SURVEY OF GEOCHEMICAL SELF-PATTERNING Self-Patterning 1 (2...  SURVEY OF GEOCHEMICAL SELF-PATTERNING

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  • SURVEY OF GEOCHEMICAL SELF-PATTERNING PHENOMENA

    Enrique Merino

    Department of Geology, Indiana University, Rloomington, Indiana 47405, U.S.A.

    Self-patterning phenomena involving chemical reactions take place in all rock types--sedimentary, igneous, and metamorphic. Because apparent exampl-es of geochemical. self-organization can- not as a rule be-directly proven by experiment, it is often dif- ficult even to establish that they are true cases of self-pat- terning, let alone identify the mechanisms involved. Examples (with variable certainty) are: (1) Iron oxide can form concen- tric bands in chert nodules in limestones, and regularly spaced, cigar-shaped, lumps in shales. (2) Agates (aggregates of micro- crystalline quartz fibers) may display all of the following: (a) banding looking remarkably like Liesegang banding; (b) adja- cent bands that consist alternatingly of non-twisted and twisted quartz fibers; and (c) a distinctive chevron interference pat- tern, visible observing the agate between crossed polarizers: (3) Precambrian iron formations display alternating bands on one or more size scales, often with great lateral extent. (4) Mis- sissippi-Valley-type ores often display striking alternating bands (Q 1 cm) of any one or two of the minerals fluorite, dolo- mite, galena, sphalerite, and others; grain-size banding is also found. (5) Sets of evenly-spaced seams and stylolites, with spacings of up to tens of centimeters, are common in limestones and many other sedimentary rocks. (6) The conspicuous, roughly regular, light and dark mineral banding (schistosity, cleavage, or Eoliation) in most regionally metamorphosed rocks. (7) The regular banding exhibited by some contact metamorphic rocks (skarns). (8) Inch-scale, planar bands with considerable lateral extent are found in basic complexes such as the Stillwater in Montana and the Skaergaard in Greenland. (9) Many graniti.~ rocks contain "orbicules" (roughly spherical, crystalline aggregates up to several tens of centimeters across) consisting of concen-

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  • t r i c , , a l t e r n a t i n g bands (% 0.1-1 (:m) of two m i n e r a l s . (10 ) Cr-yst;ils of p l a g i o c l . a s e f e l d s p a r wit11 o s c i l l a t o r y co11111osiI ion;11 zon ing can o c c u r i n i n t e r n ~ e d i c l t e v o l c a n i c r o c k s .

    S e l f - o r g a n i z a t i o n i n g e o l o g i c a l s y s t e m s I ~ a s been r e c o g n i z e d s p o r a d i c a l l y o v e r t h e l a s t s e v e r a l decades . I n 1922 f o r example

    Ifedges ant1 Myers [ I ] ment ioned n a t u r a l o c c u r r e n c e s of goJ~d /c lua r t z i ~ ~ t e r b a n d i n g s a n ~ l a t t e m p t e d t o a c c o u n t f o r them a s c a s e s o f s e l f - p a t t c r n i n y , ; i n 1918 I ) av i s [ 2 ] , t o e x p l a i n t h e f o r m a t i o ~ i of t h e bedded c h c r t s of t h e ] : r anc i scan I . 'or~untion o f C a l i f o r n i a 131, t r i e d t o show ex1)c.r i m e r ~ l a l l y t l ln t s i l i ca -ge l / c l a y s y s t e m s , i n i - t i a l l y u n i f o r m l y mixeti , c a n r e s o l v c t l l e ~ n s e l v e s s p o n t a n e o u s l y i n t o a l . t e r n a t i n g l a y e r s of s i l i c a g e l (wlr ic l~ , t l r roug l~ d e l ~ y d r a t i o n and r i p e n i n g , would l a t e r become m i c r o c r y s t a l l i n e q u a r t z o r c h e r t ) and c l a y ; and i n 1932 Ilartman and Dickey [ 4 ] s u g g c s l c d , I i ;~sed nn t h e i r - e x p e r i m e n t s , t h a t L ie segang- type i n t e r d i f f u s i o n c o u l tl c a u s e t l ~ e c l 1 n r a c t e r i s t : i c b a n d i n g of P recambr ian i r o n f o r m a t i o n s . Karl j(dr, 1,ieseg;lng l ~ i ~ l ~ s e l ~ f [ 5 , 6 ] had s l ~ e c u l a t e d t h a t a g a t e band- i n g cc~ultl form by c l i f i u s i o n o f m e t a l s t h rough lumps o f s i l i c a g e l i n r h y o l i t i c and a n d e s j t i c vo lca~ i i c : r o c k s , and Knopf [ 7 ] had s u g g e s t e d "phenomena s i m i l a r t o t l ~ o s e opera t - . ive i n t h e forma t i o n of L i e s e g a n g ' s r i n g s " t o a c c o u n t f o r a1 t e r n ; ~ t i n ~ l y banded s p h e r - i c a l s t r u c t u r e s i n metamorphic r o c k s .

