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演化史。这些信息对于理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，这些同位素的使用受到与生产单一测量相关的高成本和高劳动强度的限制。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和廉价的生态测年，从而增加了可在一项研究中确定年代的样本数量。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(100MyTime标尺)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~3He可以用作宇宙成因测年工具之前，需要克服几个技术挑战。首先，有必要发展分析方法，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离子在3He峰上的“拖尾”或在高4He压力下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，后者将在测量未知气体的过程中使用标准气体入口的“尖峰”来解决，以在该He气压下校准仪器。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的非宇宙来源的识别和校正，如从流体包裹体继承和通过6Li(n，α)+Time-3He产生。非技术解释确定岩石何时首次出现在地球表面是许多地球科学学科中的一个重要问题。例如，冰川消退时沉积的巨石提供了过去气候变化的重要记录。通过测量宇宙射线与岩石最上层几十厘米的元素相互作用产生的不寻常同位素的积累量，可以确定这些巨石的“表面暴露年龄”，也就是它们第一次沉积的时间。这项建议旨在改进现有的“表面暴露测年”方法，利用几种常见矿物中稀有的氦同位素的外观。总体目标是开发一种广泛适用且成本低廉的技术，这种技术可能会导致对改变地球表面过程的时间和速度的更好理解。

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.