Multistage melt–rock interaction and redox evolution during chromitite formation in the Bulqiza ophiolite (Albania): Constraints from Mӧssbauer spectroscopy, Fe[sbnd]Mg isotopes and chromitite geochemistry

Fe[sbnd]Mg isotopes integrated with Fe³⁺/∑Fe data provide robust constraints on the redox state of the oceanic lithospheric mantle, as both are sensitive to oxygen fugacity and mineral–melt equilibria. Fe isotopes track valence changes, while Mg isotopes reflect melt–rock interaction during partial melting and metasomatism. Applied to ophiolites, these proxies offer detailed insights into mantle melting, melt transport, and redox evolution. The Bulqiza ophiolite massif in Albania, a major chromite-bearing ultramafic complex in the Balkans, presents an ideal natural laboratory to investigate mantle processes during the evolution of the Mesozoic Tethys Ocean. This study integrates mineral chemistry, LA-ICP-MS trace element data, Mössbauer spectroscopy, and Fe and Mg isotope analyses to reconstruct mantle compositional changes and the genesis of ultramafic assemblages. Harzburgites and dunites from Bulqiza show high forsterite (Fo_91–97) and Ni contents indicative of subsolidus equilibration between olivine and magnesiochromite. Two partial melting stages are identified: an initial ∼25 % melting forming harzburgite, followed by ∼30 % melting producing dunite and chromitite. Variations in spinel Cr# [100Cr/(Cr + Al + Fe³⁺)] and oxygen fugacity suggest that dunite-chromitite formation resulted from focused melt–rock interaction. Chromitites are enriched in Cr₂O₃ (53–61 wt%) and display variable FeO (12–21 wt%), reflecting mantle source heterogeneity and dynamic melt extraction. Isotopic trends, including a negative correlation between δ²⁶Mg and Cr# and a positive δ⁵⁶Fe-δ²⁶Mg relationship, indicate progressive melt–rock interaction and oxidation. Mössbauer spectroscopy reveals Fe³⁺/∑Fe ratios of 0.075–0.253 in magnesiochromite, documenting variable redox conditions during formation. Together, these data support a multistage genesis involving high-degree melting, melt percolation, and redox evolution within an upwelling asthenospheric mantle, consistent with localized mantle flow beneath a slow-spreading ridge or transform margin rather than a subduction-related setting.