We report new helium abundance and isotope results for submarine basaltic glasses from the Central Indian Ridge (CIR) between the Marie Celeste (16.7 degrees S) and Egeria fracture zones (FZ) (20.6 degrees 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 He-3/He-4) 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 He-3/He-4 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 He-3/He-4 ratios are found near the Egeria FZ, and there is a marked increase to values of similar to 11 R-A between similar to 19 degrees and 20 degrees S. The lowest He-3/He-4 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 He-3/He-4 ratios can be explained by closed system radiogenic He-4 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 similar to 34 Ma or (2) trace element enriched MORB mantle. In contrast, the high He-3/He-4 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 (similar to 1100 km to the west) toward the CIR.
Fueri, Evelyn Hilton, David R. Murton, Bramley J. Hemond, Christophe Dyment, Jerome Day, James M. D.