Historical foundations of chemical geology and geochemistry

Chemical Geology - Elsevier Publishing Company, Amsterdam Printed in The Netherlands

Review HISTOBICAL FOUNDATIONS OF CHEMICAL GEOLOGY AND GEOCHEMISTRY

AoA. MANTEN Utrecht (The Netherlands) 1 (Received December 30, 1965)

SUMMARY Roughly, the n a m e c h e m i c a l geology has b e e n u s e d f o r a s long a s c h e m i s t r y h a s b e e n a p p l i e d in geology; the n a m e g e o c h e m i s t r y was i n t r o d u c e d by S c h b n b e i n , in 1838. W h e r e a s i n i t i a l l y the n a m e s w e r e often r e g a r d e d a s s y n o n y m o u s , in our c e n t u r y t h e r e is a t e n d e n c y to m a k e a d i s t i n c t i o n b e t w e e n the two on the b a s i s of the way in which the s u b j e c t is a p p r o a c h e d . In the p r e v i o u s c e n t u r y m o s t of the w o r k dealt with m i n e r a l and r o c k a n a l y s i s a n d the o c c u r r e n c e of g e o c h e m i c a l c y c l e s . Other f i e l d s of study, such as isotope "geology", p h y s i c o - c h e m i c a l p e t r o l o g y and c r y s t a l c h e m i s t r y r e a c h e d m a t u r i t y e a r l y in the t w e n t i e t h c e n t u r y . Somewhat l a t e r s e d i m e n t a r y g e o c h e m i s t r y b e g a n to expand. The r i s e of g e o c h e m i c a l p r o s p e c t i o n took p l a c e m a i n l y in the n i n e t e e n t h i r t i e s . In the m e a n t i m e , a c o n s i d e r a b l e p r o g r e s s was a l s o a c h i e v e d in the f i e l d of a n a l y t i c a l g e o c h e m i s t r y , p a r t i c u l a r l y t h r o u g h the d e v e l o p m e n t of v a r i o u s new t e c h n i q u e s .

INTRODUCTION With the i n t r o d u c t i o n of a new i n t e r n a t i o n a l j o u r n a l to c o v e r the wide i n t e r d i s c i p l i n a r y f i e l d b e t w e e n geology and c h e m i s t r y , it is c o n s i d e r e d useful to devote a few p a g e s to a r e f l e c t i o n of how this f i e l d of s c i e n t i f i c r e s e a r c h began. It i s fully r e a l i z e d that such a s h o r t a r t i c l e can only be s o m e w h a t a r b i t r a r y in the s e l e c t i o n of its c o n t e n t s . It m e r e l y g i v e s a few p i c t u r e s of i m p o r t a n t m o m e n t s in s c i e n t i f i c h i s t o r y , without p r e t e n d i n g to be a c o n t i n u o u s , let a l o n e a c o m p l e t e , s t o r y . In t h i s s e n s e t h e r e is a:'difference b e t w g e n a s h o r t and g e n e r a l h i s t o r i c a l a c c o u n t and the n o r m a l kind of r e v i e w a r t i c l e . T h e p r e s e n t p a p e r m a y s e r v e (1) to show how a s c i e n t i f i c d i s c o v e r y or d e v e l o p m e n t n e v e r s t a n d s on i t s own, but t a k e s a p l a c e in an u n i n t e r r u p t e d c h a i n of i n t e l l e c t u a l h u m a n a c t i v i t i e s , and a l s o (2) to show to whom m o d e r n w o r k e r s in t h i s i n t e r d i s c i p l i n a r y f i e l d owe the s c i e n t i f i c b a s i s with which they a r e so f a m i l i a r that they take its p r e s e n c e a l m o s t for g r a n t e d . It is in the n a t u r e of s c i e n c e to look into the f u t u r e a n d to m a k e u s e of the knowledge a v a i l a b l e to date m a i n l y to p r o d u c e f u r t h e r a d v a n c e s in the days and y e a r s ahead. T h i s 1

Postal address: Cortezlaan 9, Utrecht (The Netherlands)

Chem. Geol., 1 (1966) 5-31

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m o m e n t is chosen for s o m e r e f l e c t i o n about the path that is behind, and for p a y i n g a t r i b u t e to the f o u n d e r s of the s c i e n t i f i c field which this j o u r n a l hopes to s e r v e . T h o s e who wish to see a d e t a i l e d s u r v e y of the s t a t e of knowledge in this field or in one of its c o m p o n e n t s a r e r e f e r r e d to the v a r i o u s books m e n t i o n e d in the list of r e f e r e n c e s (e.g., A b e l s o n , 1959; Fyfe, 1964; G o l d s c h m i d t , 1954; Mason, 1952: R a n k a m a and Sahama, 1950: Smith, 1962. G e o c h e m i c a l m e t h o d s : A d l e r , 1966; A h r e n s and T a y l o r , 1960; C h a r i o t , 1964; E w a l d and H i n t e n b e r g e r , 1952; Feigl, 1963; H e r m a n n and A l k e m a d e , 1963;Kodama, 1963; N a k a n i s h i , 1962; R i t c h i e , 1964; S a m u e l s o n , 1963; S m a l e s and W a g e r , 1960. C~'ystal e h e m i s l ~ y : Bragg, 1937; B u e r g e r , 1960; Bunn, 1961: E v a n s , 1946: H o c a r t and K e r n , 1959. S i l i c a l e s , , e o c h e m i s l r y : A h r e n s , 1954; A h r e n s and T a y l o r , 1960; E i t e l , 1954. Plo, s i c o - c k c m i e a l / ) e l r o l o g y : Bowen, 1928: T u r n e r and V e r h o o g e n , 1951. Soil c h e m i s l r v : G e d r o i t s , 1955; J a c k s o n , 1958; Van Olphen, 1963: W i n o g r a d o w , 1954. G e o e h c m isle'3, q l s edi~Jl e~la m' r,oc'/es : Degens, 1965. ( ) ~ ' ~ z i c ,~coehem isl , y : B r e g e r , 1963: Colombo a n d H o b s o n , 1964. P e l Y o l c a m g e o c h e m i s t ~ T : D o b r y a n s k i i , 1961; Nagy and Colombo, 1966. l s o l o p e g e o l o g y : Bowen, i966; H a m i l t o n , 1965; R a n k a m a , 1954, 1963; R u s s e l l and F a r q u h a r , 1960. Geoc h e m i c a l exploratio~z : G i n z b u r g , 1960; Hawkes and Webb, 1962; Kudymov, 1962; Malayuga, 1963. H v d r o g e o c h e m i s l Y y : A l e k m , 1948, 1953; S c h o e l l e r , 1956: T a g e e v a and T i h o m i r o v a , 1962. ChemiccH o c e a n o x n f f ) h v : B a r n e s , 1959. A l m o s p l z e r i c cheJJlisl~'v : Junge, 1963).

DEFINITION

OF G E O C t l E M I S T R Y

The word geochemistry was used first, in 1838, by the German chemist C.F. Sch~Snbein, professor at the University of Basel, and the discoverer of ozone. He considered the task of geochemistry to consist of the investigation of the chemical and physical properties of all geological formations and of t h e i r age r e l a t i o n s h i p s . F o u r y e a r s l a t e r , he e m p h a s i z e d that a study of the c h e m i c a l n a t u r e and o r i g i n of the m a s s e s c o m p o s i n g the e a r t h is of equal v a l u e to geology as the d e t e r m i n a t i o n of the r e l a t i v e age and f o s s i l content of g e o l o g i c a l f o r m a t i o n s . P r i o r to SchiSnbein, such i n d i c a t i o n s as c h e m i s t r y of the e a r t h or of the e a r t h ' s c r u s t and c h e m i c a l geology had b e e n u s e d . T h e s e n a m e s r e m a i n e d in use a l s o a f t e r the w o r d g e o c h e m i s t r y had b e e n coined, p a r t l y as s y n o n y m s of g e o c h e m i s t r y , p a r t l y as t e r m s of a w i d e r d e f i n i t i o n than was at that t i m e given to g e o c h e m i s t r y . A d e f i n i t i o n of g e o c h e m i s t r y which was m u c h n a r r o w e r than that by Sch~Snbein, was given in 1924 by W.J. V e r n a d s k y , who c o n s i d e r e d g e o c h e m i s t r y as only the study of the c h e m i c a l e l e m e n t s , i.e., the a t o m s , of the e a r t h ' s c r u s t and as far as p o s s i b l e of the e a r t h in g e n e r a l , t h e i r h i s t o r y and t h e i r d i s t r i b u t i o n in space in the p r e s e n t and past. He draw a c l e a r - c u t line with m i n e r a l o g y which s t u d i e s in the s a m e way the c o m p o u n d s of the a t o m s i.e., the m o l e c u l e s and c r y s t a l s . In the s a m e y e a r , F.W. C l a r k e p r o d u c e d a w i d e r definition, when he w r o t e : "Each rock m a y be r e g a r d e d .... as a c h e m i c a l s y s t e m , in which, by v a r i o u s a g e n c i e s c h e m i c a l changes can be b r o u g h t about. E v e r y such change i m p l i e s a d i s t u r b a n c e of e q u i l i b r i u m , with the u l t i m a t e f o r m a t i o n of a new s y s t e m , which, u n d e r the new conditi(ms, is i t s e l f s t a b l e in t u r n . The study (;

~'i~cm. (icoi.,

i !~ ' ; ! ; i

r-:ll

of t h e s e c h a n g e s is the p r o v i n c e of g e o c h e m i s t r y . To d e t e r m i n e what c h a n g e s a r e p o s s i b l e , how and when they o c c u r , to o b s e r v e the p h e n o m e n a which a t t e n d t h e m , a n d to note t h e i r final r e s u l t s a r e the f u n c t i o n s of the g e o c h e m i s t . . . F r o m a g e o l o g i c a l point of view the s o l i d c r u s t of the e a r t h is the m a i n object of study; and the r e a c t i o n s which take p l a c e in it m a y be c o n v e n i e n t l y c l a s s i f i e d u n d e r t h r e e h e a d s - f i r s t , r e a c t i o n s b e t w e e n the e s s e n t i a l c o n s t i t u e n t s of the e a r t h i t s e l f ; s e c o n d , r e a c t i o n s due to its a q u e o u s e n v e l o p e ; and t h i r d , r e a c t i o n s p r o d u c e d by the a g e n c y of the a t m o s p h e r e . " A c o m p a t r i o t and y o u n g e r c o l l e a g u e of V e r n a d s k y , A.E. F e r s m a n , had given, in 1922, the following definition: " T h e p u r p o s e of g e o c h e m i s t r y is the study of the e l e m e n t - a t o m in the c o n d i t i o n s p r e v a i l i n g in the E a r t h ' s c r u s t (as well a s in the p a r t s of the C o s m o s a c c e s s i b l e to our exact o b s e r v a t i o n s ) . G e o c h e m i s t r y s t u d i e s : (a) the q u a n t i t a t i v e d i s t r i b u t i o n of the c h e m i c a l e l e m e n t s in the E a r t h ' s c r u s t and t h e i r d i s p e r s i o n and local c o n c e n t r a t i o n ; (b) the c o m b i n a t i o n s of d i f f e r e n t e l e m e n t s in the d i f f e r e n t p a r t s of the E a r t h ' s c r u s t and t h e i r d i s t r i b u t i o n in s p a c e and t i m e u n d e r the i n f l u e n c e of d i f f e r e n t c h e m i c a l p r o c e s s e s ; (c) the m i g r a t i o n of e l e m e n t s and the laws of such m i g r a t i o n as d e t e r m i n e d by the d i f f e r e n t t h e r m o d y n a m i c c o n d i t i o n s of t h e i r e n v i r o n m e n t ; and (d) the b e h a v i o u r of c h e m i c a l e l e m e n t s e i t h e r in the e n v i r o n m e n t of the E a r t h ' s c r u s t or as c o m p o u n d s and p a r t i c u l a r l y as c r y s t a l s . T h i s m a y be e x p r e s s e d even m o r e s i m p l y : g e o c h e m i s t r y s t u d i e s the h i s t o r y of c h e m i c a l e l e m e n t s in the E a r t h ' s c r u s t and t h e i r b e h a v i o u r u n d e r d i f f e r e n t t h e r m o d y n a m i c and p h y s i c o - c h e m i c a l n a t u r a l c o n d i t i o n s . " In a s i m i l a r way V.M. G o l d s c h m i d t , in the y e a r s 1923~1937, f o r m u l a t e d the t a s k of g e o c h e m i s t r y as the d e t e r m i n a t i o n of the r e l a t i v e a b u n d a n c e of the e l e m e n t s and of the n u c l i d s (isotopes), the d i s t r i b u t i o n of the e l e m e n t s in the v a r i o u s s p h e r e s of the e a r t h ( a t m o s p h e r e , h y d r o s p h e r e , b i o s p h e r e and l i t h o s p h e r e ) and the d e t e c t i o n of the laws which g o v e r n the a b u n d a n c e r e l a t i o n s h i p s and d i s t r i b u t i o n of the e l e m e n t s . R a n k a m a and S a h a m a (1950), in t h e i r s t a n d a r d work, follow the d e f i n i tion of g e o c h e m i s t r y a s u s e d by G o l d s c h m i d t . T h e i r l e a d i n g p r i n c i p l e is not the s u b j e c t of study but the v i e w p o i n t of the s c i e n t i s t a t t a c k i n g his p r o b l e m s . T h e g e o c h e m i s t d e a l s l a r g e l y with g e o l o g i c a l p h e n o m e n a and g e o l o g i c a l s u b j e c t s . H o w e v e r , his e n t i r e w o r k is b a s e d on c h e m i s t r y . His m e t h o d s and his p r o b l e m s a r e t h o s e of a c h e m i s t or a p h y s i c a l c h e m i s t . On this b a s i s , R a n k a m a and S a h a m a draw the b o u n d a r y line b e t w e e n g e o c h e m i s t r y and c h e m i c a l geology. " C o n v e r s e l y , the c h e m i c a l g e o l o g i s t e x a m i n e s his p r o b l e m s f r o m the v i e w p o i n t of a g e o l o g i s t . His is a g e o l ~ ; i c a l m a t e r i a l , and, when i n t e r p r e t i n g his o b s e r v a t i o n s , the a n a l y t i c a l r e s u l t s , etc., he a l w a y s has t h e i r g e o l o g i c a l a p p l i c a t i o n in his m i n d . F o r the g e o c h e m i s t , on the o t h e r hand, the g e o l o g i c a l o b s e r v a t i o n s , though b a s e d on the use of g e o l o g i c a l m a t e r i a l , r e p r e s e n t only a c e r t a i n p a r t of his r e s u l t s , which a r e i n t e n d e d to e s t a b l i s h laws g o v e r n i n g the a b u n d a n c e and d i s t r i b u t i o n of the e l e m e n t s . " M a s o n (1952), is of the opinion, that "in the s i m p l e s t t e r m s , g e o c h e m i s t r y m a y be defined as the s c i e n c e c o n c e r n e d with the c h e m i s t r y of the e a r t h as a whole, and of its c o m p o n e n t p a r t s . At one and the s a m e t i m e , it is both m o r e r e s t r i c t e d and a l s o m o r e e x t e n s i v e in scope than geology."

