Collaborative Research: Cosmogenic 3He Dating of Accessory Minerals

宇宙成因同位素如10Be和26Al被广泛用于确定景观特征的年龄和暴露历史。这些信息对于了解地质过程的时间和速度至关重要，例如活动断层破裂、冰川推进、土壤形成和山带侵蚀。然而，这些同位素的效用受到高成本和生产一次测量相关的高强度劳动的限制。本项目拟在磷灰石、榍石、锆石等常见附属矿物相中开发宇宙成因3He作为测年工具。如果成功，这种技术将在几个方面是独一无二的。首先，它将允许对广泛的岩石类型进行快速和廉价的宇宙起源定年，从而增加在一次研究中可以定年的样本数量。其次，因为氦是稳定的，并且有很高的生产速率，它可能被证明是有用的，与其他宇宙形成核素结合，以解决诸如古海拔测量，古代暴露和埋葬的历史（100个时间尺度），以及考古遗址和文物的年代测定等问题。在将磷灰石、榍石和锆石中的3He用作宇宙起源测年工具之前，需要克服一些技术挑战。首先，有必要开发一种分析方法，使在存在大量放射性氦的情况下对少量氦的测量不受低能氦的“尾尾”到氦峰上的影响，也不受高氦丰度时空间电荷效应的影响。基于在质量3处演示的丰度灵敏度，前者可能不是问题，而后者将在测量未知气体时使用标准气体入口的尖峰来校准该压力下的仪器。第二个挑战是验证3He的生产速度，这将使用内华达山脉和尼泊尔喜马拉雅地区冰川冰碛的已知暴露样本来完成。这项任务隐含着识别和纠正非宇宙成因的3He来源，如流体包裹体的遗传和通过6Li(n, α) +时间- 3He的产生。

Cosmogenic isotopes such as 10Be and 26Al are widely used to determine the ageand exposure histories of landscape features. Such information is critical tounderstanding the timing and rate of geologic processes such as active fault ruptures,glacial advances, soil formation and erosion of mountain belts. However, the utility ofthese isotopes is limited by the high cost and intense labor associated with producing asingle measurement. This project proposes development of cosmogenic 3He as a datingtool in common accessory mineral phases such as apatite, sphene and zircon. Ifsuccessful, this technique will be unique in several respects. First, it will allow rapid andinexpensive cosmogenic dating of a wide range of rock types, thus increasing the numberof samples that can be dated in a single study. Second, because 3He is stable and has avery high production rate, it may prove useful in combination with other cosmogenicnuclides to address problems such as paleoaltimetry, ancient exposure and burial histories(100 my time scales), and dating of archaeological sites and artifacts.Several technical challenges need to be overcome before 3He in apatite, spheneand zircon can be used as a cosmogenic dating tool. First, it is necessary to developanalytical procedures such that the measurement of small amounts of 3He in the presenceof large quantities of radiogenic 4He is not hampered by the "tailing" of low energy 4Heions onto the 3He peak, or by space charge effects at high 4He abundances. The formeris likely not a problem based on the demonstrated abundance sensitivity at mass 3, andthe latter will be addressed using a spike of standard gas inlet during the measurementof an unknown to calibrate the instrument at that He pressure. A second challenge is toverify the production rate of 3He, which will be done using samples of known exposureage from glacial moraines in the Sierra Nevada and Nepal Himalaya. Implicit in this taskis identifying and correcting for non-cosmogenic sources of 3He such as inheritance fromfluid inclusions and production via 6Li(n, alpha) + time - 3He.