This is for continuation of a project to determine the preeruptive distribution of gas in a large body of rhyolitic magma, the Bishop Tuff. First results of the PI's studies of melt inclusions and of pumice formation pressure and vesicularity suggest that a surprisingly large excess of gas was present in preerupted Bishop magma. To definitely establish the amount of gas in the Bishop magma the PIs plan comprehensive studies of individual pumice clasts from a selected set of old and new stratigraphically controlled sites. They will determine the vesicle and crystal content of individual clasts of pumice as well as the compositions of phenocrysts and the concentrations of H2O, CO2, Cl, Ba, U, Rb and other elements in both enclosed and hourglass melt inclusions in quartz and feldspar phenocrysts. The work will result in the first determination of the distribution of gas in a preeruptive body of magma. This data will then be used to test theories on gas accumulation in magma bodies and its effect on the nature of volcanic eruptions.
