OLIVINE-ORTHOPYROXENE-MELT EQUILIBRIUM AS A THERMOBAROMETER FOR MANTLE-DERIVED MAGMAS

Abstract

Empirical expressions were derived to describe the relationship between the temperature and pressure of olivine-orthopyroxene-melt equilibrium and melt composition. The input data array included analyses of melts in equilibrium with olivine (about 2800 points) and orthopyroxene (about 900 pints) obtained in experiments at pressures from 0-5 GPa under "dry" conditions and oxygen fugacity below QFM + 1 log unit. The uncertainties in melt composition for particular experiments were accounted for at the construction of the empirical model. The obtained expressions reproduce experimental parameters for melts in equilibrium with olivine and orthopyroxene with an accuracy of 40 °C and 0.3 GPa. The application of the derived expressions to experimental data on the compositions of melts coexisting with olivine and orthopyroxene at high oxygen potential and in the presence of water allowed us to assess the influence of these factors. An increase in oxygen potential does not change significantly the equilibrium parameters. The addition of 1 wt % of H2O reduces the equilibrium temperature by approximately 20 °C. If the water content of melt is lower than 2 wt %, the addition of water reduces the pressure of equilibrium of a melt of given composition with olivine and orthopyroxene; whereas the opposite effect was observed at higher H2O concentrations. The obtained expressions were used for the estimation of generation parameters for primary high-magnesia magmas from various geodynamic settings. The conditions of mantle magma generation occupy a narrow region in the P-T plane parallel to the solidus of mantle peridotite: from 4.0-5.5 GPa and 1700-1850°C for Precambrian komatiites to 1-2 GPa and 1300-1400 °C for mid-ocean ridge basalts. Magmas related to mantle plumes are located between these two extremes.