The Marion and Bouvet Rises: Remelting Gondwana’s Mantle

Abstract:

Major, trace element and isotopic data along the SW Indian Ridge identify two major geochemical provinces centered on the Marion and Bouvet Hotspots with prominent correlations between the isotopic and major element composition of basalts, ridge depth, and mineralogy of spatially associated mantle peridotites. Both consist of axial rises with elevated ridge topography. Both the Bouvet and Marion Hotspots have small volcanic fluxes, while the associated axial-rises contrast sharply in size and geochemistry. The Bouvet Rise is small, proportional to the size of the hotspot, while the Marion, with Iceland, is one of the two largest oceanic rises. A mantle plume associated with the Marion Hotspot is incapable of supporting the rise; rather it appears to be supported by a large region of anomalously depleted mantle¹. The Bouvet Plume, which likely originates above the mantle transition zone, appears to have had a direct control on the geometry of the western SWIR through time and thus likely does support the short Bouvet Rise. The bathymetric contrast between these two rises corresponds to notable differences in isotopic geochemistry: while the Marion Rise basalts exhibit a complex pattern of variability – it is a region where the classic ‘Dupal’ anomaly is well expressed, consistent with a major Archean mantle source that likely represents delaminated metasomatized sub-continental lithosphere entrained in the shallow mantle beneath the Ridge during formation of the central and SW Indian Ridges with the breakup of Gondwana. This is supported by a similar isotopic anomaly along the Rodriguez Rise on the Central Indian Ridge that also appears related to Gondwana breakup. By contrast, the mantle beneath the Bouvet Rise appears to represent largely post-Archean asthenosphere pulled from beneath the Gondwana lithosphere, interacting with a local small plume reflecting a local heterogeneity in the upper mantle.

1. Zhou, H.-y., Dick, H.J.B., 2013. Thin crust as evidence for depleted mantle supporting the MarionRise. Nature 494, 195-200.