The melt inclusion record from the rhyolitic Kos Plateau Tuff (Aegean Arc)

摘要

The >60 km~3 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 percent 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 percent range of H_2O contents (4.5-6.5 wt percent H_2O, measured independently by SIMS, FTIR, and Raman spectroscopy) and relatively low CO_2 concentrations (15-140 ppm measured by FTIR, with most analyses <100 ppm). No obvious correlations between H_2O, CO_2, 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 percent 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 CO_2-rich magma, which is preserved as andesite bands in pumices. These processes can account for the range of observed H_2O and CO_2 values and the lack of correlation between volatiles and trace elements in the melt inclusions.