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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, University of 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 the Department of GEOPHYSICS We accept this; thesis as conforming to the; required 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 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 reference and study, I furt h e r agree that permission-for 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 granted by the Head of my Department or by his representatives. I t i s understood that copying or p u b l i 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 gain s h a l l not be allowed without my w r i t t e n permission. Department of The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8 , Canada Date AW Xlt , i i ABSTRACT A zone of cru s t a l weakness associated with the Zuni Lineament i n the southern half of New Mexico has been de-lineated by lead isotope measurements, a mieroearthquake study and magnetic depth soundings. The lead r e s u l t s of Slawson and Austin (1962) revealed the possible existence of a northwest trending zone of i s o t o p i c a l l y s i m i l a r lead, bounded on either side by i s o t o p i c a l l y variable leads. This feature passes through Socorro and Alamogordo, New Mexico, and i s ; c o i n c i d e n t i n t h i s area with the Zuni Lineament. The mieroearthquake r e s u l t s (Sanford and Long,. 1965) show that a shallow c r u s t a l zone of weakness i s associated with the i s o t o p i c a l l y s imilar zone. The magnetic r e s u l t s (Schmucker, 1964; Livingstone, 1966).confirm that the structure i s shallow, and, also indicate that i t may be a region of. h i g h - e l e c t r i c a l conductivity. It i s suggested that this, zone of weakness- centred on the Zuni Lineament has served as a conduit system-for hydro-thermal mineralizing- solutions which carried the lead to i t s f i n a l s i t e s of deposition. The lead was o r i g i n a l l y i n t r o -duced into the Precambrian rocks about 1550 m.y. ago, mixed with-radiogenic rock lead of that age about 30 m.y. ago, and then, f i n a l l y emplaced. i i i TABLE OF CONTENTS ABSTRACT i i LIST OF FIGURES i v LIST OF TABLES v ACKNOWLEDGEMENTS v i CHAPTER 1 1 1.1 Outline of problem 1 1.2 Isotopic c o r r e l a t i o n with Zuni Lineament 2 1.3 Geomagnetic measurements i n south-central 2 New Mexico 1.4 Seismic results from Socorro area 6 CHAPTER 2 9 2.1 Interpretations of lead isotope data 9 2.2 Explanations of iso t o p i c d i s t r i b u t i o n 9 2.3 Seismic d e f i n i t i o n of i s o t o p i c zone 11 2.4 Geomagnetic measurements 11 2.5 Th/U contouring 13 CHAPTER 3 15 3.1 Geology of area 15 3.2 Sample preparation and analysis 15 3.3 Discussion of res u l t s / 16 3.4 Geographic d i s t r i b u t i o n of samples 21 CHATTER 4 24 REFERENCES' 27 APPENDIX A . l 29 APPENDIX A.2 31 i v LIST OF FIGURES FIG. 1.1 Model of conductivity structure of crust and 5 upper mantle between Arizona and Texas FIG. 1.2 Observed and t h e o r e t i c a l t r a v e l times f o r 8 S XS and S XP waves f o r Socorro microearthquakes FIG. 2.1a Epicentres of Socorro microearthquakes ' 12 FIG. 2.1b Projection of microearthquakes on to an E-W 12 cross-section FIG. 3.1a Isotopic plot of New Mexico leads l£ FIG. 3.1b Isotopic plot of New Mexico leads 19 FIG. 3.2 Locations of lead 1 samples and geomagnetic 22 stations LIST OF TABLES TABLE 1.1 Explanation of symbols used i n is o t o p i c work TABLE 3.1 Lead isotope ratios, f o r some New Mexico samples TABLE 3.2 Some Precambrian ages: from south-central New Mexico TABLE A . l Options available f o r program "PILOT" v i ACKNOWLEDGEMENTS The writer would f i r s t l i k e to thank Dr. W.F. Slawson for suggesting the problem and supervising the subsequent research. Thanks also go to Drs. D.H. Weichert and R.D. Russell for developing the automatic data reduction system which greatly s i m p l i f i e d the tedious business of measuring lead isotope r a t i o s . The writer i s also gr a t e f u l to Mr. P.H. Reynolds and Miss S. Newman for assistance they have provided. Miss Newman also typed the f i n a l draft of the the s i s . The work was supported by grants from the Defence Research Board and the National Research Council. A fellowship pro-vided 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 Outline of Problem. The present report i s an attempt to correlate the res u l t s of some geophysical surveys i n south and central New Mexico with geochemical and geological information. Lead isotope data (Slawson and Austin, 1962; t h i s report), magnetic depth soundings (Schmucker, 1964; Livingstone, 1966), a mieroearth-quake Investigation (Sanford and Long, 1965), a l l have a bearing on the geologists' discussion of lineaments i n New Mexico (Mayo, 1958). Constraints imposed by the di f f e r e n t models employed seriously l i m i t the possible in t e r p r e t a t i o n of the nature of the lineaments, es p e c i a l l y the nature of the Zuni Lineament. A lineament i s probably best described as a large-scale, approximately l i n e a r arrangement of str u c t u r a l features, although geologists themselves disagree over pre c i s e l y what the term means;- A lineament i s marked by some or a l l of the following:- f a u l t s , f o l d i n g , u p l i f t s , volcanics and in t r u s i v e s . Its l i n e a r nature i s believed to be due to shearing at depth. The Zuni Lineament i s thought to consist of the Guada-lupe Mts; i n western Texas; the Sacramento U p l i f t , Mt. Taylor volcanics, and Zuni U p l i f t i n New Mexico; the Defiance U p l i f t i n northeastern Arizona; and the C i r c l e C l i f f s U p l i f t , High Plateau volcanics, and the Gold H i l l s d i s t r i c t of Utah. Superimposed features such as the Sevier f a u l t and the Rio Grande graben suggest no recent movement has taken place along 2 t h e l i n e a m e n t . I t i s c r o s s e d by o t h e r l i n e a m e n t s a t numerous p o i n t s (Mayo , 1958; K e l l e y , 1955). 