces a large ion beam and a small change i n filament temperature produces a large change i n the beam i n t e n s i t y . It i s very easy to overheat the sample, leading to a r a p i d l y f a l l i n g s i g n a l or a complete los s of the sample. It was ..' found that a sin g l e tantalum filament source produced the most e a s i l y c o n t r o l l a b l e s i g n a l . The rubidium peaks often suffered from extreme peak top noise. During the period of experimentation the peak top noise was reduced by making the following changes i n procedure: 1) Recalibrating the ion exchange columns to obtain a purer Rb "cut", thus reducing impurities. 2) Oxidizing the RbCl to Rb 20 by heating the filament to a d u l l red glow p r i o r to loading i n the mass spectrometer. 3) Loading the RbCl dissolved i n d i l u t e u l t r a HC1 as suggested by Shields et a l . (1972). These modifications s i g n i f i c a n t l y reduced the peak top noise pro-blem. The Rb si g n a l remained i n general an order of magnitude n o i s i e r than the Sr s i g n a l . The noise may possibly be caused by small f l u c t u a t -ions i n the filament current or impurities on the filament causing r e t a r d -a t i o n or enhancement of i o n i z a t i o n . The following procedure was used i n determining the Rb i s o t o p i c r a t i o : 1) The sample i s taken up i n d i l u t e U l t r a HC1. One drop of sample i s dried on a sing l e tantalum filament ribbon at 1.5 amps for 3 minutes. 2) The filament i s heated to a d u l l red glow f or 10 seconds. -17-3) The filament i s loaded i n the mass spectrometer. The source region 4!s evacuated to 10 ^ t o r r . 4) The sample filament current i s increased u n t i l the presence of Rb i n the mass spectrum i s f i r s t detected. The sample i s focused to maximum i n t e n s i t y and the filament current adjusted so that the 87 most intense peak (usually Rb ) produces a 0.5 mv s i g n a l , correspond-ing to an ion current of 5 x 10 ^ amps. 5) The sample filament current i s increased u n t i l the l e a s t intense 85 Rb peak (usually Rb ) produces a 0.2 mv s i g n a l (corresponding to an ion current of 0.2 x 10 ^ amps). The s i g n a l i s focused to maximum i n t e n s i t y . '6) If the s i g n a l i s r i s i n g , the filament current may be s a f e l y increased, and data c o l l e c t e d on the 300 mv or 1 v scale of the VRE. If the s i g n a l i s stable or f a l l i n g , data i s c o l l e c t e d on the 100 mv scale. 7) Data c o l l e c t i o n begins. Background i s measured at 86 or 89.5, then several sets of 85 and 87 measurements are made by magnet current peak hopping, followed by a f i n a l background measurement. Several sets of data of t h i s type are made. Peak heights are recorded on 8 c a chart recorder equipped with an expanded scale device. The Rb O J/ Rb r a t i o i s determined by measuring the peak heights i n millimeters d i r e c t l y from the chart. -18-Rb Blanks Three t o t a l blanks were measured during the course of t h i s work, The r e s u l t s are given i n Table 4. Table 4: Rb Blank measurements Di s s o l u t i o n Date of n grams picomoles Method preparation Rb Rb HC1 15/11/75 1.6 19 HC1 21/01/76 1.8 21 HF 23/10/75 8.9 104 Six determinations were made of the amount of Rb i n various reagents used during the experiments. The r e s u l t s are given i n Table 5. Table 5: Amount of Rb i n 20 ml al i q u o t s of reagent Reagent Date n grams Rb picomoles Rb 2.5 N Quartz HC1 21/01/76 1.2 12 from ion exchange columns 2.5 N Quartz HC1 21/01/76 0.11 1.3 6 N U l t r a HC1 21/01/76 0.39 4.5 U l t r a HF 21/01/76 0.23 2.7 Quartz H 20 21/01/76 0.16 1.9 Reagent HF 21/01/76 2.1 24 Table 5 indicates that Quartz H 20 i s the purest reagent of those studied. Also, the 2.5 N Quartz HC1 appears to contain l e s s Rb than -19-the U l t r a reagents, although due to the very small Rb /Rb r a t i o the diff e r e n c e i n Rb content may be below the detection l i m i t . Two conclusions can be drawn from t h i s reagent study. F i r s t l y , no large d i f f e r e n c e was found i n the rubidium content of the U l t r a HF and the U l t r a HC1. Thus the discrepancy between the two HC1 blanks i n Table 4-3 and the HF blank i s probably spurious. A blank of 1.7 ngrams i s subtracted from a l l of the Rb concentration determinations i n t h i s study. As indicated i n Section II-6, the Rb analysis of U.S.G.S. standard PCC-1 confirmed the appropriateness of t h i s blank. The spurious Rb blank of the HF d i s s o l u t i o n i s prob-ably due to contamination by foreign material during chemical pro-cessing. Such large v a r i a t i o n s i n the measured Rb blank underline the necessity for confirmation of r e s u l t s by r e p l i c a t e analysis when analyzing for Rb at the sub-p.p.m. l e v e l . Data Reduction Method. The equations used i n determining the concentration of Rb i n the sample are given i n Table 5. The BASIC computer program used for data reduction i s given i n Appendix 3. Example. The following example i l l u s t r a t e s the data reduction procedure: Data: spike weight = 0.1426 ± 0.0001 sample weight = 0.52074 ± 0.0001 measured Rb 8 5/Rb 8 7 = 0.454 ± 0.005 P t 8 7 = 99.21 ± 0.02 P t 8 5 = 0.79 ± 0.02 -20-P s 8 5 = 7 2 . 1 6 5 4 ( 1 ) P s 8 7 = 2 7 . 8 3 4 6 ( 1 ) blank = 0.00002 umoles spike concentration = 0.03349 ± 0.0012 ymoles/gram Calculation: Ns _ 0.454(00.21) - 0.79 „ Q y 4 3 Nt " 72.1654 - 0.454(27.8346) • - 3.547 x 10~ 4 Rb moles = (0.03349) x (0.01426) x (0.743) -4 3.547 x 10~ 4 - 0.00002 = 3.547 x 10 • ,  Rb ymoles/gram = 0.52074 ,-4 - 6.43 x 10 Rb p.p.m. = 0.0549 . ,0.005 0.454 , 2 • ' %2 h = A = ( 7 T T F T X (99.21) + ( 0 . 0 2 ) * ^ = 0 . 5 0 B = k0.454 0.50 (0.454)(99.21) - 0.79 - ° * 0 1 1 %Rb± = f ( 0 . O i l ) 2 + &^1)2 + ( ° - 0 0 1 2 ) 2 + (O-OOO1 ^  + (0-0001 x2 , h Lv v 0 . 4 5 4 ; ^ V 0.03349 ; + ^0.52074 } + ( 0.01426 ) J error in measurement ratio spike sample i k • , measurement sy-LJus weieht spxtce & spike composition concentration w e * S n t weight + + .' + + + %Rb± =[(1.21 x 1 0 - 4 ) + (1.21 x 1 0 - 4 ) + (1.28 x 10" 3) + (0) + (4.9 x 1 0 - 5 ) ] h = 4.0% x 100 . . Rb p.p.m. = .0549 ± .002 (1) 85 87 The Rb and Rb abundances reported here (Catanzano et a l . , 1969) arg 5from 7a Rb/Rb measured ratio of 2.5926 ± 0.0017. The measured Rb /Rb ratio of natural Rb on the U.B.C. mass spectrometer was 2.605 ± 0.037, in agreement with the published results. -21-Table 6: Rb concentration calculations a) Rb moles '= spike concentration ( u m o x e s ) x g r ams spike x T^T gram Nt where: Ns _ # moles sample Nt # moles spike • a.-pt 8 7 - P t 8 5 P s 8 5 - Hm-Ps 8 7 where: Rm Pt Pt Ps 87 85 85 87 85 87 measured Rb /Rb ratio 87 % abundance of Rb in spike 85 = % abundance of Rb = natural % abundance of Rb in spike 85 Ps = natural % abundance of Rb 87 b) Rb ^urcoles^ _ Rb(umoles) - Rb blank (umoles) gram sample sample weight Rb(p.p.m.) = Rb ( V ^ ° ^ S ) a x atomic weight Rb sample gram sample ° c) Uncertainty in Rb concentration i s given by: A = [(Rm1 x P t 8 7 ) + (PtV-J* 2 A E" = Rm P t 8 7 - P t 8 5 T ^ . 1 0 0 X [ B 2 + ( ^ , 2 + ( i t 1 , 2 • ( | f , 2 + ( ^ ) 2 ] 'Sw 'Tw 85 87 where: Rm = measured Rb /Rb ratio Sc- = spike concentration Sw = Sample weight Tw = Spike weight ± = l a of the quantity (1) ^assuming a standard deviation of 0.0001 on a l l weight measurements (Section II-3). -22-The foregoing example points out some important aspects of the analyses which have been made. In the example the total amount of Rb determined was =.00035 ymoles, compared to a total blank of 0.00002 umoles or 6% blank Rb. As the uncertainty i n the blank i s indeterminate but may be large, and is unaccounted for in the subsequent uncertainty calculation, the uncertainty in the determined concentration may be much larger than indicated. The calculation of the uncertainty points out the contribution to the analytical uncertainty from the various experimental measurements. The uncertainty in the spike concentration i s by far the largest contribution, while the uncertainties in the sample weight and spike composition are negligible. -23-i • II-5 Isotope Dilution Analysis of Sr Strontium 89 Tracer A strontium 89 isotopic tracer produced by Union Carbide at the Oak Ridge National Laboratory was used to estimate the yield from the ion exchange columns. Strontium 89 decays by 3 decay and has a half l i f e of 50.5 days. The reported radiochemical purity of this tracer is S r 9 0 + Y 9 0 + S r 8 9 > 99% with S r 9 0 + Y 9 0 < 10%. During the last stages of the work the isotopic composition of the tracer was measured on the mass spectrometer. The tracer was found to contain isotopes of mass 85, 86, 87, 88 and 90. The 85 peak was observed at low temperatures and i s probably due to a very small quantity of Rb within the tracer. The ratio of the 88 to 90 peaks was found to change markedly with time, 90 suggesting the reported presence of Y within the tracer. Background during normal data collection i s measured at 89.4. It was found that 90 90 there i s sufficient resolution of the Sr + Y peak from the back-ground position to prevent interference. A number of standard data collection blocks were run on the 86, 87 and 88 peaks. The deter-mined ratios for 7 blocks of data are given in Table 7. . The 87/86 ratios in Table 7 have not been normalized. Table 7: Measured isotopic composition of the Sr tracer 87/86 86/88 87/88 . 0.7084 0.06697 0.04744 0.7145 0.06849 0.04394 0.6964 0.06862. 0.04779 0.7123 0.06494 0.04626 0.7184 0.06118 0.04395 0.7123 0.06392 0.04553 0.7299 0.05865 0.04281 average: 0.7132 average: 0.06468 average: 0.04610 -24-Table 7 indicates that the 87/86 r a t i o s f a l l close to the r a t i o f o r common strontium, i n d i c a t i n g that the tracer has probably suffered a small amount of Sr contamination. Most i n t e r e s t i n g are the 86/88 r a t i o s , which are much l e s s than the n a t u r a l l y occurring Sr r a t i o of 0.1194. Apparently the tra c e r has been enriched i n an isotope of 88 88 mass 88. This isotope may be Zr (half l i f e 150d) or Sr . Anoma-87 88 l o u s l y low Sr /Sr r a t i o s were measured on some sulphide samples to which the tracer had been added and which had been processed through the i on exchange columns. As the ion exchange columns would probably separate Zr and Sr, i t i s l i k e l y that the tracer i s , i n f a c t , enriched 88 i n Sr .• The consequences of this.enrichment, which was discovered only i n the f i n a l stages of t h i s work, are unfortunate. A l l samples to which the tracer was added and on which the Sr concentration was 8 8 84 determined by measuring the Sr /Sr r a t i o w i l l show anomalously high Sr concentrations, but t h i s e f f e c t i s l i k e l y to be minor. More serious-8 7 86 l y , the Sr /Sr r a t i o s of such samples w i l l be anomalously low, as 86 88 these r a t i o s are normalized, assuming a Sr /Sr r a t i o of 0.1194. 86 88 Addition of the tra c e r w i l l lower the Sr /Sr r a t i o of the mixture, 87 86 r e s u l t i n g i n an indeterminate lowering of the normalized Sr /Sr r a t i o . The magnitude of both these e f f e c t s w i l l . b e proportional to the concen-t r a t i o n of Sr i n the sample. In p r a c t i c e , i t was found, that a s i g n i f i c a n t 8 7 86 lowering of the Sr /Sr r a t i o was caused by the addition.of tracer when the sample contained l e s s than 1 p.p.m. Sr. The e f f e c t s of t h i s phenomenon with reference to s p e c i f i c samples are discussed i n Chapters I I I , IV and V. Strontium Spike A strontium 84 spike (SRM 988) which was diluted on February 13, 1974 by D. Birn ie and R.L. Armstrong, was used. Isotopic Composition. The isotopic composition on the spike as lis t e d in the N.B.S. certificate of analysis i s given in Table 8. 86 87 88 Because of the extremely low abundances of Sr , Sr and S~c in the spike, measurement of these ratios on the mass spectrometer must be done at high filament temperature and "low peak intensity... , Under these operating conditions significant variations in the measured ratios may be caused by volatization of small quantities of common strontium from the source region. The average of three determinations of the isotopic composition of the spike i s given in Table 8. Ap-parently the spike has been contaminated by a small quantity of common Sr. The measured composition of the spike i s used in the Sr isotope dilution calculations. Table 8: Isotopic composition of N.B.S. Sr spike SRM-988 86/84 88/84 87/84 N.B.S. certi f i c a t e of 0.000589 0.000386 0.000098 analysis Measured March 1976 0.00093 0.0029 0.00032 -26-Spike Concentration. The concentration of the spike determined g r a v i m e t r i c a l l y when i t was prepared i n February 1974 was 0.01130 ± 0.00001 ymoles Sr/gram. Replicate c a l i b r a t i o n s subsequent to the preparation gave concentrations of 0.01128 ymoles Sr/gram and 0.01127 ymoles Sr/gram ( B i r n i e , 197 6). During the course of t h i s work r e p l i -cate c a l i b r a t i o n s were made of the spike using the same standard as was used i n the i n i t i a l c a l i b r a t i o n s . These c a l i b r a t i o n s (February 1976) gave concentrations of 0.01120 ± 0.0002 ymoles Sr/gram and 0.01125 ± 0.001 ymoles Sr/gram i n reasonable agreement with the pre-v i o u s l y determined concentration. Accordingly a concentration of 0.0113 ymoles Sr/gram for the spike i s used for the isotope d i l u t i o n c a l c u l a t i o n s . Sr Blanks Six strontium blanks were measured i n order to determine the con-89 tamination from reagents, glassware and dust p a r t i c l e s . Sr i s o t o p i c tracer was added to 2 of these blanks and as the blank was determined 88 84 by measuring the Sr /Sr r a t i o the r e s u l t s have been discarded. In addition to these determinations of the strontium contamination, the 87 86 Sr /Sr r a t i o was measured on one of the blanks. The r e s u l t s are given i n Table 9. ._. .In addition to these measurements, the Sr content of several reagents used during the experiments was made, and the r e s u l t s are l i s t e d i n Table 10. -27-Table 9: Sr blank measurements 87 86 Blank Date ngrams Sr picomoles Sr Sr /Sr ± 26 299 0.7125 0.005 34 392 -18 211 - -14 165 Table 10: Sr content of 20 ml. aliquots of reagent Reagent Date ngrams Sr picomoles Sr 6 N Ultra HC1 March '76 1.4 16 6 N Ultra HC1 Sept. '75 2.3 26 2.5 N Q HC1 Sept. '75 4.0 45 2.5 N Q HC1 March '76 2.0 23 Q H 20 March '76 9.2 105 Ultra HF March '76 1.4 16 Reagent HF March '76 4.5 51 Column March '76 2.1 24 .1 Sept. '75 (twice through 2 ion exchange Sept. '75 column) 3 Nov. '75 4 Feb. '76 -28-Table 19 suggests a progressive reduction of the blank Sr throughout the course of the work. The data i s not s u f f i c i e n t to r u l e out an i n c o n s i s t e n t l y f l u c t u a t i n g blank,however, and so a t o t a l system blank of 19 ngrams, the average of blanks 1, 3 and 4 has been subtracted from a l l Sr concentration determin-ations. Table 10 indicates the reagents used i n t h i s work contain an i n s i g n i f i c a n t amount of Sr when compared to the " t o t a l system" blanks of Table 9-. It i s therefore l i k e l y that the Sr of the t o t a l system Sr blank i s due to fore i g n material contamination, or to Sr contamination from the Rb spike. The strontium i s o t o p i c composition of blank 1 was measured as .7125 ± .005. Two possible sources of blank strontium are (I) chalk dust and ( i i ) rock fragments from the Vancouver area. 87 86 It i s u n l i k e l y that either of these materials has a Sr /Sr as high as .712 and so the measured r a t i o of .7125 should be considered a maximum value. Strontium Mass Spectrometry Procedure. A double filament tantalum ribbon source was used for the strontium i s o t o p i c measurements. The sample was loaded as described i n the standard U.B.C. procedures (Appendix 1), with the modification that the en t i r e Sr aliquot was loaded upon the sample filament. In order to obtain a stable or r i s i n g s i g n a l with very small quantities of strontium the i o n i z i n g filament i s set at a standard -29-operating temperature (310 Ma) and the sample filament temperature i s increased slowly over a period of hours. A sketch of a t y p i c a l stron-tium run i s given i n Figure 2. Isotope d i l u t i o n analysis of strontium involves measuring the O A QC. Q Q r a t i o of Sr to Sr or Sr . An i n i t i a l measurement of t h i s r a t i o i s made as soon as a s a t i s f a c t o r y stable beam has been obtained. The i s o t o p i c r a t i o i s determined by manual magnet current switching from mass 84 to 86 or 88. Peak heights are recorded on a s t r i p chart recorder equipped with expanded scale. Background was measured at mass p o s i t i o n 89.4. 87 Rf) The Sr /Sr r a t i o i s measured on both spiked and unspiked samples using the on-line Nova 1210 minicomputer. The data i s c o l l e c t e d i n blocks, using the standard U.B.C. program i n which a "block" constitutes 85 an i n i t i a l background measurement, an i n i t i a l Rb measurement, ten sets of 88, 87 and 86 peak measurements followed by a f i n a l background and R b 8 5 measurement. The S r 8 7 / S r 8 6 r a t i o i s normalized to a S r 8 6 / S r 8 8 r a t i o of 0.1194 and corrected for Rb before printout. Blocks of data are c o l l e c t e d as soon as the Sr s i g n a l r i s e s to an i n t e n s i t y of 10 mv. Rising signals often produce a strong intense s i g n a l a f t e r 5-6 88 hours that may s t a b i l i z e at a Sr peak i n t e n s i t y as great as 0.5 87 v o l t s . At t h i s i n t e n s i t y 10 blocks of data can usually give a Sr / 86 Sr r a t i o with an uncertainty of ± 0.0001 ( l a ) . More generally, with l e s s than .5 ugrams Sr loaded on the sample filament the s i g n a l 88 s t a b i l i z e s at a Sr peak i n t e n s i t y of - 50 mv. F i f t e e n blocks of 87 86 data c o l l e c t e d at t h i s i n t e n s i t y can usually give Sr /Sr r a t i o with an uncertainty of ± 0.001 ( l a ) . - 3 0 -Timefhoiirs) Figure 2: Typical strontium mass spectrometer run showing the variation in beam intensity with sample current. -31-In the ease of spiked samples, a second determination of the S r ° 4 86 88 ~ to Sr or Sr i s made a f t e r s t a b i l i z a t i o n of the s i g n a l . I t was found that i n general 0.03 ygrams of strontium was the minimum quantity required to produce a stable beam. With due care to 87 86 avoid over-heating the sample, good Sr /Sr data can be c o l l e c t e d using as l i t t l e as 0.02 ygrams of Sr on the filament. Signal A m p l i f i c a t i o n . Mass spectrometric peaks from the U.B.C. mass spectrometer are produced by amplifying the ion current from the Faraday cup with a Cary model 31 v i b r a t i n g reed electrometer. The voltage output from the electrometer i s passed d i r e c t l y to the chart recorder and passed to a Hewlett Packard model 2212A voltage to frequency converter for on-line data processing by the Nova 1210 d i g i t a l computer. In order to examine the p o s s i b i l i t y that the electrometer produces a non-linear s i g n a l from extremely low ion currents such as were encoun-tered during the course of the work, the N.B.S. standard S.R.M.-987 was run at s i m i l a r l y low i n t e n s i t i e s . The r e s u l t s are given i n Table 11. Table 11: 87 86 Isotopic measurements of the Sr /Sr r a t i o of N.B.S. standard SRM-987 at various i n t e n s i t i e s VRE Scale VRE I n t e n s i t y ^ 87/86 lcr // Blocks 100 mv 0.048 0.70995 .0.004 11 100 mv 0.053 0.71072 .0007 5 100 mv 2.7 0.71044 0.001 12 100 mv 2.8 0.71000 o.ooi 26 300 mv 2.6 0.71052 .0004 13 1 mv 4.0 0.71063 .0001 12 average values on a scale of 0-5 -32-Table 11 indicates that the VRE + VFC conversion i s linear within the detection limit even at extremely low intensities. Measured S r 8 7 / S r 8 6 Ratios of N.B.S. Standard SRM-987. The S r 8 7 / 86 Sr ratios of the N.B.S. standard 987 measured during the course of the work are.given in Table 12. 87 86 Table 12: Sr /Sr ratios of the N.B.S. standard SRM-984 Date o 87 / c 86 Sr . /Sr la Sept. 9/75 0.71046 0.00007 Sept. 15/75 0.71035 0.00009 Oct. 29/75 0.71052 0.00006 Nov. 1/75 0.71024 0.00008 Nov. 17/75 0.71063 0.00009 Dec. 18/75 0.71041 0.00006 Jan. 8/76 0.71061 0.00009 Feb. 23/76 0.71070 0.00012 Feb. 25/76 0.71050 0.00010 April 18/76 0.71092 0.00010 average 0.71053 0.00003 The Sr u //Sr° u ratio of this standard given on the certificate of 87 analysis i s 0.71014. More recent precision measurements of the Sr / 86 Sr ratio of this standard, performed during lunar-sample-oriented studies are close to 0.71023 (Nyquist et a l . , 1975). Accordingly, a l l 87 86 Sr /Sr ratios reported in this work have been adjusted to this accepted value by subtracting a factor of 0.00030. -33-8 7 Rb C o r r e c t i o n . E r r o r s may be i n t r o d u c e d i n t o t h e measured S r / 86 S r r a t i o by t h e Rb c o r r e c t i o n f o r two r e a s o n s . Many s a m p l e s w e r e s p i k e d w i t h b o t h R b 8 7 and S r 8 4 . I n c o m p l e t e s e p a r a t i o n o f Rb and S r o n t h e i o n exchange co lumns may have a l l o w e d 87 c o n t a m i n a t i o n o f t h e S r s a m p l e s w i t h e n r i c h e d Rb . A s t h e Rb c o r -85 87 r e c t i o n assumes a n a t u r a l Rb / R b r a t i o o f 2.5926 ( C a t a n z a n o e t a l . , 1969) s u c h c o n t a m i n a t i o n c o u l d r e s u l t i n a n i n d e t e r m i n a t e e r r o r i n t h e 87 86 measured S r / S r r a t i o s . The Rb c o r r e c t i o n assumes a l i n e a r d e c a y o f t h e Rb s i g n a l i n t e n s i t y d u r i n g e a c h b l o c k o f measu remen t s . T h i s i s g e n e r a l l y t r u e f o r s i g n a l s o f l o w i n t e n s i t y . A t g r e a t e r i n t e n s i t i e s , h o w e v e r , t h e decay o f t h e Rb s i g n a l i s more n e a r l y e x p o n e n t i a l . T h i s 8 7 86 c o u l d r e s u l t i n a n i n d e t e r m i n a t e l o w e r i n g o f t h e c o r r e c t e d S r / S r r a t i o . I n o r d e r t o a v o i d t h i s , b l o c k s w h i c h s u f f e r e d f r o m a n e x c e s s i v e l y l a r g e Rb c o r r e c t i o n w e r e d i s c a r d e d . I n c a s e s where s u f f i c i e n t d a t a were c o l l e c t e d , b l o c k s were r e j e c t e d w i t h Rb c o r r e c t i o n s g r e a t e r t h a n 0.002 ( 0.3%). F o r c a s e s i n w h i c h o n l y a l i m i t e d number o f measurements were o b t a i n e d , b l o c k s o f d a t a w i t h a Rb c o r r e c t i o n as l a r g e as.0.03 (5%) were r e t a i n e d . I n o r d e r t o d e t e r m i n e i f t h e Rb c o r r e c t i o n may have p r o -87 86 duced an e r r o r i n t h e S r / S r r a t i o s , t h e c o r r e l a t i o n c o e f f i c i e n t o f 87 86 t h e m a g n i t u d e o f t h e Rb c o r r e c t i o n w i t h t h e S r / S r r a t i o was c a l c u l a -t e d f o r e a c h s a m p l e . V a l u e s o f t h e c o r r e l a t i o n c o e f f i c i e n t v a r y be tween -0.76 and +0.62. None o f . t h e c o r r e l a t i o n s a r e s i g n i f i c a n t a t t h e 90% c o n f i d e n c e l e v e l . I t i s b e l i e v e d t h a t t h e Rb c o r r e c t i o n has n o t c a u s e d 8 7 86 e r r o r s i n t h e measured S r / S r r a t i o s . -34-Data Reduction The Sr concentration of spiked samples i s u s u a l l y determined by measuring the S r 8 4 / S r 8 ^ r a t i o of the sample-spike mixture. The number 86 88 84 of ymoles of Sr + Sr + Sr i n the sample i s given by: (86 + 88 + 84) (Ra) t - [(Rb) x ( R c ) t ] Sr (ymoles) = r . — - — ; C ( R b > m x ( R c ) J - (Rc)_ (1) where: (Ra) f c = S r 8 4 / S r (Ra) s - S r 8 4 / S r <*>>» = S r 8 4 / S r 1 ( R c ) t = S r 8 6 / S r ( R O s = S r 8 6 / S r T l * moles i of S r( 8 4 + 8 6 + 8 8 ) o f spike added 84 88 Certain samples were so overspiked that the Sr /Sr r a t i o was measured on the mass spectrometer instead of the S r 8 4 / S r 8 ^ r a t i o . In 84 88 84 86 these cases the Sr 7Sr°° was converted d i r e c t l y to the Sr /Sr r a t i o by the following r e l a t i o n : S r 8 4 = S r 8 4 f S r 8 ^ S i * * I** X ( f r W * (2) 88 86 where (Sr /Sr ) r e f e r s to the r a t i o of the spike/sample mixture and i s given by: , S r 8 8 •- S r 8 8 s a + S r 8 8 t (—80") — 8 6 5 T Sr •« Sr s a + Sr t (3) where the subscripts sa and t r e f e r to the sample and spike r e s p e c t i v e l y . -35-88 86 As the value of (Sr /Sr ) A i s a function of the Sr concentration, 84 88 for samples i n which the Sr /Sr r a t i o was measured an i t e r a t i v e 84 86 88 86 procedure was used to determine the Sr /Sr r a t i o . The Sr /Sr r a t i o was i n i t i a l l y assumed to be 8.3752 (natural Sr) and subsequently decremented to convergence. In order to correct f o r Sr i s o t o p i c f r a c t i o n a t i o n i n nature and 87 86 during mass spectrometry the Sr /Sr r a t i o s measured during data 86 88 c o l l e c t i o n are normalized to a Sr /Sr r a t i o of 0.1194 by the mini-86 88 computer. As the Sr spike contains Sr /and Sr i n a proportion 87 86 s i g n i f i c a n t l y d i f f e r e n t than natural Sr the Sr /Sr r a t i o s of spiked 86 88 samples must be renormalized to the true Sr /Sr r a t i o of the sample-spike mixture. This r a t i o i s given by the inverse of equation (3). 