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Comparative Study of the mobility of major and trace elements during alteration of an andesite and a rhyolite to bentonite, in the islands of Milos and Kimolos, Aegean, Greece

Christidis Georgios

Πλήρης Εγγραφή

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Περίληψη

Progressive alteration by seawater of an andesite in the Aegean Island of Milos and an ignimbritein the Aegean Island of Kimolos, Greece, formed bentonites with or without zeolites. Both profilesare 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 Caand Si depends on the chemistry of the parent rock. In the rhyolitic profile, alteration is controlled bygain 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 bothprofiles. Thorium is immobile in the rhyolitic profile but is leached in the andesitic profile. Also, the rareearth elements (REE) display fractionation in both profiles; the degree of fractionation increases with thedegree of alteration to bentonite. Fractionation of the REE in both profiles and mobility of Th in theandesitic 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 prototithsmisleading. Mass balance calculations showed that in the smectite-rich zones the water:rock (WR) ratiomight be as high as 13:1 in both profiles, while in the zeolite-bearing zones it is about 5.5:1. Such WRratios explain the observed extensive mass transfer and suggest that the pore fluid chemistry mightoverprint the chemical characteristics of the parent rocks controlling smectite and bentonite chemistry

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