Helium isotope variations between Réunion Island and the Central Indian Ridge (17°–21°S): New evidence for ridge–hot spot interaction

Abstract

We report new helium abundance and isotope results for submarine basaltic glasses from the Central Indian Ridge (CIR) between the Marie Celeste (16.7°S) and Egeria fracture zones (FZ) (20.6°S); the adjacent Gasitao, Three Magi, and Rodrigues ridges; and for olivine separates from lavas and cumulate xenoliths from the Mascarene Islands (Reunion, Mauritius, and Rodrigues). Helium isotope ratios in basaltic glasses range from 7.1 to 12.2 R A (where R A = air 3He/4He) and lie between values of Mid‐Ocean Ridge Basalt (MORB) (8 ± 1 R A) and samples from Reunion Island (11.5 to 14.1 R A). The highest 3He/4He values (up to 12.2 R A) are found in glasses recovered off axis from the Three Magi and Gasitao ridges. Along the CIR axis, MORB‐like 3He/4He ratios are found near the Egeria FZ, and there is a marked increase to values of ∼11 R A between ∼19° and 20°S. The lowest 3He/4He values (<8 R A) are found immediately south of the Marie Celeste FZ, where incompatible trace element ratios (e.g., La/Sm) are highest. These low 3He/4He ratios can be explained by closed system radiogenic 4 He ingrowth in either (1) a fossil Reunion hot spot mantle component, embedded into the subridge mantle when the CIR migrated over the hot spot at ∼34 Ma or (2) trace element enriched MORB mantle. In contrast, the high 3He/4He ratios observed on the CIR axis adjacent to the Gasitao Ridge, and along the off‐axis volcanic ridges, are consistent with flow of hot spot mantle material from Reunion (∼1100 km to the west) toward the CIR.