1.2 I s o t o p l c C o r r e l a t i o n w i t h Z u n i L i n e a m e n t . I n a l e a d i s o t o p e s t u d y o f what a p p e a r e d t o be a g e o -l o g i c a l l y s i m p l e a r e a , S l a w s o n and A u s t i n (1962) d i s c o v e r e d a r e m a r k a b l e c o r r e l a t i o n be tween t h e g e o g r a p h i c d i s t r i b u t i o n o f t h e l e a d s a m p l e s and t h e i r i s o t o p i c c o m p o s i t i o n . L e a d s o f an e s s e n t i a l l y 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 (x r a n g i n g f r o m a b o u t 18.4 t o 19.0 - see T a b l e 1-1 f o r e x p l a n a t i o n o f s y m b o l s ) were f o u n d t o o c c u r w i t h i n a n o r t h w e s t t r e n d i n g b e l t , bounded on e i t h e r s i d e by l e a d s o f a v a r i a b l e i s o t o p i c c o m p o s i t i o n - (x r a n g i n g f r o m a b o u t 19.2 t o 25.4). The b e l t , l y i n g a l o n g a l i n e j o i n i n g A l a m o g o r d o and S o c o r r o , New M e x i c o , c o i n c i d e s i n t h i s a r e a w i t h t h e Z u n i L i n e a m e n t . 1.3 G e o m a g n e t i c M e a s u r e m e n t s i n S o u t h - c e n t r a l New M e x i c o . In g e o m a g n e t i c m i d - l a t i t u d e s , t h e e x t e r n a l f i e l d o f g e o m a g n e t i c v a r i a t i o n s i s f o u n d t o be f a i r l y u n i f o r m o v e r l a r g e a r e a s . I f t h e v a r i a t i o n s m e a s u r e d by a c h a i n o f 3-component m a g n e t o m e t e r s ( m e a s u r i n g t h e h o r i z o n t a l (H) and v e r t i c a l (Z ) componen t s o f t h e e a r t h ' s f i e l d as w e l l as t h e a n g l e (D) be tween H and t r u e n o r t h ) a r e f o u n d t o be c o n s i s -t e n t l y d i f f e r e n t f r o m s t a t i o n t o s t a t i o n , t h e d i f f e r e n c e s c a n be a t t r i b u t e d t o t h e e f f e c t o f i n h o m o g e n e i t i e s i n t h e e l e c t r i c a l c o n d u c t i v i t y d i s t r i b u t i o n w i t h i n t h e e a r t h . S i m p l e m o d e l s o f t h e e a r t h ' s c o n d u c t i v i t y , s t r u c t u r e c a n t h e n be s e t up and t h e s u r f a c e f i e l d s t h a t w o u l d be p r o d u c e d by them c a l c u l a t e d . The p a r a m e t e r s o f t h e m o d e l s a r e a d j u s t e d t o 3 t = 0 at any time t - t 0 • 4.55 (present) t (age of Earth) 206 20T a Q = 9.56 207 20T b Q = 10.42 208 20T 235 20T 238 20T 232 20T •V W Ve Xt ocVe Xt We c Q = 30.00 Ve^o ocVe X t o We^o decay scheme _ Q —I constant (xlO yrs ) P b 2 0 6 X = .1537 u 2 3 5 - P b 2 0 7 A = .9722 T h 2 3 2 - . P b 2 0 8 X'= .0499 Table 1.1 Explanation of symbols used i n isotopic work. 4 o b t a i n a g r e e m e n t be tween t h e c a l c u l a t e d and o b s e r v e d r e s u l t s . I f a g r e e m e n t I s o b t a i n e d , t h e n t h e m o d e l i s c o n s i d e r e d p e r h a p s t o be an a p p r o x i m a t e r e p r e s e n t a t i o n o f t h e e a r t h ' s c o n d u c t i v i t y s t r u c t u r e T h e r e a r e two m a j o r l i m i t a t i o n s t o t h i s m e t h o d . The f i r s t i s t h a t o n l y s i m p l e m o d e l s c a n be u s e d , o t h e r w i s e c a l c u l a t i o n s o f t h e s u r f a c e f i e l d s a r e f a r t o o c o m p l i c a t e d . The s e c o n d l i m i t a t i o n i s t h a t , by u s i n g s i m p l e m o d e l s , t h e p o s s i b i l i t y : e x i s t s t h a t two o r more m o d e l s may f i t t h e o b -s e r v e d d a t a . N o n e t h e l e s s , i t i s p o s s i b l e t o make some d e d u c t i o n s a b o u t t h e e a r t h ' s c o n d u c t i v i t y s t r u c t u r e . A m a g n e t i c d e p t h s o u n d i n g s t u d y by S c h m u c k e r (1964) a l o n g t h e • s o u t h e r n New M e x i c o - T e x a s b o u n d a r y r e v e a l e d t h e e x i s t e n c e o f a c o n d u c t i v i t y a n o m a l y t o t h e s o u t h e a s t o f t h e i s o t o p i c a n o m a l y and a p p r o x i m a t e l y a l i g n e d w i t h i t . The r e g i o n be tween L a s C r u c e s , New M e x i c o , and C o r n u d a s , T e x a s was found- t o be a t r a n s i t i o n zone be tween g- r a t i o s o f a b o u t 0.05 t o t h e west and o f 0.15-0.20 t o t h e e a s t . The c h a r a c t e r o f t h e l o n g - p e r i o d m a g n e t i c v a r i a t i o n s ( > 1 h r ) r e c o r d e d a c r o s s t h e t r a n s i t i o n zone r e q u i r e s a l a r g e - s c a l e change i n t h e e a r t h ' s c o n d u c t i v i t y a t a d e p t h o f a few h u n d r e d k i l o -m e t e r s . S c h m u c k e r h a s i n t e r p r e t e d t h e r e s u l t s as a s t e p d i s c o n t i n u i t y i n t h e deep m a n t l e s t r u c t u r e . The s t e p m o d e l by i t s e l f , h o w e v e r , d o e s n o t p r o v i d e c o m p l e t e a g r e e m e n t b e t w e e n o b s e r v e d and c a l c u l a t e d s u r f a c e f i e l d s , so i t i s n e c e s s a r y t o assume t h e e x i s t e n c e o f a h i g h l y c o n d u c t i v e r e g i o n ( F i g u r e 1.1) b e tween L a s C r u c e s and C o r n u d a s . The 5 STATIONS TU Tucson LAC Las Crucea ORO Orogrande COR Cornudas SWE Sweetwater FIG. 1.1 Model of conductivity structure of crust and upper mantle between Arizona and Texas (after Schmucker, 1964) 6 region i s at a very shallow depth, trends north-south and i s about 100 km. wide. 1.4 Seismic Results from Socorro Area. Some seismic evidence for the zone of weakness believed associated with the lineament i s presented by Sanford and Long (1965). They have used an array of from three to f i v e high-magnification seismographs, located 5 km. west of Socorro, to record nearby microearthquakes (S-P<2.5 sec.) at