Contrasting behaviour of the Bouvet and Shona hotspots as recorded in MORB from the Southwest Indian, American-Antarctic and southern Mid-Atlantic ridges

New Hf-Sr-Nd-Pb isotope and ICP-MS trace element data are presented for MORB from the western Southwest Indian Ridge (SWIR; 0-11°E), the American Antarctic Ridge (AAR) and the southernmost Mid-Atlantic Ridge (SMAR), as well as for Bouvet Island. These, combined with existing literature data, including for He isotopes, permit us to assess and compare the effects of these two anomalies on the regional asthenospheric mantle composition. The key observations are that, while both the Bouvet and Shona hotspots have roughly similar isotopic affinities intermediate between HIMU and EM, their effect on the composition of regional MORB differs significantly. The Shona hotspot (≈51°S on the SMAR) has a fairly symmetrical and localised (< 300 km from hotspot) effect on MORB isotope and trace element geochemistry (Douglass et al., 1999; Le Roux et al., 2002; Andres et al., 2002), including He isotopes (e.g., Kurz et al., 1998; Sarda et al., 2000). The Bouvet hotspot, on the other hand is associated with a much broader and asymmetric zone of geochemical enrichment extending nearly 1500 km from the central AAR to Narrowgate along the western SWIR, which separates oblique from orthogonal spreading segments. There is a gradual increase in 3He/4He from the central AAR (9-10 R/Ra at 6-7°W) to a maximum of ≈15 at about 7°E (compared to 12.5 for Bouvet Island lavas and 12.0 for the Shona maximum), but then decreases to typical MORB values of ≈8 R/Ra almost immediately east of 7°E. Enrichment in lithophile isotope systems and trace elements is also asymmetric, again starting in the central AAR but extending all the way to Narrowgate at 15°E. Moreover, strongly enriched and highly depleted compositions are present throughout this entire interval, with the most enriched compositions being present furthest east. It is possible that the location of the putative Bouvet plume close to the triple junction may facilitate the wide distribution of plume material, with the 3He-enriched signature being restricted to a zone near the plume. However, there is also evidence for highly variable melting conditions in this region and also for mineralogical variations that suggests that any model attempting to explain the expression of Bouvet must account for highly variable melting of a lithologically heterogeneous plume.