87 86 The normalization factor f o r the Sr /Sr r a t i o s which have been pre-v i o u s l y normalized to 0.1194 i s given by: .1194 + ( S r 8 6 / S r 8 8 ) m - ,1194 N = 2 ' j (4) ._ 86 #_ 88. (Sr /Sr )m 87 88 Thus the Sr /Sr r a t i o of the sample-spike mixture i s given by: ( S r 8 7 / S r 8 6 ) m = ( S r 8 7 / S r 8 6 ) R x N (5) , 8 7 / c 86. , c 8 7 / 0 86 where: (Sr /Sr ) = measured Sr /Sr r a t i o normalized to .1194 87 86 The Sr /Sr r a t i o of the sample can be determined d i r e c t l y from (5) by: ,„ 87,_, 86. 87 86. r. , ymoles 86 spike ymoles 87 spike .... (Sr /Sr ) = (Sr /Sr ) l l + - — — — K — — 1 - - — — — K — — (6) s mL ymoles 86 samplej ymoles 86 sample -36-From (6) and (1) and the known S r 8 6 / ( S r 8 6 + S r 8 8 + S r 8 4 ) r a t i o of natural Sr the concentration of Sr can be found by the following r e l a t i o n s : 86 Sr 8 7umoles = ( — ^ -^-) x ( S r 8 6 + S r 8 8 + S r 8 4 ) (7) t o t a l measured . (86 + 88 + 84) , r . S r 8 7 . „ 86 , n = Sr + ( — ^ ) x Sr ymoles (8) Sr ymoles L g r°° s -1 „ „ „ „ i „ c ~ , - measured Sr ymoles - blank Sr ymoles sample Sr ymoles = K z :—r— e (9) sample weight v ' gram sample Sr p.p.m. = sample Sr ymoles x at. wt. Sr ( } gram v / The atomic weight of Sr used i n equation 10 i s to a small extent 87 86 a function of the Sr /Sr r a t i o . As the e f f e c t of these changes on the strontium concentration i s n e g l i g i b l e a constant Sr atomic weight of 87.63 has been used i n the c a l c u l a t i o n s . The following example i l l u s t r a t e s these c a l c u l a t i o n s : Spike data: spike concentration = 0.0113 ymoles/gram spike weight = 0.21276 grams ( S r 8 4 / S r 8 8 + 8 6 + 8 4 ) spike = 0.99618 ( S r 8 6 / S r 8 8 + 8 6 + 8 4 ) spike = 0.00093 ( S r 8 8 / S r 8 8 + 86 + 84 } s p i k e = 0 . 0 Q 2 9 87 (Sr / t o t a l Sr) spike = 0.00032 Sample Data: S r 8 6 / S r 8 6 + 8 8 + 8 4-- = 0.10602 S r 8 8 / S r 8 6 + 88 + 84 - Q^1M S r 8 4 / S r 8 6 + 88 + 84 = 0 > 0 0 6 0 4 sample weight = 0.50302 -37-Isotopic Measurements: 87/86 = 0.7101 84/86 = 8.62 88/86 = 8.3752 Calculation: i • Q , Q , , O O N .99618 - (8.62 x 0.00093) n n.. , n n 0 1 0-,, moles (86 + 84 + 88) = n,.„, _ x 0.1130 x 0.21276 sample (8.62 x .10602) - 0.00604 x 0.99968 = 0.002616 Q C. ymoles (Sr ) sample = 0.002616 x 0.10602 = 0.0002773 88 ymoles (Sr ) sample = 0.002616 x 0.88794 = 0.002323 ymoles ( S r 8 6 ) spike « (0.01131 x 0.21276 x 0.99968) x 0.00093 = 2.237 x l O - 6 88 — ft ymoles (Sr ) spike = (0.01131 x 0.21276 x 0.99968) x 0.0029 = 6.976 x 10 , e 86. 88. 2.237 x 10~6 + 2.773 x 10~4 - . 9 r. n„ (Sr /Sr ) = ^ -~ = 1.2003 6.976 x 10 + 2.323 x 10 0.12003 - .1194 N = ° ' 1 1 9 4 + I = o 9974 0.12003 ( S r 8 7 / S r 8 6 ) . = 0.7101 x 0.9974 = 0.70825 mix ( S r 8 7 / S r 8 6 ) — . . , • = 0.70825 1 + 2.773 x 10 - 4  N-6. 2.237 x 10~v~] - (0.00130 x .21276) x 0.00032 sample 2.773 x 10~~Zf = 0.7112 87 -4 -4 Sr moles = 0.7112 x (2.773 x 10 ) = 1.974 x 10 / , , n s (0.002616 + 0.0001974) - 0.00022 . Sr (ymoles/gram sample) = Q 5 0 3 Q 2 = °-°°516 Sr p.p.m. = 0.00516 x 87.63 = 0.452 The BASIC computer program used in these computations is given in Appendix 3. -38-II-6 Rb and Sr Analyses of U.S.G.S. Standard PCC-1 In order to check the r e l i a b i l i t y of the isotope dilution analyses of Rb and Sr, the U.S.G.S. standard PCC-1 was analyzed for Rb and Sr. Previous analyses reported in the literature are given in Table 13. Table 13:Previous Rb and Sr analyses of U.S.G.S. standard PCC-1 p. p. m. Sr p.p.m. Method Reference 0.07.5 0.369 isotope dilution Stueber & Ikramuddin (1974) 0.064 0.38 isotope dilution Chappell et a l . (1969) 0.077 0.37 x-ray fluoresence Chappell et a l . (1969) 0.062 0.42 isotope dilution Delaeter & Abercrombie (1970) a 055 0.36 isotope dilution Pankhurst & O'Nions (1973) A 0.2 gram sample of standard PCC-1 was prepared for Rb and Sr isotope dilution analyses according to the procedure described in sections II-2. The results are given in Table 14. Table 14: Rb and Sr analyses of U.S.G.S. standard PCC-no blank correction blank correction (1.7 ngms. Rb; 19 ngms Sr) Rb (p.p.m.) 0.077 0.066 Sr (p.p.m.) 0.443 0.315 The determined Rb concentration i s in good agreement with previously reported values, suggesting a blank of 1.7 ngrams i s suitable. The deter-mined Sr concentration i s slightly below previously reported concentrations suggesting a blank of 19 ngrams may in some cases be an overestimate. II-7 Decay Constant and Calculation of Ages A l l age determinations reported here are made by the classical isochron 87 T1 method (Nicolaysen, 1961). The decay constant of Rb used is 1.42 x 10 y r " 1 (Armstrong, 1974; Huster, 1974). II-8 Previous Low-level Rb and Sr Analyses Many groups of workers have done Rb and Sr geochemistry on materials containing less than 1 p.p.m. Sr. Table 15 l i s t s some of these workers with their reported Rb and Sr blanks. -AO-Table 15: Previously reported Rb and Sr blanks Reference Institution Rb blank (ngrams) Sr blank (ngrams) Steuber and Ikramuddin (1974) Miami University 4-10 10-70 Mark et a l . (1973) U.C.L.A. 2 2 Papanastassiou and Wasserburg (1973) C.I.T. 0.02 0.2 Compston et a l . (1971) Australian National University 0.45-0.14 0.90-0.19 DeLaeter Abercrombie (1970) Western Australian Institute of Technology 10 100 Brueckner (1974) Lamont-Doherty 0.3-0.07 1-10 The blank levels of 19 ngrams Sr and 1.7 ngrams Rb obtained during the course of this work have not approached the levels achieved in the lunar-sample-oriented clean laboratories of C.I.T. and Australian National University. Nevertheless the blank levels approach the levels of Brueckner (1974) and Stueber and Ikramuddin (1974). Previous Rb and Sr analyses by isotope dilution at U.B.C. carried out i n the Geophsics Department Laboratory have had reported Sr blanks of from 50 ngrams (Birnie, 1976) to 500 ngrams (Athaide, 1974), significantly greater than the values reported here. -41-CHAPTER III THE APPLICATION OF THE RB/SR DATING TECHNIQUE TO THE PINE POINT LEAD-ZINC DEPOSIT I I I - l Introduction The S r 8 7 / S r ^ ^ r a t i o s and Rb and Sr concentrations of s p h a l e r i t e , galena and associated carbonate mineral separates from the Pine Point Pb-Zn deposit have been determined. In t h i s chapter previous work on the Pine Point deposit i s summarized, with attention focused on the paragenesis, the age, and the o r i g i n of the deposit. Subsequently, the a n a l y t i c a l r e s u l t s are presented and discussed i n r e l a t i o n to the age and o r i g i n of the m i n e r a l i z a t i o n . III-2 Geologic Setting Pine Point i s a Pb-Zn deposit of the M i s s i s s i p p i V a l l e y type on the south shore of Great Slave Lake. The geologic s e t t i n g of Pine Point has been described by many workers including McConnell (1889), Cameron (1950), Douglas (1959), Belyea and Norris (1962), Norris (1965), -42-Jackson and Beales (1967) , Jackson and Folinsbee (1969), and S k a l l (1975). Lead-zinc m i n e r a l i z a t i o n occurs within reef and o f f - r e e f f a c i e s of the Devonian Pine Point Group ( S k a l l 1975). C o r r e l a t i v e with the Pine Point Group i s the Muskeg Formulation to the south, composed of evaporites and interbedded carbonates (Klingspor, 1969); and the Horn River Formation to the north which i s l a r g e l y shale (Norris, 1965). Apparently the Pine Point area',-was a topographic b a r r i e r that r e s t r i c t e d c i r c u l a t i o n i n the Elk Point basin to the south r e s u l t -ing i n the formation of the Muskeg evaporites (Law, 1935) . The Pine Point Group i s undelain by the Devonian Keg River Formation composed of green shale. These sediments form part of a conformable Paleozoic sequence which trends northwest and dips gently to the south (Norris, 1965). The dips of up to 12 feet per mile ( S k a l l , 1975) or 2 meters per ... kilometer have been a t t r i b u t e d to u p l i f t and t i l t i n g associated with the Nevadan (150 m.y.) and Laramide (65 m.y.) orogenies (Klingspor, 1969). This Paleozoic sequence of the Great Slave Lake area i s o v e r l a i n uncon-formably by Cretaceous shales to the north and Pleistocene sands to the west. A zone of en-echelon normal f a u l t s within the Precambrian rocks which underlie the Paleozoic sediments s t r i k e south-west along the southern shore of the East Arm of the Great Slave Lake. Aeromagnetic data suggest these f a u l t s continue beneath the Paleozoic cover (Norris, 1965). Campbell (1950) considers that these f a u l t s may have influenced sedimentation into the Paleozoic. This suggestion has been substantiated by the d e t a i l e d stratigraphy of S k a l l (1975). -43-M i n e r a l i z a t i o n occurs within the porous Presqu'ile dolomite which, as shown by S k a l l (1975), i s not a reef core but rather a diagenetic a l t e r a t i o n of several reef and o f f - r e e f f a c i e s of the Pine Point Group. The porosity was apparently produced by karsting _ during a Middle Devonian u p l i f t . Sphalerite and galena m i n e r a l i z a t i o n has been l o c a l i z e d by t h i s porosity. The ore bodies are of two types: massive and tabular, and show sharp contacts with the surrounding host rocks. III-3 Paragenesis A knowledge of the c r y s t a l l i z a t i o n h i s t o r y of the sulphides i s important i n assessing t h e i r s u i t a b i l i t y for dating by the Rb/Sr method. Formation of the minerals at one period of time from solutions with a constant Sr /Sr r a t i o i s a pr e r e q u i s i t e f o r obtaining a v a l i d isochron r e l a t i o n s h i p . Previous work on the textures, f l u i d i n c l u s i o n s and i s o t o p i c compositions of the Pine Point sulphides and rela t e d carbonates provide information on t h e i r c r y s t a l l i z a t i o n h i s t o r y . Textures The i n t e r p r e t a t i o n of t e x t u r a l evidence i s r a r e l y conclusive due to the ease of r e c r y s t a l l i z a t i o n and m o b i l i z a t i o n of sulphides at low temperatures. In the case of Pine Point, i n t e r p r e t a t i o n of the textures i s further complicated by the p o s s i b i l i t y of r e c r y s t a l l i z a t i o n of the associated carbonate host rocks. -44-Sphalerite i s the main ore mineral at Pine Point, occurring c h i e f l y i n three forms: i ) colloform; i i ) s t a l a c t i t i c and i i i ) c r y s t a l l i n e (Paterson, 1975). Roedder (1968) points out that colour bands within "colloform" s p h a l e r i t e o u t l i n e c r y s t a l facets i n d i c a t i n g the s p h a l e r i t e grew as m i c r o c r y s t a l l i n e crusts and did not p r e c i p i t a t e from a c o l l o i d a l s o l u t i o n . S t a l a c t i t i c s p h a l e r i t e i s often s i m i l a r l y c o n c e n t r i c a l l y banded and often contains a hollow core, usually f i l l e d i n part by s k e l e t a l galena. C r y s t a l l i n e s p h a l e r i t e occurs i n v e i n l e t s , as encrustations upon carbonate fragments or disseminated within c r y s t a l l i n e dolomite. Galena occurs mainly as coarse c r y s t a l s , disseminated and often replacing the carbonate host rocks, or l i n i n g vugs. Galena c r y s t a l s also occur i n the cores and i n cracks r a d i a t i n g from the cores of colloform s p h a l e r i t e . This occurrence was interpreted by Lebadev (1972) as second-ary c r y s t a l l i z a t i o n of galena i n cracks due to shrinkage of the ZnS c o l l o i d a l g e l . If the t e x t u r a l evidence interpreted by Roedder (1968) does i n fa c t i n d i c a t e the s p h a l e r i t e c r y s t a l l i z e d d i r e c t l y from s o l u t i o n , these textures must be interpreted as simultaneous c r y s t a l l i z a t i o n of coarse galena with m i c r o c r y s t a l l i n e s p h a l e r i t e . A t h i r d t y p i c a l occurrence of galena at Pine Point i s as hopper or s k e l e t a l c r y s t a l s within the cores of s t a l a c t i t e s . The d e s c r i p t i o n of carbonate textures i s more d i f f i c u l t i n that d i f f e r e n t generations of carbonate are not e a s i l y d i s t i n g u i s h a b l e . Several dolomite phases seem c l e a r l y defined. Fine c r y s t a l l i n e dolomite occurs i n s t r a t i g r a p h i c a l l y defined horizons. Both F r i t z and Jackson (1972) and S k a l l (1975) consider t h i s dolomite to be a r e s u l t of early diagenesis i n a supra-t i d a l environment. A second type of dolomite i s the coarsely c r y s t a l l i n e -45-Presqu'ile type i n which the ore i s l o c a l i z e d . S k a l l (1975) notes that s t r a t i g r a p h i c a l l y d i s t i n c t "diagenetic" dolomite layers of type 1 pass through t h i s coarse c r y s t a l l i n e "Presqu'ile" dolomite suggesting the l a t e r dolomitization affected the limestone units only. From s t r a t i -graphic considerations he argues that the formation of the Prequ'ile type dolomite occurred p r i o r to, and was unrelated to, m i n e r a l i z a t i o n . A t h i r d type of dolomite, which i s milky white and coarse c r y s t a l l i n e , occurs i n v e i n l e t s and l i n i n g vugs. This white sparry dolomite i s s p a t i a l l y r e l a t e d to m i n e r a l i z a t i o n and may possibly have been p r e c i p i t -ated from the ore solutions. C a l c i t e occurs i n c a v i t i e s and replacing the host rock. From te x t u r a l considerations i t i s not possible to d i s t i n g u i s h more than one type of c a l c i t e , although Beales (1975) notes that white c a l c i t e occurs often with sulphide minerals, apparently a r e s u l t of c o - p r e c i p i t -ation; whereas translucent coarse c r y s t a l l i n e c a l c i t e does not show any c o - p r e c i p i t a t i o n textures and i s probably a l a t e r phase. F r i t z (1969) notes that c a l c i t e i s the l a t e s t phase p r e c i p i t a t e d and indicates that no c o - p r e c i p i t a t i o n of c a l c i t e and dolomite i s known. Co- p r e c i p i t a t i o n textures of s p h a l e r i t e , galena and milky white dolomite have b een noted by Beales (1975) • Paterson (1975) notes that galena occasionally shows rounded c r y s t a l faces i n d i c a t i n g s o l u t i o n a f t e r i n i t i a l p r e c i p i t a t i o n . Replacement of dolomite and sp h a l e r i t e by galena are also common textures. This t e x t u r a l evidence indicates that galena, sp h a l e r i t e and dolomite did c o - p r e c i p i t a t e from s o l u t i o n during at l e a s t one stage of the mineral-i z a t i o n . Minor s o l u t i o n and replacement of early sulphides did occur at -46-l a t e r stages, although the preservation of o r i g i n a l textures such as s t a l a c t i t i c s p h a l e r i t e suggests t h i s phenomenon was not widespread. I t i s not possible from t e x t u r a l evidence to determine the i n t e r v a l between i n i t i a l p r e c i p i t a t i o n of the sulphides and l a t e r r emobilization. In s e l e c t i n g samples for geochronological study i t may be important to select mineral separates from hand specimens that show c l e a r evidence of c o - p r e c i p i t a t i o n of the minerals involved. 87 86 In i n t e r p r e t i n g Sr /Sr i n i t i a l r a t i o s of sulphide assemblages i t i s of i n t e r e s t to know i f the strontium i n m i n e r a l i z i n g f l u i d s may have e q u i l i b r a t e d with host rock strontium p r i o r to deposition of the deposition of the sulphide assemblage. Beales (1975) and Anderson (1975) considered the question of s o l u t i o n of carbonate host rock by m i n e r a l i z i n g f l u i d s as part of a study of the mechanism of the p r e c i p i t a t i o n of the ore. Beales (1975) observed that i n one specimen microc a v i t i e s i n dolomite f i l l e d with white sparry dolomite showed c r y s t a l faces and good f o s s i l forms on t h e i r surfaces. Quite close to t h i s texture s p h a l e r i t e and galena f i l l s the c a v i t i e s . Although t h i s observation does not r u l e out s o l u t i o n of the host rock by the mineralizing..^luids, i t does suggest that such s o l u t i o n was not extensive. S k a l l (1975) from s t r a t i g r a p h i c considerations suggested that s o l u t i o n of the Presqu'ile dolomite was r e l a t e d to karsting during u p l i f t i n the middle of Devonian and i s un-r e l a t e d to m i n e r a l i z a t i o n . Although not conclusive, t h i s evidence'suggests that widespread s o l u t i o n of the host rocks did not occur during mineral-87 86 i z a t i o n , and so the i n i t i a l Sr /Sr r a t i o of a sulphide assemblage may represent the i n i t i a l r a t i o of the mineralizing s o l u t i o n , and not of the surrounding host rocks. -47-F l u i d Inclusions Roedder (1968) has studied f l u i d i n c l u s i o n s i n s p h a l e r i t e and carbonates from Pine Point. F l u i d i n c l u s i o n s within the c r y s t a l l i n e s p h a l e r i t e phase showed a wide v a r i a t i o n i n homogenization temperature (30°"» within i n d i v i d u a l c r y s t a l s ) . The in c l u s i o n s have very low freezing temperatures suggesting the ore f l u i d was very s a l i n e . Similar r e s u l t s were obtained for in c l u s i o n s within the dolomite while in c l u s i o n s i n c a l c i t e were much l e s s s a l i n e . This evidence suggests that s p h a l e r i t e and at l e a s t one phase of dolomite co-precipitated or p r e c i p i t a t e d under s i m i l a r conditions from f l u i d s of s i m i l a r composition while at l e a s t one phase of c a l c i t e i s of d i f f e r e n t o r i g i n . Stable Isotopes The i s o t o p i c r a t i o of sulphur i n the Pine Point sulphides has been measured by Baadsgaard et a l . (1965), Evans et a l . (1965), Folinsbee et a l . (1965, 1966), Robertson and (Humming (1966), and Sasaki and Krause (1969). The l a t t e r reported r e s u l t s based on 156 determinations of the 3 A 32 S /S r a t i o s of samples from numerous ore zones and surrounding rocks. 34 They observed a <5S value of +20.1 ± 2.6 (la) r e l a t i v e to the t r o i l i t e n o o / phase of the Canon Diablo meteorite (S /S = 22.220). The sulphur i s o t o p i c composition shows a remarkable uniformity and a pronounced enrichment i n the heavy isotope. This sulphur i s o t o p i c composition i s i d e n t i c a l with the sulphur i s o t o p i c composition of anhydrite i n the Elk Point Basin to the south. Although t h i s may suggest a genetic r e l a t i o n s h i p , i t requires reduction of the sulphate without f r a c t i o n a t i o n . -48-The uniform composition suggests the reduced sulphur came from a re s e r v o i r of uniform i s o t o p i c composition, or was thoroughly mixed p r i o r to sulphide p r e c i p i t a t i o n . Slight differences i n the sulphur i s o t o p i c composition of galena (618.4°/oo) and sph a l e r i t e (621.6°/oo) correspond to equilibrium p r e c i p i t a t i o n of these minerals. F r i t z (1969) and F r i t z and Jackson (1972) have measured the i s o t o p i c composition of oxygen and carbon from Pine Point carbonates. Unfortunately t h e i r type subdivisions of the carbonates i s based not only upon t e x t u r a l paragenetic considerations but also upon geochemical d i f f e r e n c e s . Accordingly only very general conclusions can be drawn from t h e i r data. They note that the Presqu'ile type dolomites and the dolomites associated with m i n e r a l i z a t i o n have i d e n t i c a l i s o t o p i c compos-i t i o n s . If the t e x t u r a l l y d i s t i n c t "white sparry dolomite" (Section II-3) i s i n fact i s o t o p i c a l l y i d e n t i c a l to the Presqu'ile dolomite,two possible conclusions may be drawn. The carbon and oxygen isotopes may have e q u i l i b r a t e d during ore p r e c i p i t a t i o n . A l t e r n a t e l y , the combination of factors producing a given i s o t o p i c composition such as composition of the f l u i d s and f r a c t i o n a t i o n processes may have f o r t u i t o u s l y produced carbonate of i d e n t i c a l i s o t o p i c composition. As the spread i n the values •I Q 1 O i s large (SO = 13 to -5(PDB); SC = 2 to +4 (PDB) t h i s i s a l i k e l y explanation. P r e c i p i t a t i o n of the ore without isotope e q u i l i b r a t i o n between Presqu'ile dolomite and the mineralizing f l u i d s i s thus not ruled out. F r i t z and Jackson (1969) and F r i t z (1969) also note that the i s o t o p i c composition of c a l c i t e shows a wide v a r i a t i o n , but appears to be d i f f e r e n t from dolomite. S p e c i f i c a l l y , c a l c i t e described as "secondary" -49-by F r i t z (1969) shows a 60 (PDB) range from -16 to -10 and a 6C-LJ(PDB) range from -6 to -4 . F r i t z (1969) suggests that l a t e c a l c i t e may have been p r e c i p i t a t e d by percolating surface waters with a l i g h t e r oxygen composition. The dolomite described as " s u p r a t i d a l " , apparently the r e s u l t of diagenetic dolomitization ( S k a l l , 1975), has a l i g h t e r oxygen i s o t o p i c composition than the "Presqu'ile" type dolomites, supporting the conclusion of S k a l l (1975) that these rocks are a r e s u l t of d i s t i n c t periods of dolomitization. Thus the oxygen and carbon isotope evidence cannot be used to dispute the four carbonate stages suggested from t e x t u r a l considerations (section II-3a)'• and does support the concepts of an early diagenetic dolomitization and d i s t i n c t period of c a l c i t e p r e c i p i t a t i o n . In summary, t e x t u r a l , stable isotope and f l u i d i n c l u s i o n studies suggest that s p h a l e r i t e and galena did co - p r e c i p i t a t e , probably with sparry dolomite. Some remobilization has occurred, so that samples selected f o r geochronological study should display t e x t u r a l evidence of c o - p r e c i p i t a t i o n . III-4 Age of M i n e r a l i z a t i o n S t r a t i g r a p h i c and t e x t u r a l considerations indicate that the min e r a l i z a t i o n i s epigenetic and thus post-Givetian (360 m.y.) The only estimate of the age of the deposit i s from the Pb i s o t o p i c data. The i s o t o p i c composition of the lead from Pine Point galena was f i r s t reported as "ordinary" by Folinsbee et a l . (1965), Baadsgaard et a l . (1965) and Robertson and Cumming (1966). Cumming and Robertson (1969) reported eight lead i s o t o p i c r a t i o s measured using the sing l e filament -50-s o l i d source technique. Their r e s u l t s , normalized to absolute,are shown i n Table 16. Also given i n Table 16 are the absolute lead i s o t o p i c r a t i o s of samples PP-11 and PP-41 measured at U.B.C. by P. Shore using the solid-source single-filament s i l i c a - g e l t e c h ni-•e- o j „ 207 / n 204 206 / T 3, 204 , ^,208, que. Figures 3a and 3b are Pb /Pb vs Pb /Pb and Pb / ™ 204 „,_206/T,, 204 . . . . Pb vs Pb /Pb p l o t s of the data. Figures 3a and 3b suggest that the i s o t o p i c r a t i o s do generally define a l i n e a r trend. Although t h i s feature may be a r e s u l t of geo-l o g i c a l f a c t o r s , the narrowness of the spread of data suggests the l i n e a r trend i s probably due to un c e r t a i n t i e s i n measuring the very 204 204 small Pb peak, and the l i n e s defined are thus "Pb error l i n e s " (Kanasewich, 1968, Cumming and Robertson, 1969). The narrowness of the spread of i s o t o p i c r a t i o s i s remarkable considering the ore i s emplaced i n upper c r u s t a l rocks and the chances of mixing ore lead with lead of d i f f e r e n t provjdpance during emplacement are great. This uniformity of Pb i s o t o p i c composition suggests that i f the lead was not derived from a uniform source region the s o l u t i o n carrying the lead was thoroughly mixed p r i o r to ore deposition. Stacey and Kramers (1975) approximated t e r r e s t r i a l lead isotope evolution using a two-stage model i n which lead existed i n a uniform environment with a U/Pb r a t i o of 7.9 and a Th/Pb r a t i o of 33.21 from 4.57 b.y. ago (age of the earth) to 3.7 b.y. ago. A f t e r t h i s i n i t i a l stage the lead was emplaced i n a uniform environment with a U/Pb r a t i o of 9.74 and a Th/Pb r a t i o of 36.84 u n t i l i s o l a t i o n i n an environment free of parent isotopes. Figure 4 i s a p l o t of Stacey and Kramer's model and the Pine Point lead isotope data. -51-••a-, galena leads(table 15). , , , , , 1 ! , ! 