New isotopic data for Mid-Atlantic Ridge basalts from the Arkhangelsk-Sierra Leone fracture zone (central Atlantic)

Abstract

This study presents Sr, Nd, and Pb isotopic data for basalts collected from the MAR axis between the Arkhangelsk transform fault and 5° N. The isotopic data for basalts from the MAR segment between the Arkhangelsk and Vernadsky faults indicate the presence of N-MORB depleted tholeiites, which are similar to some basalts from the MAR segment between the Cape Verde and Marathon faults, at the location of the 14° N geochemical anomaly. Broad positive along-axis gradients in isotopic values (and negative for ¹⁴³Nd/¹⁴⁴Nd) are observed from south to north, superimposed by isotopic variations in the 9.6° N anomalous basalts. A north-south increase in (La/Sm) _n suggests a smaller degree of partial melting. The range of isotopic variations in basalts south of the Bogdanov fault is similar to that of basalts recovered at the more southerly 1.7° N anomaly. P-MORB compositions with a large proportion of the HIMU-like component are widely distributed in the vicinity of the 5.4° N rise. The isotopic ratios decrease gradually (with increasing ¹⁴³Nd/¹⁴⁴Nd) farther to the north toward the Bogdanov transform fault. A similar trend is observed for (La/Sm) _n . Therefore, the basalts recovered to the south and north of Bogdanov fault have different sources of radiogenic isotope enrichment: 5.4° N and 9.6° N rises. Basalts recovered to the north of the Vernadsky transform fault have higher ¹⁴³Nd/¹⁴⁴Nd and lower ²⁰⁶Pb/²⁰⁴Pb at similar ⁸⁷Sr/⁸⁶Sr ratios. We conclude that the localized manifestations of enriched basalts with isotopic characteristics close to the HIMU-like plume-derived component, including the 14° N and 1.7° N geochemical anomalies, may have been formed by microplumes entrained at the base of the upper mantle by asthenospheric mantle flows arranged into a system of cylindrical convective cells, 450–500 km in diameter. Northward of the Bogdanov transform, the upper mantle is contaminated with lower continental crust material, which was entrained during tectonic erosion of the lithosphere beneath the equatorial segment of Gondwanaland during its late stage opening.