Water transport by subduction: Clues from garnet of Erzgebirge UHP eclogite
Water transport by subduction: Clues from garnet of Erzgebirge UHP eclogite
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DOI：
10.2138/am-2017-5920
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发表时间：
2017-05
影响因子：
3.1
通讯作者：
E. Schmädicke;J. Gose
中科院分区：
文献类型：
--
作者：
E. Schmädicke;J. Gose
摘要关于地幔水收支的一个关键问题是，榴辉岩化的洋壳俯冲实际上有多少水循环进入地幔。只有石英榴辉岩中的水相稳定，柯石英榴辉岩中的水相不稳定，水向更深部位的运移主要受绿辉石和石榴石中的构造水控制。在这里，我们探索石榴石是否可以作为评估这些水的数量的代理。关于柯石英榴辉岩中石榴石的水分含量的现有数据从几个数量级到大约2000ppm不等。这意味着，最大的块状岩石水含量高得不切实际(wt%级别)。来自ErzgeBirge的新数据表明，石榴石(43-84ppm)、绿辉石(400-820ppm)和大部分柯石英榴辉岩(约280-460ppm)中储存了中等数量的结构性水。石榴石的水分含量较高，但这些不是主要特征。它们与流体包裹体中的分子水有关，可归因于变质峰后榴辉岩相流体的侵入。流体的流入还导致了靠近流体包裹体的石榴石区域吸收了额外的结构水。只有在原生缺水的情况下，这种二次H2O的结合才可能发生，这表明石榴石所含的水比它能够储存的水要少。这在一定程度上是令人惊讶的，因为超高压变质作用导致榴辉岩相含水矿物的分解，释放出相对较高的H2O。根据ErzgeBirge石英榴辉岩判断，5-10%的水合矿物(±等量的橄榄石+钙角闪石)脱水产生1500-3000ppm的水。我们推测，释放出的水中最大的一部分可能是由于动力学原因而逸出的，并在悬壁中水化了超高压剥离切片。取决于物理条件，折返过程中榴辉岩中的水流入(1)产生流体包裹体，同时增加名义上无水矿物的结构水含量--如在ErzgeBirge中--和/或(2)它可能导致有水矿物的逆行。我们得出的结论是，榴辉岩将中等数量的水(百万分之几百)输送到超过100公里的地幔深处，相当于大约1%的钙质角闪石。
Abstract A key question concerning the water budget of Earth’s mantle is how much water is actually recycled into the mantle by the subduction of eclogitized oceanic crust. Hydrous phases are stable only in quartz eclogite not coesite eclogite so that water transport to greater depths is mainly governed by structural water in omphacite and garnet. Here we explore if garnet can be used as a proxy to assess the amount of this water. Available data on the water contents of garnet in coesite eclogite vary over orders of magnitude, from a few up to ca. 2000 ppm. By implication, the maximum bulk-rock water contents are unrealistically high (wt% level). New data from the Erzgebirge indicate moderate amounts of structural H2O stored in garnet (43–84 ppm), omphacite (400–820 ppm), and in the bulk coesite eclogite (ca. 280–460 ppm). Higher garnet water contents occur, but these are not primary features. They are related to molecular water in fluid inclusions that can be attributed to eclogite-facies fluid influx postdating the metamorphic peak. Fluid influx also caused the uptake of additional structural water in garnet domains close to fluid inclusions. Such secondary H2O incorporation is only possible in the case of primary water-deficiency indicating that garnet hosted less water than it was able to store. This is insofar astonishing as comparably high H2O amounts are liberated by the breakdown of prograde eclogite-facies hydrous minerals as a result of ultrahigh-pressure (UHP) metamorphism. Judging from Erzgebirge quartz eclogite, dehydration of 5–10% hydrous minerals (±equal portions of zoisite+calcic amphibole) produces 1500–3000 ppm water. We infer that the largest part of the liberated water escaped, probably due to kinetic reasons, and hydrated exhuming UHP slices in the hanging-wall. Depending on the physical conditions, water influx in eclogite during exhumation (1) produces fluid inclusions and simultaneously enhances the structural water content of nominally anhydrous minerals—as in the Erzgebirge—and/or (2) it may give rise to retrograde hydrous minerals. We conclude that eclogite transports moderate quantities of water (several hundred parts per million) to mantle depths beyond 100 km, an amount equivalent to that in ca. 1% calcic amphibole.