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

Abstract

[1] 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 R A (where R A = air 3 He/ 4 He) and lie between values of Mid-Ocean Ridge Basalt (MORB) (8 ± 1 R A ) and samples from Réunion Island (11.5 to 14.1 R A ). The highest 3 He/ 4 He 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 3 He/ 4 He 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 3 He/ 4 He 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 3 He/ 4 He ratios can be explained by closed system radiogenic 4 He 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 3 He/ 4 He 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.

Tectonic Background of Four Hydrothermal Fields Along the Central Indian Ridge

“…They proposed that the geochemical variations of zero-age basalts may primarily reflect periodic processes rather than the spatial distribution of mantle heterogeneities. Füri et al (2011) reported significantly higher 3 He/ 4 He rations between 19 and 20 along the axis, as well as along the Gasitao Ridge, Three Magi Ridges and Rodrigues Ridge, consistent with lateral flow of hot spot mantle from Reunion toward the CIR. However, basaltic lavas sampled at CIR-15B axis, i.e., the intersection of the CIR and the off-axis volcanic structures, show typical N-MORB-like trace element ratios.…”

“…CIR-S15 and S16, sometimes known as the Rodrigues Segment (RS), are sandwiched between the Egeria and Marie-Celeste Fracture Zones and both the on-and off-axis areas are shallower than the adjacent normal segments. The unusual elevated and smooth bathymetry, and the geochemical and geophysical characteristics, suggest a hotspot influence on ridge processes, although the Reunion hotspot is presently located 1,000 km away from the CIR (Mahoney et al 1989;Murton 2005;Nauret et al 2006;Füri et al 2011). The Rodrigues Ridge, an WNW-ESE trending volcanic feature (Fig.…”

Release of subducted sedimentary nitrogen throughout Earth’s mantle

“…He-isotopes -the canonical tracer of mantle plume involvement in petrogenesis -are higher (>9 R A ) than typical DMM values of 8 ± 1 R A (where R A = air-like 3 He/ 4 He) at Réunion Island and along the off-axis ridges. In contrast, the CIR exhibits DMM-like 3 He/ 4 He values along axis, except at the point where the projection of the submarine ridges meets the ridge axis ~19.9 °S (Füri et al, 2011;Fig. S-1).…”

“…The highest and lowest values are -0.10 and -3.8 ‰ giving an on-axis δ 15 N mean value of -2.1 ± 1.1 ‰ (1σ; n = 7) or -1.7 ± 1.6 ‰ (n = 8 if D9-2 is included). Notably, sample D9-2 has a 3 He/ 4 He ratio (7.25 R A ), which falls in the nominal DMM range (8 ± 1 R A ; Graham, 2002) characteristic of most samples on the CIR ridge-axis (Füri et al, 2011). The only on-axis sample (DR10-1) of the present sample suite with a 3 He/ 4 He value higher than DMM ( 3 He/ 4 He = 10.31 R A ) is from the region where the projection of the off-axis ridge impinges the spreading centre (~19.9 °S): it has a δ 15 N value of -0.10 ‰ -the second highest value of the on-axis samples.…”

“…Samples were processed by vacuum crushing with released gases analysed using a noble gas mass spectrometer (Barry et al, 2012). All samples have been analysed previously for He isotopes, major/minor and trace element chemistry (Füri et al, 2011).…”