Collaborative Research: Decoding thermal and magmatic history of mafic and ultramafic rocks through systematic studies of cation diffusion in pyroxene

斜方辉石和单斜辉石是地球上地幔和下地壳中的两种主要造岩矿物。辉石的主量元素和微量元素组成已被广泛用于推断含辉石岩石经历的热历史和岩浆史。天然岩石中辉石的一个共同特征是存在出溶片层。在存在出溶片层的情况下，没有阳离子在辉石中和通过辉石中扩散的模型，这阻碍了对含辉石岩石热历史的解释。这项合作研究的重点是Al、Ca、Zr和Hf在辉石中的扩散。该项目包括三个主要任务：(1)出溶薄片在辉石中对阳离子扩散的作用的理论和数值研究；(2)铝、钙、锆和Hf在辉石中扩散的实验研究；(3)地球化学应用。其结果将是一套在具有出溶片层的辉石颗粒之间的扩散传质模型。这些扩散模型是通用的，也可用于研究岩石学和地球化学领域以外的复合材料的化学和力学性质。Al、Ca、Zr和Hf的扩散系数将被用来建立广义的传质模型，该模型可用于量化稀土元素(REE)和高场强元素(HFSE)在沿地幔绝热不平衡熔化之后的亚固相线再平衡过程中的分馏。将开发新的Ca-In-OPX温度计和Al-In-OPX温度计的封闭温度模型。这些新模型将用于研究不同构造环境下橄榄岩中辉石中稀土元素和高场强元素的分布和分馏。拟议工作的更广泛影响将集中在研究生和本科生培训以及对高生产率女科学家的支持上。在这项拟议的工作中建立的层板内和层间扩散模型是通用的，可以用于研究复合材料在地球科学以外的领域的化学和机械性能。在镁铁质和超镁铁质岩石中，辉石中常观察到主量元素和微量元素的分带，这可能为辉石岩石经历的热历史和岩浆史提供重要线索。天然岩石中辉石的一个共同特征是存在出溶片层。虽然稀土元素(REE)和高场强元素(HFSE)在辉石中的扩散已经取得了显著的进展，但对铝在辉石中的扩散却知之甚少。目前还没有关于钙在斜方辉石中扩散的公开数据，也没有阳离子在出溶层存在的辉石中和通过辉石中扩散的模型。本次合作研究的重点是Al、Ca和HFSe在辉石中的扩散和分布及其在地球化学中的应用。该项目包括三个主要任务：(1)出溶薄片在辉石中对阳离子扩散的作用的理论和数值研究；(2)铝、钙、锆和Hf在辉石中扩散的实验研究；(3)地球化学应用。大部分扩散实验将在1个大气压下进行。压力对Al、Ca、Zr和Hf在辉石中扩散的潜在影响尚不清楚，但将进行一些更高压力的实验。用27Al(p，γ)28Si核反应测量实验装药中Al的扩散分布。用卢瑟福背散射法测量了Ca、Zr和Hf的扩散分布。任务1的结果是一套具有出溶片层的辉石颗粒间的扩散传质的微观和宏观模型。这些扩散模型是通用的，也可用于研究岩石学和地球化学领域以外的复合材料的化学和力学性质。结合已发表的分配和扩散数据，任务2中的Al、Ca、Zr和Hf扩散系数将被用来开发广义的质量传递模型，该模型可用于量化沿地幔绝热不平衡熔融之后的固相线下再平衡过程中REE和HFSe的分馏。作为任务3的一部分，将开发新的Ca-In-OPX温度计和Al-In-OPX温度计的关闭温度模型。这些新模型将用于研究不同构造环境下橄榄岩中辉石中稀土元素和高场强元素的分布和分馏。拟议工作的更广泛影响将集中在人力资源方面，其中包括研究生和本科生培训以及对一名高生产率女科学家的支持。这项拟议的工作将构成研究生博士论文的主要部分。该项目将为本科生提供实验室分析和计算机建模经验，并为高级论文项目提供研究机会。在这项拟议的工作中开发的层板内和层间扩散模型是通用的，可以用于研究复合材料在地球科学以外的领域的化学和机械性能。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Orthopyroxene and clinopyroxene are two major rock-forming minerals in the Earth’s upper mantle and lower crust. Major and trace element compositions of the pyroxenes have been widely used to infer thermal and magmatic histories experienced by pyroxene-bearing rocks. A common feature of pyroxene in natural rocks is the presence of exsolution lamellae. There is no model for cation diffusion in and through pyroxene in the presence of exsolution lamellae, which hinders the interpretation of thermal history of pyroxene-bearing rocks. The focus of this collaborative study is Al, Ca, Zr, and Hf diffusion in pyroxene. The project consists of three main tasks: (1) theoretical and numerical studies of the role of exsolution lamellae on cation diffusion in pyroxene; (2) experimental studies of Al, Ca, Zr, and Hf diffusion in pyroxene; and (3) geochemical applications. The outcome shall be a set of models for diffusive mass transfer across pyroxene grains that have exsolution lamellae. These diffusion models are general and can also be used to study chemical and mechanic properties of composite materials outside the field of petrology and geochemistry. Al, Ca, Zr and Hf diffusion coefficients will then be applied to develop generalized mass transfer models that can be used to quantify rare earth element (REE) and high-field strength element (HFSE) fractionation during disequilibrium melting along a mantle adiabat followed by subsolidus re-equilibration. New models for closure temperatures of the Ca-in-opx thermometer and the Al-in-opx thermometer will be developed. These new models will be used to study the distribution and fractionation of REE and HFSE in pyroxenes in peridotites from different tectonic settings. The broader impacts of the proposed