40Ar/39Ar dating of high temperature geothermal systems: First attempt on hydrothermally altered pyroxenes from the Saintes archipelago (Lesser Antilles arc, Guadeloupe)

We investigate the potential of 40Ar39Ar 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 degrees C/200 ka) and a maximal lifetime of 650 ka for the Saintes hydrothermal system. We investigate the potential of 40Ar39Ar 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 degrees C/200 ka) and a maximal lifetime of 650 ka for the Saintes hydrothermal system.

Automated summary

The study investigates the potential of 40Ar39Ar systematics in secondary K-bearing minerals formed from hydrothermally altered volcanic pyroxenes in a fossil geothermal system in the Saintes archipelago. It presents the first example of dating hydrothermal muscovite crystallization in altered rhyodacites, providing new insights into high temperature fluid circulation. The model of muscovite substitution by illite during cooling helps interpret the perturbation of K-Ar and Ar-Ar ages by illitization.