A comparison of trace element concentrations in chromite from komatiites, picrites, and layered intrusions: implications for the formation of massive chromite layers

Abstract

By examining the minor and trace element contents of chromites from three intrusions; the Bushveld Complex (South Africa), the Stillwater Complex (USA) and the Great Dyke (Zimbabwe), and comparing these chromite compositions to those of magmas from which they could have formed (komatiites and picrites) we conclude that: a) the variations in Ti, V, Sc and Ga contents across stratigraphy and across individual layers do not support the model of magma mixing leading to chromite-only crystallization; b) the chromites from the lowest levels of the intrusions could have crystallized from komatiite liquids that were contaminated with continental crust; c) the Great Dyke chromites have the highest Cr# and lowest incompatible element contents and formed from a liquid closest to komatiite; d) all of the chromites, except those of the Dunite Succession of the Great Dyke have equilibrated with a liquid that had also crystallized pyroxene; e) the Great Dyke and Stillwater chromites show a narrower range in composition than the Bushveld chromites; f) Chromites from the western limb of the Bushveld Complex contain much higher V contents than all the other chromites. This requires either, that the fO2 was lower in the western Bushveld or that the chromites equilibrated with a V-rich magma. We favor a model where chromite and silicate minerals crystallized in cotectic proportions (~2:98). The chromite, silicates and transporting liquid are emplaced into the magma chamber. During emplacement the chromite and silicate separated due to viscous particle flow to form a massive chromite layer overlain by silicates.

From the mantle source to the crustal sink: magmatic differentiation and sulfide saturation of the Paleoproterozoic komatiites of the Central Lapland Greenstone Belt, Finland

“…We conducted mantle melting simulations to estimate S content of the komatiitic parental melt. Sulfur content is not reported for the Kevitsa dyke (Puchtel et al 2020) and hence it cannot be evaluated similarly to the other elements of the 1 3 a K Cu D spl−melt of 0.66 based on measurement from chromites in low-Ti picrites (Barnes et al 2023a). For clinopyroxene, we used the low-P (< 1 GPa) calibrations for terrestrial silicate systems of Bédard (2014) for…”