[S-IT17] Property and role of liquids inside terrestrial planets

[SIT17-P04] Melting phase equilibrium relations in the MgSiO₃-SiO₂ system at 13.5 GPa

*Takuya Moriguti¹, Yusuke Yachi¹, Akira Yoneda¹, Eiji Ito¹ (1.Institute for Planetary Materials, Okayama University)

Keywords:MgSiO3-SiO2 system, Enstatite chondrite model, Mantle, Multi anvil, High pressure and high temperature, Pressure calibration

Melting relations in the system MgSiO₃-SiO₂ were investigated at 13.5 GPa and up to 2900 ºC using the Kawai-type of multi anvil apparatus (KMAA) in this study. The melting relations of the system MgO-SiO₂ as a representative of the mantle have been extensively studied since a pioneering work by Bowen and Anderson (1914). However, almost all of these works have been carried out on the compositions ranging from MgO to MgSiO₃ because the bulk mantle composition has been assumed to be peridotitic or close to that derived from CI chondrite. On the other hand, enstatite chondrite (E-chondrite) has been recommended as the source material of bulk Earth (e.g., Javoy et al., 2010) because the isotope compositions of the Earth, Moon and E-chondrite are indistinguishable over O, N, Mo, Ru, Os, Cr, and Ti. In order to understand the mantle differentiation in the E-chondrite model, it is indispensable to clarify the melting relations in the system MgSiO₃-SiO₂ at high pressures. Nevertheless, there have been limited works on the melting behavior of the system MgSiO₃-SiO₂ under high pressures. In this study, we aimed to determine the melting relations in the system MgSiO₃-SiO₂ at 13.5 GPa as precisely as possible using the KMAA. We found that the X_Si value (SiO₂/(SiO₂+MgO) (mol)) of the eutectic composition increases from 0.556 to 0.61 at a range from 1 to 13.5 GPa (down to ~400 km depth in mantle) (Hudon et al., 2005; Dalton and Presnall, 1997; Moriguti et al., under review), which are close to those of E-chondrites (0.533 - 0.577). This implies that if bulk Earth composition corresponds to E-chondrites, the upper mantle composition resulting from fractional crystallization of a terrestrial magma ocean would be around 0.6 in X_Si, significantly enriched in SiO₂ compared with estimates of the current upper mantle composition, 0.47. In addition, melting relations in the MgSiO₃ - SiO₂ system up to 13.5 GPa show that the liquidus phase is enstatite if E-chondrites melted in the magma ocean. Consequently, it is impossible to produce the present upper mantle composition through simple fractional crystallization processes without efficient Si partitioning out from a silicate mantle to molten iron (the core).

In the present study, the sample heating was conducted up to 2900 °C. In this presentation, the special endeavor to pressure calibration at such extreme temperature are also shown. In the multi anvil apparatus, the generated pressure is substantially altered on heating the specimen due to the increases of the thermal expansion and the fluidity of the pressure medium as shown in previous works conducted at high temperatures, > 1000 °C (e.g., Gasparik, 1989; Ito and Takahashi, 1989). However, the high temperature effect to the generated pressure under > 2000 °C has not been known well. Therefore, the temperature effect was also evaluated at 2500 and 2850 °C on the basis of phase transition between coesite and stishovite in SiO₂ determined by Zhang et al. (1996). Compared with the generated pressure at room temperature (10.5 GPa), the pressure increases by 2.1 and 3.0 GPa were recognized at 2500 °C against the press load of 2.25 MN and at 2850 °C against that of 2.4 MN, respectively.

References
Dalton, J.A. and Presnall, D.C. (1997) Geochim. Cosmochim. Acta 61, 2367-2373.
Gasparik, T. (1989) Contrib. Min. petrol., 102, 389-405.
Hudon P. et al. (2005) J. Petrol., 46, 1859-1880.
Ito, E. and Takahashi, E. (1989) J. Geophys. Res. 94, 10637-10646.
Javoy M. et al. (2010) Earth Planet. Sci. Let. 293, 259-268.
Moriguti, T. et al. (under review) Am. Min.
Zhang, J. et al. (1996) Phys. Chem. Minerals 23, 1-10.