th Lunar and Planetary Science Conference ( 2009 )

Introduction: The most important parameter for understanding the dynamic state of the interiors of rocky planets is the average temperature of the planet (typically referred to as the potential temperature, TP, which is the fictive temperature the planet would have if it’s interior were decompressed along a solid adiabat to the surface of the planet). Because there are no direct ways for estimating interior temperatures of a planet, the next best approach is to use erupted magmas as indirect windows into the mantle. Temperatures of erupted magmas, however, yield minimum bounds on potential temperature for the following reasons. First, erupted magmas have often cooled and chemically fractionated, hence these effects must be corrected for. Second, even in the case of primary magmas, there is substantial cooling during adiabatic decompression melting due to the absorption of the latent heat of fusion. The original temperature of the solid mantle must then be calculated by extrapolating magmatic temperatures back in depth until the mantle solidus is intersected. This requires an estimate of the average pressure of magma extraction from the mantle. While there are a number of existing magma thermometers, we are not aware of robust barometers that are widely applicable to planetary magmas over a range of pressures. Here, we combine a new method of estimating melting pressures with refined methods of estimating temperatures applicable to a wide range of mafic compositions ranging from Fe-poor (terrestrial) to relatively Fe-rich (e.g., Martian) compositions. The barometer uses the P-dependence of silica activity on the following silica buffer, ol + SiO2 = opx, which likely dominates mantle-melting on all rocky planets. The thermometer is based on olivine-saturation. Methods: To calibrate the barometer, we assembled a database (with the aid of the LEPR database http://lepr.ofm-research.org) of 433 experimental basaltic liquid compositions in equilibrium with olivine and orthopyroxene ranging from pressures of 1 atm to 7 GPa and temperatures from 1100 to 1800 o C; the experimental systems included terrestrial mafic and ultramafic compositions, multiple saturation experiments done on lunar, martian, and eucritic meteorite or crustal compositions, as well as water-bearing experiments. Details are presented in [1]. The calibrated barometer is as follows: