Hafnium isotope compositions of abyssal peridotites from the Southwest Indian Ridge

We report Hf isotope compositions of clinopyroxene mineral separates from plagioclase-free peridotites from the Southwest Indian Ridge (SWIR; dredge VAN7-85: 52.25^oS, 15.40^oE). The clinopyroxenes have depleted rare earth element patterns and low Na-Ti contents consistent with limited (if any) post-melting re-fertilization. The clinopyroxene Hf-isotope ratios display a limited variability (ɛ_Hf = 17.6-19.4, ɛ_Nd = 12.3- 13.3) and plot at the radiogenic end of the MORB field in ɛ_Hf - ɛ_Nd space, consistent with estimates for the depleted MORB mantle. The spatially adjacent SWIR basalts have less radiogenic compositions than the peridotites and plot at the enriched end of the MORB field (ɛ_Hf = 6.3-10, one sample with ɛ_Hf = 17, and ɛ_Nd = 2.9-7.8). The availability of Hf-Nd isotope data on geographically associated peridotites and basalts allows us to place tighter constrains on the compositional variability of the mantle beneath SWIR. A regression line through the peridotite and associated basalt data in ɛ_Hf - ɛ_Nd space has a slope of 1.21 (R² = 0.94), similar to the slope of the mantle array. The data suggests that these peridotites may represent the depleted endmember in the source of the adjacent basalts and that the enriched endmember lies within, or at the unradiogenic extension of the mantle array in ɛ_Hf - ɛ_Nd space. Importantly, this regression line passes very close to the Bouvet hotspot basalt data, consistent with a Bouvet hotpot influence on the SWIR, as previously proposed. As the presumed enriched endmember is not observed in the peridotite compositions we sampled, we speculate that it must be a low solidus lithology that preferentially melts during upwelling and MORB melting. Finally, assuming a Bouvet-type enriched component in the source of the SWIR basalts, then up to 40% of the Hf budget in the basalts comes from the depleted endmember, as this is represented by these SWIR peridotites. Assuming the enriched component has an average Bouvet lava composition and greater melt productivity than the depleted peridotitic component, we estimate less than 5% enriched component in the mantle source of the SWIR basalts.