PGE distribution in 2.7-Ga layered komatiite flows from the Belingwe greenstone belt, Zimbabwe

Abstract

The well-preserved Anderson's and Onias's komatiite flows from the late Archaean Belingwe greenstone belt (∼ 2.7 Ga), Zimbabwe, were sampled in the SASKMAR drill hole. Anderson's Flow possesses a 1.1-m-thick B1 subzone and a clinopyroxene spinifex subzone, whereas Onias's Flow has spinifex and cumulate layers typical of komatiite flows. Platinum-group element (PGE) abundances throughout these two flows provide constraints on the formation of layered komatiite flows. All samples from both flows have similar chondrite-normalized PGE patterns that are enriched in Rh, Pt and Pd relative to Os and Ir, which are attributed to large degrees (>30%) of partial melting of the upper mantle. Their Pd/Ir ratios are highly variable (2.0-31 in Onias's Flow and 2.3-61 in Anderson's Flow). PGE variations within both flows, except the spinifex A zone and B1 subzone of Anderson's Flow, together with those of major and other trace elements, are consistent with a model involving olivine fractionation. The spinifex A zone of Anderson's Flow has low Pd/Ir ratios (6-8) which occur in an interval having negative Eu anomalies caused by hydrothermal alteration. The Pt fractionation from Pd and Rh is documented from their relationship to MgO contents: behaviours of Pd and Rh are similar and more incompatible than Pt at the early stage of relatively slow cooling but less incompatible at the late stage during the formation of komatiite flows. PGE in the B1 subzone of Anderson's Flow do not follow an olivine fractionation trend. Pd/Ir ratios in this subzone increase from 28 at the top to 61 at the base. These unusually high values can be accounted for by late-stage development of the subzone and strong fractionation. © 1994.