The significance of water in the genesis of ophiolitic chromitites

Many ophiolitic chromitites contain hydrous silicate phases, implying that hydrothermal processes were involved in their genesis. This observation challenges the standard magmatic/ melt-rock reaction model for mantle chromitite genesis. This review explores three possible relationships between hydrous fluids and the genesis of chromitites formed in the mantle section of ophiolites. The models are described and critiqued in the light of our current experimental, geochemical and field-based understanding of ophiolitic chromitites, using in particular evidence from chromitites from the well-studied mantle section of the Oman ophiolite.

Matveev and Ballhaus (EPSL, 2002) and more recently Su et al. (Science China Earth Sci., 2021) have proposed an immiscibility model in which chromite is preferentially partitioned into a hydrous fluid and thus separates from the parental silicate melt. Here it is argued that there are problems with the scalability of this model. In addition there are petrographic and geochemical inconsistencies which do not conform to the proposed hydrothermal fluid-silicate melt partitioning.

Johan et al. (Eur. J. Mineral., 2017) and Arai and Akizawa, (Amer. Mineral., 2014) have championed the view that chromite can be precipitated directly from a hydrothermal fluid. This is evidenced by chromite-diopsidites in the Oman ophiolite. Recent experimental studies also support this view through the complexing of Cr²⁺ in a chlorine-rich fluid. However, this model is also an inadequate explanation for the majority of mantle chromitites because chromite-diopsidite occurrences are extremely rare in the mantle section of ophiolites, of small volume, and the chromitites formed have a different composition from typical ophiolitic mantle chromitites.

For these reasons, the preferred model and the one advocated in this paper derives from the work of Edwards et al. (GSA Spec Pap. 2000) in which water is present as a dissolved species in the parental melt from which the chromitites form. It is argued that mantle chromitites are associated with hydrous MORB and boninitic melts in which the presence of water modifies the structure of the melt to permit an increased solubility of Cr during partial melting. The model is supported with Cr solubility data from wet mantle melting experiments and from the modelling of Cr partitioning during mantle melting, using data from harzburgites from the mantle section of the Oman ophiolite. As the hydrous melts percolate through mantle harzburgites they become more siliceous through orthopyroxene dissolution triggering chromite crystallisation. The same melts also crystallise the hydrous phases sodic amphibole and micas.