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An integrated geophysical study of the Zuni lineament in New Mexico Blenkinsop, John 1966

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AN INTEGRATED GEOPHYSICAL STUDY OF THE ZUNI LINEAMENT IN NEW MEXICO by JOHN BLENKINSOP B.Sc,  U n i v e r s i t y o f B r i t i s h Columbia, 1964  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE  REQUIREMENTS FOR' THE DEGREE OF MASTER OF SCIENCE i n t h e Department of GEOPHYSICS  We accept this; t h e s i s as conforming t o the; r e q u i r e d standard  THE  UNIVERSITY OF BRITISH COLUMBIA September, 1966  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 o f the r e q u i r e m e n t s  f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t 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 study,  I f u r t h e r agree t h a t p e r m i s s i o n - f o r  and  extensive copying of t h i s  t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by t h e Head o f my Department o r by h i s r e p r e s e n t a t i v e s .  I t i s understood that  copying  or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n  permission.  Department o f The U n i v e r s i t y o f B r i t i s h Vancouver 8 , Canada  Date  AW  Xlt ,  Columbia  ii  ABSTRACT A zone o f c r u s t a l weakness a s s o c i a t e d  with the Zuni  Lineament i n the southern h a l f o f New Mexico has been del i n e a t e d by l e a d i s o t o p e measurements, a mieroearthquake study and magnetic depth soundings. Slawson and A u s t i n  (1962) r e v e a l e d  a northwest t r e n d i n g  The l e a d r e s u l t s o f the p o s s i b l e e x i s t e n c e o f  zone o f i s o t o p i c a l l y s i m i l a r l e a d ,  bounded on e i t h e r side by i s o t o p i c a l l y v a r i a b l e l e a d s . feature and  passes through Socorro and Alamogordo, New Mexico,  i s c o i n c i d e n t i n t h i s area with the Zuni Lineament. ;  mieroearthquake r e s u l t s (Sanford  i s o t o p i c a l l y s i m i l a r zone. Livingstone,  The  and Long,. 1965) show that  a shallow c r u s t a l zone o f weakness i s a s s o c i a t e d  1964;  This  with the  The magnetic r e s u l t s (Schmucker,  1966).confirm that the s t r u c t u r e i s shallow,  and, a l s o i n d i c a t e that i t may be a r e g i o n  of. h i g h - e l e c t r i c a l  conductivity. I t i s suggested that t h i s , zone o f weakness- centred on the  Zuni Lineament has served as a c o n d u i t  system-for hydro-  thermal m i n e r a l i z i n g - s o l u t i o n s which c a r r i e d the l e a d t o i t s f i n a l s i t e s of deposition.  The l e a d was o r i g i n a l l y  intro-  duced i n t o the Precambrian rocks about 1550 m.y. ago, mixed with-radiogenic then, f i n a l l y  rock l e a d o f that age about 30 m.y. ago, and  emplaced.  iii TABLE OF CONTENTS ABSTRACT  i i  LIST OF FIGURES  iv  LIST OF TABLES  v  ACKNOWLEDGEMENTS  vi  CHAPTER 1  1  1.1  O u t l i n e o f problem  1  1.2  I s o t o p i c c o r r e l a t i o n w i t h Zuni Lineament  2  1.3  Geomagnetic measurements i n s o u t h - c e n t r a l New Mexico  2  1.4  S e i s m i c r e s u l t s from Socorro a r e a  6  CHAPTER 2  9  2.1  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 data  9  2.2  E x p l a n a t i o n s of i s o t o p i c d i s t r i b u t i o n  9  2.3  S e i s m i c d e f i n i t i o n o f i s o t o p i c zone  11  2.4  Geomagnetic measurements  11  2.5  Th/U c o n t o u r i n g  13  CHAPTER 3  15  3.1  Geology of a r e a  15  3.2  Sample p r e p a r a t i o n and a n a l y s i s  15  3.3  Discussion o f results  3.4  Geographic d i s t r i b u t i o n o f samples  /  16 21  CHATTER 4  24  REFERENCES'  27  APPENDIX A . l  29  APPENDIX A.2  31  iv LIST OF FIGURES FIG. 1.1  Model of c o n d u c t i v i t y s t r u c t u r e of c r u s t and upper mantle between A r i z o n a and Texas  5  FIG. 1.2  Observed and t h e o r e t i c a l t r a v e l t i m e s f o r S S and S P waves f o r S o c o r r o microearthquakes  8  X  X  FIG. 2.1a E p i c e n t r e s of Socorro microearthquakes  '  12  F I G . 2.1b P r o j e c t i o n o f microearthquakes on t o an E-W cross-section  12  F I G . 3.1a I s o t o p i c p l o t of New Mexico l e a d s  l£  FIG. 3.1b I s o t o p i c p l o t o f New Mexico l e a d s  19  F I G . 3.2  22  L o c a t i o n s o f lead stations  1  samples and geomagnetic  LIST OF TABLES TABLE 1.1  Explanation work  of symbols used i n i s o t o p i c  TABLE 3.1  Lead i s o t o p e r a t i o s , f o r some New Mexico samples  TABLE 3.2  Some Precambrian ages: from New Mexico  TABLE A . l  O p t i o n s a v a i l a b l e f o r program "PILOT"  south-central  vi  ACKNOWLEDGEMENTS  The w r i t e r would f i r s t for  s u g g e s t i n g the problem and s u p e r v i s i n g the subsequent  research. for  l i k e t o thank Dr. W.F. Slawson  Thanks a l s o go t o Drs. D.H. Weichert and R.D. R u s s e l l  d e v e l o p i n g the automatic data r e d u c t i o n system which  g r e a t l y s i m p l i f i e d the t e d i o u s business o f measuring isotope r a t i o s .  lead  The w r i t e r i s a l s o g r a t e f u l t o Mr. P.H. Reynolds  and Miss S. Newman f o r a s s i s t a n c e they have p r o v i d e d .  Miss  Newman a l s o typed the f i n a l d r a f t o f the t h e s i s . The work was supported by g r a n t s from the Defence Research Board and the N a t i o n a l Research C o u n c i l .  A fellowship pro-  v i d e d by the C a l i f o r n i a Standard Co. i s g r a t e f u l l y  acknowledged.  I  CHAPTER I  1.1  O u t l i n e o f Problem. The present r e p o r t i s an attempt  t o c o r r e l a t e the r e s u l t s  of some g e o p h y s i c a l surveys i n south and c e n t r a l New Mexico with geochemical data  and g e o l o g i c a l i n f o r m a t i o n .  Lead i s o t o p e  (Slawson and A u s t i n , 1962; t h i s r e p o r t ) , magnetic  soundings  depth  (Schmucker, 1964; L i v i n g s t o n e , 1966), a m i e r o e a r t h -  quake I n v e s t i g a t i o n  (Sanford and Long, 1965), a l l have a  b e a r i n g on the g e o l o g i s t s ' d i s c u s s i o n o f lineaments i n New Mexico (Mayo, 1958).  