Water in orthopyroxene from abyssal spinel peridotites of the East Pacific Rise (ODP Leg 147: Hess Deep)
Water in orthopyroxene from abyssal spinel peridotites of the East Pacific Rise (ODP Leg 147: Hess Deep)
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DOI：
10.1016/j.lithos.2015.06.011
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发表时间：
2015-09-01
期刊：
影响因子：
3.5
通讯作者：
Schmaedicke, Esther
中科院分区：
文献类型：
--
作者：
Hesse, Kirsten T.;Gose, Juergen;Schmaedicke, Esther
东太平洋隆起赫斯深的深海尖晶石橄榄岩第147条)对其主要、次要和微量元素矿物化学性质以及结构水在正辉石中的掺入进行了研究。岩石部分为蛇纹石化辉石，含有橄榄石、正辉石、斜辉石和尖晶石等原生矿物。正辉石的mg# （Mg/(Mg + Fe)）在0.90 ~ 0.92之间，Al2O3在0.5 ~ 2.9 wt %之间。在尖晶石橄榄岩稳定场中，残余辉锌矿经历了高度的熔体去除。计算出的平均部分熔化度为17.5%（范围：16.4-17.8%）。正辉石和斜辉石的微量元素数据反映了这种强烈的损耗，这是深海橄榄岩的弹性性质的特征。地幔在1000摄氏度左右的再平衡温度表明，在熔体提取之后，在挖掘到海底之前，岩石在尖晶石橄榄岩消退过程中经历了显著的冷却。正辉石的含水量为86 ~ 233wt . ppm H2O，平均浓度为142wt . ppm H2O。这些结果是通过直接测量海底橄榄岩获得的第一个关于亚太平洋地幔含水量的数据。水含量与矿物化学、地层、熔融程度、地幔平衡条件或氧化态无关。计算出的熔融后橄榄岩含水量在40至100 wt. ppm H2O之间变化。与大西洋中脊的橄榄岩相比，147号Leg的东太平洋隆起样品的水浓度比153号Leg的样品略低，而比209号Leg的含量高得多（Gose et al., 2009; Schmadicke et al., 2011）。在Leg 147中，最强的OH吸收带出现在3420 cm（-1）处，而来自MAR橄榄岩的正辉石（Leg 153和209）的最强吸收带在3566和3522 cm（-1）处。Leg 147橄榄岩的地幔平衡温度低于Leg 209样品（接近1250℃），但接近Leg 153样品（950 ~ 1000℃）。Leg 147橄榄岩样品的部分熔融程度与Leg 209样品重叠。根据这些数据，我们得出结论，为了在枯竭的橄榄岩中获得高达233 wt. ppm的相对较高的含水量，在熔体去除后，水必须重新进入正辉石结构。我们认为，在构造岩挖掘之前，在尖晶石相条件下，水含量发生了再平衡，因为升高的温度促进了扩散，升高的压力促进了氢的吸收。从尖晶石相深度至少200 K的显著等压冷却推断，熔体移除和隆升之间的时间跨度延长，有助于水含量的再平衡，并可能解释高浓度。从尖晶石相深度的挖掘是快速的，并伴随着进一步的冷却，以防止再平衡到低压组合和减压引起的失水。(C) 2015 Elsevier B.V.版权所有
Abyssal spinel peridotites from Hess Deep, East Pacific Rise COD!) Leg 147) were investigated concerning their major, minor, and trace element mineral chemistry and the incorporation of structural water in orthopyroxene. The rocks are partially serpentinized harzburgites containing primary minerals of olivine, orthopyroxene, clinopyroxene, and spinel. Orthopyroxene is enstatitic with Mg# (Mg/(Mg + Fe)) between 0.90 and 0.92 and Al2O3 from 0.5 to 2.9 wt.%. The residual harzburgite experienced high degrees of melt removal in the spinel peridotite stability field. The average degree of partial melting was calculated to be 17.5% (range: 16.4-17.8%). Trace element data of ortho- and clinopyroxenes reflect this strong depletion, characteristic for the restitic nature of abyssal peridotites. Mantle re-equilibration temperatures around 1000 degrees C indicate that, after melt extraction and before exhumation to the ocean floor, the rocks experienced significant cooling in the spinel peridotite fades. Water contents of orthopyroxene range from 86 to 233 wt. ppm H2O with an average concentration of 142 wt. ppm H2O. These results represent the first data on water contents in the sub-pacific mantle obtained by direct measurements of sub-oceanic peridotite. The water contents are not related to mineral chemistry, stratigraphy, melting degree, mantle equilibrium conditions or oxidation state. Calculated post-melt peridotite water contents vary between 40 and 100 wt. ppm H2O.Compared to Mid-Atlantic Ridge peridotites, the East Pacific Rise samples of Leg 147 contain somewhat lower water concentrations than samples from Leg 153 and considerably higher contents than those of Leg 209 (Gose et al., 2009; Schmadicke et al., 2011). In Leg 147, the strongest OH absorbtion band occurs at 3420 cm(-1), wheras orthopyroxene from MAR peridotite (Legs 153 and 209) has its strongest absorbtion band at 3566 and 3522 cm(-1). The mantle equilibrium temperature of Leg 147 peridotites is lower than that of Leg 209 (similar to 1250 degrees C) but close to that of Leg 153 samples (950-1000 degrees C). The high degree of partial melting of Leg 147 peridotite samples overlaps with the Leg 209 samples. In accordance to this data we conclude that in order to obtain relatively high water contents of up to 233 wt. ppm in depleted peridotite, after melt removal water must have re-entered the orthopyroxene structure. We suggest that re-equilibration of water contents took place under spinel-facies conditions before exhumation of the tectonite, since elevated temperatures enhance diffusion and elevated pressures facilitate hydrogen uptake. The extended time span between melt removal and uplift inferred from significant isobaric cooling of at least 200 K at spinel-facies depth facilitates re-equilibration of water contents and may explain the high concentrations. Exhumation from spinel-facies depth was fast and accompanied by further cooling such that re-equilibration to lower pressure assemblages and decompression-induced water loss were prevented. (C) 2015 Elsevier B.V. All rights reserved.