Does deformation lead to misinformation? How much can granitic rocks deform before accessory minerals are geochemically disturbed?

变形会导致错误信息吗？

• 批准号: 2342159
• 负责人: Johannes Haemmerli
• 金额: $ 46.87万
• 依托单位: Washington State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2024
• 资助国家: 美国
• 起止时间: 2024-05-15 至 2027-04-30
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2342159&HistoricalAwards=false
• 关键词: Does; deformation; lead; misinformation; How

How Earth’s early crust formed and evolved is one of the most debated questions in Earth Science. A major problem is that all of these ancient rocks have experienced at least one episode of alteration, deformation and/or metamorphism. So no pristine rocks (protoliths) exist. However, many studies have used whole rock geochemistry and isotope tracers, and more recently the isotope signatures of accessory phases as important anchor points for models of how the crust has evolved and grown over time. Various theories and processes have been proposed for the formation of Earth's earliest stable crust. The debate over interpreting these data revolves around a core inquiry: whether the deformation and metamorphism of crustal rocks alters certain geochemical markers commonly used in reconstructing crustal history. The primary goal of this study is to test the hypothesis that certain elements and isotopes in deformed (ancient) rocks accurately reflect the original geochemical signatures of their protolith. This hypothesis will be tested by evaluating if the isotope signatures of whole rock and accessory minerals remain unchanged with increasing strain, and hence faithfully record the original protolithic (isotope) composition, or if deformation leads to the disturbance of isotope systems on a mineral and/or whole rock scale. Three different sequences of rocks, varying in degrees of deformation, will be studied to understand the extent to which deformation may alter their geochemical signatures on a whole rock, and (sub)-mineral-scale. These sequences range from initially undeformed (protolith) rocks to highly deformed ones, which closely resemble the world's oldest rocks. The findings of this study will help researchers use and interpret geochemical, particularly isotopic, data from deformed ancient rocks. This research will support student training, international collaboration, and continued development of analytical facilities at the Peter Hooper GeoAnalytical Lab and the Radiogenic Isotope & Geochronology Lab at Washington State University. Two PhD students (one female), as well as undergraduate students will be trained in field work and cross-disciplinary research spanning from mineralogy to structural geology to geochemistry.Isotope signatures in whole rock samples and in some accessory minerals, such as zircon, apatite, allanite, and titanite, are frequently used to reconstruct the formation and evolution of the Earth’s crust. However, virtually all Archean and Proterozoic rocks have experienced one or more episodes of deformation and metamorphism following their emplacement, and it is unclear to what degree these rocks retain their original (i.e., protolithic) isotope ratios after experiencing these tectono-thermal events. This research combines a geochemical and structural approach to understand to what degree granitic rocks can be deformed without changing their protolithic geochemical fingerprint at a whole rock- and (sub-)mineral-scale. This project will focus on testing a primary hypothesis: that whole rocks and accessory minerals in variably deformed granitic rocks faithfully retain their original geochemical isotope signatures and hence can be used to reconstruct long-term crustal processes, such as the formation of Earth’s earliest stable crust. The following key questions will be investigated: 1) what accessory minerals are involved in deformation-induced mineral reactions? 2) does deformation lead to open-system processes, and if so, how does this vary with degree of deformation? and 3) to what extent do Lu-Hf, Sm-Nd, and Rb-Sr isotope systematics in accessory minerals of variably deformed rocks faithfully preserve their initial isotope compositions? Three different field localities in which the same granitic bodies are exposed across strain gradients, spanning from undeformed to highly deformed will be studied. Via comparison of isotope data of the above systems on an accessory mineral- and whole rock-scale between deformed granitic rocks and their protoliths can be tested if deformed rocks retain their original isotope signatures on different scales (mineral vs. whole rock) or if the above isotope systems become disturbed upon deformation and hence cannot be trusted to reflect the protolithic composition. High-spatial resolution in-situ isotope and elemental analyses via EPMA and LA-(MC)-ICP-MS will be conducted on all major and accessory phases in order to gain an in-depth understanding of how minerals geochemically communicate with each other during deformation and if and to what degree trace elements are mobilized and redistributed on different scales. The results of this study will offer key understandings into how geochemical and particularly isotopic information from deformed and metamorphosed rocks can be interpreted and applied to reconstruct large-scale crustal processes.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.

