Geodynamic processes on early Earth, especially the interaction between the crust and deep mantle, are poorly constrained and subject to much debate. The rarity of fresh igneous materials more than 3 billion years old accounts for much of this uncertainty. Here we examine 3.27-billion-year-old komatiite lavas from Weltevreden Formation in the Barberton greenstone belt, which is part of the Kaapvaal Craton in Southern Africa. We show that primary magmatic compositions of olivine are well preserved in these lavas based on major and trace element systematics. These komatiitic lavas represent products of deep mantle plumes. Oxygen isotope compositions (δ18O) of the fresh olivine measured by laser fluorination are consistently lighter (about 2‰) than those obtained from modern mantle-derived volcanic rocks. These results suggest a mantle source for the Weltevreden komatiites that is unlike the modern mantle and one that reflects mantle heterogeneity left over from a Hadean magma ocean. The anomalously light δ18O may have resulted from fractionation of deep magma ocean phases, as has been proposed to explain lithophile and siderophile isotope compositions of Archaean komatiites.
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Field work by G.R.B. and K.K. was funded by a grant from the National Science Foundation. Laboratory work by H.B. was partially provided by the strategic priority research program (B) of CAS (XDB18010104) and China NSFC grant 41490635. We would like to thank E. Marshall and J. Barnes of the Stable Isotope lab in the University of Texas at Austin Department of Geological Sciences for their assistance performing oxygen isotope measurements.
The authors declare no competing financial interests.
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Byerly, B., Kareem, K., Bao, H. et al. Early Earth mantle heterogeneity revealed by light oxygen isotopes of Archaean komatiites. Nature Geosci 10, 871–875 (2017). https://doi.org/10.1038/ngeo3054