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Isotopic investigation of conformable lead deposits. Ostic, Ronald George 1963

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ISOTOPIC INVESTIGATION CONFORMABLE LEAD  OP  DEPOSITS  by RONALD GEORGE OSTIC B.A.,  The U n i v e r s i t y  of Toronto,  1957  A THESIS SUBMITTED IN PARTIAL FULFILMENT  OF  THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in  the Department of PHYSICS  We  accept  required  this  t h e s i s as conforming  t o the  standard  THE UNIVERSITY  OF B R I T I S H COLUMBIA  August,  1963  I  In presenting  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of  the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and mission for extensive purposes may  study.  I f u r t h e r agree that per-  copying of t h i s t h e s i s f o r s c h o l a r l y  be granted by the Head of my Department or  his representatives.  I t i s understood that copying, or  c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be without my w r i t t e n p e r m i s s i o n .  Department of  Physics  The U n i v e r s i t y of B r i t i s h Columbia,, Vancouver 8, Canada. Date  August  15,  1963-  by publi-  allowed  The U n i v e r s i t y of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF  THE  FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY of RONALD GEORGE OSTIC PUBLICATIONS  B.A., U n i v e r s i t y of Toronto, 1957  R.G. O s t i c : I s o t o p i c Composition of Eastern A u s t r a l i a n Leads. (Abstract) Journal of Geop h y s i c a l Research, 67, 1651-1652 (1962).  THURSDAY, AUGUST 15, 1963 at 10:00  IN ROOM 206 CHEMICAL ENGINEERING BUILDING  R.D. R u s s e l l , E.R. Kanasewich and R.G. O s t i c : Quantitative I n t e r p r e t a t i o n of Anomalous Lead Isotopes. Chemistry of the Earth's Crust, Vernadsky Centenary Symposium, Acad, of Sciences, U.S.S.R. (1963). R.G. O s t i c , R.D. R u s s e l l and P.H. Reynolds: New C a l c u l a t i o n f o r the Age of the Earth. Submitted to Nature, J u l y , 1963.  A.M.  A  COMMITTEE IN CHARGE Chairman: F.W. Dalby J.A. Jacobs K.C. Mann  W.H.  D..M.  White  Myers R.D. R u s s e l l J.C. Savage W.F. Slawson  External Examiner: J.R. Richards Department of Geophysics A u s t r a l i a n National U n i v e r s i t y , Canberra  ISOTOPIC INVESTIGATION OF CONFORMABLE LEAD DEPOSITS ABSTRACT  (1) primary leads l i e very c l o s e l y to a unique s i n g l e stage growth curve i n a plot of the r a t i o s Pb207/ Pb204 against Pb 06/pb204 and i n a plot of Pb 08/ 2 0 4 against Pb206/ 204. (2) leads away from t h i s growth curve are anomalous. 2  The purpose of t h i s thesis i s to investigate the isot o p i c composition of primary leads. The leads analysed were selected by R.L. Stanton i n accordance with h i s geologic d e s c r i p t i o n f o r conformable deposits. I t was a n t i c i p a t e d that, of a l l a v a i l a b l e t e r r e s t r i a l leads, these were the least l i k e l y to have been modified i s o t o p i c a l l y by c r u s t a l processes. Detailed analyses and i n t e r p r e t a t i o n s f o r f i f t y - s i x samples from nine d i f f e r - £ ent d i s t r i c t s i n Eastern A u s t r a l i a , Tasmania, Canada and New Zealand are presented. A p r e c i s i o n of better than 1" .05 per cent i n the measurement of the Pb206/ Pb204, pb207/pb204 and pb208/ 204 r a t i o s was achieved f o r a l l samples by means of an intercomparison technique* derived from that used by F. K o l l a r and R.D. R u s s e l l , and improved methods f o r reducing the data.  2  )  Pb  Pb  From the f i t of the conformable leads to a s i n g l e stage growth curve, values of 4.52 + .03 Gyr. and 4.54 t -02 Gyr. have been c a l c u l a t e d f o r the age of the earth. With the i d e n t i f i c a t i o n of a unique growth curve for primary leads, severe r e s t r i c t i o n s are placed on anomalous lead i n t e r p r e t a t i o n s . These are i l l u s t r a ted i n the i n t e r p r e t a t i o n s of anomalous leads studied by the w r i t e r .  Pb  As expected, the geologic c r i t e r i a used by Stanton to i d e n t i f y conformable deposits have been found to be s u f f i c i e n t to i d e n t i f y deposits which are very uniform i n i s o t o p i c composition. The findings of t h i s research "' i n d i c a t e that the c r i t e r i a , although remarkably good, are not completely adequate f o r i d e n t i f y i n g primary leads. In p a r t i c u l a r , leads from two conformable depos i t s (Manitouwadge and Rosebery) were found to be anoma-; lous, and those from two other deposits (Hall's Peak and Bathurst) may be anomalous. The single-stage lead model was found to be a very good approximation f o r i s o t o p i c development of the leads from the remainder of the conformable deposits studied by the w r i t e r , as w e l l as f o r leads analysed by other workers at the U n i v e r s i t y of B r i t i s h Columbia and assumed to be primary. These r e s u l t s suggest that p r i mary leads have formed i n a region f o r which there are very narrow l i m i t s of + .67, and T . 87« to the v a r i a t i o n s i n u 38/pb204 and Th/U r a t i o s . They substantiate the hypotheses of R u s s e l l and Stanton that: 2  GRADUATE STUDIES F i e l d of Study:  Geophysics  Advanced Geophysics Applied Geophysics Geomagnetism Modern Aspects o f Geophysics Radioactive and I s o t o p i c Processes i n Geophysics  J.A. Jacobs R.D. R u s s e l l J.A. Jacobs D.C. Tozer T. Watanabe R.D. R u s s e l l  Related Studies:  1  Electromagnetic Theory Quantum Mechanics Waves  G.M. V o l k o f f W. Opechowski J.C. Savage  ii  ABSTRACT  The  purpose of t h i s  composition  of primary  thesis i s to investigate  leads.  s e l e c t e d b y R. L . S t a n t o n  i n accordance  d e s c r i p t i o n f o r conformable that, o f a l l a v a i l a b l e likely  terrestrial  with h i s geologic I t was  Canada a n d New  l e a d s , t h e s e were t h e l e a s t  districts  Zealand  samples  i n Eastern Australia,  are presented.  A precision  processes.  Tasmania, of better  . 0 5 p e r c e n t i n t h e measurement o f t h e Pb^O^/Pb^^",  p 207/p 204 b  a  n  d  p 208/pb204 b  r a  tios  was a c h i e v e d f o r a l l s a m p l e s  b y means o f a n i n t e r c o m p a r i s o n t e c h n i q u e used  anticipated  a n a l y s e s and i n t e r p r e t a t i o n s f o r f i f t y - s i x  from nine d i f f e r e n t  D  deposits.  were  t o have b e e n m o d i f i e d i s o t o p i c a l l y b y c r u s t a l  Detailed  than i  The l e a d s a n a l y s e d  the i s o t o p i c  b y P. K o l l a r  and R. D. R u s s e l l ,  d e r i v e d from  that  and i m p r o v e d methods f o r  reducing the data.  As e x p e c t e d , identify  the g e o l o g i c c r i t e r i a used  conformable  d e p o s i t s have b e e n f o u n d  to i d e n t i f y d e p o s i t s which are very u n i f o r m composition. criteria,  The f i n d i n g s o f t h i s  although  remarkably  found  two c o n f o r m a b l e  t o be  sufficient  i n isotopic  research indicate  that the  good, a r e n o t c o m p l e t e l y  adequate f o r i d e n t i f y i n g p r i m a r y from  by Stanton t o  leads.  In particular,  leads  d e p o s i t s (Manitouwadge a n d R o s e b e r y ) were  t o be anomalous,  and t h o s e  from  two o t h e r d e p o s i t s  ( H a l l ' s Peak and B a t h u r s t ) may be a n o m a l o u s .  The  s i n g l e - s t a g e l e a d m o d e l was f o u n d  approximation  f o r isotopic  t o be a v e r y  good  development o f t h e l e a d s from the  iii  remainder o f the conformable d e p o s i t s as w e l l a s f o r l e a d s  analysed  University of B r i t i s h  s t u d i e d by the w r i t e r ,  by other  workers a t the  C o l u m b i a a n d assumed t o be p r i m a r y .  T h e s e r e s u l t s s u g g e s t t h a t p r i m a r y l e a d s have f o r m e d i n a r e g i o n f o r which there ±  a r e v e r y n a r r o w l i m i t s o f i .6$ a n d  .8$ t o t h e v a r i a t i o n s i n U 38/pb204 2  substantiate  a n  d . /u Tn  ratios.  t h e h y p o t h e s e s o f R u s s e l l and S t a n t o n  They  that:  (1) p r i m a r y l e a d s l i e v e r y c l o s e l y t o a u n i q u e s i n g l e - s t a g e g r o w t h c u r v e i n a p l o t o f t h e r a t i o s Pb2p7/pb 04 a g a i n s t Pb206/ 204, and i n a p l o t o f Pb208/ b204 a g a i n s t Pb206/ 204 (2) l e a d s away f r o m t h i s g r o w t h c u r v e a r e a n o m a l o u s . 2  P b  P  P b  ;  Prom t h e f i t o f t h e c o n f o r m a b l e l e a d s growth curve,  values  o f 4.52 ± .03 G y r .  to a single-stage  a n d 4.54 ± .02 G y r .  have b e e n c a l c u l a t e d f o r t h e age o f t h e e a r t h . identification  o f a unique growth curve f o r p r i m a r y  severe r e s t r i c t i o n s a r e p l a c e d pretations.  on anomalous l e a d  These a r e i l l u s t r a t e d  anomalous l e a d s  With the leads,  inter-  i n the i n t e r p r e t a t i o n s of  studied by the w r i t e r .  ACKNOWLEDGEMENTS Much c r e d i t  x  f o r t h i s t h e s i s i s due t o P r o f e s s o r R. D.  R u s s e l l o f t h i s u n i v e r s i t y , a n d D r . R. L . S t a n t o n U n i v e r s i t y o f New S o u t h Wales, who f o r m u l a t e d and  d i r e c t e d the research  analysed,  with  the exception  s u p p l i e d by Dr. Stanton. supply not  samples,  this  study  A l l samples  f r o m Manitouwadge, were  o f the p r i m a r y l e a d system The e x p e r i m e n t a l  To t h e s e  the p r e c i s i o n achieved  isotope  state.  research  b y P r o f e s s o r R u s s e l l , and d i r e c t l y  work o f D r . P. K o l l a r . for  o f those  the problem,  Were i t n o t f o r h i s w i l l i n g n e s s t o  have b e e n u n d e r t a k e n .  supervised  to i t s present  of the  could was  followed the  men g o e s a good d e a l o f c r e d i t  i n the present  measurement o f l e a d  abundances.  P r o f e s s o r J , A. J a c o b s made i t p o s s i b l e f o r t h e w r i t e r t o come t o t h e U n i v e r s i t y o f B r i t i s h stay here both p r o f i t a b l e  and e n j o y a b l e  l e c t u r e s and t h r o u g h p e r s o n a l The  Columbia,  interest  a n d made t h e  through h i s s t i m u l a t i n g i n his  w r i t e r wishes t o acknowledge w i t h  students.  thanks the advice  and  e n c o u r a g e m e n t o f D r . E . R. K a n a s e w i c h a n d D r . J . C. Savage,  and  e s p e c i a l l y o f D r . W. P. S l a w s o n , who a s s i s t e d i n t h e  supervision during a pleasure  t h e absence o f P r o f e s s o r  t o be a s s o c i a t e d w i t h  D r . J . R. R i c h a r d s  Australian National University during research. the  His assistance,  Geology Department,  appreciated.  Russell.  the f i n a l  I t was of the  months o f t h e  and t h a t o f D r . R. E . D e l a v a u l t o f  i n sample p r e p a r a t i o n was g r e a t l y  T h a n k s a r e a l s o e x t e n d e d t o D r . R. M. F a r q u h a r  o f t h e U n i v e r s i t y o f T o r o n t o who s u p p l i e d t h e s a m p l e s  from  xi Manitouwadge. helped this  record  The  s p e c t r a l data  t h e s i s , and  o f Mr.  acknowledged w i t h my  wife  years,  who and  technical assistance  finally  and  appreciated.  the  most o f t h e  Mr.  P.  separated  him  from i t by  ,  r  s  -~  •.  was  for  three  this thesis.  received from the National  .  is  i s reserved  typing  who  draft for  a laboratory for  Petroleum Research Foundation, •  Hanmer,  Neukirchner,  A s p e c i a l thank you  husband w i t h  Financial, assistance, Council  typed  J . L e e s and  thanks.  shared her  and  o f M i s s S.  Research  sincerely  TABLE OP CONTENTS  ABSTRACT  i i  L I S T OF FIGURES  vi  L I S T OP TABLES  viii  ACKNOWLEDGEMENTS CHAPTER  iv  1  x  CONFORMABLE DEPOSITS  1  Introduction  1  P r i m a r y and C o n f o r m a b l e L e a d s  9  Geologic CHAPTER  2  Descriptions  15  PRECISION MEASUREMENT OP LEAD ISOTOPE 26  RATIOS Introduction  26  Mass S p e c t r o m e t e r P r e c i s i o n  27  CHAPTER  3  ISOTOPIC CHARACTERISTICS OP CONFORMABLE LEADS AND THE PRIMARY LEAD SYSTEM  Single-stage  L e a d Model  42 42  Within-Deposit Variations i n Isotopic Composition o f Conformable Lead D e p o s i t s C h a r a c t e r i s t i c s o f t h e P r i m a r y Lead System CHAPTER 4  CONCLUSIONS  49 58 76 82  APPENDIX I Compensation  f o r Uniform V a r i a t i o n In  Peak H e i g h t  87  APPENDIX I I Pressure  82  Scattering Correction  APPENDIX I I I  87 94  A New C a l c u l a t i o n f o r t h e Age o f t h e E a r t h f r o m L e a d I s o t o p e Abundances  94  First  97  Calculation  TABLE OP CONTENTS  (continued)  v  APPENDIX I I I ( c o n t i n u e d ) Second C a l c u l a t i o n  103  Conclusion  105  APPENDIX I V I n t e r p r e t a t i o n o f Anomalous L e a d s BIBLIOGRAPHY  107 107 120  L I S T OP FIGURES  vi  3  Pig.  1.1  SUITES OF ANOMALOUS LEADS  Fig.  1.2  ORDINARY L E A D S — D A T A FROM RUSSELL FARQUHAR ( I 9 6 0 )  AND  ORDINARY L E A D S — D A T A FROM RUSSELL FARQUHAR ( i 9 6 0 )  AND  Fig.  1.3  4  Fig.  1.4  LOCATION OF AUSTRALIAN CONFORMABLE DEPOSITS  Fig.  1.5  GEOLOGY OF THE COBAR D I S T R I C T (FROM THOMP-  Fig. Fig.  1.6 3.1  20  GEOLOGY OF THE BATHURST-NEWCASTLE DISTRICT, NEW BRUNSWICK (FROM SMITH AND SKINNER ( 1 9 5 8 ) )  22  PLOT SHOWING THE RELATIONSHIP BETWEEN THE  RATIOS P b 2 0 . 7 / 2 0 4 Pb  A  N  D  P b  206/  Q  AND t = 4 . 5 3 G y r .  P b  2 0 4 FOR  0  c  3.2  16  SON (1953))  CONFORMABLE LEADS, AND THE SINGLE-STAGE GROWTH CURVE FOR a = 9 . 5 6 , b =10.42, aV=9.19 Fig.  5  PLOT SHOWING THE RELATIONSHIP BETWEEN THE RATIOS P b 2 0 8 / p 2 0 4 AND P b 2 0 6 / P b 2 0 4 poR CONFORMABLE LEADS, AND THE SINGLE-STAGE GROWTH CURVE FOR a = 9 . 5 6 , c = 2 9 . 7 1 , a V = 9 . 1 9 ,  68  b  0  Th/U=4.10 AND t = 4 . 5 3 G y r .  0  0  Fig.  Pig.  Pig.  Fig.  4.1  4.2  1.1  1.2  DISTRIBUTION OF THE THORIUM/URANIUM RATIO FOR CRUSTAL ROCKS AS DETERMINED BY DIRECT CHEMICAL AND RADIOMETRIC ANALYSES  80  DISTRIBUTION OP THE THORIUM/URANIUM RATIO FOR THE F I N A L STAGE OF DEVELOPMENT OP ANOMALOUS LEADS  8l  PEAK HEIGHT VARIATIONS IN A T Y P I C A L TRIMETHYL-LEAD SPECTROGRAM, WITH CORRECTIONS POR CUBIC DECAY  84  PEAK HEIGHT VARIATIONS IN A TYPICAL TRIMETHYL-LEAD SPECTROGRAM,WITH CORRECTIONS FOR 5 t h ORDER DECAY  86  PHOTOGRAPHIC COPY OP A TYPICAL TRIMETHYLLEAD SPECTROGRAM  88  Fig.  II.1  Fig.  I I . 2 PRESSURE SCATTERING C O E F F I C I E N T S  Pig.  III.l  69  RESULTS OF THE F I R S T CALCULATION FOR.THE AGE OF THE EARTH, PROM DATA POR CONFORMABLE LEADS  91  100  L I S T OP FIGURES ( c o n t i n u e d )  Fig.  Pig.  III.2  III.3  v i i  RESULTS OF THE CALCULATION OP THE AGE OP METEORITES  102  PLOT SHOWING THE RELATIONSHIP BETWEEN, THE RATIOS P b 2 0 7 / P b 2 0 4 AND P b 2 0 6 / P b 2 0 4 FOR CONFORMABLE LEADS AND THE SINGLESTAGE GROWTH CURVE FOR aQ=g.^6t b =10.42, aV=9.23, AND t = 4 . 5 2 G y r .  106  o  Q  Fig.  Pig.  Pig.  I V . 1 THE DEVELOPMENT OP TWO-STAGE AND THREESTAGE ANOMALOUS LEAD SUITES  110  I V . 2 ISOTOPIC COMPOSITION OP LEADS PROM AND MOUNT PARRELL, TASMANIA  114  I V . 3 ISOTOPIC COMPOSITION WADGE  ROSEBERY  OP LEADS PROM MANITOU116  L I S T OP TABLES  GEOLOGIC TIME SCALE AND GEOLOGIC AGES FOR CONFORMABLE DEPOSITS DISCUSSED I N T H I S THESIS, ASSUMING ALL DEPOSITS ARE SYNGENEIC . (TIME SCALE AFTER J . L . KULP  (1961),)  MASS SPECTROMETER S T A B I L I T Y DURING A SINGLE ANALYSIS AS INDICATED BY THE STANDARD DEVIATIONS OF MEAN PEAK HEIGHTS FROM ANALYSES OF HALL'S PEAK SAMPLES MASS SPECTROMETER S T A B I L I T Y DURING A SINGLE ANALYSIS AS INDICATED BY THE AVERAGE STANDARD DEVIATIONS OF MEAN LEAD ISOTOPE RATIOS FOR A SINGLE ANALYSIS S T A B I L I T Y OP THE MASS SPECTROMETER OVER PERIODS OP APPROXIMATELY ONE MONTH AS INDICATED BY REPLICATE ANALYSES OP STANDARD SAMPLES S T A B I L I T Y OF THE MASS SPECTROMETER OVER A THREE YEAR INTERVAL AS INDICATED BY REPLICATE ANALYSES OF SAMPLE 1 REPRODUCIBILITY OF MEASUREMENT WITH A TYPICAL MASS SPECTROMETER AS PUBLISHED IN TABLE 2 ( b ) BY BURGER, NICOLAYSEN AND DE V I L L I E R S (1962) EXAMPLE  OP LOOP CLOSURE CALCULATIONS  MASS SPECTROMETER  "MEMORY" E F F E C T S  THE AGE OF METEORITES USING P b 2 0 6 / 2 0 4 AND P b 2 0 7 / P b 2 0 4 (DIAGONAL LINEAR REGRESSION WITH NO CONSTRAINTS ON PRIMORDIAL ABUNDANCES) P b  D  A  T  A  THE AGE OF METEORITES USING p b 2 0 6 / 2 0 4 AND Pb207/Pb204 DATA (DIAGONAL LINEAR REGRESSION THROUGH a =9.56, b = 1 0 . 4 2 ) p b  0  SYMBOLS AND CONSTANTS DETERMINATION  o  USED I N AGE  ISOTOPIC COMPOSITION OF SAMPLES RELATIVE TO THEIR D I S T R I C T STANDARD SAMPLES (LOOP CLOSURE ERRORS) RELATIVE ISOTOPIC COMPOSITION OF D I S T R I C T STANDARD SAMPLES (LOOP CLOSURE ERRORS)  L I S T OP TABLES  (continued)  ix  Table  3.6  ISOTOPIC COMPOSITION OP GALENAS RELATIVE TO BROKEN H I L L STANDARD  53  Table  3.7  APPARENT VALUES OP URANIUM/LEAD AND THORIUM/URANIUM RATIOS FOR CONFORMABLE DEPOSITS, CALCULATED FOR t = 4 . 5 5 G y r .  60  APPARENT VALUES OP URANIUM/LEAD AND THORIUM/URANIUM RATIOS FOR CONFORMABLE DEPOSITS, CALCULATED FOR t = 4 . 5 3 G y r .  6l  APPARENT VALUES OP URANIUM/LEAD AND THORIUM/URANIUM RATIOS FOR LEADS ANALYSED AT THE UNIVERSITY OP B R I T I S H COLUMBIA AND ASSUMED TO BE PRIMARY  73  q  Table  3.8  q  Table  3.9  Table I I . 1  Table  Ill.l  Table  III.2  LEAD ISOTOPE RATIOS FOR BROKEN H I L L STANDARD SAMPLE  93  DEFINITION OF SYMBOLS  97  SUMMARY OF RESULTS FOR THE SECOND CALCULATION  104  1  CHAPTER 1 CONFORMABLE DEPOSITS  Introduction Variations  of lead  isotope  because o f t h e r a d i o a c t i v e Uranium-238, active and is  uranium-235,  decay o f uranium and thorium. a n d thorium-232 d e c a y t h r o u g h s t a b l e n u c l e i lead-206,  s e r i e s t o form the  lead-208 r e s p e c t i v e l y . stable,  process. lead,  and i s not  lead  and t h e h a l f l i v e s  isotopes  i n the  isotope,  lead-204,  radioactive  the relative  change s u b s t a n t i a l l y d u r i n g of thenatural  geophysicists.  lead-207,  ear^h o f these isotopes o f  a b u n d a n c e s were among t h e  interest  lead  radio-  and abundances o f t h e u r a n i u m and  a r e such t h a t  Investigations isotope  A fourth  p r o d u c e d b y a n y known  The p r o p o r t i o n s  thorium isotopes,  abundances o c c u r I n n a t u r e  abundances o f  geologic  time.  variations of lead first  isotopic studies to  A s e a r l y a s 1907  Boltwood used  lead,  u r a n i u m a n d t h o r i u m c h e m i c a l a n a l y s e s t o e s t i m a t e t h e age o f minerals.  Since then these  a b o u t t h e age o f t h e e a r t h , terrestrial in  the  crust  orogenic The  objects,  studies  thed i s t r i b u t i o n of radioactive  and mantle,  and the  Nier,  duration  events i nd i f f e r e n t continental first  information  i t s relationship to extraelements  and r e l a t i o n s h i p o f units.  mass s p e c t r o m e t e r m e a s u r e m e n t s o f i s o t o p i c  c o m p o s i t i o n s o f common l e a d s and  have p r o v i d e d  Thompson a n d Murphy  were c a r r i e d o u t  (1941).  a t o r i e s have p a r t i c i p a t e d i n l e a d  by Nier  Now o v e r t h i r t y  isotope  studies,  (1938), labor-  and approx-  2 i m a t e l y two ature.  thousand  (i960)  reveals several  of the observed  striking features.  i s the remarkable  l e a d - 2 0 7 / l e a d - 2 0 4 and geologically  related  illustrated  quantitative  linear  lead isotope ratios Of t h e s e t h e most  leads.  interpretation  of  Such s u i t e s o f anomalous l e a d s  i n F i g u r e 1.1  K a n a s e w i c h and  Much h a s b e e n done on  their  ( s e e f o r example, K a n a s e w i c h  Ostic  (1962),  (1963)).  Schutze  There  (1962),  (1962),  i s evidence  that  systems,  "  t h a t t h i s d e v e l o p m e n t i s f r o m p r i m a r y l e a d s w h i c h were o f  a different and of  ratios  l e a d - 2 0 6 / l e a d - 2 0 4 f o r many s u i t e s  they r e p r e s e n t the development of l e a d s i n c r u s t a l and  import-  r e l a t i o n s h i p between the  B u r g e r , N i c o l a y s e n and de V i l l i e r s Russell,  and  and b y Cannon e t a l . ( 1 9 6 1 ) .  An e x a m i n a t i o n  are  liter-  Much o f t h i s d a t a h a s b e e n summarized b y R u s s e l l  Farquhar  ant  a n a l y s e s have b e e n r e p o r t e d i n t h e  origin.  The  object of t h i s t h e s i s i s to present  e v a l u a t e c e r t a i n l i n e s o f evidence c o n c e r n i n g the  nature  these primary l e a d s .  Considerable l e a d s c a n be lead  i n f o r m a t i o n about  o b t a i n e d from the g r o s s d i s t r i b u t i o n of  isotope ratios.  and F a r q u h a r  the nature of the  (i960),  e x c l u d i n g samples which have been i n F i g u r e s 1.2  A l s o i n these f i g u r e s are s i n g l e - s t a g e growth r e p r e s e n t i n g the i s o t o p i c  c a n be  observed  A l l the Toronto data presented i n R u s s e l l  a s b e i n g anomalous, h a v e b e e n p l o t t e d  different  primary  accepted  curves  composition of leads developed  times from p r i m o r d i a l l e a d .  considered to i l l u s t r a t e  Each  1.3.  and  of these  at  curves  the growth o f l e a d s i n the  FIGURE  3  1.1  SUITES OF ANOMALOUS LEADS  BROKEN H I L L — D a t a f r o m R u s s e l l , U l r y c h a n d K o l l a r (1961)  16 15  SUDBURY—Data  from U l r y c h  and R u s s e l l  ( i n press)  16 Pb2Q7 Pb20¥  15 OZARK D O M E — D a t a  from Bate e t a l .  (1957)  16 15 -  WHITE SEA REGION  ( K a r e l i a ) — D a t a from  et a l .  Vinogradov  (1959)  16 15 14  16  18 -  P b  20 206/ 204 P b  22  24  26  FIGURE 1.2 ORDINARY LEADS—DATA PROM RUSSELL AND PARQUHAR (I960) 1  i  I  i  |  i  Pb-204  I  i  6 same u r a n i u m - t h o r i u m - l e a d c l o s e d s y s t e m f r o m t h e a t i o n of the  earth u n t i l  the  time of  form-  time o f t h e i r d e p o s i t i o n as  lead  ores.  Only minor departures  from a  s i n g l e - s t a g e development  in  a common u r a n i u m - t h o r i u m - l e a d c l o s e d s y s t e m seem t o h a v e occurred three  f o r the  s a m p l e s i n F i g u r e s 1.2  explanations  single-stage (1) The  be  g i v e n f o r these  spread  leads  represents  ratios characteristic  spent  mean v a l u e  t  on  the  either  spread  isotope  first  cluded  represents  spread  spread  i n the  and  This  side of  the  about a  5$  f o r the  thorium/  to t h i s  e r r o r s i n t h e measurement o f  That  not  i s to  abundance o f l i e on  say,  cause. lead  lead-204.  a unique  growth  l e a d s w h i c h have  been  e a r t h ' s c r u s t have b e e n i n a d v e r t e n t l y i n -  i n the p l o t s as a r e s u l t  of our  inability  to  an u n a m b i g u o u s c r i t e r i o n f o r r e c o g n i s i n g them.  T h i s t h e s i s c o n s i d e r s the separately.  on e i t h e r  the  and  system i n which  i s attributed  in particular,  anomalous.  establish  of the  s i d e o f t h e mean v a l u e  i f a l l the  ratios,  are  altered  from  p a r t of t h e i r h i s t o r i e s .  ( 3 ) L e a d s whose i s o t o p e r a t i o s do curve  least  departures  f o r the uranium/lead r a t i o ,  lead ratio,  (2) The  At  d i f f e r e n c e s i n the u r a n i u m / l e a d  w o u l d amount t o a b o u t a 7$  spread  1.3.  development:  thorium/lead the  can  and  Fifty-six  have been a n a l y s e d were c a r e f u l l y  above t h r e e  samples from n i n e  different  mass-spectrometrically.  s e l e c t e d on  explanations  The  the b a s i s of c e r t a i n  districts  leads  analysed  geologic  7 criteria  i n an  attempt  to exclude those from d e p o s i t s  were l i k e l y  t o have b e e n m o d i f i e d  processes.  A large part  niques used to o b t a i n  of the  has  been p o s s i b l e t o  were n o t  sufficient  to  and  thorium/lead  primary leads  The isotope at the  abundances.  show t h a t t h e  limits  on  the  .05$  in  writer's research s t u d i e s by  R.  D.  f o r m s an  geologic  criteria I t has  been analysed  had  been p o s t u l a t e d ,  integral part  i n 1957.  