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

挥发性元素是在低温下挥发(变成气体或蒸汽)的元素。这些元素的例子包括H、C、O、N和许多其他元素。它们对在地球上产生宜居条件以及它们如何影响地球的动态--特别是它们对岩石的削弱作用--至关重要。尽管挥发性元素很重要，但我们对挥发性元素在地球不同部分(如大气/海洋、地壳、地幔)之间的分布情况，以及这种分布在地球历史上可能发生的变化，都没有很好的了解。卤素元素(F、Cl、Br、I)是一种挥发性元素，被用来追踪地球表面和内部之间的挥发性循环。最近的研究表明，大陆上部地壳有一个独特的卤素浓度特征，很可能是由于岩石的近地表风化而产生的。这一特征类似于上地幔的特征，潜在地意味着风化的上地壳岩石循环进入地幔。然而，这一特征也可能是由于在地球内部(变质作用)发现的较高压力和温度下，沉积物重结晶时伴随着挥发性元素的损失而产生的。这个项目将量化卤素和氯同位素在沉积岩的变质脱水过程中的表现，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，这些元素会系统性地下降。这些数据将帮助我们理解卤素(也就是挥发性元素)如何在地球内部重新分布。该项目将支持一名第一代博士生和两名本科生的研究，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，在地幔熔融期间，相对于类似不相容的亲石元素，大陆上部地壳中的卤素是枯竭的，这可能是由于火山脱气，或许是由于地壳形成时的一些化学风化。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循环在控制地幔卤素收支方面发挥了作用。更好地了解卤素，包括氯同位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生研究人员，他将确定卤素元素(F、氯、溴、碘)和氯同位素在陆源沉积岩变质脱水过程中的表现。研究人员将分析四套具有良好特征的变质泥岩中的全部卤素浓度(F、Cl、Br、I)：英国加里东期泥岩；缅因州奥纳瓦岩体接触光环的变质火山岩；新西兰奥塔戈片岩；以及意大利伊夫雷亚带的变质火山岩。这些变质岩经历了亚绿片岩相到麻粒岩相的变质条件，并将使研究人员能够确定在变质脱水反应期间卤素是否以及何时彼此分馏。伴随着相同岩石的氯同位素和总有机碳含量，他们将能够评估有机质对变质岩中卤素丰度的影响程度，以及变质作用可能如何改变这些标志。该奖项反映了NSF的法定使命，并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。

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.