17.4 17.6 17.8 18.C 18.2 18.4 18.6 18.8 19.0 19.2 PB206/PB204 F i g u r e 3b : P l o t o f P b 2 0 8 / P b 2 0 4 v s . P b 2 0 6 / P b 2 0 4 o f P i n e P o i n t g a l e n a l e a d s ( T a b l e 15) . Pb 2 0 8 Pb 2 0 4 Figure 4: Growth curve of Stacey and Kramers (1975). The Pine Point lead i s p l o t t e d f o r reference. 17.0 13 O 19.0 17.0 18.0 p B 2 0 6 / p B 2 0 4 Figure 5a: p B 2 0 $ / p B 2 0 4 Figure 5b: Figures 5a,b: Growth curves of Doe and Zartmann (1976). Curve a represents the i s o t o p i c growth of lead within a mantle or lower c r u s t a l environment. Curve b represents the i s o t o p i c composition of leads within an orogenic zone. Curve c represents the i s o t o p i c growth of lead within a contin-e n t a l environment. The Pine Point leads are p l o t t e d for reference. -54-TABLE 16: Isotopic composition of lead from Pine Point Sample P b 2 0 6 / P b 2 0 4 P b 2 0 6 / P b 2 0 4 P b 2 0 8 / P b 2 0 4 Reference 6009a 6009a 6009b 4 7 8 9 io : c . i i PP-11 PP-41 (2) (2) 18.15 18.18 18.17^ 18.18' 18.20 18.23 18.17 18.12 18.16 18.11 17.95 15.61 15.61 15.61 15.65 15.65 15.68 15.63 15.59 15.61 15.51 15.37 38.12 38.25 38.23 n.d. 38.27 38.24 n.d. 37.94 38.08 37.83 Cumming & Robertson (1969) (t h i s work) The entry n.d. indicates the r a t i o s were not reported. The procedures used i n processing and determining the i s o t o p i c composition of samples PP-11 and PP-41 are given i n Appendix 5. The f i t of the "common" Pine Point lead to the model derived from "conformable" lead i s o t o p i c r a t i o s i s astonishingly good, and i t i s tempting to i n t e r p r e t the h i s t o r y of the Pine Point leads i n the l i g h t of t h i s observation. The age indicated from the model of Stacey and Kramers i s 337 m.y. There are several d e f i c i e n c i e s to t h i s approach. Stacey and Kramers (1975) acknowledge that c r u s t a l rocks do e x i s t which f a l l below t h e i r two-stage model growth curve. Accordingly the "worldwide d i f f e r e n t i a t i o n event" at 3.7 b.y. does not represent a r e a l geological event but i s rather a contrived model to produce a best f i t to the s t r a t i f o r m lead i s o t o p i c r a t i o s . Armstrong (1968) pointed out that a closed but mixing (1) (2) -55-crust-upper mantle system can produce leads which f a l l on a growth curve that l i e s between an upper mantle growth curve (low apparent U/Pb r a t i o ) and a c r u s t a l growth curve (high apparent U/Pb r a t i o ) . Recently Doe and Zartman (1976) have extended t h i s concept by es t a b l i s h i n g a model which provides f or transfer of material between mantle, lower crust and continental crust. They i n t e r p r e t lead i s o t o p i c r a t i o s of ores and rocks according to t h e i r provenance. Figures 5a and 5b are plo t s of the growth curves postulated by Doe and Zartman (1976). Curve (a) represents the i s o t o p i c growth of lead within a mantle, or lower c r u s t a l environment. Curve (c) r e -presents the growth of leads within a continental environment subject to r aised U/Pb and Th/Pb r a t i o s . Leads which f a l l between bounding curves may be c h a r a c t e r i s t i c of an orogenic zone i n which erosion, sedimentation, vulcanism and plutonism e f f e c t i v e l y homogenize leads of diverse i s o t o p i c compositions and past h i s t o r i e s . Also shown i n Figures 5a and 5b are the Pine Point i s o t o p i c r a t i o s . They f a l l close to the growth curve characterized by leads from an "orogenic" environment. The important point i s that the leads f a l l below the curve representing c r u s t a l material i n d i c a t i n g they have not resided for an extended period of time within an environment enriched i n Th or U. If the leads were derived from c r u s t a l sediments as suggested by Beales and Jackson (1968) and supported by Doe and Zartman (1976), i t i s u n l i k e l y that the sediment received abundant input from cratonic sources. An alternate source for 238 232 the leads i s the U and Th depleted lower crust. -56-I n summary, t h e age e s t i m a t e b a s e d on t h e two s t a g e m o d e l o f S t a c e y and K r a m e r s must be c o n s i d e r e d t e n t a t i v e . H o w e v e r , some c o n s t r a i n t s have been p l a c e d upon t h e h i s t o r y o f t h e Pb p r i o r t o m i n e r a l i z a t i o n by i t s i s o t o p i c c o m p o s i t i o n . I I I-5 T h e o r i e s o f O r i g i n o f t h e Ore Two g e n e r a l t h e o r i e s f o r t h e o r i g i n o f t h e o r e have been p r e s e n t e d . J a c k s o n and B e a l e s (1967) and B e a l e s and J a c k s o n (1968) s u g g e s t e d t h a t t h e o r e f o r m a t i o n was a n a t u r a l p r o d u c t o f t h e s e d i m e n t a r y b a s i n e v o l u -t i o n , w i t h a c c u m u l a t i o n o f s u l p h u r f rom e v a p o r i t e s t o t h e s o u t h and m e t a l s f r o m s h a l e s t o t h e n o r t h r e s u l t i n g i n s u l p h i d e p r e c i p i t a t i o n w i t h i n t h e h o s t r o c k ( " s t r a t a f u g i c " o r i g i n ) . I t i s i n t e r e s t i n g t o n o t e t h a t J a c k s o n and B e a l e s (1967) c o n s i d e r e d P i n e P o i n t t o be a " M i s s i s s i p p i V a l l e y " t y p e o r e d e p o s i t , and assumed t h a t t h e l e a d i s o t o p i c c o m p o s i t i o n w o u l d be 206 207 " a n o m a l o u s " ( e n r i c h e d i n Pb and Pb and s h o w i n g a w i d e r a n g e i n i s o t o p i c c o m p o s i t i o n ) , and t h a t t h e s u l p h u r i s o t o p i c c o m p o s i t i o n w o u l d 34 show w i d e v a r i a t i o n s o f e n r i c h m e n t i n S , as i n o t h e r M i s s i s s i p p i V a l l e y t y p e d e p o s i t s . A l t h o u g h n e i t h e r a s s u m p t i o n p r o v e d c o r r e c t , B e a l e s and J a c k s o n (1968) were a b l e t o r a t i o n a l i z e an " o r d i n a r y " l e a d i s o t o p i c c o m p o s i t i o n and a homogeneous s u l p h u r i s o t o p i c c o m p o s i t i o n w i t h t h e same m o d e l . T h i s s i t u a t i o n p o i n t s ou t t h e v e r y l i m i t e d g e n e t i c c o n s t r a i n t s imposed by t h e i s o t o p i c e v i d e n c e . C a m p b e l l (1967) s u g g e s t e d t h e o r e may have been p r o d u c e d by m i g r a t i o n o f m e t a l - c a r r y i n g f l u i d s , w h i c h o r i g i n a t e d f r o m deep s e a t e d i n t r u s i o n s , up f a u l t s i n t h e P r e c a m b r i a n ba semen t . S k a l l (1975) c o n -c l u d e d t h a t t h e s u l p h u r i s p r o b a b l y d e r i v e d f rom t h e a d j a c e n t s e d i m e n t a r y -57-p i l e and that reactivated Precambrian f a u l t s exerted a s t r u c t u r a l control upon the m i n e r a l i z a t i o n . The geologic h i s t o r y of Pine Point remains obscure. The following sections of t h i s chapter document an attempt to date the m i n e r a l i z a t i o n d i r e c t l y using the Rb and Sr concentrations and Sr i s o t o p i c composition of galena, s p h a l e r i t e and re l a t e d dolomite. I1I-6 Sample Description Two samples of ore (PP-1 and PP-4) from the Pine Point Pb-Zn deposit were obtained from A.J. S i n c l a i r and G. Medford. Clean galena, s p h a l e r i t e and carbonate mineral separates were prepared from these samples as described i n Chapter I I . Sample PP-1 consists of numerous tapered s p h a l e r i t e s t a l a c t i t e s (Paterson, 1975). A c e n t r a l tube which runs through the s t a l a c t i t e s i s p a r t i a l l y f i l l e d with c r y s t a l l i n e galena. Between the s t a l a c t i t e s two d i s t i n c t carbonate phases are found: i ) milky white sparry c r y s t a l -l i n e dolomite, and i i ) coarse c r y s t a l l i n e c a l c i t e . Numerous medium sized (=: 3 m.m.) euhedral s p h a l e r i t e c r y s t a l s are enclosed i n the sparry dolomite. Voids which have not been f i l l e d by carbonate materials are l i n e d with c r y s t a l l i n e galena. Mineral separates of carbonate, sphaler-i t e and galena from sample PP-1 which were prepared following the pro-cedure described i n Chapter II are l i s t e d i n Table 17. Sample PP-4 consists of massive coarse galena c r y s t a l s i n a carbon-ate matrix. As these exceptionally large galena c r y s t a l s commonly contain carbonate i n c l u s i o n s , pure mineral separates were prepared by cleaving the -58-c r y s t a l s into cubes smaller than the smallest carbonate i n c l u s i o n s observed i n polished section. During the procedure, i t was found that some of the cleavage faces of the galena are tarnished. The carbonate associated with t h i s massive galena i s of three types: i ) grey bladed dolomite, i i ) c a l c i t e , and i i i ) rust-stained massive dolomite. Mineral separates from sample PP-4 are l i s t e d i n Table 18. Within the sample, dolomite o u t l i n e s a cubic form, which may be a pseudomorph of galena. Thus the p o s s i b i l i t y must be con-sidered that the galena has been remobilized and r e c r y s t a l i z e d a f t e r i n i t i a l p r e c i p i t a t i o n , possibly obscuring the o r i g i n a l Sr i n i t i a l r a t i o . Sample PP-2 consists of numerous tapered s t a l a c t i t e s . Voids between the s t a l a c t i t e s are f i l l e d with c o l l i f o r m s p h a l e r i t e c r y s t a l s and galena cubes. An i r r e g u l a r mass of coarse c r y s t a l l i n e c a l c i t e f i l l s what may have been a void between the s t a l a c t i t e s . This c a l c i t e mass i s rimmed with a t h i n layer of f i n e l y c r y s t a l l i n e s p h a l e r i t e . In order to confirm the Sr i s o t o p i c measurements on c a l c i t e from samples PP-1 and PP-4, two separate c a l c i t e samples, PP-24 and PP-25, were prepared from t h i s hand specimen. Table 17: Mineral separates from sample PP-1 Sample Mineral Description PP-11 PP-12 PP-13 PP-14) PP-15) PP-16) PP-17) galena l-2mm galena c r y s t a l s from the cores of the s p h a l e r i t e s t a l a c t i t e s and the walls of voids between s t a l a c t i t e s s p h a l e r i t e euhedral s p h a l e r i t e c r y s t a l s enclosed i n milky dolomite carbonate indiscriminate mixture of c a l c i t e and sparry dolomite dolomite milky white sparry dolomite c a l c i t e coarse c r y s t a l l i n e c a l c i t e T.able 18: Mineral separates from sample PP-4 Sample Mineral Description PP-41 galena coarse c r y s t a l l i n e galena enclosed i n PP-41a Fe oxide stained dolomite PP-43 carbonate indiscriminate mixture of grey dolomite, Fe stained dolomite and c a l c i t e III-7 A n a l y t i c a l Results The r e s u l t s of Rb and Sr analyses and Sr i s o t o p i c composition determinations of the Pine Point sulphides and r e l a t e d carbonates are given i n Tables 19, 20 and 21. S r 8 7 / S r 8 ^ Measurements A l l r e p l i c a t e Sr /Sr r a t i o s were reproduceable within a n a l y t i c a l 87 uncertainty, with the exception of samples PP-12-1 and PP-12-2 (Sr / S r 8 6 = 0.7133 ± 0.001; 0.7107 ± 0.001). Preferred r a t i o s of a l l samples have been calculated by weighing r e p l i c a t e measurements by the inverse of the variance. In order to determine i f the Rb c o r r e c t i o n affected the measured r a t i o s the c o r r e l a t i o n c o e f f i c i e n t of the magnitude of the Rb c o r r e c t i o n with the Sr /Sr r a t i o was calculated f o r each set of measurements. Values of the c o r r e l a t i o n c o e f f i c i e n t vary between -0.76 and +0.62. Although some of the c o e f f i c i e n t s are large, none are s i g n i f i -cant at the 95% confidence l e v e l . -60-Table 19: Rb and Sr concentrations and Sr /Sr ratios of mineral separates from sample PP-1. Sample I Mineral Date Prepared S r 8 7 / s r 8 6 la I Of blocks Sr(ppm) Rb(ppm) PP-12-1 sphalerite 15/10/75 0.7133 0.0010ll) 4 0.069 n. d. 0.028 PP-12-2 sphalerite 15/10/75 0.7107 0.0010 11 n.d. PP-12-3 sphalerite 15/11/75 0.7121 0.0009 19 0.072 0.023 PP-12-4 sphalerite 10/02/76 0.7120 0.0019 12 0.082 0.110 PP-12-5 sphalerite 10/02/76 0.7114 0.0023 15 n.d. PP-11-1 galena 15/11/75 0.7095 0.0086 6 0.014 0.00014 PP-11-2 galena 10/04/76 0.7090 0.0033 9 n.d. n.d. PP-13-1 carbonate 15/10/75 0.7147 0.0001 20 10.7 0.132 PP-13-2 carbonate 15/10/75 0.7148 0.00O8 10 n.d. n.d. PP-14 dolomite 10/04/76 0.7097 0.0002 12 n.d. n.d. PP-15 dolomite 10/04/76 0.7095 0.0002 12 n.d. n.d. PP-16 calcite 10/04/76 0.7153 0.0001 12 n.d. n.d. PP-17 calcite 10/04/76 0.7152 0.0002 12 n.d. n.d. (1) The uncertainty given here is equal to ( l / l / v i + l / v + l / v + 1/ V 4> where v 1...v 2 are the variances of each block of measurements. A l l other^uncertainties in this column are the standard deviations of the ratios of each set of blocks. In every case the larger uncertainty is used. (2) n.d. ™ not determined. -61-Table 20: Rb and Sr concentrations and Sr /St ratios of mineral separates from sample PP-4. Sample 1 Mineral Date Prepared S r 8 7 / S r 8 6 la « of Blocks Sr (ppm) Rb(ppm) PP-41-1 PP-41-2 PP-41-3 PP-41-4 PP-41-5 PP-41-6 galena galena galena galena galena galena 24/09/75 24/09/75 24/09/75 15/11/75 10/02/76 10/02/76 0.7107 0.7101 0.7103 0.7125 0.7123 0.7130 0.0016 0.0020 0.0023 0.0011 0.0015 0.0012 12 7 12 14 8 12 n.d. n.d. 0.43 0.20 n.d. 0.10 n.d. n.d. n.d. 0(-.001) ' n.d. 0.0009 PP-41a-l PP-41a-2 PP-41a-3 PP-41a-4 PP-41a-5 galena galena galena galena galena 15/11/75 10/02/76 10/02/76 24/09/75 24/09/75 0.7128 0.7121 0.7117 n.d. n.d. 0.0014 0.001 0.004 n.d. n.d. 19 10 3 0.041 n.d. 0.053 n.d. n.d. 0.00009 n.d. 0.0037 0.0045 0.0049 PP-4 3 carbonate 15/11/75 0.7125 0.0001 16 89.0 0.039 1) (1) A negative Kb concentration was obtained by correcting for a blank of 1.7 ngrams. -62-Rb and Sr Analyses Table 19 indicates that three analyses of sphalerite (sample PP-l2)were made. Two of these analyses (PP-12-1 and PP-12-3) are in excellent agreement. The third analysis (PP-12-4) shows a slightly higher Sr concentration and a much higher Rb concentration, suggesting the sample may have been contaminated. The "preferred" concentration o f Rb and Sr are the means of analyses PP-12-1 and PP-12-3. N o Rb and Sr analyses were made of the dolomite and calcite samples from sample PP-1. It is l i k e l y that the concentrations of Rb and Sr in these carbonate samples do not differ greatly from the concentrations determined for sample PP-13, which is an indiscriminate mixture of the dolomite and calcite. The "preferred" Rb and Sr concentrations of the dolomite and calcite have therefore been set equal to the measured concentrations of sample PP-13. Table 21: S r 0 / / S r 0 0 ratios of calcite from sample PP-2 Sample # Mineral Date Prepared S r 8 7 / S r 8 6 la # of Blocks Averaged PP-24 calcite 10/04/76 0.7155 0.0004 8 PP-25 calcite 10/04/76 0.7149 0.0005 10 -63-Sample PP-41 and PP-41a are both galena separates from the same hand specimen. The measured Sr concentration of sample PP-41 ranges from 0.43 to 0.10 p.p.m. while the measured Sr concentrations of two analyses of sample PP-41a are 0.041 p.p.m. and 0.053 p.p.m. These r e s u l t s suggest that either the strontium i s inhomogeneously d i s t r i b u t e d within the galena, or a v a r i a b l e contamination component i s present. The measured Rb concentration of sample PP-41-4 i s negative due to the blank cor r e c t i o n , i n d i c a t i n g that the Rb concentration of t h i s sample i s below the l i m i t of detection. "Preferred" Rb and Sr concentrations of these samples are the means of the r e p l i c a t e analyses. 87 86 "Preferred" Rb and Sr analyses and Sr /Sr r a t i o s of mineral separates from samples PP-1 and PP-4 are given i n Tables 22 and 23. Isochron r e l a t i o n s h i p s 87 86 87 86 Figure 6 i s a conventional Rb /Sr -Sr /Sr plo t of the "preferred" analyses of sample PP-1 given i n Table 22. It i s poss-i b l e that the di f f e r e n c e i n Sr is o t o p i c composition between the sphale-r i t e (PP-12) and sparry dolomite (PP-14, PP-15) i s due to i n - s i t u decay 87 of Rb within the sp h a l e r i t e . If t h i s i s true, and i f the sp h a l e r i t e 87 86 and sparry dolomite did have the same Sr /Sr r a t i o at the time of t h e i r formation, then a determination of the age of mi n e r a l i z a t i o n i s possi b l e . A best f i t regression to the data (York, 1966) gives a slope of 0.00235 ± 0.00085 corresponding to an age of 165±60 m.y. Due to the very small concentrations of Rb and Sr i n sample PP-12,there may be s i g n i f i c a n t 87 86 87 error i n the measured Rb /Sr r a t i o . It i s also possible that the Sr / 86 Sr r a t i o of the sparry dolomite (0.7096) i s not representative of the 87 86 Sr /Sr r a t i o of the sp h a l e r i t e at the time of i t s formation. For these reasons the indicated age must be considered t e n t a t i v e at t h i s time. Table 22: Preferred Sr /Sr ratios and Rb and Sr concentrations of mineral separates from sample PP-1 Sample Mineral Sr ppm + or Rb ppm + or S r 8 7 / S r 8 e ( ) l a R b 8 7 / S r 8 6 PP-12 sphalerite 0.0705 0.02 ( A ) 0.025 0.003 0.7120 0.0005 1.05 ± .3 PP-11 galena 0.014 0.001 0.00014 .00001 0.7091 0.003 0.11 ± 0.01 PP-14) dolomite n.d. n.d. n.d. n.d. 0.7096 0.0001 0.03 ± 0.03 ( 3 ) PP-15) PP-16) c a l c i t e n.d. n.d. n.d. n.d. 0.7153 0.0001 0.03 ± 0.03 PP-17) (1) Uncertainties are the deviation from the median for replicate analyses. In cases where replica t e analyses were not made the uncertainty has been calculated from known uncertainties i n the weighing, spike c a l i b r a t i o n , and isotopic measurements. (2) Mean value arrived at by weighting the measured r a t i o by the inverse of the variance. (3) See text for a description of the derivation of this value. (4) An uncertainty of 50% i n the magnitude of the blank correction has been applied. Table 23: Preferred Sr°'/Sr ratios and Rb and Sr concentrations of mineral separates from sample PP-4 (2) Sample Mineral Sr ppm ± ( 1 ) Rb ppm ± ( 1 ) S r 8 7 / S r 8 6 la Rb 8 7/Sr 8 6 ± ( 1> PP-41 galena 0.24 0.16 0 ( 3 ) . 