at t h a t time, the but  no  R.  L.  of the  S e v e r a l hundred existence  single geologic  c l a s s of  a c l a s s of leads i n New  of  volcanic-sedimentary  him  while  U n i v e r s i t y of Western Ontario an N.  R.  C.  suggested t h a t the  post-doctoral leads  ( i . e . conformable deposits)  search  through the that  leads  was  (London),  origin  revealed  he  of  and  existence of  which Trips  the  Queen's U n i v e r s i t y  fellow,  and  on  these  volcanic-sedimentary  m i g h t be  sample book a t t h e from nine  visiting  had  lead  S o u t h W a l e s , A u s t r a l i a , i s an e x a m p l e .  t o T o r o n t o were made by  t r i p s he  origin,  leads  leads  S t a n t o n proposed the  I n 1954  leads  of primary  isotopic  composition.  lead  Stanton which began  i f t h e y were o f p r i m a r y  as  also  the  been i n v e s t i g a t e d to determine  (Kingston),  used  uranium/  systems i n which  R u s s e l l and  U n i v e r s i t y of Toronto  Bathurst,  the  With t h i s p r e c i s i o n ,  v a r i a t i o n s of  r a t i o s f o r the  tech-  developed.  had  of  crustal  t h e s i s i s devoted t o the  i d e n t i f y primary leads.  been p o s s i b l e t o p l a c e lead  through  a p r e c i s i o n of b e t t e r than  measurement o f l e a d i s o t o p e it  isotopically  which  primary.  U n i v e r s i t y of  conformable deposits,  A Toronto rep-  8  r e s e n t i n g a t o t a l of f o r t y - o n e  analyses,  f i t s i n g l e growth  c u r v e s ( i . e . a s i n g l e uranium-thorlum-lead system) t o the e x p e r i m e n t a l e r r o r of a few measurement of the l e a d r a t i o s .  t e n t h s of one At the  within  per cent i n the  same time, no  sig-  n i f i c a n t d i f f e r e n c e s were observed between samples from same conformable d e p o s i t .  the  I t appeared t h a t these l e a d s were  primary. Only w i t h improved p r e c i s i o n i n measurement c o u l d  any  r e a l d i f f e r e n c e s between conformable l e a d s and " t h e o r e t i c a l " primary l e a d s be  observed.  d i r e c t e d h i s a t t e n t i o n and  F o r t h i s reason, R. D.  Russell  the a t t e n t i o n of graduate students  F. K o l l a r and J . S. Stacey, at the U n i v e r s i t y of B r i t i s h Columbia, towards the c o n s t r u c t i o n of more p r e c i s e mass spectrometers.  Improved t e c h n i q u e s i n sample  were s i m u l t a n e o u s l y d e v i s e d and  preparations  under h i s d i r e c t i o n by T. J .  a method of i n t e r c o m p a r i s o n was  Ulrych,  developed t o f u r t h e r  Improve the p r e c i s i o n of measurement.  When the w r i t e r a r r i v e d  at the l a b o r a t o r y ,  many samples had  a l r e a d y been c o l l e c t e d by  R. L. Stanton, and  samples from two  conformable d e p o s i t s  been a n a l y s e d .  The  w r i t e r ' s t h e s i s p r o j e c t has been to  analyse many more l e a d s from conformable d e p o s i t s , i n t e r p r e t the r e s u l t s i n the lead isotope  had  abundances.  and  l i g h t of e x i s t i n g t h e o r i e s  to of  9 Primary  and Conformable  The  Leads  e x i s t e n c e o f well-documented  indirect  anomalous l e a d  evidence f o r the previous existence o f primary  f r o m which t h e s e anomalous l e a d s d e v e l o p e d . has  o f t e n been p r e s e n t e d t o e x p l a i n  l e a d s i s that primary l e a d s form are brought  or at a later  i nthis crustal  t i o n s t o the primary lead, formed.  system  activity.  system, and On t h e way  Radiogenic  lead  i s added i n v a r y i n g p r o p o r -  and a s u i t e  o f anomalous l e a d s i s  A r e t h e r e , however, a n y u n a l t e r e d p r i m a r y l e a d s a t  the earth's surface?  I f so, t h e y  s h o u l d be f o u n d  i n crustal  d e p o s i t s c o n t a i n i n g l e a d which h a s n o t been mixed w i t h d e r i v e d from c r u s t a l have  that  t h e y come i n c o n t a c t w i t h  rocks o f higher r a d i o a c t i v i t y .  developed  A hypothesis  i na sub-crustal  time,  leads  t h e f o r m a t i o n o f anomalous  t o t h e s u r f a c e by t e c t o n i c  to the surface, crustal  lines i s  suggested  that  sources.  S t a n t o n and R u s s e l l  d e p o s i t s which s a t i s f y  lead  (1959)  t h i s c r i t e r i o n may  include: (1)  orthomagmatic d e p o s i t s i n mafic r o c k s d e r i v e d from beneath continental crust; (2) s e d i m e n t a r y d e p o s i t s d e r i v e d f r o m d e p t h b y b a s a l t - a n d e s i t e v o l c a n i s m a l o n g , o r a t some d i s t a n c e f r o m c o n t i n e n t a l margins, and q u i c k l y i s o l a t e d i n v o l c a n i c sediments. A p a r t i c u l a r c l a s s o f d e p o s i t s t o which S t a n t o n and Russell  (1959) gave a t t e n t i o n  deposits.  T h e s e d e p o s i t s , w h i c h were f i r s t  and Thompson The  i s the "conformable"  class of  r e c o g n i z e d by King  (1953), may have s h a r e d a common t y p e o f o r i g i n .  f o l l o w i n g d e s c r i p t i o n s o f t h e main f e a t u r e s o f form and  occurrence o f conformable (I960).  d e p o s i t s are a b s t r a c t e d from  Stanton  10 The  form  margins are parallel  of the d e p o s i t s i s l e n t i c u l a r .  ill-defined,  t o the  and  their  stratification  l e n t i c u l a r i t y may  be  on t h e  state  structural  m a j o r l e n s e s may  flat  Frequently  l a r g e dimensions  are  i n the e n c l o s i n g sediments.  o r on f o l d  form, depending  The  of a s e r i e s of c l o s e l y  smaller lenses i n p a r a l l e l ,  sub-parallel,  The  entirely  of the e n c l o s i n g sediments.  be made up  the  spaced,  o r en e c h e l o n  arrange-  ment .  These d e p o s i t s o c c u r shales, rence  tuffs,  or marine  the c o n t a c t of these  In  most c a s e s t h e  deposited near limestone  Succeeding biotite  with  original  o l d shore  and  carbon,  shales, tuffaceous  sequences.  i s i n metamorphosed f i n e  to  of  i n marine  The  s h a l e s and  most f r e q u e n t siltstones,  a major fragmental  as i n d i c a t e d by  either  singly or  close  volcanic unit.  e n c l o s i n g sediments have  lines,  occur-  the  been presence  together.  r e g i o n a l metamorphism has u s u a l l y b e e n below  grade,  though o c c a s i o n a l l y h i g h e r grades  have  developed.  The of  three outstanding f e a t u r e s of the f i e l d  the conformable  d e p o s i t s are t h e i r  mental v o l c a n i c rocks  structural fairly  formation,  associations.  The  basic—approximating  A high quartz content, t a k e n by  Stanton  association with  (and d e r i v a t i v e s ) ,  sediments of near-shore  and  parent  occurrence  their  frag-  occurrence  in  their highly variable  t u f f s are g e n e r a l l y  to a n d e s i t i c - b a s a l t i c  composition.  which i s o c c a s i o n a l l y observed,  t o have been i n d u c e d by  the  later  is  addition  11  or release of s i l i c a . Stanton  The n e a r - s h o r e  environment  i n which  (i960) p o s t u l a t e s t h a t t h e c o n t a i n i n g s e d i m e n t s h a v e  formed a r e : (1) o f f - r e e f a r e a s and t h e i r e q u i v a l e n t s , p o s s i b l y where s l i g h t d e p r e s s i o n s and o t h e r f e a t u r e s i n h i b i t c i r c u l a t i o n slightly; (2) l a r g e l a g o o n s , s u c h a s t h o s e d e v e l o p e d a b o u t i s l a n d s o f t h e E a s t a n d West I n d i e s , t h e Solomons, a n d s i m i l a r a r e a s . These e n v i r o n s  are c h a r a c t e r i z e d by high organic  medium t o l o w r a t e o f d e t r i t a l  sedimentation,  activity,  a  and t h e develop-  ment o f r e d u c i n g c o n d i t i o n s i n t h e e n c l o s i n g muds. Many o f t h e c o n f o r m a b l e d e p o s i t s h a d p r e v i o u s l y b e e n interpreted  as hydrothermal replacement d e p o s i t s , t h a t i s ,  hydrothermally of c h e m i c a l l y chemical  d e r i v e d o r e which i s d e p o s i t e d s u i t a b l e sedimentary l a y e r s .  suitability  i s often accentuated  Stanton  structures.  In place  postulates biological  Since  i n fold or  of hydrothermal  activity.  activity,  He i n t e r p r e t s t h e  conformable d e p o s i t s as d i f f e r i n g from normal l a y e r s o n l y by t h e i r h i g h  such  by p h y s i c a l deform-  a t i o n , many o f t h e d e p o s i t s a r e now f o u n d associated  a t t h e expense  sulphide content.  sedimentary These have  r e s u l t e d from the b i o l o g i c a l c o n c e n t r a t i o n of v o l c a n i c basem e t a l and s u l p h u r  compounds w i t h i n c e r t a i n  sedimentary f a c i e s . sedimentation If  volcanism  phates,  very  The c o n t r i b u t i o n s o f b o t h  are e s s e n t i a l  to the formation  had not provided  a sufficient  specific v o l c a n i s m and  of the deposits.  quantity of sul-  t h e d e p o s i t s c o u l d n o t have formed; n o r c o u l d  h a v e f o r m e d were n o t c o n d i t i o n s s u i t a b l e f o r t h e r a p i d multiplication  of sulphate-reducing b a c t e r i a .  they  1 2 The  most f a v o u r a b l e e n v i r o n m e n t s f o r s u l p h i d e d e p o s i t i o n  are those  t h a t c o n t a i n the h i g h e s t c o n c e n t r a t i o n s of  organic matter. and  These are found  i n off-reef  Sulphide  and  equivalent facies,  deposition will  is  a supply of base-metal  It  i s submitted  by  i n zones p a r a l l e l  not  occur  and  Stanton  and  i n these  combined  areas u n t i l  sulphur i n the  phides w i l l  be  precipitated  are p r o v i d e d .  Fumarolic  there  water.  increase rapidly,  q u i c k l y as l o n g as the  activity  form  Once t h e  the normal p o p u l a t i o n of  i n the o r g a n i c ooze s h o u l d  coast  lagoons.  t h a t v o l c a n i s m — c h i e f l y i n the  sulphates are provided,  bacteria  to the  i n large  of f u m a r o l i c a c t i v i t y - - s u p p l i e s the base-metals. metal  sedimentary  base-  sulphur and  sul-  sulphates  I s g e n e r a l l y more i n t e n s e  immediately  p r e c e d i n g or succeeding major p e r i o d s of e x p l o s i v e  volcanism.  The  found  conformable  immediately  The  d e p o s i t s are, t h e r e f o r e , f r e q u e n t l y  b e l o w o r above a p y r o c l a s t i c  r e l a t i o n s h i p s which Stanton  conformable  d e p o s i t s and  opposed t o l o c a l geophysicist.  unit.  p o s t u l a t e s between  r e g i o n a l g e o l o g i c s t r u c t u r e (as  s t r u c t u r e above) are o f prime i n t e r e s t  F o r the  below i n Stanton's  own  the  sake o f c o m p l e t e n e s s ,  they  to  the  are s t a t e d  words.  "With t h e i r n e c e s s a r y v o l c a n i c a f f i l i a t i o n s I t i s c l e a r t h a t a l l s u c h o r e b o d i e s s h o u l d be l o c a l i z e d along zones of volcanism. Similarly, as t h e y a l l o c c u r i n n e a r - s h o r e marine sediments, t h e y s h o u l d be r e g i o n a l l y d i s t r i b u t e d a l o n g t h e m a r g i n s o f seas, o r about i s l a n d s o c c u r r i n g i n them. and  " B o t h t h e s e c o n d i t i o n s — a zone o f v o l c a n i s m surrounding near-shore areas of sedimentation--  13  are s a t i s f i e d almost e x c l u s i v e l y by t h e v o l c a n i c arcs of island festoons. I t therefore follows that orebodies o f the present type should occur a l o n g t h e l e n g t h s o f s u c h a r c s , and h e n c e t h a t t h e y s h o u l d b e a r a s p a t i a l and g e n e t i c r e l a t i o n ship with r e g i o n a l s t r u c t u r e o f a major s c a l e . Thus, though h a v i n g o n l y an i n c i d e n t a l c o n n e c t i o n w i t h any p a r t i c u l a r l o c a l f e a t u r e o f d e f o r m a t i o n , t h e d e p o s i t s have a fundamental c o n n e c t i o n w i t h broad t e c t o n i c f e a t u r e s . " *  Several relationships f o r lead from  Stanton's  t h e o r i e s o f conformable  r e l a t e s the source o f the l e a d along volcanic earthquakes,  island  arcs.  volcanic  It  that  this  Stanton  ores to fumarolic  activity  The a s s o c i a t i o n o f deep and i s l a n d  Russell  i s of uniform  this  focus  a r c s i s w e l l known  and W i l s o n  p . 2 9 2 ) , a n d a deep s o u r c e  i s anticipated that  inferred  deposits.  activity,  ( s e e f o r example J a c o b s , Geology",  i s o t o p e s c a n be  " P h y s i c s and  of lead  i s suggested.  s o u r c e c o u l d be t h e u p p e r  isotopic  composition,  a closed  ation of the earth.  T h e r e f o r e , i f l e a d s c o u l d be " e x t r a c t e d "  this  source  at different  times,  from  and t h a t i t  has been e s s e n t i a l l y  from  system  mantle,  t h e time  a n d n o t be  a t any time by o t h e r l e a d s , t h e i s o t o p i c  o f form-  "contaminated"  abundances o f these  l e a d s w o u l d l i e o n t h e same g r o w t h c u r v e s .  An a n a l y s i s o f  t h e s e l e a d s w o u l d t h e n p r o v i d e u s w i t h two c o m p l i m e n t a r y of  information.  I t would  model, and a l s o p r o v i d e  shed  some l i g h t  be  reasonable,  former *  (i960),  and t r a c e p. 2 5 .  of the  I f the h y p o t h e s i s appears t o  one c o u l d t h e n u s e c o n f o r m a b l e  coast l i n e s  Stanton  on t h e s i n g l e - s t a g e  a means t o c h e c k t h e v a l i d i t y  volcanic-sedimentary hypothesis.  types  continental  d e p o s i t s t o map  growth.  14 There are now  a number of well-documented s u i t e s of  anomalous l e a d s i n the l i t e r a t u r e . the  I t i s quite evident  study of these l e a d s t h a t m i g r a t i n g  s o l u t i o n s can p i c k  s i g n i f i c a n t q u a n t i t i e s of r a d i o g e n i c l e a d from the rock through which they pass. the f o r m a t i o n  The  from up  crustal  r e s u l t of t h i s p r o c e s s  is  of a d e p o s i t of an anomalous s u i t e of l e a d s i n  which the i s o t o p i c composition  v a r i e s with the a d d i t i o n of  v a r i a b l e q u a n t i t i e s of r a d i o g e n i c l e a d . l e a d s p r e c i p i t a t e d from s u l p h a t e s t o be very u n i f o r m  On the o t h e r hand,  i n sea water are expected  i n i s o t o p i c composition.  t h a t the d i s c o v e r y of uniform  Note, however,  i s o t o p i c composition  i n con-  formable d e p o s i t s would not c o n c l u s i v e l y prove t h e i r v o l c a n i c sedimentary o r i g i n .  F o r example, hydrothermal s o l u t i o n s  c o u l d have r i s e n through lineaments i n which they had opportunity  little  to p i c k up r a d i o g e n i c l e a d , o r a hydrothermal  d e p o s i t c o u l d have been homogenized i n p o s t - d e p o s i t i o n a l metamorphism. The  premise t o be t e s t e d i n t h i s t h e s i s i s t h a t conform-  able l e a d d e p o s i t s c o n t a i n s i n g l e - s t a g e can t h e r e f o r e be dated,  (ordinary) leads,  i f the v o l c a n i c - s e d i m e n t a r y  c o r r e c t , the model l e a d age w i l l be the same as the age  of the bedding i n which the d e p o s i t o c c u r s .  the f i n a l ,  and perhaps most d e f i n i t i v e ,  o r i g i n of conformable d e p o s i t s .  and  theory i s geologic  This  provides  i s o t o p i c check on  the  15 Geologic  Descriptions  Samples t o be of the  n i n e groups o f samples two  examined,  Six and f i v e Kollar,  a r e f r o m Canada, and one  and t h e i r  samples  i s f r o m New  Zealand.  a r e shown i n  from Broken H i l l D i s t r i c t ,  s a m p l e s f r o m Mount I s a , Q u e e n s l a n d , R u s s e l l and U l r y c h  (I960).  1.1.  New  South  Wales,  were a n a l y s e d by  The B r o k e n H i l l  and Mount  d e p o s i t s o c c u r i n r o c k s on t h e e a s t e r n m o s t e x p o s u r e  Broken H i l l  K i n g and O ' D r i s c o l l  Pratten  (1961).  The  (1953),  g i v e n b y K i n g and  s i x major  lenses  s t r a t i f o r m and c o n c o r d a n t w i t h t h e  c l o s i n g h i g h l y metamorphosed W i l l y a m a * imentary rocks.  Thompson  and C a r r u t h e r s and  sulphides occur within  which are e s s e n t i a l l y  of  The p r i n c i p a l f e a t u r e s o f  d e p o s i t s have b e e n  (1953),  Of  Eastern  apparent ages a r e g i v e n i n T a b l e  Precambrian rocks i n A u s t r a l i a . the  s i x are from  g e o g r a p h i c a l l o c a t i o n s o f the samples  F i g u r e 1.4,  Isa  range  m i n e r a l i z a t i o n and a wide g e o g r a p h i c a l d i s t r i b u t i o n .  Australia, The  s t u d i e d were s e l e c t e d t o have a wide  Series of  en-  metasedM*  C h e m i c a l a n a l y s e s o f t h e s e sediments by  Richards  (IQ63) suggest that the o r i g i n a l  s e d i m e n t s were  volcanic  greywackes.**  volcanic  arc s t r u c t u r e p a s s i n g through the Broken  Hill  District  i s i n d i c a t e d by the i n c i d e n c e o f s e v e r a l  small  The p o s s i b l e f o r m e r p r e s e n c e o f a  ser-  pentine bodies.  x x*  W i l l y a m a means " c l e f t r o c k " o r " b r o k e n r o c k " i n t h e a b o r i g i n a l language, R, L . S t a n t o n — p e r s o n a l c o m m u n i c a t i o n .  FIGURE 1 . 4  16  LOCATION OF AUSTRALIAN CONFORMABLE  DEPOSITS  • a  MOUNT ISA  H ALL'S* BROKEN HILL  PEAK  COBAR  CAPTAIN'S  CO*  FLAT  ROSEBERY  •  TABLE  17  1.1  GEOLOGIC TIME SCALE AND GEOLOGIC AGES FOR CONFORMABLE DEPOSITS DISCUSSED IN THIS THESIS, ASSUMING A L L DEPOSITS ARE SYNGENETIC. (TIME SCALE AFTER J . L . KULP (1961).) Beginning of I n t e r val (mill i o n years)  Era  Period  C.enozoic  Quaternary  1 63  Tertiary  Mesozoic  Paleozoic  Deposit  Cretaceous  135  Jurassic  181  Triassic  230  Permian  280  Hall's  Peak  Pennsylf vanian  310  Carbon-J iferous j l» M i s s i s s ippian  345  Devonian  1+05  Silurian  1+25  Cobar  Ordovician  5oo  Captain's  Flat  Bathurst  600?  Cambrian  Rosebery  Precarabrian-"-  1500. -1700  Broken  1600'-1700  Mount I s a  3000? -jc- P r e c a m b r i a n  d a t e s NOT  after  Kulp.  Hill  Manitouwadge  18 The  l e a d - z i n c o r e s o f Mount I s a have  banding p a r a l l e l rocks  and  are  sediments.  to the b e d d i n g of the  conformable with  These  enclosing  volcanic  dust.*  Mine  w i t h i n the  above  and  s i x from  the  Rosebery-Williams-  to  sixty  analysed.  e t a l . (1953).  C a m b r i a n Dundas S e r i e s  feet.  The  en  echelon,  zone o f  shale u n i t  and and  sulphide  which l i e s  con-  f t . t h i c k n e s s of b l a c k carbonaceous  shale,  above a much g r e a t e r t h i c k n e s s o f b e d d e d p y r o c l a s t i c  mater-  ial.  Carey  which these  200  Hall  lenses, arranged  within a tuffaceous  below a  i n the  d e p o s i t s have b e e n g i v e n by  a number o f f l a t  lenses occurs  Crox-  devitrified  o f w e s t e r n T a s m a n i a ) have b e e n  v a r y i n g i n t h i c k n e s s up  formably  R o s e b e r y M i n e and  (both of which are  Rosebery d e p o s i t s e x i s t of  the  sediments.  ford-Tullah district  consist  fine  Large volcanic u n i t s also e x i s t  Seven samples f r o m the  D e t a i l s of these  of  s e d i m e n t s have r e c e n t l y b e e n shown by  b e l o w t h e Mount I s a  Mount F a r r e l l  sedimentary-  particular unit  (1962) t o c o n s i s t m a i n l y o f e x t r e m e l y  ford  and  one  well-developed  (1953) s t a t e s t h a t t h e e u g e o s y n c l i n a l a x i s a l o n g rocks  accumulated  greywackes of Middle Middle  and  U p p e r C a m b r i a n age,  C a m b r i a n i t became t h e  i s l a n d s which trough."**  arc  *  R.  L.  xx  Carey  locus of a l i n e  supplied pyroclastic  Thus, t h e r e  and  a p p e a r s t o be  s u l p h i d e s of Rosebery with island  " r e c e i v e d a two-mile t h i c k n e s s  the  (1953), page  1118.  during  the  of v o l c a n i c  volcanic d e t r i t u s to an  a s s o c i a t i o n of  carbon-bearing  affiliation.  Stanton--personal  and  communication,  of  sediments  the of  the  19 Mount F a r r e l l bery.  i s l o c a t e d l e s s than  s i x m i l e s from  T h i s I s a v e i n mine o c c u r r i n g w i t h i n a few  o f t h e b e d d e d r o c k and m o d e l age geologic  massive p y r o c l a s t i c  f o r R o s e b e r y was age  found  quantitative  and  analysed  hundred  boundary.  i n an  i n t e r p r e t a t i o n t o the  s a m p l e s were  attempt  isotopic  feet  When t h e  t o be much y o u n g e r t h a n  of the e n c l o s i n g sediments,  f r o m Mount F a r r e l l  Rose-  the  obtained  to o b t a i n a  composition  of  the  Rosebery d e p o s i t .  Seven o f the  samples a n a l y s e d  of Captain's F l a t , east  of Canberra,  of f l a t  which i s s i t u a t e d Australia.  en e c h e l o n  they  age.  are e n c l o s e d .  The  Single  S.  with  south-  as a  series  constitution  sulphide l e n s e s conform with carbonate  Interbedded  stratigraphic  Silver  Peak and  the  b e a r i n g s h a l e s i n which  with these  Is a thick  been a s s i g n e d a Lower  series Silurian  d e p o s i t , Cobar.  i n Figure  1.5.  some f r a g m e n t a l  p o i n t s out  level  of the  the  Queen  Bee,  T h a r s i s d e p o s i t s of Cobar  A l l of these d e p o s i t s occur C.  S.  A.  Groups o f  These are c h i e f l y  district,  S.  A.  Group a p p e a r s  Thompson t o form  the  in  sediments,  s h a l e y and  v o l c a n i c component.  t h a t t h e C.  not  orebodies.  A l s o a n a l y s e d were s i x s a m p l e s f r o m  o r o t h e r o f t h e C o b a r and 3hown  similar  samples have been a n a l y s e d from  Cobar,  A.  miles  sulphides occur  are of very  r o c k s which has  Eastern Australia. C.  and  20  about  orebody  O r d o v i c i a n - S i l u r i a n b o u n d a r y i s known t o o c c u r  f a r below the  Great  The  of the carbon  of p y r o c l a s t i c  The  l e n s e s and  to the Rosebery o r e s . bedding  are from E l l i o t ' s  one as  sandy  (1953) a  con-  types  FIGURE 1 . 5 GEOLOGY OF THE COBAR D I S T R I C T (FROM THOMPSON  2-0 (1953))  21 cordant  s u c c e s s i o n w i t h the  C o b a r Group, b u t  stratigraphic  groups are l a r g e l y complex f o l d i n g . ian,  but  obscured The  fossils  i n the form  three p r i n c i p a l  C.  S.  relationships within the f a u l t i n g  A.  Group i s b e l i e v e d t o be The  of l e n s e s , w i t h i n which the  of the conformable  c l e a r d i s c o r d a n c y and He  suggests  t h a t the  are p r o b a b l y Queen Bee  a p p e a r t o be  and  Stanton  some o f t h e type,  S.  t h a t those  A.  and  evidence  Great  tuffs)  i s not  and  Mineral D i s t r i c t ,  suggests  New  and here  6 orebody,  and  (1958) and t h e i r map  a s F i g u r e 1.6,  has  Fossil  and  the  writer.  the 12  Brunswick  third  sample i s  orebody.  The  been d e s c r i b e d by  (Fig.  Six  Bathurst-Newcastle  are from  S m e l t i n g No.  of the m i n e r a l d i s t r i c t  Skinner  (probably  t h i s d e p o s i t by  B r u n s w i c k , two  S m e l t i n g No.  S o u t h Wales,  an U p p e r P e r m i a n age.  samples a n a l y s e d from  from Brunswick Mining geology  and  conformable  minor carbonaceous s h a l e s .  good, b u t  Of t h e t h r e e  and  Cobar  o f S i l v e r Peak  e n c l o s i n g r o c k s are greywackes  s a m p l e s have b e e n a n a l y s e d f r o m  Mining  zones.  not.  The  redistributed  states  s e v e r a l show  l e a d - z i n c d e p o s i t l o c a t e d i n n o r t h e r n New  Australia.  as  deposits  H a l l ' s Peak i s a s m a l l , h e a v i l y c r o s s f a u l t e d pyritic  by  Silur-  sulphides occur  r e l a t e d to shear  s u l p h i d e s o f C.  concordant,  are  o f c o n t a c t s and  sediments o r as v e i n l e t s .  a p p e a r t o be  the  d e p o s i t s are g e n e r a l l y  ( p e r s o n a l communication) t h a t a l t h o u g h clearly  of  these  by  are very r a r e .  