Résumé
We investigate the potential of 40Ar-39Ar systematics in secondary K-bearing minerals developed at the expense of hydrothermally altered volcanic pyroxenes from a fossil geothermal system located in the Saintes archipelago (Lesser Antilles magmatic arc).Our work reports the first example of dating hydrothermal muscovite crystallization developed during pseudomorphic transformation of pyroxenes within altered rhyodacites at temperatures above 300 °C. The white micas display a well-defined Ar/Ar plateau age of 2.59 ± 0.12 Ma, interpreted as the best age estimate for high temperature fluid circulation in the Saintes geothermal paleo-reservoir. A model of muscovite substitution by illite during cooling supports the interpretation that illitization perturbs the K-Ar and the Ar-Ar ages only when muscovite is highly illitized and/or if illitization is significantly younger than muscovite crystallization. The whole temporal dataset implies a fast cooling rate (>150 °C/200 ka) and a maximal lifetime of 650 ka for the Saintes hydrothermal system.