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How do sedimentary rocks become part of the lower continental crust?

沉积岩如何成为下大陆地壳的一部分？

基本信息

批准号：
2148886
负责人：
Roberta Rudnick
金额：
$ 35.36万
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：
2022
资助国家：
美国
起止时间：
2022-01-01 至 2024-12-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=2148886&HistoricalAwards=false
关键词：
How do sedimentary rocks become

项目摘要

Sedimentary rocks are important constituents of the continental crust. Whereas most reside in the upper crust where they form, geologic processes can transport sedimentary rocks into the lower crust. This is important because sedimentary rocks contain up to ten-times higher concentrations of the radioactive heat-producing elements K, Th, and U, compared to the rocks that otherwise dominate the lower crust. Therefore, the addition of sedimentary rocks to the deep continental crust has a profound impact on its heat production, the geothermal gradient within the crust, and the global inventory of heat-producing elements. There are different geological processes by which sedimentary rocks might enter the lower crust and each has implications for how common lower crustal sedimentary rocks may be. This project seeks to determine the processes responsible for transport of sedimentary rocks into the lower crust beneath southern New Mexico and central Montana, where such rocks are returned to the surface as accidental fragments in volcanoes (a poor-man’s drill hole), allowing scientists to study their history. The pathway and timing of sedimentary rock transport into the lower crust will be determined through geochemical analyses and radiometric dating. The project supports a female PhD student from a significantly under-represented demographic group, several undergraduate research assistants from UC Santa Barbara, two summer interns from California State University North Ridge and a senior female principal investigator. The PI and PhD student will train the undergraduate assistants and interns in geochemical research methods and provide career advice and guidance. The project seeks to determine the process(es) by which sedimentary rocks are emplaced into lower continental crust. Determining process will shed light on how common metasedimentary rocks might be the deep crust. Several tectonic scenarios are possible, which can potentially be distinguished on the basis of the prograde P-T paths followed by the rocks, as well as by the ages of detrital zircons and age and compositions of metamorphic minerals (e.g., monazite) they contain. The project focusses on two suites of lower crustal metapelite xenoliths carried in Cenozoic basalts from Kilbourne Hole and Potrillo maar, New Mexico, and the Eocene magmatic province of Montana. The prograde path these rocks followed will be ascertained using conventional thermobarometry, pseudosections, and Zr-in-rutile and Ti-in-zircon thermometry, and Ti-in-quartz thermometry. U-Pb dating of accessory phases will constrain the depositional age and timing of metamorphism. The study of these two sample suites, the lessons learned and the methods developed, will pave the way for future investigations of additional lower crustal metapelites in order to determine the dominant processes involved in sediment emplacement into the deep continental crust. The project will support the research of a female PhD student who is from a significantly under-represented group. It will also support several undergraduate research assistants from UC Santa Barbara and two summer interns from California State University North Ridge; both are Hispanic-serving institutions. The undergraduates will be trained in geochemical research methods and mentored in terms of career planning.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.

沉积岩是大陆地壳的重要组成部分。尽管大多数沉积岩存在于它们形成的上地壳中，但地质作用可以将沉积岩运送到下地壳中。这一点很重要，因为沉积岩含有的放射性发热元素K、Th和U的浓度比下地壳中占主导地位的岩石高出10倍。因此，沉积岩的加入对大陆地壳深部的产热、地壳内部的地热梯度以及产热元素的全球存量有着深远的影响。沉积岩进入下地壳有不同的地质过程，每一个过程都对下地壳沉积岩的普遍程度有影响。该项目旨在确定沉积岩进入新墨西哥州南部和蒙大拿州中部下地壳的过程，这些岩石在火山（穷人的钻孔）中作为意外碎片返回地表，使科学家能够研究它们的历史。将通过地球化学分析和放射性测年来确定沉积岩迁移到下地壳的路径和时间。该项目支持一名来自代表性明显不足的人口群体的女博士生、几名来自加州大学圣巴巴拉分校的本科研究助理、两名来自加州州立大学北岭分校的暑期实习生和一名高级女首席研究员。PI和博士生将在地球化学研究方法方面培训本科生助理和实习生，并提供职业建议和指导。该项目力求确定沉积岩侵位进入下陆壳的过程。确定过程将揭示如何常见的变质沉积岩可能是地壳深部。可能存在几种构造情景，可以根据岩石遵循的逆冲P-T路径以及碎屑锆石的年龄和变质矿物的年龄和成分（例如，独居石（monazite）。该项目的重点是两套下地壳的变岩捕虏体进行新生代玄武岩从基尔本洞和Potrillo马尔，新墨西哥州，和始新世岩浆省蒙大拿州。这些岩石遵循的迁移路径将使用常规的温压法、伪剖面法、金红石中的锆温度计、锆石中的钛温度计和石英中的钛温度计来确定。副相的U-Pb定年将制约沉积时代和变质作用发生的时间。对这两套样品、所吸取的经验教训和所制定的方法的研究，将为今后调查更多的下地壳变岩铺平道路，以便确定沉积物侵位进入大陆地壳深部的主要过程。该项目将支持一名女博士生的研究，她来自一个代表性严重不足的群体。它还将支持来自加州大学圣巴巴拉分校的几名本科生研究助理和来自加州州立大学北岭分校的两名暑期实习生;这两所大学都是为西班牙裔服务的机构。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。