C o n s t r a i n t s imposed by the d i f f e r e n t  models employed s e r i o u s l y l i m i t the p o s s i b l e  interpretation  of the nature of the lineaments, e s p e c i a l l y the nature of the Zuni Lineament. A lineament i s probably best d e s c r i b e d as a l a r g e - s c a l e , approximately l i n e a r arrangement o f s t r u c t u r a l f e a t u r e s , although g e o l o g i s t s themselves the term means;- A lineament following:-  d i s a g r e e over p r e c i s e l y what  i s marked by some or a l l of the  f a u l t s , f o l d i n g , u p l i f t s , v o l c a n i c s and i n t r u s i v e s .  I t s l i n e a r nature i s b e l i e v e d t o be due t o s h e a r i n g at depth. The  Zuni Lineament i s thought  t o c o n s i s t o f the Guada-  lupe Mts; i n western Texas; the Sacramento U p l i f t , Mt. T a y l o r v o l c a n i c s , and Zuni U p l i f t  i n New Mexico; the Defiance  i n n o r t h e a s t e r n A r i z o n a ; and the C i r c l e C l i f f s U p l i f t , P l a t e a u v o l c a n i c s , and the Gold H i l l s d i s t r i c t  Uplift High  of Utah.  Superimposed f e a t u r e s such as the S e v i e r f a u l t and the Rio Grande graben  suggest no r e c e n t movement has taken p l a c e along  2 the  lineament.  numerous  points  Isotoplc  1.2  In  a  of  the  of  an  lead  symbols) belt,  were  along  coincides  a  in  ranging  line this  Geomagnetic In  large  component vertical angle  If  (D)  between  different  can  attributed  electrical models  of  H from to  station  conductivity the  and the  calculated.  earth's  surface The  the  to  effect  be  variable  Socorro,  that  parameters  of  the  New  Mexico,  by  field  a  chain  horizontal field are  Mexico.  as  within  be  the  are  of  3and  as  be  the  consis-  differences  can  in  the  earth. then  p r o d u c e d by  models  over  well  found to the  of  (H)  inhomogeneities  would  belt,  New  uniform  conductivity,structure  fields  isotopic The  25.4).  fairly  the  distribution  of  trending  external  station, of  ranging  Lineament.  the  north)  Leads  explanation  a  to  earth's  and t r u e  the  of  measured  (measuring of  (x  South-central  found to  variations  components  tently be  the  is  magnetometers (Z)  in  distribution  a northwest  Zuni  geo-  discovered  (1962)  for  1-1  19.2  mid-latitudes,  variations  areas.  the  a  composition.  Alamogordo and  with  be  geographic  leads  from about  Measurements  geomagnetic  geomagnetic  Table  by  to  composition  within  side  joining area  the  isotopic  see  occur  appeared  and A u s t i n  isotopic  -  bounded on e i t h e r  lying  up  their  19.0  found to  c o m p o s i t i o n - (x  1.3  and  at  Lineament.  what  between  similar  to  18.4  of  lineaments  1955).  Zuni  Slawson  correlation samples  with  study  area,  other  Kelley,  1958;  isotope  essentially  from about  c r o s s e d by  (Mayo,  simple  remarkable  is  Correlation  lead  logically a  It  Simple be  them  adjusted  to  set  3  t = 0 (present)  t - t • 4.55 (age o f E a r t h )  at any time t  0  206  a  Q  = 9.56  207 20T  b  Q  = 10.42  208 20T  c  Q  = 30.00  20T  235 20T  Xt  238 20T  ocVe  232 20T  W  2 3 5  Th  Table 1.1  2 3 2  Xt  ocVe  Pb  2 0 6  -  Pb  2 0 7  -.  Pb  2 0 8  X t  o  We^o  We  decay scheme  u  Ve^o  Ve  •V  constant  X  _Q  —I  (xlO y r s  )  = .1537  A = .9722 X'= .0499  E x p l a n a t i o n of symbols used i n i s o t o p i c work.  4 obtain  agreement  If  agreement  to  be  an  Is  between  the  obtained,  approximate  calculated  then  the  and  model  representation  of  observed  is  the  results.  considered earth's  perhaps  conductivity  structure There first  is  are  that  two only  calculations of The  second  the  :  data.  deductions  about  isotopic region was  of  to  the  across the  of  a  be  the  it  and  of  itself,  in  however,  observed  necessary  the  to  and  assume  (Figure  1.1)  - Texas  anomaly  zone  interpreted  deep does  mantle not  calculated the  and  the  between Las  a  (1964)  revealed  with  The  it.  Cornudas, g- r a t i o s  east.  few  about  character  recorded change  hundred  results  as  The  a  step  step  so  in  kilo-  model  agreement  fields,  and  the  Texas  of  The  hr)  a highly  Cruces  the  of  complete  surface  the some  large-scale  the  of  ob-  make  structure.  existence  fit  southeast  (>1  of  provide  the  Schmucker  between  a  models,  structure.  aligned  a depth  complicated.  may  the  variations  at  too  to  The  otherwise  boundary  to  requires  method.  simple  by  New M e x i c o ,  zone  has  far  models  0.15-0.20 t o  conductivity  used,  possible  approximately  Cruces,  be  using  more is  this  conductivity  New M e x i c o  a transition  Schmucker  by  or  to  are  sounding study  transition  discontinuity  region  two  earth's  and  Las  west  earth's  between  that,  long-period magnetic  meters.  by  that  can  fields  conductivity  between  the  is  depth  anomaly  found- t o  0.05  surface  the  the•southern  existence  models  Nonetheless,  A magnetic along  limitations  simple  limitation  possibility exists served  major  it  is  conductive  Cornudas.  The  5  STATIONS TU LAC ORO COR SWE  Tucson Las Crucea Orogrande Cornudas Sweetwater  FIG. 1.1 Model of c o n d u c t i v i t y s t r u c t u r e of c r u s t and upper mantle between A r i z o n a and Texas ( a f t e r Schmucker, 1964)  6 r e g i o n i s at a very shallow depth, t r e n d s n o r t h - s o u t h and i s about 1.4  100 km.  wide.  Seismic R e s u l t s from Socorro Area. Some seismic evidence f o r the zone o f weakness b e l i e v e d  a s s o c i a t e d with the lineament i s presented by Sanford and Long ( 1 9 6 5 ) .  