Application of chlorite thermometry to estimation of formation temperature and redox conditions

Diverse applications of chlorite thermometry have been considered for better understanding the formation process in nature. Here, an approach which combined a semi-empirical thermometer (Inoue et al., 2009) with the method of Walshe (1986) was tested to estimate the redox conditions (log f(O2)) and the formation temperature, using the literature data from Niger, Rouez and St Martin and new data for chlorite which coexists with pink-coloured epidote in the Noboribetsu geothermal field. The log f(O2) predicted for the former data sets were compatible with those estimated by Vidal et al. (2016), suggesting that the present approach is valid for quantifying the variations in log f(O2). The Noboribetsu chlorites have lower Fe/(Fe + Mn + Mg) and greater Fe3+/Sigma Fe ratios than those observed in adjacent propylite rocks. The peculiar mineral assemblage and chemical composition are attributed to the formation under higher f(O2), conditions and possibly low Fe concentration in the alteration fluids.