Collaborative Research: Does partial melting of metasedimentary rocks lead to differentiation of heat-producing elements in continental crust?

Heat from inside Earth shapes how mountains form and how continents grow and change over millions of years. Much of this heat comes from the decay of uranium, thorium, and potassium deep within the crust. It remains unclear what happens to these elements when the crust is heated (metamorphosed) and eventually melts. Some studies suggest the elements move into the melted portion of the rock, while others infer that the elements remain in the solid portion of the crust. The answer to this question is important because it affects how hot the deep crust stays long after mountain-building events have ended. This, in turn, affects a range of important Earth processes, including volcanic activity, earthquakes, and the concentration of economic mineral deposits. This project will use excellent exposures of rocks in the Himalaya that were heated until they partly melted. The chemistry and ages of rocks will be measured at each stage of the melting process to understand where the heat-producing elements occur. The project will train graduate and undergraduate students in field and laboratory techniques. The project will build new research collaborations with scientists in Nepal and Canada. This project quantifies the net transport of the heat-producing elements (HPE) uranium, thorium, and potassium during partial melting of pelitic sediments, using the Nepal Himalaya as a natural laboratory. This region exposes a transect from subsolidus pelitic schists through migmatites and associated granitic dike and sill networks into kilometer-scale plutons, interpreted as the structurally highest accumulation of melts generated lower in the system. Hypotheses for HPE behavior during pelite anatexis range from incompatible behavior (concentration in the melt), to compatible behavior (retention in the solid residue), to no net fractionation, to contrasting behavior between elements (e.g., Th retained while U is mobilized). This project tests these competing hypotheses through systematic whole-rock and mineral geochemical analysis across the full schist-migmatite-pluton transect, combined with field mapping, petrography, geochronology, and thermobarometry to place compositional data within a well-constrained tectonic, metamorphic, and magmatic history. Comparison of subsolidus schist and pluton compositions will constrain bulk HPE transport through the system, while sampling of the melt-transfer dikes and sills will test for compositional fractionation during melt migration. Petrological modeling will quantify ranges of melt fraction and HPE partition coefficients consistent with the empirical dataset. Results will clarify the extent to which pelitic sediments retain HPE through high-grade metamorphism and anatexis, with implications for geothermal gradients during and after orogenesis and for predictive models of crustal thermal evolution. The project also provides a framework for understanding transport and fractionation of critical elements (e.g., Li, Rb, Cs, Ba, Zr, Nb, Hf, Ta, and REE) that correlate with HPE behavior in magmatic systems. This project supports graduate and undergraduate training at both institutions and builds international collaborations with researchers in Nepal and Canada. 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.