Water disequilibrium in olivines from Hawaiian peridotites: Recent metasomatism, H diffusion and magma ascent rates
Water disequilibrium in olivines from Hawaiian peridotites: Recent metasomatism, H diffusion and magma ascent rates
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DOI：
10.1016/j.gca.2015.01.030
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发表时间：
2015-04
影响因子：
5
通讯作者：
A. Peslier;M. Bizimis;M. Matney
中科院分区：
文献类型：
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作者：
A. Peslier;M. Bizimis;M. Matney
限制氢在橄榄石（上地幔的主要矿物）中的分布和流动性对于我们了解地球的地球动力学至关重要，因为这种微量元素会影响橄榄岩的熔化、流变以及电导率和热导率。为此，对来自盐湖陨石坑和巴利岛（夏威夷欧胡岛）的新鲜且特征良好的橄榄岩捕虏体中的橄榄石（代表大洋地幔岩石圈的样本）进行了 FTIR 分析。水浓度从核心到边缘以及橄榄石颗粒裂缝附近逐渐减少，最好的解释是捕虏体上升到其宿主岩浆表面期间的氢损失。这些剖面的扩散建模允许计算扩散时间，进而用于估计捕虏体宿主霞岩在 0.2–25.3 m s−1 的平均上升速率。这些速率与大陆玄武岩岩浆的速率相似。扩散模型进一步表明，橄榄石核心的水含量是保存的地幔值，并且在每个捕虏体中是不均匀的。此外，3225 cm−1OH 带（由于 Mg 空位中的 H）相对于其他 OH 带（特别是 Ti-H 缺陷）沿剖面的差异行为证明了 H 在橄榄石缺陷中不均匀分布。对这些缺陷轮廓进行建模，计算出 Mg-H 缺陷的扩散速率比 Ti-H 缺陷的扩散速率大约快 1.3-6.8 倍。单个捕虏体中的橄榄石核之间和橄榄石颗粒内的地幔中氢的不均匀分布证明了这些样品中水的不平衡状态。盐湖陨石坑橄榄岩橄榄石记录了两个过程；最近的交代作用是由熔体带来水，然后在宿主岩浆上升过程中失水，但两者都没有持续足够长的时间使水达到平衡。观察到的 H 的不均匀分布和亲石元素的均匀分布之间的脱钩表明，通过初期熔融交代作用将水添加到橄榄岩的过程可能被主体霞岩从地幔中取出样品并将其带到地表所中断。
Constraining the distribution and mobility of H in olivine, the main mineral of the upper mantle, is crucial to our understanding of Earth’s geodynamics because this trace element influences melting, rheology, and electrical and thermal conductivities of peridotite. For this purpose, the olivines from fresh and well-characterized peridotite xenoliths from Salt Lake Crater and Pali (Oahu, Hawaii), representing samples of the oceanic mantle lithosphere, were analyzed by FTIR. Water concentrations decrease from core to edge and near fractures of olivine grains, and are best interpreted as H loss during xenolith ascent to the surface in its host magma. Diffusion modeling of these profiles allowed the calculation of diffusion times, which were in turn used to estimate the average ascent rates of the xenolith host nephelinite at 0.2–25.3 m s−1. These rates are similar to those of continental basaltic magmas. Diffusion modeling further shows that the water contents at the core of olivines are preserved mantle values and are heterogeneous within each xenolith. In addition, the discrepant behavior of the 3225 cm−1OH band (due to H in a Mg vacancy) relative to the other OH bands (in particular the Ti-H defect) along profiles evidences that H is heterogeneously distributed amongst olivine defects. These defect profiles are modeled to calculate that the diffusion rate of the Mg-H defect is about 1.3–6.8 times faster than that of the Ti-H defect. The heterogeneous distribution of H in the mantle between olivine cores in single xenoliths and within olivine grains testifies of a state of disequilibrium for water in these samples. The Salt Lake Crater peridotite olivines record two processes; recent metasomatism by a melt bringing water followed by water loss during ascent in the host magma, neither having lasted long enough for water to reach equilibrium. The observed decoupling between the heterogeneous distribution of H and the homogeneous distribution of lithophile elements suggests that the process of water addition to the peridotite via incipient melt metasomatism was likely interrupted by the host nephelinite removing the samples from the mantle and bringing them to the surface.