Compositional heterogeneity and melt transport in mantle beneath Mid-Atlantic Ridge constrained by peridotite, dunite, and wehrlite from Atlantis Massif

Abstract

Peridotitic, dunitic, and wehrlitic rocks drilled at Atlantis Massif (Mid-Atlantic Ridge), 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 transported parental melts of mid-ocean ridge basalt (MORB) beneath 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 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 materials of the Atlantis Massif bear at least two end-members with regard to the extent of melting: refractory mantle material with higher Cr#, and less refractory mantle material with lower Cr#. To quantitatively evaluate mantle melting and melt transport mechanisms in the suboceanic mantle beneath the section of the Mid-Atlantic Ridge, a one-dimensional, steady-state decompressional mantle melting model was carried out employing rare-earth element concentrations of clinopyroxene. Our modeling results demonstrate the presence of refractory peridotites molten in the presence of garnet, but crustal materials are limited in the Atlantis Massif. We advocate that the refractory mantle materials underwent little recent melting beneath the paleo-ridge axis, and alternatively had been subjected to ancient partial melting. The refractory mantle materials probably contributed to the initiation of the detachment fault and affected the extreme variations in MORB chemistry in the Mid-Atlantic Ridge.