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A comparative study of lherzolite nodules in basaltic rocks from British Columbia Littlejohn, Alastair Lewis 1972

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A COMPARATIVE STUDY OF LHERZOLITE NODULES IN BASALTIC ROCKS PROM BRITISH COLUMBIA by ALASTAIR LEWIS LITTLEJOHN B . S c , U n i v e r s i t y of Aberdeen, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In the Department of Geology if We accept t h i s t h e s i s as conforming to the req u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1972 In presenting t h i s thesis i n p a r t i a l f ulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by h i s representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. ALASTAIR L. LITTLEJOHN Department of GEOLOGY The University of B r i t i s h Columbia Vancouver 8, Canada Date <2)9th. Februry 1972 . i i ABSTRACT L h e r z o l i t e nodules i n b a s a l t i c rocks from three l o c a l i t i e s i n B r i t i s h Columbia i n c l u d e rocks of mantle o r i g i n and c r y s t a l cumulates. P a r t i a l chemical analyses show that the compositional ranges of the minerals are narrow f o r both major and minor elem- ents and f a l l w i t h i n the ranges reported f o r l h e r z o l i t e nodules elsewhere. Each s u i t e i s c h a r a c t e r i s e d by a d e f i n i t e range of concentrations of some elements. O l i v i n e i n nodules from C a s t l e Rock and Jacques Lake show f a b r i c s r e s u l t i n g from deformation i n the s o l i d s t a t e p r i o r to t h e i r i n c o r p o r a t i o n i n t o t h e i r host rocks but those from N i c o l a Lake are undeformed. The d i s t r i b u t i o n of i r o n and magnesium between c o e x i s t i n g phases i s examined using an i d e a l i o n i c s o l u t i o n model. D i f f e r e n c e s i n the d i s t r i b u t i o n c o e f f i c i e n t s between the s u i t e s are probably due to d i f f e r e n t temperature and pressure c o n d i t i o n s at the source of the nodules. The d i s t r i b u t i o n of i r o n and magnesium between c o e x i s t i n g s p i n e l and o l i v i n e gives nominal temperatures of formation of 838°C f o r N i c o l a Lake nodules, 1085°C f o r Jacques Lake nodules and> l 6 0 0°C f o r C a s t l e Rock nodules. D i f f e r e n c e s among the s u i t e s i n the d i s t r i b u t i o n , of N i , Co, Mn and Zn between c o e x i s t i n g s i l i c a t e s are independent of v a r i a t i o n s i n composition and are apparently due to d i f f e r e n t c o n d i t i o n s of formation. The C a s t l e Rock and Jacques Lake l h e r z o l i t e s are r e s i d u a l fragments of the upper mantle l e f t a f t e r e x t r a c t i o n of an under- sat u r a t e d b a s a l t i c l i q u i d from p a r e n t a l mantle rock. The source of the C a s t l e Rock nodules probably l i e s at g r e a t e r depth than that of the Jacques Lake nodules. The N i c o l a Lake nodules are c r y s t a l cumulates and formed at an e a r l y stage of b a s a l t genesis w i t h i n the upper mantle or lower c r u s t . TABLE OF CONTENTS . i l l CHAPTER 1 CHAPTER 2 CHAPTER 3 CHAPTER 4 CHAPTER 5 CHAPTER 6 CHAPTER 7 CHAPTER 8 CHAPTER 9 CHAPTER 10 BIBLIOGRAPHY APPENDIX 1 APPENDIX 2 Page I n t r o d u c t i o n . 1 The L o c a l i t i e s of Nodules i n B r i t i s h Columbia. 4 (a) Nodule l o c a l i t i e s . h (b) Petrography of the host rocks . 4 Petrography of the Nodules. 9 (a) General. 9 (b) Petrographic d e s c r i p t i o n s . 9 (c) Secondary t e x t u r a l f e a t u r e s . lh P e t r o f a b r i c Study. 20 (a) General. 20 (b) D e s c r i p t i o n of the f a b r i c s . 21 (c) D i s c u s s i o n . 26 M i n e r a l Compositions. 36 (a) General. 36 (b) O l i v i n e . 37 (c) Orthopyroxene. 37 (d) Clinopyroxene. 4o (e) S p i n e l . The D i s t r i b u t i o n of Iro n and Magnesium Between C o e x i s t i n g M i n e r a l s . 4-8 (a) Theory. 48 (b) R e s u l t s f o r c o e x i s t i n g s i l i c a t e s . 50 (c) The e f f e c t s of temperature and pressure. 52 (d) The d i s t r i b u t i o n between s p i n e l and o l i v i n e . 56 (e) R e s u l t s f o r c o e x i s t i n g s p i n e l and pyroxenes. 6 l The D i s t r i b u t i o n of Trace Elements Between C o e x i s t i n g S i l i c a t e s . 63 (a) -Theory. 63 (b) The d i s t r i b u t i o n of Ni.Mn.Co and Zn. 65 (c) Other elements. 73 The O r i g i n of the Nodules. 76 (a) Temperature and pressure. 76 (b) The nature of the source. 83 The Upper Mantle i n B r i t i s h Columbia. 9^ Conclusions. 98 100 A n a l y t i c a l Techniques. 107 E r r o r Propagation i n Temperature C a l c u l a t i o n s . 112 LIST OP TABLES . i v Table. Page 1 Modes of Nodules w i t h Analysed M i n e r a l s . 36 2 P a r t i a l Chemical Analyses of O l i v i n e s . 38 3 P a r t i a l Chemical Analyses of Orthopyroxenes. 39 4 P a r t i a l Chemical Analyses of Clinopyroxenes. 4 l 5 P a r t i a l Chemical Analyses of S p i n e l s . 45 6 Values of K-q f o r C o e x i s t i n g O l i v i n e and Pyroxenes wi t h A n a l y s i s of Variance. 51 7 Values of Kp f o r C o e x i s t i n g S p i n e l and O l i v i n e w i t h A n a l y s i s of Variance. 58 8 Temperatures of Formation of C o e x i s t i n g S p i n e l and O l i v i n e . 58 9 Values of K„ f o r C o e x i s t i n g S p i n e l and Pyroxenes w i t h A n a l y s i s of Variance. 62 10 (Tr/Cr) Ratios of Analysed M i n e r a l s . 67 11 Trace Element D i s t r i b u t i o n C o e f f i c i e n t s . 70 12 A n a l y s i s of Trace Element Variance between Jacques Lake and C a s t l e Rock S u i t e s . 71 13 Values of k™ and k c f o r C o e x i s t i n g Pyroxenes w i t h A n a l y s i s of Variance. 75 LIST OF FIGURES Figure - Page 1 L o c a l i t i e s of U l t r a m a f l c Nodules In B r i t i s h Columbia. 5 2 B a s a l t Coating around L h e r z o l i t e Nodule. 7 3 Modes of U l t r a m a f l c Nodules from B r i t i s h Columbia. 10 4 Aggregate of S p i n e l and Clinopyroxene. 12 5 Exsolved Clinopyroxene i n Orthopyroxene. 12 6 Exsolved S p i n e l i n Orthopyroxene. 13 7 Exsolved S p i n e l i n O l i v i n e . 13 8 M i n e r a l o g i c a l Banding i n C a s t l e Rock L h e r z o l i t e . 15 9 Alignment of Sheared S p i n e l Grains. 15 10 Reaction of Orthopyroxene at Margin of Host. 17 11 Porous-looking Outer Rim of Clinopyroxene. 17 12 F l u i d I n c l u s i o n s i n O l i v i n e . 19 13 O l i v i n e F a b r i c Diagram f o r JL-A. . . 2 3 14 O l i v i n e F a b r i c Diagram f o r J L - 5 0 . 23 15 O l i v i n e F a b r i c Diagram f o r JL-24. 24 16 S t r a i n e d O l i v i n e w i t h Abundant Kink Bands i n Jacques Lake L h e r z o l i t e . 25 17 O l i v i n e F a b r i c Diagram f o r NL-8. 24 18 Mutually I n t e r f e r i n g Grain Boundaries between O l i v i n e s and Orthopyroxene i n N i c o l a Lake L h e r z o l i t e . 25 19 O l i v i n e F a b r i c Diagram f o r S-2. 28 20 O l i v i n e F a b r i c Diagram f o r S-4. , 28 21 O l i v i n e F a b r i c Diagram f o r CRi8. 29 22 S t r a i n - f r e e , R e c r y s t a l l i s e d O l i v i n e i n C a s t l e Rock L h e r z o l i t e . 30 LIST OP FIGURES (continued) v i F i gure Page 23 E n s t a t l t e F a b r i c Diagram f o r CR -8. 29 24 V a r i a t i o n of Na 2 0 and T i 0 2 w i t h A l 2 0 o in. Clinopyroxene. 42 25 V a r i a t i o n of A l 2 0 o ,CaO and Na 2 0 w i t h MgO:FeO i n Clinopyroxene. 44 26 Composition of Analysed S p i n e l s . 4? 2? D i s t r i b u t i o n of I r o n and Magnesium between C o e x i s t i n g O l i v i n e and Orthopyroxene. 54 28 R e l a t i v e Proportions of Zn, Co, N l and Mn between C o e x i s t i n g O l i v i n e , Orthopyroxene and Clinopyroxene.68 29 R e l a t i v e Proportions of Pb and Cu between C o e x i s t i n g O l i v i n e , Orthopyroxene and Clinopyroxene.74 30 Composition of Analysed Pyroxenes i n terms of M g S i 0 3 - CaSi.03 - AI2O3. 78 31 V a r i a t i o n of AlgO^ between S p i n e l and Pyroxenes. 80 32 R e l a t i v e S t a b i l i t i e s of Various U l t r a m a f i c M i n e r a l Assemblages. 81 33 Various L h e r z o l i t e S o l i d i . 90 34 Part of the L i q u i d u s Diagram of the System Fo - Di - S i 0 2 at 20Kb. Pressure. 93 35 Major S t r u c t u r a l Features Related to Recent Volcanism i n B r i t i s h Columbia. 97 . v i i ACKNOWLEDGEMENTS I would l i k e to express my thanks and a p p r e c i a t i o n to Dr. H.J. Greenwood f o r suggesting t h i s t o p i c and s u p e r v i s i n g the work. Thanks are a l s o due to Dr. J . Souther who provided the C a s t l e Rock specimens, to Dr. K. F l e t c h e r who gave h i s advice f r e e l y on atomic a b s o r p t i o n techniques and to Mr. J . Harakal who d i d most of the probe work. The advice and help of Dr. R. D e l a v a u l t , Miss S. B a r r and Mr. A. D h i l l o n during the a n a l y t i c a l p o r t i o n of the study, and that of Mr. C, F l e t c h e r on the use of computers i s g r e a t l y a p p r e c i a t e d . The lo a n of a t y p e w r i t e r from Mr. G. C a r g l l l was i n v a l u a b l e . The r e c e i p t of an N.R.C. Post-graduate Scholar- ship d u r i n g the period of t h i s study i s g r e a t l y a p p r e c i a t e d . CHAPTER 1 I n t r o d u c t i o n .1 U l t r a m a f l c nodules occur i n b a s a l t i c rocks throughout the world (Forbes and Kuno 19651 1 9 6 ? ) . The most common type of nodule i s s p i n e l p e r i d o t i t e , c o n s i s t i n g of v a r i o u s proportions of o l i v i n e , d i o p s i d e , e r i s j a t l t e and s p i n e l . Garnet, hornblende, ph l o g o p i t e , a n o r t h i t e and more r a r e l y m e l i l i t e and l e u c i t e a l s o occur as primary phases i n u l t r a m a f l c nodules (Green 1 9 6 8 ) . Gabbroic and g r a n u l i t i c rocks d e r i v e d from the subvolcanlc basement o f t e n occur i n a s s o c i a t i o n with the u l t r a m a f l c types. Gabbro appears to be the commonest type of nodule found i n b a s a l t s of a l l types, but u l t r a m a f l c nodules u s u a l l y occur i n b a s a l t i c rocks of a l k a l i c a f f i n i t i e s - (White 1 9 6 6 ; Forbes and Kuno 1 9 6 7 ) . Only r a r e l y are they found i n t h o l e i i t i c b a s a l t s . Despite t h e i r r e l a t i v e s c a r c i t y , u l t r a m a f l c nodules are important since they may y i e l d i n f o r m a t i o n on the nature of the upper mantle and on the genesis of b a s a l t s . Ross e t . a l . (1954) f i r s t drew a t t e n t i o n to the r e l a t i o n s h i p between u l t r a m a f l c nodules, dunites and the upper mantle. They found that u l t r a m a f l c . nodules - jaround^ the world have a uniform mineralogy and'chemistry and suggested that the nodules were de r i v e d from a uniform mantle p e r i d o t i t e . However, White ( 1 9 6 6 ) , Jackson ( 1968) and Kuno ( 1969) s t u d i e d s u i t e s of nodules from Hawaii and found that they could be grouped acco r d i n g to mineralogy, chemistry and the c h a r a c t e r of the host rock. These authors have d i v i d e d Hawaiian nodules i n t o f o u r groups. These are (a) a l h e r z o l i t e s e r i e s which forms part of the .2 upper mantle; (b) a d u n i t e - w e h r l i t e - p y r o x e n i t e s e r i e s which forms c r y s t a l cumulates i n the lower parts of the Hawaiian magma r e s e r v o i r s ; (c) an e c l o g i t e s e r i e s which forms pockets i n the dominantly l h e r z o l i t i c mantle; (d) a gabbro s e r i e s which forms part of the c r u s t . Studies by Yamaguchi ( 1 9 6 4 ) , Kuno ( 1 9 6 7 ) , Aoki (1968) and I s h l b a s h i (1970) have shown that there i s a l s o a wide v a r i e t y of nodule types i n Japan. No s i n g l e hypothesis seems adequate to e x p l a i n the o r i g i n of a l l u l t r a m a f i c nodules. Several hypotheses have been advanced to e x p l a i n the o r i g i n of these r o c k s , tnSLuding^(a) they are fragments of the mantle; (b) they are c r y s t a l l i n e r e sidues of p a r t i a l l y melted mantle; (c) they are products of c r y s t a l s e t t l i n g , formed during ascent of t h e i r b a s a l t i c h o s t s ; (d) they are products of c r y s t a l s e t t l i n g , formed a t some e a r l y stage of b a s a l t formation, not n e c e s s a r i l y the present host rock; (e) they are.fragments of some e a r l i e r formed u l t r a m a f i c body i n the c r u s t . That any one' of the above hypotheses holds f o r a l l nodules i s not g e n e r a l l y accepted. However there i s s t i l l controversy over the o r i g i n of any p a r t i c u l a r type of nodule. L h e r z o l i t e , the commonest u l t r a m a f i c v a r i e t y , i s of p a r t i c u l a r i n t e r e s t as i t i s analogous to " p y r o l i t e " , a h y p o t h e t i c a l mantle rock (Ringwood 1 9 6 6 , 1 9 6 9 ) . White ( 1966) considers l h e r z o l i t e nodules to be residue from f u s i o n of the p r i m i t i v e mantle. Jackson (1968) considers l h e r z o l i t e to be part of a heterogeneous mantle and dunlte to be the r e f r a c t o r y r e s i d u e . C a r t e r (1970) suggests that most l h e r z o l i t e nodules are r e f r a c t o r y residue from the upper mantle but that those r e l a t i v e l y r i c h i n i r o n . 3 are cumulates. Kuno and Aoki (1970) have found a wide v a r i e t y of l h e r z o l i t e compositions throughout the world and suggest that t h i s i s a r e s u l t of d i f f e r e n t degrees of p a r t i a l m elting i n the mantle which i s i t s e l f composed of l h e r z o l i t e w i t h a r e l a t i v e l y low Mg«Fe r a t i o . O'Hara ( 1 9 6 3 , 1 9 6 7 . 1968) and Brothers ( i 9 6 0 ) support the hypothesis that l h e r z o l i t e and other nodules are cognate. An a c c i d e n t a l o r i g i n i s considered, i n ge n e r a l , to be u n l i k e l y , although Instances are known where u l t r a m a f l c nodules, i n c l u d i n g l h e r z o l l t e s , are thought to be x e n o l l t h s of sub-volcanic s t r a t i f o r m complexes (Fuster e t . a l . 1 9 6 9 ) . In order to examine some of the problems o u t l i n e d , p a r t i c u l a r l y w i t h regard to l h e r z o l i t e , s u i t e s of nodules from B r i t i s h Columbia have been c o l l e c t e d and s t u d i e d by the w r i t e r . The o b j e c t i v e s were twofoldj (a) to des c r i b e the occurance and types of nodules which have been found i n B r i t i s h Columbia; and (b) to determine the d i f f e r e n c e s and s i m i l a r i t i e s between s u i t e s and r e l a t e these i f p o s s i b l e to the source of the nodules. The nodules and host rocks were stud i e d by standard petrographic techniques. P e t r o f a b r i c s t u d i e s on the o l i v i n e and e n s t a t i t e of the nodules were c a r r i e d out using a 4-axes u n i v e r s a l stage. The minerals of the nodules were separated and analysed by means of atomic-absorption spectrophotometry, e l e c t r o n microprobe and by wet-chemical means. CHAPTER 2 The L o c a l i t i e s of Nodules In B r i t i s h Columbia. (a) Nodule l o c a l i t i e s . The l o c a l i t i e s of u l t r a m a f l c nodules which have been documented i n the l i t e r a t u r e i n B r i t i s h Columbia are shown on P i g . 1. Of these, s u i t e s from C a s t l e Rock, Jacques Lake and N i c o l a Lake were stu d i e d . The host rocks were not studi e d i n d e t a i l . (b) Petrography of the host rocks. (1) C a s t l e Rock. C a s t l e Rock i s i s a small peak on the northern f l a n k of the K l a s t l l n e P l a t e a u , kO miles east of Telegraph Creek i n Northern B r i t i s h Columbia. I t i s one of s e v e r a l Quaternary v o l c a n i c centers which occur i n the region (Souther 1 9 7 ° )• Rounded nodules from*2 to 6 inches i n diameter are found i n a v o l c a n i c b r e c c i a , made up of sub-rounded fragments of a black f i n e - g r a i n e d a l k a l i b a s a l t i n a matrix of palagonite. The contact of the nodules and b r e c c i a i s sharp; i n some specimens there I s a t h i n f i l m of b a s a l t c o a t i n g the nodule. This f i l m , p e t r o g r a p h i c a l l y i d e n t i c a l to the b a s a l t fragments, c o n s i s t s of small l a t h s of p l a g i o c l a s e (An£0) set i n a matrix of g l a s s . o l i v i n e , magnetite and minor clinopyroxene. The f i l m appears to have prevented the ." ; S:w- d i s i n t e g r a t i o n of the nodules during the e x p l o s i v e e x t r u s i o n of the b r e c c i a . The b r e c c i a a l s o contains small x e n o l l t h s of d i o r i t e ; presumably these are fragments of an und e r l y i n g i n t r u s i v e body. ( i i ) Jacques Lake. The nodules a t the Jacques Lake l o c a l i t y are found i n a 1 Castle Rock 2 Jacques Lake 3 Haggen's Point 4 Boss Mountain 5 Nicola Lake 0 (00 200 300 miles small d i s s e c t e d cone 4 miles south of Quesnel Lake i n C e n t r a l B r i t i s h Columbia. The host rock i s a coarse t u f f e x h i b i t i n g crude l a y e r i n g i n places. The t u f f i s made up of rock fragments of v a r i o u s types and s i z e s , cemented by a brownish-green matrix c o n s i s t i n g of small rock fragments and p a r t l y d e v i t r i f l e d g l a s s . The rock fragments c o n s i s t of sedimentary, p l u t o n i c , metamorphic and v o l c a n i c x e n o l i t h s which are.presumably r e p r e s e n t a t i v e of the c r u s t beneath Jacques Lake. No s i n g l e type i s predominant and the s i z e and frequency of each k i n d are h i g h l y v a r i a b l e . U l t r a m a f i c nodules, mainly l h e r z o l i t e , are found i n f r e q u e n t l y throughout:*: the cone. -Many of the-nodules are coated w i t h a t h i n f i l m of b a s a l t c o n s i s t i n g of sm a l l l a t h s of a l t e r e d p l a g i o c l a s e i n a f i n e - grained matrix of glass.magnetite and o l i v i n e ( F i g . 2 ) . This c o a t i n g appears to be the o r i g i n a l b a s a l t w i t h i n which the nodules were suspended before the e x t r u s i o n of the t u f f and has served a s i m i l a r purpose to the c o a t i n g around the C a s t l e Rock nodules. The nodules are g e n e r a l l y rounded or sub-rounded and range from 1 to 15 inches i n diameter. Most are l e s s than 6 1" inches i n diameter. The cone i s Quaternary i n age and appears to be s i m i l a r ' t o s e v e r a l other cones and flows of a l k a l i b a s a l t which occur i n the area (Cambell 1 9 6 1 ) . ( i i i ) N i c o l a Lake. • U l t r a m a f i c nodules are found i n s c a t t e r e d boulders 3 miles south of N i c o l a Lake i n C e n t r a l B r i t i s h Columbia. The host rock i s a, dark grey, f i n e - g r a i n e d , v e s i c u l a r b a s a l t . I t c o n s i s t s F i g . 