c o n s t i t u e n t s of the  formations  Smith  2, p . 151), r e p r o d u c e d  shows t h e d i s t r i b u t i o n o f t h e  rock  form-  FIGURE  1.6  22  GEOLOGY OF THE BATHURST—NEWCASTLE DISTRICT, NEW BRUNSWICK (FROM SMITH AND SKINNER (1958))  Scale of Miles o  '.'j  SV  Argillaceous sadimanUry rocks  j Silicaous *ok*nk  rocks •  2 'Porphyry'  Silicaous matasedimantary rocks  Principal mineral deposits .;. X Compilation  . , 4 7 00  • ,\.  •< .<^V jt  Geological  ->./r  /.>//• •  .  •  by C. W. Sm/t/i Survey o/ Canada  23 ations relative phide  t o the sulphide deposits.  deposits occur  ("porphyry"-volcanic) canic  centre.  i n the i n t e r l a y e r e d  Most o f t h e s u l sedimentary-volcanic  complex s u r r o u n d i n g t h e s i l i c e o u s  The g e o l o g y  o f t h e B r u n s w i c k M i n i n g and  Smelting o r e b o d i e s has been d e s c r i b e d i n d e t a i l Rancourt of  (1958).  Both these  by L e a and  d e p o s i t s are u n d e r l a i n by rocks  O r d o v i c i a n age, a n d a r e e n c l o s e d i n a s e r i e s o f  rocks interbedded with quartz sizes.  vol-  "porphyry"  bodies  sedimentary  of various  B a s i c i n t r u s i o n s o f p o s s i b l e D e v o n i a n age o u t c r o p i n  p r o x i m i t y t o t h e No. 6 o r e b o d y and i n t h e a r e a o f t h e No. orebody.  The o v e r a l l  s t r i k e s o f the bedding  12  i n the sediments  as w e l l a s t h e s c h i s t o s i t y o f t h e v o l c a n i c r o c k s a r e conformable  t o the t r e n d o f the v o l c a n i c sediments.  structure  of the surrounding  conformable the  locus of interbedded  The by  t o the o u t l i n e  geology  Pye (1957).  the D i s t r i c t appears  sedimentary  The p r e v a l e n t  s e r i e s appears  t o be  o f t h e v o l c a n i c complex a s w e l l a s  sediment.  o f t h e Manitouwadge a r e a h a s b e e n d e s c r i b e d T h i s area forms a s m a l l but important  o f Thunder Bay, n o r t h o f Lake S u p e r i o r .  t o be a n i s l a n d  sediments surrounded c a n i c s are hornblende  o f Keewatln metavolcanics  b y Algoman i g n e o u s  rocks.  part of It  and m e t a -  The  metavol-  s c h i s t s which o c c u r below and i n t h e  u p p e r p a r t o f t h e s e r i e s o f m e t a s e d i m e n t s a s a number o f p a r a l l e l horizons,  separated from  each other by t h i n  of metasediments, o r laminated hornblende structure  i s assumed t o be r e l i c t  schist.  sedimentary  layers  The  latter  stratification.  24 Orientated  l e n t i c u l a r f r a g m e n t s o f what a r e  metamorphosed l a v a s w i t h i n the believe tuff.  t h a t the  schist  rocks  and  cut  diabase dikes,  sharply  a r e b e l i e v e d t o be  Samples a n a l y s e d f r o m Geco, and  include  one  lead geologists  i s a h i g h l y metamorphosed  Numerous p o s t - o r e  Keweenawan i n age,  schist  thought to  two  across  the  youngest  from Lun-Echo.  taken to  i n the  2 zone,  i n the  t a k e n f r o m a v e i n i n Geco p r o p e r t y  post-ore  and  probably  S t a n t o n r e l a t e s the to fumarolic hypothesis fumarolic  activity  i s c o r r e c t , the  same a s t h a t hypothesis composition  analysed.  the  Bay  New  Zealand.  trench is  the  isotopic  t e s t e d by two  This  occurs  Creek  fault.  arcs.  composition  If  kinds  of  this  arcs  in is  the  This  isotopic  leads.  sample o f f u m a r o l i c i s l a n d has  encrustation been  obtained  sample i s f r o m W h i t e I s l a n d , w h i c h i s i n  o f f the  north-east  t o the  shore of North  active andesitic  western margin of the  the  deposits  of l e a d  a comparison of the  W h i t e I s l a n d I s an  e x t e n d s on  known a s  volcanic island  a modern v o l c a n i c  of Plenty  on  Pox  fifth  which  o f modern v o l c a n i c i s l a n d  lead-bearing  vent of  and  lying  be  of the  O n l y one from the  post-diabase  A  o f l e a d i n modern c o n f o r m a b l e d e p o s i t s .  can  one  source of l e a d i n conformable  along  encrustations  area.  A l l these deposits f i t  Stanton's c l a s s i f i c a t i o n f o r conformable d e p o s i t s . sample was  be  consolidated  rocks  f r o m W i l l r o y No.  to  water-laid  which are  a l l other  be  White  volcano  Island Trench.  N o r t h I s l a n d o f New  Rotorua-Taupo graben.  Island,  This  Z e a l a n d where i t  A zone o f  volcanicity  •  '  150 m i l e s l o n g c o i n c i d e s w i t h t h i s major s t r u c t u r a l  25  feature,  and many o f the deep-focus earthquakes o f New Zealand are a s s o c i a t e d with i t .  The i s l a n d ' s h i s t o r y o f a l t e r n a t i n g  v o l c a n i s m and quiescence dates back a t l e a s t i n t o the P l e i s t o cene.  The sample was taken from the edge o f the vent o f a  fumarole on B i g Donald Mound.  CHAPTER 2  26  PRECISION MEASUREMENT OP LEAD ISOTOPE RATIOS  Introduction P r e c i s e measurements of l e a d i s o t o p e r a t i o s have been r e q u i r e d t o examine the f o l l o w i n g c h a r a c t e r i s t i c s i m p l i e d by Stanton's d e s c r i p t i o n f o r the f o r m a t i o n o f conformable d e p o s i t s : (1) e v e r y conformable d e p o s i t i s v e r y u n i f o r m i n i s o t o p i c composition; (2) l e a d Isotope r a t i o s f o r a l l conformable d e p o s i t s l i e on unique growth c u r v e s i n lead-207/lead-204 vs. lead-206/ lead-204 p l o t s , and i n lead-208/lead-204 v s . lead-206/ lead-204 p l o t s ; (3) the model l e a d ages o f a l l d e p o s i t s are the same as the g e o l o g i c age of the e n c l o s i n g sediments. U n t i l t h i s work was done, e x i s t i n g a n a l y s e s of conformable l e a d s confirmed the I n t e r p r e t a t i o n s g i v e n by Stanton.  Only  w i t h improved p r e c i s i o n i n measurement of the i s o t o p i c abundances c o u l d any r e a l d i f f e r e n c e s between t h e o r y and p r a c t i c e be found. The p r e c i s i o n o f a measurement o f the i s o t o p i c compo s i t i o n o f a sample by a mass spectrometer can best be by the r e p r o d u c i b i l i t y of t h i s measurement.  Judged  Most l a b o r a t o r i e s  d o i n g l e a d a n a l y s e s have been able t o reproduce r a t i o s r e l a t i v e t o lead-204 t o w i t h i n a few t e n t h s o f one p e r c e n t .  Tech-  n i q u e s developed at the U n i v e r s i t y o f B r i t i s h Columbia have enabled the w r i t e r t o improve t h i s p r e c i s i o n by a f a c t o r of ten, so t h a t r a t i o s r e l a t i v e t o lead-204 are r e p r o d u c i b l e t o w i t h i n a few hundredths of one p e r c e n t . A good mass spectrometer has been n e c e s s a r y f o r t h i s work, but the achievement of p r e c i s i o n a n a l y s e s depends more  on t e c h n i q u e used,  t h a n on i n s t r u m e n t a t i o n .  t h e measurement o f r a t i o s  reproducible  Two  Without  relative  per  i n p e a k h e i g h t s , and  be  cent.  slightly  by  compensated f o r  These are the u n i f o r m  the e f f e c t s of p r e s s u r e  Compensations f o r these e f f e c t s  would  normally overlooked  mass s p e c t r o m e t r i s t s have b e e n c o n s i d e r e d and i n the r e d u c t i o n of the data.  procedures  t o lead-204  o n l y t o w i t h i n o n e - f i f t h o f one  f e a t u r e s o f mass s p e c t r o g r a m s  the  variation  scattering.  improved  the  precision  o f measurement, b u t f a r more i m p o r t a n t h a s b e e n t h e  adoption  of a t e c h n i q u e f o r t h e i n t e r c o m p a r i s o n of samples.  With  technique,  p r e c i s i o n has been o b t a i n e d t o enable  confirm point  (1),  page 2 6 ,  p r e v i o u s l y observed.  a g e s and  the w r i t e r  than  Real departures from unique  c u r v e s have been found, model l e a d  to a h i g h e r degree  and  d i s c r e p a n c i e s observed  g e o l o g i c ages o f e n c l o s i n g  growth between  sediments.  f i g u r e s are g i v e n t o s u b s t a n t i a t e the p r e c i s i o n  Appendices  mass s p e c t r o m e t e r u s e d  are g i v e n i n  f o r a l l a n a l y s e s was  at the U n i v e r s i t y of B r i t i s h  and  Russell.  D.  claimed.  Precision  and b u i l t R.  and  I and I I .  Mass S p e c t r o m e t e r The  precise  i s o t o p e r a t i o s are d i s c u s s e d b r i e f l y ,  Further d e s c r i p t i o n s of a n a l y t i c a l procedures  to  was  I n t h i s c h a p t e r the t e c h n i q u e s employed f o r the measurement o f l e a d  this  It i s a 90°  sector,  Columbia  by F.  gas-source  designed Kollar  instrument  28 of twelve Inch r a d i u s of c u r v a t u r e . the mass spectrometer  Lead i s i n t r o d u c e d i n t o  i n the form of t e t r a m e t h y l - l e a d , and  the  t r i m e t h y l - l e a d spectrum i s recorded. Apart from b e i n g p a r t i a l l y t r a n s i s t o r i z e d , the power supp l i e s f o r t h i s instrument  are f a i r l y c o n v e n t i o n a l .  A l l are  h i g h l y s t a b i l i z e d t o keep i o n beam n o i s e to a minimum.  In-  stead of a c o n v e n t i o n a l e l e c t r o m e t e r a m p l i f i e r , t h i s mass spectrometer age  has a s e r v o - v o l t m e t e r i n which the feedback v o l t -  i s o b t a i n e d from a motor-driven  ten-turn h e l i c a l  t i o m e t e r , which i s l i n e a r to w i t h i n 0.025 p e r c e n t .  potenThe  small-  e s t s i g n i f i c a n t t r i m e t h y l - l e a d peak can be read to one p a r t i n t e n thousand on the t h r e e i n c h diameter potentiometer  shaft.  d i a l f i x e d t o the  T h i s i s a g a i n by a f a c t o r of t e n over  the u s u a l c h a r t r e c o r d e r p r e c i s i o n . T e t r a m e t h y l - l e a d was  prepared by a method e v o l v e d from  t h a t employed by C o l l i n s , Parquhar and R u s s e l l (1954). c h l o r i d e was  Lead  f i r s t prepared by l e a c h i n g galena samples w i t h  h y d r o c h l o r i c a c i d , and was T e t r a m e t h y l - l e a d was  then c o n v e r t e d t o l e a d i o d i d e .  s y n t h e s i z e d from the l e a d i o d i d e i n an  excess s o l u t i o n of methyl magnesium brdmide i n d i e t h y l e t h e r . F i n a l l y , t e t r a m e t h y l - l e a d was  separated from the o t h e r r e a c t i o n  p r o d u c t s and d i e t h y l e t h e r by means of a vapour phase chromatographic  column as d e s c r i b e d by U l r y c h (1962).  A l l t e t r a m e t h y l - l e a d samples were s t o r e d i n b r e a k s e a l tubes.  B e f o r e they were analysed, they were allowed t o f l u s h  through  the mass spectrometer  f o r approximately  t h i r t y minutes,  29  d u r i n g which time the t e t r a m e t h y l - l e a d vapour reached e q u i l i b r i u m w i t h i t s l i q u i d phase, and the mass spectrometer s u p p l i e s warmed up.  At the end o f the h a l f - h o u r f l u s h i n g time,  the sample l i n e p r e s s u r e was v a r i e d t o o p e r a t i n g p r e s s u r e and the source e l e c t r o d e s were a d j u s t e d t o maximum i o n c u r r e n t f o r minimum f i l a m e n t c u r r e n t c o n s i s t e n t w i t h s a t i s f a c t o r y i o n beam intensity.  A f t e r the source c o n d i t i o n s were e s t a b l i s h e d , the  mass spectrometer was l e f t f o r t e n minutes,  d u r i n g which time  the i o n beam i n t e n s i t y " s t a b i l i z e d " i t s e l f .  The t r i m e t h y l -  l e a d spectrum was then scanned  s e v e r a l times i n both the up-  mass and down-mass d i r e c t i o n s , a f t e r which the sample was f r o z e n back i n t o a f r e s h b r e a k s e a l f o r storage u n t i l i t was to be r e a n a l y s e d . A f t e r every a n a l y s i s , a l l greased J o i n t s were c l e a n e d , c o l d t r a p s were warmed t o a l l o w trapped t e t r a m e t h y l - l e a d t o escape, and the mercury i n the sample l i n e r e s e r v o i r was r a i s e d and lowered s e v e r a l times t o "sweep o u t " sample which may have been trapped. When samples which were expected t o be o f s i m i l a r i s o t o p i c c o m p o s i t i o n (up t o 2# d i f f e r e n c e i n r a t i o s to  relative  lead-204) were t o be a n a l y s e d i n s u c c e s s i o n , the mass  spectrometer was f l u s h e d w i t h argon f o r o n e - h a l f an hour between a n a l y s e s . was  When the d i f f e r e n c e i n i s o t o p i c  compositions  expected t o be g r e a t e r than 2$, t h e mass spectrometer was  baked f o r s e v e r a l hours between s u c c e s s i v e a n a l y s e s .  During  the baking, a f r a c t i o n o f the next sample t o be analysed was f l u s h e d through the mass spectrometer.  30 When s a m p l e s were a n a l y s e d  a s d e s c r i b e d above,  Isotopic  compositions  per  i n t h e measurement o f r a t i o s  cent  This precision Table  2.1  the  were d e t e r m i n e d  i s illustrated  with a p r e c i s i o n relative  i n Tables  2.1  H a l l ' s Peak samples.  it  That  This precision  a l l a n a l y s e s by  f o l l o w s t h a t the  isotope  average  ratios f o r a single  i s , lead  and  a n a l y s i s are  isotope ratios  reproducibility  relative .04  as  the  precision their  As  per  as p r e v i o u s l y , the  be  of  Table  Comparison of these  t h e r e h a v e b e e n no v a r i a t i o n of the interval.  and  251  significant  instrument  on  from  the  replicate  f i l a m e n t ages, changes i n the  i o n beam.  Such d a y - t o - d a y  analyses of  ( I 9 6 0 ) and 2.3.  s a m p l e s 72  obtained  by  d u r i n g the  two  and  Kollar  as l i s t e d  sets of data i n d i c a t e s changes i n the  from  variations  sample 1 b y  the w r i t e r ,  Thus  same  isotope ratios calculated  replicate  2.2  interpreted  are measured w i t h the  abundances change.  i n the  calculations  cent.  o f measured i s o t o p e r a t i o s  t h e mass s p e c t r o m e t e r  relative  of  of that  shown i n T a b l e  above must n o t  i o n beam i n t e n s i t i e s  are observed  In  to lead-204 are  emission p r o p e r t i e s occur which e f f e c t  while  2.2.  Prom t h e  a s t h e p r e c i s i o n o f measurement, a s d e t e r m i n e d  its  lead-204.  s t a n d a r d d e v i a t i o n s o f mean l e a d  precision calculated  analyses.  .04  f o r a l l analyses  i s typical  the w r i t e r .  to a p r e c i s i o n of approximately The  to  of  s t a n d a r d d e v i a t i o n s o f t h e mean t r i m e t h y l - l e a d have been l i s t e d  o b t a i n e d on  their  that  rate of a half  year  in  TABLE 2.1  31  MASS SPECTROMETER STABILITY DURING A SINGLE ANALYSIS AS INDICATED BY THE STANDARD DEVIATIONS OP MEAN PEAK HEIGHTS PROM ANALYSES,OP HALL'S PEAK SAMPLES  Peak  Mean V a l u e s  249  251  252  253  .0005 .0004 .0003 .0004 .0002 .0003 .0003 .0003 ..0004 .0003 .0003  .0047 .0029 .0017 .0031 .0028 .0013 .0039 .0009 .0035 .0025 .0050  .0018 ,0019 .0024 .0022 .0029 .0028 .0022 .0019 .0034 .0024 .0027  .0054 .0028 .0036 .0046 .0044 .0023 .0051 .0032 .0053 .0043 .0063  ±.0029 (.0120)  ±.0024 (.0110)  ±,0003 (.026$)  -.0043  (.0680)  TABLE 2.2 MASS SPECTROMETER STABILITY DURING A SINGLE ANALYSIS AS INDICATED BY THE AVERAGE STANDARD DEVIATIONS OP MEAN LEAD ISOTOPE RATIOS FOR A SINGLE ANALYSIS  Ratio  P  b206/p 204 D  *.007 (.0380)  Pb °7/Pb 2  2 0 4  ±.006 (.0370)  Pb  208/ 204 Pb  ±.013 (.0340)  TABLE  32  2.3  S T A B I L I T Y OP THE MASS SPECTROMETER OVER PERIODS OP APPROXIMATELY ONE MONTH AS INDICATED BY REPLICATE ANALYSES OF STANDARD SAMPLES  Sample No.  Date  1 1 1 1 1  March 9/60* March 10/60* March 14/60* M a r c h 16/60* M a r c h 31/60*  p b  Mean Ratios Standard Deviations**  72 72 72 72 72 72 72 Mean Ratios  Standard Deviations**  10/62 11/62 11/62 16/62 31/62 Nov. 28/62 Nov. 29/62 Oct. Oct. Oct. Oct. Oct.  Standard Deviations**  **(T=  M±  V  p b  - x)" " n - 1 2  207  / p b  204  p b  208  / p b  204  16.098 16.095 16.116 16.108 16.129  15.523 15.519 15.550 15.517 15.547  36.014 36.013 36.093 36.036 36.085  16.109  15.531  36.048  ±.016 (.1030)  ±.038 (.1070)  18.350 18.328 18.354 18.352 18.343 18.332 18.370  15.703 15.675 15.700 15.703 15.696 15.677 15.725  38.402 38.343 38.430 38.438 38.422 38.372 38.435  18.347  15.697  38.406  ±.017 (.1090)  ±.036 (.0940)  18.523 18.519 18.530 18.506 18.521 18.501 18.491  15.758 15.757 15.766 15.742 15.761 15.731 15.726  38.857 38.867 38.877 38.799 38.855 38.768 38.732  18.513  15.749  38.822  ±.014 (.0760)  A n a l y s i s by K o l l a r x  204  ±.014 (.0760)  Mean Ratios  x  / p b  ±.014 (.0860)  O c t . 6/61 O c t . 12/61 O c t . 16/61 O c t . 18/61 O c t . 20/61 Nov. 3/61 D e c . 6/61  251 251 251 251 251 251 251  206  (i960)} o t h e r  ±.016 (.1000) analyses  ±.056 (.1440) a r e by the w r i t e r .  33 The r e p r o d u c i b i l i t y of measurement of sample 1 over a three year i n t e r v a l i s given i n Table 2.4.  The variations  observed i n t h i s table are greater than those observed over an i n t e r v a l of one month, but a l l measured r a t i o s are within one-half of one per cent of t h e i r mean values.  The reproduc-  i b i l i t y over t h i s longer period i s very s i m i l a r to that observed at other laboratories,  Replicate analyses by Burger,  Nicolaysen and de V i l l i e r s (1962) on a s o l i d source mass spectrometer are l i s t e d i n Table 2.5 f o r comparison, The data of Tables 2,3 and 2.4 indicate that the r e l a t i v e isotopic compositions of samples analysed within a month cannot be mors precise than one-tenth of one per cent.  This  p r e c i s i o n decreases to o n e - f i f t h of one per cent as the time i n t e r v a l increases to a year or more, or i f the analyses are c a r r i e d out by d i f f e r e n t analysts. In order to minimise the e f f e c t of day-to-day variations i n the measured lead isotopic abundances, K o l l a r and Russell developed' a technique f o r intereomparing samples,  This,  method i s analogous to the methods used f o r the preoision measurement of differences In oxygen and sulphur isotopes. The method f i r s t followed by K o l l a r to measure one sample r e l a t i v e to another was to alternate  the two samples a f t e r  measuring two spectral p a i r s from each,  A t o t a l of twelve  p a i r s of spectra were recorded (six p a i r s f o r each sample), Later, K o l l a r changed to alternating samples after recording three spectral p a i r s instead of two, i n order to obtain a  TABLE  34  2.4  STABILITY OP THE MASS SPECTROMETER OVER A THREE YEAR  INTERVAL AS INDICATED BY REPLICATE ANALYSES OF SAMPLE 1  Analyst  Date  Kollar  Ostic  206  Pb  207/ 204 Pb  Pb  208/ 204 pb  March 9/60  16.098  15.523  36.014  March 10/60  16.095  15.519  36.013  March 14/60  16.116  15.550  36.093  March 16/60  16.108  15.517  36.036  March 31/60  16.129  15.547  36.085  Dec. 4/61  16.119  15.532  36.005  6/6l  16.101  15.506  36.015  Dec. 7/6l  16.106  15.498  36.010  March 31/62  16.080  15.478  35.913  April  2/62  16.057  15.462  35.869  April  3/62  16.064  15.465  35.875  Dec,  Kanasewich  Pb /Pb204  Ulrych  July, 1962  16.064  15.466  35.941  Ostic  Oct. 3/62  16.114  15.511  36.043  Oct. 4/62  16.106  15.503  36.028  Oct. 5/62  16.097  15.516  36.043  Oct. 9/62  16.100  15.499  36.028  May 28/63  16.127  15.527  35.946  May 29/63  16.052  15.492  35.835  16.096  15.506  35.988  Sinclair  Ostic  Average Ratios Standard Deviation*  V  n -1  ±.0236 (.15*)  *.0263 (.17*)  i.0746 (.21*)  35  TABLE 2.5 REPRODUCIBILITY OP MEASUREMENT WITH A TYPICAL MASS SPECTROMETER AS PUBLISHED IN TABLE 2 (b) BY BURGER, NICOLAYSEN AND DE VILLIERS (1962)  No.  Date o f A n a l y s i s  P  b206  /Pb  204  p 207/ 204 b  Pb  p b  208  / p b  204  1  29.4.1955  16.35  15.58  36.16  2  6.6.1955  16.36  15.55  36.14  3  25.7.1957  16.31  15.55  36.35  4  25.6.1958  16.44  15.53  36.36  5  11.12.1958  16.41  15.58  36.38  6  12.10.1959  16.24  15.43  36.30  16.35  15.54  36.28  Average Standard Deviation*  ±.071  ±.056  (.430)  (.360)  ±.107 (.290)  36 b e t t e r p r o p o r t i o n of measuring t o h a n d l i n g time. the time of measurement was  In t h i s  about three t o t h r e e and  hours, of which two hours was  way  one-half  the a c t u a l time used t o r e c o r d  the twelve p a i r s of s p e c t r a . The w r i t e r found t h a t b e t t e r r e s u l t s were o b t a i n e d by extending t o approximately n i n e t y minutes the time r e q u i r e d t o put a sample i n t o the mass spectrometer,  allow f o r equilibrium  c o n d i t i o n s t o form, remove the sample a f t e r a n a l y s i s , c l e a n the apparatus f o r the next  sample.  and  To complete an  i n t e r c o m p a r i s o n i n a reasonable amount of time, i t was p r e f e r a b l e t o adopt  the f o l l o w i n g a l t e r n a t i n g technique.  sample B was measured r e l a t i v e t o sample A of s i m i l a r composition,  then  sample A was  first  isotopic  analysed, then sample B  analysed, and f i n a l l y sample A was  re-analysed.  p a i r s were recorded each time f o r sample A, and p a i r s were recorded f o r sample B.  The  When  was  Pour s p e c t r a l seven  spectral  t o t a l time taken f o r  an i n t e r c o m p a r i s o n of s i m i l a r samples was  s i x and  one-half  hours, of which o n l y two hours were used i n r e c o r d i n g the f i f t e e n spectral pairs.  F l u s h i n g techniques, where necessary,  i n c r e a s e d the time of an i n t e r c o m p a r i s o n t o t w e n t y - s i x hours at most. A check on the accuracy of i n t e r c o m p a r i s o n s was by a r o u t i n e l o o p i n g technique. f o l l o w e d by K o l l a r .  T h i s technique was  obtained also  Each loop c o n t a i n e d a n a l y s e s of a s t a n -  a r d sample (S) and two  samples (A and B), whose i s o t o p i c  compositions were t o be determined  r e l a t i v e t o t h a t of the  37 standard.  Three i n t e r c o m p a r i s o n s formed a c l o s e d l o o p .  These Intercomparisons (2)  B intercompared  were:  (1)  w i t h A, and  Measured r a t i o s f o r the two the same day were averaged,  A intercompared (3)  w i t h S,  S intercompared  w i t h B.  a n a l y s e s of the same sample on I t was  assumed t h a t these  average  r a t i o s would have been observed f o r t h a t sample had i t been analysed at the same time t h a t the second had been a n a l y s e d .  sample of the  day  T h i s i s e q u i v a l e n t t o assuming a l i n e a r  " d r i f t " f o r the mass spectrometer d u r i n g each day's o p e r a t i o n . Computed l e a d i s o t o p e r a t i o s f o r each day were t a b u l a t e d , and d i f f e r e n c e s i n them f o r each day c a l c u l a t e d as shown i n Table 2.6.  These d i f f e r e n c e s were then added, and the  d i s t r i b u t e d e v e n l y among the t h r e e . compositions of two dard sample. Table 2.6  In t h i s way,  the  totals isotopic  samples were o b t a i n e d r e l a t i v e t o a s t a n -  The magnitudes of the d i s t r i b u t e d e r r o r s i n  are t y p i c a l of a l l measurements by the w r i t e r .  When a s u i t e of samples from the same d i s t r i c t was analysed, one of the samples was dard.  s e l e c t e d as a d i s t r i c t  Loops were formed among the samples w i t h the  standard sample i n c l u d e d i n every l o o p . *  t o be stan-  district  The d i f f e r e n c e s i n  i s o t o p i c composition of a l l samples of the d i s t r i c t were then c a l c u l a t e d r e l a t i v e t o the d i s t r i c t district  standard sample was  l a b o r a t o r y standard sample.  *  standard.  Finally,  the  analysed i n a loop c o n t a i n i n g the The  i s o t o p i c composition of the  K o l l a r and R u s s e l l s t a r t e d out t o connect the samples t o gether t o form a g r i d r a t h e r than a s e r i e s of l o o p s through a d i s t r i c t standard.  