09:30 〜 09:45
[SMP27-03] 高圧含水鉱物 phase D中へのAlの固溶及び安定領域への影響
キーワード：高温高圧実験、Al-bearing phase D、高圧含水鉱物
It is known that water is supplied into the Earth's interior by subducting slab. The dense hydrous magnesium silicate (DHMS) phases play an important role in transporting water to further depths through slab subduction. On the other hand, the components of oceanic crust such as Si, Fe and Al also supplied into the Earth’s interior. Particularly, Al is one of the important elements because it increases the melting temperature of minerals.
Phase D of Mg-endmember (Mg-phase D, ideal formula MgSi2O6H2), which is one of the important DHMS, is stable in the mantle transition zone and the lower mantle. The stability is limited only in low temperature region (<1200℃) such as subducted slab (Frost and Fei, 1998). Recently, phase D of Al-endmember (Al-phase D, ideal formula Al2SiO6H2) was discovered (Pamato et al, 2014), which is stable at temperatures up to 2000℃. Since these two phases have a similar crystal structure, it is considered to have solid solution between Mg- and Al-phase D end members. However, the actual solid solution has not been reported yet. In this study, we investigated the possible existence of the solid solutions between these two phases under the mantle transition zone condition.
High-temperature and high-pressure experiments were conducted using a Kawai-type high pressure apparatus, MAPLE600 at Hiroshima University. The experimental conditions were 20 GPa, 1200-1600℃, which are corresponding to the conditions in the mantle transition zone. The starting materials of the mixtures of MgO, Al2O3 and Mg(OH)2 powders were prepared in the MgSiO3-Al2O3-H2O system with intermediate compositions between Mg- and Al-phase D end members.
The results at 1200℃, 20 GPa showed that no solid solution between Mg- and Al-phase D was observed. On the other hand, we observed that Mg-phase D incorporates Al3+ and H+ by decreasing Si4+. In addition, the existence of the solubility limit was found in the phase D. These results indicate that this phase D is considered to be a third end member. Since this phase D has not been reported yet, we determined the thermal stability at 20 GPa.
The result showed that this phase D (H2O ~15 wt%) is stable up to ~1200℃ which is almost the same as that of Mg-phase D, in spite of 1.5 times H2O content compared to Mg-phase D (H2O ~10 wt%). This implies that this phase D can be one of the important H2O reservoirs in the mantle transition zone.
Phase D of Mg-endmember (Mg-phase D, ideal formula MgSi2O6H2), which is one of the important DHMS, is stable in the mantle transition zone and the lower mantle. The stability is limited only in low temperature region (<1200℃) such as subducted slab (Frost and Fei, 1998). Recently, phase D of Al-endmember (Al-phase D, ideal formula Al2SiO6H2) was discovered (Pamato et al, 2014), which is stable at temperatures up to 2000℃. Since these two phases have a similar crystal structure, it is considered to have solid solution between Mg- and Al-phase D end members. However, the actual solid solution has not been reported yet. In this study, we investigated the possible existence of the solid solutions between these two phases under the mantle transition zone condition.
High-temperature and high-pressure experiments were conducted using a Kawai-type high pressure apparatus, MAPLE600 at Hiroshima University. The experimental conditions were 20 GPa, 1200-1600℃, which are corresponding to the conditions in the mantle transition zone. The starting materials of the mixtures of MgO, Al2O3 and Mg(OH)2 powders were prepared in the MgSiO3-Al2O3-H2O system with intermediate compositions between Mg- and Al-phase D end members.
The results at 1200℃, 20 GPa showed that no solid solution between Mg- and Al-phase D was observed. On the other hand, we observed that Mg-phase D incorporates Al3+ and H+ by decreasing Si4+. In addition, the existence of the solubility limit was found in the phase D. These results indicate that this phase D is considered to be a third end member. Since this phase D has not been reported yet, we determined the thermal stability at 20 GPa.
The result showed that this phase D (H2O ~15 wt%) is stable up to ~1200℃ which is almost the same as that of Mg-phase D, in spite of 1.5 times H2O content compared to Mg-phase D (H2O ~10 wt%). This implies that this phase D can be one of the important H2O reservoirs in the mantle transition zone.