09:15 〜 09:30
[SIT20-14] Stability of hydrous minerals in hydrous pyrolite system at high pressure: implication for water transportation into the deep mantle
キーワード：hydrous minerals, pyrolite, high pressure , phase relation, subducting slabs
Water in the nominally anhydrous minerals (NAMs) and hydrous minerals transported into the deep mantle by subducting peridotite is of great significance in the geochemical and dynamical processes in the mantle. The stability of hydrous minerals and water solubility in NAMs control the deep water cycling and slab dynamics. Along a cold slab geotherm, hydrous minerals form as main water hosts while NAMs (e.g. olivine and its high-pressure polymorphs) remain dry [1]. NAMs can accommodate weight precent levels of water after the dehydration of hydrous minerals [2]. Water contents of NAMs control slab deformation, thus stability of hydrous minerals plays an important role on mantle dynamics and water distribution in the deep mantle. Considering limited water transported into the deep mantle, phase relations of hydrous minerals coexisting with NAMs could be applied for a realistic slab condition. However, most previous studies investigated phase relations in MgO-SiO2-H2O systems with high water content (5-20 wt% H2O) where hydrous minerals often coexist with hydrous fluid or melt [3-4].
We previously have determined the stability of hydrous minerals in MgO-SiO2-H2O system with 2 wt% H2O up to 28 GPa and we revealed that the phase relations in MgO-SiO2-H2O systems depends on water content and Mg/Si ratio in the starting materials. The stability of hydrous minerals changes when NAMs coexist [5]. In this study, we further explored the phase relations in more complicated systems to study the effect of other major components (e.g. Al2O3, FeO, CaO) in subducting peridotite on stability of hydrous minerals when they coexist with NAMs using a multi-anvil press up to 22 GPa and 1400 °C. Two starting materials, FeO-MgO-Al2O3-SiO2 +2wt% H2O (FMASH) and pyrolite + 2wt% H2O controlled by Ni-NiO buffer were prepared to assess the phase relations of hydrous simplified pyrolite and realistic pyrolite, respectively.
In FMASH system, at 16 GPa and 900 °C, super-hydrous phase B and phase D were observed coexisting with ringwoodite, stishovite and majorite. Above 1000 °C, hydrous minerals decomposed to form an assemblage of wadsleyite + clinoenstatite + majorite. Above 18 GPa at 1200 °C, phase D + ringwoodite + stishovite + majorite were stable.
In hydrous pyrolite system, the thermal stability of hydrous minerals decreased compared to that in FMASH system. At 18-20 GPa, super hydrous phase B and phase D were stable along with ringwoodite, stishovite, majorite and davemaoite below 1100 °C. At 22 GPa and 1200 °C, phase D was found to coexist with ringwoodite, stishovite majorite and davemaoite. When the temperature exceeded 1200 °C, phase D dehydrated. Compared to our previous phase relations in a simplified pyrolite system (Mg1.3SiO3.3 + 2 wt% H2O) [5], thermal stability of super hydrous phase B is largely decreased while phase D has enlarged P-T stability. The dehydration temperature of hydrous minerals in FMASH systems was nearly consistent with that in Mg1.3SiO3.3 + 2 wt% H2O, but ~ 100 °C lower in hydrous pyrolite system. We proposed that phase relation and stability of hydrous minerals in hydrous slabs are largely depends on the water content, Mg/Si ratio and the major component like CaO and Al2O3. The major water carrier in the cold subducting peridotite is phase D to the depth of uppermost lower mantle.
[1] Ohtani and Ishii, Prog. Earth Planet. Sci. 2024. 11: 1-20.
[2] Litasov et al., Phys. Chem. Miner. 2011.38:75-84.
[3] Frost, The Geochemical Society. 1999.
[4] Ohtani et al., Phys. Chem. Miner. 2000. 27:533-544.
[5] Zhu et al., 2024, AGU2024 abstract.
We previously have determined the stability of hydrous minerals in MgO-SiO2-H2O system with 2 wt% H2O up to 28 GPa and we revealed that the phase relations in MgO-SiO2-H2O systems depends on water content and Mg/Si ratio in the starting materials. The stability of hydrous minerals changes when NAMs coexist [5]. In this study, we further explored the phase relations in more complicated systems to study the effect of other major components (e.g. Al2O3, FeO, CaO) in subducting peridotite on stability of hydrous minerals when they coexist with NAMs using a multi-anvil press up to 22 GPa and 1400 °C. Two starting materials, FeO-MgO-Al2O3-SiO2 +2wt% H2O (FMASH) and pyrolite + 2wt% H2O controlled by Ni-NiO buffer were prepared to assess the phase relations of hydrous simplified pyrolite and realistic pyrolite, respectively.
In FMASH system, at 16 GPa and 900 °C, super-hydrous phase B and phase D were observed coexisting with ringwoodite, stishovite and majorite. Above 1000 °C, hydrous minerals decomposed to form an assemblage of wadsleyite + clinoenstatite + majorite. Above 18 GPa at 1200 °C, phase D + ringwoodite + stishovite + majorite were stable.
In hydrous pyrolite system, the thermal stability of hydrous minerals decreased compared to that in FMASH system. At 18-20 GPa, super hydrous phase B and phase D were stable along with ringwoodite, stishovite, majorite and davemaoite below 1100 °C. At 22 GPa and 1200 °C, phase D was found to coexist with ringwoodite, stishovite majorite and davemaoite. When the temperature exceeded 1200 °C, phase D dehydrated. Compared to our previous phase relations in a simplified pyrolite system (Mg1.3SiO3.3 + 2 wt% H2O) [5], thermal stability of super hydrous phase B is largely decreased while phase D has enlarged P-T stability. The dehydration temperature of hydrous minerals in FMASH systems was nearly consistent with that in Mg1.3SiO3.3 + 2 wt% H2O, but ~ 100 °C lower in hydrous pyrolite system. We proposed that phase relation and stability of hydrous minerals in hydrous slabs are largely depends on the water content, Mg/Si ratio and the major component like CaO and Al2O3. The major water carrier in the cold subducting peridotite is phase D to the depth of uppermost lower mantle.
[1] Ohtani and Ishii, Prog. Earth Planet. Sci. 2024. 11: 1-20.
[2] Litasov et al., Phys. Chem. Miner. 2011.38:75-84.
[3] Frost, The Geochemical Society. 1999.
[4] Ohtani et al., Phys. Chem. Miner. 2000. 27:533-544.
[5] Zhu et al., 2024, AGU2024 abstract.