Signatures of komatiite reactive melt flow through the Archaean Kaapvaal cratonic mantle

Abstract

Abundant Archaean komatiite and basalt erupted through evolving cratons, indicating melt transfer through the ancient mantle lithosphere. However, this process has rarely been identified in cratonic peridotite xenoliths, in contrast to exposed Phanerozoic mantle sections where melt-rock reactions are well-documented. We present a combined microstructural and mineral chemical investigation of eight coarse-grained (up to 20 mm), silica-rich, spinel facies peridotites from the Kaapvaal craton. These peridotites exhibit mild to strong silica-excess with 30–55 vol% orthopyroxene. Microstructural evidence of former melt presence is abundant in all samples, including low apparent dihedral angles, irregular grain boundaries, and extremely elongate grains. Despite varying silica-excess, all peridotites are highly refractory, with olivine Fo-content of 92.9 ± 0.3, reconstituted whole rock Mg-number of 92.9 ± 0.4, and negligible TiO₂ concentrations. Thermobarometry and comparisons with experimental compositions and thermodynamic models suggest a continuum of reactions in open systems, where evolving komatiite melt sourced from greater depth interacted with precursor mantle lithosphere at 2–3 GPa. We propose that silica-excess in cratonic spinel peridotites results from high time-integrated (i.e., aggregated) melt flux through melt channels, without requiring a highly silicic melt. Evidence for reactive flow of komatiite melt through cratonic mantle supports an intraplate setting for many Archaean greenstone belts and a co-evolution of Archaean crust and mantle.