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GEOCHEMICAL CYCLES There was already interest in the distribution of elements in the earth's crust even before the concept of chemical elements was clearly developed. This holds particularly true for ore geologists. Full development of geochemical work, however, only began in the first half of the nineteenth century. As early as 1793, A. yon Humboldt tried to derive the chemical composition of plants from their inorganic surroundings. Exact knowledge of the i m p o r t a n c e of p l a n t s in the g e o c h e m i c a l c y c l e of the e l e m e n t s , p a r t i c u la.rly of c a r b o n , oxygen, p o t a s s i u m and p h o s p h o r u s , s t a r t e d with the work of J . A . B . D u m a s (1837) and J. B o u s s i n g a u l t (1851; D u m a s and B o u s s i n g a u l t , 1841) in F r a n c e , and of K. S p r e n g e l (1828, 1837) and J. von L i e b i g (1840) in G e r m a n y . In l a t e r y e a r s this s u b j e c t m a d e m u c h a d v a n c e in R u s s i a , p a r t i c u l a r l y by W.J. V e r n a d s k y , who, in 1918, founded a b i o g e o c h e m i c a l r e s e a r c h i n s t i t u t e in Kiev. T h e f a m o u s book of C.G.C. Bischof, Lehrbuch der physilealische~l und che~nischeu Geologie (1846--1854) has b e e n e p o c h - m a k i n g in showing the i m p o r t a n c e of w a t e r for the d i s t r i b u t i o n of the e l e m e n t s and in d e m o n s t r a t i n g that c y c l i c p r o c e s s e s a l s o o c c u r in the i n o r g a n i c world. T h e f o u r - v o l u m e w o r k p r e s e n t s a c o m p l e t e s u m m a r y of the c h e m i c a l work c a r r i e d out by p e t r o l o g i s t s and other g e o l o g i s t s in the p r e - m i c r o s c o p i c e r a . Bischof r e g a r d e d the e a r t h as a " v a s t c h e m i c a l l a b o r a t o r y " and he q u e s t i o n e d " w h e t h e r t h e r e m a y be any other s c i e n c e of geology t h a n that of a c h e m i c a l and a p h y s i c a l geology". F o r s e v e r a l d e c a d e s B i s c h o f ' s book w a s a s t a n d a r d work. Only at the end of the c e n t u r y was it s u p e r s e d e d by a n o t h e r g e o c h e m i c a l c l a s s i c , Allgemeine und chemische Geolo.yie, by J. Roth, which a p p e a r e d in 1879--1893. F u r t h e r i m p o r t a n t w o r k s f r o m that t i m e a r e Chenzische Mineralogie, by R. B r a u n s , i s s u e d in 1896, and Treatise on Meho~o~'phism by C.R. v a n H i s e , p u b l i s h e d in 1904. Also in the t w e n t i e t h c e n t u r y a g r e a t deal of i m p o r t a n t work has b e e n done about g e o c h e m i c a l c y c l e s . W i t h i n the f r a m e of this s h o r t r e v i e w , h o w e v e r , only a few g e n e r a l p o i n t s can be m e n t i o n e d . G o l d s c h m i d t (1926a, 1930, 1933b), when c o n s t r u c t i n g a g e o c h e m i c a l p i c t u r e of the w o r l d ("Weltbild"), d i s t i n g u i s h e d four s t a g e s of g e o c h e m i c a l d i f f e r e n t i a t i o n in the e a r t h . The f i r s t stage i n c l u d e s all p r o c e s s e s r e s p o n s i b l e for the d i f f e r e n t i a t i o n of m a t t e r to f o r m the g e o c h e m i c a l s p h e r e s of which the e a r t h is b e l i e v e d to be c o m p o s e d (see the s e c t i o n on the G e o c h e m ical s t r u c t u r e of the e a r t h ) . The s e c o n d g e o c h e m i c a l d i f f e r e n t i a t i o n of m a t t e r is what h a p p e n s when a m a g m a c r y s t a l l i z e s . The t h i r d d i f f e r e n t i a t i o n chiefly c o m p r i s e s p h y s i c o - c h e m i c a l p r o c e s s e s in a q u e o u s s o l u t i o n s which r e s u l t in the f o r m a t i o n of the v a r i o u s s e d i m e n t s . R e a c t i o n s b e t w e e n the b i o s p h e r e , on the one hand, and the l i t h o s p h e r e , the h y d r o s p h e r e and the a t m o s p h e r e , on the o t h e r hand, f o r m the fourth stage in the g e o c h e m i c a l d i f f e r e n t i a t i o n of the e a r t h . R a n k a m a and S a h a m a (1949) d i s t i n g u i s h b e t w e e n a m i n o r or e x o g e n i c cycle in the m i g r a t i o n of m a t t e r a n d a m a j o r c y c l e ( F i g . l ) . T h e l a t t e r i n c l u d e s f o r m a t i o n by c h e m i c a l d i f f e r e n t i a t i o n of both the p r i m a r y i g n e o u s r o c k s and the s e d i m e n t s and t h e i r d i a g e n e t i c p r o d u c t s ( s t a g e s two and t h r e e of G o l d s c h m i d t ) . At the d e e p e r l e v e l s of the e a r t h ' s c r u s t , h o w e v e r , p r o c e s s e s of s t i l l a n o t h e r kind take p l a c e which cannot be i n c l u d e d u n d e r the concept of d i f f e r e n t i a t i o n . E v e n though t h e s e p r o c e s s e s t e n d to c a u s e a c h e m i c a l Chem. Geol., ] (l!~ti6) 5-3]

m ZnO~"

cycle

closed earth's surface

. I / IGNEOUS ROCKS

~ •

METAMORPHIC ROCKS

~--.-~ SEDIMENTARY ROCKS

--¢ ~

major cycle closed

Fig.1. V e r y s i m p l i f i e d r e p r e s e n t a t i o n of the m a j o r and m i n o r c y c l e s of m a t t e r d i s t r i b u t i o n of the e l e m e n t s among v a r i o u s r o c k s , they w o r k e s s e n t i a l l y in the opposite d i r e c t i o n , endeavouring to l e v e l - o f f the c h e m i c a l d i f f e r e n c e s a l r e a d y p r o d u c e d . T h e y f o r m the endogenic cycle of m a t t e r . The endogenic and exogenic c y c l e s f o r m the m a j o r cycle of m a t t e r . The m i n o r or exogenic cycle, which s t a r t s f r o m s o l i d c r y s t a l l i n e r o c k s and ends in s e d i m e n t a r y r o c k s , thus f o r m s but a p a r t of the m a j o r cycle of m a t t e r in n a t u r e . The m i n o r cycle t a k e s place under the d i r e c t influence of h y d r o s p h e r i c and a t m o s p h e r i c agents, and, unlike the m a j o r cycle, which is c l o s e d , it is l a r g e l y open and t a k e s place only in one d i r e c t i o n ; it is c l o s e d only for s e d i m e n t a r y r o c k s , but for all other r o c k s it is i r r e v e r s i b l e .

MINERAL AND ROCK ANALYSIS Of fundamental i m p o r t a n c e to the development of g e o c h e m i s t r y has been the c h e m i c a l a n a l y s i s of m i n e r a l s and r o c k s . P r o g r e s s in this field was c l o s e l y connected with the development of a n a l y t i c a l c h e m i s t r y . Consequently, g e o c h e m i s t r y was, to a c e r t a i n extent, f r o m the beginning an exact s c i e n c e , d i r e c t e d m a i n l y to quantitative work. In 1821, J . J . B e r z e l i u s d e s c r i b e d m i n e r a l o g y as t h e " c h e m i s t r y of the e a r t h ' s c r u s t " . I n i t i a l l y m i n e r a l and r o c k a n a l y s e s w e r e p r i m a r i l y c a r r i e d out to e s t a b l i s h the d i s t r i b u t i o n of the main e l e m e n t s in the e a r t h ' s c r u s t . The f i r s t e s t i m a t e s about this d i s t r i b u tion w e r e m a d e by D b b e r e i n e r , but these w e r e l i t t l e m o r e than intelligent g u e s s e s . I t w a s B e r z e l i u s and his school in Sweden who s t a r t e d the c o l l e c tion of b a s i c data. Valuable w o r k in the c o l l e c t i o n of a n a l y t i c a l data was a l s o c a r r i e d out by Bou~ (1843) and l~.lie de Beaumont (1847}; both w e r e i n t e r e s t e d in the q u a l i t a t i v e as well as the quantitative a s p e c t s of the d i s t r i b u t i o n of the c h e m i c a l e l e m e n t s in the c r u s t of the e a r t h . A f i r s t a t t e m p t to e x p r e s s the a v e r a g e c o m p o s i t i o n of the e a r t h ' s c r u s t in t e r m s of m i n e r a l s , was made by De S e r r e s (1863). A v e r y i m p o r t a n t stage was the d i s c o v e r y of the s p e c t r a l laws by G. Kirchhoff and R. Bunsen, in 1860-1861. T h i s led to the d i s c o v e r y of s e v e r a l new e l e m e n t s , such as c e s i u m (by R. Bunsen, in 1860-1861), r u b i d i u m (by R. Bunsen, in 1861), t h a l l i u m (by W. C r o o k e s , in 1861), indium (by F. Reich and H.T. R i c h t e r , in 1863), g a l l i u m (by L. de B o i s b a u d r a n , in 1875), s c a n d i u m (by L . F . Nilson, in 1879). Study of the d i s t r i b u t i o n of the t r a c e e l e m e n t s was begun by W.N. H a r t l e y and H. R a m a g e , in 1897, G. E b e r h a r d , in 1908, and W.J. V e r n a d s k y , in 1910. Chem. Geol., 1 (1966) 5-31