They have used an a r r a y of from t h r e e t o f i v e  h i g h - m a g n i f i c a t i o n seismographs, l o c a t e d 5 km. to r e c o r d nearby microearthquakes of about  600 a year.  west of S o c o r r o ,  (S-P<2.5 sec.) at the r a t e  Prom s u r f a c e e x p l o s i o n s ,  f o r the g e o l o g i c s e t t i n g and e l e v a t i o n of the  corrections individual  s t a t i o n s have been determined and a p p l i e d to the r e c o r d s . For an i n d i v i d u a l earthquake, the d i r e c t i o n o f the e p i c e n t r e i s o b t a i n e d by comparing  the c o r r e c t e d P a r r i v a l times at  each s t a t i o n of the a r r a y . travelled  The wave i s assumed to have  i n a s t r a i g h t • l i n e from the focus t o the  station  with a v e l o c i t y of 6 km./sec, and the d i s t a n c e t o the focus i s found from the S-P 0.25.  i n t e r v a l , assuming  Poisson's r a t i o i s  E r r o r s i n - t h e r e l a t i v e l o c a t i o n s of the f o c i  b e l i e v e d to range from  <1  km.  t o <C 4 km.,  depending  q u a l i t y of the r e c o r d s and the number of s t a t i o n s the shock.  are on the  recording  E r r o r s i n the a b s o l u t e l o c a t i o n s are unknown.  About 25% of the microearthquakes observed have S P and S S  a r r i v a l s a s s o c i a t e d with them.  The a r r i v a l s are  The two types o f seismic waves that t r a v e l through the e a r t h are P waves ( l o n g i t u d i n a l waves) and S waves ( t r a n s v e r s e waves). When e i t h e r type o f wave encounters a v e l o c i t y d i s c o n t i n u i t y , both P and S waves are r e f l e c t e d from i t . An S S wave i s an i n c i d e n t S wave r e f l e c t e d as an S wave from a c r u s t a l d i s c o n t i n u i t y , while an S P wave i s an i n c i d e n t S wave r e f l e c t e d as a P wave from the same d i s c o n t i n u i t y . X  7 believed a  depth  to of  be  located  in  (Sanford  of  and  km.  The  feature  is  shocks  Sierra  km.  not  if  associated  to  with  could  indeed  the  been  of  come of  Socorro  on  earthquakes Socorro  from a the  possible,  at  found  three  north  mean d e p t h  is  discontinuity  have  km.  35  They  the  figure  confined  crustal  S phases  Ladrones,  deep  latter  S  1962).  Holmes,  18  from a  Probable  weak  the  continuity 4.8  km.  18  seismograms  reflections  dis-  foci so  region.  were  the  A plot  0  f  i travel shown S P x  times in  and  nuity  The  Figure S S a  h  of  ment  in  bution most  h-  data,  9 km.,  and  of  the  (68%)  of  In  order  a model  a s s u m e d no geneous foci.  All  The somewhat  the  focal  to has  and  three  other  for  a  focal  depth  equally  well,  so  occur  a  of  between useful  used. of  model  and  the  delineation  of  the  recent  Zuni  has  disconti-  for of  of  crustal  studies and  focal  adjust-  the  distri-  confirms  from the  observed  model  has  a relatively of  energy  contribute  been  a  4 km.  little  particular  release  Lineament  for  9 km.  waves,  actual  foregoing material disconnected  This  will  curves  drawn  information  symmetrical  is  microearthquakes 4 and  seismic  assumptions  is  Examination  the  shocks  velocity  curve  depths  been  theoretical  a horizontal  possible.  obtain  such  One  data  is  with  for  km.  18  the  Z  them  interval  from  the  refraction  crust,  between  Z of  bracket  S-P  together  1.2,  depth  curves  the  reflections  x  at  depth  against  to  from  the  differences  structure.  a review  of  having  bearing  an  homo-  a  a  series  explanation  on of  of the it.  • 1 0..5  1.0 S-P  1..5  Interval  2,0  2.5  (sec)  FIG. 1.2 Observed and t h e o r e t i c a l t r a v e l t i m e s f o r S^S and S P waves f o r Socorro microearthquakes ( a f t e r Sanford & Long, 1965) X  9 CHAPTER IX 2.1  I n t e r p r e t a t i o n of Lead Isotope Data. The  two  models most f r e q u e n t l y used i n the i n t e r p r e t a -  t i o n of l e a d i s o t o p e measurements are o u t l i n e d i n the  appendix.  The  s i n g l e - s t a g e model f u r n i s h e s a simple  interpretation for  the  small number of leads that appear to s a t i s f y the model.  The  i n t e r p r e t a t i o n i s almost c e r t a i n l y too simple,  p r o v i d e s , i n some cases, an approximation to the h i s t o r y of the l e a d .  The  yet  actual  two-stage model i s a l s o a crude  r e p r e s e n t a t i o n of the development of a l e a d ; given  the  complex g e o l o g i c a l h i s t o r y of most areas, i t i s u n l i k e l y that o n l y two  events have a f f e c t e d the l e a d t h e r e .  the equations f o r the two-stage leads solved completely,  (eq. A.3)  while the equations f o r the  However,  can  be  three-  stage,  f o u r - s t a g e , e t c . l e a d s cannot be solved f o r unique  values  of the v a r i a b l e s .  2.2  Explanations Slawson and  explanation tion.  of Isotopic D i s t r i b u t i o n . Austin  (1962) have o f f e r e d a  simple  f o r the observed p a t t e r n of i s o t o p i c d i s t r i b u -  They p o s t u l a t e that the lineament r e p r e s e n t s  a zone  of v e r t i c a l c r u s t a l weakness which has acted as a conduit hydrothermal m i n e r a l i z i n g s o l u t i o n s .  for  Their interpretation  i s t h a t s i n g l e - s t a g e l e a d from a p e r i o d of m i n e r a l i z a t i o n 1600  m.y.ago was  mixed i n T e r t i a r y time with  radiogenic  l e a d , formed by the decay of uranium and thorium p e r i o d 1600  m.y.  ago  over the  to the time of the mixing, to produce  anomalous leads of s i m i l a r i s o t o p i c compositions.  The  leads  10 were then t r a n s p o r t e d upward by m i n e r a l i z i n g s o l u t i o n s , evidence The  f o r which has been presented  by Smith ( 1 9 6 3 ) .  