Experimental Determination of Argon Diffusion Kinetics and Mechanisms in Plagioclase

钾到氩的放射性衰变提供了确定地质历史事件年代的最重要手段之一。因为氩是一种气体，它可以从地质材料中逸出，即，矿物，特别是在高温下，通过扩散过程。扩散过程可以用几个动力学参数进行数学描述，这些参数可以通过实验室研究确定，知道这些参数的值可以确定岩石随时间的热历史。岩石的热历史已被证明是理解一系列现象的关键，包括地壳沿着主要断层的大规模运动，深盆地中碳氢化合物的热成熟，岩浆侵入的加热效应，以及来自其他行星体的陨石的热历史，这反过来又决定了生命所必需的有机分子是否能在撞击和行星际运输中幸存下来。氩扩散的动力学参数已被确定为一些矿物，但斜长石，在地壳中最丰富的矿物，直到现在还在很大程度上未被研究。该项目将使用新的激光加热技术和超灵敏质谱法来确定各种产状的斜长石的动力学参数，从而为确定岩石在大幅度扩大的地质环境中的热历史提供强有力的新工具。（质量37、39和40）从中子辐照的斜长石中脱气，并用惰性气体质谱仪测量。的动力学参数的活化能和频率因子（在无限的温度下的扩散系数）的增量加热数据定义的线性Arrhenius关系的回归。这个项目的一个高优先级是区分扩散机制与实验文物的性质。这是通过单晶的循环增量加热来解决的，强调温度低于500 °C，并分析再辐照样品以区分加热过程中的畴耗尽和畴破坏。扩散的长度尺度进行了研究，通过分析一系列的晶体尺寸，和激光烧蚀研究与高空间分辨率映射出同位素梯度。使用透射电子显微镜，如出溶层流和反相边界的微观结构的重要性进行评估。正在开发分析和应用数据的软件，将在赠款期结束时向公众提供。

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.