9

S p e c t r o s c o p i c e x a m i n a t i o n of s o t a r and s t e l l a r r a d i a t i o n , the chem-i c a l a n a l y s i s of m e t e o r i t e s and the k n o w l e d g e that w a s c o l l e c t e d about the c o m p o s i t i o n of the e a r t h led a l r e a d y in the n i n e t e e n t h c e n t u r y to tile b e l i e f that the t m i v e r s e is c h e m i c a l l y u n i f o r m , with the s a m e e l e n l e n t s , ) c e u r r i n g t h r o u g h o u t the c o s m o s . Only once has an e l e m e n t b e e n d i s c o v e r e d , which w a s not p r e v i o u s l y known to o c c u r on the e a r t h . T h i s w a s h e l i u m , f i r s t d e t e c t e d in the s u n ' s s p e c t r u m by the a s l r o n o m e r J a n s s e n , in 1868. ,rod r e c o g n i z e d as a new e l e m e n t by N. L o c k y e r , in 1869. N e a r l y t h i r t y y e a r s l a t e r , in 1895. h e l i u m was i d e n t i f i e d on the e a r t h by R a m s a y . as the ,,as e v o l v e d when u r a n i n i t e is h e a t e d with an i n o r g a n i c acid. Ill fact, the e v o l u tion of an i n e r t g a s f r o m u r a n i n i t e had a h ' e a d y been o b s e r v e d a few y e a r s e a r l i e r , by H i l l e b r a n d , but he thought it to be n i t r o g e n . T h e f i r s t p u b l i s h e d c o l l e c t i o n s of c h e m i c a l a n a l y s e s el r o c k s w e r e c o m p i l e d by Roth (1861, 1869, 1873). T h e n e e d f o r c o m p l e t e a n a l y s e s of m i n e r a l s and r o c k s was f i r m l y s t r e s s e d by the A m e r i c a n W . F . H i l l e b r a n d , who, in 1900, p u b l i s h e d one of the f i r s t a c c o u n t s of s i l i c a t e a n a l y s i s . A v e r y i m p o r t a n t p u b l i c a t i o n f r o m this p e r i o d is the s u m m a r i a i n g w o r k of F.W. C l a r k e , of the U . S . G e o l o g i c a l S u r v e y , The Dull o t Gcoche mistm, p u b l i s h e d in 1908, and f o r s e v e r a l d e c a d e s the s t a n d a r d r e f e r e n c e w o r k . E a r l i e r C l a r k e had begun w e l l by i n t r o d u c i n g the c h e m i c a l i n v e s t i g a t i o n of the e a r t h into A m e r i c a . T h i s b e g a n in 1884, with his a p p o i n t m e n t as Chief C h e m i s t to the U . S . G e o l o g i c a l S u r v e y . B e f o r e that t i m e , this f i e l d of r e s e a r c h w a s l a r g e l y confined to the E u r o p e a n continent. In 1922, C l a r k e i s s u e d , t o g e t h e r with H.S. W a s h i n g t o n , The ,.l~,uraL,u Chemical Composilio~ of l;4~zeol~s Roc/es, and in 1924 they p u M i s h e d The ComposilioJz of lhc" Earth's C~'asl. T h e s e w o r k s , t o g e t h e r with a few o t h e r p u b l i c a t i o n s by W a s h i n g t o n (1920, 1925) p r e s e n t e d a r e a s o n a b l e a p p r o x i m a t i o n (though not u n a s s a i l a b l e ) of the a v e r a g e c h e m i c a l c o m p o s i t i o n of the v a r i o u s r o c k t y p e s , the d i f f e r e n t p a r t s of the l i t h o s p h e r e , and the l i t h o s p h e r e as a whole. W a s h i n g t o n a l s o s h o w e d that the a v e r a g e c h e m i c a l c o m p o s i t i o n of the r o c k s in the P a c i f i c O c e a n r e g i o n s is a h n o s t i d e n t i c a l to that of r o c k s f r o m the A t l a n t i c O c e a n r e g i o n s , but s u b s t a n t i a l l y d i f f e r e n t f r o m the a v e r a g e for the total l i t h o s p h e r e . In 1924, a l s o the l a s t ed i t io n of The Data of Gc~',)chcmisl~3' a p p e a r e d , m o r e or l e s s m a r k i n g the end of a c e n t u r y el g e o c h e m i c a l w o r k d e a l i n g with the c h e m i c a l a s s a y of the d i r e c t l y a c c e s s i b l e p a r t s of lhe e a r t h . In 1925, C l a r k e r e t i r e d f r o m his p o s i t i o n in the U . S . G e o l o g i c a l S u r v e y . 1 In the m e a n t i m e , a new p e r i o d in i n o r g a n i c a n a l y t i c a l g e o c h e m i s t r y had a l r e a d y s t a r t e d in 1920. T h i s p e r i o d is c h a r a c t e r i z e d by the i n t r o d u c t i o n of X - r a y and optical s p e c t r o g r a p h y . X - r a y s p e c t r o g r a p h y w as f i r s t u s e d to a n a l y z e t r a c e e l e m e n t s by the Swede A. Hadding, in 1922. In the p e r i o d 1923.M929, t h i s l e e h n i q u e was u s e d v e r y ~.'xtensively f o r the a n a l y t i c a l study I In recent years, the U.S. (ieologieal Survey has been publishing a new edition of Thc~

Dala qf (;cochemislr.v, in the form of a large number of separate monographie contributions, written by specialists in the vat'ious subjects. Modern data about the abundance and distribution of elements in the earth's e c u s t can also be found in Mason (1958), Taylor (1964), Vinogradov ( 1 9 6 2 ) and Wedepohl (in press): data about the distribution of the elements in igneous cocks are given in l'uylor (1!~14'}, Turekian and Wedepohl (1961) anti Vinogradov (1962); for the distribution of elements in some sediments see Turekian and Wedepohl (19;~1). Dttta about the ,:.ompositi~m (0[ mete~rites ~lre given by Mason (l.qti2) and Vinogcadov (1362). 10

(!hem.

C,¢.~,I . I I l l ) G ( ) ) . ~ 3 1

of the d i s t r i b u t i o n of r a r e e l e m e n t s in m i n e r a l s , w h i l e X - r a y c r y s t a l l o g r a p h y f u r n i s h e d d a t a on the a t o m i c and i o n i c r a d i i of the r a r e r e l e m e n t s (cf. the s e c t i o n on C r y s t a l c h e m i s t r y ) . T h e r e a s o n f o r c o n c e n t r a t i n g on the r a r e r e l e m e n t s w a s , of c o u r s e , t h a t both a n a l y t i c a l a n d s t r u c t u r a l d a t a w e r e l a c k i n g f o r m a n y of t h e m , so that d e t e r m i n a t i o n s of such d a t a n a t u r a l l y had a high p r i o r i t y a m o n g g e o c h e m i c a l i n v e s t i g a t i o n s ( G o l d s c h m i d t , 1954). A f t e r 1929, X - r a y s p e c t r o g r a p h y w a s in m a n y c a s e s s u p e r s e d e d by o p t i c a l s p e c t r o g r a p h y . T h e c r e d i t f o r h a v i n g b r o u g h t t h i s m e t h o d into f r u i t i o n g o e s to V.M. G o l d s c h m i d t and the s c h o o l w h i c h he l e d in G 6 t t i n g e n in the y e a r s 1929--1935. In m o s t c a s e s it h a s a m u c h h i g h e r s e n s i t i v i t y and t h e r e b y the n e c e s s i t y of c h e m i c a l c o n c e n t r a t i o n f r o m the c r u d e m i n e r a l s o r r o c k s c o u l d f r e q u e n t l y be e l i m i n a t e d . S i n c e then, n o n - d e s t r u c t i v e and i n s t r u m e n t a l t e c h n i q u e s have s t r o n g l y g a i n e d in i m p o r t a n c e , with f u r t h e r d e v e l o p m e n t of the X - r a y s p e c t r o g r a p h , f l a m e p h o t o m e t e r , and n e u t r o n a c t i v a t i o n . T h e p r e s e n t s t a g e in i n o r g a n i c a n a l y t i c a l g e o c h e m i s t r y b e g a n with the development of the electron microprobe, by Castaing, in France, and introduced into petrology to a large degree by the Cambridge school of mineralogy and petrology. Focussing electrons in a way which is impossible with photons, enables the analyst nowadays to study a volume of material as small as 10-15cm 3. The use of electrons has also led to notable developments in the field of electron microscopy. Our century has also seen the development of organic geochemistry (sometimes referred to as "biogeoehemistry"). Its rise has, to an important degree, been determined by the discovery of the technique of chromatography. As early as 1903, chromatography was used by the Russian botanist Tswett, as a method of separating different natural substances, achieved by first placing them on adsorbent columns and then removing them selectively by elution with suitable solvents. Not until 1931, however, was the importance of chromatography fully realized. Major applications in chemistry did not take place until after World War If. By then column chromatography had been supplemented by paper chromatography (1943), which permitted a greater variety of separations from extremely minute samples. More recently, other thin-layer materials and gas chromatography have been introduced. All find application in modern geochemistry. The importance of mass spectrometry also in the field of organic geochemistry should be emphasized. This technique, largely employed by petroleum geochemists, has found much application also by the students of meteorite organic matter, one of the most interesting new aspects of organic geochemistry. Polarimetry, and more recently spectro-polarimetry,have also found application in the detection and characterization of opticallyactive organic compounds in sedimentary rocks and carbonaceous meteorites.

CRYSTAL CHEMISTRY A l r e a d y in 1815, J . N . F u c h s had o b s e r v e d that in g e h l e n i t e i r o n and c a l c i u m could m u t u a l l y r e p l a c e e a c h o t h e r ; he a l s o p o i n t e d out the a n a l o g y with p o t a s s i u m - a n d a m m o n i u m a l u m . F o u r y e a r s l a t e r , E. M i t s c h e r l i c h o b s e r v e d the s i m i l a r i t y in c r y s t a l f o r m of the p o t a s s i u m a n d a m m o n i u m s a l t s of a r s e n i c and p h o s p h o r i c a c i d . S e v e r a l i n v e s t i g a t o r s a f t e r w a r d s c o n c e r n e d t h e m s e l v e s with the p r o b l e m of i s o m o r p h i c r e p l a c e m e n t of e l e m e n t s in Chem. Geol., 1 (1966) 5--31

11

c r y s t a l s and p r o p o s e d h y p o t h e s e s for an e x p l a n a t i o n of the d i s c o v e r i e s . H o w e v e r , a final s o l u t i o n could only be r e a c h e d a f t e r M. von L a u e ' s d i s c o v e r y of X - r a y d i f f r a c t i o n (for which he r e c e i v e d a Nobel p r i z e , in 1914) and the f u l l e r i n t e r p r e t a t i o n t h e r e o f by W.H. B r a g g and his son W.L. B r a g g , in the y e a r s 1912--1913. T h i s m a d e p o s s i b l e the d e t e r m i n a t i o n of the a t o m i c s t r u c t u r e of s o l i d s u b s t a n c e s . But it took s o m e t i m e b e f o r e g e o c h e m i s t s r e a l i z e d the i m p o r t a n c e of this d i s c o v e r y . When the l a s t e d i t i o n of C l a r k e ' s The Data of Geochenlislry a p p e a r e d in 1924, it c o n t a i n e d n o t h i n g about c r y s t a l - s t r u c t u r e d e t e r m i n a t i o n s . Once m o r e , it was V.M. G o l d s c h m i d t who saw the i m p o r t a n c e of c r y s t a l c h e m i s t r y , and in p a r t i c u l a r , the i m p o r t a n c e of ionic r a d i i for geology, which is so m u c h a s c i e n c e of s o l i d s . T h e e n t r y of any atom or ion into the l a t t i c e s of c r y s t a l s was found to depend upon its s i z e . The p a r t i c l e s d i s s o l v e d in a m a g m a or in an a q u e o u s s o l u t i o n a r e thus s o r t e d by the l a t t i c e s of c r y s t a l l i z i n g m i n e r a l s a c c o r d i n g to t h e i r s i z e s , which can be d e f i n e d with s u f f i c i e n t a c c u r a c y by t h e i r a t o m i c or ionic r a d i i . G o l d s c h m i d t found that s t a n d a r d v a l u e s of t h e s e r a d i i could be d e t e r m i n e d for the v a r i o u s p a r t i c l e s which o c c u r in c r y s t a l s t r u c t u r e s ; the s t a n d a r d s t a t e c h o s e n by G o l d s c h m i d t and l a t e r adopted by o t h e r s c i e n t i s t s b e i n g that of the ions in a l a t t i c e of the s o d i u m - c h l o r i d e type ( G o l d s c h m i d t , 1923, 1924, 1954). In addition to d i s c o v e r i n g the r e l a t i o n s b e t w e e n c r y s t a l s t r u c t u r e and the s i z e s of a t o m s and ions, G o l d s c h m i d t a l s o d e v i s e d a r e s e a r c h plan which led to a m a x i m u m of r e s u l t s in a m i n i m u m of t i m e . T o g e t h e r with his a s s o c i a t e s at the U n i v e r s i t y of Oslo, he d e t e r m i n e d the c r y s t a l s t r u c t u r e of m a n y c o m p o u n d s , in the y e a r s 1922~1926. He t h e r e b y e s t a b l i s h e d the e x t e n s i v e b a s i s on which the g e n e r a l laws which g o v e r n the d i s t r i b u t i o n of e l e m e n t s in c r y s t a l l i n e s u b s t a n c e s had to be founded. The r e s u l t s w e r e laid down in a s e r i e s of nine p a p e r s , u n d e r the c o l l e c t i v e title Geochemische Verteilungsg e s e t z e der E l e m e n l e ( G o l d s c h m i d t , 1923, 1924; G o l d s c h m i d t and T h o m a s s e n , 1924; G o l d s e h m i d t et al., 1925a,b, 1926a,b; G o l d s e h m i d t , 1926b, 1937a); n o t w i t h s t a n d i n g the title, m o s t of the c o n t e n t s of the f i r s t eight of t h e s e p a p e r s c o n c e r n the c r y s t a l s t r u c t u r e s of i n o r g a n i c c o m p o u n d s . T h e n i n t h p a p e r s u m m a r i z e s the i n f o r m a t i o n , then c o l l e c t e d , on the a b u n d a n c e and d i s t r i b u t i o n of i n d i v i d u a l e l e m e n t s . An e x c e l l e n t s u m m a r y of the f i r s t eight a r t i c l e s can be found in G o l d s c h m i d t (1929b). The a p p e a r a n c e , in 1937, of Atomic St)'~wture of Minerals, by W.L. B r a g g , m a y be c o n s i d e r e d as the r e a l i n t r o d u c t i o n of c r y s t a l s t r u c t u r e s to w o r k e r s in the g e o - s e i e n c e s . More r e c e n t p u b l i c a t i o n s in c r y s t a l c h e m i s t r y i n c l u d e Az~iroff and B u e r g e r (1958), B u e r g e r (1942, 1960), Bunn (1961), E v a n s (1963), Gay (1959), H o c a r t and K e r n (1959).

