V13A-3090
A Clinopyroxene-Plagioclase Geobarometer for A-type Silicic Volcanic Rocks

Monday, 14 December 2015
Poster Hall (Moscone South)
John Wolff1, Alexander Andrew Iveson1, Klarissa Davis2, Thomas A Johnson2, Shawn Gahagan2 and Ben S Ellis3, (1)Washington State University, Geology, Pullman, WA, United States, (2)Washington State University, Pullman, WA, United States, (3)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland
Abstract:
Constraining the crustal storage depths of magmas is important in understanding volcanism. The reaction: anorthite (pl) = Ca-Tschermak’s (cpx) + silica (Q or liq) has a large volume change and hence offers potential as a geobarometer, but has not been extensively exploited as such. One of the chief barriers to its wide application is consistent estimation of melt silica activity for assemblages that lack quartz. We have skirted this problem by confining attention to metaluminous silicic compositions (SiO2 > 60% by weight), for which silica activity during crystallization is presumed to be close to 1, and calibrated the barometer for the range 0 - 2 GPa using the LEPR database and additional experiments from the literature. Additional improvement is obtained by excluding hydrous phase-bearing assemblages. Despite the analytical uncertainties present in older experimental investigations, with knowledge of temperature, and clinopyroxene, plagioclase and host melt compositions, pressures for amphibole- and biotite-free dacites and rhyolites can be estimated to ±0.17 GPa (1 sigma). The limitations of the barometer render it most applicable to intraplate, A-type rhyolites. Application to one such system, the Snake River Plain rhyolites, indicate that both melt-hosted phenocrysts and clinopyroxene-plagioclase aggregate grains found in these rhyolites formed at low pressures, <0.5 GPa. This is consistent with isotopic evidence for a shallow crustal origin for Snake River Plain rhyolites.