Clinopyroxene Compositions from Belingwe Komatiites: Constraints on Alteration and Komatiite Genesis

Previous studies have demonstrated that chemical analysis of relict primary igneous clinopyroxene crystals from komatiite lavas provide a means to investigate pre-metamorphic magmatic compositions and to assess similarities between komatiites and mafic magmas from modern plume or subduction zone environments. For this study we have analyzed major and trace element from two komatiite samples from the Belingwe greenstone belt (2.7 Ga), Zimbabwe, for major and trace element composition using the electron microprobe and laser- ablation ICP-MS. Clinopyroxene crystals within komatiites are typically small (generally between 0.03-0.7 mm) subhedral and euhedral bladed crystals located interstitially between larger polyhedral olivine. We interpret these textures to indicate crystallization of pyroxene relatively late in the crystallization history. Although groundmass and some olivine show indications of serpentinization clinopyroxenes typically appear unaltered, and narrow iron-rich rims most likely relate to late crystallization. Clinopyroxenes are relatively homogenous in composition with Mg# 71-80 and average composition of Ca0.8Mg0.9Fe0.30Si2O6 and coexist with olivine of Fo90-92. Comparison between measured pyroxene trace element contents and a hypothetical pyroxene calculated to be in equilibrium with whole rock compositions suggest that mid and heavy REE, Ti, Zr and Y have remained largely immobile during alteration and metamorphism, whereas Sr is depleted. Interestingly there are indications of gain of LREE during alteration, although this might also reflect late crustal contamination. Belingwe komatiites do not show the low Ti/Zr ratios recorded in Barberton komatiite samples and inferred to resemble modern boninites.