3:30 PM - 3:45 PM
[SIT19-07] Phase relations in a natural hydrous basaltic composition under the lower mantle conditions
Keywords:lower mantle, hydrous basalt, water transportation, Al-rich phase H
It has been believed that basaltic crust materials completely dehydrate at relatively low pressures and would not play any important roles in delivering water into deep mantle upon subduction of slabs (Litasov and Ohtani, 2005). Recently, however, the formation of some new hydrous phases, i.e., Fe-Ti oxyhydroxide, Al-rich phase D, and Al-rich phase H (Al-PhH, hereafter), in basaltic compositions was reported in hydrous basaltic compositions under the relatively low temperature and at pressures to 26 GPa (Liu et al., 2019), which shed light on the possibility of water transport to the mantle transition region and uppermost lower mantle. Water has a significant effect on the physical properties of Earth’s interior, and it is important to understand detailed phase relations in hydrous basalt compositions under the lower mantle conditions to address the possibility of transportation of water into the lower mantle.
In this study, attempts were made to reveal phase relations in a hydrous basalt composition at pressures of 27 and 30GPa and at temperatures 1000, 1200, and 1400 for up to 36hours, using natural hydrous basalt (JB-1b with a water content of 2.59wt%). The formation of Al-rich MgSiO3 bridgmanite, Al-rich stishovite, CaSiO3 perovskite, NAL phase, K-rich hollandite, and Al-PhH was confirmed in the hydrous basalt composition, depending on the pressure and temperature conditions. As for the hydrous phase, the presence of Al-PhH is confirmed at 1200℃ at 27GPa and at 1200 and 1400℃ at 30GPa. Based on the obtained compositional data sets and mineral proportions evaluated by mass-balance calculations, it is shown that the hydrous basalt may retain 1.5-3.0wt% water as Al-PhH under the pressure and temperature conditions, close to those expected in cold slab subducted into the uppermost lower mantle. This indicates that the hydrous mineral may exist in cold subducting slabs under the lower mantle conditions, and when water is retained by hydrous minerals at low pressures, Al- PhH transports some amount of water into even deeper regions of the Earth’s mantle. Further experiments at higher pressure and temperature are currently being pursued using sintered diamond anvils, the results of which will also be shown in the present talk.
In this study, attempts were made to reveal phase relations in a hydrous basalt composition at pressures of 27 and 30GPa and at temperatures 1000, 1200, and 1400 for up to 36hours, using natural hydrous basalt (JB-1b with a water content of 2.59wt%). The formation of Al-rich MgSiO3 bridgmanite, Al-rich stishovite, CaSiO3 perovskite, NAL phase, K-rich hollandite, and Al-PhH was confirmed in the hydrous basalt composition, depending on the pressure and temperature conditions. As for the hydrous phase, the presence of Al-PhH is confirmed at 1200℃ at 27GPa and at 1200 and 1400℃ at 30GPa. Based on the obtained compositional data sets and mineral proportions evaluated by mass-balance calculations, it is shown that the hydrous basalt may retain 1.5-3.0wt% water as Al-PhH under the pressure and temperature conditions, close to those expected in cold slab subducted into the uppermost lower mantle. This indicates that the hydrous mineral may exist in cold subducting slabs under the lower mantle conditions, and when water is retained by hydrous minerals at low pressures, Al- PhH transports some amount of water into even deeper regions of the Earth’s mantle. Further experiments at higher pressure and temperature are currently being pursued using sintered diamond anvils, the results of which will also be shown in the present talk.