Japan Geoscience Union Meeting 2019

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-MP Mineralogy & Petrology

[S-MP33] Physics and Chemistry of Minerals

Wed. May 29, 2019 9:00 AM - 10:30 AM A07 (TOKYO BAY MAKUHARI HALL)

convener:Seiji Kamada(Frontier Research Institute for Interdisciplinary Sciences, Tohoku University), Masahiro KAYAMA(Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University), Chairperson:Seiji Kamada(FRIS, Tohoku Univ.)

10:15 AM - 10:30 AM

[SMP33-06] Drop-solution enthalpy measurement of MgSiO3 majorite

*Hiroshi Kojitani1, Masamichi Noda2, Toru Inoue2,3, Masaki Akaogi1 (1.Department of Chemistry, Faculty of Science, Gakushuin University, 2.GRC, Ehime University, 3.Dept. of Earth and Planetary systems Science, Hiroshima University)

Keywords:garnet, majorite, enthalpy, thermodynamic stability, thermodynamic calculation

Silicate garnet is one of the major Earth's mantle constituent minerals. With increasing depth, pyroxene components dissolve into the silicate garnet. MgSiO3 majorite (Mj) is an important endmember to express such the pyroxene component dissolving in the silicate garnet solid solution. Recently, we experimentally determined isobaric heat capacity and entropy of MgSiO3 Mj. The stability field of MgSiO3 Mj in the MgSiO3 system thermodynamically calculated using these new thermodynamic data of MgSiO3 Mj together with its enthalpy data measured by Yusa et al. (1993) and Saikia (2008) is not consistent with that determined by previous high-pressure phase relation experiments. In this study, the standard enthalpy of MgSiO3 Mj was determined by drop-solution enthalpy measurement and the obtained enthalpy value was applied to examine the phase stability field of MgSiO3 Mj by thermodynamic calculation.

MgSiO3 Mj samples for the drop-solution calorimetry was the same as those used in the heat capacity measurement, which were synthesized at 19 GPa and 2173 K using a high-pressure apparatus at Ehime University, GRC. The sintered polycrystalline samples were crushed into powder. The drop-solution calorimetry was performed using a Calvet-type high-temperature calorimeter (SETARAM, HT-1000). The powdered sample with weight of about 4 mg was compressed into a pellet and it was dropped into the lead borate solvent (2PbO·B2O3) placed in the calorimeter which was kept at 978 K. The summation of heat content from room temperature to 978 K and solution enthalpy at 978 K, namely drop-solution enthalpy (ΔHd-s), was measured. To hasten the solution of the samples, the solvent was stirred by bubbling with Ar gas.

ΔHd-s (MgSiO3 Mj) was determined to be 69.57±0.77 kJ/mol from the average of five data. This value is the middle between 80.0±2.5 kJ/mol (Yusa et al., 1993) and 59.5±3.2 kJ/mol (Saikia, 2008). From the difference between ΔHd-s (MgO) (33.74±0.99 kJ/mol, Kojitani et al. 2012) +ΔHd-s (SiO2 quartz) (40.05±0.36 kJ/mol, Akaogi et al. 1995) and our ΔHd-s (MgSiO3 Mj), the formation enthalpy from the oxides was obtained as 4.2±1.3 kJ/mol. The thermodynamic calculation of the P-T phase diagram by adopting the present enthalpy value suggests that the stability field of MgSiO3 Mj would expand to lower temperature region than that determined by previous high-pressure high-temperature experiments and thermodynamic calculations.