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 degrees E and 88 de grees 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 a verage sample spacing is similar to 10 km, He-3/He-4 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 restric ted He-3/He-4 range, between 7 and 9 R-A. Elevated He-3/He-4 ratios on the plateau are evidence for a deep mantle plume component that has been inject ed into the sub-ridge mantle beneath the region. Ridge segments southeast o f the plateau show lower He-3/He-4 (7.6-8.3 R-A) than segments to the north west (8.3-9.0 R-A), suggesting either a systematically varying 'background contamination' of the MORE asthenosphere in the region, or localized input of a 'low He-3/He-4' component along the southeastern ridge segments, possi bly derived from the distant Kerguelen hotspot. A double peak structure exi sts in the regional He-3/He-4 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 He-3/He-4 ratios (up to 14.1 R-A) are present all along the ridge segment immediately to the northwest of the AS P plateau (segment H), where anomalies in K/Ti ratio are also observed. The high He-3/He-4 ratios persist to the northern end of segment H, beyond whi ch they abruptly drop to MORE background levels, which apparently persist a ll the way to the Rodrigues Triple Junction. This pattern suggests that the high-He-3/He-4 ASP plume supplies a shallow north-northeastward mantle Bow toward adjacent portions of the SEIR. There is a systematic difference in He-3/He-4-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 portio ns of the ASP plume, where removal of He (relative to Ti) has been less eff icient 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 higher relative He/T i ratio compared to the inner portions of the plume that feed portions of t he sub-ridge mantle beneath the ASP plateau, (C) 1999 Elsevier Science B.V. All rights reserved.