The Effects of Radiation Damage and Crystallography on Helium Diffusion in Minerals: An Integrated Bulk and Laser Ablation Depth Profiling Study

热年代学——利用同位素和核地球化学工具研究地质样品的温度历史——已经发展成为地球系统研究的一个基本部分。正如20世纪70年代和80年代地球化学界在开发新的同位素示踪剂作为地幔和地壳演化探针方面取得了巨大成就一样，热年代学界现在正在推动温度范围和温度历史重建精度的迅速扩大。这样的重建在理解与社会相关的地球系统演化方面有各种各样的应用。重建沉积盆地的热演化史对于确定油气潜力至关重要。更一般地说，我们知道上地壳岩石的热历史受到侵蚀过程的强烈影响。这些过程反过来又受到气候变化的影响，因此低温测温法——本研究项目的重点——已成为我们在数百万年的时间尺度上重建过去气候波动的主要手段。这些信息对我们建立长期气候变化至关重要，可以帮助我们更好地将当前的气候趋势与地球历史上更早的气候趋势进行比较，并更全面地评估极端气候变化的社会影响。矿物同位素系统在热时计中的实际应用受到氦扩散动力学知识质量的限制。最近对磷灰石和锆石的实验研究表明，辐射损伤对氦的扩散率有很大影响。最近对锆石的研究表明，锆石具有很强的氦扩散各向异性（与c晶体学方向平行的数量级更大）。研究人员提出了一项研究，旨在系统地量化这些对氦在各种矿物上扩散的影响，这些矿物已证实或预计具有(U-Th)/He热时计值：独居石、金红石、钛矿、xenotime和锆石。他们将通过热退火和质子辐照制备不同程度辐射损伤的样品，进行两种扩散实验：1)我们更熟悉的增量加热、体扩散方法，目前大多数氦扩散数据都是通过这种方法获得的；2)一种新开发的激光烧蚀深度剖面方法，可以在预定的晶体取向上对实验室诱导扩散剖面进行询问。该项目的最终目标将有助于建立一种模式，为单个晶体分配适当的独特扩散参数，以便温度年代学家可以更自信地使用测量日期进行热历史重建。

Thermochronology - the study of temperature histories for geologic samples using the tools of isotope and nuclear geochemistry - has evolved into a fundamental part of earth system research. Just as the geochemistry community in the 1970's and 1980's led an explosion in the development of new isotopic tracers as probes of mantle and crustal evolution, the thermochronology community is now driving a rapid expansion in the temperature range and precision of temperature history reconstructions. Such reconstructions have a variety of applications to understanding aspects of earth system evolution relevant to society. Reconstructing the thermal history of sedimentary basins is essential for our determination of oil and gas potential. More generally, we know that thermal histories are of rocks in the upper crust are strongly influenced by erosional processes. Those processes are, in turn, influenced by climatic variations such that low-temperature thermochronometry - the focus of this research project - has become our primary means of reconstructing past climate fluctuations on timescales of millions of years. Such information is essential for us to establish long-term climate variations that can help us to better compare current climate trends to those further back in earth history, and to more fully evaluate the societal implications of extreme changes in climate. The practical utility of mineral-isotopic systems for thermochronometry, is limited by the quality of our knowledge of helium diffusion kinetics. Recent experimental studies of apatite and zircon suggest that radiation damage substantially affects the diffusivity of helium. Recent studies of zircon suggest a strong helium diffusion anisotropy (orders of magnitude greater parallel to the c crystallographic direction). The investigators propose a study aimed at systematically quantifying both of these influences on helium diffusion on a variety of minerals with proven or anticipated value for (U-Th)/He thermochronometry: monazite, rutile, titanite, xenotime, and zircon. They will prepare samples with various levels of radiation damage through thermal annealing and proton irradiation for two kinds of diffusion experiments: 1) the more familiar incremental heating, bulk diffusion approach through which most of the currently available helium diffusion data were obtained; and 2) a newly developed laser ablation depth profiling approach that enables the interrogation of laboratory induced diffusion profiles in predetermined crystallographic orientations. The ultimate goal of the project will be to contribute to the development of a schema for assigning appropriately unique diffusion parameters to individual crystals so that themochronologists can more confidently use measured dates for thermal history reconstructions.