First data on melt inclusions in olivine from 3.3 Ga komatiites of Pioneer Complex, Weltevreden formation: evidence for near-surface contamination of primary komatiite

Komatiites are ultramafic volcanic rocks that originated from hot magnesium-rich magmas with high-degree mantle melting. Numerous recent studies of melt inclusions in komatiites olivine phenocrysts (Sobolev et al., 2016, 2019, this meeting; Asafov et al., 2018, 2020) provide evidence that the most primitive komatiite melts are enriched in H2O and Cl being depleted by incompatible elements (K, Rb, Ba, U, and Pb). These results were interpreted as a compositional feature of mantle plumes sources of komatiites produced by recycling degassed seawater altered lithosphere to the deep mantle. The surface contamination has been detected in all studied suites but was not investigated in detail. Here we present such a study for the komatiites of Weltevreden formation, Barberton Greenstone Belt (3.3 Ga), South Africa. The first data on melt inclusion and host high-Mg olivine phenocrysts were obtained for komatiites from Pioneer complex, Weltevreden formation. The experiments were carried out in the Vernadsky Institute. Fresh olivine grains about 0.5 mm in size were heated for 3 minutes and quenched at a temperature of 1300-1330oC and 1 bar C-O-H gas pressure corresponding to QFM buffer. Homogenized melt inclusions were brought to the surface, polished and studied by EPMA for major and minor elements, by RAMAN microscope for water contents, and SIMS for D/H ratios and water contents. Host olivines (Fo 93.4-92.5 mol%) were studied by EPMA. We also plan to obtain LA-ICP-MS data for trace elements for melt inclusions and host olivines. Reversing of post-entrapment crystallization allowed reconstructing the composition of the trapped komatiite melts, which contain 26-24 wt.% MgO and 0.5-0.8 wt.% water. Compared to previously reported compositions of primitive komatiite melts from the Saw Mill complex of Weltevreden formation, studied melts experienced a significant AFC process. They show a strong positive linear correlation between K and Cl contents and suggest contamination of the component enriched by K, Cl, Na, and probably by H2O with a relatively high D/H ratio. The most primitive komatiite melts from the Saw Mill complex of Weltevreden formation are not contaminated by this component and thus likely represent mantle source composition.