Geochemical and tectonic characterization of peridotites and chromitites from the Troodos ophiolite, Cyprus

The Troodos ophiolite (Cyprus) represents a well-preserved supra-subduction zone mantle section, providing critical insights into mantle melting and chromitite formation during subduction initiation. This study integrates petrography, mineral chemistry, whole-rock and trace element geochemistry, platinum-group element analysis, and melt composition modeling to investigate melt extraction and melt–rock interaction in harzburgites, dunites, and chromitites. Harzburgites record ＞30% melt depletion, characterized by highly forsteritic olivine (Fo_91–92) and high-Cr# spinels, while dunites reflect focused melt percolation. Chromitites, distinguished by high-Cr# spinels and magnesian clinopyroxenes (Mg# 94–99), show pronounced enrichment in iridium-group PGEs (ΣPGE up to 231 ppb) and distinctive PGE fractionation patterns, including IPGE dominance and negative Pt anomalies, indicative of chromite-controlled fractionation during crystallization from boninitic melts. These melts, with elevated Al₂O₃ (11.6–14.3 wt%) and variable FeO/MgO ratios (0.78–1.21), originated from a depleted mantle source metasomatized by slab-derived fluids, with localized higher fO₂ during melt–rock interaction (+1.42 to +3.41 ΔFMQ). These data support a two-stage mantle evolution model involving initial MORB-like melting forming a refractory lithosphere, followed by infiltration of boninitic melts during subduction initiation, which drove chromitite formation and mantle re-enrichment.