Collaborative Research: What Controls the Mobility of the Heat Producing Elements in Continental Crust?

Silicon-rich continental crust is unique to Earth and is critical for habitability, but the processes that drive the long-term stability of this crust are unclear. The most enduring tracts of continental crust have resided at the Earth’s surface for billions of years and are characterized by enrichment of uranium (U), thorium (Th) and potassium (K)—the heat-producing elements—in the upper crust. This research project will address the question: what controls the mobility of the heat-producing elements through continental crust? The researchers will measure concentrations of U and Th in rocks and minerals across two temperature profiles in exhumed sections of middle and lower continental crust. Combined with constraints on the pressure-temperature-time evolution of these rocks, they will discriminate between competing mechanisms for the mobilization of the heat-producing elements during melting of the continental crust. The research will catalyze international collaboration between scientists in the US and Switzerland, foster the training of a graduate student, and engage undergraduates in academic research. Characterizing how the heat producing elements are mobilized in continental crust is fundamental to understanding crustal evolution, the temperature and mechanical structure of crust, Earth’s heat budget and chemical differentiation of the planet. Using a suite of complementary techniques, the researchers will test five hypotheses—Equilibrium and Disequilibrium Melting, Mineral Shielding, Melt Buffering and Rejuvenation—for the distribution of the main heat producing elements, U and Th, across two well-characterized temperature profiles: contact aureoles of the Mafic Complex, Ivrea Zone, Italy and the Big Jim plutonic complex, Washington, USA. In-field Gamma Ray Spectrometer measurements will provide bulk-rock U and Th concentrations at a sampling density inaccessible to conventional geochemical techniques. Metamorphic zircon and monazite U/Th-Pb dates + trace-element abundances obtained by laser-ablation split-stream petrochronology will allow assessment of the timescale over which accessory mineral dissolution occurred. Petrologic constraints will be derived from P-T pseudosections, optimal thermobarometry, and trace-element thermometry. In conjunction, the latter two techniques will be used to reconstruct the peak metamorphic conditions and the timing and quantity of melt removal. The complete dataset will allow rigorous testing of the five 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.