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

Progressive alteration by seawater of an andesite in the Aegean Island of Miles and an ignimbrite in the Aegean Island of Kimolos, Greece, f ormed bentonites with or without zeolites. Both profiles are dominated by migration of alkalis and uptake of Mg, Fe and H2O, while Al 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 t he chemistry of the parent rock. In the rhyolitic profile, alteration is controlled by gain of Mg, Fe2+ and Ca and loss of Na, K and Si, whi le in the andesitic profile by gain of Mg and Fe2+ and loss of Na, K a nd Ca. In both profiles, significant uptake of SO4= was not observed. Moreover Zr, Nb, V and Ni are immobile and have been enriched residual ly, 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 o f Th in the andesitic profile are related to the existence of monazite (rhyotitic profile) and apatite (andesite profile). The REE and Th ap pear to partition into phosphates rather than smectite. The mobility o f Y coupled with the immobility of Nb increases the Nb:Y ratio with ad vancing alteration, rendering discrimination diagrams that use this ra tio to determine the nature of the protoliths misleading. Mass balance calculations showed that in the smectite-rich zones the water:rock (W R) ratio might be as high as 13:1 in both profiles, while in the zeoli te-bearing zones it is about 5.5:1. Such WR ratios explain the observe d extensive mass transfer and suggest that the pore fluid chemistry mi ght overprint the chemical characteristics of the parent rocks control ling smectite and bentonite chemistry.