    I 'erhaps l lecause t h e o r e t i c a l phys ico -chemica l e x p l a n a t i o n s o f Ljrsegclng I l e r i o d i c p r e c i p i t a t i o n d i d n o t improve much f rom t h e e a r l y work [ I , 8 ] t o t h a t of P r a g e r [ 9 ] , and a l s o b e c a u s e geo- l o g i c a l t h i n k i n g h a s been r i g l ~ t l y perruea t e d s i n c e t h e 1 7 ti1 cen- t u r y hy t h e s t r a t i g r a p l ~ i c p r i n c i p l r of s u p e r p o s i t i o n - - w h i c h s t a t e s t h a t any l a y e r i n a s e q u e n c e i s o l d e r i l lan t h e o n e on t o p of i t - -and l ~ e c a u s e of t h i s I I r i n c i p l e ' s unspoken i m p l i c a t i o n t h a t c.aclr l a y e r b a s i c a l l y "knows" o r " c a r e s " n o t h i n g , from a g e n e t i c s t a ~ ~ c l p o i n t , a b o u t t l ~ e l a y e r s t h a t u n d e r l i e and o v e r l i e i t , i t h a s been d i f f i c u l t t o i m ; ~ g i ~ ~ r l ~ l ~ a t l a y e r s c.an e x i s t t h a t l i n k e d g e n e t i c a l l y t o e a c h o t l i c r a11d t h ; l t , t l ~ e r e f o r e , f a l l o u t s i d e o f t l ~ e f i e l d of a p p l i c a b i l i t y o f t l ~ e p r i n c i p l e of s u p e r p o s i t i o n . 1 ,ayers g e n e r a t e d by s e l f - o r g a n i z a t i o n a r e o f t l ~ i s kinti . The r e -

    c e n t t h e o r e t i c a l p r o g r e s s made Ily p h y s i c a l c l ~ e m i s t s i n e x p l ~ a i n - i n g t h e dynamics o f s e l f - o r g a n i z a t i o n - - a p r o g r e s s we do n o t i n - t rn ( l t o r e v i e w l ~ e r e ( b u t s e e , i o r example , 110-121 and tlrc pro- c c e d i n g s of t l~ i s and p r e v i o u s workshops on n o n - e q u i l i b r i u m phcnomcnn)--alony, will1 s e v e r a l v e r y r e c e n t a r t i c l e s i n t h e geo- 1 .ogical l i t e r a t u r e [ 1 3-21] point: t o t l ~ e widesp read o c c u r r e n c e o f wl~a t corll~d Ile c a l l e d geocl~emi c a l s e l f -o rgan i z a t i o n .