2 B a s a l t c o a t i n g around l h e r z o l i t e nodule i n Jacques Lake t u f f . . 8 of small l a t h s of unzoned p l a g l o c l a s e (AngQ.), rounded o l i v i n e g r a i n s , i n t e r s t i t i a l g l a s s and minor magnetite. Often partly- corroded xenocrysts of o l i v i n e and pyroxene are found. The b a s a l t i s a l k a l i n e i n c h a r a c t e r . The age of the b a s a l t i s unknown but i s p o s s i b l y T e r t i a r y . The boulders appear to have been brought to t h e i r present p o s i t i o n by g l a c i a l a c t i o n from the north where there i s a l a r g e volume of T e r t i a r y b a s a l t . (K. C. McTaggart pers. comm.). . 9 CHAPTER 3 Petrography of the nodules, (a) General, A l l the nodules studied c o n s i s t of v a r i o u s proportions of o l i v i n e , clinopyroxene, ..orthopyroxene and s p i n e l . O l i v i n e i s the dominant phase; orthopyroxene g e n e r a l l y exceeds clinopyroxene; s p i n e l i s a minor phase. The modes of 29 nodules from the three l o c a l i t i e s are shown on a t e r t i a r y diagram ( F i g . 3)» F°r t h i s purpose s p i n e l i s omitted but a l l nodules c o n t a i n from 0 . 5 to 2% (by volume) s p i n e l . Modes were c a l c u l a t e d by point-counting from 30 to 300 g r a i n s i n t h i n s e c t i o n , depending on the s i z e of the nodule. Some of the r e s u l t s f o r the smaller nodules may not be accurate ( p a r t i c u l a r l y those from N i c o l a Lake),but are i n c l u d e d f o r comparison. Modes of other nodules from B r i t i s h Columbia are shown f o r comparison. The data are taken from S o r e g a r o l i ( 196?) and Tredger ( 1969) f o r Boss Mountain and Haggen»s Point > r e s p e c t i v e l y . . A l l the nodules s t u d i e d are l h e r z o l i t e s . The nomenclature i s i n accordance w i t h the c l a s s i f i c a t i o n of Jackson ( 1 9 6 8 ) . According to White (1966) and Kuno and Aoki (1970) l h e r z o l i t e i s the predominant u l t r a m a f i c rock found as nodules at other l o c a l i t i e s around the world. (b) Petrography of the nodules. A l l the nodules from each s u i t e have a l l o t r l o m o r p h i c g r a n u l a r t e x t u r e s , although there are some d i f f e r e n c e s between the s u i t e s . The C a s t l e Rock and N i c o l a Lake nodules are medium- Dunite Orthopyroxenite Fig. 3 Modal composition of ultramafic nodules from British Columbia. • Jacques Lake O Boss Mountain a Castle Rock © Haggen's Point S Nicola Lake .11 g r a i n e d , whereas the Jacques Lake ones are coarse-grained and are very f r i a b l e . O l i v i n e forms an i n t e r l o c k i n g mosaic of rounded g r a i n s ranging i n s i z e from 0 . 5 to 2 . 0 0 mm. i n diameter. In many of the N i c o l a Lake specimens some g r a i n s have mutually I n t e r f e r i n g boundaries. In some of the Jacques Lake specimens o l i v i n e may be up t o 5*00 mm. i n diameter. In a l l s u i t e s i t i s pale green. Orthopyroxene forms subhedral g r a i n s some of which are l a r g e r - than the o l i v i n e and p a r t l y enclose i t and others that are s m a l l , anhedral and are i n t e r s t i t i a l to the o l i v i n e . I t - i s dark brown i n hand specimen and c o l o u r l e s s i n t h i n s e c t i o n . Clinopyroxene forms small anhedral g r a i n s , g e n e r a l l y hot more.than 0 . 5 mm. i n diameter, i n t e r s t i t i a l to both o l i v i n e and orthopyroxene. I t tends to occur as aggregates of three or four g r a i n s . I t i s a b r i g h t emerald green i n hand specimen and c o l o u r l e s s or pale green i n t h i n s e c t i o n . S p i n e l occurs as i r r e g u l a r l y shaped g r a i n s i n t e r s t i t i a l to and p a r t l y e n c l o s i n g the s i l i c a t e s , p a r t i c u l a r l y the pyroxenes ( F i g . 4 ) . I t i s black i n hand specimen and reddish-brown i n t h i n s e c t i o n . E x s o l u t i o n l a m e l l a e are found i n some of the s i l i c a t e s . Lamellae of clinopyroxene In orthopyroxene ( p a r a l l e l to ^100} of the host) are found only In the l a r g e s t orthopyroxenes. They are t h i n and pinch out towards the boundary of the host ( F i g . 5 ) . Both pyroxenes and very r a r e l y o l i v i n e c o n t a i n t h i n l a m e l l a e of a brown i s o t r o p i c mineral which i s presumed to be s p i n e l ( F i g s . 6 , 7 ) . In the pyroxenes the lamellae are s u b - p a r a a i e l F i g . 4 Aggregate of s p i n e l (brown) and clinopyroxene ( g r a i n s w i t h cleavage); g r a i n s w i t h no cleavage showing are o l i v i n e ; specimen S - 2 . X - 3 0 ; plane p o l a r i s e d l i g h t . P i g . 5 E x s o l u t i o n of clinopyroxene i n orthopyroxene ( d a r k ) ; specimen J L - 5 0 * X - 3 O ; crossed n i c o l s . . 13 F i g . 6 Exsolved s p i n e l (brown) i n orthopyroxene ( g r a i n s w i t h cleavage); specimen S - 3 . X - 3 0 ; plane polaeised l i g h t . F i g . 7 S p i n e l (brown lamellae) exsolved i n o l i v i n e ( y e l l o w ) ; specimen J L - 5 0 . X - 3 0 ; crossed n i c o l s . .14 to {001^ and In the o l i v i n e they are p a r a l l e l to the (010) cleavage. Many of the nodules show signs of deformation such as kink bands i n the o l i v i n e s . A d i s c u s s i o n of deformation f o l l o w s i n Chapter 4 . Some specimens from C a s t l e Rock are l a y e r e d . Specimen s-4 has two w e l l - d e f i n e d bands, each about 5 nun. t h i c k , of clinopyroxene which are separated by a t h i c k e r band of o l i v i n e ( F i g . 8 ) . In other nodules there i s an i l l - d e f i n e d l a y e r i n g marked by t h i n disseminated s t r i n g e r s of s p i n e l ( F i g . 9) or orthopyroxene. Other nodules are massive. Specimens from Jacques Lake do not i n general show l a y e r i n g . R a r e l y , banding s i m i l a r to that observed i n S-4 above was seen i n the f i e l d . Unfortunately these specimens could not be broken out of the host t u f f . None of the N i c o l a Lake nodules show l a y e r i n g but t h e i r small s i z e might make t h i s d i f f i c u l t to see. (c) Secondary t e x t u r a l f e a t u r e s . Most of the nodules from a l l three s u i t e s show evidence of d i s e q u i l i b r i u m between the e n c l o s i n g rock and the primary m i n e r a l s . U s u a l l y r e a c t i o n has taken place a t the margin of the nodule', although i n some specimens ( p a r t i c u l a r l y from Jacques Lake) b a s a l t has been able to permeate the whole rock so that the m inerals of the i n t e r i o r have been a f f e c t e d . O l i v i n e shows the l e a s t e f f e c t s of r e a c t i o n w i t h the host. Where In contact w i t h b a s a l t , the margin of the o l i v i n e g r a i n s commonly shows a f i n e - g r a i n e d rim of secondary o l i v i n e and magnetite. . 15 F i g . 8 M i n e r a l o g i c a l banding i n specimen S-4 from C a s t l e Rock. The b r i g h t green bands are d i o p s i d e ; l i g h t e r green i s o l i v i n e . * ** i " . - J . - t ' j 7. F i g . 9 Sheared s p i n e l (brown) c u t t i n g through s i l i c a t e s ; specimen S-2 from C a s t l e Rock. . 16 S p i n e l g r a i n s i n contact w i t h the host have a dark margin, presumed to magnetite. Orthopyroxene, where i n contact w i t h the host rock, has a rim of rim of f i n e l y d i v i d e d m a t e r i a l of high b i r e f r i n g e n c e set i n a dark c r y p t o c r y s t a l l i n e matrix ( F i g . 1 0 ) . This appears to be a r e s u l t of lncongruent melting of orthopyroxene which has produced o l i v i n e and g l a s s . The clinopyroxene of some nodules i n contact w i t h the host rock has a t h i n rim of secondary clinopyroxene. In the Jacques Lake nodules and:; r a r e l y i n those from C a s t l e Rock, the c l i n o - pyroxenes have a porous-looking outer zone which may be up to a t h i r d of the diameter of the g r a i n i n width ( F i g . 1 1 ) . The e n t i r e g r a i n extinguishes uniformly and has uniform b i r e f r i n g e n c e . The spongy zone i s r i d d l e d w i t h an extremely f i n e - g r a i n e d dark m a t e r i a l which appears to be p a r t i a l l y d e v i t r i f l e d g l a s s . This rim i s e n t i r e l y d i f f e r e n t from the rims observed a t the margins of the nodules which are of secondary pyroxene w i t h d i f f e r e n t e x t i n c t i o n from the parent. According to White ( 1966) s i m i l a r features are a r e s u l t of a d e p l e t i o n i n the j a d e i t e component of the pyroxene thuss j a d e i t e - b e a r i n g d i o p s i d e —> j a d e i t e - p o o r dlopside + f e l d s p a r The r e a c t i o n rims occur throughout the rock and f o r reasons given i n Chapter 8, they are considered here to be a r e s u l t of p a r t i a l m e l t i n g . A l l the r e a c t i o n phenomena des c r i b e d here have been reported elsewhere ( W i l s h l r e and Binns 1 9 6 1j Talbot e t . a l . 1 9 6 3 ; .17 F i g . 10 Reaction of orthopyroxene at margin of host. The h i g h l y b i r e f r i n g e n t m a t e r i a l i s secondary o l i v i n e ; specimen S - 4 . X - 1 0 0 ; crossed n i c o l s . Fig. 1 1 Porous-looking outer rim of clinopyroxene (on r i g h t ) . The orthopyroxene (on l e f t ) i s u n a f f e c t e d . The mineral with no cleavage showing i s o l i v i n e ; specimen JL-24. X - 3 0 ; plane p o l a r i s e d l i g h t . .18 Yamaguchi 1964; White 1966; K u t o l i n and P r o l o v a 1 9 7 0 ) . The r e a c t i o n t e x t u r e s i n d i c a t e d i s e q u i l i b r i u m between nodules and magma at a high c r u s t a l l e v e l but do not preclude e q u i l i b r i u m at g r e a t e r depth. F l u i d i n c l u s i o n s were found i n o l i v i n e s i n most nodules from a l l the s u i t e s . Irf some cases they were a l s o observed i n pyroxenes. Many of these are two phase (gas + l i q u i d ) i n c l u s i o n s . Roedder (1965) has found two phase f l u i d i n c l u s i o n s i n o l i v i n e s from many l o c a l i t i e s . Most of these are C 0 2 ; H 20 i s found i n some nodules. The composition of the i n c l u s i o n s i n the B.C. nodules i s unknown. Most of the i n c l u s i o n s are found along f r a c t u r e s ( F i g . 12) and along the cleavage of the o l i v i n e . In some cases planes of i n c l u s i o n s cut across f r a c t u r e s ( F i g . 1 2 ) . They are most abundant where the b a s a l t has penetrated the nodule. They thus appear to be secondary and may have formed a f t e r the nodules were captured by t h e i r h osts. Some I n c l u s i o n s may be primary, \ but d i s t i n g u i s h i n g these from secondary ones i s u n c e r t a i n . F i g . 12 F l u i d I n c l u s i o n s a l i g n e d along f r a c t u r e s i n o l i v i n e ; specimen J L - 5 0 . X-400. .20 CHAPTER 4 P e t r o f a b r l c Study. (a) General. Previous work on u l t r a m a f i c nodules has revealed that the o l i v i n e i n many nodules has a prefered o r i e n t a t i o n (Turner 1942} Brothers 1959,I960j Talbot e t . a l . 1963s C o l l e e 1963; Black and Brothers 1 9 6 5 ; Brothers and Rodgers 1 9 6 9 ) . These authors have shown that there i s a v a r i e t y of o l i v i n e f a b r i c s i n nodules and have attempted to draw analogies between the o l i v i n e o r i e n t a t i o n patterns found i n nodules and those found i n o l l v i n e - b e a r i n g rocks from other environments. Thus Brothers (I960) compared o l i v i n e f a b r i c s i n nodules to those i n flow- banded t r o c t o l i t e s and b a s i c dykes and suggested the nodules formed by c r y s t a l s e t t l i n g i n a moving magma. Other workers consider that the f a b r i c s of o l i v i n e from nodules are s i m i l a r to those of rocks which have been deformed. In order to determine whether o l i v i n e s from nodules i n B r i t i s h Columbia have a prefered o r i e n t a t i o n , and i f so, whether such an o r i e n t a t i o n could be r e l a t e d to the source of the nodules, f a b r i c diagrams f o r o l i v i n e of three nodules from C a s t l e Rock, three from Jacques Lake and one from N i c o l a Lake were prepared. An e n s t a t i t e f a b r i c diagram of one of the C a s t l e Rock nodules was a l s o prepared. The o r i e n t a t i o n of the o l i v i n e and e n s t a t i t e p r i n c i p a l o p t i c d i r e c t i o n s was measured on a 4-axes u n i v e r s a l stage. Two of these were determined i n t h i s way, the t h i r d being found by c o n s t r u c t i o n . The diagrams were constructed on the lower .21 hemisphere of a Schmidt net. Measurements were made on every grain intersected on suitably spaced lines of traverse in order to minimise sampling errors, (b) Description of the fabrics. Figs. 13, 14 and 15 are the fabric diagrams for the Jacques Lake nodules. A l l three nodules have similar olivine fabrics, although there are some differences between each one. The main element of the fabrics i s the three mutually perpendicular maxima. In specimen JL-24 (Fig. 15) and JL-A (Fig. 13) this i s modified. In these fabrics (3 forms a weak partial girdle. In addition JL-24 (Fig. 15) has a fabric in which forms a well- developed girdle while s t i l l retaining the strong maximum. Kink bands in the olivines are common, particularly i n the larger grains (Fig. 16). The grain boundaries tend to be straight •and to have t r i p l e grain boundary-angles of 120°. The olivine from the Nicola Lake nodule has a very weak prefered orientation in which the three principal optic directions are mutually perpendicular. Each maximum i s ill-defined and the fabric i s essentially random (Fig. 17). Kink bands in these olivines are also f a i r l y common but polygonisation i s not a textural feature of the olivines of this nodule. The texture of this nodule, and also others of this suite, i s typical of cumulate rocks in which the grain boundaries are mutually interfering (Fig. 18). Figs. 19, 20 and 21 are the fabric diagrams for the olivines of the Castle Rock nodules. The main feature of these i s that b' forms a strong maximum perpendicular to a girdle. °< and (3 form less prominent maxima within the girdle. Specimen Explanation of F i g s . 1 3 , 14, 15 and 1? O l i v i n e f a b r i c diagram of specimen JL-A from Jacques Lake. 50 g r a i n s . Contours at 2 , 4 , 6 , 8$ of 1$ area. . Maximum concentrations are 8,8 ,10$ f o r , /3, 0 r e s p e c t i v e l y . Mode: o l . 7 7 . opx. 1 1 , cpx. 1 1 , s p i n . 1 . F i g . 14 O l i v i n e f a b r i c diagram of specimen J L - 5 0 from Jacques Lake. 50 g r a i n s . Contours a t 2, 4, 6 , 8$ of 1$ area. Maximum' concentrations are 6 , 10, 10$ f o r ©<; [3 / 5 r e s p e c t i v e l y . Mode* o l . 6 3 , opx. 19. cpx. 17, s p i n . 1. F i g * "1? O l i v i n e f a b r i c diagram of specimen JL-24 from Jacques Lake. 50 g r a i n s . Contours a t 2V 4,':6,^8,1,10$' of 1$ area. Maximum concentrations are 14, 8, 10$ f o r <* } /3 , X r e s p e c t i v e l y . Modes o l . 9 0 , opx. 2, cpx. 7» s p i n . 1. P i g * 17 O l i v i n e f a b r i c diagram of specimen NL-8 from N i c o l a Lake. 50 g r a i n s . Contours at 2, 4 , 6 , , , 8 $ of 1$ area. Maximum concentrations are 8 , 8 , 10$ f o r ot f3} ^ r e s p e c t i v e l y . Modei o l . 8 3 , opx. 7» cpx. 9» s p i n . 1. ' ' Fig. 13 Fig. 14  F i g . 16 St r a i n e d o l i v i n e with abundant kink bands; specimen J L - 5 0 from Jacques Lake. X - 3 0 j crossed n l c o l s . F i g . 18 Mutually i n t e r f e r i n g g r a i n boundaries between o l i v i n e and orthopyroxene (grey and y e l l o w ) ; specimen NL-8 from N i c o l a Lake. X - 3 0 ; crossed n i c o l s . ,26 S-4 i s l a y e r e d and has been described p r e v i o u s l y . The o r i e n t a t i o n of the l a y e r i n g i s shown i n P i g . 2 0 . The plane of the l a y e r i n g and the ^3 - <* g i r d l e c o i n c i d e and X i s perpendicular to the l a y e r i n g . Many of the o l i v i n e s of the C a s t l e Rock nodules have ki n k bands. These are found only i n the l a r g e r g r a i n s . The s m a l l e r ones are s t r a i n - f r e e and form a mosaic w i t h s t r a i g h t g r a i n boundaries meeting at 120°, ( F i g . 2 2 ) . In a d d i t i o n t o the o l i v i n e , an e n s t a t i t e f a b r i c diagram f o r sample CR-8 was constructed ( F i g . 2 3 ) . The e n s t a t i t e i n t h i s nodule has a poor prefered o r i e n t a t i o n . There i s a suggestion that y i s perpendicular t o an i l l - d e f i n e d ^ " °< g i r d l e , but more data are r e q u i r e d to confirm t h i s . Comparison w i t h the o l i v i n e f a b r i c from the same rock ( F i g . 21) shows that there i s apparently no correspondence between the o l i v i n e and the e n s t a t i t e f a b r i c s . This i n i n c o n t r a s t to the f i n d i n g s of C o l l e e (1963) and Rodgers and Brothers ( 1 9 6 9 ) . (c) D i s c u s s i o n . According to Rodgers and Brothers (1969) there are f i v e o r i e n t a t i o n r u l e s f o r o l i v i n e i n u l t r a m a f l c nodules. These are t (1) o< maximum perpendicular to a f3 ~Y g i r d l e . (2) X maximum perpendicular to a ~°< g i r d l e . (3) V and f3 maxima perpendicular to a cx g i r d l e . ( 4 ) ft , ) °* mutually perpendicular. (5) No obvious o r i e n t a t i o n . T r a n s i t i o n a l f a b r i c s a l s o occur. The f a b r i c s which are found i n l h e r z o l i t e nodules are (1) and ( 4 ) . This study has shown that .27 E x p l a i n a t i o n of F i g s . 19. 2 0 , 21 and 2 3 . F i g * 19 O l i v i n e f a b r i c diagram of specimen S-2 from C a s t l e Rock. 100 g r a i n s . Contours a t 1, 2, 4 , 6 , 8, 10$ of 1$ area. Maximum concentrations are 6 , 7, 12$ f o r <=< , /3 } a r e s p e c t i v e l y . Modej o l . 77. opx. 1 6 , cpx. 6 , s p i n . 1 . F i g . 20 O l i v i n e f a b r i c diagram f o r specimen S -4 from C a s t l e Rock. 50 g r a i n s . Contours at 2, 4 , 6 , 8, 12$ of 1$ area. Maximum concentrations are 6 , 12, 18$ f o r ^ / ^ , 0 r e s p e c t i v e l y . Mode: o l . 72, opx. 15. cpx. 12, s p i n . 1. The great c i r c l e i s the plane of the l a y e r i n g . F i g . 