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.

作为一个移动元素，大洋地壳中的 W 收支可能会受到热液蚀变的严重影响，但迄今为止还没有关于蚀变大洋地壳 (AOC) 的 W 数据。在这项研究中，我们展示了第一个通过同位素稀释和 MC-ICPMS 获得的高精度 W 浓度数据，以及高精度的 Th、U、Ta、Zr、Nb、Lu 和 Hf 数据，用于完整剖面的改变上层大洋地壳 (IODP-Borehole 1256D) 一直到基底辉长岩。由于 W、Th、U 和 Ta 具有相似的熔体不相容性，因此选择后者作为研究 W 富集的框架。

在研究的蚀变洋壳部分，保存的蚀变类型从低温（< 150 °C）海水蚀变熔岩到高温（> 400 °C）蚀变花岗岩岩脉。在整个深度序列中，钨在 Th、U 和 Ta 上被系统地富集。铀也在局部热液富集，而钍和钽在整个深度序列中都完全不动。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，在极端流体蚀变的位置发现了最强的 W 富集。

通常，W/Th、Ta/W 和 W/U 的元素比率与原始 MORB 比率仅显示出很小的重叠，甚至可能超过报告的关于 W 富集的全球弧形熔岩的值。因此，许多弧形熔岩中 W 的选择性富集可能部分继承自改变的洋壳。尽管如此，还是有一些弧形的例子，其中俯冲洋壳在微量元素预算中占主导地位，但熔岩没有显示出明显的 W 富集。我们提出了三个模型来解释这个难题，(a) W 在附属相中的保留，(b) U 和 Th 中俯冲带流体相对于 W 的选择性富集，以及 (c) 1256D 孔对 AOC 的不完整采样，留下 W未取样的大洋地壳中的枯竭部分。