    Tire p u r p o s e o f t l ~ i s c o n t r i b u t i o n i s t o t i e s c r i b e s e v e r a l l i k e l y o r c e r t a i n c a s e s of geochemica l s e l f - l ~ ; ~ t t c ~ r l i i ~ i g ant1 l o

    suggc'st d i r e c t i o n s fclr f ~ i t r ~ r c , world i g n e o l ~ s . The c a s e s p r e s e n t e d below c o v e r a wide s p e c t r r ~ n l of s i z e s c a l e s , and a l l a p p e a r co i n v o l v e f i r s t - o r d e r p l ~ a s e t r a n s i t i o n s . Some of t h e c a s e s have beer1 o n l y g e o l o g i r n l c u r i o s i t i e s u n t i l now, t l~orrgh t ~ o t h e e x t e n t t h a t t h e y arc! t r u e c a s e s o f s e l f - p a t t e r n i n g , Lhey i l l u s t r a t e t h e conll,l e x i t y and v a r i e t y of t h e 111ecl1nn isms c j p e r a t i n g i n r o c k s ant1 t h u s s l ~ o u l d g a i n i m p o r t a n c e f o r geul o g i s t s .

    R e c a ~ l s e a l ~ p a r e n t examples o f geochemica l s e l f - o r g a n i z a t i o n c a n n o t a s ;I r u l e be r e p e a t e d experirlrent:;ll l y , t h e f i r s t d i f f i c r l l t t a s k is t o e s t a b l i s h t h a t t h e y a r e t r u e c a s e s of s e l i - p a t t e r n i n j : ,

    I t l ~ a t is, c a s e s where t h e r o c k o r s y s t e m was indeed rrnpattel-ne(l ~ o h e g i n w i t h . E s t a b l i s h i n g t h a t a g i v e n t e x t u r a l p a t t e r n i s n c a s e o f t r u e se l f -o rgan jza t - io11 s h o u l d he clone i n g e n e r a l 11y a cornbi- n a t i o n of c a r e f u l f i e l d , p e t r o g r a p h i c , and chemicnl o l ~ s c r v n t i o n s .

    The second t , lbk i s t o c b o o s e t h e mechanismr, involved i n g e n e r a t i n g n g i v e n c a s e o f s e l f - o r g a n i z a t i o n s o t l l a t t hey a r e c o ~ ~ s i s t e r ~ t wit11 f i e l d e v i d e n c e and w i t h t h c o v e r a l l p r o c e s s e s known i n d e p e n d e n t l y t o have o p e r a t e d i n t h e g e n e s i s of t he e n - c a s i n g r o c k s .

    I IKON OXTDE CONCEN'I'UTIONS I N SEDIMENTARY ROCKS

    t ( : o r ~ c c n t r a t i . o n s of f e r r i c o x i d e (hematite, g o e t h i t e ) can h e f o ~ i n d i n a v a r i e t y o f s e d i m e n t a r y r o c k s fo rming s p a t i a l p a t t e r n s

    i - - c u n c e n t r i c hands , p l a n a r Ix~n t l s , o r columns. H e m a t i t e concerl-

    i t r i c bands a r e o f t e n p r e s e n t i n c h e r t c o n c r e t i o n s and noc lu l (~s i n l i m e s t o n e s (Fig. l a ) , and must t h e n Ile a c a s e o f t t -uc s r l f - o r g a n - i z a t i o n (and o n e t h a t t ook p l a c e d u r i n g d i a g e n e s i s ) , becalise t h e

    v e r y c o n c r e t i o n s a r e t h e m s e l v e s o f d i a g e n e t i c o r i g i n . C o n c e n t r i c o r p l a n a r I ~ a n d s of h e m a t i t e c a n a l s o o c c u r i n c l a s t i c rocks s u c h a s t h e Hnraboo Q u a r t z i t e o f Id isconsin a11cI t h e z c b r a r o c k s f r o m E a s t Kimber ley, I J e s t e r n A u s t r a l i a (F ig . 111, s e e d e t a i l s i n ( :he

    i c ; lp t ion ; s e e r e f . [ 2 1 a ] ) . I n b o