Hydrous silicate melts and the deep mantle H2O cycle
Hydrous silicate melts and the deep mantle H2O cycle
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DOI：
10.1016/j.epsl.2022.117408
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发表时间：
2022-02-11
影响因子：
5.3
通讯作者：
Lord, Oliver T.
中科院分区：
文献类型：
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作者：
Drewitt, James W. E.;Walter, Michael J.;Lord, Oliver T.
我们报道了深部上地幔到浅部下地幔条件下的含水硅酸盐熔体的从头算原子模拟，并用它们来参数化MgO-SiO_2-H2O(MSH)三元系的密度和粘度。根据MSH体系的相关系，地幔的原生含水部分熔体含有40-50摩尔%的H2O。我们的结果表明，这些熔体在地幔过渡带的上界和下界具有正浮力，除非在非常富铁的成分中，大于或接近75%的镁被铁取代。含水的部分熔体也是高度无粘性的。我们的结果表明，如果在410公里处发生熔融，当瓦兹利石转变为橄榄石时，熔体将被浮力熔化，熔体堆积是意想不到的。考虑过渡带上下部分熔体向上迁移的地幔环流箱式模型表明，上地幔以牺牲过渡带为代价而变得有效水化，因此在地幔循环的时间尺度上，上地幔、过渡带和下地幔之间H2O浓度的巨大差异很难维持。0.02~0.04wt%H2O的MORB源地幔可能代表了过渡带和下地幔的H2O含量，形成了0.5~1大洋量级的块状地幔H2O含量，这与地球化学约束和俯冲吸入作用的估算相一致。(C)2022年提交人(S)。爱思唯尔出版公司(Elsevier B.V.)
We report ab initio atomistic simulations of hydrous silicate melts under deep upper mantle to shallow lower mantle conditions and use them to parameterise density and viscosity across the ternary system MgO-SiO2-H2O (MSH). On the basis of phase relations in the MSH system, primary hydrous partial melts of the mantle have 40-50 mol% H2O. Our results show that these melts will be positively buoyant at the upper and lower boundaries of the mantle transition zone except in very iron-rich compositions, where greater than or similar to 75% Mg is substituted by Fe. Hydrous partial melts will also be highly inviscid. Our results indicate that if melting occurs when wadsleyite transforms to olivine at 410 km, melts will be buoyant and ponding of melts is unexpected. Box models of mantle circulation incorporating the upward mobility of partial melts above and below the transition zone suggest that the upper mantle becomes efficiently hydrated at the expense of the transition zone such that large differences in H2O concentration between the upper mantle, transition zone and lower mantle are difficult to maintain on timescales of mantle recycling. The MORB source mantle with similar to 0.02-0.04 wt% H2O may be indicative of the H2O content of the transition zone and lower mantle, resulting in a bulk mantle H2O content of the order 0.5 to 1 ocean mass, which is consistent with geochemical constraints and estimates of subduction ingassing. (c) 2022 The Author(s). Published by Elsevier B.V.