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

正辉石和斜辉石是地球上地幔和下地壳中两种主要的造岩矿物。辉石岩的主微量元素组成被广泛用于推断含辉石岩的热历史和岩浆历史。天然岩石中辉石的一个共同特征是析出片层的存在。在脱溶片层存在的情况下，辉石中和通过的阳离子扩散没有模型，这阻碍了含辉石岩热史的解释。本次合作研究的重点是Al， Ca， Zr和Hf在辉石中的扩散。本项目包括三个主要任务：(1)脱溶片层对辉石中阳离子扩散作用的理论和数值研究；(2) Al、Ca、Zr、Hf在辉石中扩散的实验研究；(3)地球化学应用。结果应该是一套模型的扩散传质在辉石颗粒具有脱溶片。这些扩散模型具有普遍性，也可用于研究岩石学和地球化学以外的复合材料的化学和力学性质。然后应用Al， Ca， Zr和Hf扩散系数来建立广义传质模型，该模型可用于量化稀土元素（REE）和高场强元素（HFSE）在沿地幔绝热体的不平衡熔融过程中的分异，然后是亚固体再平衡。新模型的关闭温度的Ca-in-opx温度计和Al-in-opx温度计将开发。这些新模式将用于研究不同构造背景橄榄岩辉石中REE和HFSE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的培训和对高生产力女性科学家的支持上。在本工作中所建立的层合板内部和通过层合板的扩散模型是通用的，可用于研究地球科学以外的复合材料的化学和力学性能。辉石的主微量元素分带现象在基性和超基性岩石中普遍存在，为辉石岩的热演化史和岩浆演化史提供了重要线索。天然岩石中辉石的一个共同特征是析出片层的存在。虽然稀土元素（REE）和高场强元素（HFSE）在辉石中扩散的定量研究取得了重大进展，但对Al在辉石中扩散的定量研究却知之甚少。目前还没有关于钙在正辉石中的扩散的公开资料，也没有在脱溶层存在的情况下，在辉石内部和通过辉石的阳离子扩散的模型。重点研究Al、Ca和HFSE在辉石中的扩散分布及其地球化学应用。本项目包括三个主要任务：(1)脱溶层对辉石中阳离子扩散作用的理论和数值研究；(2) Al、Ca、Zr、Hf在辉石中扩散的实验研究；(3)地球化学应用。大多数扩散实验将在1-atm压力下进行。压力对Al， Ca， Zr和Hf在辉石中的扩散的潜在影响尚不清楚，但将进行一些高压实验。Al在实验电荷中的扩散曲线将用27Al(p, γ)28Si核反应来测量。用卢瑟福后向散射光谱法测量Ca、Zr和Hf的扩散谱。任务1的结果是一套微观尺度和宏观尺度的模型，用于具有脱溶层的辉石颗粒的扩散传质。这些扩散模型具有普遍性，也可用于研究岩石学和地球化学以外的复合材料的化学和力学性质。结合已公布的分配和扩散数据，任务2中的Al， Ca， Zr和Hf扩散系数将用于开发广义传质模型，该模型可用于量化沿地幔绝热体非平衡熔融过程中REE和HFSE的分馏，然后是亚固体再平衡。作为任务3的一部分，将开发Ca-in-opx温度计和Al-in-opx温度计的闭合温度的新模型。这些新模式将用于研究不同构造背景橄榄岩辉石中REE和HFSE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的培训以及对一名高产女科学家的支持。建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，并为高年级论文项目提供研究机会。在本工作中所建立的层合板内部和通过层合板的扩散模型是通用的，可用于研究地球科学以外的复合材料的化学和力学性能。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.