Extraction of Cr during mantle melting: Experimental constraints on the role of peridotite composition and the effect of water

Chromium (Cr) is considered to be one of the most economically important critical raw materials. The most economically significant mineral for Cr exploitation is Chromite which can be found in podiform Chromite deposits that are associated with ophiolite complexes from Supra-Subduction Zone (SSZ) settings. In Europe, such deposits are located in northwestern Greece and the Balkans in general. The precise nature of podiform chromitite generation is not fully understood and still under debates and could be the result of a complex sequence of enrichment processes.To improve our understanding of Cr extraction during mantle melting a series of experiments were performed on natural peridotite samples from the Pindos Ophiolite, Greece. Our experiments were conducted under pressure (1.0-1.5 GPa) and temperature (1350-1400°C) conditions that are relevant for the genesis of intra-oceanic arc magmas. To study the effect of hydrous conditions, the amount of water in the system was varied from 1 to 10 wt%. The samples had various modal proportions of olivine, ortho- and clinopyroxene and a different degree of alteration.All experiments produced Cr-enriched melts (up to 7000 µg/g Cr) with a positive dependence of Cr content on the degree of melting and on the concentration of water added to the system. Based on the obtained results we have developed forward numerical models to constrain the Cr-enrichment process in magmas during mantle melting. Our study demonstrates that previous estimates on the extraction efficiency of Cr during mantle melting in SSZ should be reconsidered.