Electron spin resonance age determination of a travertine sample from the southwestern part of Turkey

Abstract

Travertine powdered samples collected from Denizli in the southwestern part of Turkey appear to be suitable for dating by ESR analysis due to presence of a g=2.0006 ESR signal. In general, ESR spectra of natural travertine show two absorption lines: g=2.0006 and g=2.0056. 60Co gamma-ray irradiations enhance the g=2.0006 signal but not g=2.0056. The signal amplitude of the g=2.0056 line remains constant and shows no dose dependence after annealing at 150–180°C for periods from 15 min to 2 days. Line g=2.0056 disappears in turn after an annealing of 15 min at 400°C. Thermal annealing experiments at different temperatures for different annealing times show that the only travertine ESR signal stable enough for dating of ages over the past 10 Ma, is the one at g=2.0006. The present ESR dating result is 1.11±0.11 Ma. The g=2.0006 signal used for dating appears to correspond with the 280°C TL peak, but at elevated temperatures the mean life, accumulated dose and age of the latter is significantly less than that measured by post irradiation analysis using the ESR signal. This can be attributed to coming from other carbon-containing paramagnetic centers. In fact, gamma-ray absorbed dose rate dependence of the ESR signal has been studied, and no dependence was observed over a 12-fold change in dose rate.