Vema Fracture Zone (Central Atlantic): Temporal variations of mantle composition and of accretion processes at ridge axis

The Vema Fracture Zone in the Central Atlantic offers a unique opportunity for studying how the processes of formation of oceanic lithosphere vary with time. An uplifted, relatively complete and undisturbed section of oceanic upper lithosphere is exposed on the southern wall of the transform valley along a seafloor spreading flow line for a distance of about 300 km corresponding to almost 20 m.y. (if we assume an average spreading half-rate of 15 mm/y). Close spaced sampling (at 6-8 km horizontal intervals) of the basal portion of this exposed lithospheric section, for a stretch of almost 200 km, produced a set of mantle-derived peridotites serpentinized to various degrees. These samples preserve relicts of mantle-equilibrated "primary" phases such as olivine, opx, cpx and spinel and give us the opportunity of studying temporal variations of the composition of the mantle for a ~10 m.y.-long interval of time. The peridotites are mostly protogranular or porphyroclastic harzburgites and lherzolites. In a portion of the lithospheric sliver, corresponding to a 2-m.y. time interval, the ultramafic rocks are strongly deformed with amphibole-bearing mylonites prevailing. Preliminary electron and ion probe data on major and trace element chemistry of the mantle-equilibrated minerals in porphyroclastic peridotites, show significant compositional variations with age. For example, the Cr/Al ratio of spinel and the Al2O3 content of opx vary systematically with distance from ridge axis, suggesting both long- (several m.y.) and short- (< 1 m.y.) wavelength systematic variations in the composition and degree of melting of the upwelling mantle. These variations probably relate to systematic temporal changes of the thermal structure of the mantle below the Mid-Atlantic Ridge. Stretches of the lithospheric sliver where amphibole-bearing ultramafic mylonites are dominant may correspond to intervals of prevalent a-magmatic extension at ridge axis.