Compositional variability and alteration dynamics of chromitites under greenschist metamorphism in the Kızıldağ ophiolite, southern Türkiye

This study presents a complete alteration sequence in chromitites from the Kızıldağ ophiolite (southern Türkiye), documenting three distinct chromite types that record processes related to Neotethyan subduction. Chromitites exhibit predominantly lenticular forms and occasionally banded textures, embedded within mantle peridotites and the mantle-crust transition zone, specifically harzburgite and dunite. Our analysis categorizes chromite grains into three groups: fresh, altered, and porous. Fresh chromites are characterized by high contents of Cr₂O₃ ranging from 42.69 to 57.08 wt% and Al₂O₃ from 12.46 to 25.34 wt%. In contrast, altered chromites, typically located along fractures or at the edges of fresh grains, show significant variability in chemical composition, with Cr₂O₃ content between 43.34 and 63.94 wt% and Al₂O₃ between 2.91 and 14.84 wt%. Porous chromites, distinguished by their porous structure, present Cr₂O₃ contents from 44.58 to 52.66 wt% and Al₂O₃ from 3.00 to 8.38 wt%. Notably, altered and porous chromites demonstrate substantial enrichments in Fe and trace elements such as Mn, Ni, Zn, Co, and Ga, indicating a shift due to metamorphic processes. The observed high Cr# (0.68–0.93) and low Mg# (0.25–0.65) values in rims of the altered and porous chromite further illustrate the metamorphic influence on chromite, with greenschist-facies conditions indicated by the Cr–Al–Fe³⁺ diagram at temperatures below 450 °C. The alteration dynamics reflect a progression that initiates with the olivine matrix and advances through reactions involving fresh chromites. This progression results in mass loss, the formation of chlorite, and the porous structures. Our findings contribute to the understanding of metamorphic alteration processes in chromitites and provide a detailed account of the geochemical trends (such as Fe/Mg exchange and trace element enrichment) that reflect greenschist-facies metamorphism. The evidence of chlorite formation and the prevalence of porous textures highlight the influence of metamorphic fluids over serpentinization, offering a model for chromite alteration in supra-subduction zones. This research elucidates the metamorphic history of chromitites in the Kızıldağ ophiolite.