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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。虽然大多数沉积岩存在于它们形成的上地壳中，但地质过程可以将沉积岩输送到下地壳中。这一点很重要，因为与主导下地壳的岩石相比，沉积岩含有的放射性发热元素 K、Th 和 U 的浓度高出十倍。因此，沉积岩向大陆地壳深处的添加对其产热、地壳内的地温梯度以及全球产热元素的库存有着深远的影响。沉积岩可能通过不同的地质过程进入下地壳，每种地质过程都对下地壳沉积岩的常见程度产生影响。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方地壳的过程，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。沉积岩迁移到下地壳的路径和时间将通过地球化学分析和放射性测年确定。该项目支持一名来自明显不足的人口群体的女博士生、加州大学圣巴巴拉分校的几名本科生研究助理、加州州立大学北岭分校的两名暑期实习生和一名高级女性首席研究员。 PI和博士生将为本科生助理和实习生提供地球化学研究方法方面的培训，并提供职业建议和指导。该项目旨在确定沉积岩进入下大陆地壳的过程。确定过程将揭示深部地壳中变沉积岩的常见程度。可能存在几种构造情景，这些构造情景可以根据岩石所遵循的顺行 P-T 路径、碎屑锆石的年龄以及它们所含的变质矿物（例如独居石）的年龄和成分来区分。该项目重点研究来自新墨西哥州 Kilbourne Hole 和 Potrillo maar 以及蒙大拿州始新世岩浆省新生代玄武岩中的两套下地壳变泥岩捕虏体。这些岩石所遵循的前进路径将使用传统的温压测定法、伪剖面、金红石中的锆和锆石中的钛测温法以及石英中的钛测温法来确定。副相的 U-Pb 测年将限制沉积年龄和变质作用的时间。对这两个样本组的研究、吸取的教训和开发的方法将为未来研究其他下地壳变泥岩铺平道路，以确定沉积物侵位到深部大陆地壳中的主要过程。该项目将支持一名来自代表性严重不足的群体的女博士生的研究。它还将为加州大学圣巴巴拉分校的几名本科生研究助理和加州州立大学北岭分校的两名暑期实习生提供支持；两者都是为西班牙裔服务的机构。本科生将接受地球化学研究方法方面的培训，并在职业规划方面得到指导。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。