38  TABLE 2.6 EXAMPLE OP LOOP CLOSURE CALCULATIONS  Pb  Sample No.  249 250  206  /pb  204  pb  207  /pb  204  pb  208  /pb  204  18.5129 18.5262  15.7429 15.7629  38.8477 38.8850  -.0133  -.0200  -.0373  +.0017  +.0075  +.0057  (249-250)  -.0116  -.0125  -.0316  250 251  18.5116 18.5062  15.7401 15.7415  38.8149 38.7990  +.0054  -.0014  +.0159  +.0017  +.0075  +.0057  (250-251)  +.0071  +.0061  +.0216  251 249  18.5245 18.5216  15.7612 15.7624  38.8718 38.8674  +.0029  -.0012  +.0044  +.0017  +.0075  +.0057  (251-249)  +.0046  +.0063  +.0101  A + B + C - total error  -.0050  -.0226  -.0170  A » (249-250)  Distributed Error Corrected  B = (250-251)  Distributed Error Corrected  C = (251-249)  Distributed Error Corrected  DIFFERENCES IN ISOTOPE RATIOS  A(206/204)  A(207/204)  A(208/204)  249  -.0046  ^.0063  -.0101  250  +.0071  +.0061  +.0216  251  0  0  0  Sample No.  39 district  s t a n d a r d was  standard,  and  the d i s t r i c t  through  then  obtained r e l a t i v e  t h i s the  isotopic  s a m p l e s were c a l c u l a t e d  to the laboratory-  compositions  relative  of a l l  t o the l a b o r a t o r y  standard.  Some o f t h e d a y - t o - d a y v a r i a t i o n s w h i c h have observed eter.  c o u l d be  due  to  "memory e f f e c t " o f t h e mass  I t i s b e c a u s e o f t h e mass s p e c t r o m e t e r  f l u s h i n g t e c h n i q u e s were a d o p t e d samples o f v e r y d i f f e r e n t  i n the  isotopic  the  least  r a d i o g e n i c samples  replicate  analyses of  analyses are l i s t e d 372  sample 372 p i c k e d up analysed  was  was  The  flushed with  removed.  The  lead.  was  was  one  p i c k e d up b y  t o contaminate  per cent  the  which  after was  the  the f o l l o w i n g analyses  f l u s h e d through  of the analysed l e a d .  sample  t h a t t h e mass  per cent of  sample 372  were  these  a trace of  t r a c e o f sample 372  argon  372),  between  r e s u l t s of  f o r o n l y 30 m i n u t e s f o l l o w i n g e a c h  t r a c e o f sample 371  approximately  analysed  a n a l y s e d between r e p l i c a t e  T h i s time  i n the  f o r e i g h t y minutes  ,3  of  371),  and  d e s p i t e the f a c t argon  Only  (sample  S i m i l a r a n a l y s e s were c o n d u c t e d  when sample 371 sample 3 7 2 .  370  Clearly,  amounts t o a p p r o x i m a t e l y  spectrometer The  2.7.  sample 3 7 0 ,  analysis of  spectrometer  of  i n Table  was  r e m a i n e d i n t h e mass s p e c t r o m e t e r  second  day  (samples  sample 3 7 0 .  memory t h a t  composition.  Sample 372  memory e f f e c t s o b s e r v e d .  spectrom-  intercomparison  i n t e r c o m p a r i s o n o f t h e most r a d i o g e n i c sample and  been  the  mass  analysis. amounts t o This  level  of cross-contamination i s n e g l i g i b l e  when i n t e r c o m p a r i s o n i s  used,  composition  and  samples d i f f e r  i n isotopic  by  less  than  TABLE  2.7  MASS SPECTROMETER  Order  of  Sample  Analysis May 23, 1963  May 24, 1963  "MEMORY" E F F E C T S  Isotope  206/204  207/204  208/204  1  370  13.323  14.530  33.355  2  372  29.883  17.661  47.125  3  370  13.369  14.549  33.392  1  372  29.847  17.663  47.084  2  371  13.444  14.554  33.461  3  372  29.707  17.617  46.948  ±.007  ±.006  ±.013  Average S t a n d a r d D e v i a t i o n  four per cent of t h e i r r a t i o s Three of  relative  independent  r e l a t i v e t o lead-204.  e s t i m a t e s c a n be made o n t h e p r e c i s i o n  i s o t o p e r a t i o s which have been o b t a i n e d by i n t e r -  comparing t h r e e samples i n a l o o p . analysis  i s approximately  the i s o t o p e  ratios  are less The  r e l a t i v e t o lead-204.  by a f a c t o r  then c a l c u l a t e d ,  /2~.  I s that  (and t h u s t h e r e l a t i v e are  t o lead-204.  single  Every  sample i s t o be  i n isotope  o f the differences  ratios will  be  o f i s o t o p e r a t i o s b y t h e f a c t o r /IT. the differences isotopic  obtained to a precision  relative  in a  c o u l d be e x p e c t e d  Since differences  the p r e c i s i o n  than the p r e c i s i o n net result  The p r e c i s i o n  .04 p e r c e n t i n t h e measurement o f  a n a l y s e d t w i c e , so t h i s p r e c i s i o n improved  Ratios  No.  In isotopic  compositions  c o m p o s i t i o n s o f t h e samples)  o f .04 p e r c e n t i n t h e r a t i o s  41  Loop c l o s u r e e r r o r s p r o v i d e us w i t h a second measure o f the p r e c i s i o n o f measurements w i t h i n a l o o p .  The d i s t r i b u t e d  e r r o r (see Table 2.6) i s e q u a l t o the d i s p e r s i o n i n the measurement o f the r a t i o s .  The loop c l o s u r e e r r o r s f o r a l l  a n a l y s e s are l i s t e d w i t h the c a l c u l a t e d i s o t o p e d i f f e r e n c e s i n Table 3.1.  They are g e n e r a l l y s l i g h t l y l e s s than .04 p e r  cent o f the a b s o l u t e i s o t o p e r a t i o s . I f one assumes t h a t a l l samples o f a p a r t i c u l a r s u i t e o f samples are o f the same i s o t o p i c composition,  and any d i f -  f e r e n c e s measured between the samples are then i n t e r p r e t e d t o be e r r o r s i n measurement, one can o b t a i n a r a t h e r p e s s i m i s t i c value f o r the p r e c i s i o n o f measurement.  The l a r g e s t  dif-  f e r e n c e s f o r a s e t o f samples from a comformable deposit, which were measured by the w r i t e r and were p r o b a b l y o f the same i s o t o p i c composition, were those f o r C a p t a i n ' s P l a t . was the f i r s t d e p o s i t s t u d i e d by him.  This  The standard d e v i a t i o n s  of the r a t i o s r e l a t i v e t o lead-204 are s l i g h t l y l e s s  than  ,04 p e r cent o f the a b s o l u t e r a t i o s (see Table 3.4) i n d i c a t i n g t h a t the p r e c i s i o n must have been b e t t e r than  .04 p e r c e n t .  42  CHAPTER 3 ISOTOPIC CHARACTERISTICS OP CONFORMABLE LEADS AND THE PRIMARY LEAD SYSTEM  Single-stage  Lead Model  I n t e r p r e t a t i o n s of l e a d i s o t o p e abundances are  always  based on the f o l l o w i n g assumptions: (1)  at the b e g i n n i n g of g e o l o g i c time, a l l l e a d was of a unique i s o t o p i c composition; l e a d of t h i s composition has been c a l l e d p r i m o r d i a l o r p r i m e v a l l e a d ; (2) any observed v a r i a t i o n s i n l e a d i s o t o p e r a t i o s are e n t i r e l y the r e s u l t of the a d d i t i o n of d i f f e r e n t amounts of r a d i o g e n i c l e a d t o the p r i m e v a l l e a d . I t f o l l o w s from these assumptions t h a t the amounts o f l e a d i s o t o p e s f o r any the  relative  l e a d sample can be g i v e n  by  equations: Pb  P B  206 / 2 0 4 P B  207/  P B  = x = a  204.  Y  .  + a  Q  B O  J:  r\„\'t  +  \  '° >t V*e" dt  L  AT  ( 3 - 1 )  J o  Pb  where t  0  2 0 8  /pb  204  i s the age  = z = c  2  dt  z  A  J  Jo  of the e a r t h , X, V  c o n s t a n t s of U 38, u 35 and 2  wV'e  +  0  Th ^ , a * 2  2  0  and b  \  and  Q  are the decay  n  c  are  Q  isotope  r a t i o s of p r i m o r d i a l l e a d , and V, W and a are  present  values  ( t = 0) f o r the r a t i o s U 3 / P b 2  5  2 0 4  ,  the the  Th 3 /Pb 2  2  2 0 4  and u 38/u 35. 2  2  E q u a t i o n s (3.1) are r i g o r o u s , but very g e n e r a l because V and W are u n s p e c i f i e d f u n c t i o n s of both time and p l a c e .  The  premise to be t e s t e d i n t h i s t h e s i s i s t h a t conformable l e a d  43 d e p o s i t s c o n t a i n s i n g l e - s t a g e (primary) l e a d s which developed i n the same uranium-thorium-lead  system.  To these  assumptions,  t h e r e f o r e , we add the f o l l o w i n g : (3)  V and W are the same f o r a l l samples and have been constant from time t u n t i l time t ; (4) V and W have been i d e n t i c a l l y zero from time t u n t i l the p r e s e n t ( t = 0 ) . 0  S o l u t i o n s t o e q u a t i o n s (3.1) x y  - a  + a V(e o  + v(e  0  + w(e  v t  xHt  c u r v e s " f o r primary l e a d s .  e**)  ° -  e  o - *  v t  H t  e  These t h r e e e q u a t i o n s (3.2)  are  -  u  c  b  z = c  are then:  )  >.  )  (3.2)  J  are one form of the "growth  When a , b , 0  0  c , t 0  Qt  \, V ,  V  and a  known, and x, y, and z are measured f o r a primary l e a d ,  e q u a t i o n s (3.2)  can be s o l v e d e x p l i c i t l y f o r t , V and W.  "model age" so c a l c u l a t e d i s the time a t which the l e a d  The was  removed from the primary l e a d system f o r which the p r e s e n t v a l u e s of the r a t i o s  u235/p 204 b  a n d  Th  232/ 204 Pb  are  V and  W.  Numerical d e t e r m i n a t i o n s are very dependent on the c h o i c e of  the e i g h t parameters  U 38/y235 2  al.  (1957)  p a r t i n 500.  (1947),  0  c , t , X, V , 0  0  X" and a.  The  Fleming e t a l .  (1952),  and  Senftle  f a i l e d t o f i n d any v a r i a t i o n g r e a t e r than one Values f o r  A l d r l c h and W e t h e r l l l Farquhar  0  r a t i o f o r t e r r e s t r i a l samples i s remarkably c o n s t a n t .  Searches by Inghram et,  a , b ,  (i960).  V  (1958),  and V  have been reviewed by  and t a b u l a t e d by R u s s e l l  and  S e v e r a l d e t e r m i n a t i o n s of \ agree to w i t h i n  one p a r t i n 500.  The u n c e r t a i n t y i n A  i s one p a r t i n s i x t y .  1  A s l i g h t l y l a r g e r u n c e r t a i n t y o f about 2 p e r cent appears i n X".  Glendenin  (1961) r e p o r t e d t h a t Argonne N a t i o n a l L a b o r a t o r y  planned t o measure the decay c o n s t a n t s V  and V  e n r i c h e d sources o f U 35 and T h 3 2  anticipated that  2  these experiments  w i l l reduce  2  t  I  t  l  s  f o r highly  the u n c e r t a i n t y i n these two  c o n s t a n t s t o the same o r d e r as t h a t f o r \, The  i s o t o p i c composition o f l e a d i n the t r o i l i t e  (iron  s u l p h i d e ) phase o f i r o n m e t e o r i t e s i s g e n e r a l l y used as p r i m o r d i a l l e a d i s o t o p e abundances a , b Q  c  and c . 0  No s i g n i f -  i c a n t amounts o f uranium and thorium have ever been found i n i r o n meteorites.  Lead i s more c o n c e n t r a t e d i n the t r o i l i t e  phase than i n the i r o n phase, and f o r t h i s reason the r e s u l t s f o r the t r o i l i t e phase are more r e l i a b l e .  Even here, because  of the small q u a n t i t i e s o f l e a d i n v o l v e d , t h e r e i s some unc e r t a i n t y due t o l a b o r a t o r y contamination.  Nevertheless,  f a i r l y good agreement has been reached by f o u r d i f f e r e n t l a b o r a t o r i e s on seven a n a l y s e s o f f i v e d i f f e r e n t m e t e o r i t e s . The v a l u e s adopted and c  Q  i n t h i s t h e s i s , namely a  = 29.71, a r e the weighted  0  = 9.56, b  weighted The  = 10.42  v a l u e s o b t a i n e d by Murthy and  P a t t e r s o n (1962) from the above-mentioned a n a l y s e s . authors quoted  0  These  an estimated u n c e r t a i n t y of 1.5* f o r the  values. f i n a l parameter t o be s p e c i f i e d f o r the s i n g l e - s t a g e  model i s " t " , 0  to specify t  0  the "age o f the e a r t h " .  I d e a l l y , one would hope  on the b a s i s o f t e r r e s t r i a l common l e a d meas-  45 urements a l o n e . formed  The c a l c u l a t i o n s o f t h e age o f t h e e a r t h p e r -  (1942), Holmes (1946, 1947), Houtermans (1947),  by G e r l i n g  Bullard  and S t a n l e y (1949), a n d C o l l i n s ,  (1953) were i n e s s e n c e c a l c u l a t i o n s o f t experimentally determined it  R u s s e l l and P a r q u h a r 0  from the s l o p e s o f  i s o c h r o n s * o f known a g e .  However,  i s d o u b t f u l i f a m e a n i n g f u l v a l u e f o r t h e s l o p e c a n be  determined  even  above-mentioned reliable.  i f i s o c h r o n s do e x i s t , calculations  Similarly,  single-stage  reason the  a r e n o t c o n s i d e r e d t o be v e r y  the c a l c u l a t i o n s of Russell  (1956) a r e s u s p e c t . from  and f o r t h i s  Although  and A l l a n  they allow f o r s l i g h t  development  departures  f o r t h e l e a d s by assuming  that  the uranium/lead  ratio  differentiation,  t h e i r c a l c u l a t i o n s are very sensitive to  experimental  c a n v a r y l i n e a r l y w i t h t i m e due t o  error.  Other e s t i m a t e s o f t restrial  and m e t e o r i t i c  meteoritic  (x, y) which  lead  i n a closed  eralization  t.  are possible  results.  primordial lead  lead  Q  i f one c o m b i n e s  The u s u a l c o m b i n a t i o n i s o f  ( a , b ) with a t e r r e s t r i a l 0  Q  i s assumed t o have d e v e l o p e d f r o m system from  ter-  time t  0  until  When t i s g e o l o g i c a l l y  t h e time o f min-  determined,  the isochron  Thus, Houtermans  (1953)  to calculate  and P a t t e r s o n e t a l .  (1955) o b t a i n e d a v a l u e o f 4.5 x 10^ y e a r s  0  .  primordial  e q u a t i o n c a n be u s e d  for  t  common  t h e age o f t h e e a r t h .  * E l i m i n a t i o n o f V f r o m t h e f i r s t two e q u a t i o n s (3.2) y i e l d s This Isochron equation p r e d i c t s a l i n e a r y-b _ e ^ ^ - e ^ * r e l a t i o n s h i p b e t w e e n x a n d y f o r samples /"i't TE\ g e o l o g i c age w h i c h d e v e l o p e d x-a a ( e o-e ) d i f f e r e n t uranium-lead systems. !  Q  0  1  0  o  f  l  n  s  a  m  e  46 A third alone.  estimate  of t  isolated  meteoritic these  linearly.  T h e r e a r e now o v e r  lead isotope analyses available  i s d e f i n e d as the "primary  age  f o r m e t e o r i t e s " , o r the "geochron".  and  R u s s e l l (1963) have u s e d  of l e a s t  square  "geochron". The pass  the accepted  Q  five  (1962)  assessed  i n T a b l e s 3.1 and 3.2.  by c o n s t r a i n i n g the "geochron" t o  p r i m o r d i a l abundances change  i n Iron meteorites.  carefully  i n an x - y  isochron of zero  Kanasewich  These r e s u l t s a r e p r e s e n t e d  only  i s removed, i n d i c a t i n g t h a t t h e  i n t h e stone m e t e o r i t e s developed  observed  line  t h e m e t e o r i t i c d a t a i n a number  s l i g h t l y when t h i s c o n s t r a i n t lead  fifty  analyses i n order t o evaluate the slope of the  values obtained f o r t through  they  i n the l i t e r a t u r e ,  do i n d e e d d e f i n e a v e r y good s t r a i g h t  This line  that  s y s t e m s o f a common age, s h o u l d have x a n d  y values that are related  plot.  c a n be made f r o m m e t e o r i t i c r e s u l t s  Any number o f m e t e o r i t i c l e a d s , t o t h e e x t e n t  represent  and  Q  from  lead like  Murthy and P a t t e r s o n  the stone m e t e o r i t i c data  that  (1962) have  a n d have s e l e c t e d  which they f e e l b e s t f i t the assumptions d e f i n i n g t h e  "geochron".  Prom t h e s e  values they found  t  Q  = 4.55 *.03 x 10^y,.  In view o f the u n c e r t a i n t i e s i n v o l v e d i n a c c e p t i n g a l l the data, the  value  adopted  i n t h i s t h e s i s i s t h a t o b t a i n e d by Murthy and  Patterson.  Symbols a n d c o n s t a n t s u s e d  i n o b t a i n i n g ages f o r p r i m a r y  l e a d s and f o r c a l c u l a t i n g c h a r a c t e r i s t i c s o f t h e p r i m a r y s y s t e m a r e summarized i n T a b l e troduced ties  3.3.  lead  The u n c e r t a i n t i e s i n -  i n c a l c u l a t e d v a l u e s f o r t , V a n d W due t o u n c e r t a i n -  i n these  constants are d i s c u s s e d with the r e s u l t s (see  C h a r a c t e r i s t i c s of the Primary  Lead  System).  TABLE  3.1  47  THE AGE OP METEORITES USING p b /Pb AND Pb 07/pb 04 DATA (DIAGONAL LINEAR REGRESSION WITH NO CONSTRAINTS ON PRIMORDIAL ABUNDANCES) 2 o 6  2  2 0 4  2  Slope and Standard Deviation  Number of Analyses  Source  5  Patterson  0.6027  1  (Gyr.)*  .0090  4.58 * .03  A l l American Data  24  0.6045 * .0120  4.58 ± .03  A l l Russian Data  28  0.5578 ± .0293  4.47 * .08  A l l the Data  52  0.5950 * .0117  4.56 * .03  TABLE 3.2 THE AGE OP METEORITES USING P b / P b (DIAGONAL LINEAR REGRESSION THROUGH a 2 0 6  Number of Analyses  Source  i  2 0 4  0  ANb P b 0 7 / P b DATA - 9 . 5 6 , b - 10.42) 2  204  Q  Slope and Standard Deviation  t  0  (Gyr.)  5  0.5954 * .0087  4.56 * .02  A l l American Data  24  0.5922 * .0095  4.55 * .02  A l l Russian Data  28  0.5998 ± .0160  4.57 * .03  A l l the Data  52  0.5944 ± .0082  4.56 ± .02  Patterson  *  1 Gyr. - I O  9  years.  i 1  TABLE 3.3  48  SYMBOLS AND CONSTANTS USED IN AGE DETERMINATION  Isotope Ratio pb  206  P b  20  P b  u  208  238  204  204  /Pb  aVe A t  aV  Pb  Decay  to  x  U235/ 204 Th232  At Time  At Time t  204  /pb  /pb  Present t =0  204  /pb  7 / p b  At  204  Constants  aVe^o  V  Ve V t  Ve* *o  W  We V't  We*"to  Value  i n 10~9  (years)"*  1  1  0.1537**  u 38  0,9722***  n  2  0.0499****  *  u238/u235  i37.8 e t a l . (1955) e t a l . (1952)  xx  Kovarik  xxx  Fleming  xxxx  Picciotto  =  tt  S t  and W i l g a i n  Parent  (inghram  (1956)  (1947))  235  Atom  49 W i t h i n - D e p o s i t V a r i a t i o n s i n I s o t o p i c Composition Conformable Lead D e p o s i t s Primary l e a d s d e r i v e d from the same primary  of source at the  same time w i l l have the same i s o t o p i c c o m p o s i t i o n . conformable  Thus, i f  d e p o s i t s are composed o f u n a l t e r e d primary  of the same age, every conformable i s o t o p i c composition.  leads  deposit i s very uniform i n  Lead a n a l y s e s by the w r i t e r c o n f i r m  i s o t o p i c u n i f o r m i t y of these d e p o s i t s . All  samples w i t h i n a d i s t r i c t have been  intercompared,  and d i f f e r e n c e s i n i s o t o p e r a t i o s from those of the standard sample have been c a l c u l a t e d . i n Table 3.4.  The d i s t r i c t  each set o f d a t a .  district  These r e s u l t s are  listed  standard sample i s l i s t e d f i r s t  in  The b r a c k e t e d f i g u r e s a f t e r each d i f f e r e n c e  are the loop c l o s u r e d i s t r i b u t e d e r r o r s (see Table 2.6).  Dif-  f e r e n c e s i n i s o t o p e r a t i o s which a r e l e s s than twice the loop c l o s u r e e r r o r s are not c o n s i d e r e d t o be  significant.  Further,  d i f f e r e n c e s i n l e a d i s o t o p e r a t i o s r e l a t i v e t o lead-204 are not expected  t o be more p r e c i s e than  T h e r e f o r e , any d i f f e r e n c e s i n p b Pb208/p 204 b  40).  / P b ° 4 , Pb207/pb204 and 2  which are l e s s than .014,  t i v e l y , are not c o n s i d e r e d t o be No  2 o 6  .04 per cent (see page  .012  and  .026,  respec-  significant.  s i g n i f i c a n t within-deposit variations i n isotopic  composition were observed f o r C a p t a i n ' s F l a t , Rosebery, H a l l ' s Peak, Broken H i l l o r Mount I s a .  Perhaps C a p t a i n ' s F l a t  sample 141 i s d i f f e r e n t from the d i s t r i c t an apparent  d i f f e r e n c e i s observed  standard, but s i n c e  o n l y i n the Pb °6/pb 04  r a t i o , the d i f f e r e n c e c o u l d e a s i l y be due  2  2  t o experimental  error.  50  TABLE 3.4 ISOTOPIC COMPOSITION OP SAMPLES RELATIVE TO THEIR DISTRICT STANDARD SAMPLES (LOOP CLOSURE ERRORS)  Sample No, A ( P b  206 204) /pb  A.(Pb207/ 204) Pb  A  (Pb208  / p b  204)  Captain s Plat 1  160  136 141  147  159 162 165  -.0131 .005; -.0201 .005 +.008| .007 +.0071 .007 -.0001 .000 +.0101 .000  +.001(.066] -.009(.006 +.008(.001 +.0031.001 -.000(.003 +.oi3(.ob3;  -.005(.023] -.047(.02-.oio(.oo? -.024(.008] ^.024(.030 +.016(,030;  +.005(.004;  -.002(.003]  +.ooi(.oo4  +.007(.003 -.004(.001  -.001(.007] +.019(.007 -.02l(.003 -.01l(.003  Rosebery  72 71 73 75 77  8 Cobar, C.S.A. Mine  242 243 244 245 246 247  +^010(.004 -.002(.004  -,005(.001 -.009(.006 -.007(.006;  -,006(.006, +.008(.006 0  -.Ol6(.Ol4  -.023(.oi4;  +.035(.016 +.036(.0l6 +.028(.015 +.029(.015 +.oo8(.oii; -.075  + .008(.0071 + .009(.007 + .009(.007 .006(.007 + ,002(.008 + T h a r s i s Mine -.028  +.008(.007] +.007(.007 +.009(.001 +.007(.001  0t.Cobar Mine -.024  -.014  -.046  -.020(.001)  -,004(,003)  -.005(.003)  Queen Bee Mine -.029(.001)  -.002(.003)  -.034(.003)  +.005(.002]  +.001(.004]  -,ooi(.ooi;  -.028  116*  117*  S i l v e r Pk. "  118 119  H a l l ' s Peak  251 248 249 250 25 2** 253  +.007(.004] -.005C.002 +.007(.002] -.002 -,002(.004)  (Table continued on page * **  -.006(.008 +.oo6(.oo8; 51.)  -.002 -.004(.002)  -.010(.006 +.022(.006; -.010 -,Ol6(,004)  P i n a l a n a l y s i s t o c l o s e loop not completed. Measured r e l a t i v e t o No. 251, but not i n a l o o p .  TABLE 3.4  Sample No.  A(Pb  2 o 6  /Pb  2 0 4  )  51  (CONTINUED)  A (Pb  2 0  ?/Pb  2 0 4  ) A(Pb  2 o 8  /Pb  2 0 4  Bathurst  22 5 12  Mount  368 363 364 365 366 367  -,oo8(.ooo  +.003(.003 -.015(.003  0 +,002(.004] -.001(.00 +.0371.003; +.044(,003 -.012(.00l!  0 +.009(.006' +.009(.006 +.008J.007 +.0151.007 -,002(.000  +.013(.000 Farrell  +.062(.001) -,029(.001) 0 +.008| ,009 +.008! ,009 +.0501 ,007 +.054i ,007 -.014) ,007)  Manitouwadge  0 371 0 -.0161 ,011' 332 -.074(.006) -.0141 .011 369 -.069(.006 -.0121 .002 370 -.066(.010 +3.12K .002 372 H i l l +16.510(.010) Broken ( a n a l y s e s by K o l l a r ) 0 30 +.006( ,010) + .001 31* -.016 025, -.005( .028) 34* -.004 025 -,007( .028) 44 + .006 004 +.0151 .010) -,004( .020) -.Oil 034 49 Mount Isa ( a n a l y s e s by K o l l a r ) 1 (Broken) (Hill) 6 0  59 60 . 61 062 63  +.0 4(.020) +.12Q(.003) +.104(.0©3) +.116(,006) +.104(.0O6) 7  *  +.096(.003) +.104(.003)  +.034(.0I1 -,06l(.04l -,050(.04l +.057(.011 -,037(.044]  ,034; •005, ,00* ,00* .004]  +.115(.044; +.22o(.0l6 +.177(.0l6 +.1731 .017 +.1851 .017  +.059 .001) +.073 ,001)  +.150(.006) +.186(.006)  +.0371 +.0601 +.0481 +,072( +.067!  Mount Isa ( a n a l y s e s by O s t l c ) 1 (Broken) (Hill) 0 0  59 60  -.074(.021" -,062(.021 -.032J.005 +13.756(.005  Compared with Sample No, 49 r a t h e r than d i r e c t l y Sample No, 1.  with  )  TABLE 3.5  52  RELATIVE ISOTOPIC COMPOSITION OP D I S T R I C T STANDARD SAMPLES (LOOP CLOSURE ERRORS)  Sample Location No.  1 160 72 160 242  22 251 72 368  1  Broken  A(Pb  2 o 6  /Pb  2 0 l +  )  Hill 0  A(Pb  2 0  VPb  2 0 4  )  0  A(Pb  2 0 8  /Pb  2 0 4  0  Captain's  Plat  +2.063(.007)  +.222(.005)  +2.510(,007)  Rosebery  +2.256(.007)  +.207(.005)  +2.409(.007)  Captain's Plat  0  0  0  Cobar , . . (C.S.A.) +.033(.005)  +.009(.004)  =.032(.0O8)  +.110(,007)  +.021(.005)  ~.063(.007)  H a l l ' s P e a k + .3H(.007)  ~.010(.005)  +.194(.007)  Bathurst  Rosebery  0  0  0  Mount P a r rel!  +.327(.001)  +.033(.000)  +.453(.007)  0  0  Broken  Hill 0  371  Manitouwadge  22  Bathurst  0  0  0  White Island  +.596(.009)  -.035(.006)  +.555(.011)  384  =2.642(.045)  -.963(.021)  -2.467(.068)  53  TABLE 3.6 ISOTOPIC COMPOSITION OP GALENAS RELATIVE TO BROKEN H I L L STANDARD  Pb206  Sample MANITOUWADGE, O n t a r i o 332 W i l l r o y Mine 369 ( T - 6 5 8 ) L u n E c h o Mine 370 (T^659) Geco Mine 371 ( T - 8 5 2 J W i l l r o y Mine 372 ( T - 1 0 2 2 ) Geco V e i n Deposit BROKEN H I L L , N. S. W. 1 M a i n Lode 30 C e n t e n n i a l Mine 31 P i n n a c l e s Mine 34 M a i n L o d e , B r o k e n H i l l 44 L i t t l e B r o k e n H i l l 49 G l o b e Mine MOUNT ISA, 59 B l a c k 60 L e v e l body 6 1 Level body 62 L e v e l body 63 L e v e l  QUEENSLAND Star No. 1 1 , No. 2 OreNo. 1 2 , No. 5 Ore* No. 9,  No. 7 O r e -  No. 9  CAPTAIN'S FLAT, N. S. W.  136 450 L e v e l ,  2550S  141 1230 L e v e l , 6 5 O S 147 2030 L e v e l , 6 0 0 S 159 910 L e v e l , 9 0 0 S  160 910 L e v e l , 1 7 O O S 1 6 2 910 L e v e l , 4 5 0 S 165 1550 L e v e l , 6 5 O S COBAR, N, S, W.  242 243 244 245 246 247  C. C C. C. C. C.  116 T h a r s i s Mine  117 G r e a t C o b a r Mine  207  / P b  204  P b  208  / p b  204  14.563 14.565 14.567 14.579  33.527 33.539 33.569 33.601  29.983  17.700  47.357  16.116 16.122 16.111 16,109 16.131 16.II2  15.542 15.543 15.526 15.538 15.548 15.531  36.068 36.102 36.007 36.018 36.125 36.031  16.212  15.601  36.218  16.220  15.615  36.254  16.220  15.590  36.245  16.232 16.220  15.614 15.609  36.241 36,253  18.166 18,159 l8,187 18.186 18.179 18.179 18,189  15.765 15.755 15.772 15.767 15.764 15.764 15.777  38.573 38.531 38.568 38.554 38.578 38.554 38.594  18.184  15.745  18.192  15.769  18.183  119 Queen Bee Mine o n page  P b  13.399 13.404 13.407 13.473  18.188  Peak Mine  (Table continued  204  18.212 15.773 38.546 1 8 . 