earth elements (REE) display fractionation in both profiles; the degree of ... Key Words--Alteration, Andesite, Bentonite, Element Mobilization, Mordenite, ...
Clays and Clay Minerals. Vol. 46, No. 4, 379-399, 1998.
C O M P A R A T I V E S T U D Y OF THE M O B I L I T Y OF M A J O R A N D T R A C E E L E M E N T S D U R I N G ALTERATION OF A N A N D E S I T E A N D A R H Y O L I T E TO B E N T O N I T E , IN THE I S L A N D S OF MILOS A N D KIMOLOS, A E G E A N , G R E E C E GEORGE E. CHRISTIDIS Technical University of Crete, Department of Mineral Resources Engineering, Kounoupidiana, 73 t00 Chania, Crete, Greece Abstract--Progressive alteration by seawater of an andesite in the Aegean Island of Milos and an ignimbrite in the Aegean Island of Kimolos, Greece, formed bentonites with or without zeolites. Both profiles are dominated by migration of alkalis and uptake of Mg, Fe and H20, while A1 and Ti are immobile. The relative removal of alkalis controls the formation of either smectite or zeolites. The behavior of Ca and Si depends on the chemistry of the parent rock. In the rhyolitic profile, alteration is controlled by gain of Mg, Fe 2§ and Ca and loss of Na, K and Si, while in the andesitic profile by gain of Mg and Fe 2§ and loss of Na, K and Ca. In both profiles, significant uptake of SO4= was not observed. Moreover Zr, Nb, V and Ni are immobile and have been enriched residually, while Sr, Rb and Y are lost in both profiles. Thorium is immobile in the rhyolitic profile but is leached in the andesitic profile. Also, the rare earth elements (REE) display fractionation in both profiles; the degree of fractionation increases with the degree of alteration to bentonite. Fractionation of the REE in both profiles and mobility of Th in the andesitic profile are related to the existence of monazite (rhyolitic profile) and apatite (andesite profile). The REE and Th appear to partition into phosphates rather than smectite. The mobility of Y coupled with the immobility of Nb increases the Nb : Y ratio with advancing alteration, rendering discrimination diagrams that use this ratio to determine the nature of the prototiths misleading. Mass balance calculations showed that in the smectite-rich zones the water:rock (WR) ratio might be as high as 13:1 in both profiles, while in the zeolite-bearing zones it is about 5.5:1. Such WR ratios explain the observed extensive mass transfer and suggest that the pore fluid chemistry might overprint the chemical characteristics of the parent rocks controlling smectite and bentonite chemistry. Key Words--Alteration, Andesite, Bentonite, Element Mobilization, Mordenite, Phosphates, REE Fractionation, Residual Enrichment, Rhyolite, Smectite, Water:Rock Ratio.
INTRODUCTION
1970; S u r d a m 1977; Taylor a n d S u r d a m 1981; Steefel a n d v a n C a p p e l l e n 1990), w h i l e s m e c t i t e g e n e r a l l y f o r m s in the initial stages o f the alteration ( S h e p p a r d a n d G u d e 1968, 1973; D i b b l e a n d Tiller 1981; H a y a n d G u l d m a n 1987). L e a c h i n g o f Si f r o m acidic r o c k s m i g h t lead to b e n t o n i t e s w i t h o u t silica p o l y m o r p h s (Zielinski 1982; C h r i s t i d i s a n d Scott 1997). W h e n excess Si r e m a i n s in the s y s t e m , o p a l - C T f o r m s ( H e n d e r s o n et al. 1971; C h r i s t i d i s a n d D u n h a m , 1997). T h e c h e m i s t r y o f the p a r e n t r o c k c o n t r o l s b o t h the type a n d the c o m p o s i t i o n o f r e a c t i o n p r o d u c t