New chemical and isotopic data for basalts from the axial segment of the Mid-Atlantic ridge between the Vema and Mercury fracture zones

Abstract

The whole-rock geochemistry and isotopic composition of the basalts dredged from the Mid-Atlantic Ridge (MAR) segment between the Mercury and Vema fracture zones during cruise 32 of the R/V Professor Logachev were studied. In addition to typical basalts with moderate petrochemical parameters, there are high-Ca and low-Na types of basalts representing higher degree melts and high-Fe varieties generated at great depths. The basalts derived by high degrees of melting exhibit a close spatial association with the central portions of three on-axis rises on the rift valley, which reach a height of 300–500 m above the floor of the valley and represent the loci of the subaxial mantle upwelling. The position of the Northern rise at the northern intersect of the Mercury transform fault is inconsistent with focused upwelling. The basalts with a more radiogenic Nd, Sr, and Pb composition are identified above the loci of the subaxial upwelling. They are compositionally similar to basalts, which have HIMU-like affinities and are interpreted to be derived under the influence of the 14° N plume between the Marathon and Cape Verde fracture zones. The relatively high isotopic ratios of these basalts anti-correlate with their low (La/Sm) _n values typical of the depleted N-MORB-type basalts. The results show that enriched melts contributing to basalts from the central portions of on-axis rises are associated with one of the flows of plume material channeled from the 14° N anomaly to these rises. The partial melting of the rising plume material causes the formation of localized accumulation of magmas at structural barriers and creates new centers of upwelling, which, being enhanced by the regular upwelling, lead to increased magma production and changes in the composition of basalts.