Constraining latent heat of hydration using combined thermal- and garnet diffusion modelling

Hydration and retrogression are common processes in many reworked polymetamorphic terranes. The incorporation of water into a new, hydrated assemblage obliterates precursor high-grade parageneses and releases significant latent heat proportional to the degree of hydration. The hydration of dry gneisses (containing 1–2 wt.% water) at greenschist-facies conditions, may result in an approximate doubling of the bulk water content (4–5 wt.% water). This equates to a gain of 20–40 g of water per kg of retrogressed rock. Considering an average latent heat release of 4 kJ per g of water accommodated in hydrous minerals (Connolly & Thompson, 1989), hydration leads to the release of 80–160 kJ per kg of retrogressed rock. Consequently, this effect causes a significant, albeit short-lived, perturbation of the local thermal conditions. Cooling of the affected rock pile is delayed while the additional energy enhances thermally-activated processes such as diffusive loss of radiogenic Argon. This, in turn, may contribute to a significant rejuvenation of apparent ⁴⁰Ar/³⁹Ar ages in white mica of up to ~10 % (Schorn et al., 2023). In this study, we present results of multicomponent diffusion modelling of garnets hosted in pervasively hydrated micaschist from the polymetamorphic eclogite-type locality (Koralpe–Saualpe, Austria). Using simple thermokinematic models, we explore exhumation paths that encompass variable degrees and conditions of hydration, along with latent heat to, constrain a set of plausible, yet non-unique, temperature–time histories for the investigated rocks. These cooling paths serve as input for garnet diffusion modelling (Fig. 1), used to determine an appropriate set of thermal parameters for the investigated samples. Our findings contribute to discussions on the broader implications of pervasive fluid–rock interaction in a collisional setting, emphasizing the significance of the often-overlooked effects of retrogression in reworked metamorphic terranes.

Figure 1 - Multicomponent diffusion modelling of a polyphasic garnet. Yellow dots: EMPA data; red line: initial composition; blue line: calculated composition.

REFERENCES

Connolly, J. A., & Thompson, A. B. (1989). Fluid and enthalpy production during regional metamorphism. Contributions to Mineralogy and Petrology, 102(3), 347-366.

Schorn, S., Moulas, E., & Stüwe, K. (2023). Hot when wet: the consequences of exothermic hydration on geochronology (No. EGU23-5769). Copernicus Meetings.