Distribution of H sub 2 O, CO sub 2 , Cl, and S in a Large Body of Rhyolitic Magma. Annual Progress Report

摘要

The Plinian air-fall unit of the rhyolitic Bishop Tuff is crudely stratified and internally varied. The size and sorting of lumps of pumice increase upwards in several visible air-fall layers. With increasing stratigraphic height pumice is increasingly fibrous (long vesicles) and rich in crystals. Most quartz crystals from fibrous lumps have abundant, large, round but irregular inclusions of glass (quenched melt) free of vapor bubbles and are coated with a relatively thick rind of coarsely vesicular glass. Most quartz crystals from lumps rich in spherical vesicles are coated with a thin veneer of finely vesicular glass and have small, partly faceted (negative crystal) inclusions of glass which commonly are fractured and contain vapor bubbles. Individual lumps of pumice contain a textural range of quartz crystals. Analyses of some melt inclusions incorporating a 400 exp 0 C bakeout to remove absorbed volatiles suggest that the large irregular inclusions have less than about 0.5 wt % of H sub 2 O. However, more analyses of more thoroughly documented inclusions are needed to be sure. 31 refs., 6 figs., 1 tab. (ERA citation 11:001493)