PHYSICO-CHEMICAL PETROLOGY The f i r s t e x p e r i m e n t s in geology w e r e c a r r i e d out by S i r J a m e s Hall, a c o n t e m p o r a r y and c o m p a t r i o t of Hutton. T h e t h e o r e t i c a l c o n s i d e r a t i o n s of the l a t t e r w e r e the d i r e c t i n d u c e m e n t to H a l l ' s e x p e r i m e n t s , but he r e c e i v e d no s u p p o r t f r o m Hutton. " T h e r e a r e s u p e r f i c i a l r e a s o n i n g m e n , who without t r u l y knowing what they see, think they know t h o s e r e g i o n s of the e a r t h which can n e v e r be seen, and who judge of the g r e a t o p e r a t i o n s of the m i n e r a l k i n g d o m f r o m h a v i n g k i n d l e d a f i r e and looked i n t o the b o t t o m of a l i t t l e c r u c i b l e " (Hutton, 1795, p.251). J2

Chen~. G¢~ol., I (1966) F~31

Hall (1812, p.72) r e m a i n e d convinced of the p r o g r e s s which would b e c o m e p o s s i b l e in geology b e c a u s e of the i n c r e a s i n g knowledge of c h e m i s t r y . He continued his e x p e r i m e n t s , however, only a f t e r the death of Hutton. A c c o r d i n g to Hutton's t h e o r y , c r y s t a l l i n e g r a n i t e s and b a s a l t s c r y s t a l l i z e d f r o m a melt. In o r d e r to t e s t this, Hall (1805), in his f i r s t s e r i e s of e x p e r i m e n t s , m e l t e d and cooled b a s a l t s and l a v a s . A f t e r the f i r s t melting, t h e s e s o l i d i f i e d to g l a s e o u s s u b s t a n c e s , but a f t e r a second melting and a g r a d u a l cooling, by slowly extinguishing his coal f i r e , he did, indeed, obtain c r y s t a l line p r o d u c t s . A second s e r i e s of e x p e r i m e n t s dealt with the effects of heating under p r e s s u r e (Hall, 1812). He w i s h e d to t e s t whether high p r e s s u r e could p r e v e n t the d i s i n t e g r a t i o n of l i m e s t o n e when heated, and could make m e l t i n g p o s s i b l e . He did his e x p e r i m e n t in m e l t e d - u p b a r r e l s of r i f l e s , and f r o m l i m e s t o n e powder obtained c r y s t a l l i n e m a r b l e s which could not be d i s t i n g u i s h e d f r o m n a t u r a l m a r b l e s even by stone m a s o n s . Guthrie, who had developed the t h e o r y of eutectic (synchronous c r y s t a l l i z a t i o n ) , was the f i r s t to s u g g e s t that this t h e o r y could be a p p l i e d to the c r y s t a l l i z a t i o n of g r a n i t e (Guthrie, 1884), and T e a l l (1888) i n t e r p r e t e d g r a p h i c g r a n i t e as an eutectic s t r u c t u r e . T h i s study was continued by the Norwegian Vogt (1884, 1888, 1890, 1904, 1908, 1921-1923)o He i n v e s t i g a t e d m e t a l l u r g i c a l s l a g s and c o m p a r e d these with n a t u r a l r o c k s . It was e s t a b l i s h e d in this way that a r e l a t i o n s h i p e x i s t s between the c h e m i c a l c o m p o s i t i o n of a melt and the n a t u r e of the m i n e r a l s which c r y s t a l l i z e out of it. Vogt a l s o s t u d i e d the influence that the addition of v a r i o u s oxides has on the nature of these m i n e r a l s and on the o r d e r of t h e i r s e p a r a t i o n . His works on the f o r m a t i o n of m e l i l i t e , ~[kermanite, gehlenite, wollastonite, p y r o x e n e and olivine a r e w e l l known and f o r m one of the foundation stones of m o d e r n p h y s i c o - c h e m i c a l p e t r o l o g y ( L o e w i n s o n - L e s s i n g , 1954). Up until 1904, when V o g t ' s c l a s s i c work a p p e a r e d , he b a s e d his opinions on the idea of l i q u i d - m a g m a t i c differentiation. L a t e r , however, he changed his view and b e c a m e a p r o t a g o n i s t of the c r y s t a l l i z a t i o n - d i f f e r e n t i a t i o n t h e o r y , with which the name of Bowen is c l o s e l y a s s o c i a t e d . Another v e r y i m p o r t a n t a p p l i c a t i o n of p h y s i c a l c h e m i s t r y to geoc h e m i s t r y and geology, a r o u n d the turn of the century, was the monumental w o r k of J.H. van 't Hoff on the f o r m a t i o n of s a l t m i n e r a l s f r o m e v a p o r a t i n g s e a w a t e r and the m e t a m o r p h i c p r o c e s s e s connected with it. His work was continued, on a m o r e g e n e r a l b a s i s , by Boeke (1915), one of his students. Although c l a s s i c a l t h e r m o d y n a m i c s was a l r e a d y well developed by the m i d d l e of the nineteenth century, through the work of, for e x a m p l e , J . P . Joule, K. Clusius and L o r d Kelvin, it l a s t e d until 1911 when it was i n t r o d u c e d into the e a r t h s c i e n c e s through the famous d o c t o r a t e t h e s i s of, again, V.M. G o l d s c h m i d t , Die Kontaktmetarnorphose irn Kristianiagebiet, p r e s e n t e d at the U n i v e r s i t y of Oslo (at that t i m e s t i l l c a l l e d C h r i s t i a n i a ) . In this work, G o l d s c h m i d t a p p l i e d the phase r u l e to the m i n e r a l changes which a r e induced by contact m e t a m o r p h i s m in s h a l e s , m a r l s , and l i m e s t o n e s . He showed that the changes can be i n t e r p r e t e d in t e r m s of the p r i n c i p l e s of c h e m i c a l e q u i l i b r i u m . Until 1922, G o l d s c h m i d t did much m o r e work on the s u b j e c t of r o c k m e t a m o r p h i s m , and this a l s o s t i m u l a t e d other Scandinavian r e s e a r c h e r s to work on r e l a t e d p r o b l e m s . Another most i m p o r t a n t institute, where the line of p h a s e - e q u i l i b r i u m r e s e a r c h was f u r t h e r developed in p e t r o l o g y , was no doubt the C a r n e g i e Chem. Geol., 1 (1966} 5--31

13

I n s t i t u t i o n of W a s h i n g t o n . In 1910, N.L. Bowen c a m e to that I n s t i t u t i o n , w h e r e he s t a r t e d a p h a s e - e q u i l i b r i u m study of a s i l i c a t e s y s t e m . The r e s u l t s which he o b t a i n e d e n a b l e d him to get a d o c t o r ' s d e g r e e and t h i s s t i m u l a t e d h i m to continue t h i s kind of r e s e a r c h , which he did for forty y e a r s . He followed the s u c c e s s f u l s y s t e m of m a k i n g e x p e r i m e n t a l s t u d i e s in the l a b o r a t o r y f i r s t and then c a r r y i n g t h e s e r e s u l t s into the f i e l d to check t h e m with the r o c k s t h e m s e l v e s , thus e v a l u a t i n g the f a c t o r s that l e a d to f a i l u r e of e q u i l i b r i u m u n d e r n a t u r a l c o n d i t i o n s , t o g e t h e r with the m a g n i t u d e and d i r e c t i o n of t h e i r effects. The e x p e r i m e n t s p r o v i d e d him with a c h e m i c a l b a s i s for h y p o t h e s e s on r o c k g e n e s i s and the field t e s t s helped to modify the h y p o t h e s e s to give a c l o s e r a p p r o a c h to the m e c h a n i s m of r o c k g e n e s i s . As such he has given a c l a s s i c a l e x a m p l e of f u n d a m e n t a l g e o - s e i e n t i f i c work. T h e book which he p u b l i s h e d in 1928, The E~,olution qf the Igneous Rocks, is s t i l l the b e s t i n t r o d u c t o r y l i t e r a t u r e to those i n t e r e s t e d in the o r i g i n of the i g n e o u s r o c k s . In addition to Bowen, a l s o s c i e n t i s t s like F. S e h a i r e r , O.F. T u r t l e and H.S. Yoder have c o n t r i b u t e d to the r e p u t a t i o n of the C a r n e g i e I n s t i t u t i o n as a leading geochemical r e s e a r c h center. In o r d e r to c l a s s i f y and group the g r e a t v a r i e t y of m e t a m o r p h i c r o c k s s y s t e m a t i c a l l y , E s k o l a (1915, 1920), following a study of the s a t u r a t i o n d i a g r a m s of Van ' t Hoff, i n t r o d u c e d the concept of m i n e r a l f a c i e s or m e t a m o r p h i c f a c i e s . In a p a r t i c u l a r f a c i e s , he i n c l u d e s all r o c k s which exhibit a unique and c h a r a c t e r i s t i c c o r r e l a t i o n b e t w e e n chemi:: al and m i n e r a l o g i c a l c o m p o s i t i o n , in such a way that r o c k s of a g i v e n c h e m i c a l c o m p o s i t i o n a l w a y s have the s a m e m i n e r a l o g i c a l c o m p o s i t i o n , and d i f f e r e n c e s in c h e m ical c o m p o s i t i o n f r o m r o c k to r o c k a r e r e f l e c t e d in s y s t e m a t i c d i f f e r e n c e s in t h e i r m i n e r a l o g i c a l c o m p o s i t i o n (of. W i n k l e r , 1965, p.16). T h e concept b e c a m e widely known t h r o u g h B a r t h et al. (1939). H e r e E s k o l a s t a t e s that the s i g n i f i c a n c e of this p r i n c i p l e is b a s e d on the o b s e r v a t i o n that the m i n e r a l p a r a g e n e s i s of m e t a m o r p h i c r o c k s in m a n y c a s e s c o n f o r m s to the laws of c h e m i c a l e q u i l i b r i u m . N u m e r o u s i n t e r n a t i o n a l d i s c u s s i o n s and e l a b o r a t i o n s w e r e the r e s u l t , and the concept w a s e n l a r g e d e s p e c i a l l y by F . J . T u r n e r . Since E s k o l a defined his five o r i g i n a l f a c i e s ( s a n i d i n e , h o r n f e l s , g r e e n s c h i s t , a m p h i b o l i t e and eclogite), a d i v i s i o n into s u b f a c i e s has in s o m e c a s e s b e e n found to be n e c e s s a r y , while s e v e r a l a d d i t i o n a l f a c i e s have a l s o b e e n p r o posed. The c o n t r o l of the m i n e r a l o g i c a l c o m p o s i t i o n of r o c k s by t h e i r c h e m ical c o m p o s i t i o n could be i l l u s t r a t e d by m e a n s of t r i a n g u l a r c o m p o s i t i o n d i a g r a m s . The a p i c e s of the d i a g r a m r e p r e s e n t the t h r e e c o m p o n e n t s that m a i n l y a c c o u n t for the o b s e r v e d m i n e r a l o g i c a l v a r i a t i o n w i t h i n the f a c i e s . T h e s e c o m p o n e n t s , as u s e d by E s k o l a , a r e A1203, CaO, and (Mg,Fe)O, and he n a m e d the d i a g r a m s t h e m s e l v e s as t r i a n g u l a r A C F d i a g r a m s . The w o r k of W. N e r n s t in G e r m a n y and of G.N. L e w i s in the U.S.A. gave us the concept of "the m i n i m u m of f r e e e n e r g y " , which d e f i n e s the t r e n d of all p h y s i e o - e h e m i e a l c h a n g e s in n a t u r e , and thus, as e m p h a s i z e d e s p e c i a l l y by A.E. F e r s m a n , g o v e r n s the f o r m a t i o n and s t a b i l i t y of m i n e r a l s and r o c k s ( G o l d s e h m i d t , 1954). Bowen and his a s s o c i a t e s g e n e r a l l y did not i n c l u d e w a t e r as a c o m p o nent in t h e i r e x p e r i m e n t s . In l a t e r y e a r s , the o p i n i o n has g a i n e d g r o u n d that a b e t t e r i n s i g h t can be o b t a i n e d into the p r o c e s s e s which have b e e n p o s s i b l e in the e a r t h , if e x p e r i m e n t s a r e m a d e with n a t u r a l m a t e r i a l s , i n c l u d i n g w a t e r W y a t t and S a b a t i e r (1959) used, a p a r t f r o m s y n t h e t i c m a t e r i a l s , c l a y i s h s e d i m e n t s , which they h e a t e d to 800°C and a w a t e r p r e s s u r e of 1800 a i m . 14

('hem. (;col.. 1 (1966) 5-31

T h e y found that m e l t s of g r a n i t i c c o m p o s i t i o n o r i g i n a t e d in e q u i l i b r i u m with c r y s t a l s of ( F e , M g ) m i n e r a l s . E x t e n s i v e e x p e r i m e n t s have a l s o b e e n c a r r i e d out by W i n k l e r (1957) and W i n k l e r and Von P l a t e n (1958, 1960, 1961a,b).

