40Ar/39Ar dating of the KBS Tuff in Koobi Fora Formation, East Rudolf, Kenya

Abstract

40Ar/39Ar and conventional K–Ar age determinations on volcanic sanidine–anorthoclase crystal concentrates from pumice lumps contained within the KBS Tuff in the Koobi Fora Formation at East Rudolf indicate an apparent age near 2.42 Myr. Recomputation of data obtained in 1969 from this tuff suggests a more accurate apparent iisochron age of 2.42±0.01 Myr for the feldspar, coincident with total fusion conventional K–Ar and 40Ar/39Ar dates of ∼ 2.4 Myr from the same sample. Age spectra and total fusion dates obtained from other samples of the KBS Tuff confirm this conclusion. Thus, apparent ages of < 2.4 Myr derived from some of our samples and conventional total fusion K–Ar apparent ages in the range 1.6–1.8 Myr obtained from the KBS Tuff by other workers are regarded as discrepant, and may have been obtained from samples affected by argon loss. This latter conclusion is compatible with our sector interpretation of the age spectrum analyses of KBS pumice feldspars. Argument for there being more than one tuff complex or an admixture of older pumice in the KBS Tuff is not supported by our work.