17:15 〜 19:15
[SIT20-P13] Phase relations in the system CaAl2O4-MgAl2O4 up to top-lower-mantle conditions
キーワード：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.
[1] Ishii T, Miyajima N, Criniti G, Hu Q, Glazyrin K, Katsura T. Earth Planet Sci Lett. 2022;584:117472.
[2] Irifune T, Ringwood AE. Earth Planet Sci Lett. 1993;117(1–2):101–10.
[3] Akaogi M, Hamada Y, Suzuki T, Kobayashi M, Okada M. Phys Earth Planet Inter. 1999;115(1):67–77.
[4] Kimura F, Kojitani H, Akaogi M. Phys Earth Planet Inter. 2020;106632.
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.
[1] Ishii T, Miyajima N, Criniti G, Hu Q, Glazyrin K, Katsura T. Earth Planet Sci Lett. 2022;584:117472.
[2] Irifune T, Ringwood AE. Earth Planet Sci Lett. 1993;117(1–2):101–10.
[3] Akaogi M, Hamada Y, Suzuki T, Kobayashi M, Okada M. Phys Earth Planet Inter. 1999;115(1):67–77.
[4] Kimura F, Kojitani H, Akaogi M. Phys Earth Planet Inter. 2020;106632.