Equilibrium and Kinetic Isotopic Fractionation of Lithium in Mantle Minerals

地幔矿物中锂的平衡和动力学同位素分馏

• 批准号: 0810129
• 负责人: Maureen Feineman
• 金额: $ 17.78万
• 依托单位: Pennsylvania State Univ University Park
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-07-01 至 2012-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0810129&HistoricalAwards=false
• 关键词: Equilibrium; Kinetic; Isotopic; Fractionation; Lithium

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.

智力上的优点。超镁铁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会在运输过程中，由于与熔体或携带它们的流体反应而发生变化。由于锂在高温下具有高度移动的性质，因此锂可能为了解该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粒间和粒内锂同位素分馏。特别是，单斜辉石往往比共存的橄榄石更轻，更易变，特别是在亏损捕虏体中。拟议的实验研究旨在确定这是否是地幔锂同位素地球化学的固有特征，或者这是否反映了喷发前或喷发期间发生的过程。例如，在橄榄石和单斜辉石的锂不同的扩散速率可以解释这两个阶段之间的锂同位素的明显分馏，如果一个阶段能够更充分地平衡与宿主熔体的捕虏体动员和运输的时间尺度。通过这项研究，我们打算解决以下问题：1）Li及其同位素在地幔矿物中的平衡分布是什么？2)锂元素在橄榄石和单斜辉石中的扩散速率是多少？以及3）在固态扩散过程中Li同位素分馏到什么程度？拟议的研究构成了第一次尝试，以实验验证假定缺乏平衡同位素分馏的锂在高温下，以确定锂在橄榄石中的扩散速率，并评估在何种程度上锂同位素分馏在固态扩散。如果实验结果表明橄榄石中Li的扩散相对于捕虏体活动的时间尺度是缓慢的（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价上地幔Li同位素地球化学的有价值的工具。与此同时，定量的单斜辉石中的锂扩散率将使我们能够评估地幔熔体相互作用的时间尺度之前和捕虏体运输过程中。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同位素异质性的程度，在运输之前和运输期间地幔结核的反应性蚀变的时间尺度和程度，以及橄榄石和单斜辉石-轴承熔岩的冷却速率。该项目将使PI能够建立研究技术和设施。合作。大卫埃格勒（宾夕法尼亚州立大学）和詹姆斯布伦南（美国）。（多伦多）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;冈山大学）进行。作为博士生国际教育和文化交流的一部分，促进了地球化学界的东西方知识交流。