0.001 0.7120 0.0006 0 0.02 PP-41a galena 0.047 D.006 0.0044 0.001 0.7122 0.0004 0.27 0.07 PP-43 carbonate 89.0 1.0 0.039 0.005 0.7125 0.0001 0.001 0.001 (1) Uncertainties are the deviation from the median for replicate analyses; In cases where replicate analyses' • were not made the uncertainty has been calculated from known uncertainties in the weighing, spike calibration and isotopic measurements. (2) Mean value arrived at by weighting the measured ratio by the inverse of the variance. (3) Of two samples analyzed one gave a negative Rb concentration and one a very low concentration of 0.0009 p.p.m. when corrected for a blank of 1.7 ngrams. -66--67-The strontium and rubidium data from sample PP-4 do not in d i c a t e an isochron r e l a t i o n s h i p . Within a n a l y t i c a l uncertainty, both galena samples are i d e n t i c a l i n Sr i s o t o p i c composition to the surrounding carbonate. III-8 Interpretation of Results 87 86 A knowledge of the Sr /Sr r a t i o of the carbonate host rocks i s 87 86 important i n order to int e r p r e t the Sr /Sr r a t i o s of the sulphides and associated sparry dolomite and c a l c i t e presented here. Medford »' 87 86 (personal;communication) has measured the Sr /Sr r a t i o s of the Pre-squ'ile dolomite. He found S r 8 7 / S r 8 6 r a t i o s from 0.7080-0.7089 ;, (SRM-987 = 0.7102) for these dolomites. Veizer (1971) has measured S r 8 7 / S r 8 6 r a t i o s of 0.7079 - 0.7085 (Eimer and Amend = 0.70814) for Devonian carbonates from A u s t r a l i a . The general agreement of these ft 7 PtfN sets of data suggests that the i n i t i a l Sr /Sr r a t i o of the Pine Point Group carbonate was close to 0.708 and has not been a l t e r e d sub-sequently. O r i g i n of C a l c i t e The i s o t o p i c composition of c a l c i t e from samples PP-1 and PP-2 shows a uniformly high r a t i o of 0.7148 ± 0.0004 to 0.7154 ± 0.0005. As previously discussed, t e x t u r a l , stable i s o t o p i c and f l u i d i n c l u s i o n evidence suggests the c a l c i t e was formed l a t e r than the ore minerals and associated dolomite. The Sr i s o t o p i c evidence supports t h i s con-c l u s i o n , i n d i c a t i n g that the c a l c i t e p r e c i p i t a t e d from a s o l u t i o n i s o t o p i c a l l y d i s t i n c t from the sparry dolomite or host rocks. The -68-c a l c i t e probably p r e c i p i t a t e d from l a t e meteoric ground waters which had a Sr /Sr r a t i o close to 0.7150. I n i t i a l r a t i o of mineralizing solutions Previously discussed t e x t u r a l and stable isotope evidence suggests that white sparry dolomite may have p r e c i p i t a t e d from the ore-forming • solutions. In sample PP-1 white sparry dolomite i s c l o s e l y associated with s p h a l e r i t e m i n e r a l i z a t i o n . Although the evidence i s not conclusive, 87 86 i t i s l i k e l y the Sr /Sr r a t i o of the sparry dolomite (0.7096) i s i n d i c a t i v e of the r a t i o of the ore forming solutions. The S r ^ 7 / S r 8 ^ r a t i o of carbonate material associated with sample PP-4 i s s i g n i f i c a n t l y higher. Textural evidence however suggests that the galena and carbonate of t h i s sample may have r e c r y s t a l l i z e d subsequent to the i n i t i a l period of ore deposition and thus t h i s r a t i o may not represent the Sr i s o t o p i c composition of the o r i g i n a l m i neralizing solutions. Although t h i s study has not c l e a r l y demonstrated that the solutions were i s o t o p i c a l l y w e l l -mixed with respect to strontium, the uniform lead i s o t o p i c composition of the galena reported by Cumming and Robertson (1969) and uniform sulphur i s o t o p i c composition reported by Sasaki and Krause (1969) suggests that t h i s may have been so. Accordingly the high i n i t i a l r a t i o measured on sample PP-4 probably does not represent the strontium i s o t o p i c composition of the mineralizing f l u i d s and may be representative of a mixture of strontium from the mineralizing f l u i d s (0.7096) and strontium from the l a t e formation waters which p r e c i p i t a t e d c a l c i t e (0.7150). In the follow-ing discussion the strontium i s o t o p i c composition of the m i n e r a l i z i n g f l u i d s i s assumed to be 0.709 6. -69-This assumed Sr /Sr r a t i o of the mineralizing solutions i s s i g n i f i c a n t l y higher than the r a t i o of the host rocks ( 0.7080). Thus i t can be concluded that the Sr i n the mineralizing f l u i d s and the l a t e r formation waters which p r e c i p i t a t e d c a l c i t e d i d not e q u i l i b r a t e i s o t o p i -c a l l y with the host rocks. This supports the conclusion of Beales (1975) that large scale s o l u t i o n of the carbonate host rocks did not occur during m i n e r a l i z a t i o n . The i s o t o p i c composition of the mineralizing f l u i d s cannot be used to place constraints upon t h e i r o r i g i n . It i s generally accepted that the sulphur within the Pine Point mineral deposits originated within the Muskeg evaporites (e.g. S k a l l , 1975). Two theories for the source of the metals are i ) shales north of the Pine Point Group; and i i ) the Pre-cambrian basement. The Sr i s o t o p i c evidence cannot be.used to.discriminate between these theories as appropriate combinations of f l u i d s from the Muskeg evaporites and f l u i d s from e i t h e r the shale basin or the Precambrian 8 7 86 basement could conceivably r e s u l t i n a f l u i d with a Sr /Sr r a t i o of 0.7096. Age of M i n e r a l i z a t i o n Rb/Sr age of m i n e r a l i z a t i o n i s 1651-60 m.y. If t h i s i s not erroneous, i t supports the conclusion of most.workers that the m i n e r a l i z a t i o n i s epigenetic. It i s i n t e r e s t i n g that the indicated age f a l l s close.to the time of the Nevadan-Columbian orogeny. According t o . S k a l l (1975), u p l i f t and t i l t i n g of the Paleozoic sequence during t h i s time period may have provided the hydrodynamic conditions for the migration of ore f l u i d s . -70-III-9 Conclusions The Rb/Sr age of the Pine Point m i n e r a l i z a t i o n i s 165 ± 60 m.y. This age i s based upon Rb and Sr analyses of s p h a l e r i t e , which show a high Rb^ 7/Sr 8^ r a t i o . The data suggest that s p h a l e r i t e may be a su i t a b l e mineral for dating ore deposits by the Rb/Sr method, although further analyses with very low and w e l l - c o n t r o l l e d blanks are necessary to confirm t h i s . Three generations of carbonate associated with the Pine Point m i n e r a l i z a t i o n have been observed: i ) Presqu'ile dolomite; i i ) white sparry dolomite; and i i i ) c a l c i t e ; each characterized by an i s o t o p i c a l l y d i s t i n c t Sr composition. -71-CHAPTER IV THE A P P L I C A T I O N OF THE R B / S R DATING TECHNIQUE TO THE BLUEBELL L E A D - Z I N C DEPOSIT IV-1 Introduction 87 86 Measurement of the Sr /Sr r a t i o s and the Rb and Sr concentrations of s p h a l e r i t e , p y r r h o t i t e , galena and associated carbonate mineral separ-ates has been attempted. In t h i s chapter, a f t e r a d e s c r i p t i o n of the geo-l o g i c s e t t i n g of the B l u e b e l l Mine, these a n a l y t i c a l r e s u l t s are presented and discussed. IV-2 Geologic Setting of the B l u e b e l l Mine The B l u e b e l l Mine,on the east shore of Kootenay Lake, i s located near the center of the Kootenay Arc s t r u c t u r a l b e l t . The m i n e r a l i z a t i o n occurs as sulphide replacements of the B l u e b e l l limestone, l o c a l i z e d by fractures and bedding planes (Irving,1957). The B l u e b e l l limestone has been correlated with the Badshot formation (Fyles,1967) of Lower Cambrian age. The host rocks have been intensely metamorphosed and f a l l within the s i l l i m a n i t e zone (Crosby,1960). -72-IV-3 Conditions of M i n e r a l i z a t i o n Ohmoto and Rye (1970) studied the textures, mineralogy, f l u i d i n c l u s i o n s and stable isotopes of the B l u e b e l l m i n e r a l i z a t i o n . They concluded that the m i n e r a l i z a t i o n occurred i n 3 periods. Period 1 was characterized by the formation of Knebelite (Mn-Fe oxide). Period 2 was the main period of sulphide formation, i n which sphaler-i t e , p y r r h o t i t e , quartz,and carbonates were pr e c i p i a t e d . During period 3 quartz, carbonates,pyrite and minor other sulphides were pr e c i p i t a t e d i n vugs. The f l u i d i n c l u s i o n studies of Ohmoto and Rye (1970) indi c a t e that period 2 minerals were deposited at temperatures higher than 450° C,.4 while the l a t e r v u g - f i l l i n g minerals (period 3) were deposited at temperatures between 450°C and 320°C. This decrease i n temperature was accompanied by a decreasing s a l i n i t y of the ore-forming solutions. Ohmoto and Rye found that the oxygen i s o t o p i c compositions of quartz and carbonates associated with period 3 m i n e r a l i z a t i o n show wide v a r i a t i o n s . This suggests that the mineralizing f l u i d s e q u i l -ibrated with the host limestone to varying degrees. The carbon and oxygen i s o t o p i c composition of period 3 minerals approaches that of the B l u e b e l l limestone, suggesting almost complete i s o t o p i c e q u i l i b r a -t i o n with the host rocks occurred i n the f i n a l stages of m i n e r a l i z a t i o n . The presence of primary f l u i d i n c l u s i o n s and open vugs p a r t i a l l y f i l l e d with c a l c i t e and quartz p r e c i p i t a t e d contemporaneously with the sulphides suggests that the m i n e r a l i z a t i o n has not been excessively remobilized or r e c r y s t a l i z e d since i t s formation, and thus may be -73-s u i t a b l e f or dating by the Rb-Sr method. The v a r i a t i o n i n oxygen i s o t o p i c composition indicates the ore f l u i d s were not well mixed and may therefore 87 86 have had v a r i a b l e i n i t i a l Sr /Sr r a t i o s . For t h i s reason i t may be necessary to obtain mineral separates from a si n g l e hand specimen i n order to obtain an isochron r e l a t i o n s h i p . IV-4 Age of M i n e r a l i z a t i o n The B l u e b e l l m i n e r a l i z a t i o n cuts mafic dykes which have not been affected by the regional s i l l i m a n i t e grade metamorphism (Ohmoto and Rye, 1970). These dykes are s i m i l a r to those on the west shore of Kootenay Lake which are believed to be Cenozoic (Fyles, 1967). Ohmoto and Rye (1970) conclude that the B l u e b e l l m i n e r a l i z a t i o n i s Cenozoic. IV-5 Petrography of Samples Two samples of the B l u e b e l l ore were taken for study. Sample RJ-1 consists of massive sphalerite and p y r r h o t i t e with minor galena, quartz and c a l c i t e . Quartz occurs as f r e e l y terminating c r y s t a l s within vugs and as intergrowths with s p h a l e r i t e . C a l c i t e occurs mainly within the vugs. Massive p y r r h o t i t e i s surrounded by the sphalerite-quartz intergrowth. Sphalerite, the p r i n c i p l e ore mineral within the specimen, occurs both as intergrowths with quartz and as massive blebs. Small pods of coarse c r y s t a l l i n e galena are surrounded by the s p h a l e r i t e -quartz material. -74-Sample RJ-2 consists mainly of c r y s t a l l i n e galena, which occurs as medium-sized (<lcm) cubes. Cleavage faces of the galena show a brown * or i r i d e s c e n t t a r n i s h . Minor p y r r h o t i t e , quartz and carbonate are also present. IV-6 Sample Preparation Mineral separates of s p h a l e r i t e , p y r r h o t i t e , galena and carbonate were prepared from sample RJ-1, and galena and carbonate separates were prepared from sample RJ-2. These separates were o r i g i n a l l y prepared using a l e s s rigorous cleaning and hand picking procedure than described i n Chapter I L The rock was broken on an aluminum p l a t e and pure sulphide grains were separated with tweezers from the rock fragments. The c o l l e c t e d sepa-rates were then ground to 100-325 mesh using a diamond mortar and an agate mortar and stored for use. After i n i t i a l Rb and Sr analyses and Sr is o t o p i c composition determinations of these separates had been made the p u r i t y of the samples was questioned. Consequently, the samples were leached i n 2.5 N HC1 for 3 minutes and then washed with q u a r t z - d i s t i l l e d ILjO and placed i n the u l t r a s o n i c bath for 1/2 hour. A second set of Rb and Sr analyses and Sr i s o t o p i c composition determinations were made on these leached samples. Following these measurements, a l l the sulphide separates were re-sieved to 100-325 mesh i n order to remove the f i n e sulphide grains which may have suffered from excessive leaching. Further Rb and Sr -75-analyses and Sr i s o t o p i c composition determinations were made on these sieved and leached samples. IV-7 Results The r e s u l t s of a l l Rb and Sr analyses and Sr i s o t o p i c composition determinations made on sulphides and rela t e d carbonates from the Blue-b e l l mine are given i n Tables 24 and 25. In tables 24 and 25 those samples which were processed from the o r i g i n a l preparations, without leaching or sieving are l a b e l l e d "A". Those samples which were leached but not sieved are l a b e l l e d "B", and those samples which were leached and sieved are l a b e l l e d "C". IV-8 Discussion of A n a l y t i c a l Results S r 8 7 / S r 8 6 Ratios 87 Tables 24 and 25 indic a t e that the determinations of the Sr°'/ 86 Sr r a t i o of the sulphide mineral separates were not reproducible. Several sources of indeterminate error have been i d e n t i f i e d . As discussed i n Chapter I I , during the l a t t e r stages of t h i s 89 work i t was discovered that the " c a r r i e r f r e e " Sr tracer which was added to several suites of samples contains Sr with an anomalously 86 88 low Sr /Sr°° r a t i o (see Table 7). Addit ion of an unknown quantity of the tracer to the samples has therefore resulted i n an indeterminate 87 86 lowering of the normalized Sr /Sr r a t i o , and the r e s u l t s must be 87 8fi discarded. Figure 7 shows the Sr /Sr normalized r a t i o s of the sulphide samples l i s t e d i n Tables 24 and 25. A l l samples which have an anomalously Table 24: Rb and Sr concentrations and S r 8 7 / S r 8 6 ratios of mineral separates from sample RJ-1 Sample Mineral Date Processed Nomalized Sr87/Sr8« Sr ppm (no blank lo correction) RJ-U-1A sphalerite 15/06/75 0.7109 0.001 0.49 RJ-11-2A sphalerite 02/07/75 0.7.146 0.0001 0.58 RJ-11-3B sphalerite 15/11/75 0.7066 0.003 0.045 RJ-11-4C sphalerite 20/01/76 0.6683 0.006 n.d. RJ-11-5C sphalerite 20/01/76 n.d. n.d. 0.024 RJ-12-1A galena 15/06/75 0.7015 0.002 0.075 RJ-13-1A pyrrhotite' 15/06/75 0.7112 0.0002 0.828 RJ-13-2A pyrrhotite 02/07/75 0.7136 0.0003 0.601 RJ-13-3B pyrrhotite 15/11/75 0.7067 0.005 0.137 RJ-13-4C pyrrhotite 20/01/76 0.6971 0.005 n.d. RJ-13-5C pyrrhotite 20/01/76 0.6978 0.005 0.035 RJ-14-1 carbonate 15/06/75 0.71253 0.0001 130.4 RJ-14-2 carbonate 02/07/75 0.7126, 0.0001 129.0 SJ-14-3 carbonate 15/11/73 1 n.d. n.d. n.d. Sr ppm (19 ng blank correction) Rb ppn (no blank correction) Rb ppm (1.7 ng blank correction) Sr 8 9 tracer addition? 0.789 0.564 0.100 n.d. -0.003 n.d. n.d. 0.005 n.d. 0.005 0.45 n.d. n.d. 0.54 n.d. n.d. 0.006 0.018 0.015 n.d. n.d. n.d. -0.01 0.011 0.008 0.038 1.9(1> n.d. n.d. 0.002 n.d. 0.002 yes yes no yes yes yes I I yes yes no yes yes yes no no Table 25: Rb and Sr concentrations and Sr /Sr ratios of mineral separates from sample RJ-2 Saapla Mineral Date Processed Noraallsed o 87.. 86 Sr /Sr Sr ppn Rb ppm Sr ppm (19 ng Rb ppn (1.7 ng (no blank blank (no blank blank correction) correction) correction) correction) - «9 Sr tracer addition BJ-21-1A galena 15/06/75 0.7000 0.0005 n.d. n.d. n.d. n.d. yes RJ-21-2A galena 15/06/75 0.7009 0.002 0.087 0.051 n.d. n.d. yea RJ-21-3B galena 07/09/75 0.7110 0.0005 n.d. n.d. n.d. n.d. yes RJ-21-4B galena 07/09/75 0.7109 0.001 0.134 0.095 n.d. n.d. yes RJ-21-5B galena' 07/09/75 0.7128 0.004 n.d. n.d. n.d. n.d. yes RJ-21-6B galena 23/07/75 0.7132 0.005 0.122 0.083 n.d. n.d. no RJ-21-7B galena 15/11/75 0.7095 0.003 0.014 -0.027 0.0039 0.0039 no RJ-21-8C galena 20/01/76 0.6905 0.002 0.024 -0.013 0.0037 0.0007 yes I I RJ-22 carbonate 20/01/76 0.7128 0.0002 n.d. n.d. n.d. n.d. no -78-CO a. CO co Gi t/3 0.72 0.71 0.708 0.70 0.69 0.68 0.87 • =Sr tracer added •=Sr89 tracer not added • 9 sphalerite galena pyrrhotite galena Figure 7: S r 8 7 / S r 8 6 ratios of sulphide samples from the Bluebell mine 89 showing the effects of addition of "carrier free" Sr tracer. -79-low S r 0 / / S r 0 D ratio (<0.702) suffered from addition of the tracer. 87 86 Another possible source of error in the Sr /Sr isotopic measurements is unstable running conditions. The i n i t i a l blocks of data usually suffer from inst a b i l i t y , and often produce spurious 87 86 ratios. Table 26 l i s t s the measured Sr /Sr ratio of those sulphide 89 samples to which the Sr tracer was not added, the number of blocks obtained and the V.R.E. scale at which the blocks were obtained. Table 26 indicates that on 3 of the indicated measurements less than 3 blocks of data were collected. Due to beam instability during the i n i t i a l data collection stages these measurements should be considered unreliable. Table 26: Number of blocks of data collected on sulphide samples to which the Sr tracer was not added. Sample c 87 / c 86 Sr /Sr Sr p.p.m. #. of Blocks V.R.E. Scale RJ-11-3 0.7066 0.0059 2 lOOmv RJ-13-3 0.7067 0.10 2 lOOmv RJ-21-6 0.7135 0.083 1 lOOmv RJ-21-7 0.7098 0 (-0.027) ( 1 ) 9 lOOmv (1) Negative Sr concentration was obtained by applying a blank correction of 19 ngrams. Another source of indeterminate error in the measurements is the con-tribution of blank Sr, of poorly known Sr isotopic composition, to the sample. These contamination effects are further discussed in the next section. -80-Sr Concentration of Sulphide Samples Tables 24 and 25 i n d i c a t e that the measured Sr concentrations of the sulphide samples were not reproduceable. Replicate analyses of samples RJ-11, RJ-13 and RJ-21 a l l showed a progressive decrease i n concentration with time. This may be due to e i t h e r improvements i n sample handling or to the leaching procedures. The important point i s that the most recent analyses (January, 1976) a l l showed negative Sr concentrations when corrected for a Sr blank of 19 ngrams. It i s possible that t h i s blank i s inappropriately large for t h i s suite of samples. A blank processed with the samples had a measured Sr content of 20 ngrams but unfortunately 89 the Sr tracer was added to a blank and the samples. As discussed i n 88 Chapter II , enrichment of the t r a c e r i n Sr makes these analyses un-88 84 r e l i a b l e i n that concentrations based on the Sr /Sr r a t i o w i l l be overestimated. The data suggest that the Sr concentration of the s p h a l e r i t e , p y r r h o t i t e and galena separates i s below the l i m i t of detection. Due to the very small concentration of Sr i n the sulphide samples, 8 7 the proportion of blank Sr to sample Sr i s large. Therefore, the Sr / 86 Sr determinations given i n Tables 24 and 25 cannot be considered r e l i a b l e . Rb Analyses Rb analyses of the sulphide samples given i n Tables 24 and 25 show reasonable r e p r o d u c i b i l i t y . The Rb analysis of sample RJ-12 (galena) was c a r r i e d out during the early stages of the work, before the standardized Rb procedure given i n Chapter II had been developed. The sample was run at an excessively high temperature r e s u l t i n g i n a f a l l i n g s i g n a l and loss of the sample. The anomalously high measured Rb concentration i s probably -81-a r e s u l t of these poor running conditions. As only a small amount of galena was present i n sample RJ-1, no r e p l i c a t e analysis could be made. Table 27 summarizes the r e p l i c a t e Rb analyses of samples RJ-11 (s p h a l e r i t e ) , RJ-13 (pyrrhotite) and RJ-12 (galena). Table 27: Replicate Rb analyses of sp h a l e r i t e , p y r r h o t i t e and galena from the Bl u e b e l l Mine. Sample Mineral Rb ppm Mean P r e c i s i o n Rb ppm (% deviation from the mean) RJ-11 sph a l e r i t e RJ-13 p y r r h o t i t e RJ-21 galena ;oi5 .0085 .0020 .0024 .00020 .00037 ,012 .0022 ,00028 29% 9% 32% Although the p r e c i s i o n of the r e p l i c a t e analyses expressed as % deviation from the mean i s as large as 32% i t i s believed the measured concentration of Rb i n samples RJ-11, RJ-13 and RJ-21 accurately r e f l e c t s both the order of magnitude of the Rb concentration of the samples and the r e l a t i v e Rb concentrations of the samples. Sphalerite from the B l u e b e l l deposit contains an order of magnitude more Rb than p y r r h o t i t e and two orders of magnitude more Rb than galena. Although the Sr concentration of the sph a l e r i t e was not succ e s s f u l l y determined, i t i s probably l e s s than 0.05 ppm. Thus the Rb /Sr r a t i o of the sph a l e r i t e i s l i k e l y greater than 1. This suggests that s p h a l e r i t e may be sui t a b l e for Rb/Sr dating provided the blank Sr l e v e l s can be -82-s u f f i c i e n t l y reduced to resolve the Sr i s o t o p i c composition of the sample from the blank Sr. Carbonate analyses Table 24 indicates r e p l i c a t e Sr analyses and Sr i s o t o p i c deter-minations of the carbonate sample RJ-14 showed good r e p r o d u c i b i l i t y . 87 86 Table 25 indicates the carbonate sample RJ-22 has a measured Sr /Sr 87 8fi r a t i o of 0.7128 ± 0.0002, i n agreement with the Sr / S r O D r a t i o of sample RJ-14. The Bl u e b e l l sulphides were emplaced into the B l u e b e l l limestone. 87 8fi Veizer (1971) reports Sr /Sr r a t i o s of 0.709 - 0.710 for Cambrian carbonates. It i s u n l i k e l y the S r 8 7 / S r ^ ^ r a t i o of the Bl u e b e l l limestone i s greater than 0.710. Thus the measured r a t i o of 0.7128 for the carbonate gangue may represent the i n i t i a l r a t i o of the mi n e r a l i z i n g f l u i d s or the r a t i o of a mixture of Sr from the mineralizing f l u i d s and the host lime-stones . IV-9 Conclusions This attempt to d i r e c t l y date the Bl u e b e l l m i n e r a l i z a t i o n was unsuc-c e s s f u l . The Sr analyses and i s o t o p i c measurements of the sulphide mineral separates were unsuccessful due to the extremely low concentration of Sf i n the samples. The Rb analyses of these samples were successful and indicate extremely low concentrations of 0.01-0.002 p.p.m. Rb. A few r e s u l t s suggest the Rb/Sr r a t i o of s p h a l e r i t e may be su i t a b l e for dating by the Rb/Sr method, but further analyses with very low and well c o n t r o l l e d Sr blank corrections are required. -83-CHAPTER V THE A P P L I C A T I O N OF THE R B / S R DATING  TECHNIQUE TO SOME ULTRAMAFIC NODULES V - l Introduction Small ultramafic nodules occur i n kimberlite diatremes and a l k a l i c basalts throughout the world (Wyllie, 1967). The most common nodule type i s s p i n e l l h e r z o l i t e composed of f o r s t e r i t i c o l i v i n e , s p i n e l , chrome diopside and e n s t a t i t e (Forbes and Kuno, 1967). Many such nodules, with a r e s t r i c t e d o l i v i n e composition (about Fo91-92) and a metamorphic and tectonic f a b r i c , are interpreted as pieces of the upper mantle (Kutolin, 1970; Mercier and Nicolas, 1975). Attention has been focused.upon the Sr i s o t o p i c composition of these nodules i n order to c l a r i f y t h e i r h i s -tory and provide information on the i s o t o p i c composition.of the upper mantle. As part of a project involving Rb and Sr analyses and Sr isoto^-p i c determinations for mineral separates containing very small quantities of Rb and Sr, mineral separates from l h e r z o l i t e nodules which occur.in. a l k a l i c basalts near Boss Mountain, B.C. and Jacques Lake, B.C..were analyzed for Rb, Sr and Sr i s o t o p i c composition. This chapter presents these r e s u l t s and a discussion of t h e i r s i g n i f i c a n c e . -84-V-2 Boss Mountain Lherzolite Nodules At Boss Mountain, 40 km south of Quesnel Lake, lherzolite nodules are included within the Takomkane a l k a l i basalts, which are of late Pleistocene age (Sutherland-Brown, 1957). According to Soregaroli (1968) the texture of the lherzolite nodules t which are associated with less abundant granodiorite and glassy augite nodules, i s coarse-grained with subtle layering defined by discontinuous bands of Cr-diopside. Some olivine grains show broad- twin lamellae. Soregaroli (1963) determined the Fo content of olivine from the nodules and associated groundmass using the x-ray powder photograph method. He reported a range of olivine composition of Fo85.1 to Fo85.9 for the groundmass, whereas the nodule olivine has a more restricted compositional range of F088.O - Fo87.5 on 6 samples. (One anomalously low determined composition was Fo 85.1). Soregaroli (1963) did not speculate upon the origin of the Boss Mountain lherzolite nodules. V-3 Jacques Lake Nodules Lherzolite nodules from the Jacques Lake l o c a l i t y have been previous-ly studied by Littlejohn (1972). They occur within late Quaternary tuff, 6 km south of Quesnel Lake. Littlejohn (1972) gave the following petrographic description of the tuffaceous host rocks: "The tuff is made up of rock fragments of various types and sizes, cemented by a brownish-green matrix consisting of small rock fragments and partly d e v i t r i f i e d glass. The rock fragments consist of sedimentary, plutonic, metamorphic and volcanic xenoliths..." -85-LittieJohn concludes that the Jacques Lake nodules are of upper mantle origin on the basis of a restricted olivine composition (Fo89.5-Fo91.0), and a texture indicative of flow in the solid state. V-4 Previous Work Considerable work has been done on the Sr isotopic composition of ultramafic nodules. Table 28 summarizes the published Sr isotopic data for ultramafic nodules which are included in a l k a l i basalts. Data on nodules which are included i n kimberlites, and reported 87 86 Sr /Sr ratios of nodules for which there i s no reported correspond-87 86 ing Sr /Sr ratio of the host basalt have been excluded from Table 87 86 28. Figure 8 i s a plot of the Sr /Sr ratios of the hosts versus the 87 86 Sr /Sr ratios of the nodules. Figure 8 indicates that many of the 87 86 nodules have a higher Sr /Sr ratio than their host basalts. In a l l these cases the discrepancy in isotopic composition cannot be due 87 to in si t u decay of Rb . The disequilibrium has been interpreted by various workers (e.g. Hoffmann and Hart, 1975; Dasch and Green, 1975) as precluding a cognate relationship between the nodules and their hosts. This conclusion i s in agreement with the generally held opinion, based on the occurrence of olivine of a restricted composition (Mercier and Nicolas, 1975) and textural evidence of flow in the solid state (Carter and Ave'Lallemant 1970) , that some ultramafic nodules do not have a cognate relationship with their host basalts. 