s o l u t i o n s t r a v e l l i n g along the conduit p r o v i d e d by  lineament  the  a c q u i r e d r e l a t i v e l y l i t t l e r a d i o g e n i c l e a d from  w a l l r o c k and the leads were t h e r e f o r e d e p o s i t e d with only minor changes i n t h e i r i s o t o p i c compositions. t h a t t r a v e l l e d outward from the conduit  Those s o l u t i o n s  system a c q u i r e d  v a r i a b l e amounts of r a d i o g e n i c l e a d as they p e r c o l a t e d through the w a l l r o c k and those with v a r i a b l e I s o t o p i c  leads were t h e r e f o r e d e p o s i t e d  compositions,  Mauger, Damon and L i v i n g s t o n (1965) have the f o r e g o i n g hypothesis evidence  of the conduit  presented,  criticized  on the grounds that no  structural  system centred on the lineament  and t h a t the assumption of ascending  is  hydrothermal  m i n e r a l i z i n g s o l u t i o n s to e x p l a i n the i s o t o p i c data i s unnecessary, hydrothermal s o l u t i o n s i m p l y i n g a d i f f e r e n t g e n e s i s f o r rocks and twelve  ores.  They p o i n t out that ten df  of the i s o t o p i c a l l y s i m i l a r leads are e i t h e r  i n v o l c a n i c rocks or adjacent  to i n t r u s i v e r o c k s , whereas of  the i s o t o p i c a l l y v a r i a b l e l e a d s , only seven of the have i n t r u s i v e s i n t h e i r v i c i n i t y . Lineament may that i t may  contained  fourteen  They agree that the  Zuni  represent a v e r t i c a l zone of weakness but  feel  have served as the l o c u s f o r the ascent  neous rocks r a t h e r than m i n e r a l i z i n g s o l u t i o n s . i s o t o p i c r a t i o s are then e x p l a i n e d as being due  The  authors  observed  to the  t i o n s of e x t r a c t i o n of the l e a d from the surrounding The  of i g -  s t a t e that l e a d e x t r a c t e d from heated  condi-  rocks.  igneous  II  rocks  is  amount  more  of  likely  to  radiogenic rocks  isotopic  distribution  lineament  activity thermal  has  the  Sanford  and  to  an  east-west  It  is  apparent striking  valley.  The  boundary  the  this  support  to  the  zone  weakness,"a  than  a  extracted  source. as  along  conduit  The  observed  a result  which  of  igneous  system  for  hydro-  of  Zone.  some m i c r o e a r t h q u a k e s  and  a projection  cross-section  isotopic The  contention zone  and  coincide anomaly,  agreement  that in  the  which  occur  stresses  foci  Figure  the  which  is  between  rocks  b u i l d i n g up  Grande  well  lines  lends  represents cannot  within  a  with  quite the  of  Rio  approximately  on  2.1.  northeast  towards  lineament the  the  comprise  d i p p i n g 55°  found to  of  recorded  a  readily them.  Geomagnetic•Measurements,  2.4  Preliminary out  across  the  total  anomaly  Dr.  Ulrych  by  Precambrian  basement  Socorro.  T.J. is  a  2.lb  decreases  microearthquakes,  basement.  at  From F i g u r e  microearthquakes Most  magnetic  isotopic  interpreted  of  weakness  smaller  lead  explained  Isotopic  and  point.  strains  heat  no m i c r o e a r t h q u a k e s  is  at  withstand  of  northwest line  a  than  a  solutions.  (1965)  that  defined  of  rather  vertical  of  of  epicentres  Long  of  therefore  zone  Definition  by  the  a  mineralizing  A map'of  line  is  (containing  isotopically  absence  occurred  Seismic  2.3  the  being  "normal"  lead)  from o l d  the  in  be  field by  measurements  C.E.  revealed  depth it  of  Livingstone that  about  appears  significantly  then,  occur  in  carried  the  the  top  4 km.  that  the  above upper  and  kindly  of  just  the north  number  about  of  4 km.  Precambrian  FIG. 2.1b P r o j e c t i o n of microearthquakes on t o an E-W c r o s s - s e c t i o n  13 The magnetic  depth sounding  study by C E . L i v i n g s t o n e  r e v e a l e d no obvious deep c o n d u c t i v i t y s t r u c t u r a l changes w i t h i n the e a r t h .  Because of the low speed of the r e c o r d i n g  chart of the Askania magnetometers, the h i g h frequency  varia-  t i o n s needed f o r shallow c r u s t a l s t u d i e s were not r e c o r d e d . I t i s not impossible that a shallow, h i g h l y conductive b e l t i s a s s o c i a t e d with the i s o t o p i c anomaly and i s an e x t e n s i o n of the shallow c o n d u c t i v i t y f e a t u r e p o s t u l a t e d by Schmucker. The c r u s t a l d i s c o n t i n u i t y , found by Sanford and Long (1965) at a depth of 18 km. isotopic 2.5  Th/U  c o u l d then mark the lower boundary of the  zone. Contouring.  S i n c l a i r and Walcott  (1966) have c a l c u l a t e d the  Th/U  r a t i o s f o r the samples as r e p o r t e d by Slawson and A u s t i n and p l o t t e d contours of the v a l u e s t o show t h e i r variations.  The  geographic  c o n t o u r i n g supports evidence i n d i c a t i n g that  some s t r u c t u r a l f e a t u r e connected  with the Zunl Lineament  c o n t r o l s the i s o t o p i c d i s t r i b u t i o n of the samples i n that contours are not continuous a c r o s s the suggested but are symmetric about  the lineament.  t h a t t h e i r c o n t o u r i n g demonstrates  boundaries  The authors  feel  that there are two  centres  of m i n e r a l i z a t i o n along the lineament, one near Socorro the other about  20 m i l e s to the southeast.  p l o t t e d a histogram of the Th/U evidence of two d i s t i n c t Th/U  the  and  They have a l s o  r a t i o s and f i n d from i t  populations.  they modify the m i n e r a l i z i n g h i s t o r y proposed  Consequently by Slawson and  A u s t i n by p o s t u l a t i n g that the leads were o r i g i n a l l y d e p o s i t e d  14  i n t o two  d i f f e r e n t Th/U  previously described The  preceding  environments, then m i n e r a l i z e d  (Section  2.2).  c o n s t i t u t e s an attempt to c o r r e l a t e the  r e s u l t s of d i f f e r e n t types of g e o p h y s i c a l  s t u d i e s i n order  to r e s t r i c t the p o s s i b l e i n t e r p r e t a t i o n s o f the Lineament.  as  Zuni  15  CHAPTER I I I 3.1  Geology of Area. I s o t o p i c analyses were c a r r i e d out on nine leads from  New  Mexico, i n c l u d i n g three c o l l e c t e d from the same g e n e r a l  area that Schmucker s work covered. f  The  f o l l o w i n g d e s c r i p t i o n of the sample a r e a , encompassing  Socorro, L i n c o l n , S i e r r a , Otera and Dona Ana c o u n t i e s , i s a b s t r a c t e d from Slawson and A u s t i n (1962):  the northwest  p o r t i o n of the area i s l o c a t e d i n the b a s i n and range type mountains on the border of the Colorado  P l a t e a u , while  the  southeastern p o r t i o n c r o s s e s the north-south t r e n d i n g b l o c k f a u l t e d mountains of the New  Mexico R o c k i e s .  