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

Abstract

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 –Si– R2+ 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 –Si– R2+ diagram provides a practical tool for thermometric purposes in the applicability range of the considered models, in particular for diagenesis and low-grade metamorphism.