Trace Element Geochemistry of MORB Glasses from 14°-16°N along Mid-Atlantic Ridge

Trace element compositions of MORB glasses from 14°-16° N along the Mid-Atlantic Ridge were investigated using both solution-based and in situ laser ablation-based ICP-MS technique. Precision (RSD)for both methods based is within 5 % for all trace elements with the exception of Pb, which averaged 12 %. The results from two methods reveal a good agreement with no systematic differences in abundances (<5% for all elements).

The new geochemical data confirm that basalts in the region of the MAR between 14°-16°N are characterized of isotopic and incompatible element enrichment. Due to the absence of a well defined mantle plume in the region, the nature of the enrichment has been the topic of significant discussion and speculation. Likewise the magma supply is probably small in the region as the magmatic crust is interpreted to be very thin in most of the area studied. In combination to previous studies of major element, trace element, and isotopic compositions of basalts, gabbroic rocks and igneous and residual ultramafic rocks in the region, the new data indicates that 1) the enriched basalts have positive Ta and Nb anomalies, enriched relative to U, Th, and La; 2) basalts have relatively high SiO2 abundances compared to the global average; 3) basalts show a HIMU isotopic signature; and 4) bulk major element abundances and mineral chemistry in mantle rocks indicate that they are among the most depleted, although variably refertilized, residual mantle assemblages sampled to date along MORs. We suggest that much of the regional variation in major and trace element data, as well as isotopic data and the unusual regional geology (multiple core complexes) can be explained by melting of a sub-axial mantle that contains two end members, one highly depleted and the other enriched. These components appear to involve ancient recycled ocean crust and lithospheric mantle.