ISOTOPE "GEOLOGY" It w a s R u t h e r f o r d and Soddy (1902), who t h r o u g h a s t u d y of t h o r i u m , d e v e l o p e d the d i s i n t e g r a t i o n t h e o r y of r a d i o a c t i v e e l e m e n t s . A m o n g the f i r s t to c o n t r i b u t e to an e x p e r i m e n t a l p r o o f of t h i s t h e o r y w a s B o l t w o o d (1905a,b, 1907). He d e t e r m i n e d the a m o u n t s of u r a n i u m and l e a d p r e s e n t in p r i m a r y m i n e r a l s f r o m d i f f e r e n t g e o l o g i c a l f o r m a t i o n s and thus of d i f f e r e n t geolog-i c a l a g e s , and found that the a m o u n t of l e a d p e r g r a m of u r a n i u m i n c r e a s e d with i n c r e a s i n g age of the f o r m a t i o n f r o m w h i c h the a n a l y z e d m i n e r a l s w e r e c o l l e c t e d . Since he b e l i e v e d f i r m l y in the d i s i n t e g r a t i o n t h e o r y and s i n c e he h a s o b s e r v e d that l e a d i s p r e s e n t in a l l r a d i o a c t i v e m i n e r a l s , B o l t w o o d c a m e to the c o n c l u s i o n that a l l of the l e a d m i g h t be the final d i s i n t e g r a t i o n p r o d u c t of u r a n i u m . S t r u t t , l a t e r L o r d R a y l e i g h , found that the a m o u n t of r a d i u m p r e s e n t in a v a r i e t y of m i n e r a l s w a s p r o p o r t i o n a l to the a m o u n t of u r a n i u m and a l s o that h e l i u m w a s p r e s e n t only in u r a n i u m and t h o r i u m - b e a r i n g m i n e r a l s (Strutt, 1905). S o m e y e a r s l a t e r , he o b s e r v e d that the h e l i u m / u r a n i u m r a t i o i s a l s o r e l a t e d to g e o l o g i c a l age (Strutt, 1908, 1909a,b, 1910), but he had to note that w h i l e high v a l u e s w e r e not found in g e o l o g i c a l l y young d e p o s i t s , low v a l u e s w e r e found in old f o r m a t i o n s , i n d i c a t i n g l o s s of h e l i u m . H o l m e s (1913) u s e d the l e a d / u r a n i u m r a t i o f o r m e a s u r i n g g e o l o g i c a l age, by a s s u m i n g a d e c a y r a t e of 1 m o l e of l e a d r e p l a c i n g 1 m o l e of u r a n i u m in 8,200 m i l l i o n y e a r s . In the m e a n t i m e , J o l y had begun d e t e r m i n i n g g e o l o g i c a l a g e s by i n v e s t i g a t i n g the p l e o c h r o i c h a l o s which s u r r o u n d r a d i o a c t i v e m i n e r a l s in m i c a s ( J o l y , 1907, 1922, 1923; J o l y and R u t h e r f o r d , 1913). T h e s e h a l o s had a l r e a d y b e e n d i s c o v e r e d in 1873, but until J o l y (1907) e x p l a i n e d t h e i r o r i g i n a s b e i n g due to r a d i o a c t i v i t y , t h e i r n a t u r e and o r i g i n w e r e the s u b j e c t of much speculation. In an a t t e m p t to connect r a d i u m and u r a n i u m , B o l t w o o d (1907) d i s c o v e r e d i o n i u m . He found that the l a t t e r w a s c h e m i c a l l y i n s e p a r a b l e f r o m t h o r i u m , d e s p i t e the o b v i o u s d i f f e r e n c e b e t w e e n t h e i r a t o m i c w e i g h t s ( t h o r i u m 232, i o n i u m 230). T h i s l e d to the c o n c e p t of i s o t o p e s , i n d i v i d u a l m e m b e r s of a s i n g l e c h e m i c a l t y p e . T h e n a m e w a s s u g g e s t e d by Soddy (1912), to e x p r e s s that t h e s e m e m b e r s o c c u p y the s a m e ( i s o s ) p l a c e (topos) in the p e r i o d i c classification. T h e next m a j o r a n a l y t i c a l a d v a n c e in i s o t o p e r e s e a r c h w a s m a d e by A s t o n (1927), who f o r the f i r s t t i m e , u s e d the m a s s s p e c t r o g r a p h to s t u d y the i s o t o p i c c o m p o s i t i o n of l e a d . T h e n a t u r a l r a d i o a c t i v i t y of p o t a s s i u m and r u b i d i u m had a l r e a d y b e e n d i s c o v e r e d in 1905, but not until the l a s t few d e c a d e s w e r e m e t h o d s of d a t i n g b a s e d on it. L i b b y and a s s o c i a t e s p r o v e d , in 1947, that the r a d i o a c t i v i t y of 14 C c o u l d be d e t e c t e d and m i g h t a l s o be u s e d a s a m e t h o d of d a t i n g . In r e c e n t y e a r s the e x p a n s i o n of i s o t o p e g e o l o g y , a l s o known a s n u c l e a r geology, nuclide geology or nuclear geochemistry, has been enormous. Its a p p l i c a t i o n s now c o v e r a wide r a n g e . I s o t o p e s t u d i e s help in s o l v i n g p r o b l e m s of the m e c h a n i s m and p r o d u c t s of g e o l o g i c a l and g e o c h e m i c a l p r o c e s s e s . Chem. Geol., 1 0966) ~r-31

15

R a d i o m e t r i c dating of r o c k s and m i n e r a l s h a s b e c o m e a s t a n d a r d p r o c e d u r e . Much i m p o r t a n t i n f o r m a t i o n has b e e n o b t a i n e d , t h r o u g h i s o t o p e s t u d i e s , on the age and the mode of f o r m a t i o n of m e t e o r i t e s and about the c o n d i t i o n s of t h e i r h a b i t a t s d u r i n g t h e i r m a k i n g . Oxygen i s o t o p e s a r e u s e d in p a l a e o t e m p e r a t u r e a n a l y s i s . S t u d i e s of s t a b l e i s o t o p e s a r e a l s o of p r a c t i c a l i m p o r t a n c e , f o r e x a m p l e , the r a t i o of s u l p h u r i s o t o p e s m a y help to u n d e r s t a n d the g e n e s i s of s o m e s u l p h i d e o r e d e p o s i t s . A r t i f i c i a l l y p r o d u c e d r a d i o n u c l i d e s a r e inc r e a s i n g l y a p p l i e d to the d i f f e r e n t b r a n c h e s of g e o l o g y and to the r e l a t e d p u r e and e n g i n e e r i n g s c i e n c e s . F o r f u r t h e r d e t a i l s r e f e r e n c e is m a d e to iRankama (1954, 1963). GEOCHEMICAL STI{UCTU[{E OF THE E A R T H A f t e r A. B o i s s e , in 1850, had s u g g e s t e d for the f i r s t t i m e that m e t e o r i t e s m i g h t g i v e i n f o r m a t i o n about the b u l k c o m p o s i t i o n of the e a r t h , s e v e r a l h y p o t h e s e s about the c h e m i c a l c o m p o s i t i o n and s t r u c t u r e of the e a r t h w e r e p u b l i s h e d , of which a few s h o u l d be m e n t i o n e d in t h i s s u r v e y S u e s s (1885-1909), following e a r l i e r w o r k of the F r e n c h m a n A. D a u b r 6 e , i n t r o d u c e d the d i s t i n c t i o n b e t w e e n nife (the n i c k e l - i r o n c o r e of s i m i l a r c o m p o s i t i o n a s the i r o n m e t e o r i t e s ) , s i m a (a z o n e of d a r k and h e a v y r o c k s , c h e m i c a l l y r i c h in s i l i c a , with m a g n e s i a in s e c o n d p l a c e ) and s i a l (the o u t e r zone of the e a r t h , f o r m i n g the c o n t i n e n t a l c r u s t to a depth of s e v e r a l k i l o m e t e r s , a n d c o n s i s t i n g of light r o c k s v e r y r i c h in s i l i c a with a l u m i n a a s the m o s t a b u n dant of the r e m a i n i n g c h e m i c a l c o n s t i t u e n t s ) . W a s h i n g t o n (1925) a s s u m e d the e a r t h to c o n s i s t of a c e n t r a l c o r e e n v e l o p e d by five s h e l l s of d i f f e r e n t c o m p o s i t i o n . T h e c o r e s h o u l d c o n s i s t of m e t a l l i c n i c k e l - i r o n , c o m p a r a b l e to the m e t a l p h a s e of the m e t e o r i t i c i r o n s . T h e c o r e m e r g e s on i t s o u t s i d e into the l i t h o s p o r i c s h e l l (700 km) with s c a t t e r e d s e g r e g a t i o n s of f e r r o m a g n e s i a n o r t h o s i l i c a t e , w h i c h d e c r e a s e in n u m b e r t o w a r d s the next, the f e r r o s p o r i c s h e l l (700 kin), w h i c h c o r r e s p o n d s to the c h o n d r i t i c stony m e t e o r i t e s . T h e p e r i d o t i t i c s h e l l (1,540 km) w o u l d be c o m p a r a b l e in c o m p o s i t i o n to the a c h o n d r i t i c stony m e t e o r i t e s . T h e two o u t e r m o s t s h e l l s a r e the b a s a l t i c (40 km) and the g r a n i t i c (20 km) shells. G o l d s c h m i d t (1922a,b, 1933b) d i s t i n g u i s h e d only t h r e e s h e l l s . F i r s t , a r o u n d the n i c k e l - i r o n c o r e o r s i d e r o s p h e r e , a s u l p h i d e - o x i d e s h e l l (1,700 km), next an e c l o g i t e s h e l l o r c h a l c o s p h e r e (1,080 km) c o n s i s t i n g of c o m p r e s s e d s i l i c a t e s , and on the o u t s i d e , the 120 k m t h i c k s i l i c a t e c r u s t o r l i t h o s p h e r e . G o l d s c h m i d t c o m p a r e d the e a r t h both with m e t e o r i t e s and with the p r o d u c t s of an o r e - m e l t i n g f u r n a c e . Kuhn and R i t t m a n n (1941) p r o p o s e d a c o m p l e t e l y d i f f e r e n t h y p o t h e s i s . T h e y a s s u m e d that the e a r t h o r i g i n a t e d a s a hot g a s e o u s globe c o m p o s e d of u n d i f f e r e n t i a t e d s o l a r m a t e r i a l . Cooling, t h r o u g h h e a t r a d i a t i o n and e s c a p e of h y d r o g e n and h e l i u m f r o m the e a r t h ' s g r a v i t a t i o n a l f i e l d , c a u s e d an i n c r e a s e in s p e c i f i c weight of the o u t e r p a r t s and c o n v e c t i o n c u r r e n t s e s t a b l i s h e d t h e m s e l v e s , c a r r y i n g h e a v y m a t e r i a l down and b r i n g i n g light m a t e r i a l to the s u r f a c e . H o w e v e r , the c o n v e c t i o n c u r r e n t s did not r e a c h the e a r t h ' s c o r e and t h i s s h o u l d s t i l l c o n s i s t of u n d i f f e r e n t i a t e d s o l a r m a t e r i a l . T h e d i s c o n t i n u i t y , which s e i s m i c r e s e a r c h h a d r e v e a l e d to e x i s t at a depth of 2,900 k m w a s thought not to r e p r e s e n t a change in c h e m i c a l c o m p o s i t i o n l(i

