Pb‐Hf‐Nd‐Sr isotope variations along the Galápagos Spreading Center (101°–83°W): Constraints on the dispersal of the Galápagos mantle plume

Abstract

We report on 45 Pb, Hf, Nd, and Sr isotope ratios in basalt glasses from the Galápagos Spreading Center (GSC) from 101°W to 83°W, along with related parent and daughter element concentrations. The purpose is to delineate the effect of the Galápagos mantle plume on this NE migrating spreading ridge and the nature of the plume dispersion in the region. Two 1000 km‐long Pb‐Hf‐Nd‐Sr isotope mixing gradients symmetrically distributed about 91.5°W are observed along the GSC axis. The gradients are nearly radially distributed about the center of the Galápagos plume, located presumably beneath Fernandina Island (91.5°W, 0.4°S) on the Nazca plate. A simple model computation taking into account spreading rate variation suggests that the along‐ridge integrated plume flux is some 15% greater west than east of the 91.5°W point of symmetry. This result is counter‐intuitive considering that (1) eastward plate drag is imposed by the moving Nazca plate relative to the plume, and (2) at equal radial distance from the plume center, the GSC axis east of the 91°W FZ is offset 110 km closer to the plume and is systematically some 500 m more elevated as a result of thermal effects. The plume flow toward the east is systematically more diluted than toward the west, probably due to greater entrainment of depleted upper mantle material during the ascent and bending of the plume conduit, such as suggested by Richards and Griffiths [1989] and White et al. [1993]. The lack of clear lithosphere damming effects on the isotopic gradients at transform faults suggests that entrainment of depleted upper mantle material has occurred prior to reaching the melting zone underlying the GSC axis. The decoupled Pb‐Hf‐Nd‐Sr isotope patterns observed along the GSC relative to those previously reported over the Galápagos platform on a smaller geographic scale further indicates a non‐steady state ascent and dispersion of the Galápagos mantle plume. GSC MORB compositions in Pb, Hf, Nd, and Sr isotope space suggest the presence of two distinct components in the Galápagos mantle plume, a HIMU type and an EM1‐like type, most likely representing, respectively, a recycled mix of oceanic crust ‐ lower mantle material and recycled continental‐derived material. Only 0.05 to 0.5% of the EM1 component is required if the continental‐derived material is terrigenous sediment, and an order of magnitude lower if it is recycled pelagic sediment. The EM1 component in the dispersing plume along the GSC is a factor of 1.5 to 1.75 richer west of 91.5°W than east of it. The possible presence of distinct protolith units in the Galápagos mantle plume could not be ascertained in the absence of any detectable ridge‐transform intersection (RTI) thermal effects on the Pb‐Hf‐Nd‐Sr isotope variations along the GSC, and the lack of correlation of the Pb‐Hf‐Nd‐Sr isotope ratios with major elements. If present, the protolith units must be smaller than the scale of RTI thermal effects on the ridge, and may have been eradicated by mixing during the melting, melt segregation, and storage processes taking place beneath the GSC.