HR: 09:45h
AN: V11E-08    [Abstracts]
TI: Silicon Isotope Fractionation During Acid Water-Igneous Rock Interaction
AU: * van den Boorn, S H
EM: boorn@geo.uu.nl
AF: Utrecht University, Department of Earth Sciences, Budapestlaan 4, Utrecht, 3584 CD, Netherlands
AU: van Bergen, M J
EM: vbergen@geo.uu.nl
AF: Utrecht University, Department of Earth Sciences, Budapestlaan 4, Utrecht, 3584 CD, Netherlands
AU: Vroon, P Z
EM: pieter.vroon@falw.vu.nl
AF: Free University, Faculty of Earth and Life Sciences, Boelelaan 1085, Amsterdam, 1081 HV, Netherlands
AB: Silica enrichment by metasomatic/hydrothermal alteration is a widespread phenomenon in crustal environments where acid fluids interact with silicate rocks. High-sulfidation epithermal ore deposits and acid-leached residues at hot-spring settings are among the best known examples. Acid alteration acting on basalts has also been invoked to explain the relatively high silica contents of the surface of Mars. We have analyzed basaltic-andesitic lavas from the Kawah Ijen volcanic complex (East Java, Indonesia) that were altered by interaction with highly acid (pH~1) sulfate-chloride water of its crater lake and seepage stream. Quantitative removal of major elements during this interaction has led to relative increase in SiO₂ contents. Our silicon isotope data, obtained by HR-MC-ICPMS and reported relative to the NIST RM8546 (=NBS28) standard, show a systematic increase in &δ&&³⁰Si from -0.2‰ (±0.3, 2sd) for unaltered andesites and basalts to +1.5‰ (±0.3, 2sd) for the most altered/silicified rocks. These results demonstrate that silicification induced by pervasive acid alteration is accompanied by significant Si isotope fractionation, so that alterered products become isotopically heavier than the precursor rocks. Despite the observed enrichment in SiO₂, the rocks have experienced an overall net loss of silicon upon alteration, if Nb is considered as perfectly immobile. The observed &δ&&³⁰Si values of the alteration products appeared to correlate well with the inferred amounts of silicon loss. These findings would suggest that &²⁸Si is preferentially leached during water-rock interaction, implying that dissolved silica in the ambient lake and stream water is isotopically light. However, layered opaline lake sediments, that are believed to represent precipitates from the silica-saturated water show a conspicuous &³⁰Si-enrichment (+1.2 ± 0.2‰). Because anorganic precipitation is known to discriminate against the heavy isotope (e.g. Basile- Doelsch et al., 2006), the &δ&&³⁰Si value of dissolved silicon in the lake water must be even higher. We infer that progressive cation removal alone is inadequate to describe rock dissolution and silicification by acid fluid. Exchange of silicon between the solution and mineral phases probably accompanied the alteration process. This hypothesis is qualitatively consistent with the idea that elements in solution take part in the formation of altered silica-rich layers at mineral-solution interfaces, as invoked to interpret surface reactions during silicate mineral weathering (e.g., Adriaens et al., 1999; Hellmann et al., 2003). References
Adriaens et al., 1999. Surf. Interface Anal., 27: 8-23
Basile-Doelsch et al., 2006. Nature, 433: 399-402.
Hellmann et al., 2003. Phys. Chem. Minerals, 30: 192-197.

DE: 1039 Alteration and weathering processes (3617)
DE: 3617 Alteration and weathering processes (1039)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting