Heat Flow and the Chemical Composition of Continental Crust

Recent models for the chemical composition of the continental crust rely on various geophysical, geological, and petrological constraints and display a wide range in composition, with $$SiO_{2}$$ varying between 57 and 64 wt % and $$K_{2}O$$ varying between 1.1 and 2.4 wt %. Models that predict relatively high levels of the most incompatible elements imply extreme depletion (>70%) of these elements in the "depleted mantle." Crustal heat flow constrains the abundances of heat-producing elements (HPE: K, Th, U) within the continental crust and thus provides a relatively unambiguous and important test for these various models. The crustal radiogenic component of continental heat flow must lie within the range of $$18-48 mW/m^{2}$$ and almost certainly lies within the range of $$21-34 mW/m^{2}$$. The latter constraints limit the abundances of HPE within the crust (assuming K/U = 10,000, Th/U = 3.8, and a crust 41 km thick) to $$K_{2}O = 0.96-1.57\%$$; Th = 3.0-4.94 ppm, and U = 0.80-1.30 ppm. Models of crustal composition based largely on seismological or geological mapping data, and that predict relatively siliceous and incompatible element enriched compositions, exceed the upper limits of these constraints.