Mantle heterogeneity observed in MORB from the northern Central Indian Ridge, 8°-17° S

Abstract

The northern Central Indian Ridge (CIR), between 8° and 17° S, composed of 7 segments of which spreading rates increase southward from ~35 mm/yr to ~40 mm/yr. Although composition of several rock samples from the northern CIR was included in early studies of 1970s, no systematic study of the northern CIR was reported yet. During the expeditions for hydrothermal activities in the northern CIR conducted by KORDI, the first detailed mapping and sampling of volcanic rocks along the ridge axis were performed. Major, trace element and Sr-Nd-Pb isotope compositions in basaltic glasses from spreading axis along the northern CIR reveal two distinct enriched mantle sources. In the southern segments, specifically for segments 6 and 7, lava samples generally show high Nb/Yb and 87Sr/86Sr and low 206Pb/204Pb. The enrichment pattern coincides with the Reunion plume composition as well as enriched lava collected from the southern CIR. Thus, the enriched mantle of the southern segments can be supplied from the Reunion hotspot plume (EM2) despite of the possible role of the Marie Celeste fracture zone as a compositional boundary. Another enriched source, characterized by high La/Sm and 206Pb/204Pb and low 87Sr/86Sr, is observed in lavas from the northern segments. This enriched source is compositionally distinct from the Reunion plume component and shows geochemical affinity with FOZO component. Our data possibly suggest the ubiquity ofFOZO-type component in the MORB-forming mantle. However, enrichment process for the formation of FOZO-type mantle should be investigated for further research.