Serpentinized peridotites and gabbros in the Mid-Atlantic Ridge axial valley at 15°37′N and 16°52′N

Abstract

Serpentinized peridotites dredged in the Mid-Atlantic Ridge axial valley at 15°37′N and 16°52′N during the ridelente cruise of R.V. Jean Charcot are mantle-derived harzburgites, plastically deformed in low stress/high temperature conditions similar to those inferred for the asthenospheric mantle. This early asthenospheric event was followed by ductile, then brittle, deformation in higher stress and progressively lower temperature conditions. We argue that this deformation occurred during the tectonic uplift of the peridotites throughout the lithosphere of the axial region. The ultramafics studied have mineral compositions similar to those of other mantle-derived Mid-Atlantic Ridge harzburgites. Strongly depleted mineral compositions in samples from the 15°37′N dredge could reflect a high degree of mantle partial melting. Emplacement of peridotites in the seafloor at mid-ocean ridges may not therefore be systematically correlated with low degrees of mantle partial melting. Serpentinized peridotites dredged at 15°37′N were intruded by evolved Na- and Fe-rich apatite and zircon-bearing gabbros. These gabbros were emplaced after the ductile deformation of the peridotites, and prior to their extensive serpentinization. Similar intrusive relationships between extensively fractionated gabbros and mantle peridotites have been observed in samples from the Mid-Atlantic Ridge axial valley south of the Kane transform, in dredged samples from Southwest Indian Ocean fracture zones, and in western Alps ophiolites. We propose that such intrusive relationships are characteristic of magma-starved spreading environments, in which stretching of the axial lithosphere may locally lead to emplacement of peridotites and gabbros in the seafloor.