2 2 0 15.781 38.581 1 8 . 2 2 1 15.7803 8 . 5 18.221 15.782 3 8 . 5 7 4 18,218 15,780 38.575 18.214 15.772 38.554  S. A. Mine S. A. Mine S . A. Mine S. A. Mine S. A. Mine S. A. Mine  118 S i l v e r  /Pb  54.)  38.471  15.759  38.500  15.771  38.512  38.541  8  2  TABLE 3 * 6 (CONTINUED)  Sample  1ATHURST, N. B. 5 1» M. S. No. 6, S, of Orebody 12 1. M. S. NO. 6, N. Of Orebody 22 B. M« S* No. 12, Center of Orebody HALL'S PEAK* N. S, W, 248 249 250 251 252 253 ROSEBERY, TASMANIA  71 2 L e v e l , A Lens, 3000N  72 4 L e v e l , A Lens, 3400N 73 9 L e v e l , ¥ Lens, 300s 75 9 L e v e l , A Lens, 3750N 77 2 L e v e l , 1 Lens, 2750N 9 11 L e v e l , A Lens, 3700N 11 L e v e l , p Lens, 300S  S  MOUNT FARRELL, TASMANIA 364 365 366 367  P b  206  / f b  204  54  P b  2 0 7 / 2 0 4 Pb208 P b  /Pb  204  18,302  15.788  38,577  18,281  15.770  38.486  18,289  15.785  38.515  18,497 18.485 18.497 18.490 18,488 18.488  15.759 15*748 15.760 15*754 15*752 15*750  38.773 38.762 38.794 38.772 38.762 38.756  18.377 18,372 18,382 18.373 18,370 18,366 18,380  15.7^7 15.749 15*756 15*745 15.744 15*740 15*742  38,476 38.477 38,496 38,456 38.466 38.461 38,454  18,701 18,698 18,736 18.743 18,687 18,699  15*791 15.791 15*790 15.797 15.780 15.782  38*938 38,938 38,980 38,984 38,916 38,930  WHITE ISLAND, N, 2,—LEAD FROM FUMAROLIC ENCRUSTATION 384A 1 D i f f e r e n t F r a c t i o n s 18,879 15.749 384BJ ©f Same Sample 18.891 15*751  39*068 39*072  55 K o l l a r , R u s s e l l and U l r y c h v a r i a t i o n i n the p b  2 0 8  /Pb  2 0 4  (i960)  f e l t t h a t the range of  r a t i o f o r Broken H i l l  was g r e a t e r than experimental e r r o r .  samples  But samples 31  34  and  t o sample 49 r a t h e r than d i r e c t l y t o sample  were compared  1.  The loop c l o s u r e e r r o r was such t h a t these d i f f e r e n c e s ( r e l a t i v e t o sample 49)  were not s i g n i f i c a n t .  Further,  49 i s not s i g n i f i c a n t l y d i f f e r e n t from sample 1. o n l y Broken H i l l  sample  Thus, the  sample which may be s i g n i f i c a n t l y d i f f e r e n t  from sample 1 i s sample 44.  Since  larger closure  errors  were observed i n the other Broken H i l l l o o p s as done by K o l l a r , the w r i t e r suspects t h a t a s i m i l a r e r r o r i s i n h e r e n t loop c o n t a i n i n g within-deposit  sample 44,  and concludes t h a t no  i n the  significant  v a r i a t i o n s were observed f o r Broken H i l l .  K o l l a r , R u s s e l l and U l r y c h  (I960) s t a t e d t h a t the  v a r i a t i o n among the Mount I s a samples i s s i g n i f i c a n t . d i f f e r e n c e s between samples66^ 62 and 63,  The  however, are c e r t a i n l y  w i t h i n experimental e r r o r , and the loop c l o s u r e e r r o r f o r sample 59 i s l a r g e enough t o make i t suspect. t h e r e f o r e prepared f r e s h t e t r a m e t h y l - l e a d 60  The w r i t e r  f o r samples 59  (the extreme p o i n t s i n K o l l a r ' s a n a l y s e s ) and  them i n a loop c o n t a i n i n g  sample. 1.  w i t h K o l l a r ' s r e s u l t s f o r samples 6 l ,  and  reanalysed  These r e s u l t s , 62 and 63,  together  show t h a t no  s i g n i f i c a n t v a r i a t i o n s occur among the Mount I s a samples. The P b 0 / P b 0 2  245  and 246,  the d i s t r i c t  8  2  4  r a t i o f o r Cobar C. S. A. samples 243,  though very  s i m i l a r t o each other,  standard sample 242  244,  a l l d i f f e r from  by more than two  standard  56 deviations.  I t i s very improbable  p e r i m e n t a l e r r o r c o u l d occur.  t h a t such a c o n s i s t e n t ex-  T h e r e f o r e , the w r i t e r concludes  t h a t sample 242 i s s i g n i f i c a n t l y d i f f e r e n t i n lead-208 comp o s i t i o n from  samples 243,  244,  f e r e n c e i s o n l y +.032 * ,016 ratio.  245 and  (.08  246.  ± .04*)  T h i s small d i f -  i n the pb208/ 204 pb  Other Cobar d e p o s i t s appear to be s i g n i f i c a n t l y  f e r e n t from C. S. A. sample 242.  dif-  Again these d i f f e r e n c e s are  s m a l l , the l a r g e s t b e i n g the Pb °6/pb °4 r a t i o f o r sample 2  (Queen Bee), which i s o n l y -.029  119  2  ±  .007  (-.16  Three B a t h u r s t samples were analysed.  ±  ,04*).  Sample 5  was  prepared from a minimum q u a n t i t y o f l e a d i o d i d e (approximately 100 mg.),  and subsequent p u r i f i c a t i o n of the sample was  as e f f i c i e n t as was n o r m a l l y a t t a i n e d by the w r i t e r .  not  In f a c t ,  t r l m e t h y l - l e a d peaks were o n l y t w o - t h i r d s those o b t a i n e d f o r most samples under the same mass spectrometer o p e r a t i n g conditions.  The b r e a k s e a l tube c o n t a i n i n g sample 11 was  found t o  have a l e a k b e f o r e the sample had been analysed, and most of the sample was  pumped o f f i n an attempt  water from i t .  t o remove a i r and  F o r these reasons, the r e s u l t s f o r these  two  samples are suspect, and the small d i f f e r e n c e s i n the P b ° 8 / 2  p 204 D  r a  t i o s f o r sample 5, which c o u l d otherwise be c o n s i d e r e d  r e a l , may  be due  to the poor c o n d i t i o n of the sample r a t h e r  than t o a r e a l d i f f e r e n c e i n i t s i s o t o p i c composition. w r i t e r suspects t h a t f u r t h e r a n a l y s e s of samples from  The this  d e p o s i t would r e v e a l w i t h i n - d e p o s i t u n i f o r m i t y of the same o r d e r as t h a t observed  i n the o t h e r d e p o s i t s s t u d i e d .  57 Samples 332 and 369 from Manitouwadge are i d e n t i c a l within experimental error,  but samples 370 and 371 appear  to be s i g n i f i c a n t l y d i f f e r e n t from these,  Sample 372 i s from  a vein deposit, and Is of very d i f f e r e n t Isotopic composition. A l l Manitouwadge samples were p u r i f i e d by passing them through the same chromatographic column. was 332, 372, 371,  370 and 369.  The order of p u r i f i c a t i o n While differences between  samples 370, 371 and the other conformable leads may be r e a l , i t i s more probable that samples 370 and 371 have picked up some lead l e f t i n the chromatographic column from the puri f i c a t i o n of sample 372.  That Is,  the " p u r i f i e d " sample 371,  p u r i f i e d immediately a f t e r sample 372, contained approximately .7* lead of sample 372, and sample 370, p u r i f i e d Immediately a f t e r 371,  contained approximately ,2* lead of sample 372.  The amount of contaminating lead i n sample 369 was too small to be detected, Heal differences were suspected for.Mount F a r r e l l samples. This i s a vein deposit, rather than a conformable deposit, and was studied when the model age and growth parameters (V and W) f o r Rosebery were found to be unusual, f u l l y m the following section.  The P b  This i s discussed 2 o 6  /Pb  2 o 4  #  Pb °7/pb °4 2  2  and p b ^ / P b ^ r a t i o s f o r samples 365 and 366 were found to 2 0  2 0  d i f f e r from the d i s t r i c t standard sample by approximately  + .040 * ,007, +.012 * .006, and +.052 * ,013 (.21 * .04*, ,08 * .04*,  .13 * ,04$), respectively,  ferences were observed,  No other s i g n i f i c a n t d i f -  5  8  In conclusion,, the l a r g e s t s i g n i f i c a n t d i f f e r e n c e s i n i s o t o p e r a t i o s measured by the w r i t e r between samples from the same conformable d e p o s i t were ,08 ± ,o4 p e r cent o f the r a t i o s r e l a t i v e t o lead-204.  The l a r g e s t s i g n i f i c a n t  dif-  f e r e n c e s between samples from d i f f e r e n t conformable d e p o s i t s of the same d i s t r i c t were .16 ± . 0 4 p e r cent o f the r a t i o s r e l a t i v e t o lead-204.  C h a r a c t e r i s t i c s o f the Primary Lead System With the e x c e p t i o n o f one Manitouwadge sample (No, 372), and the Mount P a r r e l l l e a d s , a l l samples were analysed i n the b e l i e f t h a t they might be l e a d s t h a t were formed by a s i n g l e stage growth p r o c e s s .  Measured i s o t o p e abundances have thus  been i n t e r p r e t e d on the assumption o f such a growth p r o c e s s . The  i s o t o p e r a t i o s f o r a l e a d which developed  i n a single-  (3.2).  stage c l o s e d system are g i v e n e x p l i c i t l y by equations  These equations have been used t o c a l c u l a t e the apparent v a l u e s o f the parameters t , V and W f o r every sample from the observed  v a l u e s o f x, y and z. These are o n l y apparent  values  f o r t , V and W, s i n c e they depend on the v a l u e s o f >, V , A " , a  >  a  o* o b  a  n  d c  o  u s e (  *  i  n  equations  (3.2).  F u r t h e r , the  assumption of ..a s i n g l e - s t a g e development o f the l e a d s may not be v a l i d , i n which case the v a l u e s l o s e much o f t h e i r nificance.  Apparent v a l u e s f o r the r a t i o s u238/pt,204  have been c a l c u l a t e d by the r e l a t i o n s h i p s : Th/U  => W/(l + a)V.  u  2 3 8  /Pb  siga n a  2 0 4  m  - Th/U <N  S  These r a t i o s are both p r e s e n t v a l u e s f o r  the system i n which the l e a d s developed.  I f a l l l e a d s are  59 s i n g l e - s t a g e and developed i n the same system, the values calculated for  u 38/pb 2  samples, p r o v i d e d t , 0  204  and Th/U w i l l be the same f o r a l l  the c o r r e c t v a l u e s are used f o r a , b , c , 0  X, V , A" and a.  This i s equivalent  0  0  t o f i n d i n g t h a t they  l i e on the same growth curves i n the p l o t s o f y a g a i n s t x, and z a g a i n s t x. Apparent ages c a l c u l a t e d f o r the samples are used t o check the assumption t h a t the l e a d s are s i n g l e - s t a g e l e a d s , and to approximately t e s t the value  selected f o r t . 0  The model  age  f o r a deposit  should not be g r e a t e r than the g e o l o g i c age  for  the e n c l o s i n g sediments, f o r t h i s would suggest t h a t the  l e a d had been r e m o b i l i z e d .  I t i s d i f f i c u l t t o conceive  how  r e m o b i l l z e d l e a d c o u l d not p i c k up r a d i o g e n i c l e a d from the rocks through which i t moves, i n which case the assumption o f a s i n g l e - s t a g e growth p r o c e s s would no l o n g e r be  valid.  S i m i l a r l y , i f the apparent age o f the d e p o s i t i s much younger than the g e o l o g i c age o f the e n c l o s i n g sediments, the l e a d probably  would have a second stage o f growth.  Systematic  very dif-  f e r e n c e s between the model ages and g e o l o g i c ages c o u l d be due to an e r r o r i n the accepted  value f o r t , o r i n the assumption 0  t h a t primary l e a d s are s i n g l e - s t a g e The  apparent values o f t ,  samples analysed  leads.  u 38/ b 2  204  P  and Th/U f o r a l l  by the w r i t e r , and assumed t o be s i n g l e - s t a g e ,  are l i s t e d i n Table  3.7.  Mean values have been l i s t e d  rather  than i n d i v i d u a l values f o r samples which are assumed t o be i d e n t i c a l w i t h i n the l i m i t  of experimental  e r r o r (see p r e c e d i n g  TABLE  60  3.7  APPARENT VALUES OP URANIUM/LEAD AND THORIUM/URANIUM FOR CONFORMABLE DEPOSITS, CALCULATED FOR t = 4.55 Q  G e o l o g i c Age of Enclosing Sediments  Description (No. o f S a m p l e s )  Manitouwadge Broken H i l l  (3) (6)  Mount I s a (5) Rosebery  (7)  Captain's F l a t Cobar:  (7)  C. S. A.  (6)  ca. 3000 1500-1700 1600-1700 550 420 410  Tharsis (1) Great Cobar (1) S i l v e r P e a k (1) Queen B e e (1) Bathurst,  N. B.  H a l l ' s Peak White I s l a n d  (6) (1)  (3)  460 240 Modern  RATIOS Gyr.  Apparent V a l u e s Age  u238 204  2919 1616 1603 161 337 321 3 06 321 330 339 269 80 -240  8.61 8.98 9.09 8.93 8.98 9.00 8.94 8.97 8.99 8.99 9.00 8.93 8.89  /pb  Th/U  4.51 4.11 4.16 3.95 " 4.11 4.09 4.06 4.08 4.09 4.09 4.03 4.01 3.92  TABLE  61  3.8  APPARENT VALUES OP URANIUM/LEAD AND THORIUM/URANIUM RATIOS POR CONFORMABLE DEPOSITS, CALCULATED FOR t = 4.53 G y r . Q  Description (No. o f S a m p l e s )  G e o l o g i c Age of Enclosing Sediments  Apparent V a l u e s Age  Tj238 204 /Pb  Th/U  c a . 3000  2955  8.91  4.51  1500-1700  1672  9.20  4.12  Mount I s a (6)  1600-1700  1661  9.31  4.16  Rosebery  (7)  550  251  9.12  3.96  Bathurst  (3)  460  357  9.19  4.04  420  423  9.18  4.12  410  408  9.20  Man!touwadge Broken H i l l  (3) (6)  Captain's F l a t Cobar:  (7)  C. S. A. (6) Tharsis (1) Great Cobar (1) S i l v e r Peak (1) Queen Bee ( l j  H a l l ' s Peak (6) White I s l a n d  (1)  425  9.19  4.10 4.07 4.09 4.10 4.10  172  9.12  4.03  -138  9.08  3.93  393  407 416 240 Modern  9.14 9.16  9.18  62 section).  L i s t e d with these  v a l u e s are t h e accepted  ages f o r the sediments e n c l o s i n g the d e p o s i t s . i n the r e l a t i v e i s o t o p i c composition  geologic  The u n c e r t a i n t y  o f samples from d i f f e r e n t  d e p o s i t s i n t r o d u c e s an u n c e r t a i n t y o f approximately  ± 10 m.y.  i n t h e ages o f the d e p o s i t s , and ± ,02 i n the v a l u e s o f the ratios  U238 204 /Pb  and Th/U.  These u n c e r t a i n t i e s are l a r g e r f o r  Manitouwadge samples due t o the l a r g e r l i m i t o f e r r o r i n t h e i r analyses  (see Table  3.5), and the i n c r e a s e i n the dependence  of V and W on x, y and z as t i n c r e a s e s .  The estimated un*  c e r t a i n t y I n t f o r Manitouwadge i s ± 30 m.y., and f o r  U238/ Pb  2 0 4  and Th/U i s ±  .04.  Note t h a t a l l o f the apparent ages f o r the young d e p o s i t s (younger than 500 m.y. i n the g e o l o g i c time s c a l e ) l i s t e d i n Table 3.7 are younger than the g e o l o g i c age o f the e n c l o s i n g sediments.  T h i s c o n c l u s i o n cannot be drawn so d e f i n i t e l y f o r  the Precambrian d e p o s i t s due t o the l a r g e u n c e r t a i n t y i n the Precambrian g e o l o g i c time s c a l e . No independent age measurement e x i s t s f o r the Keewatin metasediments and m e t a v o l c a n i c s  o f Manitouwadge, and e s t i m a t e s  of t h e i r age can o n l y be made by d o u b t f u l c o r r e l a t i o n s t o o t h e r Keewatin r o c k s .  Catanzaro (1963) has r e c e n t l y r e p o r t e d  z i r c o n uranium-lead ages i n Southwestern Minnesota,  suggesting  a minimum c r y s t a l l i z a t i o n age o f 3300 m.y., and a p o s s i b l e z i r c o n age o f 3550 m.y.  T h i s suggests t h a t the Manitouwadge  metasediments and m e t a v o l c a n i c s  c o u l d be i n excess o f 3000 m.y.  o l d , i n which case the apparent age o f 2919  m.y, i s a g a i n t o o  63 young. Broken H i l l d e p o s i t s are e n c l o s e d by the Willyama s e r i e s of metasediments.  Age d e t e r m i n a t i o n s on the Willyama complex  i n c l u d e f i g u r e s o f 1730 m.y. f o r the 207/206 age o f Radium H i l l uranium m i n e r a l i z a t i o n , and 1510 m.y. f o r the 207/206 age f o r d a v i d i t e from Radium H i l l  (Wilson (1951)).  Greenhalgh and  J e f f e r y (195^) quote an age value f o r C r o c k e r ' s Well, (Crocker's W e l l i s 50 m i l e s t o the north-west o f Radium H i l l and about i  100 m i l e s west o f Broken H i l l ) ,  a l s o c a r r i e d out on d a v i d i t e .  The ages determined were 1628 m.y. (Pb °6/u 3 ), 1702 m.y. 2  (Pb °7/pb 2  2 0 5  ) , and  1  2  5  2 m  . . (pb  208  y  value f o r the Pb 08/ipj 232 2  1  a  g  e  i  S  /Th 32)'. 2  2  8  The very low  very f r e q u e n t l y observed, and  does not d e t r a c t from the r a t h e r good agreement between the uranium-lead ages.  These f i g u r e s support a major period-;of  t e c t o n i c a c t i v i t y a t the time l i s t e d i n Table 3.7 f o r BrokenHill  lead mineralization. The A u s t r a l i a n N a t i o n a l U n i v e r s i t y and the A u s t r a l i a n  Bureau o f Mineral..Resources have been c a r r y i n g out a j o i n t p r o j e c t i n the Mount I s a a r e a .  T h e i r f i n d i n g s have not been  p u b l i s h e d y e t , but J . R. R i c h a r d s r e p o r t s ( p e r s o n a l communication) s e v e r a l r u b i d i u m - s t r o n t i u m and potassium-argon ages i n the range 1600-1700 m . y . — f o r m e t a v o l c a n l c s and'metasediments i n the area e x t e n d i n g from Mount I s a north-west t o the  Macarthur R i v e r a r e a . The c o n s i s t e n t l y young apparent' ages f o r t h e P a l e o z o i c  64 and younger l e a d s suggests t h a t e i t h e r a l l the d e p o s i t s are e p i g e n e t i c , o r the s i n g l e - s t a g e model f o r t h e i r development i s not v a l i d , o r the v a l u e s o f the parameters used i n the c a l c u l a t i o n o f t , V and W are i n e r r o r .  The degree o f " y o u t h f u l -  n e s s " i s not the same f o r a l l the d e p o s i t s . 80 m.y. f o r Cobar and C a p t a i n ' s  I t v a r i e s from  P l a t t o 390 m.y. f o r Rosebery.  T h i s means t h a t a new c h o i c e o f parameters f o r c a l c u l a t i o n o f t , V and W w i l l not b r i n g a l l o f the apparent ages i n t o c l o s e agreement with g e o l o g i c ages.  I n p a r t i c u l a r , Rosebery w i l l  always appear t o be younger than C a p t a i n ' s Bathurst,  P l a t , Cobar and  whereas i t s g e o l o g i c age i s o l d e r than the g e o l o g i c  ages o f these d i s t r i c t s .  T h i s evidence suggests t h a t Rosebery  i s e i t h e r an e p i g e n e t i c d e p o s i t o r i t c o n t a i n s anomalous l e a d . I f i t i s e p i g e n e t i c , i t i s probably  a l s o anomalous, because a  l e a d t r a v e r s i n g uranium and l e a d - b e a r i n g probably  c r u s t a l rocks would  p i c k up a d d i t i o n a l r a d i o g e n i c l e a d .  t h i s evidence, anomalous.  On the b a s i s o f  i t i s concluded t h a t the l e a d o f Rosebery i s  Therefore,  the apparent values f o r t , u238/pb204  and Th/U c a l c u l a t e d on the s i n g l e - s t a g e assumption f o r Rosebery have no g e o l o g i c  significance.  An i n t e r p r e t a t i o n o f the Rose-  b e r y l e a d s , based on a model f o r anomalous l e a d s , i s g i v e n i n Appendix IV. I f the White I s l a n d sample c o n t a i n s modern s i n g l e - s t a g e l e a d , the age o f the e a r t h i s 4.50 for  t  0  T h i s I s the value  adopted by Houtermans (1953) and used by most European  researchers (1962),  Gyr,  (Eberhardt,  etc).  G e l s s and Houtermans (1955), Moorbath  Kanasewich (1962—Ph.D. t h e s i s ) has shown t h a t  65  J u s t i f i c a t i o n f o r the use o f 4 . 5 0 Qyr. as the  there I s l i t t l e age  o f the e a r t h from m e t e o r i t i c data  o r from t e r r e s t r i a l  values  the age 4 . 5 0 Gyr. would make  Further,  F l a t and Cobar samples 1 0 0 m i l l i o n y e a r s o l d e r than  t h e i r e n c l o s i n g sediments.  We t h e r e f o r e conclude t h a t the  White I s l a n d l e a d i s anomalous. if  3 . 1 and 3 . 2 )  samples i f one accepts P a t t e r s o n ' s  for primordial lead. Captain's  (see T a b l e s  T h i s i s not very s u r p r i s i n g  the source o f primary l e a d i s a t depth, s i n c e White I s l a n d  i s u n d e r l a i n by c o n t i n e n t a l c r u s t up t o 2 5 km. i n t h i c k n e s s , and  l e a d generated below t h i s would undoubtedly be contaminated  by l e a d generated from uranium and thorium i n the c r u s t p r i o r to i t s d e p o s i t i o n . The  P a l e o z o i c d e p o s i t s ' apparent ages are no l o n g e r  s y s t e m a t i c a l l y younger than the g e o l o g i c ages i f the age o f the e a r t h i s taken t o be 4 , 5 3 Qyr.  T h i s value o f t  Q  i s within  the u n c e r t a i n t y o f the value on m e t e o r i t i c evidence as g i v e n i n Tables Further  and  3.1  support  and by Murthy and P a t t e r s o n  3 . 2 ,  f o r t h i s value o f t  Q  i s g i v e n i n Appendix I I I  where a manuscript by O s t i c , R u s s e l l and Reynolds preprinted.  ( 1 9 6 2 ) .  (1963)  is  The c a l c u l a t i o n i s based on the s i n g l e - s t a g e l e a d  model and the.assumption t h a t a l l o r d i n a r y l e a d s were formed i n the same uranium-thorium-lead system.  E s s e n t i a l l y the c a l -  c u l a t i o n i s as f o l l o w s . F o r any s i n g l e - s t a g e l e a d sample o f i s o t o p i c ( l» x  t  X  i  y  i)'  *  n  e a  g  e  c  a  n  b  - * ^Se [e*to-(  e  X l  c a 3  -  c u l a  - a )j 0  composition  t e d by the e q u a t i o n s and t  y  i  - 1^ l o g  e  {e*'to,(  y i  . b )j. Q  66 A value f o r t  i s assumed, and from a set of s i n g l e - s t a g e l e a d s  0  ( x , 3 ^ ) , v i s c a l c u l a t e d s u b j e c t to the l i m -  of composition itation  ^  (t  A  - t y ^ ) = minimum • r e s i d u a l .  This calculation  2  X  l  i s repeated f o r s e v e r a l d i f f e r e n t v a l u e s of t .  The  0  residual  i s found as a f u n c t i o n of t , and has a minimum value f o r t 0  equal to 4.52 The  *  .03.  apparent  culated f o r t  0  0  l e a d ages f o r the Precambrian  equal to 4.55  Gyr. and 4.53  deposits c a l -  Gyr. are w i t h i n the  u n c e r t a i n t y of the g e o l o g i c ages of t h e i r e n c l o s i n g So f a r as age d e t e r m i n a t i o n s are concerned,  sediments.  t h e r e i s no  evidence  to support the c o n c l u s i o n t h a t they are not s i n g l e - s t a g e l e a d s which were d e p o s i t e d s y n g e n e t i c a l l y . The  same c o n c l u s i o n can  be drawn f o r C a p t a i n ' s F l a t and a l l Cobar d e p o s i t s s t u d i e d i f one accepts the value of 4,53  Gyr. f o r the age of the e a r t h .  B a t h u r s t and H a i l ' s Peak, however, s t i l l appear to be  either  e p i g e n e t i c , and/or c o n t a i n anomalous /lead. , a p p a r e n t v a l u e s f o r the r a t i o s u 38/Pb °4 and  The  4  i n T a b l e s 3.7  2  and 3.8  2  Th/U  show s e v e r a l s t r i k i n g r e l a t i o n s h i p s .  The v a l u e s f o r u 38/p 204 are v i r t u a l l y i d e n t i c a l f o r Broken 2  Hill,  b  C a p t a i n ' s F l a t , e i g h t Cobar samples, and B a t h u r s t ,  v a l u e s of Th/U  f o r these d e p o s i t s are a l s o v i r t u a l l y  with the e x c e p t i o n of B a t h u r s t , which has a s l i g h t l y value r e l a t i v e to the o t h e r s .  The  identical, low  This observation i s equivalent  t o the one made by R u s s e l l and Stanton  (1959)--that  conformable  l e a d s l i e v e r y c l o s e l y to a s i n g l e growth curve i n a p l o t of P b ° 7 / p b ° 4 a g a i n s t Pb 2  2  2 o 6  /Pb  2 0  \  and i n a p l o t of b ° 8 / p b 2  P  2 0 i +  67 against P b ^ / P b 20  204  .  The f i t to a single growth curve i s  i l l u s t r a t e d f o r these leads In Figures 3.1  and 3.2, Leads  from two deposits, Rosebery and White Island, have been considered to be anomalous on the basis of age data.  It i s  i n t e r e s t i n g to note that the values of t h e i r apparent U 38/ 2  Pb  2 0 4  and Th/U r a t i o s are d i f f e r e n t from those of the above  four d i s t r i c t s .  This evidence supports the hypothesis that  leads p l o t t i n g o f f a unique growth curve are anomalous. Small differences i n the apparent values of the r a t i o s U 38/p 204 and Th/U from those f o r Broken H i l l , Captain's F l a t , 2  D  and eight Cobar samples are observed f o r Mount Isa, the remaining two Cobar deposits and Hall's Peak.  Age data f o r  Hall's Peak suggests the lead i n t h i s deposit may be anomalous, but the geologic age evidence i s poor. u238/pb204  a n  a  Since the apparent  rph/u r a t i o s support t h i s conclusion, the writer  interprets r e s u l t s f o r Hall's Peak to indicate that Hall's Peak'lead i s anomalous.  