On the radiogenic heat production of metamorphic, igneous, and sedimentary rocks

Abstract

Sedimentary rocks cover ∼73% of the Earth's surface and metamorphic rocks account for approximately 91% of the crust by volume. Understanding the average behavior and variability of heat production for these rock types are vitally important for developing accurate models of lithospheric temperature. We analyze the heat production of ∼204,000 whole rock geochemical data to quantify how heat production of these rocks varies with respect to chemistry and their evolution during metamorphism. The heat production of metaigneous and metasedimentary rocks are similar to their respective protoliths. Igneous and metaigneous samples increase in heat production with increasing SiO2 and K2O, but decrease with increasing FeO, MgO and CaO. Sedimentary and metasedimentary rocks increase in heat production with increasing Al2O3, FeO, TiO2, and K2O but decrease with increasing CaO. For both igneous and sedimentary rocks, the heat production variations are largely correlated with processes that affect K2O concentration and covary with other major oxides as a consequence. Among sedimentary rocks, aluminous shales are the highest heat producing (2.9 μW m−3) whereas more common iron shales are lower heat producing (1.7 μW m−3). Pure quartzites and carbonates are the lowest heat producing sedimentary rocks. Globally, there is little definitive evidence for a decrease in heat production with increasing metamorphic grade. However, there remains the need for high resolution studies of heat production variations within individual protoliths that vary in metamorphic grade. These results improve estimates of heat production and natural variability of rocks that will allow for more accurate temperature models of the lithosphere.