09:45 〜 10:00
[SIT14-04] Al solubility in high pressure hydrous mineral, phase D and the effect for the stability region
キーワード：高密度含水マグネシウムケイ酸塩、Al-bearing phase D、高温高圧実験
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 Al2 SiO6H2) 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 and Orange-3000 at Ehime University (GRC). The experimental conditions were 20 GPa to 26 GPa, 1200-1600℃, which are corresponding to the conditions in the mantle transition zone to upper lower mantle. The starting materials for the mixtures of MgO, Mg(OH)2, Al2O3 and Al2O3 powders were prepared in the MgSiO3-Al2O3-H2O system with intermediate compositions between Mg- and Al-phase D end members.
The results at 20 GPa and 26 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, it was found that Al3+ and H+ can be replaced up to 1 pfu. These results indicate that this phase D is considered as a third end member (MgAl-phase D, ideal formula MgAlSiH3O6). Since MgAl-phase D has not been reported yet, we determined the thermal stability between 20 GPa and 26 GPa. The result showed that MgAl-phase D has the same thermal stability as Mg-phase D. Although the water content of MgAl-phase D (H2O ~ 15 wt.%) is 1.5 times that of Mg-phase D (H2O ~ 10 wt.%), the thermal stability remains unchanged, which is noteworthy. These results suggest additional hydration extending from the mantle transition zone to the upper lower mantle.
Finally, due to the high water content in MgAl-phase D, we plan to measure the hydrogen positions of MgAl-phase D through neutron experiments at J-PARC before this presentation. The intention is to reveal the reasons for the increased water content and the factors contributing to the identical temperature stability with Mg-phase D.
High-temperature and high-pressure experiments were conducted using a Kawai-type high pressure apparatus, MAPLE600 at Hiroshima University and Orange-3000 at Ehime University (GRC). The experimental conditions were 20 GPa to 26 GPa, 1200-1600℃, which are corresponding to the conditions in the mantle transition zone to upper lower mantle. The starting materials for the mixtures of MgO, Mg(OH)2, Al2O3 and Al2O3 powders were prepared in the MgSiO3-Al2O3-H2O system with intermediate compositions between Mg- and Al-phase D end members.
The results at 20 GPa and 26 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, it was found that Al3+ and H+ can be replaced up to 1 pfu. These results indicate that this phase D is considered as a third end member (MgAl-phase D, ideal formula MgAlSiH3O6). Since MgAl-phase D has not been reported yet, we determined the thermal stability between 20 GPa and 26 GPa. The result showed that MgAl-phase D has the same thermal stability as Mg-phase D. Although the water content of MgAl-phase D (H2O ~ 15 wt.%) is 1.5 times that of Mg-phase D (H2O ~ 10 wt.%), the thermal stability remains unchanged, which is noteworthy. These results suggest additional hydration extending from the mantle transition zone to the upper lower mantle.
Finally, due to the high water content in MgAl-phase D, we plan to measure the hydrogen positions of MgAl-phase D through neutron experiments at J-PARC before this presentation. The intention is to reveal the reasons for the increased water content and the factors contributing to the identical temperature stability with Mg-phase D.