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。这些信息对于了解诸如活动断层破裂、冰川推进、土壤形成和山带侵蚀等地质过程的时间和速度至关重要。然而，这些同位素的用途受到与生产单一测量相关的高成本和高强度劳动的限制。该项目建议开发宇宙成因的~3He作为磷灰石、辉石和锆石等常见副矿物相的测年工具。如果成功，这项技术将在几个方面独一无二。首先，它将允许对广泛的岩石类型进行快速而廉价的宇宙起源测年，从而增加了在一项研究中可以测定的样本数量。其次，由于~3He是稳定的，具有非常高的产率，它可能会被证明与其他宇宙成因核素结合起来有用，以解决诸如古测高、古代暴露和埋藏史(100我的时间刻度)以及考古遗址和文物的年代测定等问题。在将磷灰石、辉石和锆石中的~3He用作宇宙成因测年工具之前，需要克服几个技术挑战。首先，必须发展分析方法，以便在大量产生放射性的~4He存在的情况下，测量少量的~3He不会受到低能~4He离子“拖尾”到~3He峰或高~4He丰度时的空间电荷效应的阻碍。前者很可能不是一个问题，基于在质量为3的情况下表现出的丰度敏感性，而后者将在测量未知气体的过程中使用标准气体入口尖峰来解决，以在该He压力下校准仪器。第二个挑战是验证3He的产生率，这将使用内华达山脉和尼泊尔喜马拉雅冰川冰川已知暴露时间的样本来完成。这项任务隐含着对3He的非宇宙源的识别和校正，例如从流体包裹体继承和通过6Li(n，α)时间-3He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.