Abstract
Whole-rock oxygen isotope data from a suite of mafic and ultramafic samples from the Onverwacht Group, the basal unit of the Archaean greenstone succession of the Barberton Mountain Land, South Africa, show a range of values from δ18O ≃ +3 to δ18O ≃ +14, a range which coincides with those of Phanerozoic ophiolites and oceanic crust. When the samples are arranged in an inferred ophiolitic pseudostratigraphy, from basal serpentinized ultramafic cumulates through altered mafic extrusives, they have an oxygen isotope distribution profile which is indistinguishable from that of Phanerozoic ophiolites. The distinctive isotopic profiles and secondary mineral assemblages in Phanerozoic ophiolites are caused by hydrothermal interaction between seawater (δ18O≃0) and oceanic crust (δ18O≃ +5.8). The existence of a similar isotopic and metamorphic profile in the Onverwacht Group argues strongly that these rocks were hydrothermally altered by an Archaean ocean whose isotopic composition was indistinguishable from modern sea water.
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Hoffman, S., Wilson, M. & Stakes, D. Inferred oxygen isotope profile of Archaean oceanic crust, Onverwacht Group, South Africa. Nature 321, 55–58 (1986). https://doi.org/10.1038/321055a0
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DOI: https://doi.org/10.1038/321055a0
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