the rate of about 600 a year. Prom surface explosions, corrections for the geologic setting and elevation of the i n d i v i d u a l stations have been determined and applied to the records. For an i n d i v i d u a l earthquake, the d i r e c t i o n of the epicentre i s obtained by comparing the corrected P a r r i v a l times at each station of the array. The wave i s assumed to have t r a v e l l e d i n a s t r a i g h t • l i n e from the focus to the station with a v e l o c i t y of 6 km./sec, and the distance to the focus i s found from the S-P i n t e r v a l , assuming Poisson's r a t i o i s 0.25. Errors in-the r e l a t i v e locations of the f o c i are believed to range from <1 km. to <C 4 km., depending on the qual i t y of the records and the number of stations recording the shock. Errors i n the absolute locations are unknown. About 25% of the microearthquakes observed have S P and S S a r r i v a l s associated with them. The a r r i v a l s are The two types of seismic waves that t r a v e l through the earth are P waves (longitudinal waves) and S waves (transverse waves). When either type of wave encounters a v e l o c i t y d i s -continuity, 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 incident 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 iscontinuity, while an S XP wave i s an incident S wave r e f l e c t e d as a P wave from the same disconti n u i t y . 7 b e l i e v e d t o be r e f l e c t i o n s f r o m a c r u s t a l d i s c o n t i n u i t y a t a d e p t h o f 18 km. P r o b a b l e S S p h a s e s have b e e n f o u n d on s e i s m o g r a m s o f weak s h o c k s a s s o c i a t e d w i t h t h r e e e a r t h q u a k e s l o c a t e d i n t h e S i e r r a L a d r o n e s , 35 km. n o r t h o f S o c o r r o ( S a n f o r d and H o l m e s , 1962). T h e y c o u l d come f r o m a d i s -c o n t i n u i t y 18 km. deep i f t h e mean d e p t h o f t h e f o c i were 4.8 km. The l a t t e r f i g u r e i s i n d e e d p o s s i b l e , so t h e f e a t u r e i s n o t c o n f i n e d t o t h e S o c o r r o r e g i o n . A p l o t 0 f i t r a v e l t i m e s a g a i n s t t h e S-P i n t e r v a l f o r s u c h s h o c k s i s shown i n F i g u r e 1.2, t o g e t h e r w i t h t h e o r e t i c a l c u r v e s f o r S x P and S x S r e f l e c t i o n s f r o m a h o r i z o n t a l v e l o c i t y d i s c o n t i -n u i t y a t a d e p t h Z o f 18 km. One c u r v e i s d rawn f o r a f o c a l d e p t h h o f 9 k m . , t h e o t h e r f o r a f o c a l d e p t h o f 4 km. The c u r v e s b r a c k e t t h e d a t a e q u a l l y w e l l , so l i t t l e a d j u s t -ment i n h- and Z i s p o s s i b l e . E x a m i n a t i o n o f t h e d i s t r i -b u t i o n o f t h e f o c a l d e p t h s o f t h e m i c r o e a r t h q u a k e s c o n f i r m s most (68%) o f them o c c u r b e t w e e n 4 and 9 km. I n o r d e r t o o b t a i n u s e f u l i n f o r m a t i o n f r o m t h e o b s e r v e d d a t a , a m o d e l ha s been u s e d . T h i s p a r t i c u l a r m o d e l ha s a s sumed no r e f r a c t i o n o f s e i s m i c w a v e s , a r e l a t i v e l y h o m o -g e n e o u s c r u s t , and a s y m m e t r i c a l r e l e a s e o f e n e r g y f r o m t h e f o c i . A l l t h r e e a s s u m p t i o n s w i l l c o n t r i b u t e t o d i f f e r e n c e s b e t w e e n t h e m o d e l and t h e a c t u a l c r u s t a l s t r u c t u r e . The f o r e g o i n g m a t e r i a l h a s b e e n a r e v i e w o f a s e r i e s o f somewhat d i s c o n n e c t e d r e c e n t s t u d i e s h a v i n g a b e a r i n g on t h e d e l i n e a t i o n o f t h e Z u n i L i n e a m e n t and an e x p l a n a t i o n o f i t . • 1 0..5 1.0 1..5 2,0 2.5 S-P Interval (sec) FIG. 1.2 Observed and t h e o r e t i c a l t r a v e l times f o r S^S and S XP waves f o r Socorro microearthquakes (after Sanford & Long, 1965) 9 CHAPTER IX 2.1 Interpretation of Lead Isotope Data. The two models most frequently used i n the i n t e r p r e t a -t i o n of lead isotope measurements are outlined i n the appendix. The single-stage model furnishes a simple interpretation for the small number of leads that appear to s a t i s f y the model. The interpretation i s almost c e r t a i n l y too simple, yet provides, i n some cases, an approximation to the actual history of the lead. The two-stage model i s also a crude representation of the development of a lead; given the complex geological history of most areas, i t i s u n l i k e l y that only two events have affected the lead there. However, the equations for the two-stage leads (eq. A.3) can be solved completely, while the equations for the three-stage, four-stage, etc. leads cannot be solved for unique values of the variables. 2.2 Explanations of Isotopic D i s t r i b u t i o n . Slawson and Austin (1962) have offered a simple explanation for the observed pattern of isotopic d i s t r i b u -t i o n . They postulate that the lineament represents 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 for hydrothermal mineralizing solutions. Their i n t e r p r e t a t i o n i s that single-stage lead from a period of mineralization 1600 m.y.ago was mixed i n Tertiary time with radiogenic lead, formed by the decay of uranium and thorium over the period 1600 m.y. ago to the time of the mixing, to produce anomalous leads of similar isotopic compositions. The leads 10 were then transported upward by mineralizing solutions, evidence for which has been presented by Smith (1963). The solutions t r a v e l l i n g along the conduit provided by the lineament acquired r e l a t i v e l y l i t t l e radiogenic lead from wallrock and the leads were therefore deposited with only minor changes i n t h e i r isotopic compositions. Those solutions that t r a v e l l e d outward from the conduit system acquired variable