Mantle Heterogeneity in the Source Region of MORBs along the Nothern Central Indian Ridge

Abstract

The northern Central Indian Ridge (CIR 8°&#8211 17°S) is composed of seven segments whose spreading rates increase southward from ~35 to ~40 mm/yr. Although previous studies have analyzed a limited number of rock samples collected more than two decades ago, no systematic chemical or isotopic studies of the northern CIR have previously been conducted. During expeditions to study hydrothermal activity on the northern CIR onboard the R/V Onnuri, high-resolution multibeam mapping and systematic sampling of volcanic rocks along the ridge axis were carried out. Here, we report on major and trace element, and Sr&#8211 Nd&#8211 Pb isotopic compositions of basaltic glasses from the spreading axis along the northern CIR, which reveal two distinct enriched mantle sources. On the southern segments, the samples generally show high Nb/Yb and 87Sr/86Sr, and low 206Pb/204Pb ratios, similar to the Reunion plume and to enriched lavas previously collected from the southern CIR. These patterns suggest that the southern segments were affected by the Reunion hotspot plume (EM2) and its trail, as the CIR migrated over hotspot-modified mantle. In contrast, lavas from the northern segments are characterized by high La/Sm and 206Pb/204Pb, and low 87Sr/86Sr ratios, distinct from the Reunion plume but indicating a “focus zone” (FOZO) component. The “FOZO-like” enrichment in the northern segments cannot be attributed to an adjacent manthan two decades ago, no systematic chemical or isotopic studies of the northern CIR have previously been conducted. During expeditions to study hydrothermal activity on the northern CIR onboard the R/V Onnuri, high-resolution multibeam mapping and systematic sampling of volcanic rocks along the ridge axis were carried out. Here, we report on major and trace element, and Sr&#8211 Nd&#8211 Pb isotopic compositions of basaltic glasses from the spreading axis along the northern CIR, which reveal two distinct enriched mantle sources. On the southern segments, the samples generally show high Nb/Yb and 87Sr/86Sr, and low 206Pb/204Pb ratios, similar to the Reunion plume and to enriched lavas previously collected from the southern CIR. These patterns suggest that the southern segments were affected by the Reunion hotspot plume (EM2) and its trail, as the CIR migrated over hotspot-modified mantle. In contrast, lavas from the northern segments are characterized by high La/Sm and 206Pb/204Pb, and low 87Sr/86Sr ratios, distinct from the Reunion plume but indicating a “focus zone” (FOZO) component. The “FOZO-like” enrichment in the northern segments cannot be attributed to an adjacent man