21 O l i v i n e f a b r i c diagram f o r specimen CR-8 from C a s t l e Rock. 50 g r a i n s . Contours at 2 , 4 , 6 , 8$ of 1$ area. Maximum concent r a t i o n are 8,8 ,10$ f o r °<, fi\ 0 r e s p e c t i v e l y . Mode: o l . 6 3 , opx. 2 3 , cpx. 1 2 , s p i n . 2 . F i g - 23 E n s t a t i t e f a b r i c diagram f o r specimen CR-8 from C a s t l e Rock. 50 g r a i n s . Contours a t 2 , 4 , 6 , 8$ of 1$ area. Maximum concentrations are 6 , 6 , 8$ f o r <=><} ft > 0 r e s p e c t i v e l y . Mode: o l . 6 3 , opx. 2 3 , cpx. 1 2 , s p i n . 2 . Fig. 19 Fig. 20 ..: o29 Fig. 21 Fig. 23 . 3 0 .31 o l i v i n e i n l h e r z o l i t e nodules may a l s o f o l l o w r u l e ( 2 ) . This type of f a b r i c has only been found p r e v i o u s l y (apart from p e r i d o t i t e s ) i n dunite and h a r z b u r g i t e nodules (Rodgers and Brothers 1969) and i n an i s o l a t e d " o l i v i n e nodule" (Talbot e t . a l . 1 9 6 3 ) . The mode of t h i s nodule was not reported. R a l e i g h ( 1968) and C a r t e r and Ave»Lallement (1970) have found that deformation of o l i v i n e a t high temperature r e s u l t s i n g l i d i n g on-^Okl^ . The a x i s of e x t e r n a l r o t a t i o n i s [lOO^ . The o r i e n t a t i o n of the g l i d e plane i s dependant on temperature and s t r a i n r a t e . At high temperature and low s t r a i n r a t e the g l i d e plane i s ( 0 1 0 ) . This i s the plane on which s l i p occurs i n most n a t u r a l l y deformed o l i v i n e s , r e s u l t i n g i n the formation of k i n k bands. P o l y g o n l s a t i o n and r e c r y s t a l l ! s a t l o n r e s u l t i n the disappearance of the k i n k bands. Kink bands i n o l i v i n e are not i n themselves evidence of deformation i n the s o l i d s t a t e . B rothers (1962) has found that these bands are present i n o l i v i n e s i n some b a s a l t s and gabbros which are undeformed. They may r e s u l t from flow i n the magma from which the o l i v i n e c r y s t a l l i s e d . Both k i n k banding and p o l y g o n l s a t i o n are common i n the o l i v i n e s of the Jacques Lake and C a s t l e Rock nodules ( F i g s . 16 and 2 2 ) . This suggests that these nodules have been deformed and have r e c r y s t a l l l s e d ( R a l e i g h 1 9 6 8 ; Ragan 1 9 6 9 ; C a r t e r and Ave'Lallement 1 9 7 0 ) . The N i c o l a Lake nodules , on the other hand, are apparently undeformed si n c e evidence of r e c r y s t a l l i s a - t i o n i s absent. Evidence from the o l i v i n e f a b r i c s supports the above suggestions. The t y p i c a l f a b r i c s developed by o l i v i n e formed from a magma are types (4) and (5) and l e s s commonly type ( 1 ) of the above .32 c l a s s i f i c a t i o n (Brothers 1959» 1962, 1 9 6 4). The N i c o l a Lake o l i v i n e f a b r i c i s t r a n s i t i o n a l between types (4) and (5) and could t h e r e f o r e have r e s u l t e d from the o l i v i n e having c r y s t a l l i s e d from a magma. There i s a range of f a b r i c s shown by the Jacques Lake nodules. The f a b r i c f o r J L - 5 0 ( F i g . 15) corresponds to type ( 4 ) . JL-A ( F i g . 13) i s a modified type (4) f a b r i c i n which {3 forms a weak g i r d l e . S l i p on the system _|"okl|, [looj may e x p l a i n the v a r i a t i o n s i n the f a b r i c s of t h i s s u i t e . I f the i n i t i a l f a b r i c of the source rock was a simple pattern ( p o s s i b l y produced by c r y s t a l s e t t l i n g or perhaps by deformation) represented by J L - 5 0 ( F i g . 1 4 ), then deformation would r e s u l t In the maxima f o r « Co/oJand =• being r o t a t e d about L'0olwhich i s the zone a x i s f o r g l i d e . This would r e s u l t i n the formation of g i r d l e s i n <=< and (3 ; X would remain as a maximum. Specimen JL - 2 4 ( F i g . 15) shows t h i s to forms a weaker g i r d l e . Perhaps t r a n s l a t i o n as w e l l as r o t a t i o n same plane as ex . An exact geometrical a n a l y s i s of these f a b r i c s i s not p o s s i b l e since a sample of 50 g r a i n s i s i n s u f f i c i e n t to determine p r e c i s e angular r e l a t i o n s h i p s . F u r t h e r complications are Introduced by the f a c t that the o r i e n t a t i o n of r e c r y s t a l l l s e d o l i v i n e g r a i n s i s , , i n p a r t , c o n t r o l l e d by the I n i t i a l f a b r i c (Ave»Lallement and C a r t e r 1 9 7 0 ) . Despite the s p e c u l a t i v e nature of the above d i s c u s s i o n , the range of f a b r i c s (which are u n l i k e those produced s o l e l y by c r y s t a l accumulation), coupled w i t h evidence of r e c r y s t a l l i s a t i o n given above suggests that the Jacques Lake nodules have been some extent where forms a g i r d l e perpendicular to was i n v o l v e d i s not i n the . 3 3 deformed. R e c r y s t a l l i s a t i o n i s not complete as some of the l a r g e r o l i v i n e s are s t i l l s t r a i n e d ( P i g . 1 6 ) . This could e x p l a i n why there i s a range of f a b r i c s of the o l i v i n e s of t h i s s u i t e . A s i n g l e f a b r i c type has been found i n the C a s t l e Rock nodules. This i s type (2) of the above c l a s s i f i c a t i o n ( g i r d l e ) . F a b r i c s developed by o l i v i n e formed from a magma have been reported above. As f a r as could be determined from the l i t e r a t u r e the f a b r i c s of these o l i v i n e s have no analogues among o l i v i n e s from igneous environments. This type of f a b r i c has been found i n deformed A l p i n e p e r i d o t i t e s and other types of u l t r a m a f i c nodules (Ave*Lallement and Ca r t e r 1 9 7 0 ; Talbot e t , a l , 1 9 6 3 ; Rodgers and Brothers 1 9 6 9 ). More commonly the t y p i c a l o l i v i n e f a b r i c of a s t r o n g l y deformed p e r i d o t l t e i s one where oCX g i r d l e (type (1) of the above c l a s s i f i c a t i o n ) . The nodules from C a s t l e Rock are l a y e r e d . This has been desc r i b e d i n Chapter 3* M i n e r a l o g i c a l l a y e r i n g i n a rock can a r i s e i n a t l e a s t two ways. These a r e i (a) by accumulation of minerals i n a magma, and (b) by metamorphism and deformation or metasomatism. Both these processes may r e s u l t i n a prefered o r i e n t a t i o n of some mineral ( i n t h i s case„;. o l i v i n e ) . To produce a f a b r i c by c r y s t a l accumulation r e q u i r e s a dimensional o r i e n t a t i o n of o l i v i n e . No such o r i e n t a t i o n was found i n the C a s t l e Rock o l i v i n e s . This can be explained by : suggesting that intercumulus growth took place which masked the o r i g i n a l shape of the g r a i n s (Jackson 1 9 6 1 ) . However t h i s would s t i l l produce a f a b r i c which i s t y p i c a l of cumulate rocks. This i s not the case f o r the C a s t l e Rock nodules. . 3 4 Metamorphic banding i n p e r i d o t i t e s i s a common feature (Thayer 1969? Loney e t . a l . 1 9 7 0 ) . The banding takes the form of monomineralic v e i n s and dykes and discontinuous s t r i n g e r s of mine r a l s . Both types are found i n the C a s t l e Rock nodules ( F i g s . 8 and 9 ) . The form of the s p i n e l i n specimen S-2 i n which i t appears to be f l a t t e n e d and to cut through the s i l i c a t e s ( F i g . 9) suggests that there may have been an element of shear i n the formation of the l a y e r i n g . Loney e t . a l . ( 1970) have found that m i n e r a l o g i c a l l a y e r i n g i s s u b - p a r a l l e l or p a r a l l e l to an a x i a l plane f o l i a t i o n i n a deformed p e r i d o t i t e . The a x i a l plane i s perpen d i c u l a r to an maximum and p a r a l l e l t o a (Z-X g i r d l e . I t i s p o s s i b l e that the f a b r i c of these nodules i s r e l a t e d to 1 a s i m i l a r deformational f e a t u r e although i n t h i s case the l a y e r i n g contains a (I g i r d l e and X forms a maximum a t r i g h t angles to i t . T h i s type of f a b r i c I s c h a r a c t e r i s t i c of some deformed p e r i d o t i t e s (Ave»Lallement and C a r t e r 1 9 7 0 ) . I t I s thus suggested t h a t the C a s t l e Rock and Jacques Lake nodules have been deformed. The b a s i s of t h i s i s a comparison w i t h the f a b r i c s of deformed p e r i d o t i t e s and magmatlc o l i v i n e s and by the text u r e s of the o l i v i n e s i n these nodules. Deformation must have occured p r i o r to the nodules having been captured by the magmas which brought them to the surfa c e . The d i f f e r e n t f a b r i c types of the two s u i t e s suggests that the source rocks of the s u i t e s underwent d i f f e r e n t deformational h i s t o r i e s . F a c t o r s which i n f l u e n c e the type of f a b r i c which i s developed by the o l i v i n e of a p e r i d o t i t e are the o r i e n t a t i o n of the p r i n c i p a l s t r e s s e s w i t h respect to the o r i g i n a l f a b r i c , . 3 5 temperature, s t r a i n r a t e and the presence or absence of water (Ave'Lallement and Carter 1 9 7 0 ) . Pressure has apparently l i t t l e i n f l u e n c e on the f a b r i c type although Ave»Lallement and C a r t e r (1970) imply (p.2214) that very high pressure i s r e q u i r e d to - produce a f a b r i c s i m i l a r to that found i n the C a s t l e Rock nodules. This f a b r i c type has.not been reproduced experimentally so that the c o n d i t i o n s under which i t formed are unknown. Experimental r e c r y s t a l l i s a t i o n of o l i v i n e takes place a t temperatures above 900°C (Ave»Lallement and C a r t e r 1970) so that the deformation of these nodules probably occured a t high temperature. . 3 6 CHAPTER 5 M i n e r a l Compositions. (a) I n t r o d u c t i o n . P a r t i a l m i n e r a l analyses presented i n t h i s study were c a r r i e d out by means of atomic a b s o r p t i o n spectrophotometry, e l e c t r o n microprobe and by wet chemical means. D e t a i l s of the a n a l y t i c a l methods and the p r e c i s i o n of the r e s u l t s are given i n Appendix 1. Only the f o u r primary minerals ( o l i v i n e , orthopyroxene, clinopyroxene, s p i n e l ) were analysed. The r e s u l t s of t h i s study r e a f f i r m the f i n d i n g s of many workers t h a t the minerals of u l t r a m a f i c nodules ( p a r t i c u l a r l y l h e r z o l i t e ) have a r e s t r i c t e d compositional range. The compositional range of these minerals i s s i m i l a r i n both major and minor elements to the ranges of other published analyses. This study a l s o shows that the compositional range of the minerals i n any p a r t i c u l a r s u i t e of nodules i s c h a r a c t e r i s t i c of t h a t s u i t e . The analysed minerals were chosen from nodules which had as wide a range of modal compositions as p o s s i b l e so as to i n c l u d e a l l p o s s i b l e v a r i a t i o n s . TABLE 1 j Modes of nodules w i t h analysed m i n e r a l s . Sample L o c a l i t y 0 1 . Cpx. Opx. Spin. JL-A Jacques Lake 77 11 11 1 JL-39 it 71 16 12 1 J L - 1 0 ii 62 12 24 2 JL-B •i 68 22 8 2 J L - 5 5 ii 80 8 10 2 95 C a s t l e Rock 74 6 19 1 s-3 ii 83 5 11 1 CR-8- •i 63 12 23 2 ERC-11 H 66 17 15 2 NL-8 N i c o l a Lake 83 9 7 1 .37 (b) O l i v i n e . P a r t i a l chemical analyses of o l i v i n e s are given i n Table 2. Of the four primary m i n e r a l s , o l i v i n e has the most r e s t r i c t e d range. I t ranges from F o g o ^ to FOCJI.Q * n t n e C a s t l e Rock and Jacques Lake nodules. The o l i v i n e of the N i c o l a Lake nodule has a composition of F o ^ 1 # g . The range i s s i m i l a r to t h a t found elsewhere (e.g. White 1966). There i s no s i g n i f i c a n t d i f f e r e n c e i n the range of major elements i n o l i v i n e among the s u i t e s . The range of values f o r N i , Mn, Co, Cu and Zn and a l s o CaO and Na£0 i s s i m i l a r to the range found elsewhere (Ross e t . a l . 1954; Forbes and Banno 1 9 6 6 ; Simkin and Smith 1970; C a r t e r 1 9 7 0 ) . The Pb content of the o l i v i n e s i s v a r i a b l e and appears to be high f o r u l t r a m a f l c r o c k s . Goles (1967) suggests 0 . 5 p p m . as an average f o r u l t r a m a f l c rocks as a whole. While no data are a v a i l a b l e on the Pb content of o l i v i n e s from u l t r a m a f l c r o c ks, t h i s f i g u r e suggests that the o l i v i n e s of these nodules are enriched i n Pb (Table 2) r e l a t i v e to the o l i v i n e s of other u l t r a m a f l c r o c k s . (b)Ort'hopyroxenes. P a r t i a l analyses of orthopyroxenes are given i n Table 3 . The Jacques Lake orthopyroxenes range i n composition from Engo^zj, to E n 9 0 . 1 a n <* t n°se from C a s t l e Rock range from En^o .5 t o E n 9 i . 9 « The N i c o l a Lake orthopyroxene'has a composition of E n g ^ ^ . The C a s t l e Rock samples are s l i g h t l y more magnesian than those of Jacques Lake or N i c o l a Lake. The AlgO^ content ranges from 3«50 to 5 . 9 0 $ . With one exception (ERC - 1 1 ) , the C a s t l e Rock ortho- pyroxenes c o n t a i n l e s s AI2O3 than those of Jacques Lake (Table 3 ) . TABLE 2 P a r t i a l chemical analyses of o l i v i n e s . Sample 95 S-3 CR-8 ERC-11 Oxide(wt.$) FeO* 9.25 8.45 9.56 9.60 MgO 50.0 50.0 46.9 48.1 CaO 0.05 0.05 0.03 0.04 MnO 0.11 0.10 0.10 0.12 NiO 0.32 0.31 0.32 0.34 Na20 0.21 0.19 0.22 0.08 Fe/Fe+Mg 10.13 9.20 10.29 10.08 Minor elements i n ppm. N l 2472 2845 2511 2640 Mn 897 751 763 911 Co 119 133 122 140 Zn 53 52 49 62 Pb 80 64 28 64 Cu 1.2 0.8 0.6 0.7 * T o t a l i r o n as FeO NL-8 JL-A JL-39 JL-10 JL-B JL - 5 5 8.38 9.66 9.91 9.95 9.70 10.42 >2.8 48.4 47.9 47.1 50.8 48.3 0.04 0.05 0.03 0.04 0.05 0.04 0.09 0.12 0.12 0.11 0.12 0.16 0.24 0.33 0.35 0.33 0.31 0.36 0.19 0.33 0.6l 0.32 0.59 0.27 8.18 10.08 10.39 10.22 9.71 10.52 1895 2619 2737 2604 2465 2861 702 929 956 889 913 1027 132 140 129 125 130 137 46 48 62 55 57 48 22 35 78 80 77 72 1.1 1.2 1.2 1.2 1.0 0.7 00 TABLE 3 P a r t i a l chemical analyses of orthopyroxenes. Sample 95 S-3 CR-8 ERC-11 NL-8 JL-A J L - 3 9 JL - 1 0 JL-B J L - 5 5 Oxide(wt.$) • TiO? 0 . 0 6 0 . 0 6 0 . 0 8 0.14 0 . 10 0.22 0 . 2 6 0.14 0 . 16 0 . 1 2 A l ? 0 o 3 . 6 8 3 . 5 4 3 . 5 0 5 . 9 0 4.04 4.46 4 . 6 8 4.40 4 . 6 0 4 . 6 6 CroOi O.37 0 . 3 5 0 .31 0 . 2 6 0 . 10 0.46 0.48 0.48 0 . 3 3 0 . 3 3 Fe0* J 5 . 8 0 5 .27 5 . 7 4 5.91 6.17 6 . 4 3 6.42 6.17 6 . 2 0 6 . 5 6 MgO 32.2 3 3 . 6 3 2 . 5 3 1 . 6 2 9 . 5 3 1 . 9 3 1 . 3 3 0 . 7 3 1 . 6 31.1. CaO 0 . 76 0 . 7 3 0 .67 1 . 3 3 1 . 0 3 0 . 9 4 0.82 0 . 8 5 0 . 79 1 .3& MnO 0.11 0.11 0.11 0.11 0 .12 0.11 0 . 10 0.11 0.11 0.11 N10 0.11 0.11 0 . 12 0 .12 0 .12 0.11 0 . 10 0.11 0 . 1 0 0 . 12 Na 20 0 .21 0 . 13 0 .27 0 . 4 5 0.24 1.00 0 .32 0 .31 0.24 0 . 5 0 K 20 0 .01 0 .01 0 . 0 1 0 .01 0 .01 0 . 0 1 0 . 0 1 0 .01 0 . 0 1 0 . 0 1 Fe/Fe+Mg 9 . 2 0 8 . 0 8 9 . 0 2 Fe/Fe+Mg+Ca 9 . 0 6 7 . 9 9 8.90 Mg/Fe+Mg+Ca 89.42 9 0 . 6 0 8 9 . 7 6 Ca/Fe+Mg+Ca 1 .52 1 . 4 l 1 . 3 4 CaTs(mol.$) 3 3 2 MgTs(mol.#) 2 4 3 Minor elements i n ppm. T l 330 36O 480 Cr 2500 2380 2130 Ni 838 856 889 Mn 839 743 812 Co 63' 60 63 Zn 40 33 3^ Pb 22 27 23 Cu 1.2 1.1 1.1 9 . 5 1 10.51 1 0 . 1 6 1 0 . 3 1 1 0 . 1 5 9 . 9 1 1 0 . 5 9 9 . 2 5 10.28 9.97 10.14 9 . 9 4 9 . 7 5 10.18 8 8 . 0 9 8 7 . 5 1 8 8 . 1 6 88.19 8 8 . 2 6 8 8 . 6 6 8 7 . 0 6 2 . 6 6 2 .21 1 .87 I . 6 7 1.80 1 .59 2 . 7 6 5 0 2 2 3 3 5 7 0 5 4 2 4 4 840 600 1320 1560 840 960 720 1750 675 3130 3250 3250 2250 2250 906 934 851 820 815 761 931 823 849 854 882 - 862 866 903 70 64 66 68 65 59 64 40 38 38 36 38 46 33 42 33 29 39 33 49 27 1.1 0 . 6 1.1 0 . 5 1 .0 1 .9 2 . 0 * T o t a l i r o n as FeO .40 The above compositions are s i m i l a r to the compositions of orthopyroxenes from other s u i t e s (e.g. White 1 9 6 6 ) . The range of values f o r T i , Cr, N i , Mn, Co, Cu, Zn and a l s o Na20, K 2 0 and Cao i s s i m i l a r to that found elsewhere (Ross e t . a l . 1954; White 1966; C a r t e r 1970). There i s a d i f f e r e n c e between s u i t e s i n some of the t r a c e element contents, independent of v a r i a t i o n s i n major element concentrations. T h i s i s discussed i n Chapter 7. As w i t h the o l i v i n e s , the orthopyroxenes have a high Pb content r e l a t i v e to the average f o r u l t r a m a f l c rocks (Table 3 ) . (d) Clinopyroxenes. P a r t i a l chemical analyses of clinopyroxenes are given i n Table 4. In terms of three end-members the Jacques Lake c l i n o - pyroxenes range i n composition from D i ^ ^ ^ E n ^ ^ ^ ^ F s ^ ^ ^ to D i 4 8 . 4 E n 4 6 . 6 F s 5 . 0 ' those from C a s t l e Rock range from D i # 0 ^ 5 8 . 6 F s 5 . i 1 . to Di4-p>'pEn48,8Fsi4.,3. The C a s t l e Rock c l i n o - pyroxenes are g e n e r a l l y more magneslan and l e s s c a l c i c than those from Jacques Lake (Table 4 ) . The N i c o l a Lake specimen i s the l e a s t c a l c i c w i t h a composition of Dlii,^tjEn^itQFs^ti^, The clinopyroxenes contain from 2.78 to 7 . 1 2 $ AI2O3. With one exception (ERC-11), the C a s t l e Rock clinopyroxenes c o n t a i n l e s s AI2O3 than those of Jacques Lake. The N i c o l a Lake specimen has an intermediate concentration of AI2O3. The AI2O3 content . increase s w i t h Na20 and TiC>2 ( F i g . 24) which suggests that these elements are s u b s t i t u t i n g f o r A l i n the pyroxene s t r u c t u r e . The above compositions are s i m i l a r to the compositions of c l i n o - pyroxenes of other l h e r z o l i t e s u i t e s (e.g. White 1 9 6 6 ) . TABLE 4 P a r t i a l chemical analyses of clinopyroxenes. Sample 95 S - 3 CR-8 ERC-11 NL -8 JL-A JL-39 JL - 1 0 JL-B J L - 5 : Oxlde(wt.#) TiC-2 0 . 2 0 0 . 0 5 0 . 2 2 0.40 0.40 0 . 4 5 0.64 0.48 0 . 6 0 0 . 5 2 A 1 2 0 3 4 . 9 2 2 . 7 8 5 . 0 0 6 . 7 0 5 . 9 0 5 . 6 0 6.84 5 . 6 0 7 . 12 7 . 1 0 Cr 2 0 o 0 . 8 8 0 . 9 3 0 . 8 5 O .83 0 . 8 5 0 . 9 0 0 . 8 5 0 . 8 3 0.80 0 . 9 0 FeO*^ 2 . 4 4 2 . 2 7 2 . 2 6 3.40 2 . 9 9 2 . 7 5 2 . 6 9 2 . 7 4 2 . 6 1 2 . 7 4 MgO 1 5 . ^ 18.2 1 6 . 4 2 0 . 8 1 5 . 8 1 5 . 6 14 .6 1 5 . 5 14.3 14 .4 CaO 2 0 . 0 19.6 2 1 . 8 19.6 2 0 . 8 2 0 . 9 2 1 . 3 2 0 . 0 17.8 18 .8 MnO 0 . 0 6 0 . 0 6 0 . 0 5 0.08 0 . 0 7 0 . 0 7 0 . 0 7 0 . 0 7 0 . 0 7 0 . 0 7 N10 0 . 0 6 0 . 0 7 0 . 0 6 0.08 0.08 0 . 0 6 0 . 0 5 0 . 0 5 0 . 0 5 0 . 0 5 Na 2 0 1 . 0 0 0 . 6 3 1 .26 1 . 0 9 1 . 3 9 1 . 0 0 1 . 5 4 0 . 9 0 2 .11 1 . 3 4 K 2 0 0 . 0 1 0 . 0 1 0 . 0 1 0 . 0 1 0 . 0 1 0 , 0 1 0 . 0 1 0 . 0 1 0 . 0 1 0 . 0 1 Fe/Fe+Mg 8.19 6 . 5 4 7.18 8.55 9 . 6 0 9 . 0 2 9 . 3 7 9 . 0 2 9 .31 9 . 6 6 Fe/Fe+Mg+Ca 4 . 3 2 3 . 6 6 3 . 9 6 5 . 3 7 5.41 4.81 5 . 0 0 4 . 6 9 4.91 4 . 9 7 Mg/Fe+Mg+Ca 48.40 5 2 . 