Enrichment and transport of Cr in magmatic systems in the mantle and crust

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Chromium is a critical metal with a high importance to the economy of the EU and high rick of sustainable supply. Chromium has been mined from four different deposit types: stratiform, podiform, placer, and laterite deposits in the form of chromite mineral but only stratiform and podiform chromite deposits represent the most significant types. Only podiform Cr deposits, intimately associated with mafic-ultramafic rocks, are present and currently mined in the EU. The close association of Cr deposits with mantle rocks dictates that the source of chromium in the formation of chromite deposits must be sought in the Earth’s upper mantle. The very low concentrations of Cr needed to saturate magmas with chromite indicate that seemingly limited extraction processes both during mantle melting and during magma crystallization appear to be sufficient to generate podiform chromite deposits. However, despite numerous studies and a variety of proposed mechanisms leading to Cr enrichment, there is a lack of general consensus on a particular model which can fully explain the formation of podiform chromite deposits.In our research we aim at understanding the processes leading to the Cr enrichment in magmatic systems from the mantle to the ore deposit. To achieve this goal we propose an experimental investigation combined with a modelling approach focusing on three stages of Cr-enrichment: 1) Mantle melting; 2) Melt evolution; and 3) Melt movement, solid-state deformation and fluid mobilization. In this project we will focus on the two first steps and conduct experimental study unrevealing and quantifying the contributions of different physico-chemical parameters at mantle and crustal conditions: the chemistry of the source rocks, the degree of mantle melting, the particular distribution of chromium among the different mantle and magmatic phases (olivine, orthopyroxene, clinopyroxene and spinel) during melting and consequent crystallization. Particular attention will be paid to the role of water and redox conditions relevant for the supra-subduction tectonic environment of the ophiolitic complexes hosting podiform Cr ore deposits. The obtained experimental results will be used to calibrate thermodynamic codes and to develop a general model of chromite enrichment in natural systems.