The melt inclusion record from the rhyolitic Kos Plateau Tuff (Aegean Arc) |
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Authors: | Olivier Bachmann Paul J Wallace Julie Bourquin |
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Institution: | 1. Earth and Space Sciences, University of Washington, Mailstop 351310, Seattle, WA, 98195-1310, USA 2. Department of Geological Sciences, University of Oregon, Eugene, OR, 97403-1272, USA 3. Section Des Sciences De La Terre, Université De Genève, 13, Rue Des Mara?chers, 1205, Geneva 4, Switzerland
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Abstract: | The >60 km3 rhyolitic Kos Plateau Tuff provides an exceptional probe into the behavior of volatile components in highly evolved arc magmas:
it is crystal-rich (30–40 vol% crystals), was rapidly quenched by the explosive eruptive process, and contains abundant homogeneous
melt inclusions in large quartz crystals. Several methods for measuring major, trace and volatile element concentrations (SIMS,
FTIR, Raman spectroscopy, electron microprobe, LA–ICPMS) were applied to these melt inclusions. We found a ~2 wt% range of
H2O contents (4.5–6.5 wt% H2O, measured independently by SIMS, FTIR, and Raman spectroscopy) and relatively low CO2 concentrations (15–140 ppm measured by FTIR, with most analyses <100 ppm). No obvious correlations between H2O, CO2, major and trace elements are observed. These observations require a complex, protracted magma evolution in the upper crust
that included: (1) vapor-saturated crystallization in a chamber located between 1.5 and 2.5 kb pressure, (2) closed-system
degassing (with up to 10 vol% exsolved gas) as melts percolated upwards through a vertically extensive mush zone (2–4 km thick),
and (3) periodic gas fluxing from subjacent, more mafic and more CO2-rich magma, which is preserved as andesite bands in pumices. These processes can account for the range of observed H2O and CO2 values and the lack of correlation between volatiles and trace elements in the melt inclusions. |
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