2 6 5 1 . 2 0 5 8 . 5 5 5 0 .97 48 .16 48.19 48 .26 48 .66 4 7 . 0 6 Ca/Fe+Mg+Ca 47.28 4 4 . 0 8 45.12 3 5 . 2 6 4 3 . 2 6 46.91 46 .63 47.93 47.24 48 .52 CaTs(mol.Jg) 3 6 4 6 5 6 4 9 6 10 Minor elements i n ppm. T i 1200 300 1320 2400 2400 2700 3840 2880 36OO 3120 Cr 5990 633P 5830 5650 5820 6160 5820 5650 5^70 6160 Ni 482 5^7 497 615 608 440 398 413 361 357 Mn 488 475 413 638 529 540 563 558 542 562 Co 50 50 46 81 63 49 48 50 53 42 Zn 21 20 18 26 22 19 19 20 15 16 Pb 89 8 3 69 60 104 84 77 73 10b 77 • T o t a l i r o n as FeO 7 O A A O O 4' 3' 2-5 O O % Na20 AZ Fig. 2 4 : Variation of Na20 and T i0 2 with A l 2 0 3 in clinopyroxenes. A Jacques Lake, o Castle Rock. • Nicola Lake. 7 - O A A 55 O O A A 3 2-5 O O •3 % Ti© 2 .^3 The composition of the clinopyroxene i s one means of d i s t i n g u i s h i n g nodules of the l h e r z o l i t e s e r i e s from those of the dunite-wehrlite-gabbro s e r i e s and the e c l o g i t e s e r i e s (White 1966; Kuno 1 9 6 9 ) . Consequently the v a r i a t i o n of A I 2 O 3 , CaO and Na20wlth the r a t i o Mg0»PeO has been p l o t t e d i n P i g . 2 5 . As can be seen a l l the cllnopyroxenes from t h i s study f a l l i n the f i e l d of the l h e r z o l i t e s e r i e s . The range of values f o r T i , Cr, N i , Mn, Co, Cu, Zn and a l s o Na 20 and K2O are s i m i l a r to the values reported elsewhere (Ross e t . a l . 1954; White 1966; C a r t e r 1970). There are d i f f e r e n c e s between s u i t e s i n some of the t r a c e element contents, independant of v a r i a t i o n s i n major element c o n c e n t r a t i o n . This i s discussed i n Chapter 7. As w i t h the o l i v i n e s and ortho- pyroxenes, the clinopyroxenes have a high Pb content (Table 4) compared to the average given by Goles (1967) f o r u l t r a m a f i c r o c k s , (e) S p i n e l . P a r t i a l chemical analyses of s p i n e l s are given i n Table 5« The composition of n a t u r a l chrome-bearing s p i n e l s adheres c l o s e l y to the model formula (Mg,Fe 2 +) (Cr,Al,Fe : 3 +) 20^; the sum of the oxides i s g e n e r a l l y more than 9&% ( I r v i n e 1965). Consequently only these c o n s t i t u e n t s were analysed to determine the v a r i a t i o n s i n composition. The composition of chromian s p i n e l s may be g r a p h i c a l l y represented by means of a t r i a n g u l a r prism, each of the s i x corners corresponds to one of th;e end-members (MgCr 2 0 ^ ; F e C r 2 0^s MgFe204; Fe203? MgA^O^jFeA^Oj^). P l o t t i n g i s done by 8 7 6 i 3 5 CM 3 2 I 22 2! ^20 o . 18 17 Lherzolite series O O A ."44 O O o Wehrlite series O Lherzolite series O Wehrlite series o O 9 Fig. 25 Lherzolite series O O A Wehrjite series 8 7 6 MgO •. FeO Variation of ALp3, CaO and ISlOgO with MgO: FeO in clinopyroxenes. The lines are the boundary lines between the compositional fields of lherzolite and wehrlite series clinopyroxenes;. taken from Kuno (1969). A Jacques Lake: o Castle Rock: Nicola Lake. TABLE 5 P a r t i a l chemical analyses of s p i n e l s . Sample 95 Oxide(wt.%) MgO 20.27 FeO 8.91 F e 2 0 3 * 2.98 A l o O o 50.49 C r 2 0 3 15.38 T o t a l . 9 8 . 0 3 , Fe 2*/Fe 2 ++Mg 19.8 F e 3 +/F e ^ + A l +C r 3 • 0 Al/Fe<>Al+Cr 79.0 Cr/Fe-' +Al+Cr 18.0 • C a l c u l a t e d by assuming model formula RR 20 i } > I o n i c formula based on 32 (0) MgOJ. 6 .507 6.55^ 6.671 6.760 5.909 6 .052 5.900 6.275 6.391 5.668 Fe? t 1 .605 1.694 1.476 0.789 2.007 1.886 1.885 2.002 I . 5 6 3 1.815 Fe-* 0.729 0.852 0.498 1.335 - 0 .068 0.144 A l 12.822 9.328 13.167 13.353 14.401 13.940 14.728 13.777 14 .736 15.194 Cr 2.620 5.656 2.237 1.415 1 .655 2.102 1.463 1.965 1.151 1.151 Rot 8.112 8.248 8.14? 7.5^6 7 .915 7.940 7.755 8 .225 7 .95^ 7.483 RJ 15.926 15.836 15.902 16.103 16.056 16.042 16.164 15.801 16.031 1 6 . 3 4 5 s-3 CR-8 ERC-11 NL-8 J L - A J L-39 J L - 1 0 J L-B J L-55 19 .56 9 .01 5*03 35.19 31.82 21 .38 8.43 3 . I 7 53.35 14.19 23.58 4.43 8.42 54.19 8.43 19.0.5 11.53 58.71 10.06 19.73 10.95 57 .39 12.93 20.15 11 .29 63.57 9.24 19 .93 11.33 0.46 57.35 11.76 21.86 9.53 0.97 63.72 7.42 19.06 10.88 64.59 7 . 3 0 100.61 100.52 99 . 0 5 99.3^ 101.00 104 .25 100.93 103.30 I O I . 8 3 20.5 5.2 56.5 38.3 18.1 3.1 80.8 16.1 9.5 8.2 82.2 9.6 25.3 88.6 11.4 2 3 . 7 85.5 14.8 24.2 90.2 9.8 24.2 0.4 86.3 13.4 20.3 0.9 91.9 7.2 24.3 92.2 7.8 .46 p r o j e c t i o n as shown i n F i g . 26. The compositions of the analysed s p i n e l s are shown on the two p r o j e c t i o n s . As can be seen, the s p i n e l s f a l l c l o s e to the MgAl 2 0 ^ apex of the prism. T h i s i s s i m i l a r to other analysed s p i n e l s from l h e r z o l i t e nodules (Ross e t . a l . 1954; E l Hamad 1963; I s h i b a s h i 1969; K u t o l i n and Fr o l o v a 1970; Carter 1 9 7 0 ) . S p i n e l s from other types of nodules do not n e c e s s a r i l y p l o t i n t h i s part of the prism ( K u t o l i n and F r o l o v a 1970). Each nodule s u i t e i s c h a r a c t e r i s e d by having the composition of the s p i n e l phase l y i n g i n a p a r t i c u l a r volume of the s p i n e l prism ( F i g . 2 6 ) . The C a s t l e Rock s p i n e l s are r i c h e r i n F e 2 0 ^ and C r 2 0 3 than those from Jacques Lake and N i c o l a Lake; a l s o the C r 2 0 ^ content of the Ca s t l e Rock s p i n e l s i s more v a r i a b l e . The range of MgO v a r i a t i o n I s sm a l l . The N i c o l a Lake nodule has the l e a s t magnesian s p i n e l . There i s no d i f f e r e n c e i n the range of MgO content between the Jacques Lake and C a s t l e Rock s p i n e l s . The r e l a t i v e l y high F e 2 0 3 content of the C a s t l e Rock s p i n e l s (Table 5) suggests that these c r y s t a l l i s e d under higher f 0 2 than the N i c o l a Lake and Jacques Lake s p i n e l s ( I r v i n e 1965. 1967). . 4 7 Fig. 26 : Composition of analysed spinels. Jacques Lake -.o Castle Rock: •Nicola Lake, .48 CHAPTER 6 The D i s t r i b u t i o n of Iron and Magnesium Between C o e x i s t i n g M i n e r a l s . (a) Theory. The d i s t r i b u t i o n of ca t i o n s between c o e x i s t i n g phases has been discussed i n d e t a i l by Ramberg and DeVore (195U. Mueller (1961), Bartholome ( 1 9 6 2 ) , K r e t z (1961, 1963) and more r e c e n t l y by Grover and O r v l l l e ( 1 9 6 9 ) . The f o l l o w i n g summary i s based on the work of K r e t z ( 1 9 6 l , 1963). An e q u i l i b r i u m exchange r e a c t i o n f o r the d i s t r i b u t i o n of A and B between c o e x i s t i n g phases (A,B)M and (A,B)N can be w r i t t e n : AM +BN = BM +..AN (1) The thermodynamic e q u i l i b r i u m constant f o r such a r e a c t i o n i s : aBM•aAN KD = aAM * aBN where a pQ i s the a c t i v i t y of the appropriate end-member compound. Equation (2) can be w r i t t e n : YBM / BM YAN / AN B , A B , AA • A A KD ~ AM / AM' BN / BN ^> . A A . /\ A . . A B where X p Q i s the mole f r a c t i o n of P i n PQ and A | Q i s the , a c t i v i t y c o e f f i c i e n t of P i n PQ. I f (A,B)M and (A,B)N behave as i d e a l s o l u t i o n s , then the a c t i v i t y c o e f f i c i e n t s are u n i t y and equation (3) becomes: yBM YAN K D = A B A (l» YAM yBN A , AB .49 I f the I d e a l s o l u t i o n model i s c o r r e c t then K D i s a f u n c t i o n of temperature and pressure only. The temperature dependence of Kp i s given by: e) l n K D = £1H (5) ^ T / P R T 2 where Z^H° i s the change i n enthalpy of r e a c t i o n ( 1 ) , R i s the gas constant, and T the temperature i n °K. The pressure dependence of i s given by: / d l n K D ^ _ _ £ v o (6) \ i p _/T RT where A V° i s the molar change i n volume of r e a c t i o n (1) and P i s the pressure i n bars. As a f i r s t approximation, o l i v i n e , orthopyroxene and c l i n o - pyroxene can be t r e a t e d as i d e a l s o l i d s o l u t i o n s of the type (A,B)M and equation (4) can be used to determine the d i s t r i b u t i o n c o e f f i c i e n t w i t h respect to the exchange of i r o n and magnesium between two c o e x i s t i n g phases. The d i s t r i b u t i o n of i r o n and magnesium between s p i n e l and any of the above minerals i s considered l a t e r . The appropriate r e a c t i o n s 1S>J=H FeSiC-3 •+ MgSio . 5 0 2 = MgSiC^ + F e S i 0 . 5 0 2 (7) f o r o l i v i n e and orthopyroxene, and KD<1? = y o p x y o l (8) A F e , AMg For c o e x i s t i n g pyroxenes the r e a c t i o n i s : . 5 0 CaFeSl 2 0 6 + MgS103 = CaMgSi 2 06 + FeS103 (9) and K D (2> = X " g - X F e (10) opx cpx X .X Fe Mg For c o e x i s t i n g o l i v i n e and clinopyroxene the r e a c t i o n i s ; C a F e S i 2 0 6 + MgSig^Og = CaMgSi 2 0£ + F e S l 0 > ^ 0 2 and Y c p x v o l KD<3) - „ ^ x ' H (12) A F e , AMg (b) Results f o r c o e x i s t i n g s i l i c a t e s . Values of K D ( 1 ) , K D ( 2 ) , and K D ( 3 ) are l i s t e d i n Table 6. A l s o given i s an a n a l y s i s of the variance f o r the K J J values between the Jacques Lake and C a s t l e Rock m i n e r a l s . The N i c o l a Lake minerals are considered s e p a r a t e l y . The Kp»s were c a l c u l a t e d by assuming that a l l the i r o n i n the minerals i s i n the f e r r o u s s t a t e . While t h i s i s a good approximation f o r o l i v i n e and to some extent orthopyroxene, i t i s not so good f o r the clinopyroxenes since they may co n t a i n up to 2% f e r r i c i r o n (Ross e t . a l . 995*0. Nevertheless i t i s s t i l l u s e f u l l to c a l c u l a t e the K^'s so obtained and compare the r e s u l t s f o r each s u i t e . K Q ( 1 ) and K D ( 3 ) f o r the Jacques Lake nodules are l e s s than t h e values f o r the Ca s t l e Rock nodules. This I s a s i g n i f i c a n t d i f f e r e n c e as the F values exceed the 1% l e v e l of F (Snedecor and Cochran 19^7). A l s o , K D ( 1 ) and K D ( 3 ) f o r the N i c o l a Lake .51 TABLE 6 Values of Kp f o r c o e x i s t i n g o l i v i n e and pyroxenes, with a n a l y s i s of var i a n c e . Sample L o c a l i t y K D ( D K D ( 2 ) KD(3) JL-A Jacques Lake 0.99 0 . 8 8 1.13 J L - 3 9 • i 1.01 0.90 1.12 JL - 1 0 • i 1.01 0 . 8 8 1.15 JL-B 0.97 0.93 1.04 J L - 5 5 •I 0.98 0.91 1.11 95 C a s t l e Rock 1.25 0.87 1.26 S -3 • i 1.16 0.79 1.45 CR-8 • i 1.17 0.78 1.48 ERC-11 i i 1/08 0.89 1.20 Variance Between s u i t e s .006 .008 .127 .016 .065 Within s u i t e s .002 Degrees of Between s u i t e s 1 1 1 freedom Within s u i f e s 7 7 7 F 33.00 4 . 0 0 15.88 NL-8 N i c o l a Lake 0.89 0.90 0.84 sample are lower than any of the other s . There i s no d i f f e r e n c e i n the va l u e s of K D ( 2 ) between any of the s u i t e s . The d i s t r i b u t i o n c o e f f i c i e n t s f o r each mineral p a i r are s i m i l a r to the c o e f f i c i e n t s determined from other l h e r z o l i t e nodules (Kretz 1963s O'Hara 1963J White 1 9 6 6 ) . D i r e c t comparison w i t h other s u i t e s i s not p o s s i b l e s i n c e t o t a l i r o n was expressed as FeO. In the i d e a l s o l u t i o n model, K D i s a f u n c t i o n of pressure and temperature only. I f the minerals depart from i d e a l i t y , then Kp w i l l a l s o be a f u n c t i o n of composition, since the a c t i v i t y c o e f f i c i e n t s w i l l not be un l y . N a f z l g e r and Muan (1967) found that magnesian o l i v i n e i s s l i g h t l y n o n - i d e a l but ortho- .52 pyroxene i s i d e a l . However, i n t r o d u c i n g a c t i v i t y c o e f f i c i e n t s ;V i n equation (3) to evaluate w i l l not change the r e l a t i v e values of K Q s i n c e there i s a r e s t r i c t e d range of compositions. I t was found t h a t each Kj) i s independent of any other component i n the m i n e r a l s . Therefore the conclusions w i t h respect to the d i f f e r e n c e s i n between the s u i t e s are s t i l l v a l i d , (c) The e f f e c t s of temperature and pressure. Medaris (1969) has determined e x p e r i m e n t a l l y the p a r t i t i o n - i n g curve f o r i r o n and magnesium between o l i v i n e and or t h o - pyroxene a t 900°C. The appropriate expression i s : l o g [ 7 ^ ) 0 1 = O.I630 + 1.1128log f^ll) opx (13) ^ XMg ' \ X M g I where X p e and X M g are the mole f r a c t i o n s of i r o n and magnesium i n the minerals. There i s good agreement between t h i s curve and the t h e o r e t i c a l p a r t i t i o n i n g curve d e r i v e d by Grover and O r v i l l e ( 1 9 6 9)i who considered the exchange of i r o n and magnesium between o l i v i n e and orthopyroxene to take place between a s i n g l e s i t e i n o l i v i n e (M^ and M 2 are e n e r g e t i c a l l y e q uivalent) and a double s i t e i n orthopyroxene (M^ and M 2 are e n e r g e t i c a l l y d i s t i n c t ) . Equation (13) t h e r e f o r e appears to express s a t i s f a c t o r i l y the p a r t i t i o n i n g of i r o n and magnesium between these minerals at 900°C. Medaris a l s o found that p a r t i t i o n i n g was not s i g n i f i c a n t l y temperature dependant between 900 and 1300°C. Despite the f a c t that temperatures of e q u i l i b r a t i o n cannot be determined from the composition of c o e x i s t i n g o l i v i n e and orthopyroxene i t i s s t i l l useful/i to compare the d i s t r i b u t i o n .53 of I r o n and magnesium w i t h the experimehtallydetermined curve i n order to see i f the minerals c r y s t a l l i s e d under e q u i l i b r i u m c o n d i t i o n s . P i g . 27 i s a p l o t of ( X F e / X M g ) o l versus ( X p e / X M g ) o p x on a l o g a r i t h m i c s c a l e . The curved l i n e i s the p a r t i t i o n i n g curve determined by Medaris (1969). A l l p a i r s p l o t c l o s e t o , but s l i g h t l y below the curve. This i s i n agreement w i t h other olivine-orthopyroxene p a i r s from l h e r z o l i t e nodules (Medaris 1969). The p o i n t s f o r both the Jacques Lake and C a s t l e Rock p a i r s l i e p a r a l l e l to the curve which suggests that a l l the p a i r s c r y s t a l l i s e d under e q u i l i b r i u m c o n d i t i o n s . The e f f e c t s of pressure on the d i s t r i b u t i o n c o e f f i c i e n t has been given i n equation (6). Using molar volume data on s y n t h e t i c o l i v i n e and orthopyroxene Medaris (1969) found that Kj)(l) i s not s i g n i f i c a n t l y dependant on pressure. However t h i s c o n c l u s i o n i s based on the assumption that A V° does not change w i t h pressure or temperature. The change i n volume f o r r e a c t i o n (7) i s given by: A V ° " V M g S i 0 3 + V F e S i 0 . 5 0 2 ~ V F e S i 0 3 " v M g S i 0 . 5 0 2 (14) where V° i s the molar volume of phase j . However the orthopyroxenes c o n t a i n a considerable amount of A l ( s u b s t i t u t i n g f o r S i and (Mg.Fe)). Therefore the volumes which are used i n equation (14) should be p a r t i a l molar volumes and not the volumes of the pure end-member components. Since the A l content of the orthopyroxenes i:s d i f f e r e n t f o r each s u i t e (Table 3) , i t i s to be expected that A v ° w i l l be d i f f e r e n t f o r each s u i t e and hence the e f f e c t of pressure cannot be e n t i r e l y disregarded. I t i s p o s s i b l e that the d i f f e r - '.'54 ~  1  • • • • ••  11  " - i • • • • • — • ™ •• — » " ' •Ol o,i I.O Fe : Mg opx. 2+' Fig. 27 : Distribution of Fe and Mg between olivine and orthopyroxene. The curve is the equilibrium partitioning curve at 900°C determined by Medaris (1969). A Jacques Lake : o Castle Rock: • Nicola Lake. .55 ences i n K D ( 1 ) between the s u i t e s may be due to d i f f e r e n c e s i n pressure. I t i s not p o s s i b l e to determine the d i r e c t i o n of pressure change since the p r e c i s i o n of the a v a i l a b l e data does not warrent the c a l c u l a t i o n . K r e t z (I963) has shown that the d i s t r i b u t i o n of i r o n and magnesium between c o e x i s t i n g pyroxenes i s a f u n c t i o n of temperature and i s constant w i t h respect to pressure. K r e t z (1963) a l s o showed that K^(2) i n c r e a s e s w i t h temperature f o r igneous and metamorphic pyroxenes and that the values f o r pyroxenes from u l t r a m a f i c nodules tend to approach u n i t y . The data from t h i s study agree w i t h the l a t t e r o b s e r v a t i o n . The data suggest that a l l the pyroxene p a i r s c r y s t a l l i s e d w i t h i n the same range of temperatures since there i s no d i f f e r e n c e i n the range of values of K^(2) between the s u i t e s . I t w i l l be shown i n the next s e c t i o n that the C a s t l e Rock nodules formed at a higher temperature than those of Jacques Lake, and that the N i c o l a Lake nodule formed at a lower temperature than the other s . I t might be expected t h e r e f o r e that the K D ( 2 ) values might show t h i s . This i s not the case. Therefore i t i s l i k e l y t h a t some other f a c t o r besides temperature i s a f f e c t i n g the values of Kp ( 2 ) . This f a c t o r i s p o s s i b l y the AI2O3 content of the pyroxenes, although no d i r e c t r e l a t i o n s h i p between the AI2O3 content of the pyroxenes and K^(2) could be found.' As w i t h K p ( l ) , the pressure e f f e c t on Kj)(2) i s dependant on the q u a n t i t y A V ° . The e f f e c t s of AI2O3. on t h i s are unknown, so tha t K D ( 2 ) could change s i g n i f i c a n t l y w i t h pressure. Also,Z\H° may be a f f e c t e d by the Al 20o, content of the pyroxenes. Therefore i t i s not p o s s i b l e to determine the c o n d i t i o n s of formation of these minerals from the d i s t r i b u t i o n of i r o n and magnesium between them. No experimental or e m p i r i c a l data on the e f f e c t of temperature and pressure on the d i s t r i b u t i o n of i r o n and magnesium between c o e x i s t i n g o l i v i n e and clinopyroxene are a v a i l a b l e i n the . l i t e r a t u r e . I t i s evident from Table 6 that i r o n and magnesium are d i s t r i b u t e d d i f f e r e n t l y between these minerals i n each nodule s u i t e . I t i s assumed that t h i s i s a r e s u l t of d i f f e r e n t c o n d i t i o n s of formation. I t Is not p o s s i b l e to evaluate v a r i a t i o n s i n Kp(3) w i t h respect to pressure and temperature using data f o r the iron-magnesium end-members si n c e Al^Oo. i n the c l i n o - pyroxenes probably a f f e c t s the appropriate thermodynamic f u n c t i o n s , (d) The d i s t r i b u t i o n of i r o n and magnesium between c o e x i s t i n g o l i v i n e and s p i n e l . The d i s t r i b u t i o n of i r o n and magnesium between c o e x i s t i n g o l i v i n e and s p i n e l has been discussed i n d e t a i l by I r v i n e (1965)* 2 + A r e a c t i o n expressing the Mg-Pe exchange between o l i v i n e and s p i n e l can be w r i t t e n ; F e 2 + S i 0 . 