| In this thesis I address two major issues that relates to the formation of the oceanic lithosphere at Mid Ocean Ridges (MOR), namely (1) how do processes of lithosphere formation vary through time at a given MOR segment and (2) what genetic links can be established between subridge mantle peridotite (presumed parent) and crustal basalt (presumed daughter), in terms of Sr-Nd isotope systematics.; These two questions are addressed through geochemical and geophysical studies of an uplifted, relatively undeformed sliver of oceanic lithosphere (Vema Lithospheric Section or VLS), exposing basaltic crust and peridotitic upper mantle, and recording a >26 Myr history of lithosphere generation at a segment of the Mid Atlantic Ridge (10-11°N, central Atlantic). My dissertation shows that slow-spreading oceanic lithosphere forms through short-period (3-4 Myr) dynamic pulsations of mantle upwelling and melting (as detected from the temporal variations of degree of melting of the mantle peridotites paralleled by variations of crustal thickness calculated from gravity data). Superimposed on these short wavelength oscillations, long-period trends were detected with decreasing degree of melting from 26 to 18.5 Ma followed by a steady increase from 18.5 to 2 Ma. Ultramafic mylonites found in a ∼2 Myr interval between the two trends suggests a nearly amagmatic event at the boundary between the two mantle thermal regimes.; Small-scale heterogeneities of the mantle exposed along the VLS were detected in the Nd and Sr isotopic composition of clinopyroxenes separated from the peridotites, in contrast with >20 Myr of constant isotopic composition of the associated basaltic glasses. This behavior suggests that the heterogeneities in the mantle peridotites were homogenized in the basalts emplaced at the ridge axis, because the basalts are the integrated products of melts released from a broad subaxial mantle region.; In contrast to the VLS peridotites, highly depleted peridotites drilled at the 15°20' Fracture Zone appear to record ancient depletion events and appear to have been recently impregnated by melts that have isotopic affinities with recycled ancient continental mantle. |
