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

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经历了至少一个阶段的蚀变，变形和/或变质作用。所以没有原始岩石（原岩）存在。然而，许多研究使用了全岩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如何随时间演化和生长的模型的重要锚点。 对于地球最早的稳定地壳的形成，已经提出了各种理论和过程。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是否改变了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检验变形（古）岩石中某些元素和同位素准确反映其原岩原始地球化学特征的假设。这一假设将通过评估整个岩石和副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的原石器（同位素）成分，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的干扰来进行测试。三个不同的岩石序列，不同程度的变形，将进行研究，以了解在何种程度上变形可能会改变他们的地球化学签名的整个岩石，（子）矿物规模。这些序列的范围从最初未变形的（原岩）岩石高度变形的，这非常类似于世界上最古老的岩石。这项研究的发现将有助于研究人员使用和解释来自变形古岩石的地球化学，特别是同位素数据。这项研究将支持学生培训，国际合作，并在彼得胡珀地质分析实验室和放射性同位素地质年代学实验室在华盛顿州立大学的分析设施的持续发展。两名博士生（一名女性），以及本科生将接受实地工作和跨学科研究的培训，从矿物学到构造地质学再到地球化学。整个岩石样品和一些副矿物（如锆石，磷灰石，褐帘石和钛铁矿）中的同位素特征经常用于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在就位后都经历了一次或多次变形和变质作用，目前还不清楚这些岩石在多大程度上保留了它们的原始（即，在经历了这些构造热事件之后，这项研究结合了地球化学和结构的方法来了解花岗岩在多大程度上可以变形，而不会改变他们的原岩地球化学指纹在整个岩石和（子）矿物尺度。该项目将侧重于检验一个基本假设：在变形花岗岩中的整个岩石和辅助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的地壳过程，如地球最早的稳定地壳的形成。 主要研究以下几个问题：1）变形矿物反应中有哪些副矿物参与？2)变形是否导致开放系统的过程，如果是的话，它是如何随变形程度而变化的？变质岩石副矿物中Lu-Hf、Sm-Nd和Rb-Sr同位素系统在多大程度上忠实地保留了原始同位素组成？将研究三个不同的现场地点，其中相同的花岗岩体暴露在应变梯度，从未变形到高度变形。通过比较上述系统在副矿物和全岩尺度上的同位素数据，可以测试变形花岗岩与其原岩之间的变形岩石是否在不同尺度上（矿物与全岩）保留其原始同位素特征，或者上述同位素系统是否在变形后受到干扰，因此不能被信任来反映原岩成分。将通过电子探针和LA-（MC）-ICP-MS对所有主要阶段和辅助阶段进行高空间分辨率原位同位素和元素分析，以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调动和重新分布。这项研究的结果将提供关键的理解如何从变形和变质岩石的地球化学，特别是同位素信息可以解释和应用于重建大规模的地壳过程。这个奖项反映了NSF的法定使命，并已被认为是值得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的支持。

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.