Chem. Geol., I (1966) .r-31

but a s u d d e n change in p h y s i c a l p r o p e r t i e s a s a r e s u l t of a s u d d e n d e c r e a s e of v i s c o s i t y b e c a u s e of r e d u c t i o n of s i l i c a by h y d r o g e n w h e r e the r e a c t i o n of SiO 2 + 2H 2 ~ Si (in solution) + 2 H 2 0 t a k e s p l a c e with i n c r e a s i n g t e m p e r a t u r e . D e s p i t e , o r p r o b a b l y b e c a u s e of, i t s r e v o l u t i o n a r y n a t u r e t h i s h y p o t h e s i s h a s r e c e i v e d c o m p a r a t i v e l y l i t t l e a t t e n t i o n . "In s p i t e of i t s i n g e n ious features, Kuhn-Rittmann's earth model involves such striking mistakes that, in o u r opinion, it i s u n n e c e s s a r y to r e f u t e it." ( E g y e d , 1957): T h e l a t t e r a u t h o r t h i n k s it l i k e l y , that the d i f f e r e n t i a t i o n of the e a r t h i s r e s t r i c t e d to a zone h a v i n g a m a x i m u m depth of 900 kin; and that a l l the r e s t of the e a r t h is of h o m o g e n e o u s c o m p o s i t i o n , with the g e o p h y s i c a l d i s c o n t i n u i t i e s b e i n g p h a s e boundaries. In r e c e n t y e a r s a t t e m p t s a r e b e i n g m a d e to c o l l e c t m o r e d i r e c t i n f o r m a t i o n about the i n t e r i o r of the e a r t h , in p a r t i c u l a r about the o u t e r m o s t 1,000 k m of the e a r t h ' s r a d i u s , that p a r t of the e a r t h w h o s e p r o c e s s e s m o s t i n f l u e n c e the d e v e l o p m e n t of the e a r t h ' s c r u s t . V.V. B e l o u s s o v , p r e s i d e n t of the I n t e r n a t i o n a l Union of G e o d e s y and G e o p h y s i c s , p r o p o s e d the e x e c u t i o n of an i n t e r n a t i o n a l s c i e n t i f i c p r o g r a m m e , the U p p e r M a n t l e P r o j e c t . T h e p e r i o d 1960-1963 w a s d e v o t e d p r i m a r i l y to the p l a n n i n g and o r g a n i z a t i o n t h e r e o f . At the I.U.G.G. m e e t i n g in B e r k e l e y , C a l i f . , in A u g u s t 1963, an Int e r n a t i o n a l U p p e r M a n t l e C o m m i t t e e (U.M.C.) w a s c o n s t i t u t e d that could " . . . e n c o u r a g e and d e v e l o p i n t e r n a t i o n a l c o o p e r a t i v e i n v e s t i g a t i o n s of the Upper m a n t l e , i t s r e l a t i o n s h i p s to and i n f l u e n c e upon the d e v e l o p m e n t of the e a r t h ' s c r u s t " . At i t s f i r s t m a j o r m e e t i n g , in May 1964, the U.M.C. r e s o l v e d t h a t " . . , a l l n a t i o n a l c o m m i t t e e s a r e u r g e d to c o n s i d e r the p e r i o d f r o m 1 J a n u a r y 1965 to 31 D e c e m b e r 1967 a s P h a s e II of the I n t e r n a t i o n a l U p p e r M a n t l e P r o j e c t , a p e r i o d d u r i n g which d e f i n i t e a c c o m p l i s h m e n t s s h o u l d be a c h i e v e d in the m a i n i n t e r n a t i o n a l and i n t e r d i s c i p l i n a r y p r o g r a m s e m p h a s i z e d by the U p p e r M a n t l e C o m m i t t e e " . D e e p - d r i l l i n g p r o g r a m m e s a r e c a r r i e d out p r i n c i p a l l y by the U.S.S.R. and by the U.S.A. (Mohole P r o j e c t ) . Although in both c o u n t r i e s u n f o r e s e e n d i f f i c u l t i e s of f i n a n c i a l , t e c h n i c a l and o t h e r n a t u r e d e l a y e d the o r i g i n a l p l a n s , p r o g r e s s is b e i n g m a d e , with the U.S.S.R., at is s e e m s now, in the l e a d . A l s o C a n a d a , China, G e r m a n y , J a p a n and South A f r i c a i n c o r p o r a t e d p l a n s f o r d e e p d r i l l i n g in t h e i r p r o p o s e d p r o g r a m m e s f o r the U p p e r M a n t l e P r o j e c t . D e e p h o l e s have two m a j o r s c i e n t i f i c a s p e c t s : f i r s t , the m a t e r i a l s f r o m o t h e r w i s e i n a c c e s s i b l e d e p t h s b e c o m e a v a i l a b l e f o r study by the p o w e r f u l m e t h o d s of l a b o r a t o r y p h y s i c s and c h e m i s t r y ; s e c o n d , the hole i t s e l f p r o v i d e s a m e a n s f o r i n t r o d u c i n g i n s t r u m e n t s f o r the o b s e r v a t i o n of p h y s i c a l p r o p e r t i e s in p l a c e .

CHEMICAL OCEANOGRAPHY T h e p r e s e n c e of c o m m o n s a l t in s o l u t i o n in s e a w a t e r w a s a l r e a d y known f r o m v e r y a n c i e n t t i m e s , a s the C h i n e s e s o m e t i m e p r i o r to 2200 B.C. e x t r a c t e d it f r o m s e a w a t e r . T h e d i s c o v e r y of o t h e r e l e m e n t s in s e a w a t e r is of m u c h m o r e r e c e n t date. T h u s , b r o m i n e w a s f i r s t noted, a r o u n d the y e a r 1825, by the F r e n c h m a n A . J . B a l a r d , in b i t t e r n s o b t a i n e d a f t e r the s a l t h a d b e e n p r e c i p i t a t e d f r o m the w a t e r of s a l t m a r s h e s n e a r M o n t p e l l i e r . M o d e r n c h e m i c a l o c e a n o g r a p h y m a y be c o n s i d e r e d to have s t a r t e d with the w o r k on the E n g l i s h p h y s i c i a n A. M a r c e t , in 1819, the D a n i s h c h e m i s t Chem. Geol., 1 (1966) .5--31

17

(3. F o r c h h a m m e r , in 1865, and the G e r m a n - b o r n c h e m i s t W. D i t t m a r , who w o r k e d with 77 w a t e r s a m p l e s c o l l e c t e d by the f a m o u s B r i t i s h d e e p - s e a e x p e d i t i o n with the " C h a l l e n g e r " (1872-1876). F o r c h h a m m e r w o r k e d for 20 y e a r s to c o m p l e t e the a n a l y s i s of 200 s a m p l e s of w a t e r c o l l e c t e d for him by f r i e n d s , ship c a p t a i n s and naval o f f i c e r s . T h e a n a l y s e s of D i t t m a r took 9 y e a r s to be c o m p l e t e d . He d e t e r m i n e d the p e r c e n t a g e c o m p o s i t i o n of the d i s s o l v e d s o l i d s ( c h l o r i d e , sulphate, s o d i u m , c a l c i u m , m a g n e s i u m , a n d p o t a s s i u m ) in each of the s a m p l e s . His r e s u l t s s u p p o r t e d the f i n d i n g s of his lwo p r e d e c e s s o r s that t h e s e six c o n s t i t u e n t s make up as m u c h as 99~L of the d i s s o l v e d s a l t s in s e a w a t e r and that the r a t i o of any one of the m a j o r c o n s t i t u e n t s to the total d i s s o l v e d s o l i d s is n e a r l y c o n s t a n t in the sea, even though the total c o n c e n i r a i i o n of d i s s o l v e d solids may v a r y from place lo plac(?. Since then w o r k e r s f r o m all p a r t s of the w o r l d have c o n t i n u e d the study of sea w a t e r , but b e c a u s e of the size and c o m p l e x i t y of the d y n a m i c ocean s y s t e m , no exact, u n i v e r s a l d e s c r i p t i o n of o c e a n w a t e r can be giwm and it is doubtful w h e t h e r this will e v e r by the c a s e . Knowledge of the m i n e r a l o g i c a l and c h e m i c a l c o m p o s i t i o n of the s e a f l o o r d e p o s i t s s t a r t e d in 1851, with the c o l l e c t i o n of s a m p l e s by the U.S.Coast S u r v e y off the A t l a n t i c coast of the U.S.A., with which the n a m e of J.W. B a i l e y is c o n n e c t e d . F o r the d e e p - s e a d e p o s i t s the " C h a l l e n g e r " expedition, and p a r t i c u l a r l y iis geologist J. M u r r a y , again did p i o n e e r i n g work.

GEOCHEMISTRY OF SEI)IMENTS T h e o r i g i n a l c o n s t i t u e n t s of the e a r t h ' s c r u s t , the i g n e o u s r o c k s and t h e i r m i n e r a l s , a r e to a l a r g e extent u n s t a b l e as r e g a r d s the a t m o s p h e r e and h y d r o s p h e r e . The i n t e r a c t i o n of the l a t t e r two on the l i t h o s p h e r e l e a d s to s e d i m e n t a t i o n , i n c l u d i n g such a s p e c t s as w e a t h e r i n g , e r o s i o n , deposition, a n d d i a g e n e s i s . The c e n t r a l p r o b l e m in the g e o c h e m i c a l side of s e d i m e n t a tion is, t h e r e f o r e , the c h e m i c a l b r e a k d o w n of s o m e m i n e r a l s and the f o r m a t i o n of o t h e r s . A l s o at the o r i g i n of t h i s b r a n c h of g e o c h e m i s t r y , one will e n c o u n t e r the n a m e of the g e n i u s V.M. G o l d s c h m i d t . The u n d e r s t a n d i n g of the d i s t r i b u t i o n of e l e m e n t s in the s e d i m e n t a r y s e q u e n c e was g r e a t l y f a c i l i t a t e d by the i n t r o d u c t i o n of the concept of " i o n i c p o t e n t i a l " . T h i s is the q u o t i e n t of ionic c h a r g e and r a d i u s . It was G o l d s e h m i d t who f i r s t p o i n t e d out that t h i s ionic p o t e n t i a l p r o v i d e s a m e a s u r e for the b e h a v i o u r of an ion in w a t e r : the h y d r a t i o n of an ion b e i n g p r o p o r t i o n a l to its c h a r g e and i n v e r s e l y p r o p o r t i o n a l to its r a d i u s . The fixation of ions by c r y s t a l l i n e clay m i n e r a l s is g o v e r n e d to a l a r g e extent in both s o i l s and s e d i m e n t s by the i n t e r a c t i o n of p o l a r i z a t i o n f o r c e s a r i s i n g f r o m the e l e c t r i c a l f i e l d s a s s o c i a t e d with the s u r f a c e s of the clay p a r t i c l e s and m o r e or l e s s p o l a r i z a b l e ions or ion h y d r a t e s (of. G o l d s e h m i d t , 1954). As shown in the n i n e t e e n t h i r t i e s , by Noll (1930, 1931, 1932, 1934) and by J e n n y (1932a,b, 1941), not only is the size of the ions i n v o l v e d in ion exchange and a d s o r p t i o n on clay p a r t i c l e s i m p o r t a n t , but a l s o t h e i r p o l a r i z a b i l i t y . S e d i m e n t a r y g e o c h e m i c a l p r o c e s s e s take place at low and i n t e r m e d i a t e t e m p e r a t u r e s and a r e often c o n n e c t e d with r e a c t i o n s in a q u e o u s s o l u t i o n s . G o l d s c h m i d t (1928) r e a l i z e d , t h e r e f o r e , that the study of the d i e l e c t r i c p r o p e r t i e s of w a t e r is of g r e a t i m p o r t a n c e , b e c a u s e m o r e or l e s s o r i e n t e d l a y e r s of p o l a r i z e d w a t e r m u s t be f o r m e d on the s u r f a c e of the l a y e r l a t t i c e of clay In