amounts of radiogenic lead as they percolated through the wallrock and those leads were therefore deposited with variable Isotopic compositions, Mauger, Damon and Livingston (1965) have c r i t i c i z e d the foregoing hypothesis on the grounds that no s t r u c t u r a l evidence of the conduit system centred on the lineament i s presented, and that the assumption of ascending hydrothermal mineralizing solutions to explain the isotopic data i s unnecessary, hydrothermal solutions implying a d i f f e r e n t genesis for rocks and ores. They point out that ten df twelve of the i s o t o p i c a l l y similar leads are either contained i n volcanic rocks or adjacent to intrusive rocks, whereas of the i s o t o p i c a l l y variable leads, only seven of the fourteen have intrusives i n t h e i r v i c i n i t y . They agree that the Zuni Lineament may represent a v e r t i c a l zone of weakness but f e e l that i t may have served as the locus for the ascent of i g -neous rocks rather than mineralizing solutions. The observed isotopic r a t i o s are then explained as being due to the condi-tions of extraction of the lead from the surrounding rocks. The authors state that lead extracted from heated igneous II r o c k s i s more l i k e l y t o be " n o r m a l " ( c o n t a i n i n g a s m a l l e r amount o f r a d i o g e n i c l e a d ) i s o t o p i c a l l y t h a n l e a d e x t r a c t e d f r o m o l d r o c k s i n t h e a b s e n c e o f a h e a t s o u r c e . The o b s e r v e d i s o t o p i c d i s t r i b u t i o n i s t h e r e f o r e e x p l a i n e d a s a r e s u l t o f t h e l i n e a m e n t b e i n g a zone o f weakness a l o n g w h i c h i g n e o u s a c t i v i t y h a s o c c u r r e d r a t h e r t h a n a c o n d u i t s y s t e m f o r h y d r o -t h e r m a l m i n e r a l i z i n g s o l u t i o n s . 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 Z o n e . A m a p ' o f t h e e p i c e n t r e s o f some m i c r o e a r t h q u a k e s r e c o r d e d by S a n f o r d and L o n g ( 1 9 6 5 ) a n d a p r o j e c t i o n o f t h e f o c i on t o an e a s t - w e s t v e r t i c a l c r o s s - s e c t i o n c o m p r i s e F i g u r e 2.1. I t i s a p p a r e n t t h a t no m i c r o e a r t h q u a k e s o c c u r n o r t h e a s t o f a l i n e s t r i k i n g n o r t h w e s t and d i p p i n g 55 ° t o w a r d s t h e R i o G rande v a l l e y . The l i n e i s f o u n d t o c o i n c i d e a p p r o x i m a t e l y w i t h t h e b o u n d a r y o f t h e i s o t o p i c a n o m a l y , w h i c h i s q u i t e w e l l d e f i n e d a t t h i s p o i n t . The a g r e e m e n t be tween t h e l i n e s l e n d s s u p p o r t t o t h e c o n t e n t i o n t h a t t h e l i n e a m e n t r e p r e s e n t s a z o n e o f w e a k n e s s , " a zone i n w h i c h t h e r o c k s c a n n o t r e a d i l y w i t h s t a n d s t r a i n s and s t r e s s e s b u i l d i n g up w i t h i n t h e m . 2.4 G e o m a g n e t i c • M e a s u r e m e n t s , P r e l i m i n a r y t o t a l m a g n e t i c f i e l d m e a s u r e m e n t s c a r r i e d o u t a c r o s s t h e i s o t o p i c anoma l y by C . E . L i v i n g s t o n e and k i n d l y i n t e r p r e t e d by D r . T . J . U l r y c h r e v e a l e d t h a t t h e t o p o f t h e P r e c a m b r i a n basement i s a t a d e p t h o f a b o u t 4 km. j u s t n o r t h o f S o c o r r o . F rom F i g u r e 2. l b i t a p p e a r s t h a t t h e number o f m i c r o e a r t h q u a k e s d e c r e a s e s s i g n i f i c a n t l y above a b o u t 4 km. Mos t m i c r o e a r t h q u a k e s , t h e n , o c c u r i n t h e u p p e r P r e c a m b r i a n b a s e m e n t . FIG. 2.1b Projection of microearthquakes on to an E-W cross-section 13 The magnetic depth sounding study by C E . Livingstone revealed no obvious deep conductivity s t r u c t u r a l changes within the earth. Because of the low speed of the recording chart of the Askania magnetometers, the high frequency v a r i a -tions needed for shallow c r u s t a l studies were not recorded. It i s not impossible that a shallow, highly conductive belt i s associated with the isotopic anomaly and i s an extension of the shallow conductivity feature postulated by Schmucker. The c r u s t a l discontinuity, found by Sanford and Long (1965) at a depth of 18 km. could then mark the lower boundary of the isotopic zone. 2.5 Th/U Contouring. S i n c l a i r and Walcott (1966) have calculated the Th/U r a t i o s for the samples as reported by Slawson and Austin and plotted contours of the values to show t h e i r geographic v a r i a t i o n s . The contouring supports evidence i n d i c a t i n g that some st r u c t u r a l feature connected with the Zunl Lineament controls the isotopic d i s t r i b u t i o n of the samples i n that the contours are not continuous across the suggested boundaries but are symmetric about the lineament. The authors f e e l that t h e i r contouring demonstrates that there are two centres of mineralization along the lineament, one near Socorro and the other about 20 miles to the southeast. They have also plotted a histogram of the Th/U r a t i o s and f i n d from i t evidence of two d i s t i n c t Th/U populations. Consequently they modify the mineralizing history proposed by Slawson and Austin by postulating that the leads were o r i g i n a l l y deposited 14 into two d i f f e r e n t Th/U environments, then mineralized as previously described (Section 2.2). The preceding constitutes an attempt to correlate the r e s u l t s of d i f f e r e n t types of geophysical studies i n order to r e s t r i c t the possible interpretations of the Zuni Lineament. 