5 0 2 •+ Mg(Cr c <,Al^,Pe :^") 20i t = M g S i 0 # 5 O 2 + F e 2 + ( C r ^ , A l / 9 , F e | + ) 2 0 i f (15) where are mole f r a c t i o n s of t r i v a l e n t c a t i o n s i n the s p i n e l and <x + / S ^ = l . The thermodynamic e q u i l i b r i u m d i s t r i b u t i o n c o e f f i c i e n t f o r equation (15) l s s a .SL* . a ^ . a * K D ( 4 ) = MgSi 0 5 0 2 * F e C r 2 0 / / F e A l ^ FeFe 2 0/j, ! 3 ~P * ( l 6 ) a F e S l Q ̂  5 0 2 • aMgCr 2 0 ^' a M g A l g O ^ M g F e ^ where i s the a c t i v i t y of end-member j . This can be expressed i n compositional terms by r e p l a c i n g a^ w i t h ( X J X J ) where A^ i s the a c t i v i t y c o e f f i c i e n t arid X j i s the mole f r a c t i o n of the a p p r o p r i a t e end-member. I f i d e a l behavior i s assumed, the a c t i v i t y c o e f f i c i e n t s are u n i t y and equation ( 1 6 ) becomess oX ®P sp ^ i sp ^ xMg* ^ x FeCr20i| ,^ • ^FeAl20/4.) • ( x F e F e 2 0 4 ) K D ( 4 ) = o i sp s r - s p 77^ ? ( 1 7 ) AMg* V A M g C r 2 0 l + ; ' l A M g A l 2 0 ^ ; * V AMgFe 2 0 ^ ; Since x|P ~. = ( Fe ^ ^ / Cr \ = o ( x s p - + e t c . ; F E C R 2 ° 4 iMg + F e 2 * / ( c r + A l .+ Fe3 +) A P e 2 + <K + {3 + X = 1, equation (16) reduces to K ( 4 ) = ^ H 2 + ( 1 8 ) x^.xJJP Fe Mg This i s a r e s u l t s i m i l a r to that f o r c o e x i s t i n g s i l i c a t e s but i s d e r i v e d i n t h i s way to show the e f f e c t s of the t r i v a l e n t c a t i o n s of the s p i n e l on K j - ) ( 4 ) . Table 7 gives the values of ^ ( 4 ) c a l c u l a t e d from equation (18). The method of c a l c u l a t i n g the u n c e r t a i n t i e s i n K D ( 4 ) i s given i n Appendix 2 . The Table a l s o gives the a n a l y s i s of variance i n K D ( 4 ) between the Jacques Lake and C a s t l e Rock s u i t e s , The N i c o l a La"ke p a i r i s considered s e p a r a t e l y . As can be seen, the N i c o l a Lake sample has the highest K D ( 4 ) . There i s a TABLE 7 Values of Kp(4) for coexisting olivine and spinel, with analysis of variance. Sample Locality K D(4) lnK D(4) 95 Castle Rock 2.20 + .17 0.785 + .078 3-3 •I 2.55 .21 0.931 + .081 CR-8 i i 1.93 t" .16 0.651 .080 ERC-11 •• 0.93 + .10 -0.068 ± .037 JL-A Jacques Lake 2.77 ± .20 1.014 ± .071 JL-39 i i 2.78 ± .19 1.020 ± .069 JL-10 i i 2.81 ± .20 1.031 ± .071 JL-B • i 2.37 ± .19 0.860 ± .079 JL^555 • i 2.74 ± .19 1.004 ± .069 Between suites 1.39 Variance Within suites 0.20 Between suites 1 Variance Within suites 7 P 7.03 Nicola Lake 3.79 ± . 3 0 1.330 ± .079 TABLE 8 Temperatures of formation for coexisting olivine and spinel. Sample Locality T C 95 Castle Rock 1632 ± 184 S-3 " 2214 ± 262 CR-8 " 1822 ± 231 ERC-11 " 10697 JL-A Jacques Lake 1133 ± 99 JL - 3 9 " 1037 ± 90 JL -10 " 1113 ± 9 8 JL-B " 1142 ± 122 JL - 5 5 " 1002 ± 88 NL-8 Nicola Lake 838 ± 71 s i g n i f i c a n t d i f f e r e n c e i n the K D ( 4 ) values between the Jacques Lake and C a s t l e Rock s u i t e s since the value of F exceeds the 5$ l e v e l of F (Snedecor and Cochran 19673. With one exception ( S - 3 ) , a l l the C a s t l e Rock K D(4)'s are g r e a t e r than those of Jacques Lake. The d i s t r i b u t i o n c o e f f i c i e n t i s r e l a t e d to the f r e e energy of r e a c t i o n (15) thus: lnK D(4) = and KD(4) = exp (zM^j ( 1 9 ) where A G = and Gj i s the f r e e energy of formation of the appropriate end-member i n equation (15)» R i s the gas constant and T the temperature of formation of the c o e x i s t i n g mineral p a i r . S u b s t i t u t i n g the f r e e energy values given by Jackson (1969, p.64) (these are temperature dependant), t a k i n g R = 1.987. and s o l v i n g f o r T, we have: 5580* + 10l 8 f i - 1720y + 2400 ' (20) T = 0.90* + 2.56(0 - 3.08 J - 1.47 + 1.9871nK D(4) Temperatures d e r i v e d from equation (20) are given i n Table 8. The method of c a l c u l a t i n g the u n c e r t a i n t y i n each temperature i s given In Appendix 2. The u n c e r t a i n t i e s l i s t e d i n Table 8 represent the maximum p o s s i b l e e r r o r s i n T which can be introduced as a r e s u l t of a n a l y t i c a l e r r o r s i n the determination of Mg and F e 2 + i n s p i n e l and o l i v i n e and Cr, A l and F e 3 + i n s p i n e l . The e r r o r that may be introduced i n t o the temperature values of Table 8 due to u n c e r t a i n t i e s i n the f r e e energy.values are much l a r g e r than the v6d; a n a l y t i c a l e r r o r s . P o s s i b l e maximum u n c e r t a i n t i e s i n the fre e energies of the s p i n e l s alone could a f f e c t the temperature values by as much as ± 300°C but these u n c e r t a i n t i e s are not l a r g e enough to reverse the d i r e c t i o n of r e a c t i o n (15) and w i l l have l i t t l e e f f e c t on the r e l a t i v e temperatures (Jackson 1 9 6 9 ) . I t i s apparent from Table 8 that each nodule s u i t e formed at a d i f f e r e n t temperature ( w i t h i n a n a l y t i c a l e r r o r and assuming that a l l the minerals e q u i l i b r a t e d a t the same temperature). The nominal temperature of formation of the N i c o l a Lake s u i t e i s 838°C (assuming that sample NL-8 i s r e p r e s e n t a t i v e of the s u i t e ) . The nominal temperature of formation of the Jacques Lake s u i t e i s 1085°C (average of 5 temperatures). The nominal;'temperature.;; of formation of the C a s t l e Rock nodules i s more d i f f i c u l t t o assess. There appears to be a range of temperatures but t h i s may not be r e a l as e r r o r s i n K-Q(4) , and hence i n l n K D ( 4 ) , become i n c r e a s i n g l y important where K D ( 4 ) I s s m a l l since t h i s term appears In the denominator of equation ( 2 0 ) . As the d e r i v e d temperatures seem u n r e a l i s t i c a l l y high the nominal temperature of formation of the C a s t l e Rock nodules i s taken to be l600°C. This i s somewhat a r b i t r a r y but there can be l i t t l e doubt that these nodules formed at temperatures much gr e a t e r than those of Jacques Lake and N i c o l a Lake. The e f f e c t of pressure on K D ( 4 ) i s that g iven i n equation ( 6 ) . I r v i n e (1965) has shown that the pressure e f f e c t i s n e g l i g i b l e at moderate pressues. Data to evaluate K^(4) a t high pressure i s l a c k i n g , but any v a r i a t i o n i n Kp(4) due to pressure i s not l i k e l y to reverse the r e l a t i v e temperatures s i n c e any volume change w i l l be small compared to the enthalpy change of equation ( 5 ) . . 6 1 : (e) The d i s t r i b u t i o n of i r o n and magnesium between c o e x i s t i n g s p i n e l and pyroxenes. Reactions equivalent to (15) can be w r i t t e n f o r orthopyroxene and;;'.clinopyroxene: PeS10 3 + Mg(Cr o (.,Al / 9 , P e 3 + ) 2 0 ^ = MgSi0 3 + Fe( Cr^ ,A1^ . F e ^ O ^ (21) and C a F e S i 2 0 6 + Mg(Cr c < . A l ^ tFe^)20^ = CaMgSigO^ + FetCr^.Al^.FeJ+JgO^ Analogous expressions f o r the d i s t r i b u t i o n c o e f f i c i e n t s are: X§gX.Xp£2-f K D ( 5 ) = o p x s p (23) x P e ^ . x M g and K D ( 6 ) = X^ X.X|P2+ — (24) x c p x x s p ' Fe*' Mg Table 9 l i s t s the values f o r Krj(5) and K D ( 6 V f o r the analysed p a i r s and a l s o the v a r i a t i o n i n these f o r the Jacques Lake and C a s t l e Rock s u i t e s . There i s no s i g n i f i c a n t d i f f e r e n c e i n these values between the s u i t e s . The values f o r ERC-11 are low i n comparison to the others. This may i n d i c a t e that the s p i n e l i n t h i s nodule i s not i n e q u i l i b r i u m w i t h the pyroxenes (and a l s o w i t h the o l i v i n e s ince K D ( 4 ) i s low). T e x t u r a l l y , however, the s p i n e l appears to be i n e q u i l i b r i u m , so t h a t the anomalous Kj)»s may be the r e s u l t .62 TABLE 9 Values of K D ( 5 ) and K D ( 6 ) , w i t h a n a l y s i s of variance, Sample L o c a l i t y K D ( 5 ) K D ( 6 ) JL-A Jacques Lake 2 .74 3.14 J L - 3 9 " 2.78 3.08 JL - 1 0 " 2.84 3.32 JL-B " 2.32 2.48 J L - 5 5 " 2.71 2 . 9 9 95 C a s t l e Rock 2 . 4 4 2.76 S-3 " 2 . 9 3 3.71 CR-8 " 2.24 2 . 8 5 ERC-11 " 1 .00 1.13 Between s u i t e s .631 .307 W i t h i n s u i t e s .313 .547 Variance Degrees of W i t h i n s u i t e s 7 7 freedom Between s u i t e s 1 1 F 1.96 .1.78 NL-8 N i c o l a Lake 2 . 8 9 3.12 of an imperfect a n a l y s i s . As can be seen from Table 4, ERC - 1 1 i s the l e a s t c l o s e to the model formula. Expressions analogous to (20) and (6) can be used to evaluate the temperature and pressure e f f e c t s on K Q ( 5 ) and K D ( 6 ) . There i s a considerable amount of AI2O3 i n the pyroxenes. Therefore there w i l l be an exchange of A l between the pyroxenes and the s p i n e l which w i l l undoubtedly Influence the temperature and pressure e f f e c t s . This e f f e c t i s not known. .63 CHAPTER 7 The D i s t r i b u t i o n of Trace Elements Between C o e x i s t i n g S i l i c a t e s . (a) Theory. The p a r t i t i o n i n g of t r a c e elements between c o e x i s t i n g minerals has been dicussed i n d e t a i l by Mclntyre ( 1 9 6 3 ) . An exchange r e a c t i o n between two s o l i d phases (Cr,Tr)A and (Cr,Tr)B can be w r i t t e n : CrA + TrB = TrA + CrB (25) where Tr and Cr represent the t r a c e element and the element f o r which i t s u b s t i t u t e s (the c a r r i e r element) r e p e c t i v e l y . A and B r e f e r to that p o r t i o n of the mineral which does not take part i n the r e a c t i o n . Cr and Tr have the same valence. The e q u i l i b r i u m constant f o r r e a c t i o n (25) i s a T r A . a C r B K T r = (26) aCrA» aTrB where a-j i s the a c t i v i t y of end-member J i n equation ( 2 5 ) . Equation' (26) can be r e w r i t t e n : ,Tr i Cr xTrA• XCrB•A TrA *ACrB ^Tr ~ Ar>-rA,ATP-rR*'Vr.T'A,A' (27) CrA TrB CrA TrB where i s the concentration of end-member j and / ̂  i s the a c t i v i t y c o e f f i c i e n t of i i n j . . I f i d e a l behavior i s assumed, then equation (27) becomes: K _ ( X T r / x C r ) A ""Tr (x /x ) -Tr Cr B • 6 4 The d i s t r i b u t i o n c o e f f i c i e n t (K^r) i s a f u n c t i o n of temperature and pressure. The temperature dependency, given i n equation (5) i s J I > l n K T r ) _ ^ o \ h T 'P R T 2 where A H° i s the enthalpy of r e a c t i o n ( 2 5 ). R the gas constant and T i s the temperature i n °K. The pressure dependence, given i n equation (6) i s : / SlnK-r^ = _ A v o W P /T RT w h e r e i s the molar change i n volume of r e a c t i o n ( 2 5 ) . P the pressure and the other symbols are the same as before. •Unfortunately An° and Av° f o r r e a c t i o n s i n v o l v i n g t r a c e elements are unknown so t h a t t r a c e element d i s t r i b u t i o n s cannot be used i n geothermometry and geobarometry w i t h any confidence. Nevertheless i t i s s t i l l u s e f u l to determine t r a c e element d i s t r i b u t i o n c o e f f i c i e n t s and a n t i c i p a t e that v a r i a t i o n s i n these are due to v a t i a t i o n s : i n the c o n d i t i o n s of formation of the mineral p a i r i n q u e s t i o n , i f i t can be shown th a t the d i s t r i b u t i o n c o e f f i c i e n t i s not dependant on the c o n c e n t r a t i o n of any other element. In o l i v i n e and pyroxenes the c a r r i e r element f o r d i v a l e n t t r a c e elements, may be e i t h e r Mg or Fe . I t i s assumed that the M2 s i t e i n clinopyroxene i s completely f i l l e d w i t h calcium and t h a t no exchange between a d i v a l e n t i o n and calcium takes place. The choice of Mg 2 + of F e 2 + i s not e n t i r e l y a r b i t r a r y as s t a t e d by Matsui and Banno ( 1 9 7 0 ) . Burns ( 1970) has shown that .65 t r a n s i t i o n metal ions have a preference f o r c e r t a i n s i t e s i n s i l i c a t e s t r u c t u r e s . He has l i s t e d , among o t h e r s , the f o l l o w i n g s i t e preferences: o l i v i n e Ml 5 N i 2 + , C o 2 + , M g 2 + . o l i v i n e M2 ; Mn 2 +,Fe 2 +. orthopyroxene Ml ; N l 2 + , M g 2 + . orthopyroxene M2 ; Mn 2 +,Co 2 +,Fe 2 +. ] clinopyroxene Mi ; Ni 2 +,Co 2 +,Mn 2+,Fe 2 +,Mg 2 +. Thus Mg 2 + w i l l be the c a r r i e r f o r N i 2 + i n o l i v i n e and ortho- pyroxene since both tend to occupy the same s i t e i n these minerals. Since both Mg 2 + and F e 2 + occupy the Ml s i t e i n clinopyroxene the choice i s not so c l e a r . As there i s a p o s i t i v e c o r r e l a t i o n between Ni,0 and MgO i n the analysed d i o p s i d e s . i t i s assumed that 2+ ? + Mg i s the c a r r i e r f o r N i i n the clinopyroxemes a l s o . S i m i l a r l y 2+ ?.+ Fe i s the c a r r i e r f o r Mn^and C o C T i n a l l three s i l i c a t e s . I t i s assumed that a l l Co i s i n the d i v a l e n t s t a t e i n these minerals. Although "Zn i s not a t r a n s i t i o n element i t appears to behave as one i n these minerals since i t i s d i s t r i b u t e d r e g u l a r l y among , the three s i l i c a t e s ( F i g . 28). For t h i s reason Zn i s t r e a t e d i n the same way as N i , Mn and Co i h f t h i s d i s c u s s i o n . From s i z e c o n s i d e r a t i o n s i t i s l i k e l y that Z n 2 + s u b s t i t u t e s f o r F e 2 + i n these minerals. (b) The d i s t r i b u t i o n of N i , Mn, Co and Zn. Since N i , Mn, Co and Zn s u b s t i t u t e f o r Mg or Fe i n the o l i v i n e and pyroxene s t r u c t u r e , the concentration of these elements i s dependent on the concentration of the c a r r i e r element. In order to compare tr a c e element c o n c e n t r a t i o n s , the r a t i o Tr/Cr . 6 6 can be considered to be a measure of the t r a c e element content of a m i n e r a l . (Ni/Mg)xl000, *(Mn/Fe)xlOO, (Co/Fe)xlOO and (Zn/Fe)xlOO f o r o l i v i n e , orthopyroxene and clinopyroxene are l i s t e d i n Table 10. The v a r i a t i o n i n these r a t i o s between the Jacques Lake and C a s t l e R6ck- :"3uites I s examined by means of Snedecor's F t e s t and the r e s u l t s given i n Table 12. The N i c o l a Lake sample I s considered s e p a r a t e l y . The N i c o l a Lake o l i v i n e contains more Ni and Co than the other o l i v i n e s and the pyroxenes contain l e s s Mn and Co. The Jacques Lake and C a s t l e Rock o l i v i n e s c o n t a i n s i m i l a r amounts of these elements. The Jacques Lake diopsldes contain l e s s N i , Co and Zn than those of C a s t l e Rock, and the e n s t a t i t e s l e s s Mn and Co. These are a l l s i g n i f i c a n t r e s u l t s s i n c e the value f o r F exceeds the 5% l e v e l of F i n each case (Table 12). There i s thus a fundamental d i f f e r e n c e i n the concent r a t i o n of some elements i n the minerals of each s u i t e of nodules. The minerals of each s u i t e are c h a r a c t e r i s e d by a p a r t i c u l a r t r a c e element "content". The d i s t r i b u t i o n of N i , Mn, Co and Zn between o l i v i n e and the two pyroxenes i s r e g u l a r . F i g . 28 shows the r e l a t i v e concent- r a t i o n s of these elements i n the three s i l i c a t e s . The r e l a t i v e enrichmentfof these elements In c o e x i s t i n g o l i v i n e and pyroxene \3 1. R a t i o s i n v o l v i n g i r o n were c a l c u l a t e d w i t h the assumption that a l l the i r o n i s i n the f e r r o u s s t a t e . Generally the minerals ( e s p e c i a l l y cllnopyroxenes) w i l l c o n t a in some f e r r i c i r o n . This w i l l a f f e c t the magnitude of such r a t i o s but the r e l a t i v e values w i l l remain since there i s a r e s t r i c t e d compositional range which i s assumed to apply to f e r r i c c i r o n a l s p a TABLE 10 (Tr/Cr) ratios of analysed minerals. .67 (Ni/Mg)xl000 o l . opx. cpx. Sample Locality JL-A Jacques Lake 8 . 9 8 4.42 4 . 6 9 JL - 3 9 " 9.46 4 . 3 4 4.52 JL -10 " 8.81 4.41 4.42 .JL-B . • . 8.04 3 .99 4 . 2 0 JL - 5 5 " 9.91 4 .97 4.12 95 Castle Rock 8 . 2 0 4.32 5.20 S-3 " 8 . 0 3 4 . 2 3 5 . 0 0 CR-8 " 8 . 8 9 4 . 5 4 5.02 ERC-11 " 9 . 0 9 4.75 4.91 NL-8 Nicola Lake 5 .95 5.25 6.40 (Co/Fe)xl00 o l . opx. cpx. Sample Locality JL-A Jacques Lake .186 .132 .229 JL - 3 9 " .168 .136 .230 JL - 1 0 M .162 .136 .235 JL-B .172 .122 .261 JL - 5 5 " .174 .125 .197 95 Castle Rock .161 .140 .263 S-3 " .202 .146 .284 CR-8 " .164 .141 .261 ERC-11 " .187 .152 .307 NL-8 Nicola Lake .203 .133 .272 (Mn/Fe)xl00 o l . opx. cpx. 1.24 1.71 2.52 1.24 1.77 2.69 1.15 1.79 2.62 1.21 1.80 2.67 1.30 1.77 2.64 1.25 1.86 2.57 1.14 1.81 2.70 1 .03 1.82 2.35 1.22 1.79 2.42 1.08 1.77 2.28 (Zn/Fe)xl00 o l . opx. cpx. .076 .076 .089 .071 .072 .091 . 0 8 0 .079 .094 .O63 .095 .074 .061 .065 .075 .074 .089 .111 .079 .080 .114 .066 .076 .102 . 0 8 3 .087 .098 .071 .079 .095 v6'8. cpx • Mn \ A Z n \ . o Co / \ • Ni ° D n • i • ol opx Fig. 28 : Relative proportions of Mn, Zn, Co and Ni between coexisting olivine, orthopyroxene and clinopyroxene. a .69 i s a f u n c t i o n of s i z e and s i t e energy as p r e d i c t e d by Burns ( 1 9 7 0 ) . These enrichments are i n agreement w i t h the data from other u l t r a m a f l c rocks (White 1966; Mercy and O'Hara 1967; C a r t e r 1970). The d i s t r i b u t i o n of these elements between c o e x i s t i n g o l i v i n e , orthopyroxene and clinopyroxene i s examined by means of the d i s t r i b u t i o n f u n c t i o n K T r (equation 28) where Tr = N i , Mn, Co or En, Cr i s the appropriate c a r r i e r element (Mg or Fe) and A and B are c o e x i s t i n g o l i v i n e , orthopyroxene or clinopyroxene. The r e s u l t s are given i n Table 11. The v a r i a t i o n i n each K T r i s examined by means of Snedecor's F t e s t (Table 1 2 ) . This a p p l i e s to the C a s t l e Rock and Jacques Lake s u i t e s o n l y j the N i c o l a Lake r e s u l t s are considered s e p a r a t e l y . From Table 11 i t can be seen that a l l the N i c o l a Lake K N i»s are l e s s than the corresponding K^»s f o r the other two s u i t e s and that K N 1 ( o l / o p x ) and K N 1 ( o l / c p x ) f o r the C a s t l e Rock p a i r s are l e s s than the corresponding r a t i o s of the Jacques Lake s u i t e . K^ n(cpx/opx) f o r N i c o l a Lake i s gre a t e r than the others;and f o r Jacques Lake /are- l e s s than the C a s t l e Rock c o e f f i c i e n t s . K C o (ol/opx) f o r N i c o l a Lake i s greater than the C a s t l e Rock and Jacques Lake c o e f f i c i e n t s . K ^ t o l / c p x ) f o r the Jacques Lake samples a r e l e s s than those of the C a s t l e Rock minerals. These comparisons are s i g n i f i c a n t since the value of F exceeds the l e v e l of F i n each case (Table 1 2 ) . The conslstancy of the d i s t r i b u t i o n c o e f f i c i e n t s w i t h i n each s u i t e suggests that the minerals are i n e q u i l i b r i u m w i t h respect to the elements which have been di s c u s s e d . TABLE 11 Trace element dis K N i Sample Locality ol/opx ol/cpx JL-A Jacques Lake 2 .03 1.19 JL -39 2.18 2.09 JL-10 II 2.00 1.99 JL-B II 2.02 1.91 JL-55 •1 1.97 2.38 95 Castle Rock 1.90 1.58 S-3 II 1.90 1.58 CR-8 II 1.96 1.71 ERC-11 II 1.91 1.85 NL-8 Nicola Lake 1.13 0.93 KCo Sample Locality ol/opx ol/cpx JL-A Jacques Lake 1.14 0.81 JL -39 II 1.24 0.73 JL-10 II 1.20 0.73 JL-B II 1.41 0.66 JL-55 1.39 0.88 95 Castle Rock 1.19 0.63 S-3 II 1.38 0.71 CR-8 • i 1.16 O.63 ERC-11 H 1.23 0.61 NL-8 Nicola Lake" 1.53 0.75 on coefficients. Mn opx/cpx ol/opx ol/cpx opx/cpx 0.94 0.73 0.49 0 . 