Fig. 2: Variation of modal amounts of olivine and orthopyroxene in the Kaapvaal craton.

Panel a shows that deviations from the main linear trend are primarily due to addition of garnet and or clinopyroxene to the melt residue as a consequence of metasomatism. The dashed line marks the threshold above which peridotites are described as having excess orthopyroxene compared to those in melt residues. Panel b shows that the modal amount of olivine in mantle peridotites primarily varies linearly with the amount of orthopyroxene and bulk MgO/SiO₂ content. The excess amount of orthopyroxene (>25%) primarily reflects the reaction of silica in a melt or fluid with olivine and can be described by the following equation: MgFe₂SiO₄ + SiO₂ → MgFe₂SiO₆. Mantle peridotites from the Kaapvaal and Tanzania cratons studied in this work are shown by large symbols. Orthopyroxene-rich garnet harzburgites are shown by circles and garnet harzburgites and lherzolites are shown by triangles. Small symbols are for mantle peridotites from the Kaapvaal craton published by refs. ^9,100.