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

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