HELIUM RETENTION IN SPELEOTHEMS AND ITS INFERENCES FOR POTENTIAL U/TH-HE AGE DATING

The retentive character of helium within speleothems composed of calcium carbonate has yet to be well established. If helium is retentive in speleothem carbonate over geologic time, it can be used for geochronology using the U/Th-He methods. This would allow the establishment of speleothem chronologies beyond the limitations of U/Th dating methods (~500 kyrs). Here we investigate the diffusivity of helium within speleothem calcite. Three separate step-crushing experiments were conducted using a Pleistocene age speleothem from Borneo. The first experiment analyzed a homogenous aliquot of 300-325 µm crushate. The first split of the aliquot was analyzed within 24 hours of sample preparation and the second split was analyzed six months afterwards. The results indicate significant leakage of He (85% loss) in the 6 months after sample preparation. A second experiment was conducted wherein crushate was exposed to an atmosphere of He gas in a sealed chamber for 48 hours prior to analysis. The subsequent results indicate significant uptake of ambient gas into the crystal lattice. The third step crushing experiment was undertaken using a single 4.7 g piece of speleothem that had fewer fresh surfaces and fractures. Once crushed, the sample leaked small amounts of He indicating that He leakage occurs after fracture introduction to the crystal lattice.

Diffusion experiments were carried out on a speleothem using a uniform distribution of ³He created by proton irradiation. Both large pieces (> 1 mm) and small grain fractions (63-75 µm; 75-125 µm) were analyzed using isothermal, prograde and retrograde heating steps (80 to 470^oC). The material displays multiple complex He diffusion domains with similar patterns for all grain sizes analyzed. The poly-diffusion-domain behavior observed makes quantifying diffusion coefficients and temperature dependence impossible. Additional experiments on large single crystal calcites are required to better assess He diffusion behavior. Our preliminary experiments suggest that helium in speleothems is potentially retentive over geologic time but can be rapidly lost once fractures are introduced into the crystal lattice.