Oxygen Isotopes of Cr-Spinel as a Pathfinder for Metasomatism in Mantle Peridotites

Abstract Debate continues over whether Cr-spinel in mantle peridotites is formed by partial melting or metasomatism. This first systematic report of SIMS oxygen isotope data for the constituent minerals, particularly Cr-spinel, in chromitites, dunites and harzburgites from the mantle sequence of the Lycian ophiolite (SW Türkiye) provides compelling evidence for the latter. The δ18O values of Cr-spinel are relatively low across the three rock types, ranging from 2.00–4.81‰ in chromitites, 1.04–3.73‰ in dunites, and 1.89–4.87‰ in harzburgites. The δ18O values of olivine are relatively higher and more uniform, decreasing from chromitites (5.25–6.68‰) to dunites (4.99–5.65‰) and harzburgites (4.90–5.44‰). The δ18O values of orthopyroxene and clinopyroxene in the harzburgites are comparable, ranging from 5.18 to 6.63‰ and 5.14 to 6.56‰, respectively. Oxygen isotopic compositions of minerals exclude any influence from low-temperature alteration. Cr-spinel in the chromitites shows decreasing δ18O with increasing Cr# and lower δ18O when amphibole is present, suggesting that magma differentiation significantly influences Cr-spinel composition, while the presence of water can significantly lower the Cr-spinel δ18O. The δ18O values of olivine in chromitites increase with increasing Fo contents, indicating re-equilibration between olivine and Cr-spinel. In harzburgites, Cr-spinel shows a negative correlation between δ18O and Cr#, similar to the characteristics of Cr-spinel in chromitites. The negative correlation between δ18O values and Cr2O3 contents in clinopyroxene excludes the possibility of a partial melting origin. Petrographically, the Cr-spinel in harzburgites is commonly associated with clinopyroxene and contains various mineral inclusions, supporting the notion that Cr-spinel forms via metasomatism. The occurrence of olivine veins in orthopyroxene and the polymineralic (silicates + Cr-spinel) pseudomorphs of orthopyroxene are evidence for significant modifications. Therefore, mineral compositions of harzburgites in ophiolites should be attributed to metasomatism, while their refractory characteristics differ from those produced by refertilization. Notably, Cr-spinel oxygen isotopic compositions exhibit greater sensitivity to such metasomatism, contradicting beliefs of their high resistance to post-formation modification. Our study highlights the utility of oxygen isotopes in deciphering the complex petrogenetic history of mantle rocks, particularly in identifying metasomatic Cr-spinel and associated silicate minerals in peridotites.