The stability of hydrous silicates in Earth's lower mantle: Experimental constraints from the systems MgO-SiO2-H2O and MgO-Al2O3-SiO2-H2O
The stability of hydrous silicates in Earth's lower mantle: Experimental constraints from the systems MgO-SiO2-H2O and MgO-Al2O3-SiO2-H2O
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DOI：
10.1016/j.chemgeo.2015.05.001
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发表时间：
2015-12-15
期刊：
影响因子：
3.9
通讯作者：
Kohn, S. C.
中科院分区：
文献类型：
--
作者：
Walter, M. J.;Thomson, A. R.;Kohn, S. C.
我们进行了激光加热金刚石对顶砧实验，在系统MgO-SiO2-H2O（MSH）和MgO-Al 2 O3-SiO2-H2O（MASH），限制地球下地幔水合相的稳定性的块体组成。通过同步辐射粉末衍射的相鉴定揭示了高压、致密的含水硅酸盐相D和H的一组一致的稳定性关系。在MSH体系中，D相稳定在50 GPa左右，在1300 ~ 1700 K范围内与温度无关。H相在35 ~ 40 GPa之间稳定，在1600 K下，H相的反相反应发生在55 GPa附近，负的dT/dP斜率约为-75 K/GPa。在类似于30和50 GPa之间，脱水熔融发生在类似于1800 K，具有平坦的dT/dP斜率。沿固相线的一个沿着尖点在近似50 GPa处对应于固相线下相H出反应的交点，并且dT/dP熔化斜率变陡到近似15 K/GPa直到近似85 GPa。在MASH系统中，相H在实验中在近似45和115 GPa之间在所研究的所有本体组成中是稳定的，我们预计铝质相H在橄榄岩和玄武岩岩性中在大约1200 km以下的下地幔深度范围内是稳定的。在亚固相线中，铝相D是稳定的，接近55 GPa，而在更高的压力下，铝相H是稳定的含水相。氢的存在可以使桥镁石向后钙钛矿转变尖锐。周围的晶胞体积的bridgmanite增加系统的压力以上类似于55 GPa，可能代表的氧化铝含量的增加，并可能氢含量，与深度。与D相和H相平衡的硼镁石具有相对低的氧化铝含量，并且氧化铝优先分配到含水相中。MASH组合物的熔化曲线比MSH系统中的浅，dT/dP类似于6 K/GPa。D相和H相固溶体在冷的水合俯冲板中是稳定的，并且可以将水输送到最深的下地幔。然而，下地幔中的水化岩石很可能是部分熔融在所有深度沿着环境地幔地热。(C)2015年，作者。由Elsevier B. V.发布，这是CC BY许可下的开放获取文章。
We performed laser-heated diamond anvil cell experiments on bulk compositions in the systems MgO-SiO2-H2O (MSH) and MgO-Al2O3-SiO2-H2O (MASH) that constrain the stability of hydrous phases in Earth's lower mantle. Phase identification by synchrotron powder diffraction reveals a consistent set of stability relations for the high-pressure, dense hydrous silicate phases D and H. In the MSH system phase D is stable to similar to 50 GPa, independent of temperature from similar to 1300 to 1700 K. Phase H becomes stable between 35 and 40 GPa, and the phase H out reaction occurs at similar to 55GPa at 1600 K with a negative dT/dP slope of similar to-75 K/GPa. Between similar to 30 and 50 GPa dehydration melting occurs at similar to 1800 K with a flat dT/dP slope. A cusp along the solidus at similar to 50 GPa corresponds with the intersection of the subsolidus phase H out reaction, and the dT/dP melting slope steepens to similar to 15 K/GPa up to similar to 85 GPa.In the MASH system phase H is stable in experiments between similar to 45 and 115 GPa in all bulk compositions studied, and we expect aluminous phase H to be stable throughout the lower mantle depth range beneath similar to 1200 km in both peridotitic and basaltic lithologies. In the subsolidus, aluminous phase D is stable to similar to 55 GPa, whereas at higher pressures aluminous phase H is the stable hydrous phase. The presence of hydrogen may sharpen the bridgmanite to post-perovskite transition. The ambient unit cell volume of bridgmanite increases systematically with pressure above similar to 55 GPa, possibly representing an increase in alumina content, and potentially hydrogen content, with depth. Bridgmanite in equilibrium with phases D and H has a relatively low alumina content, and alumina partitions preferentially into the hydrous phases. The melting curves of MASH compositions are shallower than in the MSH system, with dT/dP of similar to 6 K/GPa. Phase D and H solid solutions are stable in cold, hydrated subducting slabs and can deliver water to the deepest lower mantle. However, hydrated lithologies in the lower mantle are likely to be partially molten at all depths along an ambient mantle geotherm. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.