A 4 Ga record of granitic heat production: Implications for geodynamic evolution and crustal composition of the early Earth

Abstract

The radiogenic heat produced by granites has a significant influence on the thermal state of the crust due to both their relatively high heat production with respect to most rock types and high abundance. However, the variations in present day heat production with age are generally based on relatively few measurements that are poorly distributed geographically. In this study, we construct a global model for the heat production of granitic rocks for the past 4 Ga using 13,400 geochemical analyses. We observe a nearly monotonic increase in radiogenic heat production from 4.0 to 2.0 Ga, which mirrors a shift from more TTG-like calcic to more alkalic compositions. This shift towards high-heat-producing granites post-2.0 Ga is often attributed to enrichment related to reworking and/or erosion. However, there is a strong correlation between granitic heat production with that of similarly-aged basalts and gabbros, which suggests a dominant mantle-level component to granite generation rather than crustal reworking. Secular cooling and mantle depletion may affect heat production, but the signal is complex and cannot easily explain the heat production with age profile. The most likely mechanism to describe the observed heat production–age pattern is one of selective preservation as a consequence of thermal stability. High heat producing terranes that were not stable during the Archean become increasingly stable towards the present. This selective preservation model has significant implications for the growth and composition of the continental crust. Ferroan, alkalic and felsic compositions were less thermally stable in the Archean due to their generally higher heat production and thus may have been more common in the early Earth than assumed by most compositional models. The temporal heat production model determined in this study can be used to improve geotherm models, particularly within ancient terranes.