Altered oceanic crust as repository for highly incompatible elements in the global geochemical cycle

与预期相反，最近的研究表明，一些高度不相容的元素比值，例如W相对于Th-U-Ta的比值，在不同的地球化学储层之间表现出很大的变化。这些发现对理解硅酸盐地球的过程具有根本意义，因为（1）高度不相容的元素是壳幔分异过程中活跃过程的重要示踪剂，（2）这些元素的质量平衡对于采用Hf-W或UTh-Pb等同位素系统来解决地球演化问题至关重要。几项关于地球上不同硅酸盐储层之间触发W-U-Th-Ta分馏过程的研究无法完全解释所观察到的变化。值得注意的是，这些研究忽略了一个重要的硅酸盐储层，即蚀变洋壳，海底热液过程能够有效地分馏这些不相容元素的比例，因此，在拟议的项目中，我们打算测试的假设，即蚀变洋壳可能是一个重要的水库触发全球地球化学循环中的W-U-Th-Ta比值的显着分馏。我们认为，蚀变洋壳是一个可能的候选人，因为即使在低元素浓度，它可以有效地控制这些元素的预算通过海底热液过程，因为蚀变洋壳构成了地球表面的重要组成部分。此外，从矿石存款研究或水的地球化学预测的选择性活动的W和U在水的流体。到目前为止，我们的工作假设的验证受到以下因素的阻碍：（1）获得完整样品集的机会有限;（2）缺乏适用于如此低浓度的分析技术。现在，由于有了同位素稀释技术，特别是对于大洋地壳中丰度低ppb的钨铀钍钽等元素，可以有足够的精确度和准确度，因此可以克服分析的局限性。我们的小组是世界上为数不多的几个小组，可以进行高精度的同位素稀释测量的W和Ta，它建议在这里扩展这些协议，以U和Th。我们建议第一次进行全面的高精度W-U-Th-Ta调查上，下大洋地壳的不同部分。这些剖面包括：（1）蚀变上洋壳（1256 D孔）唯一可用的完整剖面，一直延伸到席状岩脉-辉长岩过渡带;（2）蚀变下洋壳，包括深海橄榄岩和辉长岩（209航段）。测量将在岩石蚀变和岩石学特征良好的散装样品以及覆盖W-Th-U-Ta潜在载体的矿物分离物上进行。第一个高精度的Ta-W-1256 D样品中获得的数据在试点研究中发现显着富集的W和U相对于Th-Ta，不能解释的火成岩过程和关闭之间的MORB和弧熔岩的Ta-W差距。

Contrary to what is expected, recent studies revealed that some highly incompatible element ratios, for instance those of W relative to Th-U-Ta, exhibit large variations between different geochemical reservoirs. These findings have fundamental implications for the understanding of processes in the silicate Earth, as (1) highly incompatible elements serve as important tracers for processes active during crust-mantle differentiation, and (2) the mass balance of these elements is essential for employing isotope systems like Hf-W or UTh-Pb to address problems of Earth evolution. Several studies on processes triggering fractionation of W-U-Th-Ta between different silicate reservoirs on Earth could not entirely explain the variations observed. Notably, these studies overlooked an important silicate reservoir, namely altered oceanic crust, where submarine hydrothermal processes are capable of efficiently fractionating ratios of these incompatible elements.In the proposed project, we therefore intend to test the hypothesis that altered oceanic crust may be an important reservoir triggering significant fractionation of W-U-Th-Ta ratios in the global geochemical cycle. We argue that altered oceanic crust is a likely candidate, because even at low element concentrations it can efficiently control the budget of these elements via submarine hydrothermal processes, since altered oceanic crust makes up a significant portion of the Earths surface. Moreover, a selective mobility of W and U in aqueous fluids is predicted from ore deposit studies or aqueous geochemistry. So far, verification of our working hypothesis was hampered by (1) by limited access to complete sample sets and (2) by the lack of suitable analytical techniques for such low concentrations. The analytical limitation may now be overcome by the availability of isotope dilution techniques that permit sufficient precision and accuracy, in particular for elements like W-U-Th-Ta with low-ppb abundances in the oceanic crust. Our group is one of the few groups worldwide that can perform high precision isotope dilution measurements of W and Ta, and it is proposed here to expand these protocols to U and Th.We propose for the first time to perform a comprehensive high-precision W-U-Th-Ta survey on different sections of upper and lower oceanic crust. These sections cover (1) the only available complete section of altered upper oceanic crust (Hole 1256D) ranging down to the sheeted dike-gabbro transition, and (2) variably altered lower oceanic crust comprising abyssal peridotites and gabbros (Leg 209). Measurements will be performed on variably altered and petrologically well characterized bulk samples as well as on mineral separates covering potential carriers of W-Th-U-Ta. First high precision Ta-W-data for 1256D samples obtained in a pilot study revealed marked enrichments of W and U relative to Th-Ta that cannot be explained by igneous processes and close the Ta-W gap between MORBs and arc lavas.