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 w as carried out on various grain size fractions of specular and botryoi dal hematite in order to place constraints on their He-4 retentivities . Samples from the Vosges, Odenwald, Saarbergland, and Cumbria were us ed. Below 900 degrees C, specularite yields straight lines in Arrheniu s plots. This is consistent with volume diffusion. The diffusion dista nces correspond with the macroscopic mean grain size. By contrast, bot ryoidal hematite yields curved lines which are straight only below 250 degrees C. The actual diffusion distances appear to be commensurate w ith 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 activatio n energies range from 110 to 130 and 130 to 210 kJ/mol, respectively. Helium 4 closure temperatures (T-c) of specularite grains with diamete rs 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 degre es C. On the basis of T-c, both hematite varieties should be useful th ermochronometers. Low-temperature He-4 losses of the order a few perce nt during a few 100 m.y. are conceivable for botryoidal hematite.