A pressure-independent geothermometer for primitive mantle melts

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 Iherzolite, are used to determine the molar partition coefficient of MgO between olivine and melt, D-MgO*(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 degrees- 1830 degreesC. It is shown that the trend of D-MgO*(ol/liq) is little affected by the transi tions between different lherzolite assemblages and that one empirical equat ion, which has reciprocal temperature (in Kelvin) and mole fraction of NaO0 .5 in melt as independent variables, describes the variation of D-MgO*(ol/l iq) in pressure, temperature, and composition space and has the form InDMgO *ol/liq = (4723/T) + 2.566C(NaO0.5*)(liq) - 1.729. This expression is teste d by using it to calculate the temperatures of melting experiments containi ng olivine-saturated basalt and picrite melts and is shown to yield good re sults for multiply saturated melts, consistent with the fact that the obser ved simple trend of D-MgO*(ol/liq) is dictated by low-variance phase relati ons. With fewer phases present, the equation systematically predicts temper atures that are too high. If the value of K-DFc2+/Mg(ol/liq) (K-D) is known , the value of D-MgO*(ol/liq). can be calculated from D-MgO*(ol/liq) = 0.66 67 / ((CFcO*KD)-K-liq + C-MgO*(liq)), and hence the mole fractions of MgO, FeO, and NaO0.5 can be used to calculate the equilibrium temperature of mul tiply saturated olivine-bearing melts. This geothermometer is mainly applic able to primitive tholeiitic and mildly alkalic basalt and picrite melts, w ith small amounts of volatiles only, in the 1150 degrees- 1800 degreesC tem perature range.