Experimental Determination of Argon Diffusion Kinetics and Mechanisms in Plagioclase

钾到氩的放射性衰变提供了地质历史上最重要的测年方法之一。由于Ar是一种气体，它可以通过扩散过程从地质材料，即矿物中逸出，特别是在高温下。扩散过程可以用几个动力学参数来数学描述，这些参数可以通过实验室研究来确定，知道这些参数的值能够确定岩石随时间的热历史。岩石的热史已被证明对于理解一系列现象至关重要，这些现象包括地壳沿主要断层的大规模运动、深部盆地中碳氢化合物的热成熟、岩浆侵入的热效应以及来自其他行星的陨石的热史，这反过来又决定了生命所必需的有机分子是否能够经受住撞击和星际运输。已经确定了一些矿物的Ar扩散动力学参数，但地壳中最丰富的矿物--斜长石--到目前为止基本上还没有被研究过。该项目将使用新的激光加热技术和超灵敏质谱仪来确定不同产状的斜长石的动力学参数，从而为在显著扩大的地质环境范围内确定岩石的热历史提供强有力的新工具。从中子照射的斜长岩中脱气三种Ar同位素(质量为37、39和40)，并用稀有气体质谱仪进行测量。通过回归定义线性Arrhenius关系的增量加热数据，确定了活化能和频率因子(无限温度下的扩散系数)的动力学参数。这个项目的一个高度优先事项是区分在自然界中起作用的扩散机制和实验人工制品。解决这一问题的方法是对单晶进行循环递增加热，强调温度低于500°C，并对重新辐照的样品进行分析，以区分加热过程中的磁区耗尽和磁区破坏。扩散的长度尺度是通过分析一系列晶体尺寸来研究的，激光烧蚀研究具有高空间分辨率以绘制出同位素梯度。用透射电子显微镜评价了出溶、片层和反相界等微观结构的重要性。正在开发分析和应用数据的软件，并将在赠款期间结束时向公众开放。

The radioactive decay of potassium to argon provides one of the most important means of dating events in the geologic past. Because argon is a gas, it can escape from geologic materials, i.e., minerals, especially at elevated temperatures, through the process of diffusion. The diffusion process can be described mathematically in terms of a few kinetic parameters which can be determined by laboratory studies, and knowing the values of these parameters enables the determination of the thermal histories of rocks over time. Thermal histories of rocks have proven critical to understanding a wide range of phenomena, including large scale motions of the crust along major faults, thermal maturation of hydrocarbons in deep basins, the heating effects of magma intrusions, and the thermal histories of meteorites derived from other planetary bodies which in turn governs whether organic molecules necessary for life could survive impact and interplanetary transport. Kinetic parameters for argon diffusion have been determined for some minerals, but plagioclase, the most abundant mineral in the Earth's crust has until now been largely unstudied. This project will use novel laser heating techniques and ultrasensitive mass spectrometry to determine the kinetic parameters for plagioclase from a variety of occurrences, and thereby provide powerful new tools for determining rocks' thermal histories in a significantly broadened range of geologic environments.Three argon isotopes (masses 37, 39, and 40) are degassed from neutron-irradiated plagioclase and measured with a noble gas mass spectrometer. The kinetic parameters of activation energy and frequency factor (diffusivity at infinite temperature) are determined by regression of incremental heating data defining linear Arrhenius relationships. A high priority of this project is to distinguish between diffusion mechanisms operative in nature versus experimental artifacts. This is addressed by cyclic incremental heating of single crystals, emphasizing temperatures below 500 °C, and analysis of reirradiated samples to distinguish between domain depletion and domain destruction during heating. The length scale of diffusion is investigated by analyzing a range of crystal sizes, and laser ablation studies with high spatial resolution to map out isotopic gradients. The importance of microstructures such as exsolution lamellae and antiphase boundaries is evaluated using transmission electron microscopy. Software for analysis and application of the data are being developed and will be publicly available by the end of the grant period.