Temperature-induced 4He degassing of specularite and botryoidal hematite

• Verfasst von: Bähr, Roland [VerfasserIn]
• Verfasst von: Lippolt, Hans J. [VerfasserIn]
• Verfasst von: Wernicke, Rolf S. [VerfasserIn]
• Titel: Temperature-induced 4He degassing of specularite and botryoidal hematite
• Titelzusatz: a 4He retentivity study
• Verf.angabe: Roland Bähr, Hans J. Lippolt, Rolf S. Wernicke
• E-Jahr: 1994
• Jahr: 10 September 1994
• Umfang: 13 S.
• Fussnoten: Im Titel ist die Zahl "4" hochgestellt ; Gesehen am 02.02.2023
• Titel Quelle: Enthalten in: Journal of geophysical research. B, Solid earth
• Ort Quelle: Hoboken, NJ : Wiley, 1978
• Jahr Quelle: 1994
• Band/Heft Quelle: 99(1994), B9, Seite 17695-17707
• ISSN Quelle: 2169-9356
• DOI: doi:10.1029/94JB01055
• Datenträger: Online-Ressource
• Sprache: eng
• K10plus-PPN: 1833071565

Abstract

Degassing of radiogenic 4He 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 4He 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 (D20) and botryoidal hematite (D20/r2) 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 (Tc) of specularite grains with diameters of about 5 mm are above 200°C, and for all grain fractions in botryoidal hematite >10 μm the Tc is above 90°/160°C. On the basis of Tc, both hematite varieties should be useful thermochronometers. Low-temperature 4He losses of the order of a few percent during a few 100 m.y. are conceivable for botryoidal hematite.