Noble Gas Behavior During Upper Mantle Deformation

上地幔变形过程中的惰性气体行为。本提案寻求资金来验证变形是地幔矿物中稀有气体含量的重要控制因素的假设。这一假设是基于对圣保罗岩石（赤道大西洋）和海底高度剪切的岩石（糜棱岩）的系统研究。初步研究表明，氦含量随变形程度的不同而变化，在变形程度最大的超长岩中含量最高。真空破碎和熔融耦合实验表明，糜糜岩中的氦和氖大部分存在于矿物基质中，而不是流体或熔融包裹体中，破碎释放的氦仅占总氦的5% ~ 18%。圣保罗岩石中的He/Ne比率随变形和矿物学变化很大（约20倍），在颗粒最细的超长橄榄岩中发现了最高的He/Ne比率（和氦浓度）。认为变形岩石中氖气和氦气的高含量与地幔压力下缺陷内的扩散俘获有关。然而，目前的数据集仅限于非常少量的样本，并且需要评估矿物学和蚀变的影响。建议通过在圣保罗岩石的进一步测量来验证气体变形假说，并探索矿物学对氦、氖和氩的同位素和元素丰度的影响。圣保罗岩石样品中的氦浓度如此之高，以至于人们提出了新的激光聚变实验，通过原位测量直接确定氦的驻留位置。此外，建议在两个蛇绿岩：Josephine（美国俄勒冈州）和Oman的几个剪切带上进行系统采样。这两个蛇绿岩中的剪切带是理想的，因为变形特征被很好地记录下来，并且野外暴露足以允许取样完整的样带，从未变形到变形的橄榄岩。更广泛的影响。岩石和矿物中稀有气体同位素的测量引起了地球科学界的广泛兴趣。地幔衍生岩石中稀有气体的研究对地球深部内部的模型很重要，因为在一些海洋岛屿上发现的非放射性成因稀有气体同位素组成提供了一些为数不多的观测结果，支持地球深部未脱气储层的存在。这项研究跨越了同位素地球化学和矿物物理学之间的传统学科界限，可能会对我们对这两个领域的理解产生重要的进展。要验证变形假说，就需要重新评估地幔熔融过程中稀有气体的行为模型，以及地壳断层中的气体运移模型。这将支持麻省理工学院/WHOI海洋联合项目的一名研究生和一名博士后研究员。圣保罗岩石的工作还将涉及与两位巴西地球科学家（S. Sichel和T. Campos）的合作，并将促进巴西和世卫组织的学生和科学家之间的国际交流。

Noble Gas Behavior During Upper Mantle DeformationIntellectual merit.  This proposal seeks funds to test the hypothesis that deformation is an important control on noble gas contents in mantle minerals. This hypothesis is based on a systematic study highly sheared rocks (mylonites) from St. Paul's Rocks (equatorial Atlantic ocean) and from the ocean floor. Preliminary study shows that He content varies with degree of deformation, with the highest contents in the most deformed ultramylonites. Coupled vacuum crushing and melting experiments show that most of the helium and neon within the mylonites is contained in the mineral matrices rather than fluid or melt inclusions: only 5 to 18% of the total helium is released by crushing. He/Ne ratios in St. Paul's Rocks vary widely (~ 20x) with deformation and mineralogy, with the highest He/Ne ratios (and helium concentration) found in the finest grained ultramylonite peridotite. It is suggested that very high neon and helium contents in deformed rocks is related to diffusive trapping within defects at mantle pressures. However, the present data set is limited to a very small number of samples, and the influences of mineralogy and alteration need to be assessed. It is proposed to test the gas-deformation hypothesis with further measurements at St. Paul's Rocks and to explore the influence of mineralogy on the isotopes and elemental abundances of helium, neon and argon. Helium concentrations are so high in the St. Paul's Rocks samples that new laser fusion experiments are proposed to directly determine the helium residence sites with in situ measurements. In addition, a systematic sampling across several shear zones at two ophiolites: Josephine (Oregon, USA) and Oman is proposed. The shear zones in these two ophiolites are ideal because deformation characteristics are well documented and field exposures are sufficient to allow sampling of complete transects, from undeformed to deformed peridotite.Broader impacts.  Noble gas isotope measurements in rocks and minerals are of broad interest to the earth science community. Studies of noble gases in mantle derived rocks are important to models of the earth's deep interior because unradiogenic noble gas isotopic compositions, found in some oceanic islands, provide some of the few observations that support the existence of deep undegassed reservoirs in the earth. This research crosses the traditional disciplinary boundaries between isotope geochemistry and mineral physics, which may yield important advances in our understanding of both fields. Validation of the deformation hypothesis would require reevaluation of models for noble gas behavior during mantle melting as well as gas migration along faults in the crust. This will support a graduate student in the MIT/WHOI Joint Program in Oceanography and a Postdoctoral Investigator. The work on St. Paul's Rocks will also involve collaboration with two Brazilian geoscientists (S. Sichel and T. Campos), and will foste international exchanges between students and scientists from Brazil and WHOI.