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，例如W相对于Th-U-Ta的元素比，在不同地球化学储层之间表现出很大的变化。这些发现对于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化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使用同位素至关重要Hf-W 或 UTh-Pb 等系统可解决地球演化问题。对地球上不同硅酸盐储层之间触发 W-U-Th-Ta 分馏过程的几项研究无法完全解释所观察到的变化。值得注意的是，这些研究忽视了一个重要的硅酸盐储层，即蚀变洋壳，其中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，我们打算检验这样的假设：蚀变洋壳可能是一个重要的硅酸盐储层。储层引发全球地球化学循环中 W-U-Th-Ta 比率的显着分馏。我们认为，改变的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预算，因为改变的洋壳构成了地球表面的很大一部分。此外，通过矿床研究或水地球化学预测了 W 和 U 在水流体中的选择性迁移。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）完整样本集的获取有限；（2）缺乏适合如此低浓度的分析技术。现在可以通过同位素稀释技术来克服分析限制，这些技术允许足够的精度和准确度，特别是对于洋壳中丰度低 ppb 的 W-U-Th-Ta 等元素。我们小组是全球少数几个能够对W和Ta进行高精度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。我们首次建议进行全面的高精度W-U -上、下洋壳不同剖面的Th-Ta调查。这些剖面涵盖 (1) 唯一可用的完整剖面蚀变上洋地壳（孔 1256D），范围一直延伸到片状岩脉-辉长岩过渡带，以及 (2) 不同程度的蚀变下洋地壳，包括深海橄榄岩和辉长岩（地段 209）。测量将针对不同程度的改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进行。初步研究中获得的 1256D 样品的第一个高精度 Ta-W 数据显示，W 和 U 相对于 Th-Ta 显着富集，这无法用火成岩过程解释，并缩小了 MORB 和弧熔岩之间的 Ta-W 差距。