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发展历史。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密集劳动的限制。本项目建议开发宇宙成因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，它将允许对广泛的岩石类型进行快速和廉价的生态年代测定，从而增加了在一次研究中可以测定年代的样本数量。第二，由于3 He稳定且具有非常高的产率，它与其他宇宙成因核素结合可能被证明是有用的，以解决诸如古测高、古代暴露和埋藏历史（1亿年时间尺度）以及考古遗址和文物的年代测定等问题。在磷灰石、辉石和锆石中的3 He可以用作宇宙成因年代测定工具之前，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”或在高4 He压力下的空间电荷效应的阻碍。前者可能不是一个问题的基础上证明丰度灵敏度在质量3，和thelaten将解决使用“尖峰”的标准气体入口测量期间，未知的校准仪器在该He压力。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。在这项任务中隐含的是识别和纠正非宇宙成因来源的3 He，如继承从流体包裹体和生产通过6Li（n，α）+时间-3 He。非技术性解释确定岩石首次出现在地球表面是一个重要的问题，在许多地球科学学科。例如，冰川退缩时沉积的巨石提供了过去气候变化的重要记录。这些巨砾的“表面暴露年龄”，即它们最初沉积的时间，可以通过测量宇宙射线与岩石最上面几十厘米的元素相互作用产生的不寻常同位素的积累来确定。这项提议试图改进现有的“表面暴露年代测定”方法，利用几种常见矿物中出现的稀有氦同位素。总体目标是开发一种广泛适用且廉价的技术，这可能导致更好地理解改变地球表面的过程的时间和速率。

Technical DescriptionCosmogenic isotopes such as 10Be and 26Al are widely used to determine the age andexposure histories of landscape features. Such information is critical to understandingthe timing and rate of geologic processes such as active fault ruptures, glacial advances,soil formation and erosion of mountain belts. However, the utility of these isotopes islimited by the high cost and intense labor associated with producing a singlemeasurement. This project proposes development of cosmogenic 3He as a dating tool incommon accessory mineral phases such as apatite, sphene and zircon. If successful, thistechnique will be unique in several respects. First, it will allow rapid and inexpensivecosmogenic dating of a wide range of rock types, thus increasing the number of samplesthat can be dated in a single study. Second, because 3He is stable and has a very highproduction rate, it may prove useful in combination with other cosmogenic nuclides toaddress problems such as paleoaltimetry, ancient exposure and burial histories (100 mytime scales), and dating of archaeological sites and artifacts.Several technical challenges need to be overcome before 3He in apatite, sphene andzircon 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 pressures. The former islikely not a problem based on the demonstrated abundance sensitivity at mass 3, and thelatter will be addressed using a "spike" of standard gas inlet during the measurement ofan 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.Non-Technical ExplanationDetermining when rocks first appear at the Earth's surface is an important question inmany Earth Science disciplines. For example, the boulders deposited when a glacierretreats provide an important record of past climate change. The "surface exposure age"of these boulders, the time when they were first deposited, can be determined bymeasuring the buildup of unusual isotopes produced by cosmic rays interacting with theelements of the uppermost tens of centimeters of the rock. This proposal seeks to improveexisting methods of "surface exposure dating", using the appearance of a rare isotope ofhelium in several common minerals. The overarching goal is to develop a technique thatis broadly applicable and inexpensive and which may lead to improved understanding ofthe timing and rates of processes that modify Earth's surface.