Calcium and potassium isotopic study of igneous and metamorphic transport processes

地球科学家测量矿物和岩石的化学成分和年龄，以获得有关它们如何形成的信息。利用通过分析获得的信息，再加上矿物学、物理学和化学方面的知识，就有可能重建地球在45亿年的历史中是如何进化和变化的。这一努力有很多困难，因为岩石和矿物自形成以来，通常要经受多次高温高压。为了解释这些加热和掩埋过程引起的变化，有必要了解化学元素如何在高温高压下在矿物和岩层之间移动，这种移动也导致新矿物的形成。该项目旨在使用一种新的方法来更好地定义岩石和矿物在高温下遭受变质和熔化时的化学变化。该计划是利用两种重要化学元素——钾(K)和钙（Ca）的同位素的相对比例来区分岩石化学变化的程度，以及相应的变化过程。这种方法以前还没有在野外的岩石上使用过，但有证据表明，根据过去十年来在世界各地不同实验室进行的实验，它将是有指导意义的。这种方法还可以帮助我们了解岩浆是如何在地球上形成并产生火山爆发的。这个项目将对教育、化学和材料科学产生更广泛的影响。尽管细节上存在许多复杂性，但拟议的项目将产生直接与宏观特征相关的结果，这些特征在全球岩石暴露中被广泛观察到，并经常用于教学。为此目的，该项目将制作教材，说明这些过程及其意义，并将在网上公布，供教员使用。该项目旨在发展对高温火成岩和变质体系的新认识，强调Ca和K的同位素，并有很大的潜力推进我们对岩石形成过程中化学输送的基本认识。所建议的工作的一个重要方面是关注具有足够可预测的起始条件的现场事件，以便将其视为自然实验。这种方法与大量的实验室实验相反，在实验室实验中，运行条件可以控制，但对自然的适用性值得怀疑。目的是更好地了解在变质作用和岩浆房中，关键化学成分的流动性是如何被控制的，以及这种流动性与Ca和K同位素的分馏有何关系。这些元素的同位素效应是矿物表面微纳米尺度过程和更大尺度地质介质内运输的示踪剂。特别是，这些元素中的同位素分馏主要归因于化学扩散，因此同位素测量将有助于区分高温成岩过程中扩散（而不是流体平流）在多大程度上负责大部分化学再分配。这项研究需要对岩石和矿物进行高精度的同位素和化学表征，了解露头地质特征的起源，了解扩散、热力学、热输运、岩石学和构造地质学，了解同位素分馏机制，并能够用分析和数值方法模拟过程，从测量中提取关键信息。这种多方面和多学科的努力，对于推进地球化学和岩石学科学是必要的，为学生和高级研究人员提供了丰富的教育机会，也为发展对地球岩石形成过程的新认识提供了机会。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。

Earth scientists measure the chemical composition and age of minerals and rocks to get information about how they formed. Using the information gained through analysis, and with knowledge of mineralogy, physics, and chemistry, it is possible to reconstruct how the Earth has evolved and changed over its 4.5 billion year history. There are many difficulties in this endeavor, because typically rocks and minerals are subjected to high temperatures and pressures at multiple times since they first formed. To account for the changes that these heating and burial processes caused, it is necessary to understand how chemical elements move between minerals and rock layers at high temperature and pressures, movement that also results in new minerals being formed. This project is aimed at using a novel approach to better define how the chemistry of rocks and minerals can change when they are subjected to metamorphism and melting at high temperature. The plan is to use the relative proportions of the isotopes of two important chemical elements – potassium (K) and calcium (Ca) – to distinguish the extent to which rock chemistry has been changed, and the processes responsible. This approach has not been employed previously to rocks in the field, but there is evidence that it will be instructive based on experiments that have been performed in various laboratories around the world over the past ten years. The approach can also help us to understand how magma forms in the Earth and produces volcanic eruptions. This project will have broader impacts in education and in chemistry and materials science. Although in detail there are many complexities, the proposed project will produce results that are keyed directly to macroscopic features that are widely observed in rock exposures globally and frequently used for teaching. To this end the project will produce teaching materials to illustrate the processes and their significance that will be posted on the web and made available for instructors.This project is aimed at developing a new level of understanding of high temperature igneous and metamorphic systems, emphasizing the isotopes of Ca and K, and where there is substantial potential to advance our fundamental understanding of chemical transport during petrogenesis. An essential aspect of the proposed work is a focus on field occurrences having sufficiently predictable starting conditions that they can be treated as natural experiments. This approach is in contrast to laboratory experiments, which have been done in abundance, where run conditions can be controlled but the applicability to nature is questionable. The goal is to better understand how the mobility of key chemical constituents is controlled during metamorphism and in magma chambers, and how that mobility relates to fractionation of Ca and K isotopes. Isotopic effects in these elements are tracers of micro- and nano-scale processes at mineral surfaces and transport within geologic media at larger scales. In particular, isotopic fractionation in these elements is primarily attributable to chemical diffusion, and hence the isotopic measurements will help to distinguish to what degree diffusion, as opposed to fluid advection, is responsible for the bulk of chemical redistribution during high temperature petrogenetic processes. The proposed research requires high precision isotopic and chemical characterization of rocks and minerals, insight about the origin of geologic features exposed in outcrop, knowledge of diffusion, thermodynamics, heat transport, petrology, and structural geology, insight into isotopic fractionation mechanisms, and an ability to model the processes with analytical and numerical approaches to extract key information from the measurements. This multifaceted and multidisciplinary effort, which is necessary for advancing geochemical and petrological sciences, provides rich educational opportunities for students and advanced researchers as well as an opportunity to develop a new level of understanding of rock-forming processes in the Earth.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.