[S-IT20] Deep Earth Sciences

[SIT20-P13] Phase relations in the system CaAl₂O₄-MgAl₂O₄ up to top-lower-mantle conditions

*Jinze He^1,2, Zhaodong Liu², Takayuki Ishii¹ (1.Institute for Planetary Materials, Okayama University, 2.Synergetic Extreme Condition High-Pressure Science Center, State Key Laboratory of Superhard Materials, Jilin University)

Keywords:Calcium-ferrite phase, NAL phase, CaAl2O4-MgAl2O4 system, multi-anvil press, mantle, subducting basaltic crust

Many experimental studies have shown that under lower mantle conditions, Al-rich phases of hexagonal phase (NAL phase, with a chemical formula as AB₂C₆O₁₂) and calcium-ferrite phase (CF phase, AB₂O₄ or XYZO₄) are major phases in a subducted basaltic crust^[1-2]. Although CF phase is considered to be stable down to the bottom of the lower mantle, the stability of NAL phase is still under debate. In addition, compositional changes of these phases under lower mantle conditions are not clear yet. In the system CaAl₂O₄-MgAl₂O₄, which are important endmembers for the Al-rich phases in a basaltic composition, at 1200 °C above 15 GPa^[3-4], CaAl₂O₄-rich or MgAl₂O₄ CF phase and CaAl₂O₄-MgAl₂O₄ NAL phase are stable. The MgAl₂O₄ components in the CaAl₂O₄-rich CF phase and NAL phase increase with pressure. However, temperature dependence on chemical compositions and stabilities of these phases, which is important due to temperature variation in the mantle, has not been investigated yet. In this study, we conducted high-pressure phase-relation experiments in the system CaAl₂O₄-MgAl₂O₄ at 15-25 GPa and 1100-2000 °C using a multi-anvil press to understand compositional changes and stability of these phases in the mantle.
Our results showed that CF and NAL phases are stable in the present pressure-temperature range. As shown in the previous studies, the solubility of the MgAl₂O₄ component in the CaAl₂O₄-rich CF phase increased with increasing pressure in the temperature range of 1100-2000 °C. With increasing temperature, the MgAl₂O₄ component in the CaAl₂O₄-rich CF phase also increased. In the binary phase diagram, the MgAl₂O₄ compositional width of the stability field of the NAL phase expanded slightly from 3 mol% to 5 mol% with increasing temperature. Because the chemical composition of the CaAl₂O₄-rich CF phase systematically changes with pressure and temperature, we suggest that the compositional change can be used as a pressure calibrant at high temperatures under high pressure.

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