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.

地球早期地壳是如何形成和演化的是地球科学中最具争议的问题之一。一个主要的问题是，所有这些古老的岩石都至少经历过一次蚀变、变形和/或变质作用。所以没有原始岩石（原岩）存在。然而，许多研究使用了整个岩石的地球化学和同位素示踪剂，以及最近的附属阶段的同位素特征，作为地壳如何随时间演化和生长的模型的重要锚点。关于地球最早稳定地壳的形成，人们提出了各种各样的理论和过程。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会改变某些通常用于重建地壳历史的地球化学标志。本研究的主要目的是验证一种假设，即变形（古代）岩石中的某些元素和同位素准确地反映了其原岩的原始地球化学特征。这一假设将通过评估整个岩石和附属矿物的同位素特征是否随着应变的增加而保持不变，从而忠实地记录原始石器（同位素）组成，或者变形是否导致矿物和/或整个岩石尺度上的同位素系统受到干扰来验证。将研究变形程度不同的三种不同的岩石序列，以了解变形可能在多大程度上改变整个岩石和（亚）矿物尺度上的地球化学特征。这些序列的范围从最初未变形的（原岩）岩石到高度变形的岩石，这些岩石与世界上最古老的岩石非常相似。这项研究的发现将帮助研究人员使用和解释地球化学，特别是同位素，来自变形的古代岩石的数据。这项研究将支持学生培训、国际合作以及华盛顿州立大学Peter Hooper地球分析实验室和放射性成因同位素和地球年代学实验室分析设施的持续发展。两名博士生（一名女性）和本科生将接受实地工作和跨学科研究的培训，涵盖矿物学、构造地质学和地球化学。整个岩石样品和一些辅助矿物，如锆石、磷灰石、allanite和钛矿的同位素特征，经常被用来重建地壳的形成和演化。然而，几乎所有的太古宙和元古代岩石在就位后都经历了一次或多次的变形和变质作用，并且在经历这些构造-热事件后，这些岩石在多大程度上保留了它们原来（即原石器时代）的同位素比率尚不清楚。本研究结合了地球化学和构造方法，以了解花岗岩在整个岩石和（亚）矿物尺度上在不改变其原石器时代地球化学指纹的情况下可以变形到何种程度。该项目将重点测试一个主要假设：在不同变形的花岗岩中，整个岩石和附属矿物忠实地保留了它们原来的地球化学同位素特征，因此可以用来重建长期的地壳过程，例如地球最早稳定地壳的形成。本文将研究以下几个关键问题：1)变形引起的矿物反应涉及哪些辅助矿物？2)变形是否导致开系过程，如果是，随变形程度如何变化？3)不同变形岩石副矿物中的Lu-Hf、Sm-Nd和Rb-Sr同位素系统在多大程度上忠实地保存了它们最初的同位素组成？将研究三个不同的野外位置，其中相同的花岗岩体在应变梯度上暴露，从未变形到高度变形。通过对比变形花岗岩及其原岩在附属矿物和整体岩石尺度上的同位素数据，可以检验变形岩石在不同尺度（矿物与整体岩石）上是否保留了其原始同位素特征，或者上述同位素系统在变形后是否受到干扰，从而不能反映原石器组成。通过EPMA和LA-(MC)- icp - ms对所有主相和副相进行高空间分辨率的原位同位素和元素分析，以深入了解变形过程中矿物之间的地球化学交流方式，以及微量元素在不同尺度上是否以及在多大程度上被动员和重新分配。这项研究的结果将为如何解释和应用变形和变质岩石的地球化学特别是同位素信息来重建大规模地壳过程提供关键的理解。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。