6 8 0 . 9 6 0.70 0.46 0 . 6 6 1.00 0.64 0.44 0 . 6 8 0 . 9 5 O.67 0.45 0.67 1.12 0.73 0.49 0.67 0 . 8 3 O.67 0.45 0.67 0 . 8 3 0.67 0.49 0 . 6 7 0.91 0.57 0.44 0.77 0 . 9 4 0.68 0.50 0.74 0.82 0.61 0.47 0.78 KZn opx/cpx ol/opx ol/cpx opx/cpx 0.58 1.00 0.85 0 . 8 5 0.59 0 . 9 9 0.78 0.79 0.59 1.01 0.85 0.84 0.47 0.66 O.85 1.28 O.63 0 . 9 4 0.81 0.87 0 . 5 3 0 . 8 3 0.67 0.80 0.51 0 . 9 9 0.69 0.70 0.54 0.87 0.65 0.75 0 . 5 0 0.95 0.85 0 . 8 5 0 . 4 9 0 . 9 0 0.75 0 . 8 3 .71 TABLE 12 A n a l y s i s of t r a c e element variance between C a s t l e Rock and Jacques Lake s u i t e s . R a t i o Variance Between W i t h i n s u i t e s s u i t e s S i g n i f i c a n c e # (N.i/Mg)ol (Ni/Mg)opx (Ni/Mg)cpx (Mn/Pe)ol (Mn/Fe)opx (Mn/Fe)cpx (Co/Pe)ol (Co/Fe)opx (Co/Pe)cpx (Zn/Fe)ol (Zn/Pe)opx (Zn/Fe)cpx .031 .337 12.17 Not s i g n i f i c a n t .002 .093 46 .93 .918 .037 24.81 S i g n i f i c a n t a t 0,5%' .010 .040 1.76 Not s i g n i f i c a n t .010 .0014 7.14 S i g n i f i c a n t a t 5% .030 .010 3.00 Not s i g n i f i c a n t .0001 .0002 2.00 11 .0005 .00003 16.67 S i g n i f i c a n t a t 1% .0052 .0005 10.61 S i g n i f i c a n t a t 2,5% .00002 .00007 3.50 Not s i g n i f i c a n t .00007 .00009 1.28 11 S i g n i f i c a n t a t 0.5$ .0002 .00007 28 .50 K T r N i ol/opx N i ol/cpx N i opx/cpx Mn ol/opx Mn ol/cpx Mn opx/cpx Co ol/opx Co ol / c p x Co opx/cpx Zn ol/opx Zn ol/opx Zn opx/cpx •Degrees of freedom are 1 and 7 f o r between and w i t h i n s u i t e s r e s p e c t i v e l y #The s i g n i f i c a n t l e v e l s of F are 16.24, 12.25, 8.07 and 5.59 at the 0 . 5 , 1.0, 2.5 and 5.0$ l e v e l s r e s p e c t i v e l y (Snedecor and Cochran 1967). .033 .004 8.25 S i g n i f i c a n t a t 2.5$ .278 .029 9.59 S i g n i f i c a n t a t 2,5% .033 .009 3.68 Not s i g n i f i c a n t .007 .002 3.50 •1 .00005 .001 20.00 11 .006 .001 6.00 S i g n i f i c a n t a t 5% .018 .010 1.80 Not s i g n i f i c a n t .026 .006 4.80 11 .005 .002 2.32 t i .0002 .015 73.86 •1 .029 .004 6.86 S i g n i f i c a n t a t 5$ .044 .097 2.20 Not s i g n i f i c a n t .72 Although the d i s t r i b u t i o n of t r a c e elements among these minerals cannot be r e l a t e d q u a n t i t a t i v e l y to the c o n d i t i o n s of formation, i t i s c l e a r t h a t some elements are d i s t r i b u t e d d i f f e r e n t l y among these minerals and that the observed d i s t r i - b u t i o n cannot be r e l a t e d to d i f f e r e n t concentrations of tra c e or c a r r i e r element i n the minerals. For example, i t has been shown that N i i s d i s t r i b u t e d d i f f e r e n t l y between o l i v i n e and e n s t a t i t e i n the Jacques Lake and C a s t l e Rock s u i t e s , although the N i content of these minerals i n each s u i t e i s s i m i l a r . N i th e r e f o r e appears to be s e n s i t i v e to changes i n environment. On the other hand, the Co content of the Jacques Lake pyroxenes i s l e s s than those of C a s t l e Rock but the Co i s d i s t r i b u t e d between the minerals i n a s i m i l a r way. Co i s th e r e f o r e not s e n s i t i v e to changing c o n d i t i o n s (at l e a s t i n these m i n e r a l s ) . The N i c o l a Lake pyroxenes contain l e s s Mn than the other pyroxenes and i s a l s o d i s t r i b u t e d d i f f e r e n t l y between these minerals. Each nodule s u i t e i s apparently c h a r a c t e r i s e d by d i f f e r e n t t r a c e element behavior. The d i f f e r e n c e may be a d i f f e r e n c e i n the content of some elements i n the minerals or may be a d i f f e r - ence i n element d i s t r i b u t i o n among the minerals which presumeably r e f l e c t s d i f f e r e n t p h y s i c a l and chemical c o n d i t i o n s at the source of the nodules. The above d i s c u s s i o n I l l u s t r a t e s the importance of co n s i d e r i n g more than one element and p a i r of minerals when making i n f e r e n c e s on the c o n d i t i o n s of formation of. a s e r i e s of rocks. While one set of d i s t r i b u t i o n c o e f f i c i e n t s may not be s i g n i f i c a n t w i t h respect to changesof environment, the evidence of s e v e r a l sets .73 may r e v e a l that s u i t e s of s i m i l a r rocks have formed under d i f f e r e n t c o n d i t i o n s , (c) Other elements. Other t r a c e elements such as Cu, Pb, Cr and T i are not amenable to a treatment such as given to N i , Mn, Co and Zn as the p o s i t i o n of these elements i n the mineral s t r u c t u r e i s more u n c e r t a i n and because i n c o r p o r a t i o n of these elements i n the s t r u c t u r e i n v o l v e s a coupled s u b s t i t u t i o n . Cu and Pb are d i s t r i b u t e d I r r e g u l a r l y among the three s i l i c a t e s ( P i g . 2 9 ) . Because of t h i s , the d i s t r i b u t i o n of these elements i s considered no f u r t h e r . Tables 3 and 4 show tha t the co n c e n t r a t i o n of T i and Cr i s gr e a t e r i n the clinopyroxenes than i n the orthopyroxenes. This i s i n agreement wi t h the data of White ( 1 9 6 6 ) . There appears to be no systematic d i f f e r e n c e between the s u i t e s i n the Cr content of the pyroxenes. The exception to t h i s i s the N i c o l a Lake e n s t a t i t e which i s comparatively low i n Cr. On the other hand the the pyroxenes of the Jacques Lake nodules have a higher T i content than those of C a s t l e Rock. The N i c o l a Lake pyroxenes have a s i m i l a r T i content to those of Jacques Lake. The d i s t r i b u t i o n of T i and Cr between the c o e x i s t i n g pyroxenes i s examined by means of the Ne'rnst D i s t r i b u t i o n Law: T r A / T r f i = k T r (29) where Tr^ i s the concentration of a trace element i n phase j and krp r i s a constant at any pressure and temperature. In t h i s case A and B are orthopyroxene and clinopyroxene r e s p e c t i v e l y . :?4 cpx / \ • Pb / \ # C u / D \ / # \ / § \ / • D • \ / D D» \ / ° • \ / t \ * \ ol opx Fig. 29 : Relative proportions of Pb and Cu between coexisting olivine, orthopyroxene and clinopyroxene. .75 TABLE 13 Values of k T i and k C r f o r c o e x i s t i n g pyroxenes w i t h a n a l y s i s of va r i a n c e . Sample L o c a l i t y k T 1 k C r JL-A Jacques Lake 0.49 0.51 JL - 3 9 0.41 0.56 JL-10 " 0.29 0.58 JL-B - 0.27 0.41 JL - 5 5 " °» 23 °«37 95 C a s t l e Rock 0.28 0.42 y : ? 1.20 0.38 CR-8 " 0.36 0.37 ERC-11 . " 0-35 0.30 NL-8 N i c o l a Lake 0.25 0.10 Between s u i t e s .097 «031 Variance n W i t h i n s u i t e s .080 .006 With i n s u i t e s 7 7 Degrees of freedom Between s u i t e s 1 1 p , 1.21 5.17 The values f o r k T i and k C r are l i s t e d i n Table 13. A l s o given i s an a n a l y s i s of the variance i n k T r between the Jacques Lake and C a s t l e Rock s u i t e s . The N i c o l a Lake p a i r are considered separately, As can be seen both k T 1 and k C r are v a r i a b l e and there i s no d i f f e r e n c e between the s u i t e s . The exception i s the low k C r of vv the N i c o l a Lake p a i r . The reason f o r t h i s i s not known. The foregoing suggests that the d i s t r i b u t i o n of T i and Cr. between c o e x i s t i n g pyroxenes i s not s e n s i t i v e to d i f f e r e n t c o n d i t i o n s of formation. The higher T i content of the Jacques Lake pyroxenes i s probably a r e s u l t of a d i f f e r e n c e i n the T i content of the source rocks. .76 CHAPTER 8 The O r i g i n of the Nodules. (a) Temperature and Pressure. On the b a s i s of the d i s t r i b u t i o n of Iron and magnesium between c o e x i s t i n g mineral p a i r s I t has been shown that each s u i t e of nodules formed under d i f f e r e n t P/T c o n d i t i o n s . V a r i a t i o n s i n K-q r e f l e c t d i f f e r e n t temperatures of formation. Nominal temper- atures of formation are 838°C f o r the N i c o l a Lake s u i t e , 1085°C f o r the Jacques Lake s u i t e and l600°C f o r the C a s t l e Rock s u i t e . While there i s some doubt as to the absolute temperatures i t i s b e l i e v e d that each s u i t e formed at d i f f e r e n t temperatures and that the r e l a t i v e temperatures are c o r r e c t (Chapter 6 ) . The e f f e c t s of pressure on are unknown ,but: ;ltils b e l i e v e d that d i f f e r e n t pressures are a l s o r e s p o n s i b l e f o r v a r i a t i o n s i n the d i s t r i b u t i o n c o e f f i c i e n t s . This may be one reason f o r the comparatively high C a s t l e Rock temperatures; the e f f e c t s of pressure were not taken i n t o account i n the c a l c u l a t i o n of the temperatures. A l s u b s t i t u t i o n i n pyroxenes a f f e c t s the values of K Q * S I n v o l v i n g pyroxenes (Chapter 6 ) , but u n f o r t u n a t e l y q u a n t i - t a t i v e estimates of pressure cannot be made on the b a s i s of v a r i a t i o n s i n Kp. A l occurs i n both s i x f o l d and f o u r f o l d c o - o r d i n a t i o n i n the pyroxene s t r u c t u r e . This i s a r e s u l t of the requirements of charge balance. The appropriate s u b s l t u t i o n s are A l * ^ + A 1 V * f o r Mg V I + S i I V and A 1 V I + N a V I 1 1 f o r 2Mg V I. I f A l occurs i n octahedral and t e t r a h e d r a l c o - o r d i n a t i o n , then a Tschermak's component w i l l appear, i n an end-member c a l c u l a t i o n . Since only p a r t i a l chemical .77 analyses were done, the amount of Tschermak's component can only be estimated. This was done by assuming that the weight percent of S i 0 2 = 100 - j> R x 0 y where R x 0 y i s the weight percent of any oxide. The c a l c u l a t i o n of the end-members was done by computer using the program PYREND (U.B.C. Dept. of Geology program; P.B. Read). The c a l c u l a t i o n of S i 0 2 by d i f f e r e n c e and the f a c t that a l l i r o n was assumed to be i n the fe r r o u s s t a t e l i m i t s the accuracy .:: of the r e s u l t s . The e r r o r i n e s t i m a t i n g S i 0 2 w i l l be the sum of a l l the e r r o r s i n the determined oxides. Because of t h i s only the r e s u l t s of the c a l c u l a t i o n of the Tschermak's components are given (Tables 3 and 4 ) , since these are the most s i g n i f i c a n t w i t h respect to pr e s s u r e h ( s e e below). CaTs i s a component of a l l the clinopyroxenes. There appears to be no d i f f e r e n c e between the s u i t e s i n the amount of CaTs i n these pyroxenes. Both CaTs and MgTs are components of the ortho- pyroxenes, except i n the N i c o l a Lake ens-tafelt.e.,. F i g . 30 shows the proportions of CaTs, MgTs, diopside and enstati'tB In the pyroxenes. Since CaTs and MgTs are present i n s o l u t i o n (except f o r N i c o l a Lake) i n dio p s i d e and e n s t a t i t e r e s p e c t i v e l y i t i s reasonable to suggest that these minerals formed a t high pressure(Boyd 1963; Kushiro and Yoder 1966; Kushiro 1 9 6 9 a ) . The s t a b i l i t i e s of A l - d i o p s i d e and A l - e n s t a t i t e c o e x i s t i n g w i t h s p i n e l and o l i v i n e are shown In F i g . 32. Because the CaTs s o l u b i l i t y i n dio p s i d e I s complexly r e l a t e d to temperature, pressure, the amount o f ^ J a d e i t e i n the dio p s i d e and the nature and composition of the c o e x i s t i n g phases, i t i s not p o s s i b l e to make p r e c i s e estimates of the pressures a t which MgTs CaTs pyrope _^ grossular | Enstatite MgSi0 3 O Diopside CaSi0 3 Fig. 30 •. Composition of analysed, pyroxenes in terms of MgSi0 3- CaSiOj- AlgOj. opx. f + Jacques Lake, o Castle Rock. c p x . • Nicola Lake. A Jacques Lake, o Castle Rock. > Nicola Lake. . 79 the d i o p s i d e s formed,(Kushiro 1 9 6 9 a ) . Boyd and England ( i 9 6 0 ) and Boyd (1963) have shown that the AI2O3 content of e n s t a t i t e c o e x i s t i n g w i t h o l i v i n e and garnet increases w i t h i n c r e a s i n g temperature and pressure due to the coupled s u b s t i t u t i o n 2A1 f o r (Mg + S i ) . S i m i l a r v a r i a t i o n s might be expected f o r e n s t a t i t e c o e x i s t i n g w i t h o l i v i n e and s p i n e l . The A^O^ content of the analysed e n s t a t i t e s v a r i e s with that of the s p i n e l s and a l s o between s p i n e l and dio p s i d e ( F i g . 3 D * This suggests that the d i s t r i b u t i o n of AI2O3 between s p i n e l and pyroxene may be a f u n c t i o n of pressure and temperature as i s the case f o r c o - e x i s t i n g pyroxene and garnet. Unfortunately there are no experimental data to confirm t h i s . The temperatures d e r i v e d fr-omtthe d i s t r i b u t i o n of i r o n and magnesium between c o e x i s t i n g o l i v i n e and s p i n e l , and the high pressures i n f e r r e d from the AI2O3 content of the pyroxenes are co n s i s t e n t w i t h the experimentally determined f i e l d of s p i n e l l h e r z o l i t e . The s t a b i l i t y f i e l d s of v a r i o u s u l t r a m a f l c mineral assemblages are shown i n F i g . 321 taken from Green and Ringwood ( 1 9 7 0 ) . S p i n e l l h e r z o l i t e has a f a i r l y wide s t a b i l i t y f i e l d which f a l l s w i t h i n upper mantle c o n d i t i o n s . On the low pressure side of the l h e r z o l i t e s t a b i l i t y f i e l d p l a g l o c l a s e i s s t a b l e and on the high pressure s i d e garnet appears. The upper temperature l i m i t of the s t a b i l i t y of l h e r z o l i t e i s , of course, the l h e r z o l i t e s o l i d u s . Many experimental s t u d i e s have been c a r r i e d out to determine the p o s i t i o n of these boundary curves. Reactions between o l i v i n e and p l a g l o c l a s e to y i e l d aluminous pyroxenes and s p i n e l have been i n v e s t i g a t e d by Kushiro and Yoder ( 1 9 6 6 ) . Such a r e a c t i o n i s : 7 0 6 0 H CL r so 4 0 ^ 3 0 7 0 60 Q . 50 4 0 3 0 O ,.80 O A A O O % A l 2 0 3 cpx. A A O O O O 4 5 6 % A l 2 0 3 opx. 7 7 Fig. 31 : Variation of A l 2 0 3 between spinel and pyroxenes. A Jacques Lake : o Castle Rock •. • Nicola Lake. 8 a .81 50 Depth in Km. IOO 150 2 0 0 O lO 2 0 3 0 4 0 5 0 6 0 7 0 Pressure in Kb. Fig. 32 : Relative stabilities of various ultramafic mineral assemblages. (after Green and Ringwood 1970 ) .82 f o r s t e r l t e + a h o r t h i t e = A l - d i o p s i d e + A l - e n s t a t i t e + s p i n e l The exact p o s i t i o n of the curve i n n a t u r a l systems i s dependant on the compositions of the r e a c t i n g phases which may vary. The r e l a t i v e s t a b i l i t i e s of p l a g l o c l a s e and s p i n e l bearing p e r i d o t i t e s is,however,as shown i n F i g . 32. Reactions d e f i n i n g the breakdown of s p i n e l and the incoming of garnet are: e n s t a t i t e -f s p i n e l = f o r s t e r l t e + pyrope and d i o p s i d e + s p i n e l = f o r s t e r l t e + g r o s s u l a r (McGregor I967). There i s no general agreement on the exact p o s i t i o n of the, boundary curve (Green and Ringwood 1970). McGregor (1970) has shown that't h e s t a b i l i t i e s of s p i n e l and garnet bea r i n g p e r i d o t i t e s i s s t r o n g l y dependant on the Cr20^yR2'C>3 r a t i o of the rock. >Spinel p e r i d o t i t e s w i t h a high Cr2C>3 J&203 r a t ^ ° a r e s t a b l e at higher pressures than those w i t h a low 0X20^*112^3 r a t i o . I t i s to be expected that s i m i l a r v a r i a t i o n s w i l l a l s o a f f e c t the lower l i m i t s of the s t a b i l i t y of s p i n e l l h e r z o l i t e . At near s o l i d u s temperatures s p i n e l l h e r z o l i t e s are s t a b l e to about 23Kb. depending on the r a t i o of t r i v a l e n t oxides (McGregor 1968, 1970). Despite the u n c e r t a i n t i e s i n the experimental data, the mineral assemblages and p a r t i a l mineral analyses suggest that the nodules formed i n the upper mantle where at near s o l i d u s temperatures, the pressure would be between 11 and 23 Kb. (equivalent to a depth between 35 and 75 Km.) ( F i g . 32). Each nodule s u i t e apparently formed under d i f f e r e n t c o n d i t i o n s w i t h i n the mantle. . 8 3 Of the three s u i t e s , the N i c o l a Lake nodules formed a t the lowest temperature and probably pressure. The e n s t a t i t e of the noduleo from t h i s s u i t e contains no MgTs (Table 3) suggesting that most of the A l i s i n f o u r f o l d c o - o r d i n a t i o n and that t h i s zxA? nodule formed at a lower pressure than those of the other s u i t e s * (Boyd 1 9 6 3 ) . The Jacques Lake nodules formed a t temperatures w e l l below those of C a s t l e Rock and s l i g h t l y above the N i c o l a Lake nodules (Table 8 ) . The r e l a t i v e pressures are not so c e r t a i n . The Jacques Lake pyroxenes contain more A^O-^ than those of C a s t l e Rock ( F i g . 