Shona and Discovery Aseismic Ridge Systems, South Atlantic: Trace Element Evidence for Enriched Mantle Sources

The Shona Ridge system (Shona Ridge–Meteor Rise–Agulhas Ridge–Cape Rise seamounts) and the Discovery Rise to the north have been argued to represent the surface expressions of the Shona and Discovery mantle plumes, based on enriched samples along the southern Mid-Atlantic Ridge, a single sample from Discovery Rise and tectonic reconstructions. Leg PS69 ANT XXIII/5 of the R.V. Polarstern dredged the first comprehensive suite of samples from the Discovery Rise and Shona Ridge system, comprising alkali basalt through trachybasalt to trachyandesite and trachyte. Although many samples are moderately to heavily altered [loss on ignition (LOI) = 3–10%)], some incompatible elements (Nb, Ba, Zr, Y, Hf) appear to have remained sufficiently immobile that their ratios reflect the mantle source composition. Immobile element ratios for least altered samples (LOI < 2%) demonstrate the geochemically enriched nature (La/Smn >2; La/Ybn >3; Zr/Nb <10; Y/Nb <2) of the underlying mantle. Samples from Discovery Rise include primitive basalt, trachyandesite and trachyte. Mafic lavas are relatively unaltered (LOI < 2%) and have high La/Smn (>4·5), Ba/Nb (>15) and Ba/Th (>130) coupled with low Zr/Nb (<6·5) and Ce/Nbn (<1). In accord with data from the single previously published Discovery Rise sample, these new samples confirm that the mantle source of Discovery Rise magmatism appears remarkably similar to that giving rise to Gough Island lavas some 400 km to the north. All lavas dredged from the aseismic Shona Ridge system are geochemically enriched relative to mid-ocean ridge basalt, with least altered samples having La/Smn >2, Zr/Nb <10 and Y/Nb <2. They are heterogeneous in their immobile incompatible trace element ratios, and can be subdivided into four geochemical groups: Group A and B lavas (found on the Shona Ridge and Agulhas Ridge) have Ba/Nb <10 coupled with Ce/Nbn <1; Group C lavas (restricted to the Meteor Rise) have Ba/Nb >10 with Ce/Nbn <1; Group D lavas (Meteor Rise and Cape Rise) have Ba/Nb>10 and Ce/Nbn >1. These differences are interpreted to reflect two physical components in the mantle source region: (1) a plume component (Shona plume; Group A and B lavas) comprising deep-seated upwelling mantle, itself heterogeneous and composed of typical, geochemically enriched, ocean island basalt-like mantle (La/Smn >1; Ba/Nb <10; Ce/Nbn <1), but including domains containing a recycled oceanic sediment component (Group C lavas); (2) a delaminated and recycled, metasomatized, subcontinental lithospheric mantle component, possibly rafted within the shallow convecting asthenospheric mantle, that has given rise to the Group D lavas. These new data are consistent with an earlier postulation that the Shona Ridge system represents the surface expression of an underlying mantle plume.