Mantle heterogeneity in the source region of mid-ocean ridge basalts along the northern Central Indian Ridge (8 degrees S-17 degrees S)

Abstract

The northern Central Indian Ridge (CIR) between 8 degrees S and 17 degrees S is composed of seven segments whose spreading rates increase southward from similar to 35 to similar to 40 mm/yr. During expeditions of R/V Onnuri to study hydrothermal activity on the northern CIR in 2009-2011, high-resolution multibeam mapping was conducted and ridge axis basalts were dredged. The major and trace element and Sr-Nd-Pb-He isotopic compositions of basaltic glasses dredged from the spreading axis require three mantle sources: depleted mantle and two distinct enriched mantle sources. The southern segments have Sr, Nd, and Pb that are a mix of depleted mantle and an enriched component as recorded in southern CIR MORB. This enrichment is indistinguishable from Re` union plume mantle, except for He isotopes. This suggests that the southern segments have incorporated a contribution of the fossil Re` union plume mantle, as the CIR migrated over hot-spot-modified mantle. The low He-3/He-4 (7.5-9.2 RA) of this enriched component may result from radiogenic He-4 ingrowth in the fossil Re` union mantle component. Basalts from the northern segments have high Pb-206/Pb-204 (18.53-19.15) and low Sr-87/Sr-86 (0.70286-0.70296) that are distinct from the Re` union plume but consistent with derivation from mantle with FOZO signature, albeit with He-3/He-4 (9.2-11.8 RA) that are higher than typical. The FOZO-like enriched mantle cannot be attributed to the track of a nearby mantle plume. Instead, this enrichment may have resulted from recycling oceanic crust, possibly accompanied by small plume activity.