Significance of large, refractory dunite bodies in the upper mantle of the Bay of Islands Ophiolite

Abstract

The origin of large mantle dunites is considered critical for models of melt migration in the mantle. Their presence is not compatible with formation synchronous to a fracture‐related melt transport event. In models of porous channel systems for melt transport, they represent a strongly coalesced, high‐flux conduit. Dunites from the lower parts of the mantle sections in the Bay of Islands Ophiolite are investigated by detailed geochemical traverses and with single samples. Dunites tend to cluster in the sense that several smaller dunites are associated with larger dunites or several dunites occur together. The chemistry of the large bodies is very depleted (Mg# in olivine 92–94, CaO in olivine 0.05–0.08%, Cr# [100 Cr/(Cr + Al)] in spinel 65–85, TiO2 in clinopyroxene 0.01–0.04%, Sm/Yb 0.2 to 0.7 relative to N‐MORB). Detailed traverses across the dunites commonly show a decrease of NiO in olivine associated with an increase in the Mg# along the harzburgite‐dunite boundary. Internally, dunite bodies are nearly homogeneous. Thickness of dunite bodies correlates with chemistry, in particular Mg# in olivine and probably Cr# and ferric iron in spinel, but not NiO in olivine. Incompatible element data for the largest dunites argue for their formation by an extremely depleted, high Mg# (boninitic?) melt. We suggest that integrated refractory melt: rock ratios in the largest dunites (up to 40 m) were below 8, because of a low abundance of refractory melts in the crust, and a lack of a systematic change of NiO in olivine with dunite width or across single dunites in detailed chemical traverses. Tectonically, the formation of depleted melts in a late stage of the spreading center is indicated. Their melt feeders failed when approaching the base of the mantle lithosphere and generated large dunites as replacive bodies. The latest expression of this magmatism are orthopyroxenite dykes, in part draining the large dunites. Since the large majority of all deeper mantle dunites are of refractory chemical nature and not akin to MORB, we caution as universally taking large dunite bodies to represent deep‐reaching channels with high melt flux and to take the abundance and size distribution of all dunites in an ophiolitic mantle section to infer melt migration mechanisms. In the Bay of Islands Ophiolite, the largest dunites in the mantle section appear to have little to do with the main constructional stage of the spreading center.