Helium isotope variations between Réunion Island and the Central Indian Ridge (17°–21°S): New evidence for ridge–hot spot interaction
Résumé
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 (Réunion, Mauritius, and Rodrigues). Helium isotope ratios in basaltic glasses range from 7.1 to 12.2 RA (where RA = air 3He/4He) and lie between values of Mid-Ocean Ridge Basalt (MORB) (8 ± 1 RA) and samples from Réunion Island (11.5 to 14.1 RA). The highest 3He/4He values (up to 12.2 RA) 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 RA between ~19° and 20°S. The lowest 3He/4He values (<8 RA) 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 4He ingrowth in either (1) a “fossil” Réunion 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 Réunion (~1100 km to the west) toward the CIR.
Origine : Fichiers éditeurs autorisés sur une archive ouverte
Loading...