Electron spin resonance characterization of calcretes from Thar desert for dating applications

Abstract

Results of a study to characterize electron spin resonance (ESR) signals from naturally occurring carbonates from Thar desert are reported. Three radiation induced signals at g=2.0071 (signal A, very weak), g=2.0012 (signal B) and g=2.0003 (signal C) with linewidths ranging from 0.5-2 G are seen. A broad signal (BL) with linewidth 7 G is also seen at 2.0038. Detailed ESR experiments involving (1) sensitivity of ESR signals to ionizing radiation, (2) intensity variation of ESR signals with microwave power at different temperatures, (3) analysis of acid insoluble residue to estimate the interference from ESR signals of silicate fraction, and (4) X-ray diffraction analysis (to study possible phase changes on heating), led to the choice of signal BL for dating. Experiments on acid insoluble residue revealed that signal C has a contribution from the E'₁ centre of quartz and signal B is the parallel component (g_∥) of the E'₁ centre. Paleodose estimates based on signal BL are stratigraphically consistent.