CORDIERITE AS A MONITOR OF FLUID AND MELT H2O CONTENTS IN THE LOWER CRUST: AN EXPERIMENTAL CALIBRATION

Abstract

Cordierite can contain variable amounts of H2O and CO2, and the total amount and relative proportions of these volatiles reflect the environment in which the cordierite was formed. In this study, cordierite has been experimentally equilibrated with a peraluminous granite melt, and the H2O contents of the phases have been determined by secondary-ion mass spectrometry (SIMS). The results show that cordierite can have a range of H2O contents at a single pressure and temperature, in this case 900 °C, 5.0 kbar. The maximum H2O contents occur at saturation, when H2O vapor is also present, at which point cordierite contains 1.69 ± 0.05 wt% and the melt contains 10.0 ± 0.42 wt%. However, as the system becomes progressively more water-undersaturated, the phases contain less H2O, and the partitioning coefficient (Dw = H2OMelt/H2OCordierite, in wt%) decreases from 5.9 ± 0.3 to 4.4 ± 0.6. The experiments show that equilibration with a volatile-undersaturated melt provides an alternative to leakage as a mechanism for producing volatile-poor cordierites. Application of this experimental calibration to cordierites from granulite migmatites predicts that the melts with which they last equilibrated contained only 2.7 3.8 wt% H2O.