Podiform chromitites do form beneath mid-ocean ridges

Abstract

Podiform chromitites are commonly found within the Moho transition zone to mantle section of ophiolites, as well as in non-ophiolitic peridotite massifs. However, they have rarely been observed in the present-day ocean floor, even though some ophiolites are considered slices of oceanic lithosphere. One of the factors controlling podiform chromitite formation is the chemistry of the host mantle peridotite. A moderately refractory harzburgite that contains chromite with an intermediate Cr# (Cr/(Cr+Al)) of 0.4–0.6 is the optimum host for chromitites. Such a harzburgite represents the most typical lithology of oceanic lithosphere where peridotite–melt reactions, another requirement for chromitite formation, are possibly common. Thus, the oceanic upper mantle is potentially a suitable host for podiform chromitites. In particular, off-ridge magmatism may lead to the formation of podiform chromitite. The apparent rarity of chromitites in the present-day ocean floor is simply a reflection of the under-sampling of mantle material from fast-spreading ridges and the center segment of slow-spreading ridges. However, in addition to ophiolitic chromitites forming at ordinary mid-ocean ridges from genuine MORB, they also form in the typical ophiolite tectonic setting, at supra-subduction zone spreading centers, from wet MORB. Future mantle drilling as an active way of sampling, on the ocean floor will possibly reveal the occurrence of podiform chromitite in present-day oceanic lithosphere, analogous to ophiolitic chromitite.