Thermoluminescence of the meteorite interior: a possible tool for the estimation of cosmic ray exposure ages

Abstract

Thermoluminescence of meteorites has been studied for nearly five decades and has provided useful information on meteorite classification and on terrestrial age estimation. In respect of natural luminescence measurement, studies so far have used TL emission in the blue region and the conventional protocols have yielded equivalent doses (D_e) of ~1000 Gy. Further, the annual cosmic ray dose to meteorites has been assumed as 0.1 Gy/year and, this does not take into account the depth dependence of cosmic rays inside meteorite interior and the efficiency of luminescence production that depends on linear energy transfer. Together, these D_e and dose rate estimates provide exposure ages of ~10,000 years, which are about two or more orders of magnitude lower compared to that estimated using cosmogenic radionuclides. Such abnormally low D_e's have been enigmatic as the analyses of kinetic parameters of luminescence of meteorites suggest a stability of luminescence signals of over a million years. The present study rigorously explored reasons for such low equivalent doses by, a) estimating D_e's for luminescence emission under different spectral emission windows, b) by using different protocols for the estimation of stability (particularly athermal) and c) by re-estimating the annual cosmic ray dose taking into account dose attenuation and the linear energy transfer effects on luminescence production efficiency. The result suggest significant improvements in understanding the relationship between natural luminescence and cosmic ray exposure ages via, a) probing the isothermal signal in the red emission band, b) a proper evaluation of the annual cosmic rays dose that was so far over estimated by a factor of ~2, and c) an estimation of the athermal fading (also termed as anomalous fading) rates. We found that athermal fading rate (g-value) decreased nonlinearly with cosmic ray exposure (CRE) ages and ranged from 9.4±1.1 to 2.6±1.1 (% per decade) for CRE age ranging from 2 to 110 Ma. We surmise that the decrease of fading rate is due to radiation damage by high energy heavy charged particle bombardment and consequent loss of defect centers. The equivalent doses increase with CRE ages accord with theoretical calculations, suggesting thereby the possibility of using a combination of g-values and D_e for constraining the CRE ages of meteorites.