TEMPERATURE-INDUCED HE-4 DEGASSING OF SPECULARITE AND BOTRYOIDAL HEMATITE - A HE-4 RETENTIVITY STUDY

Degassing of radiogenic He-4 induced by stepwise heating in a vacuum was carried out on various grain size fractions of specular and botryoidal hematite in order to place constraints on their He-4 retentivities. Samples from the Vosges, Odenwald, Saarbergland, and Cumbria were used. Below 900 degrees C, specularite yields straight lines in Arrhenius plots. This is consistent with volume diffusion. The diffusion distances correspond with the macroscopic mean grain size. By contrast, botryoidal hematite yields curved lines which are straight only below 250 degrees C. The actual diffusion distances appear to be commensurate with the microscopic size of the constituent crystals (0.1 to 10 mu m). The diffusion coefficients of specular (D-20) and botryoidal hematite (D-20/r(2)) are 10(-28) cm s(-1) and 10(-22) s(-1), and the activation energies range from 110 to 130 and 130 to 210 kJ/mol, respectively. Helium 4 closure temperatures (T-c) of specularite grains with diameters of about 5 mm are above 200 degrees C, and for all grain fractions in botryoidal hematite > 10 mu m the T-c is above 90 degrees/160 degrees C. On the basis of T-c, both hematite varieties should be useful thermochronometers. Low-temperature He-4 losses of the order a few percent during a few 100 m.y. are conceivable for botryoidal hematite.