Magma genesis and mantle sources at the Mid-Atlantic Ridge East of Ascension Island
Results from the Meteor 41-2 cruise to the Mid-Atlantic Ridge (MAR) between 6° and 11°S, are reported. Six spreading segments, which are bounded by non-transform offsets were discovered and numbered from north to south. Anomalously shallow segments (Segment 4 and 5) occur between ~9°S and 11°S, pointing to an excess volcanism in this area. For detailed geochemical and petrological investigations samples (mostly glasses) were taken every 5-10 km. The cause of the melting anomaly beneath Segments 4 & 5 could be either a hot mantle plume or a cold mantle heterogeneity. A mantle plume with a low excess temperature of about 50° C (Minshull et al., 1998) is unlikely, instead melting of a H2O enriched, fertile mantle is proposed. Higher H2O and incompatible element concentrations are known to lower the mantle solidus significantly. HREE depletion mainly in Segment 5 lavas suggest initial melting in the garnet peridotite stability field.