Melting of Bridgmanite Under Hydrous Shallow Lower Mantle Conditions
Melting of Bridgmanite Under Hydrous Shallow Lower Mantle Conditions
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DOI：
10.1029/2021jb022222
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发表时间：
2021-09-01
影响因子：
3.9
通讯作者：
Girard, Jennifer
中科院分区：
文献类型：
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作者：
Amulele, George;Karato, Shun-ichiro;Girard, Jennifer
在含水浅下地幔条件下（24-25 GPa和1673-1873 K），对对应于布里奇曼石化学计量比的氧化物混合物进行了高压和高温实验。%的水）。研究的氧化物混合物对应于MgSiO 3、（Mg，Fe）SiO 3、（Mg，Al，Si）O-3和（Mg，Fe，Al，Si）O-3。在所有运行中均观察到熔化。分配的各种元素，包括Mg，Fe，Si和H的研究。在含水下地幔条件下熔融导致熔体中的（Mg + Fe）O/SiO2比残余固体增加。残余固体通常含有大量的硅镁矾，并且熔体含有比初始材料更高的（Mg，Fe）O/SiO2比。(Mg富Fe）O的含水熔体可以解释下地幔浅层的低速异常，而残留固体中的大量斯石英可能是中下地幔地震波散射的原因，并可以解释“斯石英佯谬”。“由于富含硬硅钙石的物质只有在富含二氧化硅的源岩（MORB）熔融时才形成（而不是典型的橄榄岩[块状硅酸盐地球]），下地幔中的地震散射提供了俯冲MORB物质循环的线索。为了估计氢含量，我们使用了一种新的方法来估计水含量的不可淬火的熔体，也提出了一个新的解释的意义，超水相B包裹体在bridgmanite。结果提供了修订值的水分配之间的固体矿物和含水熔体，大大高于以前的估计。简单的语言摘要熔融是一个主要的过程，地球的化学演变。然而，在深地幔条件下，熔融的后果还没有得到很好的理解。这项研究的重点是在水的帮助下发生的浅下地幔条件下的熔融。使用新的方法研究了熔体和剩余固体之间的水分布的性质。我们发现，当熔融发生的材料与海洋地壳的成分相似，残留的固体几乎完全是斯石英（SiO2）。Stishovite显示出异常的弹性性能低于类似的800公里的深度取决于组成。在这个深度范围内观察到的强烈地震散射可能是由于存在下地幔熔融形成的富含硬硅钙石的物质。
High pressure and temperature experiments were carried out on the oxide mixtures corresponding to the bridgmanite stoichiometry under the hydrous shallow lower mantle conditions (24-25 GPa and 1673-1873 K with 5-10 wt. % of water in the starting material). Oxide mixtures investigated correspond to MgSiO3, (Mg, Fe)SiO3, (Mg, Al, Si)O-3, and (Mg, Fe, Al, Si)O-3. Melting was observed in all runs. Partitioning of various elements, including Mg, Fe, Si, and H is investigated. Melting under hydrous lower mantle conditions leads to increased (Mg + Fe)O/SiO2 in the melt compared to the residual solids. The residual solids often contain a large amount of stishovite, and the melt contains higher (Mg,Fe)O/SiO2 ratio than the initial material. (Mg + Fe)O-rich hydrous melt could explain the low-velocity anomalies observed in the shallow lower mantle and a large amount of stishovite in the residual solid may be responsible for the scattering of seismic waves in the mid-lower mantle and may explain the "stishovite paradox." Since stishovite-rich materials are formed only when silica-rich source rock (MORB) is melted (not a typical peridotitic rock [bulk silicate Earth]), seismic scattering in the lower mantle provides a clue on the circulation of subducted MORB materials. To estimate hydrogen content, we use a new method of estimating the water content of unquenchable melts, and also propose a new interpretation of the significance of superhydrous phase B inclusions in bridgmanite. The results provide revised values of water partitioning between solid minerals and hydrous melts that are substantially higher than previous estimates.Plain Language Summary Melting is a major process by which Earth evolves chemically. However, the consequence of melting is not well understood under the deep mantle conditions. This study focuses on melting under the shallow lower mantle conditions that occurs with the help of water. Nature of water distribution between the melt and remaining solids was studied using new approaches. We discovered that when melting occurs for a material with composition similar to oceanic crust, the residual solid is nearly completely stishovite (SiO2). Stishovite shows anomalous elastic properties below similar to 800 km depth depending on the composition. Observed strong seismic scattering in this depth range might be due to the presence of stishovite-rich materials formed by melting in the lower mantle.