Noble Gas Partitioning Between Mineral Interiors And Grain Boundaries

Farley和AsimowEAR-0125784尽管普遍使用惰性气体作为地球过程的示踪剂和计时器，但令人惊讶的是，人们对它们的许多基本地球化学行为知之甚少。其中不确定的特征包括稀有气体在岩石和矿物中的位置、矿物/熔体分配行为、重要和通常分析的矿物中的溶解度和扩散系数，以及湿岩石和干岩石的传输机制和速率。这一提议试图解决稀有气体地球化学的一个特别关键的特征，关于这一特征，既没有数据也没有理论了解：在平衡状态下，稀有气体如何在晶界和晶体之间分配？与所有其他元素不同，稀有气体是不带电荷的，而且原子相当大，在大多数晶格中不适合任何明显的方式。因此，提出稀有气体将分配到颗粒之间的富缺陷区域并不是不合理的。这种可能性并不被普遍认为，但如果是真的，将对几个重要的地球化学领域产生深远的影响，最引人注目的是包括地幔演化和基于惰性气体的地质年代学的研究。拟议的研究项目寻求以定量和热力学严格的方式研究稀有气体在岩石中矿物内部和晶界之间的分配行为。这项概念简单的实验技术依赖于反应堆生产的同位素，以消除对大气吸附和污染的担忧，这些担忧困扰着基于天然同位素的研究。除了产生其他稀有的同位素外，中子辐照还提供了一种与原地放射成因惰性气体生产密切类似的初始条件。尽管所有稀有气体的同位素都可以通过这种方式在许多不同的衬底中产生，但这项初步研究将只关注一个模型系统：4He和37Ar在透辉石多晶聚集体中的分配，在活塞-圆柱体装置中在高温和压力下照射后平衡。除了分配测量，样品将用电子探针、扫描电子显微镜和透射电子显微镜进行织构和化学表征，以直接评估晶界区域的物理性质、宽度和化学成分。拟议的工作将通过对这一简单系统的研究，开发和探索必要的技术，以确定晶界在惰性气体行为中的作用。如果辐照技术用于评估透辉石中He和Ar的分配，它应该普遍适用于其他惰性气体和其他材料，因此具有远远超出最初先导研究的潜力。

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.