Chem. Geol., 1 (1966) ?r-:~l

m i n e r a l s due to t h e i r e l e c t r i c a l f i e l d s and t h o s e of t h e i r a d s o r b e d i o n s . The c o m b i n a t i o n of l a y e r l a t t i c e s and a dipole l i q u i d is the c a u s e of the p l a s t i c i t y and b o n d i n g p r o p e r t i e s of clay a g g r e g a t e s , and t h e s e p r o p e r t i e s a r e , t h e r e f o r e , c l o s e l y d e p e n d e n t on all k i n d s of ion e x c h a n g e ( G o l d s c h m i d t , 1954). In the e a r l y stage of s e d i m e n t a r y g e o c h e m i c a l work, it w a s b e l i e v e d that h y d r o l y s i s w a s the m o s t i m p o r t a n t p r o c e s s in s e d i m e n t d e c o m p o s i t i o n . It was C o r r e n s and Von E n g e l h a r d t (1939), who d e m o n s t r a t e d that both acid and a l k a l i n e s o l u t i o n s p o s s e s s a m o r e s t r o n g l y d e c o m p o s i n g a c t i o n and a r e t h e r e f o r e m o r e i m p o r t a n t a g e n t s in w e a t h e r i n g . T h e m a n y i n v e s t i g a t i o n s into r a r e e l e m e n t s , in the p e r i o d of the n i n e t e e n t w e n t i e s and the f i r s t half of the t h i r t i e s , a l s o led to such a n a l y t i c a l s t u d i e s of s e d i m e n t s . G o l d s c h m i d t and P e t e r s (1931a,b, 1932a,b,c,d, 1933a, b,c, 1934; G o l d s c h m i d t , 1935; G o l d s c h m i d t et al., 1933a,b) p u b l i s h e d s e v e r a l p a p e r s , d u r i n g the G ~ t t i n g e n p e r i o d of the f o r m e r , in which they d e m o n s t r a t e d the p r e s e n c e of m a n y r a r e e l e m e n t s in the a s h of c e r t a i n c o a l s . P r e s u m a b l y this is due to the c o n c e n t r a t i o n of r a r e e l e m e n t s in c e r t a i n p l a n t p r o d u c t s (e.g., f o r e s t l i t t e r ) and in the u p p e r m o s t z o n e s of f o r e s t s o i l s . B a s i c s t u d i e s have b e e n m a d e by B r e n c h l e y (1926; B r e n c h l e y and W a r r i n g t o n , 1927; B r e n c h l e y et al., 1927), B y e r s et al., (1938a,b), M i t c h e l l (1944, 1945, 1948; M i t c h e l l and Scott, 1947), on the d i s t r i b u t i o n of t r a c e e l e m e n t s in s o i l s . T h i s s u b j e c t is of d i r e c t p r a c t i c a l v a l u e b e c a u s e of the n u t r i t i o n a l i m p o r t a n c e of t h e s e e l e m e n t s for p l a n t s , a n i m a l s and m a n , and t h e i r t o x i c i t y when p r e s e n t in e x c e s s i v e a m o u n t s . How health of a n i m a l s a n d m a n is l i n k e d to the m i n e r a l b a l a n c e of the soil is e x p l a i n e d by, a m o n g o t h e r s , V o i s i n (1959). A s u r v e y of the t r a c e - e l e m e n t c o n t e n t s of s o i l s , a r r a n g e d by e l e m e n t s , can be found in Swaine (1955). C o m p l e t e c a l c u l a t i o n s of the t o t a l q u a n t i t y of s e d i m e n t s on e a r t h w e r e p u b l i s h e d by G o l d s c h m i d t (1933a). He o b t a i n e d a v a l u e of about 3.10 s km 3. C l a r k e , in the l a s t e d i t i o n of The Data of Geochemistry (1924), found 84,300,000 cubic m i l e s (approx. 3.5 -108 km 3) to be the h i g h e s t p o s s i b l e v a l u e for the v o l u m e of d e c o m p o s e d i g n e o u s r o c k s . K u e n e n (1941), b a s i n g his c a l c u l a t i o n s on the r a t e of d e p o s i t i o n of s e d i m e n t s on the o c e a n b e d (Kuenen, 1937), a s s u m e d a t o t a l v o l u m e of 13 • 108 km 3. He r e v i s e d this v a l u e l a t e r (Kuenen, 1946) to 7. l 0 s km 3 as the o u t c o m e of g e o c h e m i c a l c a l c u l a t i o n s for all s e d i m e n t a r y m a t e r i a l s . The v o l u m e of c o n t i n e n t a l s e d i m e n t s is about 2 • 108 km 3 and, c o n s e q u e n t l y , the d e e p - s e a d e p o s i t s occupy about 5 • l 0 s km 3. O t h e r c a l c u l a t i o n s gave s l i g h t l y d i f f e r e n t v a l u e s for the l a t t e r , but all a r e of the s a m e o r d e r of m a g n i t u d e . Ronov (1948), using a new volumetric method, found that, in the European part of the U.S.S.R., limestones and dolomites constitute on the average 55% by volume of the total sediments in the Russian platform, whereas in the geosynclines surrounding the Russian platform the carbonate rocks are less abundant (Ural geosyncline 30%, Donetz geosyncline 28%, Great Caucasus Alpine geosyncline 20~0). Fig.2 shows that considerable variation occurs in the percentage of carbonate and clastic rocks of the Russian platform. The diagram indicates a periodic character of the sedimentation, with the beginning of each new cycle of sedimentation being determined by the end of the preceding tectonic cycle and uplift which stimulated the erosion. Similar periodic changes of relative amounts of carbonate and clastic sediments were also observed, by Ronov, in the Russian geosynclines. On the basis of the findings by Ronov and an examination of the stratigraphic columns of various parts of the world, Chilingar (1956a) estimated the abundance of carbonate rocks in the continental sediments at 25% by volume. This is higher than the values obtained by geochemical calculations (Clarke, 1924; Goldschmidt, 1933a; Chem. Geol., 1 (1966) 5-31

19

Kuenen, 1941). The example shows that a mutual checking of data obtained by geochemical and other geo-seientifie methods should be carried out wherever possible. 100

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Fig.2. Variation in percentage of (1) carbonate rocks and of (2) clastic rocks of the Dussian platform. (After l~onov, 1948, p.480.) An i m p o r t a n t s u b j e c t in s e d i m e n t a r y g e o c h e m i s t r y is the f o r m a t i o n of d o l o m i t e . A l r e a d y Daly (1910) d e m o n s t r a t e d that the a b u n d a n c e of d o l o m i t e s d e c l i n e s in y o u n g e r r o c k s . C h i l i n g a r (1956b) showed that s u p e r i m p o s e d on t h i s g e n e r a l d e c l i n e in n u m b e r of d o l o m i t e s (or i n c r e a s e in a v e r a g e C a / M g ratio) in going up the g e o l o g i c a l column, p e r i o d i c f l u c t u a t i o n s o c c u r . A p o s s i b l e e x p l a n a t i o n of this p h e n o m e n o n may be the p e r i o d i c f o r m a t i o n of d o l o m i t e s . The f i r s t stage of the cycle would i n v o l v e a c c u m u l a t i o n of Mg in the o c e a n s and a s i m u l t a n e o u s d e p o s i t i o n of c a l c i t i c l i m e s t o n e s . T h e s e c o n d s t a g e would be the f o r m a t i o n of d o l o m i t e s and d o l o m i t i c l i m e s t o n e s when the Mg c o n t e n t of the o c e a n s r e a c h e s a c e r t a i n l i m i t . The f o r m a t i o n of d o l o m i t e s would e v e n t u a l l y i m p o v e r i s h the s e a w a t e r of Mg and a new cycle, with a c c u m u l a t i o n of Mg, would s t a r t ( C h i l i n g a r , 1956b). O t h e r p o s s i b l e exp l a n a t i o n s for the p e r i o d i c f o r m a t i o n of highly d o l o m i t i c l i m e s t o n e s i n c l u d e v a r i a t i o n s in depth and t e m p e r a t u r e of b a s i n s of d e p o s i t i o n . A b r i e f s u r v e y of the p r e s e n t s t a t u s of knowledge in s e d i m e n t a r y geoc h e m i s t r y can be found in the book by Degens (1965).

ORGANIC GEOCHEMISTRY

C l o s e l y i n t e r r e l a t e d with s e d i m e n t a r y g e o c h e m i s t r y is o r g a n i c g e o c h e m i s t r y of which s o m e i n f o r m a t i o n was given a l r e a d y e a r l i e r , in the s e c t i o n on m i n e r a l and r o c k a n a l y s i s . T h i s is the b r a n c h of g e o c h e m i s t r y which s t u d i e s all n a t u r a l l y o c c u r r i n g c a r b o n a c e o u s s u b s t a n c e s . U n d e r the r e a l m of o r g a n i c g e o c h e m i s t r y f a l l s a l s o the study of t h o s e m i n e r a l s u b s t a n c e s (for e x a m p l e , c a l c i u m and m a g n e s i u m c a r b o n a t e s ) f o r m e d in n a t u r e t h r o u g h b i o l o g i c p r o c e s s e s , or a c c u m u l a t e d as a r e s u l t of the s p e c i a l f u n c t i o n of organic substances. 20

Chem. Geol., ] (1966) 7r-31

O v e r the l a s t 20 y e a r s , i n t e r e s t in o r g a n i c g e o c h e m i s t r y h a s b e e n g r o w i n g f a s t , a s d e m o n s t r a t e d by the i n c r e a s i n g n u m b e r of p u b l i s h e d w o r k s on s u b j e c t s r a n g i n g f r o m l i p i d s in P r e c a m b r i a n r o c k s , to the n a t u r e of k e r o g e n , the o r i g i n and e v o l u t i o n of p e t r o l e u m and c o a l , and to o r g a n i c m a t t e r in c a r b o n a c e o u s c h o n d r i t e s . It i s r i g h t to s a y that o r g a n i c g e o c h e m i s t r y w o u l d not be s u c h an a c t i v e f i e l d t o d a y , if it w e r e not f o r the m a j o r e f f o r t s m a d e by t h o s e g e o c h e m i s t s d e a l i n g p a r t i c u l a r l y with p e t r o l e u m and, to a s m a l l e ~ extent, with coal. S u r v e y s of the r e c e n t p r o g r e s s e s m a d e by o r g a n i c g e o c h e m i s t r y can be found with B r e g e r (1963) and C o l o m b o a n d H o b s o n (1964).

GEOCHEMICAL EXPLORATION F o r a s long a s m a n h a s b e e n u s i n g m e t a l s , he h a s b e e n t r y i n g to find m i n e r a l d e p o s i t s . H i s p r o s p e c t i n g w a s g u i d e d by w h a t he c o u l d o b s e r v e with the n a k e d eye on the s u r f a c e of the e a r t h . G e o c h e m i c a l e x p l o r a t i o n t e c h n i q u e s , h o w e v e r , w e r e not a p p l i e d in e s s e n c e b e f o r e w e l l into the t w e n t i e t h c e n t u r y and m o s t a r e s t i l l in a s t a g e of d e v e l o p m e n t . Of c o u r s e , the a d v a n c e of g e o c h e m i c a l e x p l o r a t i o n h a s b e e n s t r o n g l y d e p e n d e n t upon the d e v e l o p m e n t of analytical methods (spectrography, X-ray fluorescence, chromatography, e t c . ) , e s p e c i a l l y w h e r e t h e s e m a d e a p r o m p t and i n e x p e n s i v e d e t e r m i n a t i o n p o s s i b l e of s m a l l q u a n t i t i e s of e l e m e n t s , a s t h e s e o c c u r in p l a n t a s h e s , s o i l s , s t r e a m s e d i m e n t s , e t c . E s p e c i a l l y in the S o v i e t Union h a s g e o c h e m i c a l w o r k b e e n p a r t i c u l a r l y d i r e c t e d t o w a r d s t h e s e a r c h for a n d e x p l o i t a t i o n of m i n e r a l r a w m a t e r i a l s , and with g r e a t s u c c e s s .

Biogeochernical prospecting A s e a r l y a s m e d i e v a l t i m e s m i n e r a l p r o s p e c t o r s w e r e g u i d e d by c e r t a i n k i n d s of v e g e t a t i o n to find c o p p e r and o t h e r p y r i t e o r e s . In S c a n d i n a v i a , the o c c u r r e n c e of " p y r i t e p l a n t s " ( k i s p l a n t ) , w a s u s e d in p r o s p e c t i n g f o r c o p p e r , in the A l t a i , U.S.S.R., " c o p p e r g r a s s " (Gysophila patrinii) w a s known, a s w a s a z i n c f l o r a in B e l g i u m and in the Rhine a r e a . T h i s z i n c f l o r a c o m p r i s e s the g a l m e i v i o l e t Viola lutea v a r . calaminaria and penny c r e s s ( T h l a s p i calarninare). It h a d a l s o b e e n n o t e d that E u r o p e a n holly h a s an a f f i n i t y to r o c k s r i c h in a l u m i n i u m ( M a l a y u g a , 1963). T h e R u s s i a n s c i e n t i s t M.V. b o m o n o s o v r e p o r t e d in 1763 that v e g e t a t i o n g r o w i n g o v e r o r e s o r o t h e r m i n e r a l d e p o s i t s w a s p o o r and u n h e a l t h y . T h e c h r o m i t e d e p o s i t s in M a r y l a n d and P e n n s y l v a n i a w e r e d i s c o v e r e d , in 1810, b e c a u s e of the p r e s e n c e of a d e p r e s s e d f l o r a on o r e - b e a r i n g s e r p e n t i n i t e s (Singewald, 1928). B e g i n n i n g with the s e c o n d half of the n i n e t e e n t h c e n t u r y a n d e s p e c i a l l y d u r i n g the p r e s e n t c e n t u r y , m o r e s y s t e m a t i c d a t a on the g e o c h e m i c a l affinity of p l a n t s w e r e c o l l e c t e d . L i n s t o w (1929), who w r o t e the f i r s t m a j o r r e v i e w of the s u b j e c t , c i t e s m a n y e x a m p l e s of r e l a t i o n s h i p s b e t w e e n p l a n t s p e c i e s a n d g e o l o g i c a l s o i l c o n d i t i o n s , a s t h e s e w e r e g a t h e r e d by s c i e n t i s t s in the c o u r s e of a l m o s t a c e n t u r y . T h e f i r s t a t t e m p t to p r o v i d e a s c i e n t i f i c b a s i s f o r t h i s p h e n o m e n o n w a s m a d e by the R u s s i a n b o t a n i s t K a r p i n s k i i (1841). S t a r t i n g in 1936, the S w e d i s h P r o s p e c t i n g Co. h a s f o r s e v e r a l y e a r s c a r r i e d out b i o g e o c h e m i c a l s u r v e y s in c o n j u n c t i o n with i t s r e g u l a r Chem. Geol., 1 (1966) 5--31

21

g e o p h y s i c a l s e r v i c e s to m i n i n g c o m p a n i e s . D u r i n g the u r a n i u m b o o m (194&1956) the U . S . G e o l o g i c a l S u r v e y c o n d u c t e d a s u b s t a n t i a l v o l u m e of b i o g e o c h e m i c a l p r o s p e c t i n g w o r k in the w e s t e r n U.S.A. (Hawkes and Webb, 1962). In C a n a d a , p a r t i c u l a r l y W a r r e n and c o - w o r k e r s have done i m p o r t a n t w o r k in b i o g e o c h e m i c a l p r o s p e c t i n g (e.g., D e l a v a u l t and W a r r e n , 1950, 1955; H o w a t s o n and W a r r e n , 1947).