He admits, however, that t h i s i n t e r -  pretation i s not unbiased, and that i t adds l i t t l e to the present investigation. There i s no a p r i o r i reason to suppose the leads of either Mount Isa or Tharsis and Great Cobar deposits are anomalous, and the apparent ages calculated f o r these deposits on the single-stage model suggest they are not.  The observed d i f -  ferences i n the r a t i o s u 38/pb204 and Th/U f o r these deposits 2  are  thus interpreted as r e a l differences i n these r a t i o s f o r  the sources i n which they developed.  3.1  FIGURE  PLOT SHOWING THE RELATIONSHIP BETWEEN THE RATIOS P b ^ / P b AND p b /Pb FOR CONFORMABLE LEADS, AND THE SINGLE-STAGE GROWTH CURVE FOR a 0 =9.56, b = 1 0 . 4 2 , aV=9.19 AND t 0 =4.53 G y r . 2  Q  14  P  B  206  /  P  B  204  0  4  2 o 6  2 0 4  FIGURE 3.2  69  PLOT SHOWING THE RELATIONSHIP BETWEEN THE RATIOS Pb 08/Pb 2 04 A N D Pb206/ P b 204 poR CONFORMABLE LEADS, AND THE SINGLE-STAGE GROWTH CURVE FOR a0=9.56, c0=29.71, aV=9.19, Th/U=4.10 AND t0=4.53 Gyr. 2  70 i The  v a l u e s f o r u 38/pb204  apparent  2  wadge a r e v e r y d i f f e r e n t other deposits.  These  f o u r ways, o f w h i c h  suppose  and t h o r i u m / u r a n i u m  that  different  apparent uranium/lead  ^0* ^*  a  enough d e t e r m i n e d ,  n  d  a  i  n  stage l e a d s , uranium which  Rosebery,  the d i f f e r e n t  they developed.  °-i  2  \  and thorium/uranium  a  t i  ratios  e i t h e r the constants a , b , Q  the geologic  0  leads.  criteria for  c o n t a i n s anomalous  lead.  leads are single-  apparent uranium/lead  and t h o r i u m /  r e a l d i f f e r e n c e s i n the sources i n  The mean v a l u e s a n d s t a n d a r d d e v i a t i o n s  f o r these r a t i o s ,  e x c l u d i n g t h e v a l u e s f o r Man-  and t h e a n o m a l o u s l e a d s f r o m R o s e b e r y ,  and W h i t e I s l a n d r  leads.  )  itouwadge  p 204  and C o b a r  o r primary leads are not single-stage  r a t i o s may r e f l e c t  /£(*i - 5c)  U  Alter-  t o those f o r the  I f Manitouwadge and o t h e r c o n f o r m a b l e  six  The d i f f e r e n t  l e a d s w h i c h may be p r i m a r y i s n o t r i g o r o u s , a n d  Manitouwadge, l i k e  D  for a l l  e q u a t i o n s (3.2) a r e n o t a c c u r a t e l y  fourth explanation i s that  selecting  /  Manitou-  r a t i o s may r e p r e s e n t  Captain's Plat  f o r Manitouwadge t h e n s u g g e s t t h a t  The  r  Manitouwadge l e a d s a r e p r i m a r y l e a d s  o f Broken H i l l ,  development  o*  o  with the r e s u l t s  deposits studied.  and have d e v e l o p e d i n a s o u r c e i d e n t i c a l  c  f  d i f f e r e n c e s i n t h e s o u r c e f o r Manitouwadge l e a d s .  natively,  The  /rj  v a l u e s c a n be e x p l a i n e d i n a t l e a s t  o n l y one i s c o n s i s t e n t  apparent uranium/lead  n  from t h e v a l u e s c a l c u l a t e d  f o r the other conformable  real  T  0  are l i s t e d  i n Table 3 . 9 .  Hall's  Peak,  The a p p a r e n t  u238/  f o r Manitouwadge d i f f e r s f r o m t h e mean v a l u e b y  standard deviations,  a n d i t s a p p a r e n t Th/U r a t i o  differs  71 from the mean value by f o u r t e e n standard d e v i a t i o n s .  Such  l a r g e d i f f e r e n c e s are very p r o b a b l y not r e a l , and Manitouwadge does not c o n t a i n s i n g l e - s t a g e l e a d . Alternatively,  suppose t h a t conformable  stage l e a d s which developed accepted v a l u e s f o r a p 204 D  r a  0  l e a d s are s i n g l e -  i n very s i m i l a r sources.  and b  I f the  are i n e r r o r , the apparent  0  U ^/ 2  t i o s f o r the d e p o s i t s w i l l vary s y s t e m a t i c a l l y with the  c a l c u l a t e d ages f o r the d e p o s i t s . i s not observed; nificant,  Such a systematic  therefore, errors i n a  0  Thus the low apparent u ^ Q / P b  or b 2 0 4  0  variation  are n o t s i g -  ratio;  f o r Manitou-  wadge cannot be e a s i l y e x p l a i n e d by e r r o r s i n ( a , b ) . 0  0  The decay constant f o r U 38 2  (A) has been determined  t o one  p a r t i n 500, but t h a t f o r u 35 (A ) i s accurate t o o n l y one 2  p a r t i n 60,  1  Could changes i n A' Improve the f i t o f Manitouwadge  to the s i n g l e - s t a g e growth curve i n the x-y p l o t through Broken Hill,  C a p t a i n ' s P l a t and Cobar?  To t e s t t h i s , growth curves  were c a l c u l a t e d f o r s e v e r a l v a l u e s o f A' i n the range .9562 t o .9882. A'  The parameters t  Q  and V were v a r i e d s i m u l t a n e o u s l y with  so t h a t the growth curves passed through the same end p o i n t s  f o r t = 0 and f o r t = t . 0  The changes i n A ' , t  0  and V were  found t o change the time s c a l e along the growth curve, but the growth curve i t s e l f  (y = f ( x ) ) was not changed  significantly,  The m i s f i t o f Manitouwadge cannot be e x p l a i n e d by an e r r o r i n A ' . I f primary l e a d s are not s i n g l e - s t a g e l e a d s , equations (3.2)  are not v a l i d .  However, the r e s u l t s f o r o t h e r conform-  able l e a d s (with the e x c e p t i o n o f Rosebery) suggest t h a t the  72 s i n g l e - s t a g e assumption  i s a remarkably good approximation f o r  l e a d s younger than 1700  m.y.  r e q u i r e s the growth from 1700  Any model f o r primary l e a d s thus m.y.  ago to the p r e s e n t t o be  e q u i v a l e n t t o t h a t f o r s i n g l e - s t a g e growth.  E. R. Kanasewich  has analysed samples from C o b a l t , O n t a r i o , which he b e l i e v e s are  primary.  R e s u l t s f o r these a n a l y s e s are g i v e n i n Table  The apparent uranium/lead and thorium/uranium  3.9.  r a t i o s f o r these  l e a d s are very s i m i l a r t o those of the conformable d e p o s i t s . T h i s suggests primary l e a d s have developed i n a manner e q u i v a l e n t t o t h a t f o r s i n g l e - s t a g e growth from approximately 2300 m.y.  ago t o the p r e s e n t .  I f Manitouwadge l e a d i s primary and  has developed i n a source s i m i l a r t o the sources f o r o t h e r l e a d s which have been assumed primary, then the sources f o r primary l e a d s must have been c o n s i d e r a b l y e n r i c h e d i n uranium  and  impoverished i n thorium r e l a t i v e to l e a d between the time of emplacement of Manitouwadge and 2300 m.y.  ago.  They then must  have remained e s s e n t i a l l y c l o s e d systems from 2300 m.y.  ago t o  the p r e s e n t .  stone  Murthy and P a t t e r s o n (1962) observed t h a t  m e t e o r i t e s which are e n r i c h e d i n uranium  r e l a t i v e t o l e a d are  a l s o e n r i c h e d i n thorium to the same degree.  It i s unlikely  t h a t p h y s i c a l and chemical p r o c e s s e s i n primary m e t e o r i t i c b o d i e s are much d i f f e r e n t from those i n the e a r t h , and an enrichment i n uranium accompanied by an impoverishment for terrestrial  i n thorium  systems seems t o the w r i t e r t o be very u n l i k e l y .  F u r t h e r , i f primary l e a d s developed along a growth curve o t h e r than a s i n g l e - s t a g e one u n t i l  say 2500 m.y.  ago,  and  developed  along a s i n g l e - s t a g e growth curve from then u n t i l the p r e s e n t , the s i n g l e - s t a g e growth curve would not pass through p r i m o r d i a l  73  TABLE 3.9 APPARENT VALUES OP URANIUM/LEAD AND THORIUM/URANIUM RATIOS POR LEADS ANALYSED AT THE UNIVERSITY OP BRITISH COLUMBIA AND ASSUMED TO BE PRIMARY  Apparent for t = 0  Values  4.55 Gyr.  Apparent for t = 0  Values  4.53 Gyr.  Description Age ^ (No. o f Samples) (m.y.) u ^ / P b  Age Th/U (m.y.)  S u l l i v a n Mine, B. C. (2)  1338  9.02  4.07  1400  9.23  4.07  No. 331, Treadw e l l Yukon, Sudbury (1)  1615  9.03  4.07  1672  9.25  4.08  South-West P i n l a n d * * * (5)  1799  8.98  4.07  1852  9.21  4.08  No. 359 E r r i n g t o n Mine, Sudbury* (1)1917  9.02  4.30  1968  9.25  4.31  No. 489 C o b a l t , O n t a r i o (1)  2247  9.02  4.13  2294  9.27  4.13  No. 195 Lawson Mine, C o b a l t (1)  2329  9.02  4.16  2374  9.28  4.17  Average V a l u e s from T a b l e s 3.7 and 3.8  8.99  4.09  9.19  4.10  Standard D e v i a t i o n f o r V a l u e s from T a b l e s 3.7 and 3.8**  ±.05  ±.03  ±.05  ±.03  Q  x  xx  k 2 0 4  y238 204 /Pb  L e a s t r a d i o g e n i c sample o f a s e t o f anomalous l e a d s ,  ,  <T-= /£(xj-x)2 V n-1  xxx Prom W h i t t l e s (1962).  T  h  /  U  74 lead.  But the l e a d s other than Manitouwadge,  assumed primary, do- f i t  which have been  v e r y c l o s e l y t o a s i n g l e - s t a g e growth  curve p a s s i n g through p r i m o r d i a l l e a d . The f i n a l c o n c l u s i o n t o be drawn, by the p r o c e s s of e l i m i n a t i o n of a l l other p o s s i b i l i t i e s , l e a d s are anomalous.  i s that  Manitouwadge  An i n t e r p r e t a t i o n of these l e a d s , based  on a model f o r anomalous l e a d s , i s g i v e n i n Appendix IV. The r e s u l t s f o r a l l d e p o s i t s have been shown t o be c o n s i s t e n t with the h y p o t h e s i s t h a t primary l e a d s l i e c l o s e to unique growth curves i n p l o t s of Pb207/Pb204 against Pb206  / P b  204,  ^  o  f  P b  208  / P b  204  a  g  a  l  n  s  t  Pb 06 2  / P b  204  >  L  e  a  d  s  which do not l i e c l o s e t o the curves have been shown to be anomalous.  R e s u l t s l i s t e d i n Table 3*9 f o r l e a d s analysed by  o t h e r workers at U. B. C. and assumed t o be primary do, with one e x c e p t i o n , l i e w i t h i n the narrow band observed f o r conformable l e a d s . Sinclair),  The d e p o s i t s of S u l l i v a n , B. C ,  (analysed by A.  and Treadwell Yukon, Sudbury, Ontario, (analysed by v  E. R. Kanasewich) have been d e s c r i b e d by Stanton as b e i n g o f the conformable type.  E r r i n g t o n Mine, Sudbury, (analysed by  T. J . U l r y c h ) does c o n t a i n anomalous l e a d , and the r e s u l t s l i s t e d are f o r the l e a s t r a d i o g e n i c sample analysed. apparent Th/U r a t i o f o r t h i s i s much h i g h e r than any by the w r i t e r (with the e x c e p t i o n of Manitouwadge, anomalous).  The observed  which i s  I t i s p o s s i b l e , t h e r e f o r e , t h a t t h i s sample i s  a l s o s l i g h t l y anomalous.  However, U l r y c h ' s a n a l y s e s were o f  v e r y small samples, and were not done by complete i n t e r -  75 comparison.  T h e r e f o r e , the h i g h Th/U r a t i o may be due t o  experimental e r r o r . The f i t o f primary l e a d s t o unique growth c u r v e s I s q u i t e remarkable, and.suggests the occurrence o f a world-wide homogeneous source f o r the l e a d s ,  T h i s source appears t o have  remained v i r t u a l l y a c l o s e d system from a time e a r l y i n the e a r t h ' s h i s t o r y u n t i l the p r e s e n t .  Many s c i e n t i s t s have  thought o f the mantle as b e i n g molten d u r i n g a very e a r l y phase of the e a r t h ' s development and D e S i t t e r (1959)),  (see f o r example J e f f r e y s (1952),  and the uranium,  thorium and l e a d c o u l d  have been u n i f o r m l y d i s t r i b u t e d i n an upper l a y e r of the mantle d u r i n g t h i s time,  CHAPTER 4  76  CONCLUSIONS  The isotopic  purpose  o f t h i s t h e s i s has been t o i n v e s t i g a t e the  composition of primary leads.  b e e n e x a m i n e d , namely, t h a t leads,  conformable  Two h y p o t h e s e s  leads are primary  and t h a t p r i m a r y l e a d s a r e s i n g l e - s t a g e  c o n c l u s i o n s drawn h e r e  are based  formable  leads.  volcanic  l e a d s have b e e n u s e d  As e x p e c t e d ,  have  on i s o t o p i c  leads.  The  analyses o f con-  I n c e r t a i n c a s e s a n a l y s e s o f v e i n l e a d s and to c l a r i f y  the geologic c r i t e r i a  i d e n t i f y conformable  the i n t e r p r e t a t i o n s . used by Stanton t o  d e p o s i t s have b e e n f o u n d t o be  sufficient  to i d e n t i f y d e p o s i t s which a r e very u n i f o r m i n i s o t o p i c composition. formable  The f i n d i n g s o f t h i s  research indicate  l e a d s a r e sometimes v e r y s i m i l a r t o p r i m a r y  b u t o t h e r t i m e s c a n be s i g n i f i c a n t l y l e a d s from  two c o n f o r m a b l e  were f o u n d  t o be anomalous,  anomalous.  that either  and t h o s e f r o m  In particular,  two o t h e r d e p o s i t s This  deposits i s not v a l i d  or the lead of fumarblic o r i g i n  a primary-lead composition,.  i s not part  i t s present-day  for a l l  does n b t always  1  have  i s sup-  I t i s true  that  o f an I s l a n d a r c , b u t i t i s p o s s i b l e  geology  some o f t h e c o n f o r m a b l e  i s cpmparable t o t h e geology o f  d e p o s i t s a t t h e time  of t h e i r formation.  A r g u m e n t s have b e e n p r e s e n t e d whi,ch a p p e a r ;fco p r o v e White I s l a n d  suggests  The l a t t e r p o s s i b i l i t y  p o r t e d by t h e e v i d e n c e f r o m White I s l a n d .  that  leads,  Stanton's volcanic-sedimentary i n t e r p r e t a t i o n o f  the f o r m a t i o n o f conformable  White I s l a n d  con-  d e p o s i t s (Manitouwadge and R o s e b e r y )  ( H a l l ' s P e a k a n d B a t h u r s t ) may be a n o m a l o u s .  deposits,  that  sample c o n t a i n s a n o m a l o u s l e a d ,  that the  and t h e r e f o r e any  77 c o n f o r m a b l e d e p o s i t s f o r m e d n e a r W h i t e I s l a n d by s u g g e s t e d by fact  that  Stanton  some c o n f o r m a b l e d e p o s i t s h a v e b e e n f o u n d t o  hypothesis  are on  for their  is difficult  syngenetic the  are  disprove  to decide  The  earth.  age  I f the 20  as  m.y.,  epigenetic.  The  enough t o u s e  accepts  of the the  age  the  geologic 4.53  Paleozoic  of the  many c o n f o r m a b l e d e p o s i t s  i s very  they  l e a d s c o u l d more e a s i l y be lead age  ages would of  the  single-stage *  .02  value  Gyr.  are  Prom t h e  growth curve,  age  of the  age  of  appear to  been  c l o s e t o the  particular-  i t is  since  epigenetic the  I s a good v a l u e conformable  o f 4.52 these  *  in  inferred  In t h i s case,  Gyr.  or  lead found  Therefore,  syngenetic,  f i t of the values  accurately  syngenetic has  as  be  determined  However, t h e  have b e e n c a l c u l a t e d , and  f o r the  the  the  anomalous.  s u g g e s t t h a t 4.53  earth.  as the  deposits  of primary leads.  suppose t h a t  enclosing  This fact  (1963).  tempting to  ages o f t h e i r  e a r t h I s not  of a deposit.  composition  anomalous)  e a r t h i s g r e a t e r t h a n t h i s by  Richards  emphasized by  isotopic  Gyr.  model l e a d ages t o t e s t  epigenetic nature  deposits  model l e a d ages ( c a l c u l a t e d  s i n g l e - s t a g e model f o r l e a d s which are not  I f one  contain  volcanic-sedimentary  whether conformable  or epigenetic.  sediments only  ly  Stanton's  The  development.  i n good a g r e e m e n t w i t h  little  mechanism  would a l s o c o n t a i n anomalous l e a d .  anomalous l e a d does not  It  the  .03  model  for  the  leads to  Gyr.  a 4.54  and  f i g u r e s support 4.55  e a r t h lower than the  a  Gyr.  i obtained  f o r meteorites.  formable leads  are  The  independent  l a t t e r c a l c u l a t i o n s from of the  estimated  con-  geologic  ages  78 of the deposits. Many investigators have observed systematic deviations from a single-stage growth curve i n an x-y diagram f o r samples which have been assumed to be primary.  In order to explain  these observations, they postulated models which were considerably more complicated than the single-stage model defended here.  Damon (1954) assumed a two-stage growth f o r  primary lead, the f i r s t stage corresponding to growth i n the s i a l j and the second corresponding to growth i n the sima,  Bate  (1955) has taken d i f f e r e n t i a t i o n into account by allowing V to vary exponentially with time.  An equivalent model has been  presented by Vinogradov et a l . (1959) i n which d i f f e r e n t i a t i o n Is approximated  by proposing a nine-stage model i n which V  assumed a d i f f e r e n t constant value f o r each stage.  More  recently Patterson and Tatsumoto (19^3) have considered a d i f f e r e n t i a t i o n model i n order to explain the isotopic composition of lead i n d e t r i t a l feldspars.  With the increased p r e c i s i o n  of measurement achieved by the present writer, and improved sample selection, the data presented i n t h i s thesis suggests that s i g n i f i c a n t systematic deviations from a single-stage growth curve do not exist f o r primary leads.  A l l significant  d i f f e r e n t i a t i n g processes postulated i n the above references must now be assumed to occur i n a region other than that i n which primary leads are formed.  That i s , the above authors  have inadvertently included anomalous leads i n t h e i r c a l culations*  79 Thorium/uranium r a t i o s f o r c r u s t a l r o c k s have been determined  by d i r e c t chemical and r a d i o m e t r i c a n a l y s e s by  K e e v i l (19^4), W h i t f i e l d e t a l , (1959) and Rogers and Ragland (1961),  The d i s t r i b u t i o n t h a t was observed f o r t h i s r a t i o i s  i l l u s t r a t e d i n F i g u r e 4.1, r e g i o n o f the f i n a l  I t i s v e r y - s i m i l a r t o t h a t f o r the  stage of development of anomalous l e a d s as  c a l c u l a t e d by R u s s e l l , Kanasewich and O s t i c l u s t r a t e d i n F i g u r e 4.2.  (1963) and i l -  T h i s s i m i l a r i t y c o n f i r m s the c o n c l u s i o n  that anomalous l e a d s have had t h e i r f i n a l stage o f development i n the e a r t h ' s c r u s t .  The thorium/uranium r a t i o s c a l c u l a t e d  i n t h i s t h e s i s f o r l e a d s which have not been i d e n t i f i e d as anomalous (see T a b l e s 3.7, 3.8 and 3.9) have a range which i s more than two o r d e r s o f magnitude narrower than t h a t f o r c r u s t a l rocks.  observed  Thus, i f primary l e a d s have had a c r u s t a l  h i s t o r y s i m i l a r t o t h a t f o r anomalous l e a d s , there must have been very thorough homogenlzation  p r i o r to t h e i r d e p o s i t i o n .  A more p l a u s i b l e i n t e r p r e t a t i o n i s t h a t primary l e a d s have developed formable  i n a homogeneous s u b - c r u s t a l system, and t h a t conl e a d s are primary l e a d s which, i n many cases, have  had l i t t l e  o r no c r u s t a l a l t e r a t i o n .  Leads which are found t o  d i f f e r from primary l e a d s are then assumed t o have s t a r t e d out as primary l e a d s , but have subsequently been a l t e r e d by c r u s t a l processes,  Lead i s o t o p e s t u d i e s i n t h i s way p r o v i d e a  means of i n v e s t i g a t i n g m a n t l e - c r u s t a l r e l a t i o n s h i p s f o r the earth.  FIGURE  80  4.1  DISTRIBUTION OF THE THORIUM/URANIUM RATIO FOR CRUSTAL ROCKS AS DETERMINED BY DIRECT CHEMICAL AND RADIOMETRIC ANALYSES  15  T KEEVIL  (1944) A c i d i c and I n t e r mediate  10  I  |  Basic  __p.  a) H CO M  o  8 20  ROGERS AND RAGLAND  (1961)  WHITFIELD ET AL.  (1959)  15  10  •  ^ T 8 - T 4  w + .4  l o g (Th/U)  p  4-1,2  8 1  FIGURE 4.2 DISTRIBUTION OF THE THORIUM/URANIUM RATIO FOR THE FINAL STAGE OF DEVELOPMENT OF ANOMALOUS LEADS (FROM RUSSELL, KANASEWTCH AND OSTIC, ( 1 9 6 3  ) )  5 0 &  Goldfields, Saskatchewan  40-  3 0  2 0  1 0  mm,  -1.2  f////M  -.8  i—|  +.4  log  (Th/U)  + . 8  +1.2  +1.6  82  APPENDIX I  Compensation f o r Uniform V a r i a t i o n i n Peak Height Uniform  v a r i a t i o n s i n the h e i g h t of a l l recorded peaks  occur d u r i n g an a n a l y s i s of t e t r a m e t h y l - l e a d .  Significant  e r r o r s can be made because of these v a r i a t i o n s i f scanning i s done i n o n l y one d i r e c t i o n , i f the scanning r a t e i s non-uniform, o r i f the decay i s n o n - l i n e a r , u n l e s s s t e p s are taken to compensate  f o r the v a r i a t i o n s .  The u s u a l method a p p l i e d to com-  pensate  f o r these v a r i a t i o n s i s to scan i n p a i r s (as has been  done by the w r i t e r ) , and then to average the upwmassnaridcldownmass peaks.  A l t e r n a t i v e l y , l e a d i s o t o p e r a t i o s may  be computed  from the o b s e r v a t i o n s of s i n g l e scans and then averaged.  Both  methods are e q u i v a l e n t to assuming l i n e a r d r i f t d u r i n g each spectral pair,  Two  measurements, o b t a i n e d by scanning  s p e c t r a i n o p p o s i t e d i r e c t i o n s , are needed f o r one computation of the i s o t o p e r a t i o s ,  the  reliable  In the method used by the,  w r i t e r , a s i n g l e measurement of each peak, t o g e t h e r w i t h  the  time of o b s e r v a t i o n of the peaks, i s s u f f i c i e n t f o r one r e l i a b l e computation of the i s o t o p e r a t i o s .  The method i s  a l s o a p p l i c a b l e t o s p e c t r a o b t a i n e d from instruments the M e t r o p o l i t a n - V i c k e r s MS-2  by scanning i n o n l y one  I f a l l the t r i m e t h y l - l e a d peaks were observed  as  direction.  simulta-  neously, u n i f o r m v a r i a t i o n s i n peak h e i g h t s would no be a problem,  such  longer  T h i s i s i m p r a c t i c a b l e , but the e q u i v a l e n t can  be achieved i f they are observed  at d i f f e r e n t times, and  one  can c a l c u l a t e what t h e i r v a l u e s would have been at the same time.  T h i s i s what has been attempted at the U n i v e r s i t y of  83 B r i t i s h Columbia i n compensating f o r u n i f o r m v a r i a t i o n s i n peak h e i g h t .  C a l c u l a t i o n s were done i n two stages:  the "decay laws" were found,  and second, these  first,  "decay laws"  were a p p l i e d t o c a l c u l a t e what peak h e i g h t s would have been measured had a l l measurements been A l l peaks o f a spectrogram  simultaneous.  were assumed t o decay d u r i n g  an a n a l y s i s a c c o r d i n g t o the same c u b i c decay law. i s o f the form of h e i g h t x  ;  x = x ( i + at +  T h i s law  p t + 7t3) where a peak i s 2  Q  a t time t , and h e i g h t x  Q  a t zero time.  Observa-  t i o n s or the 253 peak were used t o c a l c u l a t e a, p and y by the method o f l e a s t squares.  Two o t h e r v a l u e s f o r these  coef-  f i c i e n t s were s i m i l a r l y o b t a i n e d from the 251 and 252 peak v a l u e s , and the t h r e e v a l u e s were averaged f i c i e n t s f o r the mean decay law.  t o o b t a i n the coef-  A l l peak h e i g h t s were then  e x t r a p o l a t e d t o zero time by the above f o r m u l a .  Extrapolated  v a l u e s o f the peak h e i g h t s were subsequently used t o c a l c u l a t e the l e a d i s o t o p e r a t i o s .  A l l c a l c u l a t i o n s were done w i t h the  f a c i l i t i e s of the U n i v e r s i t y o f B r i t i s h Columbia Computation Centre. C a l c u l a t i o n s f o r a p a r t i c u l a r spectrogram In F i g u r e 1,1, two  are i l l u s t r a t e d  G e n e r a l l y the n e t v a r i a t i o n s were l e s s  p e r cent o f the I n i t i a l peak h e i g h t s .  The spectrogram o f  F i g u r e 1,1 was s e l e c t e d t o show the " f i t " o f the c u b i c i n an extreme case. t h e i r average x  0  Percentage  than  curve  changes i n peak h e i g h t s from  v a l u e s have been c a l c u l a t e d f o r the 249, 251,  252 and 253 peaks, and the e x t r a p o l a t e d v a l u e s have been n o r -  FIGURE  1.1  PEAK HEIGHT VARIATIONS I N A TYPICAL TRIMETHYL-LEAD SPECTROGRAM, WITH CORRECTIONS FOR CUBIC DECAY  .85 m a l i z e d by d i v i d i n g them by t h e i r r e s p e c t i v e mean x It  i s c l e a r from F i g u r e 1.1  Q  values.  t h a t the v a r i a t i o n s i n peak h e i g h t s  have been the same f o r a l l peaks.  