How Are Ultrahigh Temperatures Attained in Continental Crust? A Petrochronological Investigation of the Basin and Range Lower Crust

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Continental crust that is rich in the element silicon appears to be unique to planet Earth and is critical for life, but the mechanisms that facilitate the long-term stability of this crust are unclear. Preservation of rocks that have attained temperatures greater than 900°C in the cores of old continents implies that ultrahigh temperature metamorphism and associated partial melting of the crust is critical for crustal stabilization. This research will address the primary question: How are extreme temperatures attained within the continental crust? The project will reconstruct the pressure-temperature-time history of rocks that were erupted from a section of continental lower crust that is actively undergoing ultrahigh-temperature metamorphism: the Basin and Range Province of the United States and Mexico. A combination of analytical techniques will discriminate between potential heat sources. The research will support an early career scientist, foster the training of a graduate student, and engage undergraduates in academic research.Characterizing how the Earth’s crust attains extreme temperatures is relevant to the development and distribution of body forces, flow of crust and chemical differentiation of the Earth. Using a suite of complementary techniques, this project will use xenoliths to test three hypotheses (crustal thickening, mantle thinning, or magmatic underplating) for the attainment of ultrahigh temperatures in continental lower crust beneath a zone of active extension. Accessory-mineral (zircon, monazite, titanite, and rutile) U-Pb dating and trace element depth-profiles will be used to derive the pressure and temperature history of the lower crust over the lifespan of the Basin and Range province. Lu-Hf garnet dates will be used to constrain the timing of prograde metamorphism. Petrologic constraints will be derived from Pressure-Temperature pseudo-sections, optimal thermobarometry, and trace-element thermometry. In conjunction, these techniques will be used to measure the timing and duration of ultrahigh temperatures in addition to portions of the prograde Pressure-Temperature evolution. The complete dataset will allow for rigorous testing of the three hypotheses.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.