A siliceous komatiitic source of Bushveld magmas revealed by primary melt inclusions in olivine

Abstract Our experimental thermobarogeochemical study of the primary melt inclusions in olivine (Fo91) from poikilitic harzburgite to dunite of the Uitkomst Complex in the Bushveld Igneous Province revealed a series of melt compositions that are the result of combined fractional crystallization and assimilation. The primary melt inclusions up to 50 μm in size are fully crystallized and composed of predominantly olivine and orthopyroxene with highly subordinate chromite, sulfide, amphibole and plagioclase accompanied by a fluid phase in shrinkage voids. The most primitive melt within the series has a re-homogenization temperature of >1450 °C and contains 22.47 wt% MgO, 12.57 wt% FeO, 51.90 wt% SiO2, 2.50 wt% CaO, 8.27 wt% Al2O3 and 1.69 wt% Na2O recalculated to equilibrium with the host olivine. This is the first direct measurement of the komatiitic parental magma of the Bushveld magmas, inferred in previous studies. The estimated maximum PT parameters of the melt corresponds to 2.5 GPa and 1590 °C implying that the primitive melt inclusions in the olivine were trapped while still in the upper mantle and before further crustal assimilation. This primitive melt shows the steepest slope (La/Yb)PM, the highest Ni and is depleted in volatile components (168 ppm H2O, 264 ppm Cl, 258 ppm F and 114 ppm S according to secondary ion mass spectrometry measurements) compared to more evolved compositions, which were progressively enriched in volatiles up to 7665 ppm Н2О, 1728 ppm Cl, 2288 ppm S, and 549 ppm F as the rehomogenization temperature fell to 1335 °C. The progressive assimilation of crustal material responsible for magma modification is also manifested by the occurrence of Ca Na volatile-rich minerals (such as amphibole, phlogopite, albite, sodalite, natrolite, pectolite, Cl-apatite and carbonate) in the interstitial assemblage and in “exotic” polyphase inclusions in cumulus olivine. The concentrations and ratios of the incompatible elements in the melt inclusions in Uitkomst olivine are similar to those in the model compositions of the ultramafic Bushveld parental magma and support a common source of ultramafic melts in the Bushveld Province. The main components of the melts, however, are not in accord with either boninitic or komatiitic lineages indicating a unique nature for the Bushveld magmas.