Noble Gas Partitioning Between Mineral Interiors And Grain Boundaries

法利和阿西莫夫-0125784尽管稀有气体作为地球过程的示踪剂和计时器被广泛使用，但令人惊讶的是，人们对它们的地球化学行为的许多基本方面却知之甚少。在不确定的特点是惰性气体在岩石和矿物，矿物/熔体分配行为，溶解度和扩散性的重要和常见的分析矿物，和运输机制和速率通过湿和干岩石的位置。这一提议旨在解决惰性气体地球化学的一个特别关键的特征，对此既没有数据也没有理论上的理解：在平衡状态下，惰性气体如何在晶界和晶体之间分配？与其他元素不同，稀有气体是不带电的，而且是相当大的原子，不适合以任何明显的方式在大多数晶格中。 因此，提出稀有气体将分配到晶粒之间的富缺陷区域中是不合理的。这种可能性通常不被考虑，但如果是真的，将对地球化学的几个重要领域产生深远的影响，最值得注意的是包括地幔演化和基于稀有气体的地质年代学研究。 该研究项目旨在以定量和严格的方法研究稀有气体在岩石矿物内部和晶界之间的分配行为。概念上简单的实验技术依赖于反应堆产生的同位素，以消除对困扰基于天然同位素的研究的大气吸附和污染的担忧。除了产生其他稀有同位素外，中子辐照提供了与自然界中相关的原位放射性惰性气体生产非常相似的初始条件。虽然所有稀有气体的同位素可以在许多不同的基板以这种方式产生，这个试点研究将集中在一个模型系统：4 He和37 Ar的透辉石多晶聚集体的分区，在高温和高压下照射后平衡的活塞缸装置。沿着配分测量，将用电子探针、SEM和透射电子显微镜对样品进行结构和化学表征，以直接评估晶界区域的物理性质、宽度和化学性质。拟议的工作将通过对这一简单系统的研究，开发和探索建立晶界在惰性气体行为中的作用所需的技术。如果辐照技术适用于评估透辉石中的He和Ar分配，那么它应该普遍适用于其他惰性气体和其他材料，因此具有远远超出初始试点研究的潜力。

Farley and AsimowEAR-0125784Despite the ubiquitous use of noble gases as tracers of Earth processes and as chronometers, many fundamental aspects of their geochemical behavior are, surprisingly, poorly understood. Among the uncertain characteristics are the siting of noble gases in rocks and minerals, mineral/melt partitioning behavior, solubility and diffusivity in important and commonly analyzed minerals, and transport mechanism and rate through wet and dry rocks. This proposal seeks to address a particularly critical characteristic of noble gas geochemistry, about which neither data nor theoretical understanding exist: at equilibrium, how do noble gases partition between grain boundaries and crystals? Unlike all other elements, the noble gases are uncharged and rather large atoms that do not fit in any obvious way within most crystal lattices. Thus it is not unreasonable to propose that noble gases will partition into the defect-rich region between grains. This possibility is not commonly considered, yet if true, would have profound implications for several important fields of geochemistry, most notably including studies of mantle evolution and noble-gas-based geochronology. The proposed research project seeks to investigate in a quantitative and thermodynamically rigorous way the partitioning behavior of noble gases between mineral interiors and grain boundaries in rocks. The conceptually simple experimental technique relies on reactor-produced isotopes to eliminate the concerns regarding atmospheric adsorption and contamination that plague studies based on natural isotopes. In addition to producing otherwise rare isotopes, neutron irradiation provides an initial condition closely analogous to in-situ radiogenic noble gas production relevant in nature. Although isotopes of all noble gases can be produced in many different substrates in this way, this pilot study will focus on just one model system: the partitioning of 4He and 37Ar in polycrystalline aggregates of diopside, equilibrated after irradiation at elevated temperature and pressure in the piston-cylinder device. Along with partitioning measurements, samples will be characterized texturally and chemically with the electron microprobe, SEM, and transmission electron microscope to directly assess the physical nature, width, and chemistry of the grain boundary region.The proposed work will develop and explore the techniques necessary for establishing the role of grain boundaries in noble gas behavior through study of this one simple system. If the irradiation technique works for assessing He and Ar partitioning in diopside, it should be generally applicable for other noble gases and other materials, and as such has potential well beyond the initial pilot study.