Abstract To understand the effects of H2O on the mineral phases forming under the pressure-temperature conditions of the lower mantle, we have conducted laser-heated diamond-anvil cell experiments on hydrous ringwoodite (Mg2SiO4 with 1.1 wt% H2O) at pressures between 29 and 59 GPa and temperatures between 1200 and 2400 K. Our results show that hydrous ringwoodite (hRw) converts to crystalline dense hydrous silica, stishovite (Stv) or CaCl2-type SiO2 (mStv), containing 1 wt% H2O together with Brd and MgO at the pressure-temperature conditions expected for shallow lower-mantle depths between approximately 660 to 1600 km. Considering the lack of sign for melting in our experiments, our preferred interpretation of the observation is that Brd partially breaks down to dense hydrous silica and periclase (Pc), forming the phase assembly Brd + Pc + Stv. The results may provide an explanation for the enigmatic coexistence of Stv and Fp inclusions in lower-mantle diamonds.

中文翻译：

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含水菱镁矿向下地幔相的相变和致密含水二氧化硅的形成

摘要 为了了解 H2O 对下地幔压力-温度条件下矿物相形成的影响，我们在 29 到 1.1 wt% H2O 的压力下对含水菱镁矿（Mg2SiO4 和 1.1 wt% H2O）进行了激光加热金刚石砧电池实验。 59 GPa 和 1200 至 2400 K 之间的温度。我们的结果表明，含水环伍德矿 (hRw) 转化为结晶致密含水二氧化硅、stishovite (Stv) 或 CaCl2 型 SiO2 (mStv)，其中含有 1 wt% H2O 以及 Brd 和 MgO大约 660 至 1600 公里之间的浅下地幔深度预期的压力-温度条件。考虑到在我们的实验中缺乏熔化的迹象，我们对观察结果的首选解释是 Brd 部分分解为致密的含水二氧化硅和方镁石 (Pc)，形成相组合 Brd + Pc + Stv。