Temperature‐induced ⁴He degassing of specularite and botryoidal hematite: A ⁴He retentivity study

Degassing of radiogenic 4 He 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 4 He retentivities. Samples from the Vosges, Odenwald, Saarbergland, and Cumbria were used. Below 900°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°C. The actual diffusion distances appear to be commensurate with the microscopic size of the constituent crystals (0.1 to 10 μm). 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°C, and for all grain fractions in botryoidal hematite >10 μm the T c is above 90°/160°C. On the basis of T c , both hematite varieties should be useful thermochronometers. Low‐temperature 4 He losses of the order of a few percent during a few 100 m.y. are conceivable for botryoidal hematite.