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

Abstract

[1] 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, TiO 2 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.

Mineralogical and geochemical constraints on the shallow origin, ancient veining, and multi-stage modification of the Lherz peridotite

“…Nickel contents of olivines of site A and B generally range from 0.38 to 0.46 wt.% NiO, and extend to lower values (reaching 0.26 wt.%) in compositelherzolites of site A, with a maximum NiO content (0.54 wt.%) in a lherzolite spatially associated with hornblendite at site B (04LH04). This range of Mg# and Ni contents is similar to those reported for in olivine-websterites and peridotites of abyssal peridotites, forearc peridotites, ophiolite sequences, and cratonic xenoliths (e.g., Kelemen et al, 1998;O'Hara and Ishii, 1998;Suhr et al, 2003;Dick et al, 2010;Warren and Shimizu, 2010).…”

Orogenic, Ophiolitic, and Abyssal Peridotites

“…However, as pointed out by Suhr et al (2003), the interpretation of these channels as high melt flux, feeder conduits for MORB is questionable in several instances, and alternatives such as the formation of melt accumulation horizons due to compaction waves (e.g., Rabinowicz and Ceuleneer, 2005) must be considered. Similar processes can be envisioned for harzburgites, especially when they are closely associated with dunites (Gervilla and Remaïdi, 1993).…”

The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?

“…Both petrological and textural evidence are ambiguous. The composition of mantle minerals is known to be modifi ed by melt percolation (e.g., Kelemen et al, 1995;Suhr et al, 2003), and a former tectonic fabric acquired during mantle fl ow can be destroyed due to melt accumulation (Rabinowicz et al, 1987).…”