Characterization of stratiform-like chromite deposits in ophiolites and their genetic link with podiform chromitites

To investigate the formation of ophiolitic stratiform-like chromitite occurring in the mantle–crust transition zone (MCTZ) and its genetic link with podiform chromitite, we studied samples from the Bursa ophiolite (NW Turkey) and compiled data on chromitite and dunites from other typical ophiolites. The stratiform-like chromitites in the Bursa ophiolite consist of rhythmically alternating chromite and olivine bands, with chromite displaying similar major element compositions to those in the mantle sequence (e.g. Cr# [Cr/(Cr + Al) × 100]) but higher Mn, Co, Sc and Ti contents, and lower V/Sc ratios. This suggests that the chromite in ophiolitic stratiform-like chromitites crystallized from evolved melts/fluids with a higher oxygen fugacity than podiform chromitites. In addition, profile analyses reveal a sharp compositional change in chromite and olivine near the contact between the chromite band/chromitite and the olivine band/dunite in banded chromitite from the Bursa ophiolite, and mantle transect amphibole-bearing chromitite–dunite collected from the Lycian ophiolite, SW Turkey. Combined with the gradual compositional variation of olivine in the olivine band/dunite section, the melt/fluid-mediated modification of olivine is required. The invariable composition of olivine in the amphibole-containing profile 18LN07-9 suggests that the fluids released from chromite surfaces serve as a good candidate for modification medium, as the crystallization of interstitial amphibole from penetrating fluids impeded further modification. Similarly, the analogous compositional trends observed in the Bursa ophiolite can also be attributed to the fluid–olivine interaction. This is supported by the positive correlation between the extent of serpentinization with the modal content of chromite in chromitite, implying that the fluids resulting from the wetting property of chromite were also involved in forming stratiform-like chromitites. Therefore, we tentatively propose a genetic link between podiform and stratiform-like chromitites: in the mantle sequence, podiform chromitites formed cumulates within magma conduits. Here, the hydrous fluids collected on the chromite surfaces penetrated into the surrounding peridotite, modifying the composition of pre-existing minerals and crystallized hydrous minerals such as amphibole. Subsequently, evolved melts migrated upward into the MCTZ and crystallized the stratiform-like chromitites in the magma chamber at the crustal level. The fluids released from chromite surfaces play a significant role in modifying mineral compositions in both podiform and stratiform-like chromitites.