87 86 In addition to these whole rock Sr /Sr measurements several 87 86 workers have reported Rb and Sr analyses and Sr /Sr measurements Table 28: S r 8 ? / S r 8 6 whole rock ratios of ultramafic nodules and their host basalts. o 87,. 86 Sr /Sr Age of Host Host S r 8 7 / S r 8 6 2 r Inclusion Inclusion Type E&A Standard Locality Reference n.r n.r (1) Te r t i a r y -Quaternary Middle Jurassic Late Tertiary 0.7035 0.7054 0.7035 0.7038 0.7036 0.7036 0.7047 0.7047 0.7047 0.7047 0.7030 0.7029 0.7029 0.001 0.001 0.001 0.001 0.001 0.001 0.7036 0.7064 0.7060 0.7093 0.7106 0.7061 Late Pleistocene 0.0005 0.7057 0.0005 0.7052 0.0005 0.7046 0.0005 0.7065 0.0006 0.7032 0.0006 0.7030 0.0006 0.7034 0.0006 0.7031 0.001 peridotite O-2??088 Galapagos l a . 0.001 peridotite " Hawaiian Is. 0.001 lherzolite 0.7091 Massif Central,Fr. 0.001 lherzolite 0.7091 Massif Central,Fr. 0.001 lherzolite 0.7091 Massif Central.Fr. 0.001 lherzolite 0.7091 Massif Central.Fr. 0.0006 sp.pyroxenlte n.r. Delgate,Australia 0.0004 eclogitc n.r. Delgate,Australia 0.0005 hbld.ecloglte 0.0005 granulite n.r. n.r. Delgate,Australia Delgate,Australia 0.7033-0.7036 0.7013- 0.0006 0.7035 0.7036 0.7033- 0.0006 0.7031 0.7036 0.0006 0.0006 0.0006 0.0006 peridotite peridotite peridotite webstetite Recent Recent Recent Recent Recent Quaternary Quaternary Quaternary Quaternary Quaternary 0.7029 0.7029 0.7029 0.7029 0.7029 0.0006 0.7050 0.0006 0.7052 0.0006 0.7039 0.0006 0.7030 0.0006 0.7036 0.0005 0.7045 0.7027-0.7034 0.7028- 0.0005 0.7049 0.7034 0.7028- 0.0005 0.7055 0.7034 0.7028- 0.0005 0.7023 0.7034 0.7028- 0.0005 0.7040 0.7034 0.0006 websterite 0.0006 websterite 0.0005 lherzolite 0.0005 lherzolite 0.0006 dunite 0.0006 wehrlite 0.0006 hartzburgite 0.0005 lherzolite 0.0005 lherzolite 0.0005 lherzolite 0.7080 0.7080 0.7080 0.7080 0.7080 0.7080 0.7080 0.7080 0.7060 0.7080 0.7080 0.7083 0.7083 0.7083 0.0005 lherzolite 0.7083 0.0005 lherzolite 0.7083 0.7080 0.7080 0.7080 0.7080 0.7080 0.7080 Quaternary 0.7028- 0.0005 0.7023 0.000! lherzolite 0.7034 Quaternary 0.7028- 0.0005 0.7031 0.000> iherzoxii:** 0.7034 Pleistocene 0.7035 0.0008 0.703'j 0.0008 lherzolite Pleistocene 0.7035 0.0008 0.7044 0.0008 lherzolite. Pleistocene 0.7035 0.0003.0.7051 0.0008 lherzolite Pleistocene 0.7033 0.0008 0.7067 0.0008 lherzolite PlelBtoccne 0.7O3J 0.0008 0.7028 0.0008 wehrlltt Pleistocene 0,703$ 0.0008 0.7035 0.0008 wehrlite Pleistocene 0.7035 0.0(108 0.7044 '0.0008 l h e r z o l i t e Pleistocene 0.7035 0.0008 0.7051. 0.0008 Ihcrznllto Pleistocene 0.7035 0.0008 0.7067 0.0008 lh e r z o l i t e •Meistoceae 0.7033 •J.OCnS C.7028 0.0008 wt\!ulite Pleistocene 0.7035 C.0008 0.7035 0.0008 wehrlite Pleistocene 0.7035 0.0008 0.7039 0.0008 wehrlite Pleistocene 0.7035 0.0008 0.7040 0.0008 wehrlite Pliocene 0.7040 0.0003 0.7089 0.0003 l h e r z o l i t e Pliocene 0.7040 0.0003 0.7083 0.0003 l h e r z o l i t e Pliocene 0.7040 0.0003 0.7068 0.0003 l h e r z o l i t e PIiocene 0.7040 0.0003 0.7069 0.0003 l h e r z o l i t e Pliocene 0.7040 0.0003 0.7069 0.0003 l h e r z o l i t e Late 0.7028 - 0.0006 0.7043 0.0008 l h e r z o l i t e Ht.Perkins,Antarctic Mt.Perkins,Ant. Mt.Perkins,Ant. Delgate,Australia Delgate.Australia Del gate, Australia La sh a ine,Tanzania Lasha ine,Tanzania Lashaine, Tnu^aiila Lashaine,Tanzania Lashaine.Tanzania New Mexico New Mexico Hew Mexico New Mexico New Mexico New Mexico Stueber&Murthy,1966 StuebertMurthy,1966 Leggo&Hutchlnaon,1968 Leggo&Hutchinson,1968 Leggo&Hutchlnson,1968 Leggo&Hutchlnson,1968 Compston&Loverlng,1969 Compston&Loverlng,1969 Compston&Lovering,l969 Compston&lovering,1969 Halpern,1969 Halpern,1969 Halpern,1969 O'Neil et al,1970 O'Ncll et al,1970 O'Neil et al,1970 HutchlnscnWawson, 1970 Hutchlnson&davson,1970 ItetchlnsonSaacwns 1970. Hutchlnson4Dawson,1970 Hutchinson&Dawson,19 70 Laughlln et al,1971 Laughlln et al.1971 Laughlln et al.1971 Laughlln et al.1971 Laughlln et al.1971 laughlln et »1,1971 <-->«ghll» «t gj,'?Tt Dreiser Weiher.Ger. Paul,1971 Dreiser.Welher,Oar. Paul.1971 Dreiser Weiher.Ger. Paul,1971 Dreiser Weiher.Ger. Paul,1971 Dreiser Weiher.Ger. Paul,1971 Drelncr Welher,G<t. Paul.1971 0.7080 0.70H0 0.7030 P.. 7080' ' 0.7080 C.706U 0.7080 0.7080 0.7080 0.7080 0.7080 0.7080 0.7080 Dreiser Welher .Ce.r . PaulTT971~" Dri-lner Wa!hcr,«i'i:. Paul,1971 Dreiser Welher .C.e.r. Paul, 1971 Drelsp.'r Welher.Cer. Paul.1971 Drplaei Weiher.Ger. Paul,1971 Breisei Weih'er.Gor. . P«u!,197.1 Dreiser Welher,Cer. Paul,1971 Pucrco Necks,New Mex. Kudo et al,1972 Puerco Necks,New Mex. Kudo et al,1972 Puerco Necks,New Mex. Kudo et al,1972 Puerco Necks,New Mex. Kudo et al,1972 Puerco Necka,New Mex. Kudo et_al,1972 Malapat H i l l , C a l i f . StullSMcMillan,1973 Ccnzolc Late Cenozolc n.r n.r. Pleistocene pleistocene Pleistocene Pleistocene Pleistocene' Pllestocene Pleistocene Pleistocene 0.7036 0.7028- 0.0006 0.7043 0.7036 0.7025 0.0002 0.7053 0.7025 0.0002 0.7049 0.7038- . 0.0002 0.7040 0.7045 0.7038- 0.0002 0.7027 0.7045 0.0008 l h e r z o l i t e 0.7080 Malapal H i l l . C a l i f . Stull&McMillan.1973 0.7038-0.7045 0.7038-0.7045 0.7038-0.7045 0.7038-0.7045 0.7038-0.7045 0.7038-.7045 0.0002 0.7062 0.0002 0.0002 0.0002 0.0002 0.0002 0.7071 0.7077 0.7093 0.7107 0.7057 0.0002 dunite 0.7080 0.0002 dunite . 0.7080 0.0002 l h e r z o l i t e 0.7080 0.0002 l h e r z o l i t e 0.7080 0.0002 l h e r z o l i t e 0.7080 0.0002 l h e r z o l i t e 0.7080 0.0002 l h e r z o l i t e 0.7080 0.0002 l h e r z o l i t e 0.7080 0.0002 l h e r z o l i t e 0.7080 0.0002 l h e r z o l i t e 0.7080 Exec.Comm.Range,Ant. Stueber&Ikramuddin,1974 Exec. Comm. Range, Ant. Stueber&Ikramuddin, 1974 Victoria,Australia Dasch&Green,1975 Victoria,Australia Dasch&Green,1975 Ii.£ • C "*C33 n.i'. r>. 7027 •>-.r n.r. n.r. n.r. 0.7033 n.r. 0.7058 n.r. n.r. n.r. Recent 6.7041 0.0001 0.7075 0.0003 lher z o l i t e 0.7081 Recent 0.7041 0.0001 0.7043 0.0002 lh e r z o l i t e 0.7081 Recent 0.7041 0.0001 0.7045 0.0002 lhe r z o l i t e 0.7081 Recent 0.7041 0.0001 0.7047 0.0002 lhe r z o l i t e 0.7081 Recent 0.7041 0.0001 0.7052 0.0002 lhe r z o l i t e 0.7081 Recent 0.7041 0.0001 0.7041 0.0002 lhe r z o l i t e 0.7081 Recent 0.7041 0.0001 0.7035 0.0002 lhe r z o l i t e 0.7081 Victoria,Australia Victoria,Australia Victoria,Australia Victoria.Australla Victoria,Australia Victoria.Australla Ross Is. .Ant. Ross ,Ant. Victoria.Australla Victoria.Australla Victoria.Australia Victoria.Australia Victoria.Australla Victoria.Australla Victoria.Australla Dasch&Green,1975 Dasch&Green,1975 Dasch&Green,1975 Dasch&Green,1975 Dasch&Green,1975 Dasch&Green,1975 Stuckless&Erlckson, 1975 Stuckless&Erlckson,1975 Burwell,1975 Burwell,1975 Burwell,1975 Burvell.1975 Burvel1,1975 Burwell.1975 Burvell.1975 ^ The entry n.r. indicates that the information was not reported -87-Figure 8: Sr 0 //Sr° D ratios of ultramafic nodules vs Sr°'/Srt'u of their basaltic host rocks. The plotted line has a slope of 1, intercepts of 0, and represents equilibration between the nodules and their hosts. -88-of mineral fractions separated from the nodules (Peterman et a l . , 1970; Paul, 1971; Kudo et a l . , 1972; Stueber and Ikramuddin, 1974; Dasch and Green, 1975; Barrett, 1975; Basu and Murthy, 1976). A l l these studies 87 86 have shown Sr /Sr disequilibrium between mineral phases of a. single nodule. 87 86 87 86 Stueber and Ikramuddin (1974) showed that Sr /Sr and Rb /Sr ratios of clinopyroxene, orthopyroxene and olivine mineral separates from Mt. Aldaz Antarctica and Kilbourne Hole, New Mexico define isochrons. By the same method Basu (1976) determined apparent ages for lherzolite xenoliths from Baja, California. Table 29 summarizes these results. 87 86 Table 29: Ages and Sr /Sr i n i t i a l ratios of ultramafic nodules. Reference Location 87 86 Age (m.y.) I n i t i a l Sr /Sr Stueber & Ikramud-din (1974) Stueber & Ikramud-din (1974) Basu & Murthy (1976) Basu & Murthy Mt. Aldaz, Antarctica Ratio 610 + 110 0.7020 + 0.0002 Kilbourne Hole, 1270 + 230 0.7024 + 0.0002 New Mexico Ba j a, California Baja, California 1050 + 40 0.7026 + 0.0001 1870 + 120 0.7030 + 0.0002 Stueber and Ikramuddin (1974) and Basu and Murthy (1976) have con-sidered the nodules to represent portions of the upper mantle and have interpreted the apparent ages as "mantle events". Basu and Murthy (1976) interpret the discrepancy in age and i n i t i a l r a t i o of nodules from the same locality as indicating derivation of the nodules from different mantle provenances. - 8 9 -, - n , o 87 / 0 86 , ^ 87 86 Several workers have determined the Sr /Sr and Rb /Sr ratios of clinopyroxene, olivine and orthopyroxene mineral separates from ul t r a -mafic nodules without finding an isochron relationship. Dasch and Green (1975) analyzed mineral separates of lherzolite nodules included in the Newer Volcanics of western Victoria. The work was carried out i n the Lunar Sample Laboratory at A.N.U., and exceptionally low blanks of 87 86 87 86 3ng Rb and 2ng Sr are reported. Their Sr /Sr and Rb /Sr ratios of olivine, enstatite, and diopside do not define an isochron. Olivine, 87 86 87 86 with the highest Sr /Sr ratio has a lower Rb /Sr ratio than enstatite. Stueber and Ikramuddin (1974) obtained similar results for a lherzolite nodule from San Carlos, Arizona. Olivine has the 8 7 8 6 8 7 8 6 highest Sr /Sr ratio but has a lower Rb /Sr than enstatite. Olivine i n a lherzolite nodule from S.E. California analyzed by Peter-87 86 87 man et a l . (1970) also has a higher Sr /Sr ratio and a lower Rb / 86 Sr ratio than co-existing enstatite. An exceptionally high Sr blank of 0.6jug. was reported with these analyses however, suggesting the ratios may suffer from a large analytical uncertainty. These results indicate that some minerals within some ultramafic nodules have not remained closed systems since their formation. There is a large body of evidence indicating that grain boundary materials and minor i n t e r s t i t i a l phases in lherzolite nodules are en-riched i n Rb. Dasch and Green (1975) reported acid washing of the olivine separate resulted in a reduction i n the Rb concentration from 0.07 to 0.05 p.p.m. Hutchinson and Dawson (1970) report the results of a microprobe study of potassium distribution within clinopyroxenes from a lherzolite nodule. It was found that potassium was concentrated -90-i n tiny spots surrounding each grain. Rubidium presumably follows potassium and i s similarly concentrated around grain boundries. Compston and Lovering (1969) found that the sum of Rb concentrations in mineral fractions could account for only 35% of the total rock Rb and concluded the remainder was distributed i n t e r s t i t i a l l y . Allsopp et a l . (1969) found that whole rock (eclogite) concentrations of potas-. sium and rubidium could not be accounted for by mixtures of the con-stituent minerals . They noted that 10% of potassium and rubidium went into solution when the eclogite was leached with HC1 and concluded that a significant fraction of rubidium was present i n an easily re-moveable phase. G r i f f i n and Murthy (1968) also performed leaching experiments on eclogite and found that 90% of Rb and 62% of the potas-sium was removed by leaching with R^ O and 2.5N HC1. These results indicate that the minor i n t e r s t i t i a l phases and grain boundary materials are enriched i n Rb. Contamination of the mineral separates with this i n t e r s t i t i a l material may account for the lack of an isochron 87 86 relationship from some nodules which show Sr /Sr mineral dis-equilibrium. 87 86 The observation that whole rock Sr /Sr ratios of ultramafic nodules are often higher than the host basalts (Table 28) suggests that the Rb i s not due to late stage contamination of the nodules by basaltic liquids. Dasch and Green (1975) suggest that the inter-s t i t i a l Rb originates in accessory minerals such as phlogopite, amphi-bole or their p a r t i a l melting products and that this i n t e r s t i t i a l Rb may have contamined the olivine during recrystalization or reheating. -91-In summary, S r u ' / S r u u disequilibrium between minerals i n u l t r a -mafic nodules is common. Apparent isochron relationships may represent mantle events. Extreme care in preparation and cleaning the mineral separates prior to Rb and Sr analyses and Sr isotopic determinations i s necessary due to the presence of Rb-rich i n t e r s t i t i a l phases. V-5 Samples Samples of olivine, orthopyroxene and clinopyroxene from a Boss Mountain lherzolite were prepared as described i n Chapter 2. Olivine, orthopyroxene and clinopyroxene separates from the Jacques Lake lherzolite nodules which had been previously prepared by Littlejohn (1972) at U.B.C. were obtained. These samples were hand picked and prepared as described in Chapter II. The following section documents the Rb and Sr analyses and Sr isotopic measurements of these samples. V-6 Discussion of Analytical Results The results of Rb and Sr analyses and Sr isotopic determinations of olivine, orthopyroxene and clinopyroxene mineral separates from samples BM, JL-10 and JL-39 are given in Table 30. I n i t i a l l y between 0.4 and 1.0 grams of each sample was prepared as described i n Chapter II Periodically, as these samples were partly exhausted, fresh samples were ground and cleaned by the same methods used i n i t i a l l y , and mixed with the remaining sample material. These new preparations were made twice, during December 1975 and January 1976. Unfortunately, -92-Table 30: Rb and Sr concentrations and Sr /Sr r a t i o s of mineral separates from Boss Mountain and Jacques Lake l h e r z o l i t e nodules. I of Sample t Mineral Date Prepared S r 8 7 / S r 8 6 la Blocks Averaged Sr(p.p.m.) Rb(p.p.m.) Rb 8 7/Sr 8 6 BM-13-1A cpx BM-13-2 B cpx 19/09/75 23/10/75 0.7022 0.7027 0.0004 0.0005 37 16 22.5 26.2 n.d. 0.269 n.d. 2.97 x 10"2 BM-21-1A olivine BM-21-2A olivine 19/09/75 23/10/75 0.7060 0.7069 0.0027 0.0004 6 10 1.56 1.25 n.d. 0.0483 n.d. 1.11 x 10" 1 BM-22-1A opx BM-22-2 A opx BM-22-3 B opx 19/09/75 23/10/75 05/02/76 0.7034 0.7056 n.d. 0.002 0.001 n.d. 7 10 1.49 1.94 n.d. n.d. n.d. 0.0197 n.d. n.d. n.d. BH-matrix whole rock 13/4/76 0.7058 0.0001 11 n.d. n.d. n.d. JL-101-1A olivine JL-101-2B olivine JL-101-3B olivine 23/10/75 05/02/76 05/02/76 0.7081 0.7092 n.d. 0.0005 0.0030 n.d. 18 24 2.13 n.d. 1.82 0.0887 0.154 n.d. 1.20 x n.d. n.d. iO" 1 JL-102-1A opx JL-102-2 B opx 23/10/75 05/02/76 0.7054 0.7063 0.0003 0.0035 12 8 2.04 0.783 0.0580 0.0284 8.25 xl0~ 2 1.05 xl0~ A JL-103-1A cpx JL-103-2 B cpx JL-103-3 B cpx 23/10/75 05/02/76 05/02/76 0.7033 0.7032 0.7031 0.0012 0.0001 0.0003 13 18 I 4 27.9 29.0 n.d. 0.0718 0.103 n.d. 7.45 x 1.02 X n.d. 10 * JL-391-1A olivine JL-391-2B olivine JL-391-3B olivine JL-391-4B olivine 23/10/75 05/02/76 05/02/76 23/01/76 0.7069 0.7098 0.7121 n.d. 0.0014 0.0005 0.0060 n.d. 16 9 5 1.40 n.d. 0.685 n.d. 0.0519 n.d. 0.0143 0.0298 1.07 x n.d. 6.06 x n.d. lO" 1 lO" 2 JL-392-1A opx JL-392-2C opx JL-392-3C opx JL-392-4 B opx 23/10/75 06/02/76 06/02/76 23/01/76 0.7066 0.7076 0.7091 n.d. 0.0022 0.0012 0.0037 n.d. 12 15 13 2.13 n.d. 1.49 n.d. 0.100 n.d. n.d. 0.0299 1.37 x 10 n.d. n.d. n.d. -1 JL-393-lA cpx JL-393-2 B cpx JL-393-3 B cpx 23/10/75 23/01/76 2 3/01/76 0.7026 0.7027 0.7027 0.0013 0.0001 0.0005 19 17 14 41.6 n.d. 41.4 0.143 n.d. n.d. 9.96 x 10 n.d. n.d. -3 -93-during the preparation period newly prepared sample was added to some but not a l l of the samples, and the samples to which freshly prepared material was added were not recorded. This procedure i s of some con-cern because the newly processed samples may not have had identical Rb and Sr concentrations as the primary sample material. In order to assist interpretation of the data those samples processed prior to the subsequent preparation are labeled A i n Table 30. Those samples processed after the f i r s t and second re-preparations are marked B and C respectively. 87 , 86 Sr /Sr Ratios. 87 86 The Sr /Sr ratios given i n Table 30 are the means of several blocks of data. The number of blocks used i n calculating these means and i n computing, the standard deviation i s l i s t e d i n column 6 of Table 30. Some blocks of data were rejected for a number of reasons. If the Sr signal became persistently unstable the data were rejected. In order to obtain the maximum number of Sr isotopic measurements from the samples containing very small quantities of Sr^ however, blocks of data which suffered from short-term ion beam i n s t a b i l i t y were re-averaged with the affected measurements discarded and these re-averaged results retained. Blocks of data were rejected i f the Rb correction became unusually large. In cases where sufficient data was collected blocks of data were rejected with Rb corrections' greater than 0.3% (0.002). For cases in which only a limited number of measurements were obtained (for example sample BM-21-1) blocks of data with a Rb correct-ion as large as 5% (-0.03) were retained. As discussed in Chapter II, -94-errors in the Rb correction may occur due to a number of factors. 87 86 These errors may produce systematic errors i n the measured Sr /Sr ratios. In order to check this, the correlation coefficient of the Rb 87 86 correction with the corrected Sr /Sr ratio was determined for each run. Values of the correlation coefficient vary between -0.67 and +0.42. None of the correlations are significant at the 90% confidence level. It i s thus believed that the Rb correction has not caused systematic 87 86 errors in the measured Sr /Sr ratios reported here. 89 Addition of the Sr tracer may have caused an indeterminate lower-87 86 ing of some of the Sr /Sr ratios given i n Table 30. Replicate Sr isotopic measurements with and without addition of the tracer made on mineral separates from samples JL-10 and JL-39 are summarized in Table 31. 87 86 Table 31: Comparison of the Sr /Sr ratio measured on samples with, and samples without, addition of the Sr89 tracer. Sample Mineral Date of Preparation Tracer Addition Sr (ppm) „ 87 ,„ 86 , ^  Sr /Sr 10-JL-101--1 olivine 23/10/75 yes 2.13 0.7081 0.0005 JL-101-•2 olivine 05/02/76 no n.d. ( 1 ) 0.7092 0.003 JL-102-•1 opx 23/10/75 yes 2.04 0.7051 0.0003 JL-102--2 opx 05/02/76 no 0.78 0.7063 0.0035 JL-391-•1 olivine 23/10/75 yes 1.40 0.7069 0.0013 JL-391-•2 olivine 05/02/76 no n.d. 0.7098 0.0005 JL-391--3 olivine 05/02/76 no 0.685 0.7121 0.0060 JL-392-•1 opx 23/10/75 yes 2.13 0.7066 0.0022 JL-392-•2 opx 06/02/76 no n.d. 0.7076 0.0012 JL-392-•3 opx 06/02/76 no 1.49 0.7091 0.0037 (1) n.d. = not determined -95-89 Table 31 indicates that a l l samples to which the Sr tracer 87 86 was added had lower measured Sr /Sr ratios than those samples to which the tracer was not added. However, the difference in isotopic composition i s well within the uncertainty of the measurements and so i t was concluded that the effect of addition of the tracer on the 87 86 Sr /Sr ~ ratios of these olivine and orthopyroxene samples was small. 87 86 A l l the measured Sr /Sr ratios of orthopyroxene and olivine separates are retained as data i n this work. The measured Sr isotopic composition of clinopyroxene separates wi-11 be unaffected by the addition of the tracer due to the high Sr concentrations i n clinopyroxene. 87 86 With the exception of sample JL-391 a l l the measured Sr /Sr ratios given i n Table 30 were reproducible within the limits of un-certainty. Rb and Sr Analyses Replicate analyses of Rb and Sr given i n Table 30 do not show good reproducibility. In general, (samples JL-101 (Rb) and JL-103 (Sr) are exceptions) those analyses made of samples prepared i n January and February 1976 show lower Rb and Sr . concentrations than analyses of the same samples prepared in September and October 1975. Table 32 l i s t s those samples for which replicate Rb and Sr analyses from the same sample (spiked with both Rb and Sr) are available. Each pair of analyses in Table 32 i s made up of one analysis of a sample processed prior to the subsequent sample preparations, and one analysis of a sample processed after the subsequent sample preparations, It i s therefore possible that the "replicate" analyses given in Table 32 are not analyses of different spli t s of the same sample but are -96-rather two analyses of the same sample material. Thus sample i n -homogeneity might be expected. Table 32 indicates that r e p l i c a t e analyses of orthopyroxene 87 86 (JL-102) are not i n agreement. The Rb /Sr r a t i o s show much better agreement, however, with a deviation of 12% from the mean. Replicate analyses of clinopyroxene (JL-103) show reasonable agreement. The r e -p l i c a t e analyses of o l i v i n e JL-391 show poor agreement with deviations from the mean of 34% and 57% for the Sr and.Rb analyses r e s p e c t i v e l y . 87 86 The Rb /Sr r a t i o shows better agreement with a deviation from the mean of 28%. 8 7 86 With the exception of sample JL-391, a l l the r e p l i c a t e Sr /Sr determinations of the ultramafic mineral separates were i n agreement within the l i m i t s of uncertainty.-(ler) . The lack of discrepancy i n r e p l i c a t e Sr i s o t o p i c composition measurements of samples i n which the Sr concentration measurements were not i n agreement i s of note. Unfortunately the Sr i s o t o p i c measurements have a large uncertainty making t h i s observation inconclusive. Three aspects of the discrepancy i n r e p l i c a t e Rb and Sr analyses have been noted: 1) most l a t e r analyses showed lower concentrations; i i ) discrepancies i n the Rb and Sr analyses are larger than d i s c r e -87 86 pancies i n the Rb /Sr r a t i o of samples spiked with both Rb and Sr; % and i i i ) no v a r i a t i o n i n Sr isotopic); composition of most samples i s associated with the discrepancies 'in r e p l i c a t e Sr concentration deter-minations. Table 32: Replicate Rb and Sr analyses of mineral separates from Jacques Lake ultramafic nodules. Sample Mineral [Sr](ppm) % deviation from the mean [Rb]ppm ; deviation from the mean „. 87 86 Rb /Sr % deviation from the mean JL-102-1 JL-102-2 opx opx 2.04 0.783 44 0. 