Komatiites of the Weltevreden Formation, Barberton Greenstone Belt, South Africa: implications for the chemistry and temperature of the Archean mantle

Abstract

The komatiites of the 3.29 Ga Weltevreden Formation, Barberton Greenstone Belt, South Africa represent extremely magnesian lavas derived from the Archean mantle. Three related papers are presented on aspects of the temperature and composition of the Archean mantle, and surface alteration processes. Petrographical and geochemical analyses confirm that olivines from the komatiites retain much of their original igneous character, containing fresh olivines, pyroxenes, and chromites. The olivines from the Weltevreden Formation are extremely magnesian, with Fo contents up to 95.6. Primitive mantle normalized Gd/Yb values near 1, and Al2O3/TiO3 values of 26-33 for these rocks are consistent with derivation from deep primitive mantle, leaving a melt residue of only olivine. Models based on the geochemistry of these rocks indicate eruption temperatures of at least 1660°C, higher than previously reported for komatiitic eruption temperatures. X-ray diffraction and electron microprobe analyses show that the dominant alteration mineral in these rocks is serpentine. The dominant serpentine mineral present, lizardite, shows a wide range of compositional variation. Observed textures and geochemical variation of the alteration products indicate these rocks were altered at a lower metamorphic grade than previously reported for the Barberton Greenstone Belt. The d7Li and d18O values of the fresh olivine separates from these rocks range from 0.5 to 10.0‰ and 3.8 to 4.2‰, respectively. The variation of d7Li values and the low d18O values of olivine separates indicates the incorporation of a subduction component in the mantle source region, which suggests the Archean mantle was heterogeneous. Temperatures of serpentinization ranging from 142 to 310°C were determined using the oxygen isotopic compositions of the whole rocks assuming fluids with d18O values ranging from -1 to 2‰. The estimated temperatures and the petrographical and geochemical data of these rocks indicate alteration at temperatures lower than previously estimated for the Barberton Greenstone Belt.