Invited talk: Scale and nature of the mantle heterogeneities at the Vema Lithospheric Section (Mid Atlantic Ridge 11°N)

A 26 Ma-long mantle stretch has been sampled at the Vema Lithospheric Section (VLS) (11° N along the MAR), providing a unique opportunity to analyze inside the mantle heterogeneities and unravel their effect on the melting regime looking at correlations between rock geochemistry and the amount of melt produced (crustal thickness). The geochemistry of mantle residua sampled at the base of the VLS reveals an overall positive correlation with the magmatic crustal thickness, in that suggesting a thermal/compositional control of the melting process. However short (sample-) scale heterogeneities appear when considering the difference between the degree of melting inferred from crustal thickness (integrated extracted melt) and the actual degree of melting calculated per each sample on the base of major and trace elements. Accordingly, the isotopic systematics of the peridotites show a sample scale strong variability, averaging close to the MORB value at the Ma scale. The observed discrepancy reveals that some samples are more depleted than the expected average degree of melting, suggesting the presence of mantle parcels that have undergone an ancient depletion event prior the upwelling and incorporation into the subridge melting region. Less depleted parcels are also present. These blobs are possibly the result of spinel-field refertilization at the end of the melting region. However, an intriguing inverse correlation between isotopic and major element systematics suggests that even such heterogeneities could have formed beneath the melting region possibly by local metasomatism that heterogeneously affected the deep mantle. Pyroxenite-derived melts may have reacted and refertilized some mantle portions then driving the subsequent melting process in the main melting region. This process can explain the paradox that the isotopically most depleted samples (those less metasomatised) are also those that record the lowest degree of melting beneath the ridge axis. A major, first-order, heterogeneity appears in the composition of the residual phases allowing to recognize a source discontinuity that cannot be related to the subridge melting event. Mantle samples older than 18.5 Ma have higher cpx Na and Al and lower Ca for a given Mg value than younger samples. Both domains are characterized by a normal mantle depletion trend correlating overall with the produced magmatic crust.