CAREER: Is Continental Crust Juvenile or Reworked? A Test of Growth Models Using the Extant Neoarchean Granitoid Record

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There currently exist more geological samples from the Moon than from our own planet during the first 500 million years of the Solar System. This dearth of samples has led to vigorous debate amongst the geological community regarding three very important aspects of Earth evolution: 1) when, 2) how, and 3) to what extent did continents emerge on the early Earth. Recent advances in technical capabilities now allow these questions to be revisited. Earth is a tectonically active planet, which means that old rocks are constantly being altered, leaving few pristine samples from the earliest phases of Earth history. By looking for very small, but important isotopic signatures in preserved rock samples, one may infer how much continental crust was around very early in Earth history. This project will fund advanced isotopic analyses of ~2.6-billion-year-old rocks in a search of evidence for great than 4.0-billion-year-old continental material, material that may have been reworked during later tectonic events. Collection of this data will allow the research team to test the hypothesis that very large volumes of ancient continental crust existed on the early Earth and to answer a fundamental question about planetary formation and evolution – when did continental crust form on Earth? This project will also support an undergraduate field research experience, which will serve to train the next generation of geoscientists in skills that employers are keenly interested in – field geology and spatial reasoning skills.This proposal will combine petrology and isotope geochemistry to test broad hypotheses regarding the growth and reworking of continental crust. Neoarchean granitoid rocks represent some of the oldest well-preserved suites of rocks on Earth. Detailed analysis of samples will address the fundamental question of when continental crust formed on Earth. This proposal will analyze suites of granitoids from three distinct North American cratons for their igneous chemical signatures. Zircon age and isotopic information (U-Pb-Hf-O) will be measured from several compositional groups to test the fidelity of commonly used tracers for crustal melting and overprinting. Analyses of xenocrystic zircons and bulk rock Nd-isotope ratios will be used to search for reworking of truly ancient (4.0 Ga) crust in the Neoarchean—a time that has been suggested to have experienced broad overprinting of the continental record. The research team focus on 142Nd analyses as these data are particularly adept at identifying Hadean crustal relics. This proposal will fund an REU field experience aimed at developing a diverse and strategically important workforce with training in field geology and spatial reasoning. The maps created and samples collected by undergraduate researchers will be used for advanced isotopic analyses conducted during the scientific study, and undergraduate participants will not only gain important—and in-demand—skills in spatially-focused critical thinking but will also be exposed to advanced geochemical techniques and datasets. These participants will finish their undergraduate experience well positioned to pursue many types of careers in the geosciences.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.