Evidence for tungsten mobility during oceanic crust alteration
Evidence for tungsten mobility during oceanic crust alteration
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DOI：
10.1016/j.chemgeo.2021.120504
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发表时间：
2021-08
期刊：
影响因子：
3.9
通讯作者：
R. Reifenröther;C. Münker;B. Scheibner
中科院分区：
文献类型：
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作者：
R. Reifenröther;C. Münker;B. Scheibner
作为一种移动的元素，W在洋壳中的收支可能受到热液蚀变的严重影响，但至今还没有关于蚀变洋壳（AOC）的W数据。在这项研究中，我们提出了第一个高精度的同位素稀释和MC-ICPMS获得的W浓度数据，沿着与高精度的Th，U，Ta，Zr，Nb，Lu和Hf数据，为一个完整的部分蚀变上洋壳（IODP钻孔1256 D）下至基底辉长岩。由于W、Th、U和Ta具有相似的熔体不相容性，因此选择Ta作为研究W富集的骨架，在所研究的蚀变洋壳剖面中，保存的蚀变类型从低温（< 150 °C）海水蚀变熔岩到高温（> 400 °C）蚀变花岗变岩脉。在整个深度序列中，钨相对于Th、U和Ta呈系统性富集。铀也是热液富集的地方，而Th和Ta都表现出完全不动的整个深度序列。根据U系列数据，大多数蚀变活动的年龄可以限制在>350 kyrs。在不同岩性单元之间的接触带中发现了最强的W富集，在极端的流体蚀变的网站。一般来说，W/Th，Ta/W和W/U的元素比值显示只有轻微的重叠与原始MORB比值，甚至可能超过全球弧熔岩的值，就W富集。因此，许多弧熔岩中W的选择性富集可能部分继承了蚀变洋壳。然而，也有一些例子弧俯冲洋壳占主导地位的微量元素预算，但熔岩没有表现出明显的W富集。我们提出了三个模型来解释这个难题，（a）保留W在辅助阶段，（B）选择性富集的俯冲带流体中的U和Th相对于W，和（c）不完整的采样AOC孔1256 D，留下W贫化部分的洋壳未采样。
Being a mobile element, the budget of W in the oceanic crust, an important geochemical reservoir, may be severely affected by hydrothermal alteration, but W data for altered oceanic crust (AOC) have not been available so far. In this study, we present the first high precision W concentration data obtained by isotope dilution and MC-ICPMS, along with high precision Th, U, Ta, Zr, Nb, Lu, and Hf data, for a complete section of altered upper oceanic crust (IODP-Borehole 1256D) down to the basal gabbros. As W, Th, U and Ta are similar melt incompatible, the latter were selected as framework to study W enrichment.In the studied section of altered oceanic crust, preserved alteration styles range from low temperature (< 150 °C) seawater altered lavas to high temperature (> 400 °C) altered granoblastic dikes. Tungsten is systematically enriched over Th, U, and Ta in the whole depth succession. Uranium is also hydrothermally enriched locally, whereas Th and Ta both behaved entirely immobile over the entire depth succession. Based on U-series data, most alteration activity can be confined in age to >350 kyrs. The strongest W enrichments were found in contact zones between the different lithological units, at the sites of extreme fluid alteration.Generally, the elemental ratios of W/Th, Ta/W and W/U show only minor overlap with pristine MORB ratios and may even exceed the values reported for global arc lavas with respect to W enrichment. The selective enrichment of W in many arc lavas may therefore be partially inherited from altered oceanic crust. Nevertheless, there are examples of arcs where subducted oceanic crust dominates the trace element budget, but the lavas do not show pronounced W enrichment. We propose three models to explain this conundrum, (a) retention of W in accessory phases, (b) selective enrichment of subduction zone fluids in U and Th relative to W, and (c) incomplete sampling of AOC by hole 1256D, leaving W depleted portions in the oceanic crust unsampled.