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

Abstract

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.

Early Cretaceous intra-plate volcanism in the Pieniny Klippen Belt – a case study of the Velykyi Kamenets’/Vilkhivchyk (Ukraine) and Biała Woda (Poland) sections

“…How ever, there are many re ports that trace el e ments are mo bi lised dur ing al ter ation pro cesses (e.g., Winchester and Floyd, 1977b;Zielinski, 1982;Summa and Verosub, 1992;van der Weijden and van der Weijden, 1995;Christidis, 1998;Malpas et al, 2001;Büchl and Gier, 2003), though, as was reported by van der Weijden and van der Weijden (1995) the changes in REE val ues are not con sid ered to be sig nif i cant. More over, the au thors re ferred em pha sized that the mo bil ity of el e ments dur ing al ter ation pro cesses is ex tremely com plex and con trolled by sev eral fac tors.…”

Chemical signature of two Permian volcanic ash deposits within a bentonite bed from Melo, Uruguay

“…Christidis (1998) showed that these chemical exchanges depend on the composition of the volcanic debris and the water/rock ratio in a marine system. Consequently, it is excluded to use the proportions of the major elements to determine the original glass composition.…”

Bigadiç (Balıkesir) Bentonit Yataklarının Mineralojisi ve Jeokimyası ve Bentonitleşme Sırasındaki Ana, Eser ve Nadir Toprak Elementlerinin Mobilizasyonu

“…Riyolitik camın bentonite dönüşümünde Cs, Rb, U ve Zr alterasyonun ilk evrelerinde kayba uğrar, Sr, Sc, Cr ve Co farklı yollarla bentonitte zenginleşir [16] [18]. İncelenen örneklerdeki küçük Zr zenginleşmeleri bentonitleşme sırasında erimeyen kalıntı mineral (biyotit) yoluyla ve Zn zenginleşmeleri ise yapısal birleşme yoluyla (dış kaynaklı) gerçekleşmiş olmalıdır.…”

“…Sıcaklığın yüksek olduğu alterasyon şartlarında altere üründe Eu +2 zenginleşmesi meydana gelebilir [20,21]. Benzer olarak andezitik volkaniklerin bentonite dönüşümü sırasında ilerleyen alterasyona bağlı olarak plajiyoklasların smektite replase olması ile altere üründe Eu zenginleşir [18][19][20][21][22] [4], CaO zenginleşmesinin iyon değişimi yoluyla gerçekleştiğine işaret eder [27,28]. Zenginleşen Ca'un bir kısmı değişebilir katyon olarak montmorillonit yapısında tutulmuş geriye kalan Ca, bentonitler içinde gözlenen kalsit oluşumunda kullanılmıştır.…”

“…Alterasyon sırasında erimeyen REE bileşenlerini tutan manyetit, zirkon, allanit vb. aksesuar mineraller ve (bazı) hidrotermal sistemlerdeki F, Cl, SO 4 gibi elemanlar bentonitte özellikle LREE'nin yerinde zenginleşmesini sağlar [16,18,20,21]. Neoforme fillosilikat ve otijenik fosfat minerallerinin varlığı ise bentonitteki REE içeriklerinin azalmasını sağlar [18,22,24,32].…”