Production and Diffusion of Cosmogenic Noble Gases: Using Open-system Behavior to Study Surface Processes

该项目旨在开发和测试一种新的地球化学方法，用于量化在过去几百万年中暴露在地球表面的岩石和沉积物的温度和暴露历史。这项技术是基于对稀有气体氦和氖的痕量同位素的测量，这两种气体是由地球表面的宇宙射线轰击在岩石和矿物颗粒中产生的。这些浓度既反映了岩石暴露在地表的时间长度，也反映了这些气体的扩散损失速率取决于温度，即暴露期间所经历的温度。首先，我们将对常见矿物石英和长石的天然和人工样品进行受控实验室实验，以建立氦和氖在这些矿物中的扩散机制、速率和温度依赖关系。原则上，这些结果将使我们能够预测这些气体在自然地质样本中的浓度，这些样本的暴露和温度历史已经从其他证据中得知。因此，该项目的第二部分将是通过比较天然岩石样品中的预测浓度和实际浓度来测试和验证我们的实验室结果和理论框架。这项研究很重要，因为重建表面材料的过去温度和表面暴露历史对于广泛的科学研究是有价值的。首先，测量地表岩石的暴露时间对于理解形成和改变地表的地质过程非常重要，包括地表侵蚀、沉积物运输和与地震有关的地表变形。其次，测量过去的温度对于了解地球在过去几百万年里的自然气候变化非常重要。例如，这些信息对于确定过去环境条件的变化如何影响全球不同地区的生物群，以及过去的气候变化如何可能受到诸如山脉逐渐发展等自然现象的控制，都是非常重要的。然而，我们量化过去温度的能力目前仅限于少数地球化学技术。如果这种新方法被成功开发和测试，它将为现有方法提供一个独立的测试，并可能有益于广泛的研究分支学科，包括定量地貌学、景观演化研究、晚新生代气候变化、冰川和冰盖变化，以及潜在的积极隆升景观的古海拔。这项基础的地球化学研究将有可能使广泛的地球科学研究人员能够解决长期存在的和全新的问题，从而使更广泛的科学界受益。此外，该项目将资助加州大学伯克利分校的一名研究生，从而为地球科学教育和地球化学及更广泛科学研究的人力资源发展做出贡献。

This project aims to develop and test a new geochemical method for quantifying the temperatures and exposure histories of rocks and sediments exposed at the Earth's surface during the past few million years. The technique is based on the measurement of trace isotopes of the noble gases helium and neon that are produced in rock and mineral grains by cosmic-ray bombardment at the Earth's surface. These concentrations reflect both the length of time the rocks have been exposed at the surface and, because the rate of diffusive loss of these gases depends on temperature, the temperature they experienced during exposure. First, we will use controlled laboratory experiments on both natural and artificially-produced samples of the common minerals quartz and feldspar to establish the mechanism, rate, and temperature dependence of helium and neon diffusion in these minerals. These results will, in principle, allow us to predict concentrations of these gases in natural geological samples whose exposure and temperature histories are already known from other evidence. Thus, the second part of this project will be to test and validate our laboratory results and theoretical framework by comparing predicted with actual concentrations in natural rock samples. This research is important because reconstructing the past temperatures and surface exposure histories of surface materials is valuable for a broad range of scientific research. First, measuring exposure durations of surface rocks is important in understanding geologic processes that act to form and change Earth's surface, including surface erosion, sediment transport, and earthquake-related surface deformation. Second, measuring past temperatures is important for understanding Earth's natural climate variability during the last few million years. For example, such information is important to establish how past changes in environmental conditions influenced biota at various regions across the globe, and how past climate changes were potentially controlled by natural phenomena such as the gradual development of mountain ranges. However, our ability to quantify past temperatures is currently limited to a small number of geochemical techniques. If this new method is successfully developed and tested, it will provide an independent test of existing methods, and potentially benefit a broad range of research sub disciplines including quantitative geomorphology, landscape evolution studies, late Cenozoic climate changes, glacier and ice sheet change, and, potentially, paleo-elevation of actively uplifting landscapes. This basic geochemistry research will potentially enable a wide array of earth science researchers to address both longstanding and completely new questions, thus benefiting the broader science community. In addition, this project will support a graduate student at UC Berkeley, thus contributing to Earth science education and the development of human resources for geochemistry and broader science research.