MgO partition between olivine and K2O-rich silicate melt: Geothermometers applicable to high potassium magmas

Potassium and ultrapotassium magmas with potential origins in the upper mantle are common and widely distributed on the Earth. Previous experimental studies have suggested that K can significantly decrease the partial melting T of the peridotite. Since K has a strong effect on the melt composition, and on the molar partition coefficient of some cations such as Mg between the olivine and melt, DMgOl/liq, it can substantially affect the T estimation of the Ol geothermometers. However, none of the presently available geothermometers have critically considered the effect of K. In this paper, we have collected a large number of composition data of the Ol-melt equilibrium pairs covering a large range of K2O contents in the melts, and have successfully calibrated three K2O-dependent geothermometers on the basis of the MgO partition between the Ol and basaltic magma, two Ol geothermometers and one melt composition geothermometer. The database covers the P range from 1 atm to 7 GPa and T range of 1087-1950 °C. Compared to some other geothermometers, our Ol geothermometers reproduce the experimental T very well, with the average of the absolute differences between the calculated T and experimental T being only 14(15) °C at room P and 25(21) °C at high P, and with these differences showing no clear dependence on the P, T, and contents of K2O and any other oxides in the melts. Furthermore, our melt composition geothermometer allows excellent T estimate for the silicate magma coexisting with Ol by using the melt composition alone. The application of our melt composition geothermometer to the Somma-Vesuvius volcanic complex in Italy demonstrates that this geothermometer can lead to reasonable T estimates for the high potassium silicate magmas, even there are small amounts of volatiles such as H2O and CO2 in the melts.