Variations with age of ultramafic composition near the Vema fracture zone, Central Atlantic

A relatively undisturbed sliver of oceanic lithosphere is exposed on the sea floor for a distance of about 300km along a sea floor spreading flow line near the Vema Fracture Zone in the Central Atlantic. Assuming a spreading half-rate of 15mm/y, the exposed lithospheric sliver represents an almost 20 m.y. time interval of accretion at the axis of the Mid-Atlantic Ridge. The basal, mantle ultramafic unit of this lithospheric section was sampled at close-spaced (~6-8 km) intervals for a stretch of almost 200 km, corresponding to roughly 13 m.y. of accretion. A study of these samples, which is in progress, will give us information on temporal variation of the thermal structure and composition of the upper mantle below a segment of the Mid-Atlantic Ridge. The first results, involving the major and trace element composition of mantle-equilibrated minerals; olivine, orthopyroxene, clinopyroxene and spinel in porphyroclastic peridotites suggest both long (several m.y.) and short ($<$ 1m.y.) wavelength systematic variations that reflect temporal variations in the composition and extent of melting of the mantle below the Mid-Atlantic Ridge. Anphibole-bearing ultramafic mylonites are dominant in a stretch of the section roughly corresponding to a 2 m.y. time interval, that may correspond to a period of prevalent a-magmatic extension at ridge axis.