Ophiolites: ancient oceanic lithosphere?

Trace elements and Li isotopic compositions of olivine from the mantle-crust transition zone of the Bursa ophiolites (including Orhaneli ophiolite and Harmancık ophiolite) in NW Turkey were measured to constrain the genesis of these dunites and chromitites. A cumulate origin for dunite can be ruled out due to the depletion of incompatible trace elements (Zr, Ti, and heavy rare earth elements) in olivine, instead the chemical signatures point to a replacive origin via melt-rock interaction. The olivine grains in the dunites have lower MnO (0.06–0.15 wt%), Co (106–137 ppm), and higher NiO (0.23–0.44 wt%) concentrations than olivine phenocrysts in MORB, suggesting these transition-zone dunites have equilibrated with extremely depleted melts. Additionally, the relatively small δ7Li variations of olivine (average δ7Li +4.8 to +8.7‰) of the Orhaneli suite indicate the Li isotopic compositions of melts percolating through these dunites are relatively homogeneous. However, the large δ7Li variations of olivine (−2.5 to +20.3‰) in Harmancık dunites can be explained by incomplete diffusive equilibration with melts percolating through these dunites, suggesting infiltration happened not long before obduction of the ophiolite. Olivine in chromitites has higher Fo (92.6–94.7) than coexisting dunites, likely induced by subsolidus Mg-Fe exchange between olivine and chromite. The higher chromite contents of the chromitites can also explain the lower concentrations of Sc, V, Co and Zn in coexisting olivine grains. Mixing of depleted mantle-derived melts and boninitic magmas is suggested to induce a compositional shift from the olivine-chromite cotectic line to the liquidus field of chromite, causing the precipitation of chromite and formation of chromitite layers in the dunites. The heavy Li isotopic compositions (+6 to +11‰) of olivine in chromitites compared to MORB, together with the estimated compositions of parental magmas (Al2O3: 9.8–11.4 wt%; TiO2: 0.22–0.38 wt%) for the chromitites, indicate an arc-like geochemical affinity, hence a subduction-related setting in which these mantle-crust transition zones formed