Long-Lasting Influence of the Discovery Plume on Tholeiitic Magmatism in the South Atlantic: Data on Basalts Recovered by Hole 513a, DSDP Leg 71

The paper presents the very first data on concentrations of major and trace elements; Sr, Nd, and Pb isotopic ratios of rocks; and the composition of olivine phenocrysts of 38-Ma basalts recovered by Hole 513a (DSDP Leg 71) in the South Atlantic. The bulk-rock samples and the chilled glasses are mildly magnesian (7–8 wt % MgO) and bear elevated FeO and low Na₂O concentrations, as is typical of MORB of the TOR-1 type. Olivine phenocrysts (Fo_84.5–88) in these rocks contain concentrations of trace elements (Ni, Mn, Cr, and Zn) that are typical of classic MORB, which are produced by partial melting of mantle peridotite. The rocks are strongly depleted in incompatible elements [(La/Sm)_n ~ 0.6] but have elevated Ba/Nb, K/Nb, and Pb/Ce ratios and Cu, Ag, and Au concentrations that are 1.5–4 times higher than in typical depleted MORB (N-MORB) and in most rift basalts in the South Atlantic. Isotope compositions of the basalts (average ratios ²⁰⁶Pb/²⁰⁴Pb ~ 18.0; ²⁰⁷Pb/²⁰⁴Pb ~ 15.6, ²⁰⁸Pb/²⁰⁴Pb ~ 38.0, ¹⁴³Nd/¹⁴⁴Nd ~ 0.5130, and ⁸⁷Sr/⁸⁶Sr ~ 0.7040) are close to those in modern tholeiites from the southern MAR segment (SMAR) north of the Agulhas Fracture Zone. The data indicate that the magmas were derived from a strongly depleted mantle source that contained a minor (~3%) admixture of an enriched component, which is discernible in the magmas of the Discovery hotspot. The composition of the source, which is more depleted than DM, and the high degrees of melting of this source explain why the basalts from DSDP Hole 513a are enriched in chalcophile elements. It is believed that spreading magmatism at 45°–48° S in SMAR as far back as 40 Ma was already affected by the Discovery hotspot. This hotspot might be related to the Tristan plume system, and its origin and long-lasting influence on spreading magmatism in the South Atlantic are regarded as evidence of the extensive effect of the Tristan plume.