Volatiles H2O,CO2, and Cl in a subduction related basalt |
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| Authors: | David M Harris Alfred T Anderson Jr |
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| Institution: | (1) Department of the Geophysical Sciences, The University of Chicago, 60637 Chicago, Illinois, USA;(2) Present address: Department of Geology, University of Oregon, 97403-1272 Eugene, Oregon, USA |
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| Abstract: | The products of the 1974 eruption of Fuego, a subduction zone volcano in Guatemala, have been investigated through study of
silicate melt inclusions in olivine. The melt inclusions sampled liquids in regions where olivine, plagioclase, magnetite,
and augite were precipitating. Comparisons of the erupted ash, groundmass, and melt inclusion compositions suggest that the
inclusions represent samples of liquids present in a thermal boundary layer of the magma body. The concentrations of H2O and CO2 in glass inclusions were determined by a vacuum fusion manometric technique using individual olivine crystals (Fo77 to Fo71)
with glass inclusion compositions that ranged from high-alumina basalt to basaltic andesite. Water, Cl, and K2O concentrations increased by a factor of two as the olivine crystals became more iron-rich (Fo77 to Fo71) and as the glass
inclusions increased in SiO2 from 51 to 54 wt.% SiO2. The concentration of H2O in the melt increased from 1.6 wt.% in the least differentiated liquid to about 3.5% in a more differentiated liquid. Carbon
dioxide is about an order of magnitude less abundant than H2O in these inclusions. The gas saturation pressures for pure H2O in equilibrium with the melt inclusions, which were calculated from the glass inclusion compositions using the solubility
model of Burnham (1979), are given approximately by P(H2O)(Pa)=(SiO2−48.5 wt.%) × 1.45 × 107. The concentrations of water in the melt and the gas saturation pressures increased from about 1.5% to 3.5% and from 300
to 850 bars, respectively, during pre-eruption crystallization. |
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