Crystallization of garnet-bearing rhyodacite under high-pressure hydrous conditions

Abstract

A high-pressure, high-temperature investigation of a glass prepared from a natural garnet-bearing rhyodacite from Victoria has been conducted under anhydrous and hydrous conditions (2, 5 and 10% by weight of water added). Garnet and quartz are near-liquidus phases from 13.5 to 27 kb with 2% water present, but with 5–10% water present garnet is the liquidus phase from 18–27 kb and the appearance of quartz is depressed significantly below the liquidus. All garnet crystals analysed over the pressure range 11 to 27 kb have > 12 mol.% grossular, higher than that found in the natural garnets. It was not possible to obtain garnets with composition corresponding in grossular content and Fe/Mg ratio to the natural garnets for the experimental conditions of this study. The anhydrous rhyodacite liquidus is higher than the corresponding andesite liquidus, but with water present the rhyodacite liquidus is markedly below the andesite liquidus, for similar water contents over the pressure range 18–27 kb. This result has important implications in considering partial melting of the down-thrust lithosphere in subduction zones. The initial hydrous melt fraction will be more silicic than andesite and may then rise, mix with, and influence composition trends of melts produced in the overlying peridotitic lithosphere.