Revealing mantle heterogeneity in a cold intra-transform spreading segment (7-8°N at Mid Atlantic Ridge)

Abstract

The Doldrums transform system (TS), located in the Equatorial Mid Atlantic Ridge (MAR) at 7-8°N, is a 110 km-wide multi-fault shear zone, with five active transform faults separated by four short intra-transform ridge segments (ITRs). The medial ITRs are substantially deeper than the peripheral rift segments, which indicate differences in the thermal conditions of the sub-ridge mantle. New chemical and radiogenic isotope data from on-axis lavas erupted across the entire transform domain reveal that the basalts from the shortest and deepest ITRs are enriched comparatively in alkalis (Na2O+K2O= 4.3 wt%; Na8 up to 3.7) and light rare earth elements (La/Sm)N = 0.86 -0.97), likely suggesting the presence of an extremely cold mantle region characterised by low degrees of partial melting. The enriched incompatible element compositions, however, are coupled with the lowest Sr and Pb isotopes in the Equatorial Atlantic magmatism (i.e., 87Sr/86Sr ~ 0.70237 and 206Pb/204Pb ~ 18) and relatively high Nd and Hf isotope ratios (143Nd/144Nd = 0.51315-0.51325; 177Hf/176Hf = 0.2832-0.28325), which indicates that incompatible element enriched components are less abundant in the mantle source of the central ITRs. Hence we infer that the mantle under the central ITRs has been melted at the MAR axis before being transported laterally into the central ITR domain during the formation of the Doldrums transform system. This mantle portion melted a second time, and to a low extent, during the opening of the cold ITR, revealing its depleted geochemical character. Therefore, MORB from intra-transform ridge segments provide a rare opportunity to constrain the isotopic composition of the depleted peridotitic mantle, a ubiquitous, but otherwise often concealed component of Earth’s mantle.