FY 1999

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

Graham, D.W., K.T.M. Johnson, L.D. Priebe, and J.E. Lupton

Earth Planet. Sci. Lett., 167, 297–310, doi: 10.1016/S0012-821X(99)00030-8 (1999)

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 higher relative He/Ti ratio compared to the inner portions of the plume that feed portions of the sub-ridge mantle beneath the ASP plateau.