3 D but whether t h i s i s due to temperature, pressure or bulk composition i s not known, A higher temperature does not necessar- i l y imply a higher pressure as the geothermal gradient i n the C a s t l e Rock r e g i o n may be steeper than i n the Jacques Lake r e g i o n . However i t i s b e l i e v e d that the pressures at which the two s u i t e s of nodules formed were not the same. The b a s i s of t h i s i n f e r e n c e i s the d i f f e r e n t d i s t r i b u t i o n s of i r o n and magnesium between o l i v i n e and orthopyroxene f o r the two s u i t e s (Chapter 6 ) . Supporting t h i s i s the d i f f e r e n t o l i v i n e f a b r i c s i n the s u i t e s (Chapter 4 ) . The C a s t l e Rock f a b r i c may be due to r e l a t i v e l y high pressure and temperature (Ave'Lallement and C a r t e r 1970) but t h i s r e q u i r e s experimental c o n f i r m a t i o n , (b) The nature of the source. The preceding d i s c u s s i o n has.shown that the l h e r z o l i t e nodules probably o r i g i n a t e d i n the upper mantle. I t i s now necessary to decide which aspect of the mantle they represent. Two hypotheses .84 are considered. (a) They are c r y s t a l cumulates which p r e c i p i t a t e d from t h e i r present host rocks. (b) They are fragments of the mantle which may have been depleted by p a r t i a l m e l t i n g . I f a cognate o r i g i n i s proposed, i t would be expected that there would be a wide range i n the mineral proportions and compositions among a s u i t e of l h e r z o l i t e nodules. This i s not the case f o r the C a s t l e Rock or Jacques Lake s u i t e s . The range of compositions of the N i c o l a Lake s u i t e i s unknown. White (1966) and Kuno (1969) have shown that d u n i t e , w e h r l i t e and gabbro nodules have a wide compositional range and that the compositional trends of these nodules i s d i s t i n c t from those of l h e r z o l i t e nodules. The v a r i a t i o n i n the w e h r l i t e nodule s e r i e s i s thought to be due to c r y s t a l s e t t l i n g from a b a s a l t i c magma at depth. The narrow compositional range of l h e r z o l i t e nodules i s due to t h e i r being fragments of the mantle. Binns (1969) has found both l h e r z o l i t e nodules and megacrysts of undoubted cognate o r i g i n i n the same l a v a flow. These mega- c r y s t s which may occur as clusters-, i n c l u d e o l i v i n e , clinopyroxene, orthopyroxene and s p i n e l and are q u i t e d i s t i n c t c h e mically (they are l e s s magnesian) from the minerals of the l h e r z o l i t e s and «gf% appear to have o r i g i n a t e d at depth. I t can be argued i n t h i s case that the l h e r z o l i t e s are not cognate but are r e s i d u a l mantle m a t e r i a l and that the megacrysts represent the e a r l i e s t c r y s t a l f r a c t i o n of a b a s a l t i c magma produced i n the upper mantle. K u t o l i n and P r o l o v a (1970) and Aoki and Kushiro (1968) have examined .85 s i m i l a r m a t e r i a l and have come to the same c o n c l u s i o n . The p e t r o f a b r i c study has shown that the C a s t l e Rock and Jacques Lake nodules have been deformed i n the s o l i d s t a t e . I f these nodules are cumulates, then the f o l l o w i n g sequence of events might have taken place. P a r t i a l m e l t i n g i n the mantle occured. The l i q u i d which was produced remained at depth while c r y s t a l f r a c t i o n a t i o n took place. The cumulates which formed were then deformed before being brought to the surf a c e . I f t h i s process d i d take p l a c e , i t would r e q u i r e quiescent c o n d i t i o n s i n the upper mantle or lower c r u s t to a l l o w a c r y s t a l p i l e to accumulate. This i s c ontrary to the c o n d i t i o n s i n f e r r e d from the o l i v i n e f a b r i c s , and a l s o contrary to c o n d i t i o n s expected dur i n g p a r t i a l f u s i o n . On the other hand, a simple two-step process whereby f r a c t i o n a l m e l t i n g and ascent of the r e s u l t a n t l i q u i d brought up fragments of the r e s i d u a l mantle rock would s a t i s f y the requirement that the nodules have been deformed. The f a b r i c I s then due to processes which operated p r i o r to or perhaps dur i n g p a r t i a l m e l t i n g . I t i s p o s s i b l e that p a r t i a l m e l t i n g could r e s u l t i n the production of a b a s a l t which remained at depth while p r e c i p i t a t - i n g c r y s t a l s and forming a l h e r z o l i t e . The magma could then have been removed and the l h e r z o l i t e deformed. A second episode of m e l t i n g could then have occured and fragments of the l h e r z o l i t e caught up i n the r e s u l t a n t b a s a l t which brought them to the su r f a c e . C a r t e r (1970) has examined l h e r z o l i t e s and other u l t r a m a f l c nodules from a s i n g l e l o c a l i t y . He found that those l h e r z o l i t e s ( " t y p i c a l 4-phase nodules") which have a mode cl o s e to the o l i v i n e apex of the olivine-orthopyroxene-cllnopyroxene diagram ( F i g . 3) g e n e r a l l y have o l i v i n e w i t h a composition more magneslan than Fo3g. Other l h e r z o l i t e s ( " a t y p i c a l 4-phase nodules") f a l l i n the c e n t r a l part of the diagram and have o l i v i n e s l e s s magneslan than Fog^.These l h e r z o l i t e s appear to be undeformed and have cumulate t e x t u r e s , as opposed to the t y p i c a l l h e r z o l i t e s which appear to r be deformed and r e c r y s a l l i s e d . C a r t e r (1970) proposes a model whereby a t y p i c a l l h e r z o l i t e s (and a l s o w e h r l i t e s and pyroxenltes) are among the cumulates formed at depth from a b a s a l t i c magma. T y p i c a l l h e r z o l i t e s are probably r e s i d u a l products of p a r t i a l m e l t i n g , although i n some cases an o r i g i n by accumulation cannot be discounted. This i s s i m i l a r to the model proposed by White ( 1 9 6 6 ) . C a r t e r ' s (1970) modellisbas;€d on experimental work by Kushiro (1969b) and on an a n a l y s i s of p o s s i b l e c r y s t a l - l i q u i d paths dur i n g p a r t i a l f u s i o n according to the methods of P r e s n a l l ( 1 9 6 9 ) . White's (1966) model i s based on petrography and mineral chemistry. Using the above model the' l h e r z o l i t e s of t h i s study f a l l i n t o the category of " t y p i c a l 4-phase nodules" (Table 2, F i g . 3 )• No w e h r l i t e s or pyroxenltes were found w i t h the Jacques Lake l h e r z o l i t e s . A comparison w i t h the above model and w i t h work by Aoki and Kushiro ( 1 9 6 8 ) , Kuno ( 1 9 6 9 ) , Blnns (1969) and K u t o l i n and F r o l o v a (1970) mentioned above suggests t h a t the Jacques Lake nodules were not part of a cumulate s e r i e s p r i o r to t h e i r i n c o r p - o r a t i o n i n t o the Jacques Lake t u f f . The f u l l range of nodule types at C a s t l e Rock i s not known but since a l l the nodules which were st u d i e d f a l l i n t o the category of " t y p i c a l 4-phase nodule" and appear to have been deformed, a n t o r i g i n by accumulation i s discounted. .87 On the b a s i s of the mode (Table 1) and the composition of the o l i v i n e ( F o ^ ^ > 2 ) specimen NL-8 from N i c o l a Lake i s a " t y p i c a l " 4-phase nodule". The modes of the other nodules from t h i s s u i t e are v a r i a b l e ( F i g . 3) although the f u l l range i s not known. The o l i v i n e f a b r i c of t h i s nodule and the textures of t h i s and other nodules from t h i s s u i t e suggest that these nodules have formed by accumulation. The temperature at which the N i c o l a Lake nodules have i n f e r r e d to have formed (nominal temperature i s 838°C) i s below any known l h e r z o l i t e s o l i d u s , even under hydrous c o n d i t i o n s ( F i g . 3 3 ) . The weight of the evidence t h e r e f o r e favours an o r i g i n by c r y s t a l s e t t l i n g and accumulation at depth f o r t h i s s u i t e . The nodules may be cognate w i t h t h e i r e n c l o s i n g b a s a l t . One e s s e n t i a l c h a r a c t e r i s t i c of a pa r e n t a l mantle rock i s that i t i s capable of producing a b a s a l t on p a r t i a l m elting at high pressure. There are two ways to consider t h i s p r o p o s i t i o n . F i r s t l y , d i r e c t experiments on the melting behavior of p o s s i b l e mantle rocks ( n a t u r a l or s y n t h e t i c ) can be made. Secondly, mixtures of b a s a l t plus r e f r a c t o r y residuum can be examined to determine .i;:-;^-;' whether these p a i r s r e s u l t i n a proposed mantle composition. D i r e c t m e l t i n g experiments on s p i n e l l h e r z o l i t e have been c a r r i e d out by Kushiro e t . a l . (1968) and by Nishikawa e t . a l . ( 1 9 7 0 ) . The r e s u l t s of these s t u d i e s and a l s o of work by Kushiro (1969b) i n d i c a t e that a s i l i c a undersaturated magma can be produced by p a r t i a l m elting of a s p i n e l l h e r z o l i t e . The type of magma which i s produced depends on the degree of p a r t i a l m e l t i n g , on the presence or absence of water, and a l s o the pressure at which me l t i n g took .88 pl a c e . The residue from such m e l t i n g may be a more magnesian l h e r z o l i t e , a h a r z b u r g i t e or a d u n i t e . These may be produced under a wide pressure and temperature range and under both hydrous and anhydrous c o n d i t i o n s . In additiom to the above experimental s t u d i e s , work on the chemical r e l a t i o n s h i p s between b a s a l t and u l t r a m a f i c nodules has been done by Kuno ( 1 9 6 9 ) , Kuno and Aokl (1970) and Jackson and Wright ( 1 9 7 0 ) . They found that a b a s a l t close t o the composition of an o l i v i n e t h o l e i i t e could be produced from a pyroxene r i c h l h e r z o l i t e , l e a v i n g a more magnesian l h e r z o l i t e as r e s i d u a l m a t e r i a l . Green and Rlngwood (1969) argue that a l k a l i o l i v i n e b a s a l t can be produced by a low degree {^20%) of p a r t i a l m e l t i n g of l h e r z o l i t e at depths of 35-70Km. With an i n c r e a s i n g degree of p a r t i a l m e l t i n g , o l i v i n e t h o l e i i t e can be produced. The above summary of recent work on b a s a l t - l h e r z b l i t e r e l a t i o n - ships shows that l h e r z o l i t e may be r e f r a c t o r y r e s i d u a l m a t e r i a l l e f t a f t e r p a r t i a l m e l t i n g of primary mantle, which may i t s e l f be l h e r z o l i t e (Kuno and Aoki 1970) or a mixture of l h e r z o l i t e and garnet p e r i d o t i t e (Jackson and Wright 1970). Thus l h e r z o l i t e nodules ( i n c l u d i n g the nodules of t h i s study) could be r e s i d u a l fragments of p a r t i a l l y melted mantle. L h e r z o l i t e nodules are not g e n e r a l l y considered to be parts of the primary mantle s i n c e they are too low i n c e r t a i n elements ( K , T i , P, Ba,Sr,Rb,Th,U and others) to produce b a s a l t on p a r t i a l m e l t i n g ( H a r r i s e t . a l . 1967; Green and Ringwood 1 9 6 9 ) . The low K 2 0 content of the minerals of these nodules supports t h i s (Tables 3 and 4 ) . .89 No chemical data on the enc l o s i n g rocks of the nodules In t h i s study are a v a i a b l e . Nevertheless, i n view of the c l o s e s i m i l a r i t i e s i n mineralogy, t e x t u r e and mineral compositions of these nodules to nodules which have been stud i e d i n r e l a t i o n to t h e i r host rock chemistry, i t i s reasonable to suggest that these rocks are fragments of the mantle. A worldwide s i m i l a r i t y of l h e r z o l i t e nodules, r e g a r d l e s s of the nature of the e n c l o s i n g r ocks, i s an argument against l h e r z o l i t e i m d d u l e s In general being cumulates. The temperatures at which theSCastle Rock and Jacques Lake nodules are; tnought^td have: formed are c o n s i s t e n t w i t h the hypothesis that the C a s t l e Rock and Jacques Lake l h e r z o l i t e s are r e s i d u a l fragments of the upper mantle. The Jacques Lake nodules appear to have formed at 1085°C. This temperature i s c l o s e to the s o l i d u s temperature of l h e r z o l i t e at high pressure where PJ J 2 Q < ]?total and above the s o l i d u s temperature of l h e r z o l i t e where P H 2 ° = P t o t a l a t h i g h P r e s s u r e (Kushiro e t . a l 1968) ( F i g . 3 3 ) . While the above temperature could be a p p l i e d to the argument that the Jacques Lake nodules formed at a b a s a l t l l q u i d u s temperature, the same cannot be s a i d f o r the C a s t l e Rock s u i t e . The c a l c u l a t e d temperatures of formation of these nodules are greater than l600°C. This i s g r e a t e r than any l h e r z o l i t e s o l i d u s at pressures where s p i n e l i s s t a b l e , even under anhydrous c o n d i t i o n s (Kushiro e t . a l . 1968; Nishikawa e t . a l . 1970). While there i s some doubt as to the absolute temperatures a t which the nodules formed, the r e l a t i v e temperatures are b e l i e v e d to be c o r r e c t (p . 6 0 ) so that these nodules appear to b e f r e f r a c t o r y and may be residue from p a r t i a l f u s i o n of the mantle. T e x t u r a l evidence described i n Chapter 3 suggests that the above 2 0 0 0 1 o I O O O • o_ e a> 5 0 0 IO 2 0 3 0 4 0 5 0 Pressure in Kb. Various lherzolite solidi. A •. Anhydrous \ B : PH 2O < piotai ( Kushiro et.al. I968 ) C : PH 20 = Ptotal J D: Anhydrous ( Nishikawa et.al. I970 ) O Fig. 3 3 : .91 d i s c u s s i o n i s reasonable. I t has been suggested that the marginal a l t e r a t i o n of the dio p s i d e s of the C a s t l e Rock and Jacques Lake nodules ( F i g . 8) i s due to the reactions j a d e i t l c d i o p s i d e —> ja d e i t e - p o o r d i o p s i d e + f e l d s p a r I f the r e a c t i o n i s of t h i s form, then another phase must have p a r t i c i p a t e d ( e i t h e r as a reactant or a c a t a l y s t ) s i n c e the r e a c t i o n occurs only at the rims of the d i o p s i d e s . E n c l o s i n g minerals such as o l i v i n e are unaffected ( F i g . 10). A f l u i d phase could have been present, but i f a f l u i d was present and p a r t i c i p a t e d i n the r e a c t i o n , then c o e x i s t i n g orthopyroxene should a l s o have > been a f f e c t e d s i n c e i t has been shown that orthopyroxene i s unstable at thermargin of the nodules. C o e x i s t i n g o l i v i n e and e n s t a t i t e are unaffected by any r e a c t i o n ( F i g , 1 0 ) , Therefore the breakdown of d i o p s i d e i s not due to r e a c t i o n w i t h a f l u i d phase and cannot be a polymorphic change due to v a r i a t i o n In the pressure and temperature s i n c e r e a c t i o n occurs only at the rims of the d i o p s i d e s . One explanation of t h i s i s that i t i s due to p a r t i a l m e l t i n g . I t i s p o s s i b l e t h a t the l i q u i d produced by p a r t i a l m e l t i n g of l h e r z o l i t e was trapped and quenched when the nodules were brought to the surfa c e . The gl a s s then d e v i t r i f l e d to form f e l d s p a r ( i n part a t l e a s t ) . Diopside i s the f i r s t phase to melt i n a l h e r z o l i t e of l i k e l y mantle composition (Kushiro 1969b; I t o and Kennedy 1 9 6 7 ) . Dickey e t . a l . (1971) have found that Cr-bearing d i o p s i d e melts incongr- u e n t l y to s p i n e l and l i q u i d above 5Kb. The e f f e c t s of A 1 2 0 3 , Cr20<j and other components on melting behavior at high pressure i n the system Di-Fo-S10 2 (a s i m p l i f i e d p e r i d o t i t e system) which .92 was stu d i e d by Kushiro (1969b) are not known but i n view of the behavior of Cr-bearing d i o p s i d e (Dickey e t . a l . 1971) they are l i k e l y to be s i g n i f i c a n t . I t i s not necessary f o r the minimum me l t i n g point of l h e r z o l i t e to be a e u t e c t i c . I n the s i m p l i f i e d p e r i d o t i t e system i t i s hot c e r t a i n whether point P (Fig.3*0 i s a e u t e c t i c or a r e a c t i o n point.cEdiptiAtOB* the Di'-Fo j o i n i s a p i e r c i n g point (Fig.34) (Kushiro 1 9 6 9 b ) . Thus mel t i n g of d i o p s i d e i n a n a t u r a l system i s a p o s s i b l e explanation of the described t e x t u r e , • • . The weight of the evidence favours the hypothesis that the source of the C a s t l e Rock and Jacques Lake nodules i s the upper mantle and that these nodules could be fragments of the mantle which has been depleted by p a r t i a l m e l t i n g . There are two l i n e s of evidence to suggest that the source of the C a s t l e Rock nodules i s l a y e r e d . The m a j o r i t y of the samples a v a i l a b l e f o r study show some m i n e r a l o g l c a l l a y e r i n g . The" o l i v i n e f a b r i c of one of the l a y e r e d specimens appears t o be r e l a t e d to the l a y e r i n g ( F i g . 2 0 ) . The massive specimens have a s i m i l a r f a b r i c which suggests that the mechanism which produced the l a y e r i n g was operative throughout the source rock, even though some of hand specimen s i z e d nodules are apparently unlayered. Mesoscopic l a y e r i n g i s t h e r e f o r e probably c h a r a c t e r i s t i c of the source of the C a s t l e Rock nodules. On the other hand, the source rock of the Jacques Lake nodules i s u n l i k e l y to be l a y e r e d , a t l e a s t on a mesoscopic s c a l e . Only a few of s e v e r a l hundred specimens observed i n the f i e l d were l a y e r e d . The o l i v i n e f a b r i c of the nodules which were st u d i e d i s d i f f e r e n t from the C a s t l e Rock f a b r i c s or other l a y e r e d u l t r a m a f l c rocks (Chapter 4 ) . CaMgSi206 CaMgSi206 Fig. 34 : Part of the liquidus diagram of the system Fo-Di-SiC>2 at 20Kb. pressure j (a) hydrous , (b) anhydrous. (after Kushiro 1969b) . .94 CHAPTER 9 The Upper Mantle In B r i t i s h Columbia. I t has been shown that the l h e r z o l i t e nodules of C a s t l e Rock and Jacques Lake are probably samples of the upper mantle. The d i f f e r e n c e s and s i m i l a r i t i e s between these s u i t e s of rocks are s i g n i f i c a n t w i t h respect to the c o n s t i t u t i o n of the upper mantle i n B r i t i s h Columbia. While much of the f o l l o w i n g d i s c u s s i o n i s s p e c u l a t i v e , i t i s nevertheless u s e f u l as a guide to what might be expected upon f u r t h e r study of the mantle i n B r i t i s h Columbia. The l i m i t e d evidence from t h i s study suggests that the upper mantle i n B r i t i s h Columbia c o n s i s t s l a r g e l y of s p i n e l l h e r z o l i t e . There i s no evidence of r e g i o n a l d i f f e r e n c e s i n mineralogy nor of m i n e r a l o g i c a l zoning. There i s evidence of chemical v a r i a t i o n s . The C a s t l e Rock pyroxenes are more magnesian and the s p i n e l s and pyroxenes are more aluminous than the corresponding minerals of the Jacques Lake nodules. T h i s may be a r e s u l t of a fundamental d i f f e r e n c e i n the chemistry of the mantle i n these.