Equilibrium and Kinetic Isotopic Fractionation of Lithium in Mantle Minerals

知识价值。超镁质捕虏体提供了有关上地幔性质的直接信息。然而，捕虏体的地球化学可能在运输过程中由于与携带它们的熔体或流体发生反应而发生改变。由于锂在高温下的高流动性，它可能为研究这一反应的时间尺度和程度提供了一个窗口。地幔捕虏体常表现出颗粒间和颗粒内的锂同位素分异。特别是斜辉石往往比共存的橄榄石更轻，变化更大，特别是在衰竭的捕虏体中。提出的实验研究旨在确定这是否是地幔Li同位素地球化学的固有特征，或者这是否反映了喷发之前或期间发生的过程。例如，如果一个相能够在捕虏体动员和运输的时间尺度上更充分地与宿主熔体平衡，那么Li在橄榄石和斜辉石中的不同扩散速率可能解释了这两个相之间Li同位素的明显分异。通过本文的研究，我们打算解决以下问题：1)锂及其同位素在地幔矿物中的平衡分布是什么？2)锂元素在橄榄石和斜辉石中的扩散速率是多少？3) Li同位素在固态扩散过程中分馏的程度如何？该研究首次尝试通过实验验证高温下Li同位素分馏不平衡的假设，确定Li在橄榄石中的扩散速率，并评估Li同位素在固态扩散过程中的分馏程度。如果实验结果表明Li在橄榄石中的扩散相对于捕虏体动员的时间尺度是缓慢的（这可能是也可能不是），那么地幔结核中橄榄石的Li同位素组成可能是评估上地幔Li同位素地球化学的有价值的工具。同时，量化锂在斜辉石中的扩散速率将使我们能够评估捕虏体运输之前和期间地幔-熔体相互作用的时间尺度。在这项实验研究中产生的数据将使未来的研究人员能够评估地幔中Li同位素非均质性的程度，地幔结核在运输之前和期间的反应性蚀变的时间尺度和程度，以及含橄榄石和斜辉石熔岩的冷却速率。更广泛的影响。这个项目将使PI能够建立研究技术和设施。协作。David Eggler（宾夕法尼亚州立大学）和James Brenan（多伦多大学）将极大地促进在宾夕法尼亚州立大学建立一个新的实验岩石学实验室。该项目将支持宾夕法尼亚州立大学的一名博士生和几名本科生的高级论文项目。分析工作将在日本（ISEI; Okayama U.）进行，作为博士生国际教育和文化交流的一部分，并促进地球化学界的东西方知识交流。

Intellectual merit. Ultramafic xenoliths provide direct information regarding the nature of the upper mantle. However, xenolith che geochemistry may be modified during transport due to reaction with the melts or fluids that carry them. Lithium may provide a window into the timescales and extent of this reaction due to its highly mobile nature at high temperatures. Mantle xenoliths often display inter-and intra-granular lithium isotopic fractionation. In particular, clinopyroxenes are often lighter and more variable than co-existing olivines, particularly in depleted xenoliths. The proposed experimental study aims to determine if this is an inherent characteristic of the Li isotope geochemistry of the mantle, or if this is reflective of processes occurring immediately prior to or during eruption. For instance, different diffusion rates for Li in olivine and clinopyroxene might explain the apparent fractionation of Li isotopes between these two phases, if one phase is able to equilibrate more fully with the host melt on the timescales of xenolith mobilization and transport. With the proposed study, we intend to address the following questions: 1) What is the equilibrium distribution of Li and its isotopes in mantle minerals?, 2) What is the rate of Li elemental diffusion in olivine and clinopyroxene?, and 3) To what extent are Li isotopes fractionated during solid-state diffusion?. The proposed research constitutes the first attempt to experimentally verify the presumed lack of equilibrium isotopic fractionation of Li at high temperatures, to determine Li diffusion rates in olivine, and to assess the degree to which Li isotopes are fractionated during solid-state diffusion. If the experimental results show that Li diffusion in olivine is slow relative to the timescales of xenolith mobilization (which may or may not be the case), the Li isotope composition of olivines in mantle nodules could be valuable tools for evaluating the Li isotope geochemistry of the upper mantle. At the same time, quantifying the rates of Li diffusion in clinopyroxene will allow us to assess the timescales of mantle-melt interaction prior to and during xenolith transport. The data generated during this experimental study will allow future researchers to assess the extent of Li isotope heterogeneity in the mantle, the timescales and extent of reactive alteration of mantle nodules immediately prior to and during transport, and cooling rates of olivine- and clinopyroxene- bearing lavas.Broader Impacts. This project will enable the PI to establish research techniques and facilities. Collaborations with. David Eggler (Penn State) and James Brenan (U. of Toronto) will greatly facilitate establishment of a new experimental petrology lab at Penn State. The project will support one PhD student and senior thesis projects for several undergraduates at Penn State. Analytical work will be conducted in Japan (ISEI; Okayama U.) as part of an international educational and cultural excgange for the PhD student, and promotes East-West intellectual exchange for the geochemical community.