[S-IT15] Mass and energy transport properties and processes in the crust and the mantle

[SIT15-P10] Phase relations in MgO-SiO₂-H₂O systems up to uppermost lower mantle conditions: Towards understanding precise water cycle and distribution in the mantle

*Jintao Zhu^1,2,3、Takayuki Ishii¹、Renbiao Tao³、Lifei Zhang² (1.Institute for Planetary Materials, Okayama University、2.MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University、3.Center for High Pressure Science and Technology Advanced Research (HPSTAR))

キーワード：hydrous phases, phase relations, Olivine polymorphs, deep water cycling

Water in the deep mantle is of great significance in the geochemical and dynamical processes, i.e., it promotes the magma generation, enhances the transformation kinetics, and softens the mantle materials. Subducting slabs transport water as OH^- and H⁺ in hydrous phases and nominally anhydrous minerals. The stabilities and water contents of these phases thus control the deep water cycling and slab dynamics. Especially, hydrous minerals stable at relatively low temperatures can accommodate significant amounts of water and their dehydration reactions can significantly affect slab dynamics due to hydrous magma formation and water supply to nominally anhydrous minerals. Most of the previous studies have focused on phase relations in systems of hydrous minerals (e.g. 1, 2). However, water contents of slabs subducted in the deep mantle are considered to be less than few wt.%, resulting in coexistence of hydrous phases with nominally anhydrous minerals such as olivine and its polymorphs. These nominally anhydrous minerals are essentially dry when coexisting with hydrous minerals along cold slab geotherma (3 ), although they can accommodate weight percent levels of water (4) after the dehydration of hydrous minerals happens. Therefore, it is essential to systematically determine phase stabilities of hydrous minerals coexisting with nominally anhydrous minerals in realistic hydrous mantle compositions. Here we conducted high-pressure experiments in MgO-SiO₂-H₂O systems at 8-25 GPa, 800-1400 ℃ using multi-anvil presses. Two types of starting materials with 2 wt% H₂O were used in the experiments (Mg/Si = 2 and ~1.3 corresponding to forsterite and simplified peridotite compositions, respectively). In the Mg/Si=2 system, wadsleyite was observed at 14-18 GPa, and the phase assmblages were transformed to ringwoodite+stishovite+super hydrous phase B at 20 GPa and ringwoodite+akimotite+super-hydrous phase B at 22 GPa under 1200 ℃. With increasing temperature, super-hydrous phase B decomposed under 1200 ℃ at 18 GPa and 1300-1400 ℃ at 20-22 GPa to form fluid/melt +wadsleyite/ringwoodite. In the Mg/Si=~1.3 system, the decomposition temperature of hydrous minerals was slightly lower. Run products above dehydration boundary consisted of wadsleyite+clinoensite at 14 GPa under 1000 ℃, wadsleyite+stishovite at 16-18 GPa under 1200 ℃, ringwoodite+stishovite at 20 GPa under 1300 ℃ and ringwoodite+akimotoite at 22 GPa under 1400 ℃. In the present hydrous systems, the dehydration boundaries were almost the same to previous hydrous minerals systems (5) and mantle systems with relatively high water contents (5-11 wt%H₂O) (2). Comparing to the water-saturated systems (20 wt% H₂O) (1), the dehydration temperature was lower but still above the transformation threshold temperature of olivine and wadsleyite, indicating dry conditions for the phase transition of olivine to its high-pressure polymorphs. Hydrous phases with more water content like phase D were absent under low termperature in this study, however, super-hydrous phase B remained stable along slab geotherma. The harzburgite layer, which is the coldest region and has a higher Mg/Si ratio than the peridotitic layer, in a subducting slab may be able to transport water as hydrous minerals more efficiently than the peridotitic layer due to higher temperature stability of hydrous minerals in the layer.