Influence of the Amsterdam/St. Paul hot spot along the Southeast Indian Ridge between 77° and 88°E: Correlations of Sr, Nd, Pb, and He isotopic variations with ridge segmentation

Abstract

The submarine Amsterdam‐St. Paul (ASP) Plateau, bisected by the Southeast Indian Ridge (SEIR), is a bathymetric high rising ∼2 km above the surrounding seafloor that includes the islands of Amsterdam and St. Paul; this excess volcanism is attributed to a mantle hot spot. We obtained new Sr, Nd, and Pb (n = 37) and He isotopic (n = 10) ratios for basalt glasses from 11 SEIR segments on and adjacent to the plateau and from three seamounts on the plateau. The results show systematic spatial variations in these isotopic ratios that correlate with physical segmentation of the ridge. Specifically, lavas from the four ridge segments on the ASP Plateau have higher 208Pb/204Pb at a given 206Pb/204Pb than SEIR basalts distant from ASP Plateau. Surprisingly, lavas from the ridge segment 100 km north of the ASP Plateau are distinguished by the most radiogenic 206Pb/204Pb (up to 19.4) and highest 3He/4He ratios (up to 14.1 RA). These are characteristics of lavas erupted at Amsterdam and St Paul Islands. The isotopic data for SEIR basalts erupted on or adjacent to the ASP Plateau provide equivocal evidence for a mantle component derived from the distant Kerguelen hot spot. Overall, the Pb‐Nd‐Sr‐He isotope variations within this data set are explained well by three mantle end‐members: (1) depleted mantle having relatively low 206Pb/204Pb and 87Sr/86Sr and high 143Nd/144Nd, which has been variably mixed with (2) material having relatively high 208Pb/204Pb and 87Sr/86Sr and low 143Nd/144Nd, a signature commonly ascribed to detached or eroded metasomatized continental lithosphere, and (3) hot spot–related mantle having elevated 3He/4He and 206Pb/204Pb but intermediate 87Sr/86Sr and 143Nd/144Nd, similar to the common or C material observed in hot spots globally. These results suggest either that the ASP hot spot is isotopically heterogeneous or that the shallow mantle or lithosphere beneath the ASP Plateau contains more continentally derived material than the SEIR mantle ≥500 km away. Perhaps, like the 39°–41°E section of the Southwest Indian Ridge, beneath the ASP Plateau there are rafts of continental material stranded within a local “tectonic corridor,” possibly present since the opening of the Indian Ocean basin.