Collaborative Research: Diffusion of High Field Strength Elements (HFSE) and Rare Earth Elements (REE) in Pyroxenes and Pyroxene-bearing Rocks

知识价值。基性和超基性岩石中高场强元素（HFSE）和稀土元素（REE）的丰度和分布为认识火成岩的成因和演化提供了重要线索。虽然这些微量元素在许多岩浆过程中共存的矿物和熔体之间的分布可以用平衡分馏来理解，但在某些情况下，共存相之间的实质性化学不平衡可能为地质过程的时间尺度提供独特而重要的见解。为了更好地理解导致地幔及幔源岩中HFSE和REE不平衡分布的各种过程，需要研究这些微量元素在主要造岩矿物中的分配系数和扩散系数。重要的基性矿物如斜辉石（cpx）和正辉石（opx）的矿物-熔体和矿物-矿物HFSE和REE分配系数可用于一系列温度、压力、熔体和晶体组成。辉石的近端透辉石和顽辉石的稀土元素弥散数据均不存在，而辉石中的稀土元素弥散数据仅存在于近端透辉石和辉辉石。REE和HFSE在辉石中的扩散速率与辉石成分的关系尚不清楚。为了建立含辉石岩岩浆岩和次固相岩浆岩中HFSE和REE的动力学分馏研究数据库，提出了一项为期3年的合作研究计划，主要包括两个部分：(1)辉石岩中HFSE和REE扩散的实验室研究；(2)三位一体蛇绿岩中共存cpx和opx橄榄岩中HFSE和REE分带的离子探针和数值研究。探讨了基性岩和超基性岩成因和分异中常见的辉石组成。大多数扩散实验的扩散曲线将用卢瑟福后向散射光谱法（RBS）测量。离子探针将测量共存的cpx和opx中的微量元素浓度曲线。结合已发表的分配和扩散数据，本研究获得的辉石中HFSE和REE的扩散系数将用于建立广义大地测速模型，该模型可用于理解三位一体橄榄岩的熔融-岩石反应过程和时间尺度，包括溶蚀和再沉淀以及亚固体再平衡。更广泛的影响。扩散是固体地球系统中一个基本的传质过程，因此是一个非常普遍的话题。本工作得到的扩散系数将广泛适用于来自地球、月球、火星和各种陨石的辉石岩和含辉石岩。所开发的溶解-再沉淀模式将有助于地球科学以外的一系列实际应用。这项研究的结果也将为不同的岩石学家、地球化学家、地球年代学家和宇宙化学家提供有价值的信息，促进地球科学的跨学科整合。研究结果将通过公开讲座以及本科和研究生课程传播给更广泛的受众。最后，建议的项目将为本科生提供实践经验，为高级论文工作提供研究机会，并为研究生提供实验，计算和教育经验。

Intellectual Merit. The abundance and distribution of high field strength elements (HFSE) and rare earth elements (REE) in mafic and ultramafic rocks provide important clues to understanding the origin and evolution of igneous rocks. While the distribution of such trace elements between coexisting minerals and melt in many magmatic processes can be understood in terms of equilibrium fractionation, there are occasions where substantial chemical disequilibria between coexisting phases might provide unique and important insights into the time scales of geological processes. To better understand the various processes that give rise to disequilibrium distributions of HFSE and REE in the mantle and mantle derived rocks, partition coefficients and diffusion coefficients of these trace elements in major rocking-forming minerals are needed. Mineral-melt and mineral-mineral HFSE and REE partition coefficients for important mafic minerals such as clinopyroxene (cpx) and orthopyroxene (opx) are available for a range of temperatures, pressures, and melt and crystal compositions. There exists no HFSE diffusion data for any pyroxene compositions, while REE diffusion data in pyroxene is only available for the near endmember diopside and enstatite. It is not known how strongly REE and HFSE diffusion rates in pyroxenes depend on pyroxene compositions. In order to establish a database for studying kinetic fractionations of HFSE and REE in pyroxene-bearing rocks during magmatic and subsolidus processes, a three-year collaborative research program is proposed that consists of two major components: (1) laboratory studies of HFSE and REE diffusion in pyroxenes; and (2) ion probe and numerical studies of HFSE and REE zoning in coexisting cpx and opx in peridotites from the Trinity ophiolite. A range of pyroxene compositions common in the origin and differentiation of mafic and ultramafic rocks will be explored. Diffusion profiles from most diffusion experiments will be measured with Rutherford Backscattering Spectrometry (RBS). Trace element concentration profiles in coexisting cpx and opx will be measured by ion probe. Together with published partition and diffusion data, diffusion coefficients of HFSE and REE in pyroxenes obtained from this study will be used to develop generalized geospeedometry models that can be used to understand the processes and time scales of melt-rock reaction, via dissolution and reprecipitation, and subsolidus reequilibration experienced by the Trinity peridotites.Broader Impacts. Diffusion is a fundamental mass transfer process in solid-Earth systems and hence a very general topic. Diffusion coefficients obtained from this work will be widely applicable to pyroxenes and pyroxene bearing rocks from the Earth, Moon, Mars, and various meteorites. The dissolution-reprecipitation model to be developed will be useful to a range of practical applications beyond Earth Sciences. Results from this study will also provide valuable information for a diverse group of petrologists, geochemists, geochronologists, and cosmochemists, promoting cross-discipline integration in Earth Sciences. Results will be disseminated to a broader audience through public lectures and undergraduate and graduate courses. Finally, the proposed project will provide hands-on experience for undergraduates, research opportunities for senior thesis work, and experimental, computational, and educational experience for graduate students.