Experimental Determination of Eutectic Liquid Compositions in the MgO‐SiO₂System to the Lowermost Mantle Pressures

Abstract

We have performed melting experiments on the MgO-SiO 2 binary system to 139 GPa in a diamond anvil cell. Both MgO-MgSiO 3 and MgSiO 3 -SiO 2 eutectic compositions were examined by textural/chemical characterizations of recovered samples. Results demonstrate that the MgO-MgSiO 3 eutectic becomes more enriched in MgO with increasing pressure and is close in composition to Mg 2 SiO 4 at the core-mantle boundary (CMB) pressure. On the other hand, the MgSiO 3 -SiO 2 eutectic is more SiO 2 -rich at higher pressure and exhibits SiO 2 /(MgO + SiO 2 ) = 0.66 at the CMB. These have profound implications for a large-scale compositional stratification possibly formed upon solidification of a magma ocean. The MgO-MgSiO 3 eutectic liquid being more MgO-rich than previously predicted suggests that only bridgmanite crystallizes up to 60 wt% solidification from a fully molten state of a pyrolitic lowermost mantle. In contrast, an SiO 2 layer may develop when melting/crystallization occurs in rocky objects with silica-rich enstatite-chondritic compositions.Plain Language Summary MgO-SiO 2 is the most important binary system to understand melting/crystallization processes in the Earth's crust and mantle as well as those in other rocky objects. Its melting phase relations, in particular, eutectic compositions, also have profound implications for a large-scale compositional stratification possibly formed upon solidification of a magma ocean. However, the eutectic compositions in the MgO-MgSiO 3 and MgSiO 3 -SiO 2 systems have not been examined experimentally at the deep lower mantle conditions yet. Here we performed melting experiments and determined both of these eutectic compositions to 135 GPa, corresponding to the pressure at the bottom of the mantle, by a combination of high-pressure-temperature generation in a laser-heated diamond anvil cell and textural/compositional characterization of recovered samples. The results demonstrate that the MgO-MgSiO 3 eutectic liquid is more MgO-rich than previously predicted. It suggests that a large volume of MgSiO 3 bridgmanite-dominant-layer may have formed during crystallization of a deep magma ocean and is possibly still preserved up to the present in the Earth's lower mantle. On the other hand, the MgSiO 3 -SiO 2 eutectic liquid implies that an SiO 2 -phase-rich layer could develop when extensive melting occurs in rocky objects with silica-rich enstatite-chondritic compositions.

Melting phase equilibrium relations in the MgSiO3-SiO2 system under high pressures

“…The XSi value of the eutectic composition increases from 0.556 to 0.61 in a range from 1 to 13.5 GPa (down to ~400 km depth in the mantle) (Dalton and Presnall, 1997;Hudon et al, 2005; this study), which are close to those of Echondrites (0.533-0.577; Wasson and Kallemeyn, 1988). Using a DAC, experimentally determined eutectic compositions (XSi) at 41, 128, and 139 GPa were 0.60, 0.66, and 0.66, respectively (Ozawa et al, 2018). Combined with all the available experimentally determined eutectic compositions, the XSi increases significantly to ~0.61 with increased pressure when lower than 50 GPa and then increases asymptotically to 0.66 up to 128 GPa (Fig.…”

“…Nevertheless, there have been limited studies on the melting behavior of the MgSiO3-SiO2 system at high pressures: e.g., at 1 GPa using the piston cylinder (Hudon et al, 2005) and at 5 GPa using the Kawai-type multianvil apparatus (KMAA) (Dalton and Presnall, 1997). On the other hand, using a diamond anvil cell (DAC), melting experiments in the MgO-SiO2 system were carried out up to 139 GPa (Baron et al, 2017;Ozawa et al, 2018). According to these studies, the MgSiO3-SiO2 system displays eutectic melting and the XSi of the eutectic compositions systematically increases with increased pressure.…”

Low Melting Temperature of Anhydrous Mantle Materials at the Core‐Mantle Boundary

“…The Eu-MFS sample is important for the accurate determination of solidus temperature, because an ideal eutectic composition would undergo complete melting without a mixed phase region of solid and liquid, providing maximum contrast in scattering properties across the melting point (Baron et al, 2017). We note that the existing studies are not in complete agreement on the eutectic composition at the lower-mantle conditions (de Koker et al, 2013;Ozawa et al, 2018). Even if the exact eutectic composition is not clearly known, the larger amount of melt generated from a near-eutectic composition would still improve the accuracy of melt detection because of the expected large production of melt at the solidus.…”

“…The chondritic composition in Andrault et al (2011) had a 0.44 mole fraction of SiO 2 , X (SiO 2 ), substantially higher than that of pyrolite (0.33). Both theoretical and experimental studies (de Koker et al, 2013; Ozawa et al, 2018) have shown that the eutectic X (SiO 2 ) value decreases from 0.4–0.42 to 0.3–0.38 with an increase in pressure from 25 to 136 GPa. Therefore, while the X (SiO 2 ) value of the chondritic composition is close to that of the eutectic composition at low pressures, the difference from eutectic composition would increase with pressure due to the decrease in eutectic X (SiO 2 ).…”

Lower Mantle Melting: Experiments and Thermodynamic Modeling in the System MgO‐SiO2

“…(2017) (Ba17, brown), Ozawa et al. (2018) (O18, green) and Ohnishi et al. (2017) (O17, purple) are included, with triangles in the latter two bracketing the eutectic composition, along with results by Belmonte et al.…”