Mantle podiform chromitites do not form beneath mid-ocean ridges: A case study from the Moho transition zone of the Oman ophiolite

Abstract Podiform chromitites from the mantle transition zone at Maqsad in the Oman ophiolite have MORB-like cr# (0.48–0.59). However they show other features which are not in keeping with a MORB-origin: some parental melts have TiO2 values lower than typical MORB, they have a higher Fe3 +/ΣFe ratio and a wider range of Fe3 +/ΣFe ratios than might be expected in MORB and their parental melts have a higher fO2 than expected for MORB. The chromitites also contain silicate melt inclusions which are hydrous and atypical of MORB and inclusions of higher cr# chromites (cr# = 0.55–0.71). These inclusions are derived from a more depleted harzburgite source than that of the harzburgites which now host the Maqsad chromitites. In addition associated MORB-like lavas display geochemical traits which are atypical of MORB. It is proposed therefore that the Maqsad chromitites were derived by mixing from parental melts which were produced by the interaction of a MORB-like melt and depleted mantle. The MORB-like melt was produced by the hydrous melting of asthenospheric mantle above a subducting slab. The trigger for chromite to appear on the liquidus of the melt was the reaction between the primary melt and depleted harzburgite, illustrating the more general relationship in which chromite appears on the liquidus in mafic and ultramafic melts as a consequence of some form of mixing process. Our preferred tectonic setting for the genesis of the Maqsad chromitites is in the context of subduction initiation in a forearc environment. We would argue on the basis of their geochemistry that they have not formed beneath a spreading ridge, despite the superficial similarity between their cr# and those of MORBs. In the absence of podiform chromitites in modern ridge environments and from ‘truly MORB ophiolites’ we propose that the very occurrence of podiform chromitites is a tectonic indicator of a subduction-related environment.