15 CHAPTER III 3.1 Geology of Area. Isotopic analyses were carried out on nine leads from New Mexico, including three collected from the same general area that Schmucker fs work covered. The following description of the sample area, encompassing Socorro, Lincoln, S i e r r a , Otera and Dona Ana counties, i s abstracted from Slawson and Austin (1962): the northwest portion of the area i s located in the basin and range type mountains on the border of the Colorado Plateau, while the southeastern portion crosses the north-south trending block-faulted mountains of the New Mexico Rockies. The Rio Grande graben passes through the sampled area. T e r t i a r y gravels and lava flows cover most of the area, although Paleozoic sediments and some Mesozoic and Precambrian rocks are exposed along the mountains. There i s some limited exposure of Laramide i n t r u s i v e s . 3.2 Sample Preparation and Analysis. The samples were prepared i n e s s e n t i a l l y the manner described by Ulrych ( I 9 6 0 ) . Galena was leached with d i l u t e HCl to form lead chloride, which was p r e c i p i t a t e d by cooling, then f i l t e r e d out and washed with cold d i s t i l l e d water. The lead chloride was re-di3;solved i n hot d i s t i l l e d water, potassium iodide added to the solution, and the r e s u l t i n g lead iodide p r e c i p i t a t e f i l t e r e d and dried. Tetramethyllead was prepared by reacting the lead iodide with methyl iodide and a Grignard reagent, methyl magnesium bromide i n ethyl 16 ether. (The purpose of the Mel i s to increase the y i e l d of tetramethyllead.) The r e s u l t i n g mixture of tetramethyllead and ethyl ether was separated into i t s constituents by vapour phase gas chromatography as developed by Ulrych ( i 9 6 0 ) . The tetramethyllead was analyzed on the 90°, 12 inch single focusing mass spectrometer designed and b u i l t by Drs. F. Kollar and R.D. Russell. A l l the samples were intercompared as described by Kollar et a l (I960) with the U.B.C. No. 1 standard* and the data reductions for four of the f i v e loops were carried out automatically, using the system described in the appendix, 3.3 Discussion of Results. The values obtained are given i n Table 3.1, and also plotted on P b 2 0 7 / P b 2 0 4 - P b ^ / P b 2 0 1 1 and P b 2 0 8 / P b 2 0 1 1 -pb206/,pt)204 g r a p n s i n p i g U r e 3.1. Straight l i n e s have been f i t t e d through the points using a weighted least-squares f i t suggested by York (1966). The weights used were the recipro-cals of the percentage uncertainties. The slope of the l i n e drawn through the nine samples i s 0.0952 ± . O O 6 7 , i n agreement with the value of 0.0931 *.0033 obtained for 63 samples by Slawson and Austin (1962), a value subsequently modified to 0.0938± .0029 by l a t e r work. The samples can be interpreted i n terms of a two-stage history (Appendix A . l ) . I f the Bosque del Apache sample i s a single-stage lead, then i t s age can be taken as t ^ , and the corresponding t2 found from equation A.4. * A s p l i t of the widely c i r c u l a t e d Toronto #1003 sample. 17 Sample no. Name x y z 501 La Bonita 25.32 16.34 42.67 502 Box Canyon 20.63 15.91 39.94 504 Hansonburg 22.30 16.06 40.64 505 Bosque del Apache 16.19 15.46 36.03 507 Linchburg 13.55 15.74 33.72 503 K e l l y 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 Table 3.1 Lead isotope r a t i o s f o r some New Mexico samples 16.5 •16.0 Pb 2 0?" 7b"204 •15.5 / • <*^^^ ^^^^ / 16 17 18 19 • 1 1 20 > 21 1 22 Li 23 1 24 25 P b 2 0 6 / Pb 2 0 /* FIG. 3.1a Isotopic plot of New Mexico leads Pb Pb 203 204 Pb 2 0 6 /pb 2 0 4 FIG. 3.1b Isotopic plot of New Mexico leads; 2 0 U s i n g e q u a t i o n A.2 an age o f 1500 m . y . i s c a l c u l a t e d f o r t h e Bosque d e l Apache s a m p l e . F o r t h i s v a l u e , oc\l f o r t h e s amp le i s f o u n d t o be 8 .80 , and i t s T h / U r a t i o i s 3.83. T a k i n g i n t o a c c o u n t t h e u n c e r t a i n t i e s i n t h e c o n s t a n t s i n e q u a t i o n A .2, a s w e l l as t h e s u s p e c t e d v a r i a t i o n s i n t h e <*.V and r a t i o s f o r t h e p o s t u l a t e d deep c r u s t a l o r u p p e r m a n t l e s o u r c e , i t a p p e a r s t h e Bosque d e l Apache l e a d may be d e s c r i b e d a s p r i m a r y - l i k e . I t i s q u i t e p o s s i b l y n o t a s i n g l e - s t a g e l e a d , bu t c a n n o t r e a d i l y be d i s t i n g u i s h e d f r o m o n e . C o n s e q u e n t l y , t h e v a l u e o f t^ i s t a k e n t o be 1500 m . y . The c o r r e s p o n d i n g v a l u e o f t 2 i s 90 m . y . w h i c h i s p e r h a p s t o o h i g h . I t i s p o s s i b l e t o c h o o s e a more r e a s o n a b l e v a l u e f o r t 2 o f 30 m . y . (Weber and B a s s e t t , 1963), b e i n g t h e a p p r o x i m a t e age o f w i d e s p r e a d v o l c a n i c a c t i v i t y ( S m i t h , 1963). The c o r -r e s p o n d i n g v a l u e o f t 1 i s t h e n 1550 m . y . I n v i e w o f t h e d e v i a t i o n i n t h e t w o - s t a g e l e a d l i n e , t h e s e a g e s a r e n o t s i g n i f i c a n t l y d i f f e r e n t f r o m t h o s e p r e v i o u s l y g i v e n , b u t s h o u l d be p r e f e r r e d t o them b e c a u s e o f t h e d o u b t f u l p r i m a r y n a t u r e o f t h e Bosque d e l Apache l e a d . The i n t e r p r e t a t i o n t h e n i s t h a t l e a d , u r a n i u m , and t h o r i u m were i n c o r p o r a t e d i n t o a c r u s t a l e n v i r o n m e n t a b o u t 1550 m . y . a g o , The Bosque d e l Apache d e p o s i t was f o r m e d a t t h i s same t i m e . T e c t o n i c a c t i v i t y 30 m . y . ago m i x e d i n v a r y i n g p r o -p o r t i o n s o l d e r l e a d and t h e r a d i o g e n i c l e a d g e n e r a t e d be tween 1550 m . y . and 30 m . y , , and t h e l e a d s were t h e n e m p l a c e d , M u e h l b e r g e r and D e n i s o n (1964) have r e p o r t e d a few Rb-S r and K-Ar a g e s f o r t h e P r e c a m b r i a n r o c k s o f s o u t h - c e n t r a l New M e x i c o . T h e y a r e g i v e n i n T a b l e 3.2. 