graben passes through the sampled a r e a .  The Rio Grande  Tertiary gravels  and l a v a flows cover most of the a r e a , although P a l e o z o i c sediments and  some Mesozoic and Precambrian rocks are exposed  a l o n g the mountains. Laramide 3.2  There i s some l i m i t e d exposure of  intrusives.  Sample P r e p a r a t i o n and A n a l y s i s . The  samples were prepared  d e s c r i b e d by U l r y c h ( I 9 6 0 ) .  i n e s s e n t i a l l y the manner  Galena was  HCl to form l e a d c h l o r i d e , which was  leached with  p r e c i p i t a t e d by c o o l i n g ,  then f i l t e r e d out and washed with c o l d d i s t i l l e d l e a d c h l o r i d e was potassium  re-di3;solved i n hot d i s t i l l e d  i o d i d e added to the s o l u t i o n , and the  l e a d i o d i d e p r e c i p i t a t e f i l t e r e d and d r i e d . was  prepared  dilute  water.  The  water, resulting  Tetramethyllead  by r e a c t i n g the l e a d i o d i d e with methyl i o d i d e  and a Grignard reagent, methyl magnesium bromide i n e t h y l  16 ether. of  (The purpose of the Mel i s to i n c r e a s e the  tetramethyllead.)  and e t h y l ether was  The r e s u l t i n g mixture  of t e t r a m e t h y l l e a d  separated i n t o i t s c o n s t i t u e n t s by vapour  phase gas chromatography as developed t e t r a m e t h y l l e a d was  by U l r y c h ( i 9 6 0 ) .  The  analyzed on the 9 0 ° , 12 i n c h s i n g l e  f o c u s i n g mass spectrometer K o l l a r and R.D.  yield  Russell.  designed and b u i l t by Drs. A l l the samples were  F.  intercompared  as d e s c r i b e d by K o l l a r et a l (I960) with the U.B.C. No. standard* and the data r e d u c t i o n s f o r f o u r of the f i v e  1 loops  were c a r r i e d out a u t o m a t i c a l l y , u s i n g the system d e s c r i b e d i n the  appendix,  3.3  D i s c u s s i o n of R e s u l t s . The v a l u e s obtained are given i n Table 3.1, and  p l o t t e d on P b p 206 , 204 g b  /  pt)  2 0 7  r  a  p  /Pb n  s  2 0 4  i n  - Pb^/Pb  pig  U r  e  3.1.  2 0 1 1  and P b  by York ( 1 9 6 6 ) .  c a l s of the percentage  /Pb  2 0 1 1  -  S t r a i g h t l i n e s have been  f i t t e d through the p o i n t s u s i n g a weighted suggested  2 0 8  also  The weights  uncertainties.  least-squares f i t  used were the r e c i p r o The  drawn through the nine samples i s 0.0952  slope of the  ± .OO67,  line  i n agreement  with the value of 0.0931 * . 0 0 3 3 obtained f o r 63 samples by Slawson and A u s t i n ( 1 9 6 2 ) , a value subsequently m o d i f i e d t o 0 . 0 9 3 8 ± .0029 by l a t e r work. The history  samples can be i n t e r p r e t e d i n terms of a two-stage (Appendix  A.l).  I f the Bosque d e l Apache sample i s  a s i n g l e - s t a g e l e a d , then i t s age can be taken as t ^ , and the corresponding t 2 found from equation  *  A split  A.4.  of the widely c i r c u l a t e d Toronto  #1003 sample.  17 Sample no.  Name  x  y  z  16.34  42.67  501  La B o n i t a  25.32  502  Box Canyon  20.63  504  Hansonburg  22.30  505  Bosque d e l Apache  16.19  15.46  36.03  507  Linchburg  13.55  15.74  33.72  503  Kelly  13.74  15.73  33.32  530  Modoc  13.20  15.66  33.99  532  Orogrande  13.96  15.73  39.31  533  Courtney  13.72  15.36  39.27  T a b l e 3.1  15.91  39.94  16.06  40.64  Lead i s o t o p e r a t i o s f o r some New Mexico samples  16.5  •16.0  •  Pb ?" 20  7b"204 <*^^^ ^^^^  •15.5  /  /  16  17 •  18  1  1  >  1  Pb  FIG. 3.1a  21  20  19  2 0 6  /  Pb  20/  *  I s o t o p i c p l o t of New Mexico l e a d s  22 Li  23 1  24  25  Pb  203  Pb 204  Pb /pb 206  FIG.  3.1b  204  I s o t o p i c p l o t o f New Mexico leads;  20 Using the  equation  Bosque  del  sample  is  Taking  into  Apache  found to  A.2,  and  ratios  as  it  as  primary-like.  lead,  but  cannot  2  It  of  age  is  30 m . y .  of  deviation  (Weber  value in  be  preferred  of  the  Bosque  The were  The  time.  1550  Bosque  m.y.  older and  Mexico.  of  t^  is  2  then  They  are  given  a  taken  in  or  may  the  <*.V  be  mantle  described  single-stage one.  to  be  1500  which  is  perhaps  reasonable  (Smith, m.y.  In  these  view  too  for  approximate The  cor-  of  the  are  not  given,  but  ages  doubtful  m.y.  value  1963).  previously  the  in  upper  1963), b e i n g t h e  line,  is  that  crustal  primary  m.y.  30  was  uranium, about  formed at  ago m i x e d i n  radiogenic leads  should  nature  lead  were  and  m.y.  1550 this  varying  generated  then  thorium  same probetween  emplaced,  (1964) h a v e r e p o r t e d a f e w  Precambrian in  lead,  environment  deposit  and D e n i s o n the  lead  the  3.83.  constants  crustal  m.y.  90  1550  of  and the  for  is  for  lead.  and the  ages  is  a more  lead  Apache  m.y,,  the  Apache  activity  then  activity  30  deep  from those  a  in  for  oc\l  Th/U r a t i o  p o s s i b l y not  choose  is  1  into del  Muehlberger K-Ar  t  Apache  lead  del  and B a s s e t t ,  interpretation  Tectonic  portions  to  t  them because  incorporated  ago,  of  two-stage  del  its  calculated  value,  d i s t i n g u i s h e d from  value  different to  be  volcanic of  the  significantly  and  the  this  is  suspected variations  quite  readily  widespread  responding  the  Bosque  is  possible  For  and  m.y.  1500  postulated  corresponding value  high. t  the  It  of  uncertainties  as  the  Consequently, The  the  for  age  8.80,  well  appears  an  sample.  be  account  equation  source,  A.2  Table  rocks 3.2.  of  south-central  Rb-Sr New  21 Location  Mineral  Method.  Age  S i e r r a Oscura L i n c o l n Cty.  whole rock  Rb-Sr  1300  m.y.  Rhodes Canyon* S i e r r a Cty.  whole. rock  RbrrSr  1430  m.y.  Hansonburg d i s t r i c t * S i e r r a Cty.  microcline biotite  Rb-Sr K-Ar  1570 1360  m.y. m.y.  *  Lead samples c o l l e c t e d from same region,  Table 3.2  Some Precambrian ages from s o u t h - c e n t r a l New Mexico (Muehlberger and Denison, 1 9 6 4 ) .  The authors have i n t e r p r e t e d these ages as r e p r e s e n t i n g a metamorphism  and g r a n i t i c i n t r u s i o n about 1570 m.y.  f o l l o w e d by f u r t h e r igneous a c t i v i t y about 1350 m.y. Taking i n t o account the 6% u n c e r t a i n t y i n the Rb-Sr the agreement between the 1570 m.y. l e a d age i s q u i t e i m p r e s s i v e . 1350 m.y.  age and the 1550  ago, ago. half-life, m.y.  The Igneous a c t i v i t y of about  ago does not appear to have been accompanied by any  lead m i n e r a l i z a t i o n . 3.4  Geographical D i s t r i b u t i o n of Samples. F i g u r e 3.2 i s a map  of the r e l e v a n t a r e a , showing the  l o c a t i o n s of the samples analyzed by Slawson and A u s t i n and the present author, as w e l l as the l o c a t i o n s of the geomagnetic s t a t i o n s used by Schmucker (1966).  (1964) and L i v i n g s t o n e  The boundaries between the i s o t o p i c a l l y s i m i l a r and  i s o t o p i c a l l y v a r i a b l e leads are a l s o drawn.  The i s o t o p i c  anomaly appears t o extend i n t o the r e g i o n i n which Schmucker  22  A FIG,  3.2  SYMBOLS USED geomagnetic s t a t i o n • "variable"  O " s i m i l a r " lead lead  L o c a t i o n s of l e a d samples and geomagnetic  stations  23 postulated possible The in  his  that  two  only  to  has that  contoured of  one  centre  of  area.  Mayo  area  could  processes  because  no means  indeed  the  Cordilleran  Front  a  crustal  Zuni  belt  anomaly,  so  it  is  certainly  related.  by  Magdalena  of  are  are  Socorro-Magdalena  region  conductivity  author  similar  results  indicate  the  present  a manner  While  surface  all  U/Pb  Sinclair  and  and W a l c o t t  conclusive,  they  mineralization,  (1958) h a s  be  a  focus  Lineament, meet  weakness.  near  Th/Pb  ratios (1966).  appear  located  to  in  suggested that  for  the the  mineralization  Morenci  belt  Magdalena,  and  producing  24 CHAPTER  Approximately were  introduced  central the  into  resulted  basement  of  the  by  lead  of  fairly  then  carried  a  of  by  lead  the  uranium  isotopic  and  of  the  and  Zuni  relatively were  Other rather  z  Austin,  constant leads  rock-lead genization  1962) ratio  been  (Heier  on  graph  isotopically  and would  isotopic  that to  Pb  2  0  only the  Otherwise, given  Rogers,  1965)  in  show much more  variable  leads  do.  zone  of  wallrock.  similar  isotopic  percolated and  through  consequently  These  leads  Pb  3-B  p.24,  explanation. 2  0  6  for  formed the  the  amounts by  The isotopically  of  radiogenic  the  homo-  inhomogeneous  crustal scatter,  The  to  from the  Figure  small lead  solutions  solutions  lead.  this to  8  was  compositions.  or  3.1b  lead  shallow  weakness  radiogenic  supports of  added  process.  ratios  of  of  produce  therefore  solutions  zone  (Figure  confirms  have  the  variable  - x diagram  and  virtually  than  lead  to  and  essentially  lead-bearing  amounts  with  with  a  in  lead,  The  Lineament,  radiogenic  deposited  variable  The  little  by  south-  Precambrian  thorium,  lead-bearing  on  and  the  mineralizing  centred  the  in  thorium  activity  composition.  hydrothermal Some  west  radiogenic  weakness  deposited  the  uniform  old  provided  acquired  similar  thorough mixing  of  and  tectonic  system  leads  Slawson  quite  of  ago,  conduit  wallrock  were  rocks  30  decay  m.y.  uranium,  a  compositions. the  lead,  Precambrian  m.y.  upward  along  acquired These  the  ago,  About  deposition.  ascended crustal  in  1550  generated  sites  the  New M e x i c o .  area  m.y.  1550  IV  rocks,  the  as  data  scatter  the of  the  Th/U points for  latter  the  25 v e r i f i e s that they have passed through the  wallrock.  Hydrothermal m i n e r a l i z i n g s o l u t i o n s f l o w i n g along conduit  the  system r a t h e r than l e a d e x t r a c t i o n by magmatic  appear t o be r e s p o n s i b l e f o r the observed p a t t e r n of distribution.  Smith (1963) has  heating  isotopic  found g e o l o g i c evidence of  hydrothermal m i n e r a l i z i n g s o l u t i o n s i n the mines of Socorro Peak, l o c a t e d w i t h i n the zone. shallow  c r u s t a l f e a t u r e and  as a l o c u s f o r the ascent  The  t h e r e f o r e c o u l d not have  of igneous b o d i e s ,  tend t o come from g r e a t e r depths. near the c o u l d not  become molten at depths <  served  s i n c e such bodies  I f the thermal  s u r f a c e of the e a r t h i s 10-50  i n c r e a s e s when pressure should  zone of weakness i s a  gradient  C°/km,, then g r a n i t e  20 km.,  a depth which  e f f e c t s are a l s o c o n s i d e r e d .  It  a l s o be p o i n t e d out that the hydrothermal s o l u t i o n s are  only considered  as the agent c a r r y i n g the l e a d from a  shallow  c r u s t a l environment to i t s s i t e of f i n a l m i n e r a l i z a t i o n , so that a d i f f e r e n c e i n the genesis necessarily Seismic  of rocks and  ores i s not  postulated. r e s u l t s and magnetic depth soundings have  pro-  v i d e d a d d i t i o n a l i n f o r m a t i o n about the zone of weakness. The  seismic work has  shown that i t can be c o r r e l a t e d with  i s o t o p i c anomaly, and  that I t extends to a depth of about 18  Geomagnetic work ( L i v i n g s t o n e , 1966) zone of weakness i s a shallow I n d i c a t e d t h a t i t may  has  confirmed that  c r u s t a l f e a t u r e , and  extend at l e a s t to the New  Texas border (Schmucker, 1 9 6 4 ) . t h a t the  The  Zuni Lineament.  the  has  Mexico -  l e a d i s o t o p e data show  zone of weakness i s a northwest t r e n d i n g  c o i n c i d e n t with the  the  belt,  I t i s , then, another  km.  26 s t r u c t u r a l f e a t u r e of the lineament.  