Geochemistry of some ultramafic komatiite lava flows from the Barberton Mountain Land, South Africa

Abstract

Thin ultramafic flows from the Komati Formation in the Barberton Greenstone Belt show striking textural variations similar to those found in some ultramafic flows from Munro Township, Ontario and Western Australia. The spinifex textured A2 zone has been divided into an A2r zone consisting of short randomly oriented olivine blades with vesicles in the interblade areas and an A2p zone made up of booklets of olivine blades forming a ‘thermotactic’ spinifex texture. The composition of the A2r zone is considered to be representative of the initial ultramafic liquid, while the variation in the composition of the A2p zone reflects the evolving residual liquid composition as the flow crystallized. Detailed modelling of major and trace element data obtained from a 3.1 m thick flow demonstrates that olivine fractionation and accumulation after extrusion probably generated the observed compositional range (25.7–31.7 wt.% MgO). The Ni content of olivine cores analysed from the B zone of this flow are consistent with these crystals having existed in equilibrium with a liquid composition represented by the random spinifex textured A2r zone. A DCr (Cr % in olivine/Cr % in ‘liquid’) of 0.5 found for these olivine crystals suggests that the high Cr content of olivines from Archaean ultramafic lavas is due to the very high Cr content of the liquids from which they crystallized. The textural and chemical data are combined to develop a cooling and crystallization model for this flow.