- areas or may be a r e s u l t of d i f f e r e n t degrees of p a r t i a l m e l t i n g . The C a s t l e Rock nodules are more r e f r a c t o r y so i f the upper mantle i n B r i t i s h Columbia was o r i g i n a l l y homogeneous,different degrees of p a r t i a l m e l t i n g have r e s u l t e d i n the d i f f e r e n c e s i n mineral chemistry. As w e l l as major element v a r i a t i o n s there are v a r i a t i o n s i n the t r a c e element concentrations i n the two areas. Both C a s t l e Rock pyroxenes c o n t a i n more Co^p'theediopsides c o n t a i n more N i and Zn, and the e n s t a t i t e s more Mn than the corresponing minerals of the Jacques Lake nodules. The upper mantle beneath C a s t l e Rock .95 appears to be enriched i n these elements r e l a t i v e t o the mantle beneath Jacques Lake. This enrichment I s probably a primary feature of the mantle i n the C a s t l e Rock reg i o n since the concen t r a t i o n of these elements i s independent of major element v a r i a t i o n s . F u r t h e r sampling of nodules and a l s o of the b a s a l t i c rocks of each area i s r e q u i r e d to support t h i s suggestion. Analyses f o r more mobile elements such as Rb, Sr and the r a r e earths would be us e f u l to t e s t the hypothesis'; that geochenical provinces e x i s t i n the upper mantle i n B r i t i s h Columbia.- Data on these elements might a l l o w one to evaluate the extent of p a r t i a l m e l t i n g and so d i s t i n g u i s h primary v a r i a t i o n s i n the chemistry of the mantle from v a r i a t i o n s due to d i f f e r e n t degrees of p a r t i a l m e l t i n g . P h y s i c a l as w e l l as chemical v a r i a t i o n s e x i s t i n the upper mantle i n ' . B r i t i s h Columbia. I t has been shown that the o l i v i n e : f a b r i c s of the C a s t l e Rock and Jacques Lake nodules have r e s u l t e d from deformation i n the s o l i d s t a t e , and that the s u i t e s have ->r* d i f f e r e n t c h a r a c t e r i s t i c f a b r i c s . The f a b r i c s are considered to have been imposed on the rocks p r i o r to t h e i r I n c l u s i o n i n t h e i r e n c l o s i n g rocks and are a r e s u l t of s t r e s s w i t h i n the mantle. Hess (1964) showed that seismic anisotropy i n the upper oceanic mantle i s caused by the alignment of o l i v i n e i n the d i r e c t i o n of flow at major f r a c t u r e zones. Keen and B a r r e t t (1971) found that the mantle west of the Queen C h a r l o t t e I s l a n d s has an anis o t r o p y r e l a t e d to the d i r e c t i o n of sea f l o o r spreading ( F i g . 35). I t i s p o s s i b l e that t h i s a n isotropy extends i n t o the mantle beneath B r i t i s h Columbia. Both the C a s t l e Rock and Jacques Lake .96 o l i v i n e s have a strong degree of prefered o r i e n t a t i o n suggesting that both regions are u n d e r l a i n by an a n i s o t r o p i c mantle. Each s u i t e has a d i f f e r e n t f a b r i c which suggests t h a t the degeee of ani s o t r o p y i s not the same i n each area. A l s o , t h e o r i e n t a t i o n of the o l i v i n e f a b r i c s i n the mantle and consequently the seismic v e l o c i t y v e c t o r s i n each area are l i k e l y to be d i f f e r e n t . Souther (1970) has suggested that the r e g i o n a l d i s t r i b u t i o n of Quaternary volcanoes i n B r i t i s h Columbia i s c o n t r o l l e d by major f a u l t s a s s o c i a t e d w i t h mantle structures>, ( P i g . 3*0 • I f the o r i e n t a t i o n of o l i v i n e i n the mantle i s a s s o c i a t e d with f r a c t u r e zones i n the mantle, then d i f f e r e n t s t r e s s regimes a s s o c i a t e d w i t h the two regions w i l l produce d i f f e r e n t f a b r i c s i n the o l i v i n e s and w i l l r e s u l t i n varyingltdegrees of a n i s o t r o p y . C a s t l e Rock appears to l i e i n a zone of extension and Jacques Lake appears to l i e i n a zone of shear above the mantle. (Fig.35). This might r e s u l t i n d i f f e r e n t f a b r i c s being produced i n the o l i v i n e s i n the mantle i n these areas. Other f a c t o r s such as s t r a i n r a t e , temperature, pressure and the presence or absence of water w i l l a l s o c o n t r i b u t e to the type of f a b r i c developed by a p a r t i c u l a r s u i t e of rocks. Which of these i s dominant i s not known. Whatever the cause, i t i s l i k e l y that v a r i a t i o n s i n the degree of an i s o t r o p y and consequently i n the seismic behavior of the mantle i n B r i t i s h Columbia are t o be expected and t h a t these may e v e n t u a l l y be i n t e g r a t e d i n t o the r e g i o n a l t e c t o n i c framework. Fig. 3 5 Major structural features related to recent volcanism in British Columbia, (after Souther 1970) Belts of Quaternary volcanoes, j Eastern limit of Tertiary and Recent transcurrent ' faulting (right lateral shear). Direction of inferred relative motion between adjacent segments of the Cordillera. A Area of mantle anisotropy. ( Keen and Barret 1971) .98 CHAPTER 10• Conclusions. The main f i n d i n g of t h i s study i s that each s u i t e of l h e r z o l i t e nodules i n b a s a l t i c rocks from B r i t i s h Columbia i s c h a r a c t e r i s e d by i t s own range of mineral compositions and f a b r i c s . The range of compositions f o r each s u i t e i s narrow and overlap to some extent. The d i s t r i b u t i o n of some elements (eg. Fe, Ni,Mn,Co,Zn) between the minerals of each s u i t e i s d i f f e r e n t and i s independent of mineral composition. The s i g n i f i c a n c e of t h i s i s that each s u i t e probably formed under d i f f e r e n t P/T c o n d i t i o n s . Comparison w i t h other s t u d i e s and wit h r e l e v a n t experimental work places the source of these nodules i n the upper mantle. This agrees w i t h the P/T c o n d i t i o n s i n f e r r e d from the mineral chemistry of these nodules. C o n s i d e r a t i o n of the te x t u r e s and o l i v i n e f a b r i c s of the C a s t l e Rock nodules suggests that these rocks are fragments of the r e f r a c t o r y upper mantle which i s l a y e r e d and has been deformed. The Jacques Lake nodules are a l s o r e s i d u a l fragments of the mantle which has been deformed but i n the Jacques Lake area I s uhlayered. The mineral chemistry of the nodules has shown that the C a s t l e Rock l h e r z o l i t e s formed at higher temperatures and probably at gr e a t e r depths than those of Jacques Lake. The N i c o l a Lake nodules are c r y s t a l accumulates, have not been deformed, and formed a t lower pressures and temperatures than the other nodules, They could be cognate w i t h t h e i r present host rocks. The d i f f e r e n t chemical c h a r a c t e r i s t i c s of the C a s t l e Rock and .99 Jacques Lake nodules suggest that the mantle i s chemically d i f f e r e n t i n these r e g i o n s . This may be a r e s u l t of d i f f e r e n t degrees of p a r t i a l melting of an o r i g i n a l l y homogeneous mantle but may a l s o be a r e s u l t of o r i g i n a l heterogeneity The strong prefered o r i e n t a t i o n of the o l i v i n e of the C a s t l e Rock l h e r z o l i t e s and, to a l e s s e r extent, those of Jacques Lake suggests that the mantle i n these two regions i s a n i s o t r o p i c . The d i f f e r e n t f a b r i c types suggest that d i f f e r e n t deformational regimes are to be found from place t o place i n the mantle. .100 BIBLIOGRAPHY Aokl,K. ( 1 9 6 8 ) . Petrogenesls of u l t r a b a s i c and b a s i c i n c l u s i o n s i n a l k a l i b a s a l t s , I k i I s l a n d , Japan. Am. Miner. 53, 241 -256. Aoki.K. and K u s h i r o , I . ( 1 9 6 8 ) . Some clinopyroxenes from u l t r a - mafic I n c l u s i o n s In D r e i s e r Weiher, E l f e l . Contr. Miner. P e t r o l . 18, 326-337. Ave'Lallement,H. and Carter,N.L. 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The c o n s t i t u e n t minerals of each nodule were separated by a combination of hand s o r t i n g and by use of a Franz magnetic separator. P u r i t y was estimated by poi n t - c o u n t i n g g r a i n s on a 1mm. transparent g r i d . The f i n a l p u r i t y of the mineral separates was greater than 9 9 . 5 $ i n most cases and never l e s s than 9 9 . 0 $ . About l g p o r t i o n s of o l i v i n e , orthopyroxene and clinopyroxene were ground to a f i n e powder ( - 100 to - 2 0 0 mesh) by hand f o r 15 minutes i n an agate mortar. A few g r a i n s of each s p i n e l were mounted i n F i b r o l a y epoxy and p o l i s h e d with t i n oxide. P r i o r to the probe analyses each mount was coated withaa t h i n l a y e r of carbon. (b) E l e c t r o n microprobe analyses. The s-piireflXanalyses were c a r r i e d out w i t h a JXA - 3 e l e c t r o n microprobe X-ray a n a l y s e r . The analyses were done by comparing I n t e n s i t i e s (measured as counts per second) of s e l e c t e d X-ray l i n e s to those from standards of known composition. In a l l cases f i r s t order Koc l i n e s were used. The voltage was 2 5 K v . f o r every element. A 10 second counting time was used i n each case,10 to '20 points on each g r a i n being analysed. The average of each s e r i e s of counts was taken as the true i n t e n s i t y . The standards were analysed before and a f t e r each run to determine i n s t r u m e n t a l d r i f t . A f t e r each run the background was determined f o r both standards and samples. Table 1 l i s t s , the elements which were determined, r e f e r e d to the a p p r opriate standard. .108 TABLE 1 Elements and standards used In e l e c t r o n microprobe analyses. Element Standard Pe Pure Fe metal Cr Pure Cr metal Mg Sy n t h e t i c s p i n e l * A l - " * Composition i s : MgO 28 . 3 0 ; AlgO^ 71.55; FeO 0.02; CaO 0.02. TABLE 2 Operating c o n d i t i o n s f o r the hollow cathode lamps. Element Wavelength^) Lamp current(ma.) S l i t ( ) Flame Co 2407 5 25 a c e t y l e n e - a i r Cu 3247 3 50 Mn 2794 10 100 N i 2320 8 50 Pb 2170 6 300 Zn 2138 6 100 Na* 5890 5 200 K* 7664 10 200 Ca# 4226 10 25 •• Mg# 2852 4 50 " Fe 3719 5 50 T i 3643 20 100 a c e t y l e n e - n i t r o u s oxide A l 3091 11 100. 11 *Cs added to samples and standards to suppress I n t e r f e r e n c e s . #La added to samples and standards to suppress i n t e r f e r e n c e s . The data were processed through the computer program EMPADR VI1 which a p p l i e s c o r r e c t i o n s f o r background, dead time i f necessary, atomic number, abs o r p t i o n and fluorescence and converts the readings to weight percent of the appropriate oxide. (Rucklidge and G a s p a r r i n i 1 9 6 9 ) . The p r e c i s i o n of the analyses f o r each element c a l c u l a t e d as .109 the standard e r r o r of the mean of each s e r i e s of counts, i s given below. For A l and Mg the e r r o r i s c l o s e to $% of the amount present and f o r Fe and Cr i t i s about 1%. The e r r o r i s c o n s i s t e n t from sample to sample except f o r A l which v a r i e s from k to 6%, Accuracy can be no b e t t e r than p r e c i s i o n so t h a t the e r r o r s i n counting alone can account f o r the d e v i a t i o n s from 100$ i n the t o t a l s . (c) Atomic-absorption analyses. A l l elements expected to have a c o n c e n t r a t i o n of l e s s than 2% were t r e a t e d as t r a c e elements i n the a n a l y t i c a l scheme. These were Co,Cu,Mn,Ni,Pb,Zn,Na,K,Ti,and Ca ( i n o l i v i n e ) . 0 .3000g of the mineral powder was d i s s o l v e d i n 5ml of HF and 1ml of HCIO^ and the s o l u t i o n evaporated to dryness at 180°C on a h o t p l a t e . The residue was taken up i n 3ml °f HC1 and the s o l u t i o n made up to 25ml w i t h d i s t i l l e d water. S e r i e s of standards of appropriate c o n c e n t r a t i o n were made up i n 1.5M HC1. The standards were a s p i r a t e d i n t o the flame of a Techtron AA-4 atomic-absorption spectrophotometer and a p l o t of c o n c e n t r a t i o n versus a b s o r p t i o n prepared. The samples were then a s p i r a t e d , a f t e r a p p ropriate d i l u t i o n i f necessary, and the c o n c e n t r a t i o n read from the graph. The operating c o n d i t i o n s of the hollow cathode lamps are summarised i n Table 2. For major elements (Mg,Fe,Al and Ca i n pyroxenes) a method desc r i b e d by Langmyhr and Paus (1968) was used. 0 .2000g of mineral powder was d i s s o l v e d i n 5ml of HF and evaporated to dryness at 180°C on a h o t p l a t e . A f u r t h e r 5ml of HF was then added and the s o l u t i o n warmed. 50ml of saturated b o r i c a c i d s o l u t i o n was then added to d i s s o l v e the p r e c i p i t a t e d f l u o r i d e s and to complex any .110 excess HF. The s o l u t i o n was made up to 100ml w i t h d i s t i l l e d water. Standards of appropriate c o n c e n t r a t i o n were made up i n the same way. Each sample s o l u t i o n was a s p i r a t e d four times i n t o the flame of a Techtron AA-4 atomic-absorption spectrophotometer, b r a c k e t i n g I t each time between standards of appropriate c o n c e n t r a t i o n . The order of a s p i r a t i o n was reversed a f t e r each set of readings. The concentr a t i o n of the elements i n each sample was c a l c u l a t e d from the f o l l o w i n g equations Ex - E l C = A + K E 2 _ E I where A i s the con c e n t r a t i o n (wt.$) i n the lower standard, K the d i f f e r e n c e i n weight between the upper and lower standards, Ex, E l and E2 are the absorbance of the sample s o l u t i o n , lower standard and upper standard r e s p e c t i v e l y . The a r i t h m e t i c mean of the four readings was taken as the con c e n t r a t i o n . Operating c o n d i t i o n s f o r the hollow cathode lamps are summarised i n Table 2. Each batch of samples i n c l u d e d a d u p l i c a t e and a blank. No c o r r e c t i o n s f o r the blank were necessary. No c o r r e c t i o n s f o r background were r e q u i r e d f o r any of the elements. The p r e c i s i o n of a l l the analyses was estimated as the standard d e v i a t i o n of the d u p l i c a t e analyses. For a l l elements the p r e c i - s i o n was b e t t e r than $%% and g e n e r a l l y about 3% of the amount present. The l a r g e s t e r r o r s were i n Mg due. to the high d i l u t i o n f a c t o r r e q u i r e d and the s e n s i t i t y of the lamp, and i n A l which Is s e n s i t i v e to the f u e l flow, (d) Determination of C^O^. Chromium was determined c o l o r o m e t r i c a l l y u s i n g an adaption of .111 the methods described i n San d e l l (1967). A 20ml a l i q u o t of the s o l u t i o n used f o r the determination of Mg e t c . by atomic-absorption was taken. 5ml of 6N H2SOij, was added and the s o l u t i o n warmed. A few drops of 0.1N KMnO^ s o l u t i o n was added to t h i s u n t i l the s o l u t i o n remained f a i n t l y pink on heating. The s o l u t i o n was b o i l e d f o r ten minutes, allowed to c o o l and O.lg of Na 20 and 10 - 20ml of 20$ NaCO^ s o l u t i o n added u n t i l a permanent p r e c i p i t a t e appeared. The s o l u t i o n was then b o i l e d f o r ten minutes, cooled and f i l t e r e d . Enough 6N Ĥ JSÔ  (10 -20ml) was added c a r e f u l l y w i t h s w i r l i n g to l i b e r a t e C0 2 u n t i l the _ TvX s o l u t i o n was approximately 0.2N i n H2S04. 1ml of diphenylcarbazide s o l u t i o n was added and the s o l u t i o n made up to 50ml w i t h d i s t i l l e d water. The purple colour so obtained was compared v i s u a l l y to a s e r i e s of standardsisolutions c o n t a i n i n g 0.2 - 5ppm Cr made up with standard K 2Cr 20y i n the same way as the sample s o l u t i o n s . D u p l i c a t e samples and a blank s o l u t i o n were run w i t h each batch. P r e c i s i o n , determined as the standard d e v i a t i o n of the average of the d u p l i c a t e s , i s 5$ of the amount present. Due to the d i l u t e s o l u t i o n s used, the l i m i t of d e t e c t a b i l i t y I s 2'60ppm. Consequently the chromium content of the o l i v i n e s was not determined. A s l i g h t e r r o r i s Introduced by usi n g t h i s method as some Cr i s d r i v e n o f f as a f l u o r i d e during the i n i t i a l decomposition. However t h i s i s not b e l i e v e d to be ser i o u s as the pyroxenes con t a i n between 0.5 and 1.0$ Cr 20^ and r e p r o d u c i b i l i t y i s only 5$. P r e c i s i o n could be improved by making up f r e s h concentrated s o l u - t i o n s , but f o r r a p i d convenient analyses, the above method i s considered adequate i n view of the e r r o r s - i n the other elements, .112 APPENDIX 2 E r r o r propagation In temperature determination. The p r e c i s i o n i n determining Mg and Pe i n o l i v i n e (based on d u p l i c a t e analyses) i s l e s s than 5% of the amount present. The p r e c i s i o n i n determining Mg and A l i n the s p i n e l i s 5% of the amount present; f o r Fe and Cr p r e c i s i o n i s 1% of the amount present. These f i g u r e s are based on counting s t a t i s t i c s . Table 1 gives the u n c e r t a i n t i e s i n r a t i o s i n v o l v i n g these elements, based on the above p r e c i s i o n s . Table 1 U n c e r t a i n t i e s i n element r a t i o s . R a t i o Uncertainty R a t i o Uncertainty o l * .004 Cr Cr + A l + Fe A l Cr + A l * Fe Fe3+ Cr + A l + Fe Mg + Fe Mg Mg + F e 2 + S P - 'OOS F e 2 + Mg + F e 2 + s p - .005 Cr ~ * Uncertainty taken as the u n c e r t a i n t y i n Cr + A l + FeJ as Fe20<j c a l c u l a t e d by assuming the model s p i n e l formula. The u n c e r t a i n t i e s i n K D ( 4 ) were c a l c u l a t e d from the expression: |dy | / f x 1 ( x 1 . . . . x n ) / / dx x/ + / f x 2 ( x 1 . . . .x^)// dx 2| + / f x n ( x 1 . . . . x n ) / /dx n/ where dy was set equal to d K D ( 4 ) , x^ to X°^, x 2 to Xp^, x^ to Xpg and x^ to X^.. dx^, d x 2 , dx^and dx^ were taken from Table 1. (these were calculated using the above equation with the appropriate substitutions). The uncertainties in the derived temperatures were calculated from the above equation where dy was set.equal to dT, x^ to x^ to the variables on the right hand side of the equation for the calculation of the temperatures _ 5580*+ 1018/9- 1720T + 2400 .90* + 2,56/3 - 3.08^- 1.47 + 1.98?lnKD(4) and dx1 to dx 7 were taken from Table 1,, and Table 7 in the text.

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