Optical bleaching and thermal stability of ESR signals in fault-related carbonates

Abstract

Recrystallized carbonates formed on fault surfaces via fault activity and calcite veins in fault zones are promising dating materials for geochronological studies using methods such as electron spin resonance (ESR) dating. However, the bleaching characteristics and thermal stability of ESR signals in these materials have not been previously reported. In this study, three carbonates associated with fault activity (recrystallized carbonate, calcite vein, carbonate bedrock) were collected to investigate their bleaching characteristics and thermal behavior. The results of bleaching experiments show that the bleaching characteristics exhibit two patterns. Of seven observed signals, four were completely bleached by sun-like light exposure (g = 2.0040, g = 1.9995, g = 2.0057, g = 2.0018) in order of decreasing sensitivity to light. The other three signals (g = 2.0006, g = 2.0022, g = 2.0031) could not be fully zeroed, and their signal intensities reached stable residual values after different exposure times depending on the signal. The results of thermal annealing experiments show that the observed signals intensities decreased in the three samples during annealing, except the g = 2.0057 line. The g = 2.0006 line is the most promising for ESR dating on account of its stable thermal behavior and low sensitivity to optical bleaching. Lastly, the g = 2.0040 signal shows thermal instability, and caution is required when using this signal for dating.