Collaborative Research: Experimental and computational constraints on the isotope fractionation of Mossbauer-inactive elements in mantle minerals

地球深部矿物的同位素组成对地球内部结构和化学演化具有重要的制约作用。地球深部矿物的同位素组成受同位素分馏过程的控制，即在不同的压力和温度下，不同的同位素在不同的矿物之间重新分配。传统的方法有三种，即理论计算、质谱和核共振散射来限制深部矿物中平衡同位素分馏。然而，每种方法都有自己的挑战。理论计算受到模型中使用的物理近似的限制，必须通过实验进行基准测试;质谱法很难确定平衡的实现，而且很耗时;核共振散射只能应用于少数穆斯堡尔活性元素。该提案描述了一种新型的多学科研究，通过将最先进的X射线光谱学与理论计算相结合，通过协作和协同努力，对难以受到传统实验方法约束的深海矿物的同位素分馏进行研究。研究人员的方法是实验基准，时间效率，直接反映了平衡同位素分馏，并可适用于几乎所有的元素。他们的目标是通过他们提出的研究回答以下问题：1）地球深部矿物中的穆斯堡尔惰性元素如何随着压力，温度和晶体结构重新分布？2)如何有效地抑制穆斯堡尔惰性元素在地球深部固溶体中的分馏？振动非谐性如何影响地幔硅酸盐的同位素分馏？该项目将支持三名早期到中期职业研究人员在位于Maona的夏威夷大学继续他们的研究（Zhang和B。Chen）和Purdue University（M. Chen）。通过该项目，该团队将在国家用户设施开发实验仪器和开源计算代码，研究套件将提供给地球科学及其他领域的国内外研究人员。本科生助理和博士后学者将参与这个项目。该项目还致力于为参与的早期职业研究人员建立职业发展途径，推动地球科学中的性别和种族平等，并扩大传统上代表性不足的少数民族对STEM的参与。地幔矿物组成元素的同位素在高P-T条件下的分馏被认为是地球分化的后果，为地幔的组成和化学演化提供了重要线索。以前对矿物之间穆斯堡尔惰性元素的同位素分馏的研究要么使用质谱测量，其中达到平衡需要额外的谨慎，要么从理论计算中估计，而没有实验的证实。研究人员已经证明，四配位Si的约化配分函数比（β因子）可以通过理论计算校准的原位高温SXD实验来限制。在这个拟议的项目中，他们将扩展该方法，首次确定非淬火深土矿物中六配位Si的实验约束β因子。他们还将应对挑战，建立一种有效的方法，通过理论计算确定固溶体中的β因子，这些方法将通过SXD实验进行基准测试，从而使他们能够研究深部矿物中六配位Ti的同位素分馏。他们提出的组合方法避免了固体溶液的直接理论建模，从而显着降低了计算成本。采用实验和理论模拟相结合的方法，研究了振动非谐性对含水层状硅酸盐中Si β因子的修正。提出的实验基准机器学习框架建立一个高度准确而有效的模型，将使他们能够提供可靠的估计非谐波矿物系统中的Si β因子。虽然他们只建议研究选择穆斯堡尔惰性元素（Si Ti）在这个建议中，同样的方法可以扩展到其他元素，如C，O，Mg和Ca。所有这三项拟议任务将为地幔矿物同位素分布的完整景观奠定基础，并将增进对地球同位素组成及其化学演变的了解。该项目由地球深层内部合作研究（CSEDI）和激励竞争性研究的既定计划（EPSCoR）共同资助。该奖项反映了NSF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。

Isotope compositions of deep-earth minerals provide crucial constraints on the earth’s internal structure and chemical evolution. The isotope compositions of deep-earth minerals are controlled by a process named isotope fractionation, in which different isotopes redistribute between different minerals under various pressures and temperatures. There are three conventional ways to constrain equilibrium isotope fractionation in deep-earth minerals, namely theoretical calculation, mass spectroscopy, and nuclear resonance scattering. However, each method has its own challenges. Theoretical calculation is limited by physical approximations used in the model and has to be benchmarked by experiments; mass spectroscopy has difficulty in determining the attainment of equilibrium and is time-consuming; and nuclear resonance scattering can only be applied to the few Mössbauer-active elements. This proposal describes a novel multidisciplinary study on the isotope fractionations of deep-earth minerals that are difficult to be constrained by conventional experimental approaches, through collaborative and synergetic efforts by combining state-of-the-art X-ray spectroscopy with theoretical calculations. The researcher's approach is experimentally benchmarked, time-efficient, directly reflects the equilibrium isotope fractionation, and can be applied to nearly all elements. They aim to answer the following questions with their proposed study: 1) How do the Mössbauer-inactive elements in deep-earth minerals redistribute with pressure, temperature, and crystal structure? 2) How to constrain the fractionation of Mössbauer-inactive elements in deep earth solid solutions efficiently? and 3) How does vibrational anharmonicity affect the isotope fractionation in mantle silicates? The project will support three early to mid-career researchers to continue their research at the University of Hawaii at Maona (Zhang and B. Chen) and Purdue University (M. Chen). Through this project, the team will develop both experimental instruments at a national user facility and open-source codes for computation, and the research suite will be available to domestic and international researchers in earth sciences and beyond. Undergraduate assistants and postdoc scholars will be involved in this project. This project is also committed to establishing the career development pathways for the involved early-career researchers, pushing for the gender and racial equality in geoscience, and broadening the participation in STEM of traditionally underrepresented minorities.The fractionation of the isotopes of constituent elements of mantle minerals at high P-T conditions are considered the ramifications of the differentiation of the Earth, offering crucial clues for the mantle’s composition and chemical evolution. Previous studies on the isotope fractionation of Mössbauer-inactive elements between minerals were either measured using mass spectroscopy in which the attainment of equilibrium requires additional caution, or estimated from theoretical calculations without corroboration from experiments. The researchers have demonstrated that the reduced partition function ratio (β-factor) of tetracoordinated Si can be constrained by theoretical-calculation-calibrated in-situ high-T SXD experiments. In this proposed project, they will extend the method to determine experimentally constrained β-factors of hexacoordinated Si in non-quenchable deep-earth minerals for the first time. They will also tackle challenges to establish an efficient approach to determine β-factors in solid solutions by theoretical calculations, which are to be benchmarked by SXD experiments and thus allows them to investigate the isotope fractionation of hexacoordinated Ti in deep-earth minerals. Their proposed combined approach circumvents direct theoretical modeling of solid solutions, which significantly reduces computational costs. Using the combined approaches of experiments and theoretical modeling, the correction to Si β-factor in hydrous phyllosilicates induced by the vibrational anharmonicity will be investigated. The proposed experimentally benchmarked machine learning framework to establish a highly accurate yet efficient model will allow them to provide reliable estimations of Si β-factor in anharmonic mineral systems. Though they only propose to study select Mössbauer-inactive elements (Si & Ti) in this proposal, the same approach can be extended to other elements such as C, O, Mg, and Ca. All the three proposed tasks will build the foundation for a complete landscape of isotope distribution in mantle minerals, and will enhance understanding of the Earth’s isotopic composition and in turn its chemical evolution. This project is jointly funded by Cooperative Studies of the Earth's Deep Interior (CSEDI) and the Established Program to Stimulate Competitive Research (EPSCoR).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.