s ( ! i j i m a 1980; Christidis a n d D u n h a m 1993, 1997). I n t e r m e diate r o c k s tend to f o r m W y o m i n g - a n d C h a m b e r s type m o n t m o r i l l o n i t e , w h e r e a s t h e i r acidic counterparts tend to f o r m beidellite a n d Tatatilla-type m o n t m o r i l l o n i t e (Christidis a n d D u n h a m 1993, 1997). H o w e v e r , this does not s e e m to b e a l w a y s the case. R e c e n t l y Christidis a n d Scott (1997) d e s c r i b e d the form a t i o n o f C h a m b e r s - t y p e m o n t m o r i l l o n i t e b y alteration o f a n ignimbrite. T h i s indicates that, in o p e n systems, the i n t r o d u c t i o n o f k e y c h e m i c a l e l e m e n t s m i g h t influence t h e c o m p o s i t i o n o f the n e o f o r m e d s m e c t i t e s and, consequently, the alteration o f the p a r e n t rock. A l t h o u g h the m i n e r a l o g i c a l a n d m a j o r c h e m i c a l features o f the f o r m a t i o n o f b e n t o n i t e s h a v e b e e n studied
T h e alteration o f v o l c a n i c glass yields smectites a n d various types of zeolites ( S h e p p a r d a n d G u d e 1968, 1973; B o l e s a n d S u r d a m 1979; I i j i m a 1980; S e n k a y i et al. 1984; H a y a n d G u l d m a n 1987; N o h a n d B o l e s 1989; A l t a n e r a n d G r i m 1990; Christidis et al. 1995), leading to the f o r m a t i o n o f e c o n o m i c deposits o f e i t h e r b e n t o n i t e s or zeolites ( M u m p t o n 1977; G r i m a n d Gtiv e n 1978). A l t e r a t i o n o f v o l c a n i c glass m a y take place t h r o u g h w e a t h e r i n g , gas p h a s e crystallization, burial diagenesis, c o n t a c t m e t a m o r p h i s m , h y d r o t h e r m a l activity, p e r c o l a t i n g g r o u n d w a t e r and, in alkaline lakes or the sea floor, in m a r i n e s e d i m e n t s (Iijima 1980; C a s a n d W r i g h t 1988). Field o b s e r v a t i o n s (Zielinski 1982; S e n k a y i et al. 1984; Christidis et al. 1995) a n d l a b o r a t o r y experim e n t s ( M o t t l a n d H o l l a n d 1978; Seyfried a n d M o t t l 1982; Shiraki et al. 1987; Shiraki a n d I i j a m a 1990) h a v e s h o w n that the c o n v e r s i o n o f v o l c a n i c glass to either s m e c t i t e s o r zeolites i n v o l v e s m o b i l i z a t i o n o f e l e m e n t s f r o m a n d to the altered glass. T h u s the loss o f alkalis a n d a h i g h M g - a c t i v i t y p r o m o t e the f o r m a tion of s m e c t i t e (Hay 1977; S e n k a y i et al. 1984). W h e n l e a c h i n g is n o t effective, zeolites crystallize usually f r o m p r e c u r s o r gels ( M a r i n e r a n d S u r d a m Copyright 9 1998, The Clay Minerals Society
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Figure 1. Simplified geological map of the sampling areas, a) Milos, modified after Fyticas et al. (1986). Key to the numbers: 1 = Metamorphic basement, 2 = Neogene sedimentary sequence, 3 = Upper Pleiocene-Lower Pleistocene pyroclastic rocks (undifferentiated), 4 = Upper Pleiocene-Lower Pleistocene lavas (undifferentiated), 5 = Upper Pleistocene chaotic horizon, 6 = Alluvial deposits, Z = Zoulias deposit, b) Kimolos modified after Fyticas and Vougioukalakis (1993). Key to the numbers: 1 = Granite, 2 = Kastro ignimbrite, 3 = Hydrothermally altered volcanic rocks, 4 = Lavas undifferentiated, 5 = Pyroclastic breccia, 6 = Ignimbrite of Prassa area, 7 = Pumice flows, 8 = Pyroclastics of Psathi area, 9 = Alluvial scree, and elluvial deposits, P = Prassa deposit.
