Analyses across a mid-ocean ridge give the scale of plume-fed heterogeneity

Abstract

The primary spatial scale of heterogeneity in the Earth's upper mantle, introduced from plume sources thousands of kilometres in scale at the base of the mantle, is critical for understanding mantle convection but is poorly constrained. Here, we demonstrate a geochemical shift in a set of lava samples collected from sites spaced approximately 10 km apart perpendicular to the Central Indian Ridge, which shows the length scale for a cross-section of concentric asthenospheric flow through the Réunion plume. This conclusion is derived from a mixing model that incorporates all expected mantle endmembers and common mantle components to reconstruct Sr–Nd–Pb isotopes and trace element profiles of lavas. The mixing model revealed that more than 95 % of the geochemical fingerprints of lavas are dictated by interactions between depleted mid-ocean ridge basalt mantle (DMM) and the prevalent mantle component of the plume (PREMA). The remaining, more specific features are contributed by varying proportions of enriched mantle components for each volcanic stage, reflecting the proposed archetypal chemical structure of the plume source at the base of the mantle. The latter additional components are concentrated in the core of the asthenospheric plume flow, which provides the primary short-length-scale (<10 km) heterogeneity in the upper mantle. Material transport from the lower mantle to the upper mantle thus occurs on a scale one order of magnitude smaller than that of seismic detection. This result provides a high-resolution constraint on solid-Earth convection models.