Hotspot–ridge interaction along the Southeast Indian Ridge near Amsterdam and St. Paul islands: helium isotope evidence

We report new helium isotope analyses for basaltic glasses recovered along the Southeast Indian Ridge (SEIR) between longitudes 77°E and 88°E. In this region, the SEIR shoals to a depth less than 1800 m as it crosses the Amsterdam–St. Paul (ASP) plateau. Atop the plateau, where the average sample spacing is ∼10 km, 3He/ 4He ratios range between 9.3 and 13.4 R A ( R A = atmospheric ratio) with significant variability over short distances. Away from the plateau, ridge segments show a more restricted 3He/ 4He range, between 7 and 9 R A. Elevated 3He/ 4He ratios on the plateau are evidence for a deep mantle plume component that has been injected into the sub-ridge mantle beneath the region. Ridge segments southeast of the plateau show lower 3He/ 4He (7.6–8.3 R A) than segments to the northwest (8.3–9.0 R A), suggesting either a systematically varying `background contamination' of the MORB asthenosphere in the region, or localized input of a `low 3He/ 4He' component along the southeastern ridge segments, possibly derived from the distant Kerguelen hotspot. A double peak structure exists in the regional 3He/ 4He spatial distribution, because there is also a strong plume-derived He isotope signal which is offset from the shallowest, thickest section of the ridge. High 3He/ 4He ratios (up to 14.1 R A) are present all along the ridge segment immediately to the northwest of the ASP plateau (segment H), where anomalies in K/Ti ratio are also observed. The high 3He/ 4He ratios persist to the northern end of segment H, beyond which they abruptly drop to MORB background levels, which apparently persist all the way to the Rodrigues Triple Junction. This pattern suggests that the high- 3He/ 4He ASP plume supplies a shallow north-northeastward mantle flow toward adjacent portions of the SEIR. There is a systematic difference in 3He/ 4He–K/Ti behavior between axial lavas from the ASP plateau and those from segment H, suggesting a higher He/Ti elemental abundance ratio in the plume source material beneath segment H compared to that beneath the ridge axis on the ASP plateau. These observations are consistent with a model in which plume material supplied to segment H is derived from the outer portions of the ASP plume, where removal of He (relative to Ti) has been less efficient than within the core of the plume located beneath the Amsterdam and St. Paul islands. These outer portions of the plume have lower temperatures and have undergone less partial melting, leading to a hig