Transcript 4He Solubility in Apatite is Low But Possibly Significant

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.