21 Location Mineral Method. Age Sierra Oscura whole Rb-Sr 1300 m.y. Lincoln Cty. rock Rhodes Canyon* whole. RbrrSr 1430 m.y. Sie r r a Cty. rock Hansonburg d i s t r i c t * microcline Rb-Sr 1570 m.y. Sierra Cty. b i o t i t e K-Ar 1360 m.y. * Lead samples coll e c t e d from same region, Table 3.2 Some Precambrian ages from south-central New Mexico (Muehlberger and Denison, 1964). The authors have interpreted these ages as representing a metamorphism and g r a n i t i c intrusion about 1570 m.y. ago, followed by further igneous a c t i v i t y about 1350 m.y. ago. Taking into account the 6% uncertainty i n the Rb-Sr h a l f - l i f e , the agreement between the 1570 m.y. age and the 1550 m.y. lead age i s quite impressive. The Igneous a c t i v i t y of about 1350 m.y. ago does not appear to have been accompanied by any lead mineralization. 3.4 Geographical D i s t r i b u t i o n of Samples. Figure 3.2 i s a map of the relevant area, showing the locations of the samples analyzed by Slawson and Austin and the present author, as well as the locations of the geo-magnetic stations used by Schmucker (1964) and Livingstone (1966). The boundaries between the i s o t o p i c a l l y similar and i s o t o p i c a l l y variable leads are also drawn. The isotopic anomaly appears to extend into the region i n which Schmucker 22 SYMBOLS USED A geomagnetic station O " s i m i l a r " lead • "variable" lead FIG, 3.2 Locations of lead samples and geomagnetic stations 23 p o s t u l a t e d h i s s u r f a c e c o n d u c t i v i t y a n o m a l y , so i t i s c e r t a i n l y p o s s i b l e t h a t t h e two a r e r e l a t e d . The p r e s e n t a u t h o r has c o n t o u r e d U/Pb and Th/Pb r a t i o s i n a manner s i m i l a r t o t h a t o f S i n c l a i r and W a l c o t t (1966). W h i l e r e s u l t s a r e by no means c o n c l u s i v e , t h e y a p p e a r t o i n d i c a t e o n l y one c e n t r e o f m i n e r a l i z a t i o n , l o c a t e d i n t h e S o c o r r o - M a g d a l e n a a r e a . Mayo (1958) has s u g g e s t e d t h a t t h e M a g d a l e n a a r e a c o u l d i n d e e d be a f o c u s f o r m i n e r a l i z a t i o n p r o c e s s e s b e c a u s e t h e Z u n i L i n e a m e n t , M o r e n c i b e l t and C o r d i l l e r a n F r o n t b e l t a l l meet n e a r M a g d a l e n a , p r o d u c i n g a r e g i o n o f c r u s t a l w e a k n e s s . 24 CHAPTER IV A p p r o x i m a t e l y 1550 m . y . a g o , l e a d , u r a n i u m , and t h o r i u m were i n t r o d u c e d i n t o t h e P r e c a m b r i a n r o c k s o f west and s o u t h -c e n t r a l New M e x i c o . Abou t 30 m . y . a g o , t e c t o n i c a c t i v i t y i n t h e a r e a r e s u l t e d i n q u i t e t h o r o u g h m i x i n g i n t h e P r e c a m b r i a n ba semen t o f t h e 1550 m . y . o l d l e a d and r a d i o g e n i c l e a d , g e n e r a t e d by t h e d e c a y o f t h e u r a n i u m and t h o r i u m , t o p r o d u c e l e a d o f a f a i r l y u n i f o r m i s o t o p i c c o m p o s i t i o n . The l e a d was t h e n c a r r i e d upward by h y d r o t h e r m a l m i n e r a l i z i n g s o l u t i o n s t o s i t e s o f d e p o s i t i o n . Some o f t h e l e a d - b e a r i n g s o l u t i o n s a s c e n d e d a l o n g a c o n d u i t s y s t e m p r o v i d e d by a s h a l l o w zone o f c r u s t a l weakness c e n t r e d on t h e Z u n i L i n e a m e n t , and t h e r e f o r e 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 f r o m t h e w a l l r o c k . T h e s e l e a d s were d e p o s i t e d w i t h e s s e n t i a l l y 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 s . O t h e r l e a d - b e a r i n g s o l u t i o n s p e r c o l a t e d t h r o u g h t h e w a l l r o c k r a t h e r t h a n t h e zone o f weakness and c o n s e q u e n t l y a c q u i r e d v a r i a b l e amounts o f r a d i o g e n i c l e a d . T h e s e l e a d s were d e p o s i t e d w i t h v a r i a b l e i s o t o p i c c o m p o s i t i o n s . The z - x d i a g r a m ( F i g u r e 3.1b o r F i g u r e 3-B p . 2 4 , S l a w s o n and A u s t i n , 1962 ) s u p p o r t s t h i s e x p l a n a t i o n . The v i r t u a l l y c o n s t a n t r a t i o o f P b 2 0 8 t o P b 2 0 6 f o r t h e i s o t o p i c a l l y s i m i l a r l e a d s c o n f i r m s t h a t o n l y s m a l l amounts o f r a d i o g e n i c r o c k - l e a d have b e e n a d d e d t o t h e l e a d f o r m e d by t h e homo-g e n i z a t i o n p r o c e s s . O t h e r w i s e , g i v e n t h e i n h o m o g e n e o u s T h / U r a t i o s ( H e i e r and R o g e r s , 1965) i n c r u s t a l r o c k s , t h e p o i n t s on t h e g r a p h w o u l d show much more s c a t t e r , as t h e d a t a f o r t h e 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 d o . The s c a t t e r o f t h e l a t t e r 25 v e r i f i e s that they have passed through the wallrock. Hydrothermal mineralizing solutions flowing along the conduit system rather than lead extraction by magmatic heating appear to be responsible for the observed pattern of isotopic d i s t r i b u t i o n . Smith (1963) has found geologic evidence of hydrothermal mineralizing solutions i n the mines of Socorro Peak, located within the zone. The zone of weakness i s a shallow c r u s t a l feature and therefore could not have served as a locus for the ascent of igneous bodies, since such bodies tend to come from greater depths. If the thermal gradient near the surface of the earth i s 10-50 C°/km,, then granite could not become molten at depths < 20 km., a depth which increases when pressure e f f e c t s are also considered. It should also be pointed out that the hydrothermal solutions are only considered as the agent carrying the lead 