058 0.025 40 8.25 x 10 1.05 x 10 -2 -1 12 JL-103-1 JL-103-2 cpx cpx JL-391-1 o l i v i n e JL-391-3 o l i v i n e 27.9 29.0 1.40 0.685 34 0.0718 0.103 0.0519 0.0143 18 57 7.45 x 10 1.02 x 10 -3 -2 1.07 x 10 6.06 x 10 -1 -2 15 28 i I -98-Excessive leaching of the mineral separates can account for these observations. As described i n Chapter II, a l l the mineral separates were leached with HF in order to remove labile Rb and Sr from the grain boundries. Excessive leaching of the grains may have removed primary Rb and Sr. Such excessive leaching could account for observa-tions i i ) and i i i ) above. As described in Chapterii, the mineral separates were stored in glass sample bottles after preparation. For chemical processing the 100 to 325 mesh samples were poured onto aluminum f o i l for weighing, It is possible that the larger size fraction was used f i r s t , and that the finer size fraction was used in the later preparations. As the finer size material may have suffered from excessive leaching this mechanism can explain observation i ) , the reduction i n Rb and Sr concentrations i n the later analyses. In the 87 86 following age calculations uncertainties in the Rb /Sr ratios are calculated from the deviation from the mean of the Rb and Sr analyses. If these deviations are caused by sample inhomogeneity as described, the uncertainties may be overestimated. The observation that discrepancies in the Rb and Sr analyses are 87 86 larger than discrepancies i n the Rb /Sr ratio of samples spiked with Rb and Sr i s tentative due to the small number of samples spiked with both Rb and Sr. Also, the conclusion that no variation i n Sr isotopic composition i s associated with the discrepancies in replicate Sr concentration determination is tentative due to the large un-certainty in many of the Sr isotopic measurements. Consequently i t i s possible these discrepancies in the Rb and Sr analyses are also due to contamination or spiking errors. -99-I n summary, t h e d i s c r e p a n c i e s i n r e p l i c a t e Rb and S r a n a l y s e s may be due t o : s ample i n h o m o g e n e i t y c a u s e d by t h e r e - p r e p a r a t i o n o f some sample s d u r i n g t h e w o r k ; sample i n h o m o g e n e i t y c a u s e d by o v e r - l e a c h i n g ; l a b o r a t o r y c o n t a m i n a t i o n ; s p i k i n g e r r o r s ; ° r any c o m b i n a t i o n o f t h e s e f a c t o r s . I s o c h r o n R e l a t i o n s h i p s 87 86 87 86 T a b l e 33 g i v e s " p r e f e r r e d " S r / S r r a t i o s and Rb / S r r a t i o s 8 7 86 f o r s a m p l e s B M , JL-10 and JL-39. The S r / S r r a t i o s have b e e n a r r i v e d a t by w e i g h t i n g r e p l i c a t e measurements a c c o r d i n g t o t h e i n v e r s e o f t h e 87 86 87 86 v a r i a n c e . F i g u r e 9 i s a Rb / S r v s Sr / S r p l o t o f " p r e f e r r e d " d a t a f r o m sam p le BM. No i s o c h r o n r e l a t i o n s h i p i s i n d i c a t e d . I t i s l i k e l y t h a t e r r o r s i n t h e Rb a n a l y s e s a r e r e s p o n s i b l e . The b e s t f i t i s o c h r o n ( Y o r k , 1966) i n d i c a t e s a n o m i n a l age o f 4.4 b . y . - 2.5 b . y . and an i n i t i a l r a t i o o f 0.7006 - 0.0017. 87 86 87 86 F i g u r e 10 i s a Rb / S r v e r s u s S r / S r p l o t o f t h e u s e a b l e d a t a 87 86 f r o m sam p le JL-10. A l t h o u g h t h e u n c e r t a i n t i e s i n . t h e Rb / S r r a t i o a r e l a r g e ( b a s e d upon t h e d e v i a t i o n f r o m t h e mean o f d u p l i c a t e a n a l y s e s ) t h e a v e r a g e v a l u e s d e f i n e a r e a s o n a b l y good i s o c h r o n . A b e s t f i t i s o c h r o n ( Y o r k , 1966) has a s l o p e o f 0.0289- 0.0008 w i t h a n i n i t i a l r a t i o o f 0.7024 t. 0.0001. The s l o p e c o r r e s p o n d s t o a n age o f 2.0-0.1 b . F i g u r e I I i s a n i s o c h r o n p l o t o f t h e d a t a f o r s ample JL-39. No c l e a r i s o c h r o n r e l a t i o n s h i p i s i n d i c a t e d . A b e s t f i t i s o c h r o n ( Y o r k , 1966) has a s l o p e o f 0.069 - 0.036 w i t h an i n i t i a l r a t i o o f 0.7020 -0.0004. The i n d i c a t e d age i s 4.7 - 2.4 b . y . The u n c e r t a i n t y i s v e r y l a r g e . -100-Table 33: Preferred. Rb and Sr concentrations and Sr /Sr ratios of samples BM, JL-10 and JL-39. Sample Mineral S r 8 7 / S r 8 6 lo- Rb 8 ?/Sr 8 6 ± ( 1 > BM-13 cpx 0.7024 0.0003 0.032 0.003 BM-21 olivine 0.7069 0.0004 • 0.099 0.012 BM-22 opx 0.7052 0.0009 0.033 0.004 JL-101 olivine 0.7081 0.0005 0.177 0.049 JL-102 opx 0.7054 0.0003 0.088 0.0052 JL-103 cpx 0.7032 0.0001 0.0088 0.0016 JL-391 olivine 0.7095 . 0.0005 0.089 0.063 JL-392 opx 0.7075 0.0010 0.104 0.060 JL-393 opx 0.7027 0.0001 0.010 0.0006 Uncertainties given here are deviations from the mean for s; on which replicate analyses were made. For samples without replicate / analyses the uncertainties have been calculated from known uncertain-ties i n the weighing, spike calibration and isotopic measurements. V-7 Interpretation of Results Origin of the Nodules Littlejohn (1972) concluded that the Jacques Lake nodules formed in the upper mantle o-nct cue not crystal cumulates. An upper mantle origin was inferred from the mineral assemblages and part i a l mineral analyses. The restricted composition of olivine and textural evidence for flow in the solid state are cited as evidence against a cumulate origin. The Sr isotopic data presented here support the conclusions of Littlejohn (1972). Strontium isotopic disequilibrium between the clino-pyroxene and olivine, and clinopyroxene and orthopyroxene i s clearly - 1 0 1 -o'.ia Figure 9: S r 8 7 / S r 8 6 vs Rb 8 7/Sr 8 6 of mineral separates from a Boss Mountain lherzolite nodule. -102-r» Lake lherzolite nodule JL-10. -103-IN f Figure 11: Sr /Sr vs Rb /Sr of mineral separates from Jacques lherzolite nodule JL-39. -104-present i n both samples JL-10 and JL-39. These minerals could not have formed as c r y s t a l cumulates of the Quaternary volcanics i n which they are enclosed as there has been i n s u f f i c i e n t time for i n s i t u 87 87 decay of Rb to produce the observed Sr i s o t o p i c v a r i a t i o n s . Any other mechanisms of producing the observed mineral i s o t o p i c d i s -equilibrium (such as separation and contamination of the o l i v i n e and orthopyroxene c r y s t a l s p r i o r to t h e i r incorporation into the ascending lava) are considered s u f f i c i e n t l y u n l i k e l y events to preclude a cognate r e l a t i o n s h i p between the nodules and t h e i r hosts. Soregaroli (1968) d i d not draw any conclusions regarding the o r i g i n of the Boss Mountain nodules. He showed, however, that the composition of the o l i v i n e from the nodules i s much more r e s t r i c t e d than the compo-s i t i o n of the o l i v i n e from the groundmass. C r y s t a l cumulates usually contain o l i v i n e of a wide range of f o r s t e r i t e content. That of the S t i l l w a t e r Complex ranges from Fo94 to Fo80., (Jackson, 1969). Thus, the r e s t r i c t e d f o r s t e r i t e content of the Boss Mountain l h e z o l i t e nodules may suggest they did not form as cumulates. This suggestion i s supported by the i s o t o p i c data presented here which indicates clea r Sr i s o t o p i c d i s e q u i l i b r i u m between clinopyroxene and o l i v i n e . Thus the data indicates that both the Boss Mountain and Jacques Lake l h e r z o l i t e nodules do not have a cognate r e l a t i o n s h i p with t h e i r hosts and are probably accidental i n c l u s i o n s of the mantle caught up i n the ascending magma. Mineral Isotopic Disequilibrium The Sr i s o t o p i c d i s e q u i l i b r i u m of the constituent minerals may 87 have three possible causes: i ) i n s i t u decay of Rb ; i i ) con--105-87 tamination of the mineral phases by Rb anoj Sr enriched fluids; or, i i i ) a combination of both of these processes. There i s some evidence that contamination may be responsible for the observed Sr isotopic disequilibrium of lherzolite nodules. As discussed previously, the high precision Rb and or analyses of Stueber and Ikramuddin (1974) and Dasch and Green (1975) of nodules from San Carlos Arizona and Victoria Australia do not define an isochron 87 86 as the olivine, which has the highest Sr /Sr ratio, has a lower 87 86 Rb /Sr ratio than enstatite. A similar result was obtained for sample JL-39 of this work, although analytical uncertainties make this observation inconclusive. This data suggest that contamination of at least some ultramafic nodules has occurred. It i s not yet clear whether the contamination has merely altered the Rb/Sr ratio of minerals whose Sr isotopic disequilibrium has been produced by in si t u decay of Rb, or whether the contamination i s solely responsible for the observed isotopic disequilibrium. A second observation that suggests contamination may be respons-ible for the observed isotopic disequilibrium i s the presence of Rb enriched i n t e r s t i t i a l material within the nodules. (Evidence for the Rb enrichment of the i n t e r s t i t i a l material i s given i n section V-4.) This i n t e r s t i t i a l material i s probably not due to late stage invasion 87 86 of the nodules by basaltic liquids, as the whole rock Sr /Sr ratios of the nodules are often higher than their host basalts 87 86 (section V-4). As the whole rock Sr /Sr ratio of nodules cannot be accounted for by appropriate combinations of the Sr within con-stituent minerals based on the measured mode (Dasch and Green, 1975; Compston and Lovering, 1969) i t can be concluded that the inter--106-s t i t i a l Rb-enriched phases have a higher Sr /Sr ratio than the host basalts, and are therefore not a result of invasion by basaltic liquids. This implies that Rb rich fluids did exist in the source regions of the nodules. Partial equilibration of the olivine and 87 orthopyroxene with these Rb and Sr enriched phases has been sug-gested by Dasch and Green (1975) and Harris et a l . (1972) as an explanation for the observed Sr isotopic disequilibrium between mineral phases of ultramafic nodules. Sr isotopic disequilibrium between mineral phasas in ultramafic nodules indicates that Sr i s not equilibrated at temperatures as high as 1000°C, at least for short periods of time (Paul, 1971). If the 87 observed disequilibrium i s due to in situ decay of Rb and i f the nodules have resided in the upper mantle since their formation u n t i l their incorporation in the basalts then such disequilibrium must be maintained over long periods of time (up to 2.0 b.y.) at upper mantle temperatures. Hofmann (1975) has determined the diffusion coefficients of Ca and Sr in a basalt melt, and concludes that equilibration of these elements on a 1 cm scale within the melt phase requires only a few years. Diffusion i n the solid phases is much slower4however, so that this data may not be directly applicable i n assessing the pos-s i b i l i t y of long term disequilibrium at upper mantle temperatures. Baadsgard and Van Breeman (1970) showed that considerable changes i n the isotopic composition of mineral phases of quartz monzonite occurred when the sample was heated in air at 1025°C for 100 hours. Hart 87 (1964) showed that loss of Sr in biotites occurred up to 1000 f t . away from the Eldora Stock. This data suggest that Sr isotopic -107-di s e q u i l i b r i u m f o r extended periods of time at temperatures at or close to the basalt liquidous i s u n l i k e l y . Such d i s e q u i l i b r i u m may have per-s i s t e d i f the nodules resided i n cooler regions of the upper mantle, however. Thus, three observations suggest that contamination may be respon-s i b l e f o r the observed i s o t o p i c d i s e q u i l i b r i u m : i ) the presence of sui t a b l e contaminating f l u i d s within the source regions of the nodules; i i ) the lack of an isochron r e l a t i o n s h i p between mineral phases of some nodules; and, i i i ) the improbability of maintaining i s o t o p i c d i s e q u i l i -brium for extended periods of time at upper mantle temperatures. The most powerful argument i n favour of the Sr d i s e q u i l i b r i u m 87 being produced by i n s i t u decay of Rb was brought forward by Stueber and Ikramuddin (1974) who suggested that i t i s u n l i k e l y that observed. isochron r e l a t i o n s h i p s are f o r t u i t o u s . The isochron r e l a t i o n s h i p s of some nodules such as those from San Carlos Arizona, V i c t o r i a A u s t r a l i a and possibly Boss Mountain, B.C. ( t h i s work) may have been obscured by l a t e stage p a r t i a l melting or reheating.. The S r 8 7 / S r 8 6 Ratio of the Mantle If the observed Sr i s o t o p i c d i s e q u i l i b r i u m of the Boss Mountain 87 and Jacques Lake nodules i s due to i n s i t u decay of Rb then the 87 86 i n i t i a l r a t i o s i n f e r r e d from the isochrons i n d i c a t e the Sr /Sr r a t i o of the parent mantle material at the time of formation. The i n i t i a l r a t i o of the Boss Mountain nodule i s poorly defined due to the 87 86 anomalously high Rb /Sr r a t i o of the clinopyroxene. The i n i t i a l r a t i o defined by the best f i t isochron i s 0.7006 - 0.0017. I t i s - 1 0 8 -possible that the true i n i t i a l ratio of this sample is closer to 0.7024, the ratio of the clinopyroxene phase. The i n i t i a l ratios of the Jacques Lake nodules are better defined due to the small analytical uncertainty of the clinopyroxene analyses. 87 86 Figure 12 is a plot of the i n i t i a l Sr /Sr ratio of nodules versus their age. The nodule data plotted i n Figure 12, taken from this work and the literature, i s summarized in Table 34. Included i n Figure 12 i s an age based upon a two-point isochron.defined by clino-pyroxene and orthopyroxene from the Victoria lherzolites (Dasch and Green, 1975). The indicated age i s about 700 m.y. with an i n i t i a l ratio of 0.7029 + 0.0002 (2o-). As discussed previously, Dasch and Green found that the olivine from these lherzolites has a lower 87 86 87 86 Rb Sr ratio and a higher Sr /Sr ratio than enstatite and con-sequently does not f a l l on the isochron. There i s evidence however that this two-point isochron may define the age of the nodules. 87 Burwell (1975) found that whole rock Rb and Sr analyses and Sr / 86 Sr ratios of lherzolite nodules from the same loc a l i t y define an isochron with an age of 650 + 125 m.y. and an i n i t i a l ratio of 0.7039 + 0.0002 (2o*) . The i n i t i a l ratios of these two sets of data do not agree. However, this situation may be explained i f the whole rock nodules analyzed by Burwell contained approximately equal pro-portions of olivine of constant Sr isotopic composition. The ages of the two sets of data agree and suggest the two-point isochron of Dasch and Green (1975) has significance. The data is thus included in Figure 12. Also plotted i n Figure 12 i s the best i n i t i a l ratio of basaltic achrondites (BABI) which may be assumed to be the i n i t i a l o B i l l i o n s of Years i n the Past Figure 12: I n i t i a l Sr /Sr r a t i o vs. age of • ultramafic nodules from world-wide l o c a l i t i e s . -110-87 86 ,Sf /Sr ratio of the primordial mantle (Papanastassiou and Wasser-burg, 1969) . The best estimate of the present day upper mantle beneath the ocean, derived from mid-ocean ridge basalts (MORB) 87 86 (Hofmann and Hart, 1975) is also plotted. The range of Sr /Sr ratios of the Garibaldi volcanics, which may be representative of the present day upper mantle beneath B.C. (N. Green, personal communication) are also plotted for reference. 87 86 Table 34 gives the reported measured Sr /Sr ratios of Sr 87 standards, and the reported Rb decay constants used i n the age calculations, for the nodule data plotted i n Figure 12. As this data was not reported in several cases, no standardization of the data in Figure 12 has been made. 87 86 The path of development of the Sr /Sr ratio from BABI to MORB is unknown. It has long been realized that the Rb/Sr ratio of mid-ocean ridge basalts is too low to account for the evolution of the 87 86 87 presently observed Sr /Sr ratio from BABI by in si t u decay of Rb (Tatsumoto et a l . , 1965; Bence, 1966; Armstrong, 1968; O'Nions and Pankhurst, 1974). As Rb from the source regions of mid-ocean ridge basalts fractionates preferentially into the melt phase, this observa-tion also applies to. the source regions of those basalts—the upper mantle. Thus the source regions of upper mantle tholeiites have ^ 87 undergone depletion in Rb relative to Sr, or enrichment in Sr from crustal sources, or probably both. In Figure 12 a straight line i s drawn from BABI to MORB for reference. - I l l -87 86 Table 3 4: Ages and Sr /Sr i n i t i a l ratios of ultramafic nodules with reported strontium standards and decay constants. Locality Age 87 , 86 Sr /Sr Strontium Decay i n i t i a l ratio Standard Constant Reference Mt. Aldaz Antarctica Kilbourne Hole, New Mexico Baja, Calirornia Baja, California Victoria, Australia Boss Mtn., B.C. Jacques Lake, B.C. Jacques Lake, B.C. 610 +110m.y. 1270 +230m.y. 1050 +40m.y. 1870 +120m.y. 700 +125m.y. 1.9-4.4b.y. 2.0 O.lb.y. 2.3-4.5b.y. 0.7020 +0.0002 0.7024 0.0002 0.7926 +0.0001 0.7030 +0.0002 0.7029 +0.0002 0.7006 +0.0017 0.7024 +0.0001 0.7020 +0.0004 E&A 0.7079 +0.0002 E&A 0.7079 +0.0002 not Stueber & reported Ikramuddin(1974) not Stueber & reported Ikranuddin (1974) not reported not not reported Basu & reported Murthy (1976) not Basu & E&A 0.7080 +0.0002 SRM-987 0.7102, SRM-987 0.7102, SRM-987 0.7102, reported Murthy (1976) Dasch & Green (1975) 1.39xlO - 1 1Dasy r - l 1.42xl0 _ 1 1(this work) y r ~ l 1.42xl0 _ 1 1(this work) yr--*-1.42xl0 - 1 1(this work) Figure 12 indicates that the i n i t i a l ratios of nodules from Baja, Jacques Lake (JL-10), and Kilbourne Hole are a l l significantly higher than the ratios that would be expected for a mantle with 87 86 linear Sr /Sr evolution. Some of the older nodules (Baja 2 and Jacques Lake JL-10) have sli g h t l y higher ratios than the younger nodules. There are several possible explanations for these observations. If the nodules are samples of a homogeneous upper mantle, the data -112-suggest that the upper mantle had a very high Sr /Sr r a t i o , of about 0.7030, 2.0 b.y. ago. There i s , however, abundant evidence of chemical inhomogeneity within the upper mantle (e.g. Sun and Hanson, 1975; Church and Tatsumoto, 1975). It i s possible that the nodules are samples of an upper mantle that i s inhomogeneous with respect to 8 7 Sr . If t h i s i s true the l i m i t e d data given i n Figure 12 may be of l o c a l s i g n i f i c a n c e only. Further accumulation of data of t h i s type, may provide a more r e l i a b l e estimate of the average mantle composition as a function of time. The Age of the Mantle Beneath B.C. If the observed i s o t o p i c d i s e q u i l i b r i u m of the mineral phases of the ultramafic nodule JL-10 i s not due to.contamination, then the ana-l y t i c a l data i n d i c a t e an age of approximately 2 b.y. More generally, the steep slopes of isochrons f or samples BM, JL-10 and JL-39 suggest that the upper mantle beneath c e n t r a l B.C. i s very o l d . V-9 Conclusions Strontium i s o t o p i c mineral d i s e q u i l i b r i u m between the clinopyro-xene, orthopyroxene and o l i v i n e phases of l h e r z o l i t e nodules from Boss Mountain, B.C. and Jacques Lake, B.C. has been demonstrated. The cause of t h i s d i s e q u i l i b r i u m i s unknown, although ..it may be. due 87 to contamination by Sr and Rb-rich f l u i d s within the source regions . of the nodules. Apparent isochron r e l a t i o n s h i p s , however, suggest the 87 dis e q u i l i b r i u m may be due to i n s i t u decay of Rb , and i f t h i s i s true, the indicated ages may represent "mantle events". The Rb and -113-Sr analyses showed poor reproducibility, underlining the d i f f i c u l t i e s involved in analysis of these phases which contain very small quantities of Rb and Sr. -114-CHAPTER V I SUMMARY AND CONCLUSIONS A n a l y s e s o f s p h a l e r i t e f rom P i n e P o i n t i n d i c a t e t h a t i t has a s u i t -a b l y l a r g e R b / S r r a t i o f o r age d e t e r m i n a t i o n . T h i s c o n c l u s i o n i s s u p p o r t e d by Rb a n a l y s e s o f s p h a l e r i t e f r o m t h e B l u e b e l l M i n e w h i c h show s p h a l e r i t e c o n t a i n s an o r d e r o f m a g n i t u d e more Rb t h a n a s s o c i a t e d g a l e n a o r p y r r h o t i t e . A n a l y s e s o f g a l e n a f rom P i n e P o i n t and B l u e b e l l i n d i c a t e t h a t i t i s u n s u i t -a b l e f o r age d e t e r m i n a t i o n due t o a l o w R b / S r r a t i o and e x t r e m e l y l o w c o n -c e n t r a t i o n s o f Rb and S r . P y r r h o t i t e f r o m t h e B l u e b e l l M i n e c o n t a i n s v e r y l i t t l e Rb and may have a R b / S r r a t i o t h a t i s i n a p p r o p r i a t e l y s m a l l f o r age d e t e r m i n a t i o n . P r e s q u ' i l e d o l o m i t e , s p a r r y d o l o m i t e , and c a l c i t e a s s o c i a t e d w i t h t h e 87 86 P i n e P o i n t m i n e r a l i z a t i o n e a c h have d i s t i n c t S r / S r r a t i o s . S p a r r y d o l o m i t e may have p r e c i p i t a t e d f rom t h e o r e f o r m i n g s o l u t i o n s and t h u s 87 86 87 86 may have a S r / S r r a t i o i n d i c a t i v e o f t h e i n i t i a l S r / S r r a t i o o f t h e o r e . B a s e d o n t h i s p r e m i s e , a t w o - p o i n t i s o c h r o n be tween s p h a l e r i t e and s p a r r y d o l o m i t e i n d i c a t e s an age o f 165- 60 m . y . f o r t h e P i n e P o i n t m i n e r a l i z a t i o n . -115-Sr i s o t o p i c d i s e q u i l i b r i u m e x i s t s between o l i v i n e , orthopyroxene and clinopyroxene i n the Boss Mountain and Jacques Lake l h e r z o l i t e nodules. Thus these nodules do not have a cognate r e l a t i o n s h i p with t h e i r host 87 86 87 86 b a s a l t s . The Rb /Sr and Sr /Sr r a t i o s of the mineral separates from the two Jacques Lake nodules define isochrons : which i n d i c a t e ages of 2.0- 0.1 and 4.7- 2.4 b.y. These ages may represent mantle events. I t i s possible, however, that the observed Sr i s o t o p i c disequilbrium i s due to s e l e c t i v e contamination of the o l i v i n e and orthopyroxene within the mantle. Many of the Rb and Sr analyses of samples containing le s s than 1 p.p.m. Rb and Sr were not reproducible. It i s l i k e l y that laboratory contamination was responsible. Extremely small and well c o n t r o l l e d blanks are necessary for analyses of these small quantities of Rb and Sr. -116-BIBLIOGRAPHY Allsopp, H.L., Nicolaysen, L.O., and Hahn-Weinheimer, P.1969. Rb/K r a t i o s and Sr i s o t o p i c composition of minerals i n ecologite and p e r i d o t i c rocks. Earth Planet S c i . L e t t . 5_, pp. 231-244. Anderson, G.M. 1973. The hydrothermal transport and deposition of galena and s p h a l e r i t e near 100 c. Econ. Geol. 68, pp. 480-492. Anderson, G.M. 1975. P r e c i p i t a t i o n of M i s s i s s i p p i Valley-type ores. Econ. Geol. 70, pp. 937-942. Armstrong, R.L. 1968. A model for the evolution of strontium and lead isotopes i n a dynamic earth. Rev. Geop. 6_, pp. 175-199. Armstrong, R.L. 1974. Proposal f o r the simultaneous adoption of new U, Th, Rb, and K decay constants f o r c a l c u l a t i o n of radiometric dates (Abstr.). Internat. Meeting for Geochronology, Cosmochronology and Isotope Geology, Abstract Volume, P a r i s , 1974. Athaide, D.J.A. 1975. A model for the evolution of the chemical systems of the earth's crust and mantle defined by radiogenic strontium d i s t r i b u t i o n , and the rubidium-strontium geochemistry of the Shulaps Range and other ultramafic bodies near southwestern B r i t i s h Columbia. Unpubl. M.Sc. th e s i s , U.B.C.,157 pages. Baadsgaard, H., Cambell, F.A., Cumming, G.L., Evans, T., Kanaswech, E., Krause, R.H., Robertson, D.K., and Folinsbee, R.E. 1966. Isotopic data from the C o r d i l l e r a and L i a r d Basin i n r e l a t i o n to the genesis of Pine Point Pb-Zn deposits. Proc. and Trans. Royal Soc. Canada, set 4, 4_, pp. 189. Baadsgaard, H., and VanBreeman, 0. 1970. Thermally induced migration of Rb and Sr i n adamellite. Eclogae Geol. Helv. JK3, pp. 31-44. Barrett, D.R. 1975. The genesis of kimberlite and associated rocks: strontium i s o t o p i c evidence. In: Physics and Chemistry of the Earth V o l . 9 (L.H. Ahrens, J.B. Dawson, A.R. Duncan, and A.J. Erlank Eds.) Pergamon Press, Oxford, pp. 637-654. Basu, R.A. and Murthy, V.R. 1976. Sr isotopes and trace elements i n sp i n e l l h e r z o l i t e xenoliths i n basalts, San Quentin, Baja, C a l i f o r n i a . (Abstr.) EOS, 57, pp. 355. Beales, F.W. 1975. P r e c i p i t a t i o n mechanisms for M i s s i s s i p p i Valley-type ore deposits. Econ. Geol. 70, pp. 943-948. -117-Beales, R.W. and Jackson, S.A. 1968. Pine Point - a s t r a t i g r a p h i c a l approach. Can. Mining Met. B u l l . 