EAR-PF: Application of Paired Ti and Fe Isotopes to Understand the Evolution of Earth's Upper Continental Crust

Johnson, Aleisha, Tempe AZ

Investigators

Abstract

Dr. Aleisha Johnson has been awarded an NSF EAR Postdoctoral Fellowship to investigate how the stable isotopes of Ti (Titanium) behave during the formation of continental crust. Her research and public service will take place at the University of Chicago under the supervision of Prof. Nicolas Dauphas. Stable Ti isotopes have been recently proposed to trace the evolution of Earth’s upper continental crust and the onset of plate tectonics. If demonstrated, to be a robust proxy, Ti isotopes would fill a unique niche because they are (1) ubiquitous in igneous rock suites and (2) insensitive to modification by weathering, fluid overprinting, or metamorphism. However, Ti isotopes appear to behave differently in tholeiitic (plume-like) and calc-alkaline (arc-like) magmas, complicating interpretations of the ancient geologic record. Dr. Johnson will measure Ti isotopes in modern arc volcanoes to understand what controls Ti isotope fractionation during the formation of crust, which will allow her to calibrate the proxy and revisit the geologic record to learn more about the composition of Earth’s ancient continents. During her tenure as an NSF Postdoctoral Fellow, Dr. Johnson will become involved as a mentor in two organizations: the first is the FLI network at University of Chicago (First-generation, Low-income, Immigrant, Ally) where she can mentor students similar to her own background to encourage diversity and inclusion in science at the private college level. She will also volunteer at the National Association for Geoscience Teachers (NAGT) annual meetings as an alum of the workshop “Preparing for an Academic Career”. Existing geochemical proxies for the composition of Earth’s early continental crust present a conundrum. Stable Ti isotopes are being used to distinguish between existing hypotheses by providing a temporal record of magmatic differentiation. As fractional crystallization progresses, the lighter isotopes of Ti are preferentially incorporated into oxides and ultimately sequestered in magmatic cumulates. Thus, felsic crust carries an isotopically heavy signature whereas mafic crust indicates minimal fractionation. The recent observation that tholeiitic and calc-alkaline magmas fractionate Ti isotopes to different extents signaled that there are important unknown variables left to consider: water content, oxygen fugacity, crystallization temperature, and mineral phase. Proper calibration in modern settings can address this challenge and perhaps identify new processes which Ti isotopes are sensitive to. Mineral-melt fractionation factors of Ti isotopes will be measured in a calc-alkaline volcanic suite from Rindjani Volcano, Indonesia. Paired Fe isotope analyses will quantify the role of oxygen fugacity and magnetite crystallization specifically. This differentiation suite will elucidate the controls on Ti isotope fractionation in arc settings, which will allow Ti isotope fractionation during crust formation to be modeled accurately for the first time. Finally, modeled Ti isotope values will be compared with the shale record to better identify the compositions of ancient crustal protoliths. This project received funds from the Geochemistry and Petrology program in the Earth Sciences division. 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.