Origin of podiform chromite deposits by multistage melting, melt segregation and magma mixing in the upper mantle

Abstract

Chromite in upper mantle environments appears to result from incongruent melting of pyroxenes in addition to being the product of direct crystallization from partial melts in small magma pockets. Partial melts, produced as a result of differing degrees of partial melting over a large region in the upper mantle beneath a spreading ridge, ascend in small channels due to differential buoyancy. En-route to higher levels, these partial melts coalesce and mix in small magma pockets as they are progressively drawn towards the ridge axis. Boninitic melts resulting from re-melting of refractory residuum near the ridge axis may interact with the evolved MORB-liquids to cause chromite crystallization. Chromite concentrations form by accumulation of chromite at the bottom of small magma chambers and are incorporated into the surrounding refractory dunite as the magmas continue to rise before entering the lower crustal magma chamber. Chromite formed in this way could be aggregated into pods as a result of confining pressure imposed upon them. The residuum, consisting of chromitite and dunite, will move away from the ridge axis and cool. The nature and distribution of the podiform deposits are determined primarily by the timing and location of their formation within the mantle column undergoing multistage melting and secondarily by the plastic deformational events accompanying and/or following melt extraction and crystallization events.