4 He diffusion in 40 Ar-retentive minerals

Mathematics – Logic

Scientific paper

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Scientific paper

4 He vacuum diffusion experiments were carried out by stepwise degassing minerals normally used in K-Ar chronometry such as hornblende, nepheline and others. The study aims at 4 He diffusion parameters for uppermost crustal temperatures and thus at He retentivities of the minerals. The results indicate long-time near-quantitative 4 He retention in hornblende, pyroxene, langbeinite and perhaps nepheline, for 0.5 mm grains at least at temperatures below about 70°C. Sanidine and muscovite lose 4 He over geologic periods at lower temperatures. Only the minerals of the first group are possible (U + Th)/He chronometers or geothermometers provided that other dating problems, such as alpha loss and U migration, can be settled. Closure temperatures for rapidly cooling 2 mm hornblende crystals are found to be in the range of 190 to 170°C. Tentative U-He indices indicate that diffusion parameters of the same order of magnitude as determined for the He-retentive minerals may be used to select minerals to be further examined for their (U + Th)/He datability, e.g. ore minerals.

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