4He/3He Thermocronometry and Noble Gas Diffusion Behavior via Proton Irradiation

（U-Th）/He定年技术和应用在过去十年中发展迅速。这种方法现在普遍用于研究从第四纪火山岩的定年到确定断层运动的时间和幅度，再到记录古地形等问题。这些应用依赖于对过期谷物的散装年龄的测量。通过质子轰击将3 He引入矿物中，然后对4 He/3 He比率进行分步加热测量，可以产生一个“年龄谱”，作为颗粒内的4 He浓度分布。年龄谱携带了关于样品冷却路径的独特定量信息，补充了批量年龄。3 He测量还提供了严格的限制氦扩散率，不能从测量天然放射性4 He单独。这项赠款支持的工作将在几个受约束的地质情况下对4 He/3 He技术进行现场测试，包括深地壳钻孔和具有已知时间的大规模折返的正常断块。将开发数值模型来解释剖面并评估该方法的公认潜在缺陷。该项目将通过进一步发展与东北质子治疗中心回旋加速器设施的物理学家的合作来加强科学基础设施。该提议支持的质子束时间可与其他感兴趣的用户共享，如果成功，最终产品将是地质年代学和地球化学方面的全新能力。在过去的几年里，法利在加州理工学院的地质年代学实验室已经被许多外部调查人员使用，这项提议将支持必要的基础设施，以便在未来几年继续使用这一事实上的多用户设施。拟议的工作还将支持加州理工学院博士生的教育。

Techniques and applications of (U-Th)/He dating have evolved rapidly in the last decade. The method is now in common use to study problems ranging from dating of Quaternary volcanic rocks, to establishing the timing and magnitude of fault motions, to documenting paleotopography. These applications have relied on measurement of the bulk age of the dated grain. By introducing 3He into minerals via proton bombardment followed by step-heat measurements of the 4He/3He ratio, it is possible to produce an "age spectrum" developed as a 4He concentration profile within the grain. The age spectrum carries unique quantitative information on the sample's cooling path that complements the bulk age. 3He measurements also provide a rigorous constraint on helium diffusivity that cannot be obtained from measurements of natural radiogenic 4He alone. Work supported by this grant will field-test the 4He/3He technique in several well-constrained geologic situations including a deep crustal borehole and a normal fault block with a large magnitude of exhumation of known timing. Numerical models will be developed to interpret profiles and to evaluate recognized potential pitfalls with the method.This project will enhance scientific infrastructure by further developing collaboration with physicists at the Northeast Proton Therapy Center's cyclotron facility. Proton beam time supported by this proposal may be shared with other interested users, and if successful the ultimate product will be fundamentally new capabilities in geochronology and geochemistry. Farley's geochronology laboratory at Caltech has been used by many outside investigators in the last several years, and this proposal will support the infrastructure necessary to continue this defacto multi-user facility for the coming years. The proposed work will also support the education of a Caltech PhD student.