t h o r o u g h l y , the m o b i l i t y o f trace e l e m e n t s i n c l u d i n g l a n t h a n i d e s , w i t h few e x c e p t i o n s (Zielinski 1982; E1liott 1993), h a v e b e e n e x a m i n e d t h o r o u g h l y only in w e a t h e r i n g profiles ( D u d d y 1980; B a n f i e l d a n d Eggleton 1989; P r u d e n c i o et al. 1995). T h i s c o n t r i b u t i o n exa m i n e s 2 alteration profiles in the v o l c a n i c islands o f M i l o s a n d K i m o l o s , A e g e a n , Greece, in w h i c h an andesite a n d a rhyolitic i g n i m b r i t e h a v e b e e n c o n v e r t e d to b e n t o n i t e in a s u b m a r i n e e n v i r o n m e n t . Its p u r p o s e is to 1) d e s c r i b e m o b i l i z a t i o n o f b o t h m a j o r a n d trace e l e m e n t s i n c l u d i n g R E E ; 2) c o m p a r e the g e o c h e m i c a l b e h a v i o r o f c h e m i c a l e l e m e n t s a l o n g b o t h profiles; 3) e x a m i n e a n d c o m p a r e the i n t e r d e p e n d e n c e o f mineralogy a n d relative m o b i l i t y o f the various c h e m i c a l e l e m e n t s in the 2 profiles; a n d 4) e s t i m a t e the w a t e r : r o c k ratios a n d d e t e r m i n e their significance d u r i n g alteration. GEOLOGICAL SETTING AND LOCATION OF THE STUDY AREAS T h e islands M i l o s a n d K i m o l o s are situated in the S W part o f the S o u t h A e g e a n Volcanic Arc (Figures l a a n d l b ) , w h i c h was created b y the s u b d u c t i o n o f the A f r i c a n Plate u n d e r the d e f o r m e d m a r g i n o f the E u r a s i a n Plate (Fyticas et al. 1986). T h e islands h a v e similar g e o l o g i c a l h i s t o r y (Fyticas 1977; Fyticas et al. 1986; Fyticas a n d V o u g i o u k a l a k i s 1993). O n M i l o s Island, the study area is located in the composite Zoulias bentonite deposit (Figure la), w h i c h consists o f 11 distinct h o r i z o n s (Christidis et al. 1995). T h e alteration profile c o m p r i s e s the p r o g r e s s i v e alteration o f an andesitic lava (Figure 2), w h i c h constitutes the l o w e s t h o r i z o n of the deposit (Christidis et al. 1995). O n K i m o l o s Island, the alteration profile was f o r m e d at the e x p e n s e o f the u n w e l d e d P r a s s a i g n i m brite (Figure l b ) , the alteration b e i n g structurally con-
trolled. T h e profile c a n b e s u b d i v i d e d into 6 different z o n e s (Figure 2), w h i c h r e p r e s e n t the gradual transition f r o m the fresh glass to a gray b e n t o n i t e and a white, zeolite-bearing bentonite. In b o t h profiles, alteration t o o k place in a s u b m a r i n e e n v i r o n m e n t , at a low t e m p e r a t u r e (Christidis et al. 1995). MATERIALS AND METHODS In b o t h profiles, s a m p l e s were collected 3 0 - 4 0 c m b e n e a t h the surface to m i n i m i z e w e a t h e r i n g a n d contamination. W h o l e - r o c k m i n e r a l o g y was d e t e r m i n e d b y X - r a y diffraction ( X R D ) u s i n g a Philips p o w d e r d i f f r a c t o m e t e r e q u i p p e d w i t h a 1710 c o m p u t e r i z e d control unit, operating at 4 0 k V a n d 30 m A , u s i n g Nifiltered CuKet radiation. S c a n n i n g speed was 1 ~ min. T h e clay m i n e r a l o g y was d e t e r m i n e d after dispersion o f the
