Low-temperature chlorite geothermometry: a graphical representation based on a T–R2+ –Si diagram

The chlorites are good indicators of rock history because their wide compositional variations are sensitive to the formation conditions, like pressure (P), temperature (T), redox state, fluid composition. Accordingly, many geothermometers based on their composition, either empirically or thermodynamically, have been proposed during the last 30 years, especially in low-temperature contexts (T < 350 °C). This paper presents a graphical tool that considerably facilitates the use of two of the most recent chlorite thermometers for low- and very-low-T chlorites. The temperature–composition relationships for low-T chlorites are represented in T–R2+–Si diagrams, allowing chlorite compositions to be predicted as a function of temperature or, conversely, temperature to be estimated from compositional fields of natural chlorites. This graphical projection is based on a comparison of the parameters (ideal chlorite compositions and calculated T) predicted by geothermometers with analyses of natural chlorites for which independent T estimates are available over a range of geological environments. The new T–R2+–Si diagram provides a practical tool for thermometric purposes in the applicability range of the considered models, in particular for diagenesis and low-grade metamorphism.