Melting relations in the MgO-MgSiO3 system under the lower mantle conditions using a double-sided CO2 laser heated diamond anvil cell

Abstract:

Seismological observations of the ultralow-velocity zones (ULVZs) suggest the presence of partial melts above the core-mantle boundary (CMB). Knowledge of the melting relations in the lower mantle is a key to understanding the chemical differentiation at the base of the mantle. While melting relations of mantle materials at relatively low pressures (below 30 GPa) have been extensively studied using a multi-anvil apparatus (e.g. Ito et al., 2004 Phy. Earth Planet. Inter.), melting experiments at higher pressures are still limited. Only a few model compositions, such as peridotite and mid-oceanic ridge basalt (MORB), were studied under the CMB conditions using a laser-heated diamond anvil cell (LHDAC) (e.g. Fiquet et al., 2010 Science, Andrault et al., 2014 Science). Since chemical heterogeneity of both major elements (Mg, Si, Fe, Al...) and minor ones (e.g. alkalis and volatiles) should have a large effect on the melting behavior, the melting phase diagrams as a function of composition are fundamental to understand the nature of the ULVZs. For melting relations in a binary system MgO-MgSiO₃, which is a major component in the lower mantle, previous experiments were performed up to only 26 GPa (Liebske and Frost, 2012 Earth Planet. Sci. Lett.). Further studies at higher pressures corresponding to the deep lower mantle conditions are required. In this study, we have determined the melting relations in the MgO-MgSiO₃ system above 30 GPa using a LHDAC. Glasses of several different compositions in the MgO-MgSiO₃ system (from 37 to 45 mol% SiO₂) were used as starting materials. A double-sided CO₂ laser heating system was used to heat the sample directly. The recovered samples were polished and analyzed by a dualbeam focused ion beam (FIB) and a field emission scanning electron microscope (FE-SEM), respectively. The eutectic compositions and the liquidus phases were determined on the basis of chemical and textural analysis of the quenched samples. Our results show that the eutectic composition at 30 GPa is about 42%, consistent with previous results by Liebske and Frost (2012). It becomes about 41% at 60 GPa, and more MgO-rich with increasing pressure. The present result should provide a baseline for a better understanding of the effects of other components on the melting relations of the mantle materials at deep mantle conditions.