HE-4 DIFFUSION IN SPECULAR HEMATITE

In order to test the chronometer qualities of specularite for the (U+T h)/He dating method, He-4 release experiments by stepwise heating of t wo specularites from the Rimbach mineralization locality in the southe rn Vosgues (France) have been carried out. The diffusion coefficients define linear Arrhenius plots within a temperature interval of 250 to 830-degrees-C, which is suggestive of volume diffusion. Extrapolation of the diffusion behavior to 20-degrees-C yields diffusion coefficient s (D20 values) smaller than 10(-26) [cm2 s-1] for both hematites with activation energies at 116 [kJ/mole]. The results of our study suggest that specularite is a very helium retentive hematite variety which is capable of quantitatively retaining radiogenic helium over geologic p eriods of time.