work will focus on graduate and undergraduate training and support of a highly productive female scientist. Models for diffusion in and through laminates to be developed in this proposed work are general and can be used to study chemical and mechanic properties of composite materials in fields outside Earth science. Major and trace element zoning in pyroxene has often been observed in mafic and ultramafic rocks, which may provide important clues to the thermal and magmatic histories experienced by the pyroxene-bearing rocks. A common feature of pyroxene in natural rocks is the presence of exsolution lamellae. Although significant progress has been made in quantifying rare earth element (REE) and high field strength element (HFSE) diffusion in pyroxene, very little is known about Al diffusion in pyroxene. There are no published data for Ca diffusion in orthopyroxene and no model for cation diffusion in and through pyroxene in the presence of exsolution lamellae. The focus of this collaborative study is Al, Ca and HFSE diffusion and distribution in pyroxene and their geochemical applications. The project consists of three main Tasks: (1) theoretical and numerical studies of the role of exsolution lamellae on cation diffusion in pyroxene; (2) experimental studies of Al, Ca, Zr, and Hf diffusion in pyroxene; and (3) geochemical applications. The majority of the diffusion experiments will be conducted at 1-atm pressure. The potential effects of pressure on Al, Ca Zr, and Hf diffusion in pyroxene are not known, but some higher-pressure experiments will be conducted. Diffusion profiles of Al in experimental charges will be measured with the 27Al(p,gamma)28Si nuclear reaction. Diffusion profiles of Ca, Zr and Hf will be measured with Rutherford Backscattering Spectrometry. The outcome of Task 1 is a set of microscale and macroscale models for diffusive mass transfer across pyroxene grains that have exsolution lamellae. These diffusion models are general and can also be used to study chemical and mechanic properties of composite materials outside the field of petrology and geochemistry. Together with published partitioning and diffusion data, Al, Ca, Zr and Hf diffusion coefficients from Task 2 will be used to develop generalized mass transfer models that can be used to quantify REE and HFSE fractionation during disequilibrium melting along a mantle adiabat followed by subsolidus re-equilibration. As part of Task 3, new models for closure temperatures of the Ca-in-opx thermometer and the Al-in-opx thermometer will be developed. These new models will be used to study the distribution and fractionation of REE and HFSE in pyroxenes in peridotites from different tectonic settings. The broader impacts of the proposed work will focus on human resources, which include graduate and undergraduate training and support of a highly productive female scientist. The proposed work will constitute a major part of the graduate student’s PhD thesis. The project will provide laboratory analytical and computer modeling experiences for undergraduate students and research opportunities for senior thesis projects. Models for diffusion in and through laminates to be developed in this proposed work are general and can be used to study chemical and mechanic properties of composite materials in fields outside Earth science.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.