Experimental Studies in Petrologic and Geochemical Kinetics

本提案要求资助继续进行一项实验研究计划，目标是在高温高压下地质材料中原子迁移的速率和机制。提出了两个特定的重点：1)同位素在熔融硅酸盐中的扩散分馏（重点是岩浆挥发物）；2)稀有气体原子在造岩矿物中的扩散和溶解度。知识价值。同位素比率被地球化学家用于许多目的，包括但不限于：估计地质系统的温度，流体和熔体的“来源”，量化元素循环，以及记录生物活动。在所有这些应用中，同位素比率的使用依赖于对能够改变它们的过程的全面计算。最近的研究表明，熔融硅酸盐中简单的化学扩散过程可以引起显著的同位素分馏，因为同位素的扩散速率略有不同。迄今为止所表征的元素（Li, Ca, Mg）在扩散率上的差异是微妙的，但地球化学家专门测量和利用同位素比率的非常小的差异。拟议的项目将主要侧重于挥发性岩浆成分（S, Cl， C， H）同位素在等温化学扩散过程中和存在温度梯度时的扩散分馏。简单的模型显示，这些元素的同位素扩散系数的预期差异将导致在非平衡岩浆脱气过程中产生大量的分馏。如果项目获得资助，将测量相关的扩散分馏因子，并将其应用于岩浆系统的定量模型。了解地质物质中稀有气体原子的迁移和分配行为，对于理解地球如何以及在多大程度上失去其挥发物至关重要。作为放射性衰变的产物，40Ar和4He在热年代学中也有广泛的应用。从岩石中提取时间-温度历史。作为对其他实验室通过对岩石形成矿物的热脱气研究获得的信息的补充，本提案提供了一个系统的实验方案，其中矿物将暴露于高温下的惰性气体大气中，并通过离子束剖面技术直接表征气体吸收。目标将是量化稀有气体原子扩散到矿物晶格中的速率，以及通过它们在矿物表面的浓度来测量它们的溶解度。利用这种方法在RPI对Ar行为的研究非常成功，因此，随着资金的持续投入，这项工作将主要针对其他惰性气体，从He和Ne开始。所得数据将用于利用定量模型评价稀有气体在地球上的储存、扩散和相间分配。拟议活动的总体智力价值在于获取有关扩散现象的基本信息，这些现象在固体地球化学中的作用尚未得到充分了解。在大多数情况下，计划中的实验还针对特定的地球化学问题，例如岩浆脱气过程中的同位素分馏以及40Ar和其他惰性气体在固体地球中的保留。更广泛的影响。除了提供关于地球材料性质的基本战略数据外，RPI实验地球化学实验室的首要作用是作为一个教育环境，让博士生学习各种技能和策略，使他们成为足智多谋、多才多艺的科学家，为在其他环境中进行独立研究做好准备。本科生定期参加实验室活动，这种模式鼓励他们应用课堂学习，帮助他们在团队环境中培养责任感，并促进对细节的关注。该项目将为RPI实验地球化学实验室提供主要支持，使该设施继续发挥其作为伦斯勒社区内外科学访客的资源的作用（最近的“外部”访客来自哈佛大学、波士顿大学和巴西联邦大学）。RPI地球与环境科学系的几位成员已经开放了对实验室的访问，这使他们独立的nsf资助的研究项目受益。该实验室还满足了通用电气公司研究GaN半导体材料在高P-T条件下的行为的研究人员的需求。RPI实验地球化学实验室与纽约州立大学奥尔巴尼分校（UAlbany）的粒子加速器设施建立了协同活动，并从中受益。RPI开发的实验技术与UAlbany设施可用的分析技术相结合，使大量的科学研究能够以每小时25美元的光束时间的适度成本完成。这里更广泛的影响是地球化学界的新分析方法的引入和可用性。

This proposal requests funding to continue a program of experimental research targeting the rates and mechanisms of atom migrations in geologic materials at high temperatures and pressures. Two specific foci are proposed: 1) fractionation of isotopes by diffusion in molten silicates (with emphasis on magmatic volatiles); and 2) diffusion and solubility of noble-gas atoms in rock-forming minerals.Intellectual Merit. Isotope ratios are used by geochemists for many purposes, including but not limited to: estimation of the temperatures of geologic systems, 'sourcing' of fluids and melts, quantifying element cycling, and documenting biological activity. In all these applications, the use of isotope ratios relies upon a full accounting of the processes capable of changing them. Recent studies have revealed that the simple process of chemical diffusion in molten silicates can induce significant isotopic fractionation because the isotopes diffuse at slightly different rates. The differences in diffusivity are subtle for the elements characterized to date (Li, Ca, Mg), but geochemists specialize in measuring and exploiting very small differences in isotope ratios. The proposed project would focus mainly upon diffusive fractionation of the isotopes of volatile magmatic constituents (S, Cl, C, H) during isothermal chemical diffusion and in the presence of a temperature gradient. Simple models reveal that anticipated differences in the diffusivities of the isotopes of these elements would lead to substantial fractionations during, for example, nonequilibrium magmatic degassing. If the project is funded, the relevant diffusive fractionation factors will be measured and applied to quantitative models of magmatic systems. Knowledge of the mobility and partitioning behavior of noble gas atoms in geologic materials