61, pp. 867-878. Belyea, H.R. and Norris, F.W. 1962. Middle Devonian and older Paleozoic formations of southern D i s t r i c t of Mackenzie and northern Alberta, Canada. G.S.C. Paper 62-15, 82 pages. Bence, A.E. 1966. 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Tectonics, reefs and s t r a t i f o r m lead-zinc deposits of the Pine Point area, Canada. Econ. Geol. Mon. 3_, pp. 59-70. Carter, N.L. and Ave'Lallement, H. 1970. Mineralogy and chemistry of the earth's upper mantle, based on the p a r t i a l fusion - p a r t i a l c r y s t a l l i z a t i o n model. Geol. Soc. Am. B u l l . 81, pp. 2021-2034. Catanzaro, E.J., Murphy, T.J., Garner, E.L., and Shields, W.R. 1969. Absolute i s o t o p i c abundance r a t i o and atomic weight of t e r r e s t r i a l rubidium. J . Res. Nat. Bur. Std. Physics and Chemistry, 73A, pp. 511-516. Chappell, B.W., Compston, W., Arriens, P.A., and Vernon, M.J. 1969. Rubidium and strontium determinations by x-ray fluoresence spec-trometry and isotope d i l u t i o n below the part per m i l l i o n l e v e l . Geochim. Cosmochim. Acta, 33, pp. 1002-1006. Church, S.E. and Tatsumoto, M. 1975. Lead isotope r e l a t i o n s i n oceanic ridge basalts from the Juan de Fuca - Gorda Ridge area, N.E. 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Ul t r a b a s i c nodules i n basalts and the upper mantle composition. Earth Planet. S c i . L e t t . ]_, pp. 330-332. Laughlin, A.W., Brookins, D.G., Kudo, A.M., and Casey, J.P. 1971. Chemical and strontium i s o t o p i c i n v e s t i g a t i o n s of ultramafic i n c l u s i o n s and basa l t , Bandera Crater, New Mexico. Geochim et Cosmochim. Acta, 35, pp. 107-113. Law, J . 1935. Geology of N.W. Alberta and adjacent areas. Am. Assoc. Pet. Geol. B u l l . 39, pp. 1927-1941. Lebedev, L.M., Baranova, N.N., and N i k i t i n a , I.B. 1971. On the forms of lead and zinc i n the Cheleken thermal brines. Geochem. Int. 8., pp. 511-516. Leggo, P.J. and Hutchinson, R. 1968. A Rb-Sr isotope study of u l t r a b a s i c xenoliths and t h e i r b a s a l t i c host rocks from the Massif Central, France. Earth Planet. S c i . L e t t . _5, pp. 71-75. L i t t l e j o h n , A.L. 1972. A comparative study of l h e r z o l i t e nodules i n basalt rocks from B r i t i s h Columbia. Unpubl. M.Sc. thesi s , U.B.C., 113 pages. Mark, R.K., Lee-Hu, C , and W e t h e r i l l , G.W. 1973. Rb-Sr studies of lunar breccias and s o i l s . Proc. Lunar S c i . Conf. 4th, pp. 1785-1795. Mattinson, J.M. 1970. Preparation of ultrapure HF, HCL, and HN03. Carnegie Inst. Washington Year Book, 70, pp. 266-267. -121-McConnell, R.G. 1889. Report on an exploration i n the Yukon and McKenzie Basins. G.S.C. Ann. Rept. 1888-1889, Part D, 163 pages. Mercier, J.C.C. and Nicolas, A. 1975. Textures and f a b r i c s of upper mantle p e r i d o t i t e s as i l l u s t r a t e d by xenoliths from b a s a l t s . Jour. Pet. 16, pp. 454-487. Norri s , A.W. 1965. Stratigraphy of Middle Devonian and older Paleozoic rocks of the Great Slave Lake region, N.W.T. Geol. Surv. Can. Mem. 322, 180 pages. Nicoloysen, L.O. 1961. Graphic i n t e r p r e t a t i o n of discordant age measurements on metamorphic rocks. Ann. N.Y. Acad. S c i . j)l_, pp. 198-206. Nyquist. L.E-, Bansal, B.M., and Weismann, H. 1975. Rb-Sr ages and i n i t i a l S r 8 7 / S r for Apollo 17 basalts and KREEP basalt 15386. Proc. Lunar S c i . Conf. 6th, pp. 1445-1465. Ohmoto, H. and Rye, R.O. 1970. The B l u e b e l l mine, B r i t i s h Columbia. I: mineralogy, parageneses, f l u i d i n c l u s i o n s and the isotopes of hydrogen, oxygen and carbon. Econ. Geol. 65_, pp. 417-437. O'Neil, J.R., Hedge, C.E., and Jackson, E.D. 1970. Isotopic i n v e s t i g a t i o n s of xenoliths and host basalts from the Honolulu volcanic s e r i e s . Earth Planet. S c i . L e t t . 8_. pp. 253-257. O'Nions, R.K. and Pankhurst, R.J. 1974. Petrogenetic s i g n i f i c a n c e of isotope and trace element v a r i a t i o n s i n volcanic rocks from the mid-Atlantic. J . Pet. 15, pp. 603-634. 87 Pankhurst, R.J. and O'Nions R.K. 1973. Determination of Rb/Sr and Sr / Sr 86 r a t i o s of some standard rocks and evaluation of x-ray fluoresence spectrometry i n Rb-Sr geochemistry. Chem. Geol. 12_, pp. 127-136. Papanastassiou, D.A. and Wasserburg, G.J. 1969. I n i t i a l strontium i s o t o p i c abundance and the r e s o l u t i o n of small time differences i n the formation of planetary objects. Earth Planet. S c i . L e t t . 5_, pp. 361-376. Papanastassigg, D.A. and Wasserburg, G.J. 1973. Rb-Sr ages and i n i t i a l Sr /Sr r a t i o s i n Apollo 15 basalts. Earth Planet. S c i . L e t t . 17, pp. 324-337. Paterson, D.M. 1975. A mineralographic i n v e s t i g a t i o n of Pine Point ores. Unpub. B.Sc.thesis,U.B.C., 135 pages. Paul, D.K. 1971. Strontium isotope studies on ultramafic i n c l u s i o n s from Dreiser Weiher, Germany. Cont. Min. and Pet. _34_, pp. 22-29. Peterman, Z.E., Carmichael, J.S.,and Smith, A.L. 1970. Strontium isotopes i n Quaternary basalts of south eastern C a l i f o r n i a . Earth Planet. S c i . L e t t . 1_, pp. 381-384. -122-Reeseman, R.H. 1968. The Rb-Sr analyses of some sulphide m i n e r a l i z a t i o n . Earth Planet. S c i . L e t t . 5_, pp. 23.26. 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Least-squares f i t t i n g of a s t r a i g h t l i n e . Can. J . Phys. 44_, pp. 1079-1086. APPENDIX I (LEAVES 124-135) NOT MICROFILMED FOR REASONS OF COPYRIGHT. Armstrong, R.L. Standard U.B.C. Procedures for Sample Dissolution and SR Mass Spectrometry. 1974. PLEASE CONTACT THE UNIVERSITY FOR FURTHER INFORMATION AT THE ADDRESS BELOW. Special Collections Division The Library University of British Columbia 2075 Wesbrook Place Vancouver, B.C., Canada V6T 1W5 -124-APPENDIX 1 STANDARD U.E.C. PROCEDURES FOR SAMPLE DISSOLUTION AND SR MASS SPECTROMETRY. (1) Author : R.L. Armstrong,1974 -125-I. Chemical Procedure. D i s s o l u t i o n Weigh out 0.10 to 0.20 gm of mineral separate or powder using the Mettler balance. Weighing can be crude for unspiked Sr i s o t o p i c compos-i t i o n run but should be precise f o r spiked runs — observe a l l pre-cautions such as rechecking exact weight and weight of weighing f o i l a f t e r loading sample i n beaker — compute r e s u l t by d i f f e r e n c e . Be sure to clean up balance area, to restore settings to zero, close doors, leave on a r r e s t , lock release knob, and put cover back on. Place weighed sample i n t e f l o n beaker or d i s s o l u t i o n bomb, record weight, sample ID, and beaker number. The beakers are usually processed i n groups of 10 and kept i n aluminum holders to f a c i l i t a t e handling. 1. Beaker set i s placed i n fume hood on warm hot plate ( s e t t i n g 4). HF i s added from p l a s t i c dropping b o t t l e i n s u f f i c i e n t amount to cover the beaker bottom to a depth of 4 mm. Caution: ground rocks and minerals may f i z z e n e r g e t i c a l l y — add acid slowly to empty side of t i l t e d beaker and slowly t i l t to s t a r t r e a c t i o n of acid with sample, i f you f i n d the reaction too v i o l e n t while pouring i n a c i d . Cover beaker with t e f l o n watchglass, allow to stew 20 min. 2. Add 1 f u l l dropper of HClO^ — c a r e f u l l y push cover to side with t e f l o n rod or metal tweezers, add HCIO^, s l i d e cover back. Allow to digest several hours or overnight. Caution: watch the HF - don't touch the watchglass with f i n g e r s , avoid s p i l l s , always wash hands a f t e r handl-ing any items i n the fume hood. 3. Remove covers, put i n s t a i n l e s s s t e e l tray, r i n s e each well with deionized water, put i n warm HN0„ wash. At a hot plate s e t t i n g 4 with 2 heat lamps evaporate to a c r y s t a l mush — white fumes indic a t e that v i r t u a l l y a l l HF has been removed. 4. Add more HF, enough to barely cover beaker bottom, and maybe a few drops HCIO^, evaporate again to c r y s t a l mush or dry (hot plate at 6 with 2 lamps). 5. Add 1 drop (or more i f i t ' s weak) of radioactive Sr. Cover beaker bottom with 2.5 N HC1. Evaporate to c r y s t a l mush or dry (hot plate at 6 with 2 lamps). 6. Remove from hot p l a t e , barely cover bottom of beaker with 2.5 N HC1 and add 2 drops HClO^. Cover with a clean p l e x i g l a s s sheet. Do not allow evaporation a f t e r t h i s point, as i t w i l l increase the acid concentration. The samples are now ready for ion exchange columns. If they s i t and cool overnight the KClO^ p r e c i p i t a t e w i l l become coarsely c r y s t a l l i n e and be easier to separate from the Sr-bearing l i q u i d . -126-Ion Exchange Prepare columns (AG 50W x 8 r e s i n , 200-400 mesh) for running: They may be l e f t backwashed, i n Q-water, undrained or i n 2.5 N HC1 ready to load a f t e r a 2-3 ml r i n s e with 2.5 N HC1. P u l l stoppers, wash o f f column ends with s q u i r t b o t t l e , i f s t i l l i n Q-water, allow water to drain completely (1.5 hours). Wash down r e s i n with =25 mil 27.5. N HC1 from burettes (-1.0 hour). Use s q u i r t b o t t l e of 2.5 N HC1 to r i n s e down any r e s i n on sides of r e s e r v o i r . The t r i c k i s to spin the columns while d i r e c t i n g a strong stream of acid down the in s i d e of the r e s e r v o i r . Several 2-3 ml washes may be needed. Columns are ready to load once a l l the acid has soaked i n . If you s t a r t at t h i s point the samples w i l l be through a couple of hours sooner than i f started a f t e r backwashing, and before Q-water i s drained. The cardboard l a b e l s are used to report the current status of the columns. Be sure they are correct for the next user. Do not allow the top of the r e s i n to dry out and shrink. While waiting for the columns to drain proceed with the next step. Pour each sample into a clean, numbered (use a water insoluble marker) 12 to 15 ml centrifuge tube — keeping track of where each sample i s . Do not put the beakers through the acid bath as they are used f o r sample c o l l e c t i o n . Be c a r e f u l to get the KC10, p r e c i p i t a t e i n the bottom of the tube — not s t i c k i n g up on the sides. T i l t , tap, wash down with 2.5 N HC1 i f necessary. (Try not to f i l l centrifuge tube more than ha l f f u l l . ) If samples are rushed to t h i s step a f t e r d i s s o l u t i o n i t i s advisable to cool them i n a 250 ml beaker of ic e water to encourage KCIO^ p r e c i p i t a t i o n . Centrifuge about 3 minutes on a se t t i n g of 5 or 6, or allow crud to s e t t l e . F i l l the burettes to t h e i r respective discard marks (established by a c a l i b r a t i o n run). Place loading tubes i n columns. Each sample i s poured from the centrifuge tube into the loading tube. Match the centrifuge tube and column numbers. As soon as pouring i s complete remove the loading rube — being u l t r a cautious not to touch the r e s e r v o i r wall with i t s ample-contaminated t i p . Return the tygon tubing to i t s place on the burette t i p , extending down into the open end of the r e s e r v o i r . If you do get a touch of sample solu t i o n on the re s e r v o i r i t w i l l have to be c a r e f u l l y washed down with small b i t s of acid from the burette. No sample should get into the bulb of the r e s e r v o i r at any time. Allow a l l of sample to soak into r e s i n before proceding to add any further a c i d . -127-Once the sample has soaked i n , the long process of adding acid begins. From t h i s point i t w i l l take about 6 hours before the Sr samples come o f f the columns. Extreme care i s e s s e n t i a l to avoid Rb contamination i n the Sr separate. At the beginning each burette should be f i l l e d to i t s pre-v i o u s l y c a l i b r a t e d discard value. Small amounts of acid are dripped onto the sides of the r e s e r v o i r while the columns are rotated — washing down the sample. About 2 or 3 ml are added each time, only enough to f i l l the column below the r e s e r v o i r . Then i t i s allowed to soak i n completely before the next aliquot of acid i s added. This i s repeated over and over again u n t i l the i r o n (yellow) i s observed to be coming into the c o l l e c t i n g beakers. The acid must be added i n such a manner as to wash down the walls of the r e s e r v o i r — a l l sample must be flushed onto the column. F a i l u r e to do t h i s properly w i l l r e s u l t i n g r i e f during the mass spectrometry. Remove markings from test tubes with acetone. The test and loading tubes are thoroughly rinsed and put i n the detergent bath. Cotton swabs may be used to dislodge the perchlorate residue i n the test tubes. The beakers are rinsed several times s t a r t i n g with deionized water (both in s i d e and out) then d i s t i l l e d water (insi d e only), then quartz water (insi d e only), If they come completely clean they are then set aside to be used for c o l l e c t i n g the f i r s t Sr a l i q u o t . If there i s any doubt about t h e i r c l e a n l i n e s s they go into the wash and a clean beaker i s set out for Sr aliqu o t c o l l e c t i o n . The operator may keep u s e f u l l y busy during column loading by going through a glass and teflon-ware washing cycle. (See "Dishwashing"). Once i r o n has come through the columns add a l l but about 30 ml of the remaining acid i n the burettes. Once t h i s has soaked i n , add the remainder of the acid to be discarded. A l l the acid w i l l pass through i n about 6 hours. Once a l l the discard acid has passed, the waste beakers are removed -being c a r e f u l not to touch the column t i p with discard s o l u t i o n — t h i s would destroy the chemical separation of Rb and Sr. If a touch does occur use a wash b o t t l e to clean o f f the column t i p . It i s a good idea to r i n s e o f f the column t i p s at t h i s point even i f no contamination i s suspected. The c o l l e c t i o n beakers are put i n place on top of the red 2 by 4 so the drips won't splash out. Be sure to check that the correct beaker i s being used for i t s designated sample. Add 10 ml acid from burettes, c o l l e c t i n beakers. Remove these beakers and put them on hot plate set at 4 with 2 lamps on. Add 1 or 2 drops of HCIO^. If you are at a l l unsure about the column c a l i b r a t i o n , c o l l e c t a second 10 ml aliq u o t and tr e a t likewise. Acid i n the waste beakers i s washed down the sink with large amounts of running water. The beakers are rinsed i n running water. Af t e r the second aliquot i s c o l l e c t e d the waste beakers are returned and the columns are flushed with 6N HC'l. Replace l u c i t e panel i n front of beakers. Remove the tygon tubing from the burette, r i n s e -128-insi d e and out with Q-water and store i n the l a b e l l e d p l a s t i c box. Keep the drip t i p clean — handle only the square-cut end and put i n box as noted on l a b e l s on box. Wash down the walls of the re s e r v o i r with 6N HC1 — aim stream of acid from s q u i r t b o t t l e about 2 mm below top of re s e r v o i r , s q u i r t strongly while r o t a t i n g column. F i l l r e s e r v o i r to about 3 mm above l e v e l of l u c i t e holder (=40 ml). Once t h i s acid has passed (1.5 hours) the columns are to be backwashed: Connect the tygon tube from the quartz water r e s e r v o i r to the bottom of the column. Add a small s q u i r t of 6N HC1 to the top of the column, open water stopcock and remove pinchcock. Once the clear water breaks to the surface i n the res e r v o i r close water stopcock. S t i r the r e s i n s l u r r y with a long p l a s t i c rod to break up lumps. Usually the column t i p s are plugged at t h i s point with rubber stoppers so that the r e s i n i s l e f t s i t t i n g i n quartz water, i - n t^ i e closed l u c i t e column holder. Close the water stopcock and acid stopcock. Wipe up s p i l l s , f l u s h discarded solutions down sink with l o t s of running water. Once the samples have evaporated down to large drops (or i f gone dry the Sr i s taken up i n a couple of drops of HC1) they are ready to transfer to planchettes. Set beakers i n an array i n front of hot pl a t e . Arrange l a b e l l e d p l a s t i c planchette boxes i n a corresponding array — these boxes may be reused i n d e f i n i t e l y , but should be well rinsed i n water before reuse. Set up another corresponding array of t e f l o n planchettes on the hot plate (set at 6 to high). Pour each sample from beaker onto appropriate planch-ette, r i n s e beaker with a couple of drops of HC1. Add a f i n a l drop of HC10,. Take to dryness, remove from hot plate to cool for a moment, store i n closed p l a s t i c boxes for mass spectrometry. Beakers go into the washing cycle. Planchettes are counted to determine which aliq u o t contains the most Sr. Usually that one i s the one run on the spectrometer. The second i s a reverse i n case the f i r s t i s contaminated or l o s t , but t h i s r a r e l y happens. Spiking A weighed quantity of spike s o l u t i o n i s added to the p r e c i s e l y weighed sample, either before or a f t e r the i n i t i a l d i s s o l u t i o n step. The spike i s drawn into a clean hypodermic needle plus syringe from the rubber — b a r r i e r b o t t l e , the syringe weighed, sample spiked and the syringe weighed again to determine the spike weight added. Subsequent procedure i s i d e n t i -c a l including measurement of Sr88, 87, 86 r a t i o s . An a d d i t i o n a l measure-ment of Sr86/Sr84 must be made using the expanded scale recorder - the pr e c i s i o n i s quite s u f f i c i e n t . The sample should be underspiked so that Sr86/Sr84 - 0.2 and the p r e c i s i o n of the Sr87/Sr86 r a t i o i s not s i g n i f i c a n t l y degraded. -129-Dishwashing (acid bath route) To begin, a l l l a b e l s should be removed with acetone and the item should be cleaned i n running tap water u n t i l no v i s i b l e d i r t remains. Use only finger or cotton swab to clean o f f t e f l o n , not a brush or scouring cleanser. Then soak several hours i n Alconox or Aparkleen bath. Rinse i n warm tap water to completely remove detergent. Rinse i n deionized water. Soak or r i n s e i n cold d i l u t e n i t r i c acid bath i n p l a s t i c dish and then allow to soak i n warm d i l u t e n i t r i c acid i n glass beaker for at l e a s t 20 min. The acid i s a 1:1 mixture of concentrated n i t r i c acid and deionized water. Remove cautiously (wear a lab coat?) from acid bath, using enamel, p l a s t i c , or s t a i n l e s s s t e e l transfer tray. Rinse i n deionized water, then b r i e f l y i n d i s t i l l e d water and f i n a l l y with a b i t of quartz d i s t i l l e d water. A i r dry or r i n s e with specpure Acetone and allow to dry. Store i n p l a s t i c boxes, or with parafilm covers, or inverted on a clean Kimwipe. In the case of used t e f l o n planchettes the old sample should be scrubbed out under running warm water with a cotton swab. Then the planch-ettes should be acid cleaned before going through the usual wash cycle. The i n i t i a l acid soak should be i n a small beaker at b o i l i n g temperature for at l e a s t % hour. The acid i s then discarded. When dealing with numerous small items i t helps to use the perforated p l a s t i c beaker to hold the items i n the acid bath. They can then be removed together by l i f t i n g out the beaker. Otherwise i t ' s a piece-by-piece job and rather tedious. Mass^Spec.tfometry-. 1"' * Loading Sample The s t a r t i n g point i s the end of a previous run with the following conditions confirmed: Hi v o l t s Standby, Filament Power Down or Off, A l l Valves Closed, Computer on Hold or Off, Teletype Off, Magnet scan and recorder on LOCAL Recorder chart and slidewire Off, VRE switch on 1 v o l t or higher - 1 3 0 -Start rotary pump. Close pyrex venting valve and when pump quiets down put l i q u i d nitrogen on pyrex cold trap. Open valve from Vacsorb to rotary pump, unless the pressure i s already low (500 jt) i n which case the valve can be l e f t closed. Meanwhile load the new sample on a clean filament assembly: Take a clean, baked-out double-filament block, loosen set screws on center filament and s l i d e i t back 2-3 mm. Lay on side on a clean surface and connect power leads to side filament posts. Assemble the glass pipette and syringe, r i n s e out the new pipette with Q-water, then take up 1 drop of water, transfer to planchette, s t i r to dissolv e sample, place a f r a c t i o n of the drop on the side filament. Dry at 1.5 amps (a higher current may be used to speed up evaporation but the current should be at 1.5 amps at the moment the filament dries out), gently increase current to obtain red-orange flow, hold f o r 10 s e c , return power to zero,remove c l i p s , recenter i o n i z i n g filament, tighten screws on both filaments. Keep loaded filament block i n a marked p l a s t i c box and/or place i t i n a convenient place near the spectrometer. Do not allow clean pipette t i p to touch f i n g e r s , table surface, or otherwise become contaminated. The used pipette can be placed with i t s t i p i n the planchette container i n a larger p l a s t i c box. A f t e r a successful run i t i s discarded. Open dry ^  tank (h turn i s enough). Open needle valve on regulator, dry N valve on spectrometer (check pressure reading - i t should f a l l ) , open metal venting valve below source can, use a small wrench to give more c o n t r o l , but do not force the valve. During venting the pressure on the N£ guage should drop to 1-2 lbs NO MORE. Once source i s vented, close the venting valve gently. Remove a l l source flange b o l t s except two opposite b o l t s adjacent to pins. Loosen b o l t s i n opposing p a i r s . Nearly remove the two b o l t s remaining - they are merely a caution to keep the flange from f a l l i n g o f f . C a r e f u l l y pry out flange, do not use any sharp t o o l s . As soon as i t i s free check to see that venting valve i s closed. Take out b o l t s , remove flange. I f you are lucky the gold gasket w i l l s t i c k . If not, take i t out c a r e f u l l y and t r y to re-locate i t on flange i f there i s s t i l l l i f e l e f t i n i t ( i . e . i f i t hasn't been torqued over 60 l b s . for the previous run). The small swelling on the gasket must match the depression i n the sealing surface - located near the t h i r d b o l t around clockwise from the top, looking at the spectrometer. Otherwise you w i l l not get more than two runs per gasket. A small dab of black wax (use a toothpick and carbon t e t r a c h l o r i d e to apply the wax and to clean of f any excess) helps keep the gasket stuck i n place. One gasket should give - i 3 i -dozens of runs and maybe hundreds. The gasket.material i s recycled so don't discard the used ones. Insert s t a i n l e s s s t e e l catch tray into source. P u l l o f f filament leads with long-nosed p l i e r s . Check to see that f i t t i n g s are t i g h t . If not, remove the leads, tighten the connections, replace the leads i n source. It i s important that a l l filament connections are s o l i d , otherwise they may cause a noisy run. Unscrew mounting nuts with s p e c i a l nut dr i v e r ( i f they get t i g h t retap the threads). Use p l i e r s or s p e c i a l threaded-end t o o l to remove filament block. Place used block — CAUTION: block i s hot — face down on bench, not on teletype as the hot block may melt the p l a s t i c . Pick up new block with p l i e r s , or screw i n block t o o l , i n s e r t into source, remove block t o o l i f used. There i s a marked box for used blocks. Screw down mounting nuts - firm but not overly t i g h t . Push filament leads back on. Be sure they are f i r m l y gripping the filament posts and that a l l connections are t i g h t . Remove tools and tray and return to large p l a s t i c box. Ca r e f u l l y s l i d e flange over pins - note l a b e l on top side. Push i n slowly u n t i l flange mates and no longer s l i d e s e a s i l y . Hold flange on u n t i l at le a s t one bolt i s i n . Start a l l b o l t s . Use bo l t s to p u l l by hand. Close DRYiLvalve:, close valve from Vacsorb to rotary pump, i f i t i s open, open VAC valve connecting source to rotary pump, open venting valve on source, close N tank needle valve and tank valve. This s t a r t s the pumpdown. At t h i s time put p l a s t i c foam bucket, f i l l e d \ with l i q u i d N2, on Vacsorb pump. Use small dewar to f i l l bucket nearly f u l l with l i q u i d Tighten b o l t s to = 10 to 20 l b s . below present torque s e t t i n g . Use a tightening pattern of opposing bol t s and cross-overs to get to the point where a l l are evenly t i g h t . Increase to present torque (35 l b s . for a fresh gasket, up to 90 l b s . for long-used gaskets) and tighten a l l b o l t s again. Close source venting valve hand t i g h t . Open toggle valve to Vacsorb pump. Top up l i q u i d N2 on Vacsorb pump. Check to see that valve from Vacsorb to rotary pump i s closed, close source VAC valve, turn o f f rotary pump, vent\ rotary pump l i n e , remove l i q u i d N£ from small trap. Leave venting valve open u n t i l trap i s warmed to room temperature. The pressure should be almost immediately i n the 0 to 10 micron range and f a l l i n g slowly, i f there are no leaks. Check for leaks with acetone. Large leaks w i l l cause a jump i n pressure. As long as leak p e r s i s t s keep increasing the torque on the b o l t s . A f t e r 120 l b s . give -132-up and s t a r t over with a new gold gasket. A bad leak may mean the gasket f e l l o f f the flange while being released. If pressure f a l l s to 1 micron, l e t pumping continue for 5 min. Optional s t a t i c leak check: Close Vacsorb toggle for 5 min., reopen. If system i s t i g h t the pressure guage should show no upward d e f l e c t i o n . If the sample i s to be run immediately following pumpdown, remove alcohol from cold finger and f i l l cold finger with l i q u i d ^ once pressure f a l l s below 1 micron. If no leaks, top up l i q u i d pump for 5 min. more. Then close toggle valve to Vacsorb pump and immediately open toggle valve to ion pump. If you want to leave the machine pumping down overnight or over a few hours, do not add l i q u i d to cold f i n g e r . Leave Vacsorb pump on source for at le a s t 15 min. before c l o s i n g toggle valve to Vacsorb and opening toggle valve to ion pump. ^ Pressure should r i s e momentarily then immediately f a l l into or below •" 10 range on source guage. Check for leaks with acetone once guage has s e t t l e d . If you s t i l l have a leak, tighten flange a b i t - check progress with acetone. Moisture i n the source may make i t seem as i f there i s a leak so be cautious, and patient to a degree. A r e a l l y large pressure surge w i l l cause the ion pump to switch o f f . The leak must be corrected and the pump evacuated by the Vacsorb pump before r e s t a r t i n g with the s t a r t / run switch i n s t a r t p o s i t i o n - u n t i l the pressure drops below the 10 scale - then switch back to run and the pumps are again protected against over-pressures. Clean up to o l s , remove l i q u i d ^ from Vacsorb pump - return l e f t o v e r s to large storage dewars but keep one small dewar f u l l f o r the cold finger i f you are s t a r t i n g a run. Check to see that N tank valve i s closed, venting valve open, rotary pump o f f . If the system i s OK i t can be l e f t pumping overnight or for days on end without attention or the run can be started immediately. Don't s t a r t unless you can be sure of finishing"! (allow about 4 hours). P e r i o d i c a l l y the Vacsorb pump should be baked out to remove accumul-ated water vapor and improve i t s performance. The bake-out jacket i s placed on the pump and i t i s switched on f o r about 4 hours. The rotary pump i s turned on, the venting valve closed, and the valve from Vacsorb to rotary pump l i n e opened. L i q u i d ^ i s put on glass small cold trap for the duration of pumping ( i t goes on any time the rotary pump i s on except for a few minutes at the beginning of each pumpdown). Aft e r bake-out i s complete the Vacsorb to pump l i n e valve i s closed, the pump switched o f f , vented, and the l i q u i d removed from the trap. If l i q u i d N2 i s l e f t on the trap l i q u i d oxygen w i l l accumulate i n i t and hinder the next pumpdown, and possibly cause an explosion of the venting valve i f i t becomes closed inadvertently. The sorption pump alone may be able to cycle the source 2 or 3 times from atmospheric pressure but things go much quicker i f i t i s helped by the rotary pump. If the source i s leaking the ion pump cannot handle the load - i t s capacity i s very l i m i t e d and i t s l i f e i n v e r s e l y r e l a t e d to the pressure at which i t works (nearly i n f i n i t e i n 10" range and below). - 1 3 3 -Running To accelerate pumpdown to running pressure the filament power may be get at 80 ma (sample) and 350 ma (ionizing). Pressure should be low 10~ in source,low 10 in tube, or better, before run begins. Empty alcohol from cold finger and f i l l with liquid ^ i f this has not already been done. From this point on the liquid N£ must be kept on - r e f i l l at least once an hour but i t i s best to top up every half hour or so. A f u l l finger w i l l last about one hour. Bring ion filament power to 320 ma (xl) and sample slowly up to 100 ma (x2). Prepare to find signal: Turn computer ON, turn teletype to LINE, turn on Heath power modules i f they are off. Press RESET, then start on Computer, with 000003^ (switches 14, 15 up, a l l others down) on front panel switches. Teletype w i l l print READY. Set magnet switches to 88 (four upper right switches on) - i t should already be there. Open beam valve (rotate over to l e f t - smoothly and firmly, do not force or f l i p carelessly). Watch the pressure gauges. Turn on high volts. Observe the meter to be sure behavior is normal. Switch VRE to 300 mv or leave at lv . Turn on servo motor (slidewire switch) and chart motor on recorder. Switch scale to REMOTE on recorder and magnet scan to REMOTE. You should have an 88 signal or at least a dying Rb at 85 and 87. Push peak offsets to see i f there has been some d r i f t i n magnet setting. Hop around to look for Rb (87 w i l l be about half the 85 signal intensity). Increase sample power towards - 120 ma slowly, looking for an 88 signal to appear and grow slowly. Once you find the 88 center the magnet oh i t , focus to increase intensity to the maximum you can achieve. Focusing: every sample requires some adjustment and a newly cleaned source may need quite a bit of fooling around. Once you have a perceptible signal, and on several occasions as intensity changes and run continues, adjust the focus controls to maximize intensity - try them a l l and go through several times to get the most intense signal. You may find that there i s a maximum of intensity at slightly lowered ion filament power (- 300-310 ma). Take advantage of i t i f possible. The less heat on the sample the longer i t w i l l last and the smoother the run. Try not to get over 130 ma on the sample filament - above that most samples burn off rapidly and the run i s lost. For the best runs the signal i s steady or rising slowly. It may take some adjustment of filament power (ion and sample) to get what you want. Dropping ion filament power may plow down too rapid an increase in intensity. -134-Some sample s a r e b o r n l o s e r s . W i t h l u c k t h e y c a n be pushed t o r u n n i n g i n t e n s i t y and u s e a b l e d a t a t a k e n - w i t h t h e s i g n a l f a l l i n g a l l t h e t i m e . When i n doub t i t i s somet imes a d v i s a b l e t o t a k e d a t a on t h e 100 o r 300 mV s c a l e b e f o r e p u s h i n g on t o h i g h e r i n t e n s i t y , and p o s s i b l e . t o t a l l o s s o f t h e r u n . Samples pushed t o o f a r may d i e r a p i d l y o r become n o i s y , m a k i n g d a t a c o l l e c t i o n i m p o s s i b l e . D u r i n g t h e b u i l d u p o f s i g n a l i n t e n s i t y c h e c k t h e 85 peak on o c c a s i o n -i t s h o u l d d i e away . I f i t p e r s i s t s y o u have a Rb c o n t a m i n a t i o n p r o b l e m . T h i s c a n a r i s e when t h e s o u r c e becomes d i r t y - t h e remedy i n t h a t c a s e i s t o p u t i n a c l e a n s o u r c e . O f t e n i t i s p e c u l i a r t o one l o n e s a m p l e . I t may be p o s s i b l e t o b u r n o f f t h e Rb and s t i l l have a u s e a b l e S r s i g n a l . T h i s c a n be t i m e c o n s u m i n g ; i t u s u a l l y w o r k s , b u t c o m p l e t e f a i l u r e t o g e t r i d o f Rb does o c c u r ( t h i s s h o u l d be v e r y r a r e ) . The s i t u a t i o n i s n o t s a t i s f a c t o r y u n t i l t h e 85 peak i s l e s s t h a n ' t h e 84 peak i n t e n s i t y , p r e -f e r a b l y by a f a c t o r o f 10 o r b e t t e r , b u t t h e e r r o r when 84 = 85 i s n o t u n a c c e p t a b l y l a r g e f o r most p u r p o s e s . Good c h e m i s t r y u s u a l l y r e s u l t s i n ; no Rb p r o b l e m s , s l o p p y c h e m i s t r y w i l l i n v a r i a b l y l e a d t o Rb c o n t a m i n a t i o n . When a s a t i s f a c t o r y i n t e n s i t y , r a t e , and d i r e c t i o n o f s i g n a l i n t e n s i t y change have been a c h i e v e d , b e g i n t a k i n g d a t a . Opt imum: 88 @ 0.70 on 1 v o l t s c a l e o f V R E , c l i m b i n g s l o w l y . I f f a l l i n g , s t a r t a t 0.9 on V R E . W r i t e on c h a r t and T e l e t y p e o u t p u t : Sample number , VRE s e t t i n g , f i l a m e n t c u r r e n t s , and s o u r c e and t u b e p r e s s u r e s i f u n u s u a l l y h i g h . L o g i n t o s p e c t r o m e t e r b o o k . D u r i n g a r u n u n d e r compute r c o n t r o l y o u may c h e c k c e n t e r i n g o f a l l peaks - 88 f i r s t , t h e n 87, t h e n 86. I f n o t w e l l c e n t e r e d a d j u s t 8-7 and 7-6 p o t s c h e c k hop t o 85 - go up t o 89 f rom 88, pause a few s e c o n d s , t h e n q u i c k l y jump t o 84 t h e n t o 85. I f y o u d o n ' t end up on 85 a d j u s t 5-6 p o t . R e c h e c k u n t i l t h e h o p p i n g i s s t a b l e and s a t i s f a c t o r y . T h i s may t a k e s e v e r a l c y c l e s t h r o u g h t h e p e a k s b u t i s a v e r y c r i t i c a l a d j u s t m e n t . To s t a r t d a t a c o l l e c t i o n by compu te r s e t magnet on 88, r e a s o n a b l y w e l l c e n t e r e d . P r e s s CONTINUE. Computer w i l l t u r n on s w i t c h e s f o r 88 so o p e r a t o r must now s w i t c h them o f f on t h e m a n u a l magnet c o n t r o l p a n e l . P r e s s CONTINUE. Computer w i l l s t a r t d a t a c o l l e c t i o n and a l l t h e o p e r a t o r has t o do i s w a t c h , t o p up l i q u i d N i n c o l d f i n g e r o c c a s i o n a l l y , and make a d j u s t m e n t s , i f n e e d e d , t o f o c u s , f i l a m e n t p o w e r , and magnet s e t t i n g . I f t h e f i r s t s e t i s OK t h e o p e r a t o r i s f r e e t o l e a v e t h e room f o r e x t e n d e d p e r i o d s o f t i m e . I f t h e r e i s s l i g h t d r i f t o f t h e magnet t h e compute r s h o u l d be a b l e t o r e c e n t e r t h e peak - j u s t w a t c h t h e f i r s t d a t a c y c l e t o be s u r e i t d o e s n ' t r u n i n t o t r o u b l e and t h a t t h e peak t o p s a r e s q u a r e , n o t d i s t o r t e d by magnet d r i f t down t h e s i d e o f t h e p e a k s . D a t a i s t a k e n i n t h e o r d e r : b a c k g r o u n d a t 89.5, Rb a t 85, t h e n one 88, 87, 86 c y c l e , p e a k c e n t e r i n g r o u t i n e , t h e n 10 88, 87, 86 d a t a c y c l e s e n d i n g up b a c k on 88, t h e n b a c k g r o u n d , 85, and b a c k t o 88 where i t s i t s w h i l e t y p i n g o u t r e s u l t s . The r a t i o s a r e c a l c u l a t e d and p r i n t e d b e f o r e s t a r t i n g p r i n t o u t o f r a t i o s t h e o p e r a t o r may make a d j u s t m e n t s t o f o c u s , p o w e r , o r peak c e n t e r i n g w i t h o u t a f f e c t i n g t h e compute r o p e r a t i o n . E v e r y s i x s e t s o f 10 p eaks a r e a v e r a g e d t o g e t h e r and -135-the r e s u l t p rinted. When a run s t a r t s the f i r s t two sets are not included i n t h i s average - the t h i r d set i s taken as the beginning of stable data. You have the f i r s t two sets on the printout i f you care to use them. The computer w i l l keep going through cycles of 6 sets of 10 peaks f o r hours on end (10 minutes per data block of 10 peaks, 1 hour for 6 s e t s ) . Usually 12 good sets i s a reasonable quantity of data for .0001 (a) p r e c i s i o n . If noisy data i s encountered and more than four of the s i x one second counts on each peak (the f i r s t f i v e seconds are always discarded) are rejected ( 1% deviation from mean) the teletype skips a l i n e and returns the carriage. Then the raw average i s printed. The screwed up format i s a warning of trouble and produces an audible change i n spectrometer behav-i o r . If the error (a) f o r r a t i o s i n any peak set exceeds .1% the i n d i v i d u a l r a t i o s are printed out for operator inspection, and s e l e c t i v e r e j e c t i o n of spurious r a t i o s . This printout i s surpressed i f the data i s OK. If the computer should f a i l to center the 88 peak (as might happen i f i t becomes completely l o s t or the magnet remote con t r o l switch has been l e f t switched o f f or i f fuse on magnet scan motor has blown) i t w i l l stop a f t e r 20 t r i e s and go to s i t on 88 and r i n g the teletype b e l l and p r i n t recentre peak. The operator must remedy the f a u l t and then press CONTINUE to return the operation to computer c o n t r o l . If a run i s bad and you want to f i d d l e with d i a l s and s t a r t over again the data c o l l e c t i o n may be stopped. Or you may decide the run i s f i n i s h e d and ready to shut down. To do t h i s press STOP, then RESET on the Computer. Switch the magnet con t r o l to 88 then press START and you are back to the beginning with computer switches o f f and under manual c o n t r o l . Then CONTINUE, magnet switches o f f , CONTINUE, w i l l return to computer con t r o l and data c o l l e c t i n g w i l l begin again. Never STOP during the printout of a number. This w i l l leave the state of the f l o a t i n g point i n t e r p r e t e r undefined and probably destroy commands i n computer memory and require felbadirig-the ^ -programs;; an-annoying iwaste of time.:r(20~min.) . At the end of run turn both filament powers down to zero, switch o f f slidewire and chart motors on recorder, VRE should be on 1 v o l t . Turn high v o l t s to standby, recorder and magnet scan to LOCAL. Turn o f f teletype. If shutdown i s only for hours leave computer and Heath nodules on and magnet set on 88. For longer shutdowns switch these o f f also. Close beam valve - slowly, c a r e f u l l y , but f i r m l y to i t s snapped over p o s i t i o n . Close ion pump toggle valve. Only with these valves closed, blow out cold finger and f i l l with alcohol, put on cover. -136-A P P E N D I X 2 R B S T A N D A R D P R E P A R A T I O N A N D R B S P I K E C A L I B R A T I O N P R O C E D U R E S A 1000 ml volumetric flask was calibrated by weighing the flask empty and then weighing the flask f i l l e d with d i s t i l l e d R ^ O . The weight by difference was 997.2 ± 0.1 gms. The density of moist air is given by: D = 1.2929(273.13/T){(B - 0.3783e)/760} where T = temperature (°k), B = barometric pressure (mm), and e = vapour pressure of moisture in air (mm). Although none of these factors was measured at the time of calibration, an assumed temperature of 20° C ± 2 C and an assumed barometric pressure corrected of.?a vapour pressure of 720 ± 50 mm gives the density of moist air as 1.1 ± 0.1 gms/liter. A buoyancy correction of 1.1 ± 0.1 grams/ l i t e r applied to the weight of H^O in the volumetric flask gives a weight of 998.3 ± 0.2 gms. A further uncertainty in the calibration was intro-duced by the thermal expansion of H o O . The calibration was carried out using d i s t i l l e d water that had equilibrated to room temperature. The Rb standard was prepared at a later date using Ultra I L j O that had also equili-brated to room temperature. ^Assuming a maximum variation in room temperature of 2 C and using 2.07 x 10 as the coefficient of thermal expansion of water, the uncertainty in measuring the volume of 1 l i t e r of H_0 is ± 0.4cc. Thus the weight of R ^ O in the calibrated volumetric flask i s 998.3 ± 0.4 grams. -137-The Rb standard was prepared i n the following manner: Rubidium chl o r i d e , N.B.S. standard SRM-984 was dried at 100° C for 12 hours. It was then weighed by di f f e r e n c e on a new piece of aluminum f o i l . The weighed amount was placed i n the c a l i b r a t e d volumetric f l a s k which was f i l l e d to the mark with U l t r a R^O. The standard was thoroughly mixed. The c a l i b r a t i o n of the spike was c a r r i e d out using the following procedure: (1) Weighed quantities of Rb standard and Rb spike were placed i n a clean t e f l o n beaker. In order to f a c i l i t a t e mixing, 25 mkQ~ of U l t r a 6 N HC1 was added, and the mixture was evaporated i i n a laminar flow hood. (2) The mixed RbCl was taken up i n a few drops of U l t r a 2.5 N HC1 and placed i n a t e f l o n planchette. (3) The i s o t o p i c composition of the mixture was determined by the standard procedure (Chapter I I ) . The spike concentration was determined by inverse isotope d i l u t i o n c a l c u l a t i o n s . -138-APPENDIX 3 "BASIC" PROGRAM FOR CALCULATION OF RB CONCENTRATIONS BY ISOTOPE DILUTION. 1REM A PROGRAM TC CALCULATE RB CONCENTRATIONS AND ERRORS IN RB 2REM CONCENTRATICNS FROM ISOTOPE DILUTION MEASUREMENTS. 3REM THE WEIGHT OF SAMPLE AND SPIKE IS ASSUMED KNOWN TO +- 0001" 4REM GMS. THIS ASSUMED UNCERTAINTY IS USED IN LINE 310. 10 PRINT "#SAMPLES" 20 INPUT I 30 DIM S<6) 40 PRINT "COMCN SPIKE IN MICROMOLES/GM,+OR-" 50 INPUT S(1),SC2) 60 PRINT "% 87 SPIKE* +-•• 70 INPUT S(3),S<4) 80 PRINT "X85 SPIKE,*-" 90 INPUT S<5),S<6> 100 PRINT "BLANK MICROMOLES RB" 110 INPUT Q 112 PRINT 114 PRINT 120 FOR Y=l TO I 125 PRINT "SAMPLE #" 126 INPUT U 130 PRINT "SPIKE WT" i 140 INPUT B 150 PRINT "SA WT" 160 INPUT C 170 PRINT "MEAS 85/87" 160 INPUT D 190 PRINT "RATIO ERROR" 200 INPUT J 210 LET A=<(<S<3}*D)-S(5})/{72.1654-{D*27.8346)))*(SC1)»3) 220 LET A1=(A-Q)/C 230 LET E=A!*85.48 240 LET F=A1*.278346 250 LET G=J/D 260 LET H=G*(D*S<3>> 270 LET Z=SQR<CH*H)+(S<6)t2>> 280 LET K=Z/<<D*S<3))-S<5)> 310 LET R1 = <((S(2)/S(1))t2> + <(.0001 /CJ12) + C«.0001/B)12) + (K*K) + CG»2i) 320 LET R2=SQR(R1) 330 LET A2«=A1*R2 340 LET £1=E*R2 350 LET Ft=F*R2 360 PRINT "SAMPLE ";U 370 PRINT "MICROMOLES P.B";A1 J"+-";A2 380 PRINT "P.P.M. RB";E;"+-••;El 390 PRINT "MICROMOLES RB 87";Fi-+-"iFl 392 PRINT 400 NEXT Y 410 END -139-APPENDIX 4 "BASIC" PROGRAM FOR CALCULATION OF  SR CONCENTRATIONS AND NORMALIZED  SR 8 7/SR 8 6 RATIOS FROM ISOTOPE DILUTION MEASUREMENTS. -140-I fEM A mOGPAM TO CALCULATE SR CONCENTRATIONS AMD NOPMAL17ED ,2 PEM SP 87/86 PATIOS fPOM PfiV SP 8 7 / 8 6 , 8 6 / 8 6 . AMD 84/86 OP 84/88 3 PEM PATIOS. £ DIM PC4),FC l f»>,SC2>.AC 10>,TC4> 7 PRINT "# SAMPLES" 9 IN PUT I ie r«INT "MICPOMOLES OF BLANK SR.+0R-" 1 I INPUT E,Z5 IS PRINT "PLANK 87 / 8 6 , + 0R-" 13 INPUT Y , Y 5 14 PRINT "SPIKE #" 15 INPUT N 16 PRINT 17 FOR H » I TO I 2 0 FPINT." SAMPLE #" 3 0 INPUT 0 40 FPINT "MEASURED 8 4 / 8 6 , 8 7 / 8 6 , 8 8 / 8 6 . 8 4 / 8 8 " 4 1 FHINT "INPUT 8 4 / 8 6 0 P 8 4 / 8 8 BUT NOT BOTH" 50 1NFUT PC 1>,PC2),RC3>,RC4> 52 FPINT "UNCERTAINTY I N 8 4 / 8 6 , 87/86, 88 / 8 6 , 84/88" 53 INPUT TC 1) , TC 2) » TC 3>, TC 4> 6e LET FC D=I 7 C LET L5= CG /450)* C C 1999*X)/450) Be LET L6=CG/515>*CC 1999*X)/515) 9 0 FPINT "GMS SPIKE" i e e INFUT M l i e FRINT "SAMPLE WTM 120 INPUT V 130 LET SC l ) - . e i13013 146 L E T SC2>=0 1S0 L E T StB>=0 iee LET F=SCN)*M 170 LET P1»P*.99968 iee LET c=9.3KeeeE-B4*Pi i9e LET c=2 .90eeeE-03*Pi 200 I F PC 13 = 0 GO TO 230 210 L E T F.1 = RC 1) 220 GO TO 240 230 LET F.1 = P.C4>*8.3752 240 FOR J=1 TO 10 250 LET ACJ> = CP1*C .996 18-C P. 1*9 . 300B0E- 04 J ) )/C C R l » . 1 0602) -6. 04000E-03) 260 LET A1 = ACJ)». 10602 270 LET E I=ACJ>* .88794 290 LET F.3=CE1 + D)/CA1*C) 3ee LET FCJ)=R3/RC3> 310 LET F2= F C J)-C5» C F C J)-1> 3 320 LET P.2=RC2>*F2 -330 »-ET K=J- 1 340 I F PC4>=0 GO TO 400 350 LET P.1=R3*RC4> 360 IF AESCFC«J>-FCK>X1.00BB0E-06 GO TO 400 370 NEXT «J 400 L E T E=F.2*CA1+C) 410 LET G=E-CP»3.20000E-04> 411 LET E1=C-.9723*Y>+10-55 412 LET E2=C9.17»Y)*.4897 413 LET A2= A l - C C 2 l * E)/IB0) 414 L E T C2=G-CCE2*E>/100> 420 L E T U2=G2/A2 421 LET~L=SCEC CC CTC 1>/RC 1> i*C 1/RC ] ) )>»2>+1.B0000E -08 ) 422 LET L1=L/CCI/RC1)>-C9.3B00BE-04)> 423 LET L2= CL 1 * 2>+C C TC 1 >/RC 1 > ) » 2) 424 L E T L3=L2+CC 1 .0Bt i lofcE-E4/ t i ) t2 ) + CC1 .0e0e0E-04 /V)t2) 425 LET L4=L3+CCS-CrCCJr-CS/SC 15) t23 \ LFT L;«=M>(L4> 436 LET 06=E/CACJ) + G*E) 440 LET A3=CCACJ)+G-E)*87.63J/W 445 LET A4=C CACJ)*G-E)/W) Hie PRINT "PPM SR 451 "PINT 452 PRINT "MICPOMOLES SR/G.M 455 FRINT "MICROMOLES SR86/GM 460 PRINT 461 PRINT 465 PRINT 466 PPINT 470 PPINT "87/86 NOLMALIZED-BLANK 471 FRINT 475 FPINT "I BLANK 476 FPINT 480 PPINT 481 PRINT ,482 FRINT 4e3 PFINT 487 NEXT H 490 END "87/B6 NORMALISED "86/88 MIX A3; "+OR-"J A3»L5 "J A4; "• OR-"; A4»L5 "IA2/V1"+0R-"; CA2/V)*L5 ";G/AII"*OR-";TC2> "1 U2 "J 06* 100 "J I/R3 READY -141-APPENDIX 5 PROCEDURE FOR CHEMICAL PREPARATION  AND PB ISOTOPIC DETERMINATION OF ORE LEADS. The following procedure i s that used by P. Shore i n determining the Pb i s o t o p i c composition of samples PP-11 and PP-41. I. Samples 1) Approximately 0.5 gms of sample i s placed i n a clean beaker on a hot plate and dissolved i n warm 6.2 N d i s t i l l e d HC1. 2) The beaker i s removed from the hot plate and allowed to cool, r e s u l t i n g i n the p r e c i p i t a t i o n of lead c h l o r i d e . 3) The supernate i s decanted and the lead c h l o r i d e i s taken up i n 40 mis. of 1.5 N HC1. The sample i s warmed to increase s o l u b i l i t y of the lead c h l o r i d e , then allowed to coo l . I I . Ion Exchange Columns 1) P r i o r to use the columns are 1.5 N HC1. 2) The 40 ml aliquot of sample r e s i n and allowed to completely soak flushed with d i s t i l l e d water and i s placed onto the ion exchange i n . -142-3) The columns are eluted under pressure with d i s t i l l e d water. 4) The 15-30 ml. aliqu o t i s c o l l e c t e d . I I I . Preparation f o r Running 1) The 15 ml. sample aliq u o t i s evaporated to dryness. 2) The chloride c r y s t a l s are dissolved i n 20 mis of HNO., then f i l t e r e d and evaporated to dryness. The weight of the lead chloride i s measured. 3) The lead chloride i s dissolved i n an appropriate quantity of 2% HNO^ to give a s o l u t i o n of-2 ug Pb per drop. IV. Mass Spectrometry One drop of the sample i s placed on a sing l e Re filament with s i l i c a gel and H PO and dri e d . The filament i s then loaded i n the 6" 90 mass spectromefZer^for i s o t o p i c measurements. 

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