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-使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基本组成部分。正如20世纪70年代和80年代的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的探针方面取得了爆炸性进展一样，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快速扩展。这样的重建有各种各样的应用，以了解地球系统演化的社会相关方面。重建沉积盆地的热历史是确定油气远景的关键。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，低温热计时法-本研究项目的重点-已成为我们重建数百万年时间尺度上过去气候波动的主要手段。 这些信息对于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的气候趋势进行比较，并更全面地评估气候极端变化的社会影响。矿物-同位素系统在热年代学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，氦的扩散具有很强的各向异性（平行于c晶向的数量级更大）。研究人员提出了一项研究，旨在系统地量化这两种影响氦扩散的各种矿物（U-Th）/He热年代学的证明或预期价值：独居石，金红石，钛铁矿，磷钇矿，锆石。他们将通过热退火和质子辐照制备具有不同辐射损伤水平的样品，用于两种扩散实验：1）更熟悉的增量加热，体扩散方法，通过该方法获得了目前大多数可用的氦扩散数据;和2）一种新开发的激光烧蚀深度剖析方法，能够在预定的时间内对实验室诱导的扩散轮廓进行询问，晶体学取向该项目的最终目标将是有助于制定一个方案，为个别晶体分配适当的独特扩散参数，使热年代学家可以更有信心地使用测得的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.