Abstract
We report here the first 40Ar/39Ar ages for komatiites and komatiitic basalts from the Barberton Mountain Greenstone Belt on the Transvaal–Swaziland border in southern Africa1. Both rock types display remarkable argon retentivity. While some variation is found among the samples, the best argon age estimate for the time of metamorphism is in the 3,450–3,490 Myr range. This age is only slightly less than that found in komatiites from the same area by Sm–Nd dating (3,540±15 Myr)2. Within the uncertainties of the decay constants involved, the two age estimates are not different. The results show that the principal pervasive greenschist metamorphism1 in the area must have occurred within 100 Myr of the eruption of the komatiite. Analytically precise ages were obtained despite potassium contents as low as 80 p.p.m. in the presence of high calcium concentrations (3–5% Ca). To our knowledge, the results represent by far the oldest reliable ages obtained for terrestrial rocks using the K–Ar system.
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Martinez, M., York, D., Hall, C. et al. Oldest reliable 40Ar/39Ar ages for terrestrial rocks: Barberton Mountain komatiites. Nature 307, 352–354 (1984). https://doi.org/10.1038/307352a0
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DOI: https://doi.org/10.1038/307352a0
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