T h i s j u s t i f i e s the use  of  an average decay law t o e x t r a p o l a t e a l l peak h e i g h t v a l u e s to zero  time. The f i t t o a c u b i c curve i n t h i s case i s f a i r , but i t i s  apparent t h a t a h i g h e r o r d e r curve would be a b e t t e r c h o i c e . The c a l c u l a t i o n s have been repeated f o r a f i f t h o r d e r decay curve, and are shown i n F i g u r e 1.2, curve are no l o n g e r polynomial, going t o a h i g h e r o r d e r c u r v e .  V a r i a t i o n s about  this  and l i t t l e would be gained  by  1.2  FIGURE  PEAK HEIGHT VARIATIONS I N A TYPICAL TRIMETHYL-LEAD SPECTROGRAM, WITH CORRECTIONS FOR 5TH ORDER DECAY 1  1  1 >°r»  • . fx  •ix  .  .  ^ Peak H e i g h t s Decay  5TH . O r d e r  • 24 9  Peak  + 25 1  Peak  O 252  Peak  X253  Peak  1  20  IN  Peak  Heights  oo  1  30 T I'M E  Removed  Curve  Observed  1 10  Decoy  with  40 MINUTES  50  ON 60  APPENDIX I I  Pressure  Scattering  87  Correction  Some i o n s i n a mass s p e c t r o m e t e r i o n beam c o l l i d e interact are  w i t h r e s i d u a l gas present  scattered.  i n the a n a l y s e r tube  T h i s s c a t t e r i n g produces t a i l s  peaks which a r e o b s e r v e d as an a p p a r e n t r i s e l e v e l underneath the adjacent peaks.  The  a t w h i c h t h e sample  i n recorder  spectrometer, We  and how  isotope  on the p r e s s u r e  t h e r e s o l u t i o n o f t h e mass  b a s e l i n e s a r e drawn on t h e s p e c t r o g r a m .  draw t h e b a s e l i n e u n d e r t h e 249 mass b y j o i n i n g  on e i t h e r  side o f i t by a s t r a i g h t  joining with a straight  line  line.  251,  the t h r e e major peaks a t masses  measured  base  scattering i n -  among o t h e r t h i n g s ,  i s analysed,  and  to the i o n  t r o d u c e s a n e r r o r i n t h e measurement o f r e l a t i v e abundances which depends,  or  the  minima  The b a s e l i n e s f o r  252 and 253 a r e drawn b y  the c o r r e s p o n d i n g base  lines  w e l l beyond the l i m i t s o f the t r i m e t h y l - l e a d  spectrum.  B a s e l i n e s were s k e t c h e d b y h a n d u n d e r t h e mass 250 and mass .254 p e a k s a s shown i n F i g u r e in  t h i s way,  pressure  II.1.  s c a t t e r i n g would cause t h e masses  252 and 253 measurements  t o be t o o l a r g e ,  measurement t o be t o o s m a l l . this  source o f e r r o r  at low p r e s s u r e s ,  When t h e b a s e s a r e drawn  and t h e mass 249  A t t e m p t s h a v e b e e n made t o k e e p  s m a l l b y o p e r a t i n g t h e mass s p e c t r o m e t e r  by h a v i n g pure t e t r a m e t h y l - l e a d  b y u s i n g a n i n t e r c o m p a r i s o n t e c h n i q u e whereby the  same f o r a l l  s a m p l e s so t h a t  A t t h e same t i m e a c o r r e c t i o n , applied f o r pressure absolute  isotope  251,  relative  samples,  the e r r o r s are  e r r o r s are  as o u t l i n e d below, has  s c a t t e r i n g b y w h i c h we  and  small. been  expect that  r a t i o s d e t e r m i n e d a r e more a c c u r a t e  the  than they  88  FIGURE I I . 1 PHOTOGRAPHIC COPY OF A TYPICAL TRIMETHYL-LEAD  SPECTROGRAM  251  r  89  a r e when t h e c o r r e c t i o n  It  i s assumed t h a t  two mass u n i t s away. tail  the t a i l s  o f p e a k s a r e o b s e r v e d up t o  L e t the f r a c t i o n  o f a peak observed as  a t t h e s i d e s o f t h e p e a k be r e p r e s e n t e d b y w, t h e f r a c t i o n  observed  one mass u n i t  fraction  observed  w,  i s ignored.  away be - r e p r e s e n t e d b y x, a n d t h e  two mass u n i t s away be r e p r e s e n t e d b y y ;  x and y a r e t h e n r e f e r r e d  coefficients.  Suppose t h a t  t o as the p r e s s u r e  scattering  t h e measured i o n i n t e n s i t i e s ( i n -  cluding attenuation factors) are A ^ , A  Q  , A  1S  A , 2  A3,  A 4 ,  for  m a s s e s 249, 250, 251, 252, 253, 254.- T a i l s f r o m mass 250 and mass 254 a r e assumed t o be n e g l i g i b l e . rected f o r pressure scattering,  V  = A 9 + WA  V  =  A A 2  0  A  0  A  3  A4°  "  A  -  A  are then:  9  l" 2  "  XA  x(A  * A3 - X A  -  2  3  + A3)  x  2  yA  -  yA  1  = 4 A  The p r e s s u r e s c a t t e r i n g c o e f f i c i e n t s a r e d e t e r m i n e d measuring  cor-  0  0 1  A  A  The i n t e n s i t i e s ,  the apparent  rises  i n b a s e - l i n e beneath  by  t h e 249,  250,  254 a n d 255 p e a k s .  T h e s e m e a s u r e m e n t s a r e made a t t h e c e n t r e s  of the peaks.  these  can then w r i t e :  Call  a ^ , an, 8 4 and a ^ , r e s p e c t i v e l y . a ^ = wAg + yA-^ a  Q  = x(A  a 4=  9  XA3 +  a c j =  XA4  + A-L) + y A yA  2  + yA3  2  We  90 S o l u t i o n s f o r x and  y from  s a t i s f y the equations f o r a . Q  assymetry mass 248  observed are  i n the  e q u a t i o n s f o r a ^ and  a ^ do  not  This inconsistency reflects  the  spectrogram  always h i g h e r than  (the base r e a d i n g s  t h o s e .read above mass 255).  w o r k a b l e compromise h a s b e e n a c h i e v e d b y first  two  equations  are then  scattering coefficients",  equations  solved f o r x  scattering  One we  find  x  w o u l d hope t o f i n d t h a t x-^ and  x 10"^  the  the p r e s s u r e  To  agree  2  x 10~\  their difference can use  x  y mm.  i s not average  scattering  samples from  mm.  o f Hg.,  calculated  and  Hg..)  last  II.2,  and  "down-  two  pected  f o r pressure  v a l u e o f x^  the p r e s s u r e  p e r cent, but y^ i s  = 0.68  x  2  and  y  2  to a f i r s t and  x , 2  = 6.50  x  x  x IO" *, 2  are both  small,  approximation  and y ^  and y  2  as  coefficients.  e f f e c t s were I n d e e d due  Captain's Plat  linear  .91  x 10~6  pressure  were a n a l y s e d mm.  0  f Hg.  at  to  T h e s e have b e e n p l o t t e d  v a r i a t i o n found  observed  to  2.48  x  s c a t t e r i n g c o e f f i c i e n t s were  scattering.  (i960), who  Typically  2  2  s e r i o u s , and  pressure  the  y-^ = y .  x IO" *, y Since y  f o r each a n a l y s i s .  Figure  of K o l l a r  the  the  ( T y p i c a l v a l u e s are x  s e v e r a l p r e s s u r e s ranging from 10  and  2  t o w i t h i n 10  = 1.90  x  check t h a t t a i l i n g  scattering,  x-j^ = x  2  p r e s s u r e = 1.0  one  and  y^  the "upiriaassppressure  2  several times y .  = 6.02  2  y ,  and  The  coefficients".  consistently 10°\  and  2  A  a s s u m i n g w = 2x.  s o l v e d f o r x^  mass p r e s s u r e are  below  a s w o u l d be  T h i s s u b s t a n t i a t e s the the  same e f f e c t s ,  scattering coefficients  slightly  but  in exwork  calculated  differently  than  92 t h e y have been c a l c u l a t e d The  here.  same s e t o f m e a s u r e m e n t s o n C a p t a i n ' s P l a t  samples  i n d i c a t e s t h e improvement i n p r e c i s i o n o f measurement if  pressure  scattering corrections  Sample 136 a n a l y s e d was f o u n d  achieved  a r e made t o a l l a n a l y s e s .  a t a p r e s s u r e o f 2.86 x 10"^ mm,  o f Hg.  t o have a Pb2 "7/pb204 r a t i o o f 16.01, when p r e s s u r e c  scattering corrections corrections  were n o t made.  t h i s t o 15.77.  reduced  Pressure  scattering  The same r a t i o f o r sample  147  ( a n i d e n t i c a l sample) a t a p r e s s u r e o f 1.02 x 10""°" mm. o f  Hg.  was 15.77 w i t h o u t  rection. relative  the c o r r e c t i o n ,  Clearly, pressure e r r o r from  and 15.71  scattering corrections  .24 (1.5$) t o .06  (0.4$),  r a n g e o f d a y - t o - d a y v a r i a t i o n s f o r t h e mass The P b  with the cor-  which i s i n the spectrometer.  same i m p r o v e d agreement i n a n a l y s e s i s f o u n d  204  a  n  d  p 208/ 204 D  significant, the  p b  since  r a  tios.  Normally  i n t h e Pb2 - 6/ <  at  approximately  the v a l i d i t y  of the  method.  The pressure  1  pb207/pb204  r a t i o i s t h e one most i n f l u e n c e d  scattering corrections.  o u r measurements i s n o r m a l l y is  s a t i s f y i n g t o note  pressure  that  a b o u t -1/3 o f one p e r c e n t .  Kollar's analysis  scattering corrections,  i s Kollar's analysis  are i g n o r e d .  by  The change i n t h i s r a t i o b y  of a s p l i t  when p r e s s u r e  It  o f sample 1, w i t h  i s i n better  P a t t e r s o n and T a t s u m o t o ' s a n a l y s i s than  )  t h i s e f f e c t i s not very  t h e samples a r e a l l a n a l y s e d  same p r e s s u r e , b u t i t d o e s i n d i c a t e  reduced the  agreement  o f t h e same  scattering  These r e s u l t s are g i v e n i n T a b l e  with sample,  corrections  I I . l . The  analysis  b y P a t t e r s o n and T a t s u m o t o was b y a s o l i d  strument  i n which the main  multiplier discrimination beam  sources  source i n -  of u n c e r t a i n t y are the  correction  and r a p i d l y v a r y i n g i o n  intensity.  TABLE I I . 1 LEAD ISOTOPE RATIOS FOR BROKEN H I L L STANDARD  Pb 206/p 204 b  p  b  207/p 204 b  SAMPLE  p b  208  / p b  204  U.B.C. w i t h o u t pressure scattering correction U.B.C. w i t h p r e s s u r e scattering correction P a t t e r s o n and T a t sumoto w i t h M c K i n n e y correction (personal communication)  16.16  15.6l  36.14  16.12  15.54  36.07  16.12  15.54  36.10  94  APPENDIX I I I  A New C a l c u l a t i o n f o r t h e Abundances* The of the  p o s i t i o n of l e a d . the p a s t ,  but  1  composition  is  the  f o r the  closed with  Isotope  f o r the  isotopic  age  com-  Numerous c a l c u l a t i o n s have b e e n r e p o r t e d i n 2  primeval  Lead  today  on m e a s u r e m e n t s o f t h e  only two *  In both,  initial  of the E a r t h from  only precise estimates that exist  e a r t h are based  time.  Age  3  are of  assumption  i s implicit  various lead  l e a d by  significance  at the  t h a t the  s a m p l e s c a n be  equations  present  isotopic  related  to  an  d e r i v e d f o r a system which  r e s p e c t t o u r a n i u m and  lead.  One  calculation  2  assumes t h a t t h e p r i m e v a l l e a d  i s o t o p e abundances are  identical  with  those  troilite  meteor-  ites,  and  that  observed  e v a l u a t e s the  found  isotopic  now  i n one  observed  of e v o l u t i o n from values.  Theory  fractionating meteorite, line,  and  determine  2. 3«  The  r e q u i r e d t o change such  second  the p r i m e v a l  and  t h a t the the  age  by  calculation  3  leads, i n the  of  straight  Russell  and  line  system.  Reynolds  a  the  present events  the straight  c a n be  Apart  and  products  absence of  uranium d u r i n g the l i f e t i m e  of the m e t e o r i t e  into  assumes t h a t  isotope r a t i o s to t h e i r  slope of t h i s  Ostic,  lead  s a m p l e s o f known age  i n stone m e t e o r i t e s r e p r e s e n t the  shows t h a t s u c h  lead  phase o f i r o n  s h o u l d have i s o t o p e r a t i o s w h i c h l i e on  x Manuscript 1.  time  o r more t e r r e s t r i a l  composition.  l e a d s now  i n the  from  used  to  the d i f -  (1963).  e g . A. Holmes: Nature, London 157, 680 ( 1 9 4 6 ) ; i b i d . 159, 127 ( 1 9 4 7 ) ; i b i d . 163, 453 ( 1 9 4 3 T 7 F. G. H o u t e r m a n s : N a t u r w i s s e n s c h a f t e n 3 3 , 1 8 5 ( 1 9 4 6 ) ; i b i d . 3 3 , 219 ( 1 9 4 6 ) ; Z. N a t u r f o r s c h , A / 2 , ~ l 2 2 ( 1 9 4 7 ) ; E . C. B u l T a r d and J . P. Stanley: Pub. F i n n i s h Geod. I n s t . 3 6 , 3 3 ( 1 9 4 9 ) . P. G. H o u t e r m a n s : Nuovo C i m e n t o 1 0 , 1 6 2 3 (1953). C. C, P a t t e r s o n , G. T i l t o n and M. G. Inghram: Science 121, 69  (1955).  95  ficulty closed of  i n p r o v i n g t h e common a s s u m p t i o n s y s t e m may b e u,sed i n t h e s e  the f i r s t  calculation  that equations f o r a  calculations,  the accuracy  i s l i m i t e d by the u n c e r t a i n t y i n a s -  s i g n i n g ages t o t h e samples used,  and o f t h e second  calculation  by  t h e u n c e r t a i n t y i n e q u a t i n g t h e age o f t h e e a r t h t o t h e age  of  the meteorite  system.  The a p p r o x i m a t e e q u a l i t y o f t h e v a l u e s  o b t a i n e d b y t h e two c a l c u l a t i o n s p r o v i d e s some e x p e r i m e n t a l justification  f o r both  procedures.  R e c e n t l y c o n s i d e r a b l e p r o g r e s s h a s b e e n made i n i n t e r p r e t i n g the isotopic The  results  suggest  the e a r t h from distinct others,  from  lead  composition a third  of terrestrial  lead  samples,  4  method f o r c a l c u l a t i n g t h e age o f  i s o t o p e abundances, which i s b a s i c a l l y  t h e two m e n t i o n e d a b o v e .  I n common w i t h t h e  t h e new method assumes t h a t t h e s a m p l e s u s e d  c a l c u l a t i o n c a n be i n t e r p r e t e d b y e q u a t i o n s  i n the  appropriate to  evolution of the lead  i s o t o p e abundances i n a s i n g l e c l o s e d  system.  however, r e q u i r e e s t i m a t e s t o be made f o r  I t does not,  any  o f t h e ages o f t h e samples used,  the  assumption  of  meteorites.  evolved to  and i t does n o t r e q u i r e  t h a t t h e age o f t h e e a r t h i s t h e same a s t h e age I t d o e s assume t h a t t e r r e s t r i a l  i n the e a r t h from  that i n the t r o i l i t e  lead  l e a d s have  similar i n isotopic  phase o f i r o n m e t e o r i t e s ,  composition  !  and i n f a c t  t h e method p r o v i d e s a n e s t i m a t e f o r t h e t i m e  a t w h i c h t h e Earth's':  l e a d had t h i s i s o t o p i c  requires the as-  4.  composition.  I t also  R. D. R u s s e l l : "Some R e c e n t R e s e a r c h e s o n L e a d I s o t o p e Abundances" i n E a r t h S c i e n c e and M e t e o r i t i c s , C h a p t e r 3 , North-Holland Publishers, 4 4 - 7 3 ( 1 9 6 3 ) .  96 sumption  that  the p r i m a r y growth p r o c e s s f o r l e a d  isotopes i n  the e a r t h o c c u r r e d i n a system o r systems i n which the lead  uranium/  r a t i o d i f f e r e d b y a v e r y s m a l l amount f r o m t h e mean v a l u e ,  an a s s u m p t i o n f o r which c o n s i d e r a b l e  evidence has been  ac-  cumulating.  The  calculation Itself  i s v e r y s i m i l a r t o one  s e v e r a l y e a r s ago by A l l a n ,  F a r q u h a r and R u s s e l l . 7  s y s t e m c a n be  assumed f o r a p a r t i c u l a r  lead  be d e t e r m i n e d f r o m b o t h t h e P b * V p b ° 4 and 2 0  2  sample,  proposed i f a  closed  I t s age  can  Pb  207/Pb204 ratios a c c o r d i n g t o the equations  (III.l)  The In the righthand  symbols  are d e f i n e d  side of equations ( I I I . l ) there  known p a r a m e t e r s , n a m e l y V and t that  a unique value of V w i l l  closely  enough).  i n T a b l e 111,1.  0  (we h a v e made t h e  describe the s u i t e  I t i s formally possible  squares a n a l y s i s o f measured l e a d  a r e two  isotope  of  un-  assumption samples  t o c a r r y out a  least  r a t i o s to determine  5.  R. S t a n t o n and R. D. R u s s e l l : "Anomalous L e a d s and Emplacement o f L e a d S u l f i d e O r e s " . E c o n . G e o l . 54,  6.  R, D. R u s s e l l , E , R. K a n a s e w i c h and R, G. O s t i c : "Quant i t a t i v e I n t e r p r e t a t i o n o f Anomalous L e a d I s o t o p e Abundances", Chemistry of the E a r t h ' s Crust, Vernedsky C e n t e n a r y Symposium, A c a d , o f S c i e n c e s , U.S.S.R. (1963). D. W. A l l a n , R, M. F a r q u h a r , and R. D, R u s s e l l : "A Note on t h e L e a d I s o t o p e Method o f G e o l o g i c a l Age D e t e r m i n a t i o n " . S c i e n c e 118, 486-488 (1953).  607 (1959).  7.  "  the 588-  97 values o f both cordant  t h e above p a r a m e t e r s w h i c h g i v e t h e most  t and t  values.  1  Such a p r o c e d u r e  I s able t o y i e l d  r e a s o n a b l y p r e c i s e v a l u e f o r t ; we h a v e f o l l o w e d two o b t a i n e d two n u m e r i c a l  estimates as d e t a i l e d  TABLE  a  procedures  Q  and  con-  below.  III.l  DEFINITION OP SYMBOLS  Isotope Ratio  p b  206  / p b  Present  204  Pb °7/Pb 2  2 0 4  (t-o)  Time t  a  X  b  y  204  aV  U235/ b204  V  Physical Constants:  a =  Tj238  / P b  P  o#e  First  (t-*o) o  b  o  Ve*'^o  137.8 x 10"  q  yr"  1  0 . 9722 x 10-9 y r " *  1  Calculation The  first  growth curve  c a l c u l a t i o n r e q u i r e s t h a t t h e mean, o r d i n a r y - l e a d  o r i g i n a t e s a t t h e p r i m e v a l abundances ( a , b ) = c  (9.56,  10.4-2) g i v e n b y M u r t h y a n d P a t t e r s o n . 8  select  suitable  have used  8.  a  aVe*to  A t  Ve***  A = 0 , 1537 V=  Primeval  0  i n order t o  s a m p l e s w h i c h a r e u n l i k e l y t o be a n o m a l o u s , we  analyses f o r galenas from  d e p o s i t s s e l e c t e d by  V . Rama M u r t h y and C, C. P a t t e r s o n : "Primary Isochron o f Z e r o Age f o r M e t e o r i t e s and t h e E a r t h " , J . o f Geophys. R e s e a r c h 67, I I 6 I - I I 6 7 (1962).  98 Stanton ' as being r e p r e s e n t a t i v e o f h i s "conformable" o r 0  pyritic  conformable"  classification.  Previous study ,5 has 4  indicated that t h i s geologic c r i t e r i o n ,  although not perfect,  more s u c c e s s f u l l y e x c l u d e s  anomalous l e a d s t h a n a n y o t h e r known.  We h a v e u s e d  analyses c a r r i e d  only isotopic  s i t y o f B r i t i s h Columbia, precise galena Table  III.2.  s i n c e we b e l i e v e t h e s e  analyses available.  t o be t h e most  T h e a n a l y s e s a r e shown i n  Two m i n e s , w h i c h s h o u l d be i n c l u d e d o n t h e b a s i s  o f t h e above c r i t e r i a , Samples from  out at the Univer-  have been e x c l u d e d  Rosebery, Tasmania  from  calculation.  (18.374, 15.746, 38.469), a r e  known t o be p a r t o f a n a n o m a l o u s l e a d s u i t e l e a d s f r o m Mount F a r r e l l ;  the  including  H a l l ' s Peak, New S o u t h W a l e s (18.490,  15.754, 38,770) i s s i g n i f i c a n t l y more r a d i o g e n i c t h a n known t o be y o u n g e r f r o m the  by  geologic relationships.  s a m p l e s l i e c l o s e t o t h e mean g r o w t h c u r v e ,  c l u s i o n i n the present  vein  calculation  samples  I n both  cases  and t h e i r i n -  increases the result  for t  Q  o n l y 1/4$. We h a v e i n t e r p r e t e d b e s t c o n c o r d a n c e b e t w e e n t a n d t ' a s  c o r r e s p o n d i n g t o t h e minimum v a l u e o f t h e e x p r e s s i o n n R e s i d u a l = 2 Wj.(ti - t ' i ) The  weight f u n c t i o n , W  t o ensure nature  that the result  value.  .  (III.2)  i sincluded after Bullard obtained i s independent  of the d e f i n i t i o n of the residual  probable 9.  i#  2  and S t a n l e y o f the exact  a n d i s t h e most  I t i sd e f i n e d by the r e l a t i o n s h i p s  R. L . S t a n t o n : "General f e a t u r e s o f t h e conformable 'pyritic' orebodies". C a n . M i n . a n d Met. B u l l . 63, 22-27  (I960).  —  1  99  = t i - t'j_,  and  where 6x and By a r e t h e e s t i m a t e d in  t h e measured r a t i o s .  8x = s y ; t h e n u m e r i c a l The and  experimental u n c e r t a i n t i e s  The p r e s e n t value  e x p r e s s i o n f o r At±  calculation  assumes  i s immaterial. i s n o n - l i n e a r i n b o t h V and t  0  ,  we h a v e e x p a n d e d i t a s a T a y l o r s e r i e s i n 8V, t h e d e v i a t i o n  f r o m a n a p p r o x i m a t e v a l u e V. Residual =  2  Then c o n d i t i o n ( i l l . 2 )  Wl^Ati  1=1  ~  6V +  + d  ...)  becomes*  2  v  = minimum. The by  best  value  o f SV c o r r e s p o n d i n g  z  6V  until  =  -  the procedure  although The  i s obtained  . A t i.  least  = 4.49,  converges t o the best V value. the process  squares  4.52,  i n Figure I I I . l .  ~5V~J  =  a s many a s t w e l v e  4.55,  e v a l u a t e d f o r each.  *  Wi  i=l  c a s e s we h a v e t r i e d ,  0  Q  a p p l y i n g r e p e a t e d l y c o r r e c t i o n s g i v e n by n  t  to a given t  In a l l  converges t o a unique  result,  i t e r a t i o n s may be r e q u i r e d .  s o l u t i o n f o r V was o b t a i n e d f o r e a c h o f 4.58,  4.61,  a n d t h e minimum  These a r e p l o t t e d  residual  on an a r b i t r a r y  The w e l l - d e f i n e d minimum a t 4.52  scale  Gyr. i s the  B o l d f a c e d symbols i n d i c a t e t h a t t h e q u a n t i t y h a s been e v a l u a t e d w i t h V. B y ^At± we mean t h e v a l u e o f t h e d e r i v a t i v e o b t a i n e d W i t h V. 1  FIGURE  100  III.l  RESULTS OF THE F I R S T CALCULATION FOR THE AGE OF THE EARTH, FROM DATA FOR CONFORMABLE LEADS  f At  =  .07  Gyr./  U  t/  y  I*  -T-^.. mir--  4.5  ,  4.6 t  0  (Gyr.)  101 best  value f o r t  Q  i n a least  v a l u e f o r a V I s 9.23 is  squares  atoms/atom.  sense.  The  The s h a r p n e s s  a measure o f t h e p r e c i s i o n o f t h e e s t i m a t e  approximation  t o the standard d e v i a t i o n of t  d i v i d i n g the h a l f - w i d t h a t double square  corresponding o f t h e minimum  of t D  Q  .  A good  i s . o b t a i n e d by-  minimum r e s i d u a l b y t h e  r o o t o f two l e s s t h a n t h e number o f s a m p l e s .  s t a n d a r d d e v i a t i o n so e v a l u a t e d i s 0.03  The  Gyr. (obtained by  r o u n d i n g up 0.028). F o r the purpose o f comparison, similar, results  we h a v e c a r r i e d  out a  though s i m p l e r a n a l y s i s o f m e t e o r i t i c data, shown i n F i g u r e I I I . 2 .  I n t h i s case  with  the stone  meteor-  i t e s were assumed t o be o f z e r o age, a n d w o u l d t h e r e f o r e l i e on  t h e i s o c h r o n o f z e r o age, g i v e n b y t h e e q u a t i o n y-bp x-a  =  0  The  residual,  e^'tp - 1 (e*t - l) a  0  '  e q u a l t o t h e sum o f t h e p e r p e n d i c u l a r d i s t a n c e s  between t h e o b s e r v e d  m e t e o r i t i c r a t i o s and t h e i s o c h r o n o f  z e r o age, was c a l c u l a t e d f o r d i f f e r e n t curves  show t h e r e s u l t  values of t  several laboratories.  obtained from  .  The two  of using analyses f o r only three  m e t e o r i t e s by P a t t e r s o n , and f o r t y - e i g h t from  0  the f i g u r e  of various  stone  types  The a g e s a n d s t a n d a r d d e v i a t i o n s  a r e 4.56  - .03 G y r . a n d 4..56  - .02 G y r .  T h i s compares w e l l w i t h M u r t h y a n d P a t t e r s o n ? who g i v e a s l o p e o f 0.59  *  o  oi  e q u a l t o 4.55  f o r the zero i s o c h r o n which corresponds *  .03 G y r .  to t  Q  FIGURE I I I . 2  102  RESULTS OF THE CALCULATION OF THE AGE OF  1 /  V | Nji  At =  .144  At  Gyr.  \  METEORITES  Patterson's Meteorites  = .C>36 G y r .  ! z-^48 ^/Meteorite s  .4  4.5  4.6 t  G  (Gyr.)  4.  103 Second  Calculation  A c r i t i c i s m of the f i r s t  c a l c u l a t i o n i s t h a t i t assumes  e x a c t knowledge o f the p r i m e v a l l e a d abundances.  a negligible  amount, b u t  e s t i m a t e d b y M u r t h y and  Substituting  (9.50, 10.36) c h a n g e s t h e r e s u l t  the o l d e r P a t t e r s o n v a l u e s  t h e 1.5$  u n c e r t a i n t y of the  Patterson could result  an  by  ratios  i n an e r r o r  of  f o r t y m i l l i o n years i n t . 0  The  f o l l o w i n g procedure  a v o i d s p l a c i n g e x c e s s i v e weight  on t h e p r i m e v a l a b u n d a n c e s .  In equations  ( I I I . 1 ) , w i t h the  as-  s u m p t i o n t h a t an a v e r a g e V p r o v i d e s a r e a s o n a b l e r e p r e s e n t a t i o n of the  samples,  a , Q  b  p r i m e v a l values, but  Q  Table  t  and  calculate  the  apparent  V-value.  use  e i t h e r of equations  which t e r r e s t r i a l mathematical  through  (III.1)  l e a d had  through  age  samples  of the  In  sample  the  III.2.  to calculate  the time  t  Using  values of t  r e s u l t s were a v e r a g e d The  t h e two  times,  through  t h e Mount I s a a b u n d a n c e s ,  at  c  the  to  give between  forced  amounted t o l e s s t h a n  0.2#.  