A geochemical and Nd isotopic study of Barberton komatiites (South Africa): implication for the Archean mantle

Abstract

The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al 2O 3/TiO 2 and CaO/Al 2O 3 ratios of 15–18 and 1.1–1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb) N ratios. In contrast, the Al-depleted komatiites show significantly lower Al 2O 3/TiO 2 ratios of 8–12, highly variable CaO/Al 2O 3 (0.19–2.81) ratios combined with (Gd/Yb) N ratios varying from 1.08 to 1.56. A Sm–Nd whole rock isochron for komatiites of the Komati Formation gives an age of 3657±170 Ma. 147Sm/ 144Nd ratios (0.1704 and 0.1964) are all lower than the chondritic value of 0.1967. The komatiite ε i,Nd (3.45) values cluster at +1.9±0.7. Trace element distribution indicates that most of the primary geochemical and isotopic features of the komatiites were preserved in line with the conservation of the primary chemical composition of clinopyroxene. High field strength element and rare earth element abundances indicate that crustal contamination and post-crystallisation processes did not disturb the primary features of komatiites. The Sm/Nd and Nb/U ratios of komatiites indicate that the Barberton greenstone belt mantle source has undergone melt extraction prior to komatiite formation. Variations of Al 2O 3/TiO 2, (Gd/Yb) N, Zr/Sm and Sm/Nd ratios of komatiites indicate that a batch melting of slightly depleted mantle source during with garnet and/or clinopyroxene remained in the residue can produce the geochemical isotopic feature of the Barberton greenstone belt komatiites. Typical geochemical fingerprints of subduction-related processes (LILE enrichment, HFSE depletion compared to REE), as known from modern subduction zones, are not observed. Komatiites exhibit Ti/Zr, La/Nb, Nb/U, Sr/Nd and Ba/La ratios comparable to those of oceanic island basalt and mid-ocean ridge basalt. (La/Nb) PMN, (Sm/Yb) PMN, positive δNb values and flat or slightly enriched REE patterns suggest that BGB komatiites are part of an oceanic plateau rather than an oceanic island such as Iceland. Therefore, an oceanic plateau or mid-ocean ridge, in connection with an oceanic plateau, such as Ontong Java plateau or Caribbean–Colombian oceanic plateau, is a suitable tectonic setting for the formation of the BGB komatiites.