Collaborative Research: Halogen and chlorine isotope behavior during metamorphism of metapelitic rocks

挥发性元素是在低温下挥发（变成气体或蒸气）的元素。这些元素的例子包括氢、碳、氧、氮和许多其他元素。它们对于在地球上创造适宜居住的条件以及它们如何影响地球动力学——特别是它们对岩石的削弱作用——至关重要。尽管它们很重要，但我们对挥发性元素在地球不同部分（例如大气/海洋、地壳、地幔）之间的分布情况并没有很好的了解，也不知道这种分布在地球历史上是如何变化的。卤素元素（F、Cl、Br、I）是挥发性元素，已被用来追踪地球表面和内部之间的挥发循环。最近的研究表明，上大陆地壳具有独特的卤素浓度特征，可能是由岩石近地表风化作用产生的。这个特征与在上地幔中看到的相似，可能意味着风化的上地壳岩石在地幔中循环。然而，这种特征也有可能是由于在地球内部发现的较高压力和温度下沉积物的再结晶过程中挥发性元素的损失（变质作用）而产生的。这个项目将量化卤素和氯同位素在沉积岩变质脱水过程中的表现，这些沉积岩最初是地球表面的泥浆。这种岩石含有高浓度的水和其他挥发性元素，随着岩石在越来越高的温度下变质，这些元素会有系统地减少。这些数据将使我们了解卤素（以及隐含的挥发性元素）如何在地球内重新分布。该项目将支持一名第一代博士生和两名本科研究人员的研究，这些研究人员来自历史上被排除在地球科学之外的群体。最近的研究表明，在地幔熔融过程中，相对于不相容的亲石元素，上大陆地壳的卤素含量减少，这可能是由于火山脱气和地壳形成时的某种化学风化作用。有趣的是，结晶的上大陆地壳具有与上地幔岩石相似的相对卤素丰度，这可能表明UCC再循环在控制地幔卤素收支方面的作用。更好地了解包括Cl同位素在内的卤素在变质作用中的行为，将有助于确定陆源沉积岩再循环对地幔卤素特征的影响程度。该项目将资助一名博士研究员，他将研究卤素元素（F、Cl、Br、I）和氯同位素在陆源沉积岩变质脱水过程中的表现。研究人员将分析四组特征良好的变质岩中的全卤素浓度（F、Cl、Br、I）：英国加里东泥岩；缅因州奥纳瓦岩体接触光圈中的变长岩；奥塔哥片岩，新西兰；以及意大利伊夫雷亚区的元apelite。这些变长岩经历了亚绿片岩相到麻粒岩相的变质条件，这将使研究人员能够确定在变质脱水反应中卤素是否以及何时相互分离。随附的氯同位素和总有机碳含量将使他们能够评估有机质对变质岩中卤素丰度的影响程度，以及变质作用如何改变这些特征。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。

Volatile elements are elements that volatilize (turn into a gas or vapor) at low temperatures. Examples of such elements include H, C, O, N, and many others. They are critically important in generating habitable conditions on Earth and in how they influence Earth’s dynamics -- particularly their rock-weakening effects. Despite their importance, we do not have a good understanding of how volatile elements are distributed between different portions of the Earth (e.g., atmosphere/ocean, crust, mantle), nor how this distribution may have changed over Earth history. Halogen elements (F, Cl, Br, I) are volatile elements that have been used to trace volatile cycling between Earth’s surface and its interior. Recent work has shown that the upper continental crust has a distinctive halogen concentration signature that was likely generated by near-surface weathering of rocks. This signature is similar to that seen in the upper mantle, potentially implicating recycling of weathered upper crustal rocks into the mantle. However, it is also possible that the signature is generated by volatile-element loss accompanying recrystallization of the sediments at the higher pressures and temperatures found in Earth’s interior (metamorphism). This project will quantify how halogens and chlorine isotopes behave during metamorphic dehydration of sedimentary rocks that started out as mud at Earth’s surface. Such rocks have high concentrations of water and other volatile elements that systematically decline as the rock is metamorphosed at higher and higher temperatures. These data will inform our understanding of how halogens (and, by implication volatile elements) are re-distributed within Earth. The project will support the research of a first-generation PhD student as well as two undergraduate researchers from groups historically excluded from Earth Science.Recent work showed that the upper continental crust is depleted in halogens relative to lithophile elements of similar incompatibility during mantle melting, which is likely due to volcanic degassing and perhaps some amount of chemical weathering at the time of crust formation. Interestingly, the crystalline upper continental crust has relative halogen abundances that are similar to those seen in upper mantle rocks and may suggest a role for UCC recycling in controlling mantle halogen budgets. A better understanding of halogen, including Cl isotope, behavior during metamorphism will allow determination of the degree that mantle halogen signatures are influenced by terrigenous sedimentary rock recycling. This project will fund a PhD researcher who will determine how halogen elements (F, Cl, Br, I) and chlorine isotopes behave during metamorphic dehydration of terrigenous sedimentary rocks. The researchers will analyze complete halogen concentrations (F, Cl, Br, I) in four well-characterized suites of metapelites: British Caledonian mudrocks; metapelites from the contact aureole of the Onawa pluton, Maine; Otago schists, New Zealand; and metapelites from the Ivrea Zone, Italy. These metapelites experienced sub-greenschist facies to granulite facies metamorphic conditions and will allow the researchers to determine if and when halogens are fractionated from one another during metamorphic dehydration reactions. Accompanying chlorine isotope and total organic carbon contents of the same rocks will allow them to evaluate the degree to which organic matter influences halogen abundances in metapelites and how metamorphism may change these signatures.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.