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

改变的洋壳作为全球地球化学循环中高度不相容元素的储存库

基本信息

批准号：
242406757
负责人：
Professor Dr. Carsten Münker
金额：
--
依托单位：
Institut für Geologie und Mineralogie
依托单位国家：
德国
项目类别：
Infrastructure Priority Programmes
财政年份：
2013
资助国家：
德国
起止时间：
2012-12-31 至 2020-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/242406757?language=en
关键词：
Altered oceanic crust repository highly

项目摘要

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.

与预期相反，最近的研究表明，一些高度不相容的元素比率，例如W相对于Th-U-Ta的比率，在不同的地球化学藏之间表现出很大的差异。这些发现对理解硅酸盐地球中的过程具有基本意义，因为(1)高度不相容的元素是壳幔分异过程中活动的重要示踪剂，以及(2)这些元素的质量平衡对于利用像HF-W或UTH-Pb这样的同位素系统来解决地球演化问题至关重要。一些关于在地球上不同的硅酸盐储集层之间触发W-U-Th-Ta分馏的过程的研究不能完全解释观察到的变化。值得注意的是，这些研究忽略了一个重要的硅酸盐储集层，即蚀变洋壳，在那里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，我们打算检验这样一个假设，即蚀变洋壳可能是一个重要的储集层，在全球地球化学循环中触发W-U-Th-Ta比值的显著分馏。我们认为，蚀变洋壳是一个可能的候选者，因为即使在低元素浓度下，它也可以通过海底热液过程有效地控制这些元素的收支，因为蚀变洋壳构成了地球表面的重要部分。此外，从矿床研究或水化学中可以预测W和U在水相流体中的选择性活动性。到目前为止，我们工作假说的验证受到以下因素的阻碍：(1)获得完整样本集的机会有限，(2)缺乏适用于如此低浓度的分析技术。现在可以利用同位素稀释技术克服分析上的限制，使其具有足够的精密度和准确度，特别是对洋壳中具有低ppb丰度的W-U-Th-Ta等元素。本课题组是世界上少数几个可以进行高精度W和Ta同位素稀释测量的小组之一，这里建议将这些协议扩展到U和Th。我们首次提出在大洋上、下地壳不同剖面进行高精度W-U-Th-Ta综合测量。这些剖面包括(1)唯一可利用的上洋壳蚀变完整剖面(1256D孔)，一直延伸到席状岩脉-辉长岩过渡；(2)由深海橄榄岩和辉长岩组成的可变蚀变下洋壳(209段)。测量将在变化不定的蚀变和岩石学特征良好的大块样品以及覆盖W-Th-U-Ta潜在载体的矿物分离物上进行。首先，在初步研究中获得的1256D样品的高精度Ta-W-数据显示，相对于Th-Ta，W和U明显富集，这是火成岩过程无法解释的，并缩小了MORBs和弧状熔岩之间的Ta-W差距。