That the  feature i s a  r e g i o n of weakness can be r e l a t e d to the i d e a that are a s s o c i a t e d with ancient deep t r a n s - c u r r e n t  lineaments  faults.  Experiments to t e s t the v a l i d i t y of some of the preceding  conclusions  ments on the  can be performed.  surface and n e a r - s u r f a c e  would r e v e a l whether or not layers r e a l l y exist.  C o n d u c t i v i t y measure-  l a y e r s of the  the p o s t u l a t e d h i g h l y  An e x t e n s i o n  of Sanford  and  crust conductive  Long's  mieroearthquake work could c o n f i r m the observed c o r r e l a t i o n between s e i s m i c i t y and  the i s o t o p i c boundary, i n a d d i t i o n  to c o n f i r m i n g the p o s i t i o n of the l a t t e r . i s o t o p e analyses features.  could d e f i n e the extent  Samples from the New  southwestern New interesting.  Further  of the i s o t o p i c  Mexico-Texas border area  and  Mexico ( S i l v e r C i t y area) would be e s p e c i a l l y  Techniques c u r r e n t l y being used i n t h i s l a b f o r  the e x t r a c t i o n bf Small amounts of l e a d from other ores c o u l d be a p p l i e d i n areas where galena available.  lead  sulphide  i s not r e a d i l y  Only by f u r t h e r experiments such as these can  the  problem of the observed i s o t o p i c d i s t r i b u t i o n of leads i n New be  Mexico and fully  i t s c o r r e l a t i o n with the Zuni Lineament ever  understood.  27 REFERENCES H e i e r , K.S., and J.J.W. Rogers. Radiometric d e t e r m i n a t i o n of thorium, uranium, and potassium i n b a s a l t s and i n two magmatic d i f f e r e n t i a t i o n s e r i e s . Geochlm. Cosmochim. A c t a , 27, 137-154, 1963. K e l l e y , V.C. R e g i o n a l t e c t o n i c s o f the Colorado P l a t e a u and r e l a t i o n s h i p t o the o r i g i n and d i s t r i b u t i o n o f uranium. U n i v e r s i t y o f New Mexico p u b l i c a t i o n s i n Geology No. 5, U n i v e r s i t y P r e s s , Albuquerque. K o l l a r , F., R.D. R u s s e l l , and T . J . U l r y c h . Precise i n t e r comparison o f l e a d i s o t o p e r a t i o s : Broken H i l l and Mt. I s a . Nature, 18_7, 754-756, i 9 6 0 . L i v i n g s t o n e , C.E. UnpublishedM.Sc. B r i t i s h Columbia, 1966.  t h e s i s , U n i v e r s i t y of  Mauger, R.L., P.E. Damon and D.E. L i v i n g s t o n . Congruent Laramide p e t r o g r a p h i c p r o v i n c e and copper me&allogenesis i n the southwestern U n i t e d S t a t e s . Appendix kHJ A E C Progress Report (COO-689^50), 1965- U n i v e r s i t y o f A r i z o n a Geochronology Lab. Mayo, E.B. Lineament t e c t o n i c s and some ore d i s t r i c t s o f the southwest. Mining Eng., 2 1 1 , 1169-1175, 1958. Muehlberger, W.R., and-R.E. Denison. Precambrian geology o f s o u t h - c e n t r a l New Mexico. New Mexico G e o l o g i c a l S o c i e t y Guidebook, F i f t e e n t h F i e l d Conference 62-69, 1964. Sanford, A.R., and C R . Holmes. Microearthquakes near Socorro New-Mexico. J . Geophys. Res., 6j_, 4449-4459, 1962 . S a n f o r d , A.R. and L.T* Long; Mieroearthquake c r u s t a l r e f l e c t i o n s , S o c o r r o , New Mexico. B u l l . Seism. Soc. Am. 55, No. 3, 579-586, 1965.  Schmucker, U. Anomalies o f geomagnetic v a r i a t i o n s i n the southwestern U n i t e d S t a t e s . J . Geomag. and G e o e l e c t . 15., No. 4, 193-221, 1964.  S i n c l a i r , A . J . and R.I. Walcott. The s i g n i f i c a n c e o f Th/U r a t i o s c a l c u l a t e d from w e s t - c e n t r a l New-Mexico m u l t i stage l e a d d a t a . E a r t h and P l a n e t a r y Science L e t t e r s ,  1, 3 8 - 4 1 , 1966.  Slawson, W.F. and C F . A u s t i n . A l e a d i s o t o p e study d e f i n e s a - g e o l o g i c a l s t r u c t u r e * Econ. G e o l . 57, 21-29, 1962. Smith, C T . P r e l i m i n a r y notes-on the geology o f part o f the Socorro Mountains, Socorro County, New Mexico. New Mexico G e o l o g i c a l Society- Guidebook, F o u r t e e n t h F i e l d Conference, 185-196, 1963.  28 U l r y c h , T . J . Unpublished, M.Sc. t h e s i s , U n i v e r s i t y o f B r i t i s h Columbia, I960. Weber, R.H., and W.A. B a s s e t t . K-Ar ages o f T e r t i a r y v o l c a n i c and i n t r u s i v e rocks i n S o c o r r o , C a t r o n , and Grant C o u n t i e s , New Mexico. New Mexico G e o l o g i c a l S o c i e t y Guidebook, Fourteenth F i e l d Conference. 2 2 0 - 2 2 3 , 1963. York, D. Least-squares f i t t i n g o f a s t r a i g h t Phys. 4_4, 1079-1086, 1966.  line.  Can. J .  29 APPENDIX A . l Lead  Models. The s i n g l e - s t a g e model:  At a time t , the e a r t h i s Q  c o n s i d e r e d t o have been in. a molten s t a t e so t h a t homogeniz a t i o n o f the U/Pb, Th/Pb and l e a d i s o t o p e r a t i o s o c c u r r e d . A f t e r c o o l i n g o f the e a r t h , the- s t a t e o f homogenization p e r s i s t e d i n a source r e g i o n i n the lower c r u s t or upper ^mantle, w i t h the lead: i s o t o p e r a t i o s changing due t o the decay o f uranium and thorium.  At a subsequent  time t ^ , l e a d  was removed from the. source;, separated from t h e U and Th, and emplaced  i n a lead deposit.  Prom the time t ^ up t o the  p r e s e n t , the i s o t o p i c r a t i o s are c o n s i d e r e d t o have remained unchanged.  The r a t i o s are g i v e n by: X  =  y •  a  b  o o  z with the symbols  +  ocV(e o Kt  e  X t  l)  + ••• V i e ^ o -•**!> +  (A.l)  WCe^o - e ^ l )  as d e f i n e d i n Table 1.1.  equations can be combined  The f i r s t two  t o y i e l d an e q u a t i o n (Equation A.2)  which may be s o l v e d y - b " "= x - a o  1 e  A t  o - e  X t  l  ~ Xt - , At c< e ° e Q  The two-stage model:  l xx  (  A  J  2  )  Once a g a i n , a deep c r u s t a l or  upper mantle r e g i o n i s p o s t u l a t e d as the source o f a s i n g l e stage l e a d emplaced  i n a c r u s t a l environment  at a time t-^.  A l s o at time t , , uranium and thorium were i n c o r p o r a t e d  into  30 t h e same environment.  