ESR Isochron Dating of Speleothems

Abstract

<p>Calcium carbonate precipitated in caves (speleothem) may contain more than 1 ppm of U but is devoid of Th or K. The major part of its natural radiation dosage (NRD) may have arisen internally from U and its short-lived daughters. By analysing adjacent, approximately coeval samples with varying U content, it is possible to construct an isochron of NRD versus U content which, when extrapolated to zero, gives the external NRD component. The slope of the isochron is a function of the age of the samples, taking into account appropriate corrections for disequilibrium between U-series daughter isotopes, and alpha-particle ESR efficiency ("a"-factor). Using speleothems which have been dated by the 230Th/234U method, the above procedure has been reversed to determine the average "a"-factor of the samples, and to establish that its values lie around 0.1.</p> <p>Several discrete and reproducible ESR peaks of approximately constant g-value have been observed in speleothem calcites. The main peak (S 1A) with g=2.0008(±2), fades at approximately 220°C and corresponds to the principal TL peak, used for dating so far. Other weaker peaks of various g-values and fading temperatures are also radiation-sensitive. Several other peaks are present which are associated with Mn2+. Low-temperature annealing has a mild effect on the main peak, probably due to retrapping. This peak varies greatly with grain size, decreasing by more than half for particles less than 100 μm.</p>