Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Chromite is one of the first phases to crystallize in ultramafic - mafic silicate melts, and can thus be used to study early magmatic processes in volcanic and plutonic environments. A better understanding of chromite crystallization is also of economic interest, since it constitutes the only source of primary Cr, and massive chromitites can host important Platinum-Group Element (PGE) concentrations (e.g. UG2, Bushveld Complex). In our ongoing work, we are studying the composition of chromites, obtained by EMPA and LA-ICP-MS, to i) constrain it’s role in the fractionation of certain elements (e.g. Os, Ir, Ru, Sc, Ga, Ni, Co, …) during early magmatic processes (i.e. fractionation of Ru by komatiitic chromite [2, 3]) and ii) investigate the nature of the parental melt involved in the formation of chromitite in both oceanic and continental settings. Using this approach, we have shown a clear similarity between the geochemical signature of chromite from ophiolitic podiform chromitites and chromite from the primitive boninites [1]. This study aims to constrain the nature of the primitive melt involved in the formation of the Stillwater Complex chromitites which have a distinctive geochemical signature compared to chromites from various primitive ultramafic - mafic melts (Fig. 1) suggesting that none of these primitive magmas are involved in their formation. Also, these dissimilarities between the geochemical signature of chromite from the various lavas and from the Stillwater Complex chromitites can be used to discriminate them from one another and thus be used as a provenance mineral.