A pressure-independent geothermometer for primitive mantle melts

Abstract

Phase equilibrium data from the CaO-MgO-Al2O3-SiO2, CaO-MgO-Al2O3-SiO2-Na2O, and CaO-MgO-Al2O3-SiO2-FeO systems on the melting behavior of plagioclase, spinel, and garnet lherzolite, are used to determine the molar partition coefficient of MgO between olivine and melt, DMgO*ol/liq, as a function of temperature, pressure, and composition. The data sets cover the pressure range 0.1 MPa to 5 GPa and temperatures of 1225°-1830°C. It is shown that the trend of DMgO*ol/liq is little affected by the transitions between different lherzolite assemblages and that one empirical equation, which has reciprocal temperature (in Kelvin) and mole fraction of NaO0.5 in the melt as independent variables, describes the variation of DMgO*ol/liq in pressure, temperature, and composition space and has the form lnDMgO*ol/liq = (4723/T) + 2.566CNaO0.5*liq-1.729. This expression is tested by using it to calculate the temperatures of melting experiments containing olivine-saturated basalt and picrite melts and is shown to yield good results for multiply saturated melts, consistent with the fact that the observed simple trend of DMgO*ol/liq is dictated by low-variance phase relations. With fewer phases present, the equation systematically predicts temperatures that are too high. If the value of KDFe2+/Mgol/liq (KD) is known, the value of DMgO*ol/liq can be calculated from DMgO*ol/liq = 0.6667 / (CFeO*liqKD + CMgO*liq), and hence the mole fractions of MgO, FeO, and NaO0.5 can be used to calculate the equilibrium temperature of multiply saturated olivine-bearing melts. This geothermometer is mainly applicable to primitive tholeiitic and mildly alkalic basalt and picrite melts, with small amounts of volatiles only, in the 1150°-1800°C temperature range. Copyright 2001 by the American Geophysical Union.