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

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

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.