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4He Solubility in Apatite is Low But Possibly Significant Peter K. Zeitler, Eva Enkelmann, Bruce Idleman Dept. of Earth & Environmental Sciences Lehigh University, Bethlehem, PA, 18015 USA The age equation used in dating rocks includes a correction term for that portion of the daughter nuclide present in a sample at the time of its formation (or closure to diffusive loss). In U-Th/He dating, this component is assumed to be negligible because of the mobility of helium and its generally low concentration in the atmosphere. However there are geological environments in which 4He is present in significant quantities, so an assessment of whether excess helium could ever be a problem in dating requires knowledge of the solubility of helium in minerals, but no relevant solubility data currently exist for helium in apatite. As part of our efforts to study radiation-damage trapping of 4He in fission tracks, we have been attempting to saturate apatite crystals with helium, and these experiments have given us insight into helium solubility in apatite. Our newest experiments at high helium partial pressures show that saturation appears to occur by 30 bars, at levels that would be quite significant for dating (see figure at right). Further experiments at lower pressures of several bars will permit us to better constrain the Henry’s Law region for solubility. We also find that analytical scatter for treated samples is far beyond normal values, suggesting that additional complexities may be involved in the incorporation of helium into apatite.