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

沉积岩是大陆地壳的重要组成部分。虽然大多数都位于形成沉积岩的上地壳，但地质过程可以将沉积岩输送到下地壳。这一点很重要，因为与控制下地壳的岩石相比，沉积岩含有高达十倍的放射性放热元素K、Th和U的浓度。因此，沉积岩的加入对大陆深部地壳的产热、地壳内的地温梯度以及全球产热元素的存量都有深远的影响。沉积岩可能通过不同的地质过程进入下地壳，每一种过程都意味着下地壳沉积岩可能有多常见。该项目旨在确定负责将沉积岩输送到新墨西哥州南部和蒙大拿州中部下地壳的过程，在那里，这些岩石在火山(穷人的钻洞)中作为意外碎片返回地表，使科学家能够研究它们的历史。沉积岩进入下地壳的途径和时间将通过地球化学分析和放射性测年来确定。该项目支持一名女性博士生、几名加州大学圣巴巴拉分校的本科生研究助理、两名加州州立大学北岭分校的暑期实习生和一名高级女性首席研究员。PI和博士生将对本科生助理和实习生进行地球化学研究方法方面的培训，并提供职业建议和指导。该项目试图确定沉积岩侵位到下大陆地壳的过程。确定过程将阐明变质沉积岩可能是地壳深部的常见原因。有几种构造情景是可能的，可以根据岩石所遵循的前进P-T路径以及碎屑锆石的年龄和它们所含的变质矿物(例如独居石)的年龄和成分来区分它们。该项目的重点是来自新墨西哥州基尔伯恩洞和波特里洛马尔的新生代玄武岩中携带的两套下地壳变辉石捕虏体，以及始新世的蒙大拿省。这些岩石所遵循的进动路径将使用常规的温压、假截面、金红石和锆石中的钛测温以及石英中的钛测温来确定。副相U-Pb定年将制约变质作用的沉积时代和时间。对这两套样品的研究、吸取的经验教训和发展的方法，将为今后调查更多的下地壳变质岩铺平道路，以确定沉积物侵位到大陆深部所涉及的主要过程。该项目将支持一名女性博士生的研究，她来自一个明显低于代表性的群体。它还将支持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；这两所大学都是为拉美裔服务的机构。本科生将接受地球化学研究方法方面的培训，并在职业规划方面进行指导。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.