Noble Gas Behavior During Upper Mantle Deformation

上地幔变形期间的稀有气体行为

• 批准号: 0948609
• 负责人: Mark Kurz
• 金额: $ 37.05万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-04-01 至 2012-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0948609&HistoricalAwards=false
• 关键词: Noble; Gas; Behavior; During; Upper

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.

上地幔变形期间的稀有气体行为智力价值。这项建议寻求资金来检验变形是对地幔矿物中稀有气体含量的重要控制这一假说。这一假设是基于对圣保罗岩石(赤道大西洋)和海底的高度剪切岩石(糜棱岩)进行的系统研究。初步研究表明，He含量随变形程度的不同而不同，变形最大的超糜棱岩中He含量最高。真空破碎和熔融耦合实验表明，糜棱岩中的大部分氦和霓都包含在矿物基质中，而不是流体或熔融包裹体中：只有5%~18%的氦是通过破碎释放出来的。圣保罗岩石的He/Ne比值随着变形和矿物学的变化而变化很大(~20倍)，其中最高的He/Ne比值(和氦浓度)出现在最细小的超糜棱岩橄榄岩中。认为变形岩石中具有很高的Ne和He含量与地幔压力下缺陷内的扩散圈闭有关。然而，目前的数据集仅限于非常少量的样本，需要评估矿物学和蚀变的影响。建议通过对圣保罗岩石的进一步测量来检验气体形变假说，并探索矿物学对氦、霓和Ar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，以至于有人提议进行新的激光聚变实验，通过现场测量直接确定氦的居留点。此外，还建议对美国俄勒冈州约瑟芬和阿曼两个蛇绿岩的几个剪切带进行系统采样。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，而且野外暴露足以对从未变形到变形的橄榄岩的完整断面进行采样。更大的影响。岩石和矿物中的稀有气体同位素测量引起了地球科学界的广泛兴趣。对地幔岩石中惰性气体的研究对地球深部的模型很重要，因为在一些海洋岛屿上发现的非放射性稀有气体同位素组成提供了一些支持地球深层未脱气储集层存在的少数观察结果。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边界，可能会在我们对这两个领域的理解方面取得重要进展。要验证变形假说，需要重新评估地幔熔融过程中惰性气体行为的模型，以及沿地壳断层的气体迁移模型。这将支持麻省理工学院/世界卫生组织海洋学联合项目的一名研究生和一名博士后研究员。关于圣保罗岩石的工作还将涉及与两位巴西地球科学家(S·西切尔和T·坎波斯)的合作，并将促进来自巴西和世界卫生组织的学生和科学家之间的国际交流。