Characterization of hydration in the mantle lithosphere: Peridotite xenoliths from the Ontong Java Plateau as an example
Characterization of hydration in the mantle lithosphere: Peridotite xenoliths from the Ontong Java Plateau as an example
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DOI：
10.1016/j.lithos.2014.11.005
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发表时间：
2015-01-01
期刊：
影响因子：
3.5
通讯作者：
Keshav, Shantanu
中科院分区：
文献类型：
--
作者：
Demouchy, Sylvie;Ishikawa, Akira;Keshav, Shantanu
本文报道了西太平洋所罗门群岛马莱塔地区翁东爪哇高原西南缘爆发的由钠长石质熔岩输送的橄榄岩（橄榄石和辉石）中氢（H）的浓度。非偏振FTIR分析表明橄榄石、斜方辉石和透辉石分别含有2-32 ppm、162-362 ppm和159-459 ppm wt H2O。在所研究的石榴石中，石榴石是无水的。氢的浓度在个别橄榄石和辉石颗粒几乎是均匀的，表明没有证据的脱水或水化的离子扩散。在二辉橄榄岩中，橄榄石中氢的浓度往往随着深度的增加而增加（基于地质温压法），这与随着水逸度的增加而增加的水溶解度作为压力的函数是一致的，但是浓度仍然远低于实验确定的水饱和值。然而，橄榄石中H的最高浓度（32 ppm wt H2O）发现于耐火尖晶石方辉橄榄岩中，其在85 km深处达到平衡，而更深的样品如高温尖晶石方辉橄榄岩和一些石榴石伊赫茨辉石，橄榄石中含氢较少。富含辉石或部分辉石的橄榄岩中的橄榄石也具有较低的氢浓度。我们解释了来自耐火尖晶石方辉橄榄岩的橄榄石中的高氢浓度是由于（1）富含水的硅酸盐熔体/流体对岩石圈地幔的同时水化和交代作用，在此期间氢跟随MREE并且尖晶石方辉橄榄岩经历了“隐形”交代作用，和/或（2）晚期“短暂”氢交代作用，在第一次“隐形”交代作用后会使岩石水合。在第二种情况下，组成的“短暂的”蒸发流体（体积分数很小的非常进化的流体，具有高挥发分浓度和瞬态特性）可能与羽活动的减少，并导致岩石圈软流圈边界的下涌。方辉橄榄岩和伊赫茨贝格之间的氢浓度的差异可能与橄榄石中的微量元素的变化引起的第一个“隐形”交代单独或相关的后期“稍纵即逝”的氢交代，引发的缺乏单斜辉石在方辉橄榄岩的二次结晶。综上所述，上地幔矿物中的H浓度可能因此与交代事件，与后来产生的变化的矿物组合和微量元素组成的橄榄岩。(C)2014爱思唯尔有限公司版权所有。
We report concentrations of hydrogen (H) in upper mantle minerals of peridotites (olivine and pyroxenes) transported by alnoitic lavas, which erupted on the southwestern border of the Ontong Java Plateau (Malaita, Solomon Islands, West Pacific). Unpolarized FTIR analyses show that olivine, orthopyroxene, and diopside contain 2-32 ppm, 162-362 ppm and 159-459 ppm wt H2O, respectively. In the studied lherzolites, garnets are anhydrous. The concentration of hydrogen within individual olivine and pyroxene grains is almost homogeneous, indicating no evidence of dehydration or hydration by ionic diffusion. In the lherzolite, the concentration of hydrogen in olivine tends to increase weakly with depth (based on geothermobarometry), consistent with the increase of water solubility with increasing water fugacity as a function of pressure, but concentrations remain well below water-saturation values determined experimentally. The highest concentration of H in olivine (32 ppm wt H2O) is, however, found in refractory spinel harzburgites, which equilibrated at depths of 85 km., while deeper specimens as the high-temperature spinel harzburgites, and some of the garnet Iherzolites, contain less hydrogen in olivine. Olivines from pyroxene- or pargasite-rich peridotites have also lower hydrogen concentrations. We interpret the high hydrogen concentrations in olivine from the refractory spinel harzburgites as due to (1) simultaneous hydration and metasomatism of the lithospheric mantle by a water-rich silicate melt/fluid, during which hydrogen follows MREE and where spinel harzburgite have experienced 'stealth' metasomatism, and/or (2) to a late 'fleeting' hydrogen metasomatism, which would hydrate the rock after this first 'stealth' metasomatism event. In the second case, the composition of the 'fleeting' percolating fluid (small volume fraction of very evolved fluids, with high volatiles concentration and transient properties) is likely to be linked to the decrease of the plume activity and resulting in the downwelling of the lithosphere asthenosphere boundary. The difference in hydrogen concentration between harzburgites and Iherzolites could be linked to variation in trace elements in olivine induced by the first 'stealth' metasomatism alone or associated to the late 'fleeting' hydrogen metasomatism, triggered by the lack of secondary crystallization of clinopyroxenes in the harzburgite. To conclude, H concentrations in upper mantle minerals may thus be correlated to metasomatic events, with the later yielding variation of the mineral assemblage and trace element composition that constitute the peridotites. (C) 2014 Elsevier B.V. All rights reserved.