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

合作研究：变质岩变质作用过程中的卤素和氯同位素行为

• 批准号: 2321367
• 负责人: Roberta Rudnick
• 金额: $ 33.09万
• 依托单位: University of California-Santa Barbara
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-09-01 至 2025-08-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2321367&HistoricalAwards=false
• 关键词: Collaborative; Research; Halogen; chlorine; isotope

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.

挥发性元素是在低温下挥发（变成气体或蒸气）的元素，包括 H、C、O、N 等，它们对于地球上的宜居条件及其影响至关重要。地球动力学——特别是它们的岩石弱化效应，尽管很重要，但我们对挥发性元素如何在地球不同部分（例如大气/海洋、地壳、地幔）之间分布以及这种分布如何没有很好的了解。可能有卤素元素（F、Cl、Br、I）是挥发性元素，可用于追踪地球表面与其内部之间的挥发性循环。最近的研究表明，上大陆地壳具有独特的卤素浓度特征。这种特征很可能是由岩石的近地表风化产生的，这可能意味着风化的上地壳岩石再循环到地幔中。然而，这种特征也可能是由挥发性元素产生的。损失地球内部较高压力和温度下沉积物的重结晶（变质作用）该项目将量化沉积岩在变质脱水过程中的行为，这些岩石最初是地球表面的泥浆。随着岩石在越来越高的温度下变质，水和其他挥发性元素会系统性地减少。这些数据将帮助我们了解卤素（以及暗示的挥发性）。该项目将支持第一代博士生以及来自历史上被排除在地球科学之外的群体的两名本科生研究人员的研究。最近的工作表明，相对于上层大陆地壳，卤素的含量已经耗尽。在地幔熔化过程中，亲石元素具有类似的不相容性，这可能是由于火山脱气以及地壳形成时的一定量的化学风化，这表明结晶的上大陆地壳具有相对的卤素丰度。与上地幔岩石中看到的类似，可能表明 UCC 循环在控制地幔卤素预算中发挥作用，更好地了解变质过程中的卤素（包括 Cl 同位素）行为将有助于确定地幔卤素特征受陆源影响的程度。该项目将资助一名博士研究员，他将确定卤素元素（F、Cl、Br、I）和氯同位素在变质脱水过程中的行为。研究人员将分析四组特征明确的变泥岩中的完整卤素浓度（F、Cl、Br、I）：英国喀里多尼亚泥岩；来自缅因州奥纳瓦岩体接触环的变泥岩；西兰岩；以及来自意大利伊夫雷亚区的变泥岩，这些变泥岩经历了次绿片岩相到麻粒岩相的变质条件，并且将允许。研究人员确定在变质脱水反应过程中卤素是否以及何时相互分馏，这将使他们能够评估有机物对变质岩中卤素丰度的影响程度以及变质作用如何影响。更改这些签名。这反映了 NSF 的法定使命，并且通过使用基金会的智力优点和更广泛的影响审查标准进行评估，被认为值得支持。