Collaborative Research: Impact of crystal defects on helium diffusion in apatite crystals in (Uranium-Thorium)/Helium isotopic dating for the Earth sciences

要了解地球表面是如何随着造山和其他构造过程而变化的，就需要精确的方法来确定地壳和沉积盆地的状态和历史，这些盆地中蕴藏着天然气和石油。一种方法是测量岩石的温度历史，这使地质学家能够追踪岩石从深处移动到地表的速度和时间。这类工具中最广泛使用的是（铀钍）/氦同位素测定矿物磷灰石的年代。在磷灰石中，由铀和钍衰变产生的氦只有在非常低的温度下才会留在颗粒中，而且通常氦的扩散对温度非常敏感，因此磷灰石上的日期是热历史的记录。尽管这项技术一直很有用，但在某些情况下，即使考虑到诸如晶粒尺寸、辐射损伤和晶粒化学等复杂因素，从单个样品中获得的同位素日期也是分散的。拟议的研究将验证一个假设，即在磷灰石日期中有时看到的多余散射起源于改变磷灰石晶体颗粒内氦扩散行为的晶体缺陷。该项目通过特别在宾夕法尼亚州伯利恒的外展努力，强调让K-12未被充分代表的学生参与STEM（科学、技术、工程和数学）地球科学教育，并通过促进本科生和研究生的培训和参与积极的研究，从而为一支强大的、具有全球竞争力的科学队伍做出贡献，从而造福社会，促进预期的社会成果。这项研究证明了对一名早期职业研究人员的支持，他是STEM中一个代表性不足的专业群体的一部分。该项目将地质学和实验研究领域联系起来，以便能够发展出更准确的地质和地球化学过程模型。它还进一步将磷灰石（铀钍）/氦定年法作为一种强有力的工具应用于研究和商业应用，这可能对资源开发产生影响。利用不同空间尺度上应用的各种分析技术，我们将从已知产生分散磷灰石（铀-钍）/氦年龄的样品中表征各种磷灰石颗粒。表征将包括电子探针分析的化学成分，使用蚀刻实验和光学显微镜对晶体缺陷进行量化，以及电子束技术。利用连续斜坡加热实验对天然样品进行扩散实验，将提供扩散氦损失的动力学参数，并表征不同类型缺陷相关氦释放的温度依赖性。为了研究晶格尺度缺陷与变形相关的具体作用，我们将杜兰戈磷灰石单晶变形，以制造具有不同位错密度的样品。对已知位错密度的样品进行氦-3离子注入和核反应分析的扩散实验。此外，在全粒水平上，质子辐照样品将通过阶梯加热实验进行分析。然后将氦扩散动力学的变化建模为晶格和快速路径扩散的组合，以获得位错密度和扩散率之间的关系。变形实验的数据也将用于构建流动规律并确定应力/位错-密度关系。为了测试这些关系，来自已知变形历史的四个剪切带的样品将使用改进的协议来确定日期，该协议包含了位错密度和扩散率之间的新关系。该项目由地球科学部的构造学、岩石学和地球化学项目共同完成。

Understanding how the Earth's surface changes in response to mountain building and other tectonic processes requires accurate methods to determine the state and history of the crust and of sedimentary basins in which gas and oil deposits are developed. One approach involves measuring the temperature history of rocks, which allows geologists to track the rate and timing at which rocks are moved from depth to the surface. One of the most widely used of such tools is (Uranium-Thorium)/Helium isotopic dating of the mineral apatite. In apatite, the helium produced from decay of uranium and thorium will remain in a grain only at very low temperatures and in general the diffusion of helium is very sensitive to temperature, such that dates on apatite are a record of thermal history. Useful as the technique has been, in some instances isotopic dates obtained from even a single sample are scattered, even taking into account complicating factors such as grain size, radiation damage, and grain chemistry. The proposed study will test the hypothesis that the excess scatter that is sometimes seen in apatite dates originates in crystal imperfections that change helium diffusion behavior within apatite crystal grains. This project benefits society and advances desired societal outcomes by its emphasis on engaging K-12 underrepresented students in STEM (science, technology, engineering and math) earth science education through outreach efforts particularly in Bethlehem, PA, and by contributing to undergraduate and graduate student training and involvement in active research, thus contributing to a strong and globally competitive scientific workforce. The research is proving support for an early career researcher who is part of an underrepresented group in STEM as the professional level. The project is bridging the fields of geology and experimental research such that more accurate models of geologic and geochemical processes can be developed. It is also further the application of the apatite (Uranium-Thorium)/Helium dating as a robust tool in both research and commercial applications, which may have implication for resource development. Using various analytical techniques applied over different spatial scales, we will characterize a variety of apatite grains from samples known to yield dispersed apatite (Uranium-Thorium)/Helium ages. Characterization will include chemical composition by electron-probe analysis, quantification of crystal defects using etching experiments and optical microscopy, and electron-beam techniques. Diffusion experiments on natural samples using continuous ramped heating experiments will provide kinetic parameters for diffusive helium loss and characterize the temperature dependence of Helium release associated with various types of defects. To study the specific role of lattice-scale defects associated with deformation, we will deform Durango apatite single crystals to create samples with varying dislocation density. Diffusion experiments on these samples of known dislocation density will be conducted by Helium-3 ion implantation and nuclear reaction analysis. In addition, at the whole-grain level, proton-irradiated samples will be analyzed by step-heating experiments. Changes in helium diffusion kinetics will then be modeled as a combination of lattice and fast pathway diffusion to obtain a relationship between dislocation density and diffusivity. Data from deformation experiments will also be used to construct a flow law and determine a stress/dislocation-density relationship. To test these relationships, samples from four shear zones of known deformation history will be dated using an improved protocol that incorporates the new relationship between dislocation density and diffusivity.The project is a share between the Division of Earth Science's Tectonics and Petrology and Geochemistry programs.