地球的早期地壳如何形成和演化是地球科学中最有争议的问题之一，一个主要问题是所有这些古老的岩石都经历了至少一次蚀变、变形和/或变质作用，因此没有原始岩石（原岩）。然而，许多研究都使用了全岩石地球化学和同位素示踪剂，最近还使用了副相的同位素特征作为地壳随时间演化和生长的模型的重要锚点。关于解释这些数据的争论围绕着一个核心问题：地壳岩石的变形和变质作用是否改变了重建地壳历史中常用的某些地球化学标记。研究的目的是检验变形（古代）岩石中的某些元素和同位素准确反映其原岩的原始地球化学特征的假设，该假设将通过评估整个岩石的同位素特征来检验。随着应变的增加，副矿物保持不变，因此忠实地记录了原始的原石（同位素）组成，或者变形导致了矿物和/或整个岩石尺度上同位素系统的扰动三种不同程度的岩石序列。将研究变形的程度，以了解变形可能在整个岩石和（亚）矿物尺度上改变其地球化学特征的程度，这些序列范围从最初未变形（原岩）岩石到高度变形。这项研究的结果将帮助研究人员使用和解释来自变形古代岩石的地球化学数据，特别是同位素数据。这项研究将支持学生培训、国际合作和分析设施的持续发展。华盛顿州立大学的彼得胡珀地球分析实验室和放射性同位素与地质年代学实验室的两名博士生（一名女性）以及本科生将接受从矿物学到跨学科研究的培训。结构地质学到地球化学。整个岩石样本和一些副矿物（例如锆石、磷灰石、铜榴石和钛矿）中的同位素特征经常被用来重建地壳的形成和演化。然而，几乎所有的太古代和元古代岩石都是如此。在侵位后经历了一次或多次变形和变质作用，目前尚不清楚这些岩石在多大程度上保留了其原始状态（即原石器）这项研究结合了地球化学和结构方法，以了解花岗岩在不改变整个岩石和（亚）矿物尺度的原石地球化学指纹的情况下可以变形到什么程度。将重点测试一个主要假设：不同变形花岗岩中的整个岩石和副矿物忠实地保留了其原始的地球化学同位素特征，因此可用于重建长期地壳过程，例如地球最早稳定地壳的形成，将研究以下关键问题：1）哪些辅助矿物参与变形引起的矿物反应？2）变形是否会导致开放系统过程，如果是的话， 3) 不同变形岩石的副矿物中的 Lu-Hf、Sm-Nd 和 Rb-Sr 同位素系统在多大程度上忠实地保留了它们的性质？初始同位素组成？将通过比较上述系统在辅助矿物和整个岩石尺度上的同位素数据来研究三个不同的现场位置，其中相同的花岗岩体暴露在从未变形到高度变形的应变梯度上。如果变形岩石在不同尺度（矿物与整块岩石）上保留其原始同位素特征，或者如果上述同位素系统，则可以测试变形花岗岩及其原岩变形后会受到干扰，因此不能反映原石成分。将对所有主要和副相进行通过 EPMA 和 LA-(MC)-ICP-MS 进行的高空间分辨率原位同位素和元素分析。深入了解矿物在变形过程中如何在地球化学上相互交流，以及微量元素是否以及在何种程度上在不同尺度上被动员和重新分布。这项研究的结果将为我们提供关键的理解。来自变形和变质岩石的地球化学信息，特别是同位素信息可以被解释并应用于重建大规模地壳过程。该奖项是 NSF 的法定使命，并且通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。