Abstract
everal geochronological methods are considered most effective in deciphering the history of possible recurring fault activity and in estimating the timing of the most recent activity. The 40K-40Ar method has been widely used to evaluate fault activity. In this study, we apply the 40K-40Ca method to authigenic illite in fault samples as a new dating method to constrain the age of faulting. Comparison of the obtained 40K-40Ca ages with those of the 40K-40Ar method is expected to provide constraints on the magnitude and thermal history of fault movement.We used the Silurian Rochester shale in West Virginia, USA, as a test sample for illite 40K-40Ca dating. Furthermore, preliminary Ca isotopic analysis of a fine-grained fraction mimicking authigenic illite using surface ionization mass spectrometry revealed a radiogenic 40Ca/44Ca ratio ranging from +6-9 epsilon-units compared to the NIST SRM 915a reference material measured at the same time. In the future, potassium and calcium will be quantified by isotope dilution method to obtain 40K-40Ca ages for the Rochester shale. In addition, the isotopic composition of calcite, which can coprecipitate with authigenic illite in fault zones, will be considered for the assumption of initial Ca isotopic ratios necessary for the calculation of model ages, with the aim of establishing this method as a new chronological approach to constrain fault movement.
