Keywords:IODP Exp 357, Mid-Atlantic Ridge, Harzburgite, Melt transport, MORB, ultra-depleted melt
Harzburgitic, dunitic, and wehrlitic rocks drilled at Atlantis Massif (Mid-Atlantic Ridge, 30°N), where mantle-derived rocks were exhumed to the sea floor via detachment faulting, were investigated employing in situ geochemical analyses. Since dunitic and wehrlitic rocks are commonly interpreted as fossil melt channels, they likely represent transported parental melts of mid-ocean ridge basalt (MORB) beneath the paleo-ridge axis. Although the rocks are severely serpentinized, primary olivines, chromian spinels, orthopyroxenes, and clinopyroxenes were found in several samples. The Cr/(Cr + Al) ratios (Cr#) of the chromian spinel show a bimodal distribution, where one group records higher Cr# (0.35–0.5) and the other group is represented by lower Cr# (0.2–0.3). The extent of mantle melting is strongly correlated to the Cr# of the chromian spinel, thus the mantle-derived lithologies of the Atlantis Massif record at least two end-members with regards to the extent of melting: refractory mantle material with higher Cr#, and enriched mantle material with lower Cr#. To quantitatively evaluate mantle melting and melt transport mechanisms in the suboceanic mantle beneath this section of the Mid-Atlantic Ridge, a one-dimensional, steady-state decompositional mantle melting model was carried out based on rare-earth element concentrations of clinopyroxene. Our modelling results demonstrate that the melting region of the mantle extends unevenly across the ridge axis, with deeper melting occurring on-axis from the garnet-stability field, and shallower melting occurring off-axis from the spinel-stability field. We propose that depleted melts generated at depths in the absence of garnet within the off-axis region were possibly focused to the ridge axis via fracture-induced melt channels and produced wehrlite. It is likely that fracture-induced melt migration was dominantly a response to a decrease of temperature as the mantle was exhumed during detachment faulting.