Heat Production and Transfer in Earth’s Continental Crust

Abstract

Planetary differentiation through tectonism reflects heat production and transfer, reducing internal energy and shaping planetary interiors. Geologic heat originates from two primary sources: primordial heat from planetary formation and the radiogenic decay of isotopes like U, Th, and K. Shearing can generate heat on local scales, but heat transfer predominantly occurs through conduction, where energy flows from hotter to cooler regions via atomic vibrations. In tectonically active areas, advection of magma and convection in fluids are more efficient mechanisms. Local heat transfer often relies on one dominant process, while large-scale systems involve a mix of conduction, convection, and advection. The causes and proportions of each heat source and mantle and crustal radiogenic contributions remain challenging to quantify. Understanding the re-distribution of heat-producing (i.e., radioactive) elements during metamorphism and crustal differentiation is achieved via combining natural observations with trace element and accessory mineral modelling. Six potential end-members were considered for the protolith of mid-crustal tonalite-trondhjemite-granodiorite packages—often considered the product of lower-crustal melting. Model results suggest heat-producing elements partition subequally between solid and melt at typical pressure-temperature conditions for crustal differentiation. Accessory minerals like apatite, feldspar, amphibole, and epidote are primary repositories for radioactive elements, with their stability in pressure-temperature space governing heat removal from the lower crust. Observations from the Archean Kapuskasing uplift reveal a similar partitioning pattern during mafic rock melting, supporting the notion that radiogenic heat equally influences mantle and crustal processes. Examining the crustal heat-production record provides insights into continental growth, thickness, and preservation. A database of crustal rocks, including trace element compositions and crystallization ages, reveals trends in heat production over time with implications for basalt formation and crustal evolution. While Archean mantle melting produced less heat-producing basalt than today due to higher degrees of partial melting, the total crustal heat-production rate has remained relatively constant. However, modern crust exhibits more significant variability, suggesting recent enrichment in heat-producing elements contrasts with a more homogenized Archean crust. Crustal growth pulses marked by mafic-to-felsic transitions imply a cyclic nature of crustal differentiation, with crustal thickness remaining stable due to self-organizing thermal processes. Trace element substitution in autocrystic zircon is a valuable tool for reconstructing deep geologic processes, but some influences on these proxies remain underexplored. Elevated titanium rims on volcanic zircon, often attributed to magma recharge, could also result from adiabatic ascent. Modelling shows that decompression melting and system expansion during ascent drive cooling, while subvolcanic boiling induces crystallization and latent heat release. Zircon growth during ascent can record these processes, and high-titanium rims may form in a single magma pulse without recharge, emphasizing the importance of multiple geochemical tools to interpret magma evolution. Ultra-high temperature (UHT) metamorphism represents the thermal extreme of crustal processes, yet its mechanisms and energy sources remain debated; common explanations include mafic underplating or mantle upwelling. The Frontenac Terrane in southeastern Ontario records UHT conditions during the Mesoproterozoic. Back-arc sedimentation between 1390–1200 Ma preceded Shawinigan (~1180–1160 Ma) and Ottawan (~1060 Ma) orogenic events. Granitic and minor mafic intrusions during Shawinigan times triggered regionally advective UHT metamorphism preserved through subsequent reheating events. The Frontenac Terrane provides critical insights into the Grenville Province assembly, with felsic intrusions providing a plausible mechanism for UHT conditions during orogenesis. Heat production and transfer fundamentally shape Earth's structure and behaviour. Radiogenic heat from U, Th, and K and primordial heat drive crustal and mantle dynamics. Although incompatible with most rock-forming minerals, heat-producing elements are preferentially incorporated into accessory minerals like apatite and zircon, depleting their source rocks. Increased melting reduces system-wide heat production as these elements concentrate in the melt. Advective heating is crucial for crustal growth, enabling magmas to ascend nearly adiabatically and providing a heat source for crustal reworking. These processes dictate metallogenic fluid generation, crustal heterogeneity, and long-term crustal stabilization.