A Slow Spreading Ridge with Ultra-Slow Melt Supply Characteristics: The Mid-Atlantic Ridge at 12°-16°N

The region between 12^°N and 16^°N on the Mid-Atlantic is characterized by large numbers of extensional core complexes exposing ultramafic and gabbroic rocks suggesting a thin overall crustal component and low melt supply. Sampling programs from previously surveyed have been expanded to include four newly discovered core complexes (Smith et. al., 2006) and adjacent ridge segments between 14 and 12^°30'N on a cruise of the research vessel James Cook in March-April 2007. We examine the distribution of ultramafic and gabbroic lithologies sampled in the entire region, the newly discovered core complexes, ridge morphology, and the results of geochemical analysis of basalts, gabbroic rocks and mantle assemblages. Basalt geochemical characteristics show variations symptomatic of high degrees of melting and mantle compositions are among the most strongly depleted along the MOR. However, evidence of mantle unroofing and a thin magmatic crust persists throughout a broad area indicating a low average melt supply. Integrated studies of major element, trace element, and isotopic variations among basalts, gabbroic rocks and igneous and residual ultramafic rocks in the region indicate that 1) the enriched basalts have positive Ta-Nb anomalies, enriched relative to La and Th, 2) basalts have relatively high SiO2 abundances compared to the global average, 3) basalts show a HIMU isotopic influence, and 4) bulk major element abundances and mineral chemistry in mantle rocks indicate that they are among the most depleted, although variably refertilized and metasmomatized, residual mantle assemblages sampled to date along MORs. The strong regional variation in major and trace element data and isotopic data and the unusual regional geology reflect magma supply that can be explained by melting of a sub-axial mantle that contains various proportion of two end members, one highly depleted and the other enriched. These components appear to involve ancient recycled ocean crust and lithospheric mantle.