is central to understanding how, and to what extent, the Earth has lost its volatiles. As products of radioactive decay, 40Ar and 4He also see widespread use in thermochronology-i.e., the extraction of time-temperature histories from rocks. As a complement to information obtained in other laboratories through thermal degassing studies of rock-forming minerals, this proposal offers a systematic experimental program in which minerals will be exposed to noble-gas atmospheres at high temperatures and the gas uptake characterized directly by ion-beam profiling techniques. The goal will be to quantify the rate at which the noble-gas atoms diffuse into the mineral lattices, as well as their solubilities as gauged by their concentrations at the mineral surfaces. Studies of Ar behavior at RPI using this approach have been very successful, so with continued funding the effort would be directed mainly toward other noble gases, beginning with He and Ne. The resulting data will be used to evaluate the storage, dispersal and interphase partitioning of noble gases in the Earth using quantitative models. The overall intellectual merit of the proposed activities lies in the acquisition of basic information about diffusion-related phenomena whose role in solid-Earth geochemistry is not fully understood. In most instances, the planned experiments also target specific geochemical problems, such as isotopic fractionation during magma degassing and retention of 40Ar and other noble gases in the solid Earth.Broader Impacts. In addition to providing basic, strategic data on properties of Earth materials, a first-order role of the RPI experimental geochemistry lab is to serve as an educational environment where doctoral students learn a variety of skills and strategies, maturing into resourceful, versatile scientists who are prepared for independent research in other settings. Undergraduates participate regularly in lab activities in a mode that encourages application of their classroom learning, helps them develop responsibility in a team environment, and promotes attention to detail. This project would provide major support for the RPI experimental geochemistry lab, allowing the facility to continue in its role as a resource for scientific visitors from both inside and outside the Rensselaer community (recent 'outside' visitors have come from Harvard, Boston University, and the Federal University of Brazil). Several members of RPI's Department of Earth and Environmental Sciences have open access to the lab that has benefited their independent NSF-funded research projects. The laboratory has also served the needs of researchers from General Electric working on the behavior of GaN semi-conductor materials at elevated P-T conditions. The RPI experimental geochemistry lab has both created and benefited from synergistic activities with the particle-accelerator facility at the State University of New York at Albany (UAlbany). The pairing of experimental techniques developed at RPI with analytical techniques available at the UAlbany facility has enabled a great deal of science to be done at a modest cost of $25/h of beam time. The broader impact here is the introduction and availability of new analytical approaches to the geochemical community.