Chemical geothermometry in CO 2-rich thermal waters. Example of the French massif central

Abstract

The present paper aims at clarifying the use of chemical geothermometers in CO 2-rich waters. Most of the conclusions developed in this text are based on geochemical studies of Massif Central thermal waters. The general chemical characteristics of these waters are presented, together with some hypotheses on their mode of formation. During the rise of the CO 2-waters from the reservoir to the surface, the pH of the deep fluid decreases as a result of cooling and mixing with shallow waters, leading to the dissolution of silicate rocks (and/or carbonate rocks), and the acquisition of reduced species (mainly iron II). Oxidation of iron II by dissolved oxygen entering the system upon mixing can strongly contribute to H∗ production. In the application of solute geothermometers, the following has to be taken into account: (1) Cooling of fluids, combined with rapid attack on rocks, gives rise to silica contents approaching the solubility of amorphous silica. (2) Sodium/potassium ratios generally decrease as roughly equal amounts of Na and K are released during simple rock dissolution. Consequently, the Na/K geothermometer often provides overestimates of deep temperatures. (3) Addition of Ca compensates to some degree the addition of K; the Na—K—Ca-geothermometer, therefore, is less sensitive to secondary dissolution. (4) Sodium/lithium ratios are unaffected by these modifications of initial solutions and appear to constitute a good thermometric index for CO 2-water, at least for the rock type encountered in the Massif Central.