ALBERT

Description: In contrast to the long narrow volcanic chains in the Pacific, Atlantic hotspot tracks, in particular in the South Atlantic, e.g. Tristan-Gough, Discovery, Shona and Bouvet, are irregular and in some cases diffuse and discontinuous. An important question is whether this irregularity results from tectonic dismemberment of the tracks or if it represents differences in the size, structure and strength of the melting anomalies. Here we present new age and geochemical data from volcanic samples from Richardson Seamount, the Agulhas Ridge along the Agulhas-Falkland Fracture Zone (AFFZ) and Meteor Rise. Six samples yielded ages of 83-72 Myr and are 10-30 Myr younger than the underlying seafloor, indicating that they are not on-axis seamounts associated with sea-floor spreading. The incompatible element and Sr-Nd-Pb-Hf isotopic compositions rangefrom compositions similar to those of the Gough domain of the nearby Tristan-Gough hotspot track to compositions similar to samples from the Shona bathymetric and geochemical anomaly along the southern Mid-Atlantic Ridge (49-55°S), indicating the existence of a Shona hotspot as much as 84 Myr ago and its derivation from a similar source region similar to that of the Tristan-Gough hotspot. Similar morphology, ages and geochemistry indicate that Richardson, Meteor and Orcadas guyots originally formed as a single volcano that has been dissected and displaced 3500 km along the AFFZ, providing a dramatic example of how plate tectonics can dismantle and disseminate a hotspot track across an ocean basin.

Description: Asymmetrically zoned hotspot tracks in the Pacifi c Ocean are interpreted to have formed from zoned plumes originating from the large-scale, lower-mantle, low-seismic-velocity anomaly (superplume?) beneath the southern Pacifi c, providing direct information about lowermantle compositional heterogeneity. New trace-element and Sr-Nd-Hf-Pb isotope data from the classic Tristan-Gough hotspot track in the South Atlantic also display a bilateral, asymmetric zonation with two distinct mantle source components, making it the fi rst zoned plume to be recognized overlying the African superplume. The plume zonation can be traced for 70 m.y., four times longer than recognized for Pacifi c zoned hotspot tracks. These fi ndings confi rm that the proposed zonation of Pacifi c hotspots is not simply a geochemical oddity, but could be a major feature of plumes derived from lower-mantle superplumes. We propose that the enriched southern Gough subtrack source with elevated 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb, but low 143Nd/144Nd and 176Hf/177Hf (DUPAL-like composition), may refl ect the African superplume composition, whereas the more depleted northern Tristan subtrack source could represent a mixture of the superplume with the surrounding depleted mantle. Our results strengthen arguments that the enriched signature (DUPAL anomaly) in the South Atlantic could be derived from the lower mantle.