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首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >Effects of melting, subduction-related metasomatism, and sub-solidus equilibration on the distribution of water contents in the mantle beneath the Rio Grande Rift
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Effects of melting, subduction-related metasomatism, and sub-solidus equilibration on the distribution of water contents in the mantle beneath the Rio Grande Rift

机译：熔化，俯冲相关的偏摩出来和亚固体平衡对RIO Grande Rift下面的地幔中水含量分布的影响

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摘要

The distribution of water in the upper mantle plays a crucial role in the Earth's deep water cycle, magmatism, and plate tectonics. To better constrain how these large-scale geochemical systems operate, peridotite and pyroxenite mantle xenoliths from Kilbourne Hole (KH) and Rio Puerco (RP) along the Rio Grande Rift (NM, USA) were analyzed for water, and major and trace element contents. These xenoliths sample a lithosphere whose composition was influenced by subduction and rifting, and can be used to examine the effects of melting, metasomatism, and sub-solidus equilibration on the behavior of water. The first result is that in KH xenoliths, olivines underwent negligible H loss during xenolith ascent, i.e. preserved their mantle water contents. These olivine water contents are used to calculate mantle viscosities of 0.5-184 . 10(21) Pa.s. These viscosity values are more than 40 times higher than those of the asthenosphere and show that KH peridotites represent samples from the lithosphere. The preservation of olivine water contents is exceptional for off-cratonic xenoliths, and the KH peridotites provide the first estimate of the average concentration of water in Phanerozoic continental mantle lithosphere at 81 +/- 30 ppm H2O. The mantle lithosphere beneath the Rio Grande rift is nevertheless heterogeneous with water contents ranging from <0.5 to 120 ppm H2O in peridotites and from 227 to 400 ppm H2O in pyroxenites. A composite KH xenolith of a harzburgite cross-cut by a clinopyroxenite vein shows this heterogeneity at the cm scale. The second contribution of this study stems from the majority of the KH peridotites and two of the RP peridotites having major and trace elements that can be explained by partial melting without any need to invoke metasomatic processes. This allows to show that, prior to modelling the water content variation of each peridotite mineral during melting, a correction for sub-solidus equilibration has to be applied to the water contents of the minerals. Sub-solidus equilibration also provides an explanation for the discrepancy between the clinopyroxene/orthopyroxene ratio of water contents in natural peridotites worldwide and in laboratory experiments on water partitioning in peridotite minerals. Finally, the cryptically metasomatized peridotites, rare at KH and abundant at RP, as well as the pyroxenites, permit to decipher the origin and water contents of the metasomatic melts that affected the continental lithosphere beneath the Rio Grande Rift. Trace element modelling of the metasomatized KH and RP peridotites are consistent with metasomatism via melts that are of subduction origin. Melts in equilibrium with peridotites contain more water at RP (similar to 1 wt.% H2O) than at KH (similar to 0.5 wt.% H2O), although this did not result in a more water-rich mantle lithosphere at RP. Rio Puerco lies within the northern Rio Grande rift, proposed to have been affected by a flat slab subduction, which may explain the more hydrous and extensive metasomatism compared to the south, where KH is located. (C) 2018 Elsevier Ltd. All rights reserved.
机译：上部地幔中的水分布在地球的深水循环，岩浆广告和板块构造中起着至关重要的作用。为了更好地限制这些大型地球化学系统如何运作，沿着Rio Grande Rift（NM，USA）的Kilbourne Hole（KH）和Rio Puerco（RP）的橄榄球和Pyroxenite Mantle Xenoliths进行水，以及主要和微量元素内容物。这些Xenoliths样本岩石圈，其组成受俯冲和剥离的影响，并且可用于检查熔化，弥扑和亚固体平衡对水的行为的影响。第一个结果是，在Kh Xenoliths中，橄榄石在Xenolith Ascent期间接受了忽略的H损失，即保留了它们的地幔水分。这些橄榄胺水含量用于计算0.5-184的搭式粘度。 10（21）PA.S.这些粘度值比哮喘圈高出40倍，表明KH PeridoTites代表岩石圈的样品。橄榄石含水含量的保存是外裂解氙气的特殊情况，KH恒星在81 +/- 30 ppm H2O下提供了Phanogoice Continental Mantle Lithers岩石圈的平均水浓度的第一估计。然而，在RIO Grande Rift下方的地幔岩石圈仍然是异质的，水含量从恒星中的<0.5〜120ppm H 2 O和在曲氧胞的227-400ppm H 2 O中。通过临床Xenite静脉交叉切割的复合KH oNoletith在CM刻度下显示出这种异质性。该研究的第二次贡献源于大部分Kh恒星和具有主要和微量元素的RP恒星的两种，其可以通过部分熔化来解释，而无需任何需要调用态化过程。这允许表明，在熔化过程中为每个恒星矿物的水含量变化建模之前，必须将亚固体平衡的校正施加到矿物质的水含量。亚固态平衡还提供了在全世界天然恒星水含量和实验室实验中的水含量与恒星矿物分配的实验室实验的差异的解释。最后，偏偏见的恒星，罕见的阶下，RP罕见，以及曲兴，允许破译弥漫融解的起源和水含量，这些熔体影响了里约热内卢裂谷下面的大陆岩石圈。弥扑KH和RP恒星的痕量元素建模与通过俯冲源的熔体融合的熔融性。恒平熔体熔体含有更多的水在RP（类似于1重量％H 2 O）（类似于0.5重量％H 2 O），但这并未导致RP的富含水性富含水的岩石圈。 Rio Puerco位于Rio Grande Rift的北部，建议受到平板俯冲的影响，这可以解释与南方相比的较为含水性和广泛的偏索，其中kh所在。 （c）2018年elestvier有限公司保留所有权利。

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