Exothermic reactions and 39Ar−40Ar thermochronology: Hydration leads to younger apparent ages

Retrogression and hydration commonly affect large swaths of Earth’s crust, causing variable degrees of chloritization, sericitization, and/or serpentinization. Hydration is a strongly exothermic process that partially opens isotopic systems, thereby distorting the recorded apparent ages and cooling histories of reworked terranes. Using a simple one-dimensional numerical model involving heat released at variable temperatures to simulate exothermic hydration, we track the cooling histories for rocks that exhume from depth. The calculated cooling paths are used to quantify apparent 39Ar−40Ar ages in muscovite, biotite, and feldspar considering 40Ar production and diffusive Ar loss. For fluid incorporation relative to chloritization of ∼10%, ∼50 kJ of latent heat are released per kilogram of rock. For this scenario and exhumation rates between 1 mm·yr−1 and 4 mm·yr−1, muscovite grains ≤100 µm in diameter yield apparent ages that are younger by up to 10%, but always exceeding the typical uncertainty of Ar dating. Biotite and feldspar display a similar distortion, even for large grains of ∼1 mm in size. The relative younging effect increases to >30% with enthalpy released, exhumation rate, and decreasing grain size, with younging reaching a maximum for hydration at approximately the nominal closure temperature of the respective thermochronometers. Using published data sets (from Sifnos, Greece, and Tian Shan, China), we suggest that rejuvenation of apparent mica ages is consistent with diffusive Ar loss due to exothermic hydration during exhumation. Our method applies to any thermally activated process, like element exchange in mineral thermometers or fission-track annealing, provided heat is released close to the characteristic closure temperature. This extends to processes beyond hydration, such as shear heating or localized magma emplacement, making our results pertinent for diverse thermochronometers and temperature-sensitive methods across a broad range of conditions.