supports the b a s i c assumption of a unique Q  values obtained,  we  was  were  Q  greatest difference  o b t a i n e d when t h e c u r v e was  a v e r a g i n g the t  and  numbers i t i s p o s s i b l e t o  d e s c r i b e d above, t h e g r o w t h c u r v e  t h e a g e s shown i n T a b l e  By  as  Then i t i s f o r m a l l y  of the  e a c h p o i n t i n t u r n , t h e two  w h i c h was  point  the p r i m e v a l r a t i o s .  and  curve.  any  the apparent  c a l c u l a t e d I n each case  T h i s agreement  interpreted  t o any  Prom t h e s e two  procedures  f o r c e d through  n e e d n o t be  i s f o r c e d to pass.  to f o r c e the curve  III.2,  Q  can correspond  which the growth curve possible  and  arrive  at  growth a  104  TABLE III". 2 SUMMARY OP RESULTS FOR THE SECOND CALCULATION Location x  y  t z  Residual  app (Gyr.)  (U238  / P b  t  Q  204) app  (Gyr.)  1. P r i m e v a l Abundances, Murthy and P a t t e r s o n  (1962) 9.56, 10.42, 29.71 4.52  9.23  450  4.5.2  3.03  9.66  580  4.48  1.63  9.00  530  4.55  1.49  8.61  1400  4.60  1.31  8.90  880  4.57  0.47  9.27  450  4.52  9.25  437  4.52  9.34  510  4.52  8.87  620  4.56  2. Manitouwadge, O n t a r i o (Mean o f 3 s a m p l e s )  13.403, 14.565, 33.545 3. B r o k e n H i l l , A u s t r a l i a (Mean o f 6  samples) 16.117, -15.539, 36.087  4. Mt. I s a , A u s t r a l i a (Mean o f 5 s a m p l e s )  16.210, 15.593, 36.219  5. S u l l i v a n M i n e , B.C. (Mean o f 2 s a m p l e s )  16.633, 15.638, 36.577  6. C a p t a i n ' s Australia  Flat, (Mean o f  7 s a m p l e s ) 18.178, 15.766, 38.564  7. C o b a r , A u s t r a l i a (Mean o f 10 s a m p l e s )  18.205, 15.771, 38.543  0.45  8. B a t h u r s t , New B r u n s w i c k (Mean o f 3 sam-  p l e s ) 18.291, 15.781, 38.526 0.42s  9. W h i t e I s l a n d , New Z e a l a n d (Mean o f 2  a n a l y s e s ) 18.885, 15.750, 39.070  -0.24  Mean * S t a n d a r d D e v i a t i o n o f Mean  9.13*0.11  4.54±.02  10<5 r e s u l t w h i c h d o e s n o t w e i g h t t h e p r i m e v a l a b u n d a n c e s so h e a v i l y . The  result  quoted  o b t a i n e d i s 4.54  - .02 G y r . , where t h e u n c e r t a i n t y  i s t h e s t a n d a r d d e v i a t i o n o f t h e mean  (rounded  up  from  0.014). Conclusion E i t h e r of the r e s u l t s obtained, .02 G y r . , p r o v i d e s a n e s t i m a t e had  t h e same i s o t o p i c  meteorites.  s i g n i n g ages t o t e r r e s t r i a l  - .03 G y r . o r 4.54  o f t h e time  composition  The p r o c e d u r e  4.52 1  when t e r r e s t r i a l  as the t r o i l i t e  avoids both  ± lead  phase o f i r o n  the n e c e s s i t y of as-  s a m p l e s and t h e a s s u m p t i o n  that  t h e age o f t h e e a r t h i s e q u i v a l e n t t o t h e age o f m e t e o r i t e s . Thus t h e agreement o f t h e p r e s e n t  result  w i t h t h e age o f  m e t e o r i t e s o b t a i n e d from: t h e s l o p e o f t h e m e t e o r i t i c i s o c h r o n (4.55 two  *  .03 G y r . ) p r o v i d e s v a l i d  events The  independent  o c c u r r e d a t a b o u t t h e same  essential  samples used  assumption  evidence  time.  of the c a l c u l a t i o n ,  can be.represented  that the  that the  by a single.growth curve,  j u s t i f i e d b y F i g u r e I I I . 3 w h i c h shows t h e . e x p e r i m e n t a l plotted  on such  a growth curve  V = 0.0670 (aV = 9.23).  calculated f o rt  Q  = 4.52  i s .  points G y r . and  III.3  FIGURE  PLOT SHOWING THE RELATIONSHIP BETWEEN THE RATIOS P b 7 / P b AND P b / P b 0 4 FOR CONFORMABLE LEADS AND THE SINGLE-STAGE GROWTH CURVE FOR a =9.56, b = 1 0 . 4 2 , aV=9.23, AND t = 4 . 5 2 G y r . 1 1 1 1 ' ' I ' 2 0  Q  2 0 4  2 o 6  2  o  G  16 2 points  -  14  Pb  2 0  -  ?  ppcW  -  12  • / 10  1  10  1  1  1  -  12  16  14 p b  206  / p b  204  18  APPENDIX IV I n t e r p r e t a t i o n o f Anomalous Equations  107  Leads  (3.1)-are v e r y - g e n e r a l a n d a r e v a l i d  f o r the  development o f a l l l e a d s .  p b  206  204  / p b  =  .  x  a  Q  r  a  +  °  -t  .t ^t o (  A e  d  t  J o  (  (3,1) Pb207  P b  208  / P b  / p b  204  =  y  204  =  z  =  b  .  o  C  q  +  J^VA'e^'t  d  t  +  ^HeA-'t  d  t  Jo The a s s u m p t i o n t h a t V a n d W have b e e n t h e same f o r a l l  time i s  c o n s i s t e n t with the r e s u l t s observed f o r primary l e a d s . r e s u l t s f o r Manitouwadge, suggest t h a t  R o s e b e r y and White I s l a n d ,  The  however,  t h e s e l e a d s have d e v e l o p e d i n s y s t e m s f o r w h i c h  V and W v a r i e d . make c e r t a i n  In order t o interpret  the r e s u l t s ,  we have t o  a s s u m p t i o n s on how V a n d W d i d v a r y .  Models f o r  a n o m a l o u s l e a d s d i f f e r o n l y i n t h e manner i n w h i c h t h e v a r i a t i o n s i n V a n d w a r e assumed t o h a v e o c c u r r e d .  The m o d e l  u s e d h e r e i s t h a t b y K a n a s e w i c h (1962) i n w h i c h V a n d W a r e assumed V  2  t o have h a d t h e v a l u e s V^^ a n d W]_ f r o m t i m e t  and W  2  from time t ^ t o t  2  t o t]_,  0  , a n d V 3 a n d W3 f r o m t i m e t  2  to t ^ .  T h i s t h r e e - s t a g e model i s u n d o u b t e d l y an o v e r s i m p l i f i c a t i o n o f t h e h i s t o r y o f t h e s a m p l e s , b u t K a n a s e w i c h h a s shown t h a t useful  a g e s c a n be c a l c u l a t e d f r o m s u c h a m o d e l .  s u m p t i o n s f o r V a n d w, x = ap + a V i ( e ^ „ A t i j D  y=b  0 +  z=c  0 + W l  e  equations  +  a  ,  2  2  (e  ,  0  x , , t  (3.1) r e d u c e t o :  v (e*tl-eAt ) + aV3(eAt2_ At )  V (e^ t _e^ tl) + V (eA't 1  2  1 - e  ^tt ) 2  o - e A " t i ) + w (e* tl- A M  2  Under t h e a s -  e  H t  e  3  + v (e*' 2_ X't )  2) +  t  3  e  3  ^(e^ t „ V t ) u  l  2  e  3  (IV  108 All started  l e a d s o f an a n o m a l o u s s u i t e out as primary l e a d s ,  that  I s , they developed  r e g i o n f o r w h i c h V]_ i s c h a r a c t e r i s t i c t i m e t ^ , when a l l s a m p l e s ( l* x  y±'  l)f  z  V and W need t  2  a r e c o n s i d e r e d t o have  f o r primary leads.  h a d t h e same i s o t o p i c  t h e y were s h u n t e d  into  a crustal  n o t be t h e same f o r a l l s a m p l e s .  > t]L t h e l e a d s i n t h i s  system  In a At  composition system  f o r which  A t any time  are then l i n e a r l y  related to  each o t h e r a c c o r d i n g t o the e q u a t i o n y - y i  e*'ti  - e*  = x - x Leads  linear  anomalous l e a d s . diagram  (IV.2)  r  a(e*ti_  x  e x t r a c t e d from t h i s  retain this  2 — . Xt ) ! t  e  system  and m i n e r a l i z e d  relationship. The l i n e  2  These  through these p o i n t s i n t h e x-y  has a s l o p e g i v e n by e q u a t i o n ( I V . 2 ) ,  l e a d s a r e i n t e r p r e t e d by t h i s two-stage t h e time  into  a t which  the crust,  and t  2  .  and i n t e r c e p t s When a n o m a l o u s  model, t ^ i s t a k e n t o  primary lead, uranium 2  2  a r e two-stage  the p r i m a r y growth c u r v e a t t i m e s t ^ and t  be  a t time t  and t h o r i u m were p u t  i s t h e time o f anomalous l e a d  mineraliza-  tion. The is  development  of a suite  o f t h r e e - s t a g e anomalous l e a d s  s i m i l a r t o t h a t f o r two-stage  systems  are postulated.  A linear  leads,  except that  relationship will  o b s e r v e d b e t w e e n x and y f o r t h r e e - s t a g e a n o m a l o u s provided  a l l samples  a l s o be leads,  r e c e i v e d e q u a l amounts o f r a d i o g e n i c  d u r i n g e i t h e r t h e second ment, o r h o m o g e n i z a t i o n development.  two c r u s t a l  I t appears  stage o r the t h i r d  stage o f develop-  o c c u r r e d a f t e r t h e second t o be g e o l o g i c a l l y  lead  stage o f  improbable  that a l l  109  samples c o u l d r e c e i v e the f i n a l  e q u a l amounts o f r a d i o g e n i c l e a d d u r i n g  g r o w t h s t a g e , and f o r t h i s r e a s o n an o b s e r v e d  linear  r e l a t i o n s h i p , i n x and y f o r t h r e e - s t a g e l e a d s i s i n t e r p r e t e d a s due t o t h e g r o w t h o f t h e s e uranium/lead The  abundances d u r i n g t h e t h i r d  s l o p e o f t h e anomalous l i n e  R =  e  a  ' ( Xt X  t  e  e  2  V  the  i n t h i s case  t  (IV.3)  2  to a line  and t ^ .  If t  g e o l o g i c e v i d e n c e o r an I n d e p e n d e n t s l o p e o f t h e anomalous l e a d  the o t h e r time The  t ^ (or t  i n F i g u r e IV. 1 .  growth curve from into  time  a crustal  growth c u r v e s u n t i l t h o r o u g h l y mixed. average  t  Q  ( o r t3)  2  age d a t i n g method,  c a n be u s e d  t  Lead  g  .  t-j_ a t w h i c h t i m e  where i t d e v e l o p s At t h i s  A f t e r the mixing,  time  t  t^,  i t i s  along  several  the leads are  2  growth c o n t i n u e s from t h e  c o m p o s i t i o n a l o n g s e v e r a l growth c u r v e s u n t i l  line  from t  to calculate  develops along the primary  when f i n a l m i n e r a l i z a t i o n o f t h e l e a d o c c u r s . lead  i s known  o f t h r e e - s t a g e anomalous l e a d s  t o time  system  time  line  which I n t e r c e p t s  ) .  development o f a s u i t e  illustrated  shunted  2  i s g i v e n by  3  Is parallel  the p r i m a r y growth curve a t t from  stage o f development.  . - e*^)  g  T h i s anomalous l i n e  is  samples i n r e g i o n s o f d i f f e r e n t  2  i s parallel  to t g .  the actual  uranium/lead particular,  time t ^  The a n o m a l o u s  t o t h e c h o r d on t h e p r i m a r y growth  I t i n t e r c e p t s t h e p r i m a r y growth curve intercept  ratio i fV  2  depending  f o r t h e second  curve  a t time  o n t h e mean v a l u e o f t h e stage o f growth.  = V-^ t h e i n t e r c e p t  i s at t  2  .  In  The  inter-  FIGURE I V . 1  110  THE DEVELOPMENT OF TWO-STAGE AND THREE-STAGE ANOMALOUS LEAD SUITES  Pb207 Pb204  •  Single-stage  +  Two-stage anomalous l e a d s  X  T h r e e - s t a g e anomalous l e a d s  P 206/pb204 D  leads  Ill pretation no  less  tj_ w i l l  2  be  f o r a two-stage 1  be  less  g  the  t o t g on t h i s c u r v e .  p r i m a r y l e a d has been shunted  ment a t t i m e a normal  radiogenic  t-j_, and crustal  at time t  the  addition only, u n t i l  first  event.  The  apparent  The  age  If V  2  g r e a t e r t h a n 4.50  that  the  lead line  positive two-stage served  i t was  time  time  this  For this  moved f r o m t h i s lead  location  developed  t ^ when f i n a l  lead  i s anomalous.  diagram  c o m p o s i t i o n o f White  assuming  t  The  by min-  c a l c u l a t e d by  reason the w r i t e r  o b s e r v e d f o r one  i n an x - y  c a l c u l a t e d by  means  a uranium-free environ-  sample, will  l e a d model i s t h u s n o t adequate  = t g = 0.  intercept  the  An  concluded  anomalous  but  any  n o t have  line  two  i n t e r c e p t s w i t h the p r i m a r y growth c u r v e .  isotopic  the  t i m e t ^ i s t h u s a minimum age f o r  A minimum v a l u e f o r t h e t i m e  2  the  slope of  to  f o u n d t o be n e g a t i v e f o r a l l v a l u e s o f  Gyr.  c o u l d n o t be  will  ±  Physically,  i n which  sample f r o m W h i t e I s l a n d  t h r o u g h White I s l a n d  into  = 0,  o f White I s l a n d l e a d ,  s i n g l e - s t a g e m o d e l , was 0  2  environment  e r a l i z a t i o n occurred.  I f 7*2  t h e s l o p e o f t h e c h o r d f r o m t±  on t h e p r i m a r y g r o w t h c u r v e i s g r e a t e r t h a n t h e 2  and  i s greater  However, t  than t .  w i t h t h e p r i m a r y g r o w t h c u r v e i s a t t-|_.  t  2  lead,  not e x i s t .  2  l e s s t h a n t-^ b e c a u s e  chord from t  to  i f v  #  g r e a t e r than t , or w i l l  t h a n V]_, tj_ w i l l  n e v e r be t  to that  information i s obtained concerning t  t h a n V^, is  i s then equivalent  t o e x p l a i n the  A ob-  Island.  of the f i r s t  event can  a t h r e e - s t a g e l e a d development  s l o p e o f t h e anomalous l e a d l i n e  be  with  in this  case  112  Is  .0459.  This  anomalous l e a d l i n e  the p r i m a r y growth curve f o r t = 1200 m.y.  An u n c e r t a i n t y  to the uncertainty Results  o f ± 400 m.y.  suggest,  the upper  intercepts  = 4.55 G y r . and aV = 8.99 a t t±  .05 i n t h e r a t i o  from White I s l a n d  were removed f r o m 1200  of ±  0  o f minimum s l o p e  i s placed  aV f o r p r i m a r y  therefore,  mantle  o n t ^ due  that  leads.  materials  i n t h i s area as e a r l y as  ± 400 m.y. a g o .  The  o l d e s t known r o c k s i n N o r t h  Cretaceous  ( c a . 100 m.y.  Island,  o l d ) , and W h i t e I s l a n d  sumed t o be no o l d e r t h a n P l e i s t o c e n e however, t h a t New Z e a l a n d anywhere f r o m within depth  (ca.  i s a continental  itself  1 m.y.  fumarolic  Note,  l a n d mass w h i c h i s i s well  The a n a l y s e d l e a d came  activity,  and o u r r e s u l t s  a t depth  i s much o l d e r t h a n a n y known e x p o s e d  on N o r t h  Island.  The l e a d may d a t e  the c o n t i n e n t a l  r o c k s a r e now f o u n d ,  from  suggest  material  on w h i c h y o u n g e r e x p o s e d  i s as-  old).  25 t o 35 km. t h i c k , and W h i t e I s l a n d  the c o n t i n e n t a l margin. through  New Z e a l a n d , a r e  that  rocks  basement  o r i t may  perhaps  d a t e o l d o c e a n i c c r u s t w h i c h i s now o v e r l a i n b y c o n t i n e n t a l material.  The l a t t e r p o s s i b i l i t y  i s suggested by t h e f a c t  a l o w v a l u e f o r aV i s n e c e s s a r y f o r t h e s e c o n d  growth  that  stage i n  o r d e r t o e x p l a i n t h e n e g a t i v e i n t e r c e p t o f t h e anomalous l e a d line  w i t h t h e p r i m a r y growth Leads from Rosebery,  when t h e i r was f o u n d and  apparent  curve.  T a s m a n i a were assumed t o be anomalous  age ( c a l c u l a t e d o n t h e s i n g l e - s t a g e  t o be t o o y o u n g i n c o m p a r i s o n  the apparent  uranium/lead  with geologic  and t h o r i u m / u r a n i u m  model)  evidence,  ratios  were  113 found  t o be  significantly  f o r other conformable t o be  very uniform  smaller than  deposits.  i n isotopic  samples o f g e o l o g i c a l l y  The  the  values  deposit i t s e l f  composition,  related  calculated  l e a d s had  and  was  found  additional  to analysed i n order  1 t o o b t a i n an a n o m a l o u s l e a d these  l e a d s was  Mount F a r r e l l ,  possible.  additional  same s e d i m e n t a r y  Real differences i n isotopic  s e r v e d between l e a d s from between d i f f e r e n t these  The  b e f o r e an  R o s e b e r y and  samples from  the  The  slope of the  x-z  The  anomalous l i n e  folded  and  and  are  plotted  i n the  x-y  by  the  l e a d data can  iferous,  i . e . about  300  i n t e r c e p t s a primary t h i s o n l y i f the  area are  lead  The  t h u s n o t be old.  growth curve  The twice  diagram leads. ~  overlain  fossiliferous  youngest l e s s than  event Carbon-  anomalous l e a d at times  line  greater  than  u 38/p 204 r a t i o f o r t h e p r i m a r y g r o w t h c u r v e 2  b  l e s s t h a n 8.89  the primary  m.y.  x-y  d i a g r a m i s .127  an u n d e f o r m e d , h o r i z o n t a l l y - l y i n g g r o u p o f age.  the  Mount F a r r e l l  o r e - b e a r i n g rocks of the  observed  the  ob-  Results f o r  i n both  d i a g r a m f o r t h e R o s e b e r y and  sediments of Permo-Carboniferous  is  3.6  Rosebery  were  Mount F a r r e l l ,  east  IV.2.  and  by  composition  of  from  s e r i e s as the  i n Table  L i n e a r r e l a t i o n s h i p s are observed  .008.  s a m p l e s came  Mount F a r r e l l .  a n a l y s e s have been l i s t e d  in Figure  interpretation  which i s a v e i n d e p o s i t l o c a t e d s i x m i l e s  of Rosebery, w i t h i n the deposit.  line  (for t  G  = 4.55  Gyr.).  Such a low  system' i s v e r y i m p r o b a b l e .  interpretation  g i v e n i s based  model r a t h e r t h a n the  two-stage  on  For t h i s  the t h r e e - s t a g e  model.  value f o r reason, lead  114  FIGURE IV.2 ISOTOPIC COMPOSITION OF LEADS FROM ROSEBERY AND MOUNT FARRELL, TASMANIA  •• .— • • •«  *  <  Mount F a r r e l l  /  1  V  ••  Rosebery  / »•  18.3  18.4  18.5 P b  206  / P D  18.6 204  18.7  115 It be  was  noted i n Chapter 1 that Rosebery  o f M i d d l e C a m b r i a n age.  s e l e c t e d f o r t , and 3  the of 200  uncertainty  m.y.,  source. the  m.y.  1790  has  The  - 130  v a l u e f o r t._ was  U 3°7pb 2  204  second  uranium/lead t i m e 1790  stage.  into  2100  suggest that  standard  f o r the primary  lead  ratio  was  m.y.  a new  less  ago  into  m.y.  and  to note that  m.y.  a r e a i n New  Wales.  South  Sample 372  Final  Manitouwadge, was More s a m p l e s  age  At  m.y.  the  perhaps  ago.  The  It i s in-  is in fair  agreement  estimated f o r the Broken  a n a l y s e d and  Creek  fault  f o u n d t o be  Hill  I n Geco very  o f anomalous l e a d from t h i s v e i n  analysed before a s a t i s f a c t o r y given.  m.y.  lead .mineralization  ages i n Tasmania.  m.y.  and  the  h o m o g e n i z e d and  from a v e i n near the Fox  c o m p o s i t i o n c a n be  that  a r e t h e o n l y e s t i m a t e s known  the younger  t o 1700  anomalous.  ratio  200  than that f o r the mantle.  system.  1790  ±  a r e g i o n f o r which  t h i s l e a d was  crustal  t o 2100  -  deviade-  at a time p r i o r  shunted  w i t h a g e s o f 1500  property,  2100  T h e r e f o r e , the r e s u l t s from Rosebery  the w r i t e r of Precambrian  be  i n the slope  s t a g e o f g r o w t h must have b e e n l e s s t h a n  - 130  teresting  to  where  standard  o c c u r r e d w i t h t h e f o r m a t i o n o f t h e d e p o s i t s 550  by  m.y.,  f o u n d t o be  and t h e  ratio  to  t h u s been  Since t _ i s g r e a t e r than t g , the uranium/lead  p r i m a r y l e a d was  figures  expected  r e s u l t s from the u n c e r t a i n t y  .05 i n t h e  the f i r s t  shunted  f o u n d t o be  s l o p e o f the anomalous l i n e  Mount F a r r e l l ago  i n age  o f 550  where t h e e r r o r i s e s t i m a t e d f r o m t h e  v i a t i o n of =  of  was  2  t h e anomalous l i n e .  t i o n of the  for  t  An age  was  i n t e r p r e t a t i o n of  I n the absence  need  their  of these analyses,  116 we c a n assume t h a t t h e s e l e a d s  are g e n e t i c a l l y r e l a t e d t o the  conformable-type leads o f the area. the  that  v e i n l e a d m i g r a t e d f r o m one o f t h e l a r g e r c o n f o r m a b l e - t y p e  deposits, in  I t i s quite probable  a n d i n so d o i n g  the enclosing  rocks  conformable d e p o s i t s  picked  up r a d i o g e n i c  lead  between t h e time o f f o r m a t i o n  and t h e time o f m i g r a t i o n .  This  o f development o f an anomalous l e a d i s e q u i v a l e n t c u s s e d f o r t w o - s t a g e and t h r e e - s t a g e i n t e r p r e t a t i o n given  generated of the method  to that  anomalous l e a d s .  dis-  The  h e r e i s based, o n t h i s m o d e l .  An a n o m a l o u s l e a d l i n e  i s drawn f r o m sample 372 t o t h e  Manitouwadge c o n f o r m a b l e l e a d s  (see F i g u r e  IV.3).  This  line  FIGURE IV.3 ISOTOPIC COMPOSITION OF LEADS FROM MANITOUWADGE  ^57  2  16 Pb207 P b  204  12  /  itoiiwadge Cor f o r m a b l e I e a d  /*~~ P r i m a r y Growth Cu r v e  / 8  10  •'  14  18 Pb206  22 / p b  204  26  30  117 i n t e r c e p t s the primary  growth curve  at a negative time.  l e a d s c a n t h u s n o t be  e x p l a i n e d as two-stage l e a d s .  o f the anomalous l i n e  i n t h e x-y  age be  d i a g r a m i s .189.  assuming t g i n e q u a t i o n  The  slope  A maximum  f o r t h e f o r m a t i o n o f Manitouwadge c o n f o r m a b l e c a l c u l a t e d by  The  deposits  (IV.3) i s z e r o .  can This  i s e q u i v a l e n t t o t h a t c o n s i d e r e d b y R u s s e l l e t a l . (1954)  limit  a s t h e maximum t i m e  o f i n c o r p o r a t i o n o f u r a n i u m and  i n t o the c r u s t a l r o c k s . more t h a n  2780 m.y.  thorium  T h u s , Manitouwadge d e p o s i t s c a n be  no  old.  S i n c e i t i s d o u b t f u l t h a t t h e K e e w a t i n , Algoman  and  Keweenawan r o c k s o f Manitouwadge have b e e n a l t e r e d  s i n c e Gren-  ville  m.y.  time  times,  a more r e a s o n a b l e  f o r the G r e n v i l l e  orogeny, as o b t a i n e d by  a n a l y s e s on f o u r g n e i s s e s f r o m Hurley  and  o l d e r measurements. m.y.,  wadge c o n f o r m a b l e  the G r e n v i l l e  A s s u m i n g t h i s c o u l d be we  find  the time  d e p o s i t s t o be  lead'line  i n t e r c e p t s the primary  aV = 8.99  ±  .05)  K-Ar  made b y  of formation of  growth curve i  (Fairburn,  (t  estimate  o f t h e age  anomalous  = 4.55  This  Gyr.,  suggests  3110  i  20  m.y.  of the Keewatin rocks can l e a d s formed  were u n a l t e r e d i s o t o p i c a l l y b y  b y w h i c h t h e v e i n l e a d s were f o r m e d .  Manitouwadge i s t a k e n t o be  0  as  Manitou-  The  20 m.y.  a s s u m i n g t h e Manitouwadge c o n f o r m a b l e  event  Rb-Sr  i n e r r o r by  2330 * 30 m.y.  a t t i e q u a l t o 3110  a t w o - s t a g e d e v e l o p m e n t and third  and  locality  t h e K e e w a t i n r o c k s o f Manitouwadge a r e o l d e r t h a n A second  This  (i960)), i s i n good agreement w i t h many  Pinson,  much a s - 50  v a l u e f o r t g i s 950  2330 * 30 m.y.,  and  The the  age  be by the  of  first-stage  118 growth i s assumed t o be i n a primary ,05 ( f o r t  0  = 4.55 G y r . ) .  system with o/V « 8,99 *  The time t-_ i s then c a l c u l a t e d t o  be 3320 * 50 m,y, Pye  (1957* page 9) s t a t e s :  "The Algoman i s r e p r e s e n t e d  p r i n c i p a l l y by l a r g e masses o f g r a n i t i c rocks, and a l s o by small s i l l s and d i k e s . . .  and a s s o c i a t e d b a s i c e r u p t i v e s .  D u r i n g t h i s p e r i o d the Keewatin and Pre-Algoman (?) rocks were i n t e n s e l y metamorphosed, and i n i t s c l o s i n g stages the m i n e r a l d e p o s i t s o f the area were formed."  The i n t e r p r e t a t i o n g i v e n  above f o r the f o r m a t i o n of Manitouwadge conformable i s i n accord w i t h t h i s d e s c r i p t i o n .  deposits  I t i s i n t e r e s t i n g t o note  t h a t the ages c a l c u l a t e d by the above model are i n good  agree-  ment with the minimum c r y s t a l l i z a t i o n age o f 3300 m.y, c a l c u l a t e d by Catanzaro  (1963) f o r z i r c o n s i n South Western Min-  nesota, and an event a t 2500 * 100 m.y. c a l c u l a t e d by K-Ar and Rb-Sr a n a l y s e s f o r samples from the same area ( G o l d i c h e t a l . (1961), G o l d i c h and Hedge (1962)). In summary, the i n t e r p r e t a t i o n f o r Manitouwadge l e a d i s as f o l l o w s .  Some time p r i o r t o 3110 * 20 m.y. ago (probably  i n the range 3320 * 50 m.y, ago) the Keewatin r o c k s c o n t a i n i n g l e a d , uranium and thorium were d e p o s i t e d a t Manitouwadge, These were i n t e n s e l y metamorphosed d u r i n g the Algoman p e r i o d at a time l a t e r than 2780 m.y. ago, (probably i n the range 2330 * 30 m.y, ago), and the conformable-type at t h i s time.  Radiogenic  d e p o s i t s formed  l e a d formed i n the e n c l o s i n g rock  from t h i s time u n t i l the Keweenawan p e r i o d o r G r e n v i l l e orogeny,  119 During t h i s period,  or at the  radiogenic  l e a d was  mixed w i t h  and  end  at the  \  of the  end  o f i t 950  l e a d o f the  p e r i o d the  ± 50  m.y.  conformable  Geco v e i n d e p o s i t  was  ago,  the  type, formed.  BIBLIOGRAPHY  1  120  A l d r i c h , L . T . a n d G.W. W e t h e r i l l : Geochronology by r a d i o a c t i v e decay. 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