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 mineralization, so that a difference i n the genesis of rocks and ores i s not necessarily postulated. Seismic r e s u l t s and magnetic depth soundings have pro-vided additional information about the zone of weakness. The seismic work has shown that i t can be correlated with the is o t o p i c anomaly, and that It extends to a depth of about 18 km. Geomagnetic work (Livingstone, 1966) has confirmed that the zone of weakness i s a shallow c r u s t a l feature, and has Indicated that i t may extend at least to the New Mexico -Texas border (Schmucker, 1964). The lead isotope data show that the zone of weakness i s a northwest trending b e l t , coincident with the Zuni Lineament. It i s , then, another 26 s t r u c t u r a l feature of the lineament. That the feature i s a region of weakness can be related to the idea that lineaments are associated with ancient deep trans-current f a u l t s . Experiments to test the v a l i d i t y of some of the preceding conclusions can be performed. Conductivity measure-ments on the surface and near-surface layers of the crust would reveal whether or not the postulated highly conductive layers r e a l l y e x i s t . An extension of Sanford and Long's mieroearthquake work could confirm the observed co r r e l a t i o n between seismicity and the isotopic boundary, i n addition to confirming the position of the l a t t e r . Further lead isotope analyses could define the extent of the isotopic features. Samples from the New Mexico-Texas border area and southwestern New Mexico ( S i l v e r City area) would be especially i n t e r e s t i n g . Techniques currently being used i n t h i s lab for the extraction bf Small amounts of lead from other sulphide ores could be applied i n areas where galena i s not r e a d i l y a v a i l a b l e . Only by further experiments such as these can the problem of the observed isotopic d i s t r i b u t i o n of leads i n New Mexico and i t s correlation with the Zuni Lineament ever be f u l l y understood. 27 REFERENCES Heier, K.S., and J.J.W. Rogers. Radiometric determination of thorium, uranium, and potassium i n basalts 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. Acta, 27, 137-154, 1963. Kelley, V.C. Regional tectonics of the Colorado Plateau and relationship to the o r i g i n and d i s t r i b u t i o n of uranium. University of New Mexico publications i n Geology No. 5, University Press, Albuquerque. K o l l a r , F., R.D. Russell, and T.J. Ulrych. Precise i n t e r -comparison of lead isotope r a t i o s : Broken H i l l and Mt. Isa. Nature, 18_7, 754-756, i960. Livingstone, C.E. UnpublishedM.Sc. t h e s i s , University of B r i t i s h Columbia, 1966. Mauger, R.L., P.E. Damon and D.E. Livingston. Congruent Laramide petrographic province and copper me&allogenesis i n the southwestern United States. Appendix kHJ AEC Progress Report (COO-689^50), 1965- University of Arizona Geochronology Lab. Mayo, E.B. Lineament tectonics and some ore d i s t r i c t s of the southwest. Mining Eng., 211, 1169-1175, 1958. Muehlberger, W.R., and-R.E. Denison. Precambrian geology of south-central New Mexico. New Mexico Geological Society Guidebook, Fifteenth F i e l d Conference 62-69, 1964. Sanford, A.R., and CR. Holmes. Microearthquakes near Socorro New-Mexico. J . Geophys. Res., 6j_, 4449-4459, 1962 . Sanford, A.R. and L.T* Long; Mieroearthquake cr u s t a l r e f l e c -t i o n s , Socorro, New Mexico. B u l l . Seism. Soc. Am. 55, No. 3, 579-586, 1965. Schmucker, U. Anomalies of geomagnetic variations i n the southwestern United States. J . Geomag. and Geoelect. 15., No. 4, 193-221, 1964. S i n c l a i r , A.J. and R.I. Walcott. The significance of Th/U ra t i o s calculated from west-central New-Mexico mult i -stage lead data. Earth and Planetary Science Letters, 1, 38 -41, 1966. Slawson, W.F. and C F . Austin. A lead isotope study defines a-geological structure * Econ. Geol. 57, 21-29, 1962. Smith, C T . Preliminary notes-on the geology of part of the Socorro Mountains, Socorro County, New Mexico. New Mexico Geological Society- Guidebook, Fourteenth F i e l d Conference, 185-196, 1963. 28 Ulrych, T.J. Unpublished, M.Sc. t h e s i s , University of B r i t i s h Columbia, I960. Weber, R.H., and W.A. Bassett. K-Ar ages of Te r t i a r y volcanic and int r u s i v e rocks i n Socorro, Catron, and Grant Counties, New Mexico. New Mexico Geological Society Guidebook, Fourteenth F i e l d Conference. 220-223, 1963. York, D. Least-squares f i t t i n g of a straight l i n e . Can. J . Phys. 4_4, 1079-1086, 1966. 29 APPENDIX A . l Lead Models. The single-stage model: At a time t Q , the earth i s considered to have been in. a molten state so that homogeni-zation of the U/Pb, Th/Pb and lead isotope r a t i o s occurred. After cooling of the earth, the- state of homogenization persisted i n a source region i n the lower crust or upper ^mantle, with the lead: isotope r a t i o s changing due to the decay of uranium and thorium. At a subsequent time t ^ , lead was removed from the. source;, separated from the U and Th, and emplaced i n a lead deposit. Prom the time t ^ up to the present, the isotopic r a t i o s are considered to have remained unchanged. The r a t i o s are given by: X = a o + ocV(eKto - e X t l ) y • b o + •• V i e ^ o -•**!> (A.l) z + WCe^o - e ^ l ) with the symbols as defined i n Table 1.1. The f i r s t two equations can be combined to y i e l d an equation (Equation A.2) which may be solved y - b 1 e A t o - e X t l " " = ~ X t Q - , A t x ( A J 2 ) x - a c< e ° e l x o The two-stage model: Once again, a deep c r u s t a l or upper mantle region i s postulated as the source of a si n g l e -stage lead emplaced i n a cr u s t a l environment at a time t-^. Also at time t , , uranium and thorium were incorporated into 30 the same environment. At a subsequent time t 2 , tectonic a c t i v i t y recurred i n the area and caused the mixing i n varying proportions of radiogenic lead generated between t.^ and t 2 with some of the single-stage lead to produce the two-stage leads. Mineralization then occurred. I f x^, y^t z^ are the r a t i o s of the single stage leads emplaced at t ^ , and x, y, z are the r a t i o s of a two-stage lead, then x = x x + ^ ( 6 ^ 1 - e A t2) y = y i + V 1 ( e A t l - e X t2)_ (A.3) z = z± + W ^ e ^ l - e^2) from which the slope of the straight l i n e defined by the leads i s : i i , X t i At p y - yi 1 e 1 - e ^ R = = — cx- X t l At 2 ( A ' 4 ) x - x ^ oc e x - e ^ The slope of t h i s l i n e is-then independent of and depends only on the i n t e r v a l t ^ to t 2 . The time t 1 can often be determined from the age of single-stage leads-from which the two-stage leads developed! Using equation A.4, t 2 can then be found. 31 APPENDIX A.2 Automatic Reduction of Data. The measuring: system of the mass spectrometer i s de-signed tomeasure accurately the i n t e n s i t i e s of the d i f f e r e n t ion beams. The ions are collected on a Faraday cup and leaked to ground across a 10" ohm r e s i s t o r . The r e s u l t i n g voltage-across the r e s i s t o r i s balanced by a servo-voltmeter consisting of a potentiometer and a d r i v i n g motor. The voltage across the potentiometer i s then proportional to the ion beam intensity... The position of the potentiometer shaft i s - also- proportional to the voltage across, the potentiometer, so the shaft position is-proportional to the ion beam i n -t e n s i t i e s . The practice i s to record the p o s i t i o n of the potentiometer shaft, rather than to measure-peak heights displayed on a chart recorder.. The shaft p o s i t i o n can be read automatically by means of an encoder, and the information put onto paper or magnetic tape. At the moment, the information from the mass spectro-meter i s punched onto paper tape i n units known as "words", with each word consisting of 6 characters. The f i r s t four characters of each word contain the shaft position (propor-t i o n a l to peak height) of the potentiometer i n the measuring system. The f i f t h character contains information such as attenuation used, scan d i r e c t i o n , and, when necessary, the "scan separate" and "scan, r e j e c t " signals. The f i n a l character i s always an "end of l i n e " punch which denotes the end of a word. Words are recorded at a rate of 2 per second, and 32 t h e r e a r e u s u a l l y a b o u t 900 words f o r a p a i r o f s p e c t r a c o n s i s t i n g o f e i g h t p e a k s i n e a c h s c a n d i r e c t i o n . I n r e c o r d i n g t h e d a t a f o r a p a i r , t h e p r o c e d u r e u s e d i s f i r s t t o g e n e r a t e t h e " s c a n s e p a r a t e " s i g n a l by p r e s s i n g t h e b u t t o n on t h e mass s p e c t r o m e t e r c o n s o l e , t o r e c o r d 10-20 w o r d s on t h e b a s e l i n e o f e a c h a t t e n u a t i o n u s e d , and t h e n t o s c a n down-mass o v e r t h e Pb^CH^.)* s p e c t r u m . B a s e l i n e s a r e a g a i n r e c o r d e d a t t h e l ow-mass end o f t h e s p e c t r u m , t h e s p e c t r u m s c a n n e d u p - m a s s , t h e b a s e l i n e s r e c o r d e d f o r a t h i r d t i m e , and t h e p a i r i s t h e n t e r m i n a t e d by a n o t h e r " s c a n s e p a r a t e " s i g n a l . Up t o 25 p a i r s may be r e c o r d e d i n t h i s m a n n e r . I d e n t i f i c a t i o n f o r e a e h s amp l e i s p u n c h e d a t t h e s t a r t o f t h e p a p e r t a p e a n d - t h e - i n f o r m a t i o n on i t- i s t r a n s f e r r e d t o m a g n e t i c t a p e , w i t h 21 wo rds o c c u p y i n g 1 r e c o r d o f m a g n e t i c tape-. The d a t a c a n t h e n be p r o c e s s e d by t h e r e d u c t i o n p r o g r a m " P I L O T " , w h i c h i s c u r r e n t l y s t o r e d on. t h e IBM 1301 d i s c f i l e a t t h e U.B>;G. C o m p u t i n g C e n t r e . The c a r d s r e q u i r e d for* t h e p r o g r a m a r e : $ J0B ( Job N o . and. name o f u s e r ) $CL0SE S . SU04 ,REMOVE t $TIME (as needed.) c o l . l 6 I $EXECUTE P I LOT TAPE * ( L i s t o f o p t i o n s i f d e s i r e d - see T a b l e A . l ) The o p t i o n s P T A P E , GO, DECK , and CALC- a r e c u r r e n t l y p r e -s e t ^ so t h a t t h e p r o g r a m w i l l a u t o m a t i c a l l y p r o c e s s i n f o r m a t i o n 33 Option Description PTAPE mass spectrometer output on paper tape MTAPE mass spectrometer output on mag. tape GO process sample DUMP output f i l t e r e d data SELECT process sample whose i d e n t i f i c a t i o n s t a r t s i n c o l . 71 of "option" card DECK outputs data cards REPEAT processes a l l samples on the mag. tape CALC calculates lead r a t i o s MAX. outputs raw peak heights, peak t a i l s , etc. SKIP,XX skips XX records on mag. tape "Negative" option NOPAP NOMAG NOGO NODMP DOALL NODECK NOCALC NOMAX Preset value (V5/66A) TRUE FALSE TRUE FALSE FALSE TRUE FALSE TRUE FALSE FALSE Table A . l Options, available f o r program/ "PILOT" 34 o r i g i n a l l y on paper, tape, producing lead r a t i o s and a deck of data cards. Any other options may be used simply by l i s t i n g them on the f i n a l card. The t o t a l time required by the program can e a s i l y be estimated. About 20 sec are used by the computer to set up the program, and the time required to process a pair and calculate lead r a t i o s from i t i s about 18 sec. Lead isotope r a t i o s can be obtained from data cards by using the following cards: $JOB (Job no. and name of user) $EXECUTE . PILOT T CARDS c o l . 16 data cards The time required by the computer i s about 20 sec to set up the program and 1 sec a pair to calculate the. lead r a t i o s . 

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