Latent heat of metamorphic reactions: boosting diffusion – hampering cooling

Reactions involving variable exchange of latent heat are ubiquitous dynamic metamorphic processes: prograde dehydration and melting reactions cause an increase of the effective heat capacity by over an order of magnitude as they advance, while melt crystallization and retrograde hydration leads to transient heat production similar to radioactive heating in the continental crust. We show results of  thermokinematic models simulating the release and consumption of latent heat in an upward advecting rock pile to constrain thermal histories akin to exhumation in active tectonic settings. We show that hydration of dry gneisses leads to a gain of 20–40 g water per kg of rock and releases 80–160 kJ/kg latent heat. This transient thermal perturbation delays cooling and enhances thermally-activated processes such as diffusive loss of radiogenic Argon, which can rejuvenate apparent ⁴⁰Ar/³⁹Ar ages in white mica by up to ~10%. Biotite and feldspar display a similar distortion, even for large grains of ~1 mm in diameter (Schorn et al., 2024). In another case study we present multicomponent diffusion modeling of garnets in hydrated micaschist from the polymetamorphic Koralpe–Saualpe locality (Austria). We explore exhumation paths for varying hydration and latent heat production to constrain temperature–time histories, with a best-fit of modelled garnet zoning pattern achieved for ~120 kJ/kg released at 550°C and an exhumation rate of 4 mm/yr. As for melting reactions, we simulate periodic sill emplacement in 5-km wide ‘hot zone’ at 25 km depth, like magma injection in a subduction-related arc setting (e.g., Annen et al., 2006). Focusing on the thermal–temporal evolution of metapelitic source rocks at depth, we investigate the thermal retardation related to the endothermic melting of mica followed by the exothermic crystallization of leftover melt in comparison to the unbuffered case. This interplay leads to a clustering of temperatures around the conditions of melt-related thermal buffering and is consistent with the predominance of mineral assemblages related to focused biotite–sillimanite breakdown in metapelites (Schorn et al., 2018), as observed at the orogen-scale in large exhumed hot orogens such as the granulite-facies domain of the Namaqua–Natal Metamorphic Province in southern Africa (Diener & Macey, 2024).

References

Annen, C., Blundy, J. D., & Sparks, R. S. J. (2006). The genesis of intermediate and silicic magmas in deep crustal hot zones. Journal of Petrology, 47(3), 505-539.

Diener, J. F., & Macey, P. H. (2024). Orogen‐scale uniformity of recorded granulite facies conditions due to thermal buffering and melt retention. Journal of Metamorphic Geology.

Schorn, S., Diener, J. F., Powell, R., & Stüwe, K. (2018). Thermal buffering in the orogenic crust. Geology, 46(7), 643-646.

Schorn, S., Moulas, E., & Stüwe, K. (2024). Exothermic reactions and ³⁹Ar–⁴⁰Ar thermochronology: Hydration leads to younger apparent ages. Geology, 52(6), 458-462.