[S-IT07] Structure and dynamics of Earth and Planetary deep interiors

[SIT07-14] Melting relations in the MgO-MgSiO₃ system under the lower mantle condition using a CO₂ laser heated diamond anvil cell

*大西 里佳¹、桑山 靖弘²、井上 徹² (1.愛媛大学大学院理工学研究科、2.愛媛大学地球深部ダイナミクス研究センター)

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 understand the chemical differentiation at the base of the mantle. While melting relations of mantle materials at relatively low pressure (below 30 GPa) have been extensively studied using a multi-anvil apparatus (e.g. Ito et al., 2004 Phy. Earth Planet. Inter.), the melting experiments at higher pressures are still limited. Only in a few model rock compositions, such as peridotite and mid-oceanic ridge basalt (MORB), the experiments were conducted 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, the experiment up to only 26 GPa was performed (Liebske and Frost, 2012 Earth Planet. Sci. Lett.). So, further studies at higher pressure corresponding to the deep lower mantle condition are required. In this study, we 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 the starting materials. A 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 44 mol% SiO₂ and it becomes about 40 mol% at 50 GPa. Above 50 GPa, it is predicted to become relatively constant, consistent with the previous result by Liebske and Frost (2012). From these results, MgO-rich melt layer may be generated by partial melting of the bulk mantle, such as pyrolite composition (i.e. 42 mol% SiO₂), at the base of the mantle.
The present result should provide basic information for better understanding on the melting relations at deep mantle conditions.