DgaDzaLre s/t r l ' e v s In his Pirotech~zia, f i r s t p u b l i s h e d in 1540, V a n n o c c i o B i r i n g u c c i o s a y s : " T h e r e a r e s o m e who p r a i s e highly a s a good s i g n c e r t a i n r e s i d u e s that w a t e r s m a k e w h e r e they a r e s t i l l , and a f t e r having s t o o d [or s e v e r a l d a y s , f r e q u e n t l y w a r m e d by the r a y s of t h e sun, t h e y show in s o m e p a r t s of t h e i r r e s i d u e s v a r i o u s t i n c t u r e s of m e t a l l i c s u b s t a n c e s . T h e r e a r e o t h e r s who u s u a l l y t a k e t h i s w a t e r and c a u s e it to ex/aporate o r d r y up e n t i r e l y by b o i l i n g it in a v e s s e l of e a r t h e n w a r e , g l a s s , o r s o m e o t h e r m a t e r i a l , and they t e s t the g r o s s e a r t h y s u b s t a n c e that r e m a i n s at the b o t t o m by t e s t i n g , by the o r d i n a r y f i r e a s s a y , o r in s o m e o t h e r way that p l e a s e s t h e m . In t h i s way (although they do not have an e x a c t proof) t h e y a p p r o a c h s o m e s o r t of knowl e d g e of the thing." (quoted f r o m H a w k e s and Webb, 1962, p.5). T h i s s e e m s to be the f i r s t w r i t t e n a c c o u n t of the p o s s i b l e a p p l i c a t i o n of c h e m i c a l t e s t s of s t r e a m w a t e r to p r o s p e c t i n g . T h e R u s s i a n s , a p p a r e n t l y , w e r e the f i r s t , in the e a r l y n i n e t e e n t h i r t i e s , to s e e the i m p o r t a n c e of c h e m i c a l a n a l y s i s of s t r e a m s e d i m e n t . T h e y u s e d s p e c t r o g r a p h i c t e c h n i q u e s to d e t e r m i n e the tin content of the fine f r a c t i o n of s t r e a m s e d i m e n t s a s a m e t h o d of finding b e d r o c k d e p o s i t s of c a s s i t e r i t e (Hawkes and Webb, 1962). T h e p i o n e e r i n g study in the w e s t e r n w o r l d was a l a r g e d r a i n a g e s u r v e y in e a s t e r n C a n a d a , in 1954, in w h i c h the r e a d i l y s o l uble h e a v y - m e t a l content w a s d e t e r m i n e d f r o m s a m p l e s of s t r e a m s e d i m e n t s c o l l e c t e d o v e r an a r e a of a p p r o x . 70,000 k m 2 ( H a w k e s et a l . , 1956).

Metallo~)~etric o~" soil surueyin,~ In 1932, S o v i e t g e o l o g i s t s m a d e the f i r s t c h e m i c a l a n a l y s i s of s y s t e m a t i c a l l y c o l l e c t e d s a m p l e s of s o i l and w e a t h e r e d r o c k , to s e e what i n d i c a t i o n s t h e s e c o u l d give in m i n e r a l e x p l o r a t i o n (cf. F e r s m a n , 1939; S e r g e y e v , 1941). T h e f i r s t r e s u l t s w e r e p r o m i s i n g , and t h r e e y e a r s l a t e r the g e o l o g i c a l staff of the C e n t r a l G e o l o g i c a l and P r o s p e c t i n g I n s t i t u t e , in M o s c o w , had d e v e l o p e d s a m p l i n g p r o c e d u r e s f o r r o u t i n e g e o c h e m i c a l s u r v e y s of s o i l s and o t h e r s e d i m e n t s and had at t h e i r d i s p o s a l good s p e c t r o g r a p h i c e q u i p m e n t to a n a l y z e the c o l l e c t e d m a t e r i a l . T h e y c a l l e d t h e i r m e t h o d " m e t a l l o m e t r i c s u r v e y i n g " . S e v e r a l f i e l d p r o j e c t s w e r e c a r r i e d out in s e a r c h of tin, o t h e r s to p r o s p e c t for c o p p e r , l e a d , z i n c , n i c k e l , c h r o m i u m , w o l f r a m , m o l y b d e n u m and b o r o n . In the w e s t e r n w o r l d , e x p e r i m e n t a l w o r k on g e o c h e m i c a l s o i l s u r v e y i n g s t a r t e d in 1947, by the U . S . G e o l o g i c a l S u r v e y ( H a w k e s and Webb, 1962). T h i s o r g a n i z a t i o n h a s s i n c e c a r r i e d out an a c t i v e p r o g r a m m e of f i e l d i n v e s t i g a t i o n . R a p i d a n d s i m p l e t e c h n i q u e s of c o l o r i m e t r i c a n a l y s i s w e r e d e v e l o p e d In the e a r l y n i n e t e e n f i f t i e s s e v e r a l l a r g e m i n i n g c o m p a n i e s a l s o b e g a n to apply r o u t i n e g e o c h e m i c a l s o i l s u r v e y s . 22

Chem. C.eol.,

I

(1.~i66} .)-:{1

Making use of the A m e r i c a n e x p e r i e n c e (Webb, 1953), B r i t i s h geoc h e m i s t s at that t i m e s t a r t e d an e x t e n s i v e p r o g r a m m e of r e s e a r c h and d e v e l o p m e n t , p r i m a r i l y in A f r i c a and the F a r E a s t , but a l s o in t h e i r own c o u n t r y (Webb, 1958a,b). A d o m i n a n t r o l e was p l a y e d in t h i s by the G e o c h e m i c a l P r o s p e c t i n g R e s e a r c h C e n t e r at the I m p e r i a l College of S c i e n c e and T e c h n o l o g y , London ( W i l l i a m s , 1956; Hawkes and Webb, 1962).

Geoche~nica l prospecting f o r petroleum In c o n n e c t i o n with p e t r o l e u m d r i l l i n g and p r o d u c t i o n , which s t a r t e d in the s e c o n d half of the n i n e t e e n t h c e n t u r y , it was noted that p e t r o l e u m is g e n e r a l l y a s s o c i a t e d with s a l i n e w a t e r . No p a r t i c u l a r a t t e n t i o n was paid to this w a t e r . One of the f i r s t s c i e n t i f i c a l l y i m p o r t a n t p a p e r s on the s u b j e c t w a s that by R o g e r s (1917). He found that o i l - f i e l d w a t e r s a r e not n e c e s s a r i l y high in s a l i n i t y and showed that n e a r the s u r f a c e s u l p h a t e w a t e r s a r e l i k e l y to occdr, that at i n t e r m e d i a t e depths b i c a r b o n a t e w a t e r s a r e c o m m o n and that at depth, in w a t e r s m o s t c l o s e l y a s s o c i a t e d with p e t r o l e u m , c h l o r i d e is g e n e r a l l y the d o m i n a n t anion. M i n o r (1925, 1934) f o c u s s e d a t t e n t i o n on g e o c h e m i c a l d i f f e r e n c e s in w a t e r at a h o r i z o n t a l p l a n e and d e m o n s t r a t e d that i s o l a t e d p a t c h e s of s a l i n e w a t e r s o c c u r within a zone of a l k a l i n e w a t e r s 75 m i l e s wide p a r a l l e l to the Gulf Coast; t h e s e p a t c h e s w e r e p r o v e d to o v e r l i e salt d o m e s . M i n o r then r a i s e d , as the f i r s t one, the idea that such g e o c h e m i c a l a n o m a l i e s m i g h t be r e l a t e d to oil f i e l d s . L a u b m e y e r (1933) i n v e s t i g a t e d m i c r o - s e e p a g e s of gas f r o m 1 and 2 m deep holes, and noted that t h e s e soil g a s e s w e r e r i c h e r in m e t h a n e o v e r known oil f i e l d s than they w e r e in a d j a c e n t b a r r e n a r e a s . H o r v i t z (1939) s u p p o s e d that s o i l s could be e n r i c h e d by m i g r a n t h y d r o c a r b o n s and t r i e d to d e m o n s t r a t e that t h e s e would f o r m a h a l o - t y p e ( r i n g - s h a p e d ) a n o m a l y a r o u n d the m a r g i n s of an oil field. T h i s is a r e m a r k a b l e p h e n o m e n o n , s i n c e at f i r s t sight, a gas a n o m a l y with the highest c o n c e n t r a t i o n of h y d r o c a r b o n s at its c e n t e r should be e x p e c t e d d i r e c t l y on top of the p e t r o l e u m deposit. M c D e r m o t t (1939) showed that w h e r e a s l i q u i d h y d r o c a r b o n s w e r e n e v e r p r e s e n t in s o i l s in b a r r e n a r e a s in a weight p e r c e n t a g e of m o r e than 0.2, this p e r c e n t a g e could go up to 2 in s i m i l a r s o i l s o v e r oil f i e l d s , l:tosaire (1940) m a i n t a i n e d that a h y d r o c a r b o n halo is only one of the p o s s i b l e p a t t e r n s that could develop at the s u r f a c e o v e r an oil field. T h e p a t t e r n d e p e n d s on the m e c h a n i s m of h y d r o c a r b o n m i g r a t i o n and this is g o v e r n e d by the o c c u r r e n c e of z o n e s of l o w e r p r e s s u r e . P r e c i p i t a t i o n of c a r b o n a t e s , s u l p h i d e s and even s i l i c a in the cap r o c k which is in contact with oil would r e d u c e the p e r m e a b i l i t y of the s e d i m e n t , thus r e s t r i c t i n g h y d r o c a r b o n effusion to the f r i n g e of a p e t r o l e u m r e s e r v o i r and would so lead to the f o r m a t i o n of a halo p a t t e r n . Soli (1957) a s s u m e d a r e l a t i o n s h i p b e t w e e n h y d r o c a r b o n c o n c e n t r a t i o n and b a c t e r i a l a c t i v i t y . In the c e n t r a l p a r t of the field they keep the a m o u n t of h y d r o c a r b o n g a s e s low, but with h y d r o c a r b o n c o n c e n t r a t i o n d i m i n i s h i n g t o w a r d s the edges, t h e r e will be a point w h e r e this is i n s u f f i c i e n t for the s u p p o r t of s p e c i f i c b a c t e r i a . If it is a s s u m e d that the p e r m e a b i l i t y on top of the p e t r o l e u m dep o s i t is r e d u c e d as a r e s u l t of r e d u c t i o n of the s u l p h a t e s to s u l p h i d e s by s u l p h a t e - r e d u c i n g b a c t e r i a , then the g a s e s will take the path of l e a s t r e s i s t ance and f o r m a h a l o - s h a p e d a n o m a l y . M o g i l e v s k i y (1959) b a s e d his h y p o t h e s i s Chem. Geol., 1 (1966) 5-31

23

on the fact that n i t r o g e n and c a r b o n d i o x id e a r e u s u a l l y p r e s e n t with the h y d r o c a r b o n g a s e s and that the c o n c e n t r a t i o n of c a r b o n dioxide in e x c e s s of 10% t e n d s to d e p r e s s the a c t i v i t y of h y d r o c a r b o n m i c r o f l o r a . I n a s m u c h as a h i g h e r c a r b o n - d i o x i d e c o n c e n t r a t i o n is u s u a l l y n e a r the p e r i p h e r y of the s t r u c t u r e , w h e r e it t e n d s to d e c r e a s e the b a c t e r i a l a c t i v i t y , the h y d r o c a r b o n c o n c e n t r a t i o n is g r e a t e r at the p e r i p h e r y . On the o t h e r hand, in the c e n t r a l p a r t of the s t r u c t u r e w h e r e the c o n c e n t r a t i o n of n o n - c o m b u s t i b l e g a s e s is at a m i n i m u m , the h y d r o c a r b o n s a r e o x i d i z e d by b a c t e r i a . T h u s , a m i n i m u m c o n c e n t r a t i o n of h y d r o c a r b o n s o c c u r s at the c e n t e r , and the g as s u r v e y s p r o d u c e a h a l o - t y p e gas a n o m a l y . Blau (1943) and De Ment (1947) d e v e l o p e d the e s t i m a t i o n of f l u o r e s c e n t m a t t e r in s o i l s , w h i c h w a s b a s e d on the a s s u m p t i o n that b a c t e r i a t h r i v i n g on a s c e n d i n g h y d r o c a r b o n s w e r e the m e d i u m that p r o d u c e d the f l u o r e s c e n t c o m p o u n d s . When c o n t i n u o u s a c c o u n t w a s t a k e n of the m y c o l o g i c a l content of the s o i l s , this m e t h o d w a s found to p r o d u c e good r e s u l t s . In 1954, H o r v i t z p r e s e n t e d the g a s - s a m p l i n g t e c h n i q u e of p e t r o l e u m e x p l o r a t i o n .

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Chore. (k,oi..

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