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

智力上的功绩。镁铁质和超镁铁质岩石中高场强元素(HFSE)和稀土元素(REE)的丰度和分布为了解火成岩的成因和演化提供了重要线索。虽然在许多岩浆作用中，这些微量元素在共存矿物和熔体之间的分布可以用平衡分馏来理解，但有时共存相之间的大量化学不平衡可能会为地质过程的时间尺度提供独特而重要的见解。为了更好地了解引起地幔和地幔岩石中高场强元素和稀土元素不平衡分布的各种过程，需要这些微量元素在主要造岩矿物中的分配系数和扩散系数。单斜辉石(CPx)和斜方辉石(OPx)等重要镁铁质矿物的矿物熔体和矿物矿物HFSE和REE分配系数可在一定的温度、压力、熔体和晶体成分范围内获得。没有任何辉石成分的HFSE扩散数据，而辉石中的稀土元素扩散数据仅适用于近端元透辉石和顽辉岩。目前尚不清楚辉石中稀土元素和高场强元素的扩散速率与辉石成分的关系有多大。为了建立一个研究含辉石岩石中高场强元素和稀土元素在岩浆和固相线下过程中动力学分馏的数据库，提出了一项为期三年的联合研究计划，该计划包括两个主要部分：(1)高场强元素和稀土在辉石中扩散的实验室研究；(2)三位一体蛇绿岩橄榄岩中高场强元素和稀土元素共存的离子探针和数值分带研究。将探索镁铁质和超镁铁质岩石的成因和分异中常见的一系列辉石成分。大多数扩散实验的扩散轮廓将用卢瑟福背散射光谱仪(RBS)测量。共存的CPX和OPX中的痕量元素浓度分布将用离子探针测量。结合已发表的分配和扩散数据，从这项研究中获得的HFSE和REE在辉石中的扩散系数将被用来开发通用的地球测速模型，该模型可以用来理解熔岩反应的过程和时间尺度，通过溶解和再沉淀，以及三位一体橄榄岩经历的亚固相线重新平衡。扩散是固体-地球系统中的一个基本的质量传递过程，因此是一个非常普遍的话题。从这项工作获得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和含辉石的岩石。将开发的溶解-再沉淀模型将对地球科学以外的一系列实际应用有用。这项研究的结果还将为不同的岩石学家、地球化学家、地质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。结果将通过公开讲座以及本科生和研究生课程向更广泛的受众传播。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.