Abstract
We have measured the composition of He, Ne and Ar in the glassy rim of a fresh oceanic basalt from the East Pacific Rise. Three splits of the sample were subjected to neutron irradiation prior to analysis to investigate the noble gas release as a function of different neutron doses. The shapes of the observed 39Ar- 40Ar degassing profiles depend critically on the irradiation dose due to a severe redistribution of excess 40Ar. This shows that the application of the 39Ar- 40Ar method for dating of glassy samples requires the utmost care. Including the He and Ne results from the irradiated specimens, it was possible to determine the concentrations of Li and Mg and to estimate upper limits for Na and F in addition to the concentrations of K, Ca and Cl which are measured routinely as a byproduct of the 39Ar- 40Ar dating method. This provides a useful extension of the 39Ar- 40Ar technique since it may help to identify the outgassing sites during stepwise heating experiments. Furthermore it allows one to study the diffusion behavior of He and Ne isotopes artificially produced during irradiation. In our samples the activation energies of 3He and 21Ne tend to increase with growing neutron doses.
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