Collaborative Research: Cosmogenic 3He Dating of Accessory Minerals

宇宙成因同位素如10 Be和26 Al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，例如活动断层断裂、冰川前进、土壤形成和山区侵蚀。然而，这些同位素的效用受到与产生单一测量相关的高成本和高强度劳动的限制。本项目建议发展宇宙成因~ 3 He作为磷灰石、辉石和锆石等常见副矿物相的定年工具。如果成功，这项技术将在几个方面是独一无二的。首先，它将允许快速和廉价的宇宙成因测年的广泛的岩石类型，从而增加了数量的样本，可以在一个单一的研究。第二，由于3 He稳定且产率很高，它与其他宇宙成因核素结合可能被证明是有用的，可以解决诸如古高度测量、古代暴露和埋藏历史（100米时标）以及考古遗址和文物的定年等问题。在磷灰石、蝶骨和锆石中的3 He可以用作宇宙成因定年工具之前，需要克服一些技术挑战。首先，有必要开发分析程序，以便在大量的放射性4 He的存在下测量少量的3 He不会受到低能4 He到3 He峰上的“拖尾”或高4 He丰度下的空间电荷效应的阻碍。基于质量3的丰度灵敏度，前者可能不是问题，而后者将在测量未知量期间使用标准气体入口的尖峰来解决，以在该He压力下校准仪器。第二个挑战是验证3 He的生产率，这将使用来自内华达州和尼泊尔喜马拉雅山脉冰川冰碛的已知测量样本来完成。这一任务的隐含意义是识别和纠正3 He的非宇宙成因来源，如流体包裹体的继承和通过6Li（n，α）+时间-3He的生产。

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.