At a subsequent  time t  2  , tectonic  a c t i v i t y r e c u r r e d i n the area and caused the mixing i n v a r y i n g p r o p o r t i o n s o f r a d i o g e n i c l e a d generated between t.^ and t  2  w i t h some o f the s i n g l e - s t a g e l e a d t o produce the two-  stage l e a d s .  I f x^, y^  M i n e r a l i z a t i o n then o c c u r r e d .  the r a t i o s o f the s i n g l e stage l e a d s emplaced y, z are the r a t i o s o f a two-stage x = x  y =  y  x  +  i  z = z  + ^ ( 6 ^ 1  ±  V (e  +  A t  1  l  - e  A t  t  z^ are  at t ^ , and x,  l e a d , then  2)  - e 2)_  (A.3)  Xt  W ^ e ^ l - e^2)  from which the slope o f the s t r a i g h t l i n e d e f i n e d by the leads i s : i  y - yi R  =  x - x ^  1 =  i  At p 1 - e ^  Xti  ,  e  — cx-  o c eX  tx l  - e At ^2  The slope o f t h i s l i n e i s - t h e n independent only on the i n t e r v a l t ^ t o t . 2  The time t  ( A  of 1  '  4 )  and depends  can o f t e n be  determined from the age o f s i n g l e - s t a g e leads-from which the two-stage be  found.  l e a d s developed!  Using equation A.4, t  2  can then  31 APPENDIX A.2 Automatic Reduction The  o f Data.  measuring: system o f the mass spectrometer i s de-  signed t o m e a s u r e a c c u r a t e l y the i n t e n s i t i e s o f the d i f f e r e n t ion  beams.  The i o n s are c o l l e c t e d on a Faraday cup and  leaked t o ground across a 10" ohm r e s i s t o r . v o l t a g e - a c r o s s the r e s i s t o r i s balanced c o n s i s t i n g o f a potentiometer  beam intensity...  by a  servo-voltmeter  and a d r i v i n g motor.  v o l t a g e across the potentiometer ion  The r e s u l t i n g  The  i s then p r o p o r t i o n a l t o the  The p o s i t i o n o f the potentiometer  i s - also- p r o p o r t i o n a l t o the v o l t a g e across, the  shaft  potentiometer,  so the shaft p o s i t i o n i s - p r o p o r t i o n a l t o the i o n beam i n tensities.  The p r a c t i c e i s t o r e c o r d the p o s i t i o n o f the  potentiometer  s h a f t , r a t h e r than t o measure-peak h e i g h t s  d i s p l a y e d on a chart recorder..  The s h a f t p o s i t i o n can be read  a u t o m a t i c a l l y by means o f an encoder, and the i n f o r m a t i o n put onto paper o r magnetic  tape.  At the moment, the i n f o r m a t i o n from the mass s p e c t r o meter i s punched onto paper tape i n u n i t s known as "words", with each word c o n s i s t i n g o f 6 c h a r a c t e r s .  The f i r s t  c h a r a c t e r s o f each word c o n t a i n the s h a f t p o s i t i o n t i o n a l t o peak h e i g h t ) o f the potentiometer system.  The f i f t h  four  (propor-  i n the measuring  c h a r a c t e r c o n t a i n s i n f o r m a t i o n such as  a t t e n u a t i o n used, scan d i r e c t i o n , and, when n e c e s s a r y , the "scan  s e p a r a t e " and "scan, r e j e c t " s i g n a l s .  The f i n a l  character  i s always an "end of l i n e " punch which denotes the end o f a word.  Words are recorded  at a r a t e o f 2 p e r second, and  32 there  are  usually  consisting In first  words scan  eight  generate  on t h e on the  over  recorded  at  in  data  for  "scan  of  the  the  each  and the  separate"  pair  is  signal.  a pair  scan  the  console,  to  then  Up t o  of  the  baselines  terminated  25 p a i r s  may  by  used  pressing  record  used,  Pb^CH^.)* s p e c t r u m . end  spectra  procedure  signal  attenuation  low-mass  of  direction.  a pair,  separate"  spectrum scanned up-mass, the time,  for  each  spectrometer  baseline  down-mass  again  the  mass  900 w o r d s  peaks  r e c o r d i n g the  to  button  of  about  Baselines spectrum,  another  be  recorded  to  are the  for  by  the  10-20  and t h e n  recorded  is  a  third  "scan in  this  manner. Identification of  the  paper  magnetic tape-.  disc  tape,  The  program  tape  "PILOT",  file  for* t h e  at  the  program  $J0B  (Job  eaeh  sample  can  21 w o r d s then  which U.B>;G.  be  is  punched at on it- i s  occupying 1 record p r o c e s s e d by  currently  the  the  start  transferred of  to  magnetic  reduction  s t o r e d on. t h e  Computing Centre.  The  IBM  cards  1301 required  are: No.  a n d . name  of  user)  $CL0SE $TIME  is  and-the-information  with  data  for  S.SU04,REMOVE (as  t col.l6  needed.)  I $EXECUTE  PILOT  TAPE *  (List  The set^  so  of  options that  the  options PTAPE,  if  desired  GO, DECK,  program w i l l  - see and  Table  CALC- a r e  automatically  A.l) currently  process  pre-  information  33 Option  "Negative" option  Description  Preset value (V5/66A)  PTAPE  mass spectrometer output on paper tape  NOPAP  TRUE  MTAPE  mass spectrometer  NOMAG  FALSE  NOGO  TRUE  NODMP  FALSE  DOALL  FALSE  NODECK  TRUE  output on mag. tape GO  process sample  DUMP SELECT  output f i l t e r e d d a t a process sample whose identification starts i n c o l . 71 o f " o p t i o n " card  DECK  outputs data cards  REPEAT  p r o c e s s e s a l l samples on t h e mag. tape  CALC  calculates lead  MAX.  outputs raw peak h e i g h t s , peak t a i l s , e t c .  ratios  SKIP,XX s k i p s XX r e c o r d s on mag. tape Table A . l  FALSE NOCALC  TRUE  NOMAX  FALSE FALSE  Options, a v a i l a b l e f o r program/"PILOT"  34 o r i g i n a l l y on paper, tape, producing l e a d r a t i o s and a deck of data c a r d s .  Any other o p t i o n s may be used  simply by l i s t i n g  them on the f i n a l c a r d . The t o t a l time r e q u i r e d by the program can e a s i l y be estimated.  About 20 sec are used by the computer t o s e t up  the program, and the time r e q u i r e d t o process a p a i r and c a l c u l a t e l e a d r a t i o s from i t i s about 18 s e c . Lead i s o t o p e r a t i o s can be obtained from data cards by u s i n g the f o l l o w i n g c a r d s : $JOB (Job no. and name o f user) $EXECUTE CARDS  .  PILOT T c o l . 16  data cards The  time r e q u i r e d by the computer i s about 20 sec t o s e t up  the program and 1 sec a p a i r t o c a l c u l a t e the. l e a d  ratios.  

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