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Mid-ocean ridge basalts (MORB) from the Central Indian Ridge (CIR) between 12 degrees and 17 degrees S show a wide range of geochemical and isotopic variations. Particularly, MORB from a segment between 14 degrees and 15 degrees S are more enriched in incompatible trace elements with more radiogenic Sr and Pb isotope and unradiogenic Nd isotope values than the lavas between 15 degrees and 16 degrees S with geochemical features of normal MORB. However, the causes for the enrichment between 14 degrees and 15 degrees S are poorly constrained. In this study, we re-examined the CIR MORB from 12 degrees to 17 degrees S with new geochemical data obtained based on high spatial resolution sampling to better understand the nature of the enriched mantle source. Our new geochemical data show that the MORB between 14 degrees and 15 degrees S, with maximum values for (La/Sm)(N) = 1.95, Sr-87/Sr-86 = 0.703526 and Pb-206/Pb-204 = 18.7558, are more enriched than those from the southern segments (16 degrees to 20 degrees S) known to be influenced by the Reunion mantle component. The new trace element and isotopic compositions of MORB suggest that three mantle end-members are required to explain the geochemical variations shown between 14 degrees and 16 degrees S: the depleted Indian-type MORB mantle, Reunion Plume (RP), and Seychelles/Madagascar-like continental crust components. Moreover, our mixing model suggests that the differences in enriched MORB signature from 14 degrees to 20 degrees S are due to variable proportions of continental material previously mixed with the RP. Our study implies that a continental component interacted with the plume into the asthenosphere, possibly beneath Madagascar or below Mauritius Island and the Mascarene plateau.

Mid-ocean ridge basalts (MORB) from the Central Indian Ridge (CIR) between 12 degrees and 17 degrees S show a wide range of geochemical and isotopic variations. Particularly, MORB from a segment between 14 degrees and 15 degrees S are more enriched in incompatible trace elements with more radiogenic Sr and Pb isotope and unradiogenic Nd isotope values than the lavas between 15 degrees and 16 degrees S with geochemical features of normal MORB. However, the causes for the enrichment between 14 degrees and 15 degrees S are poorly constrained. In this study, we re-examined the CIR MORB from 12 degrees to 17 degrees S with new geochemical data obtained based on high spatial resolution sampling to better understand the nature of the enriched mantle source. Our new geochemical data show that the MORB between 14 degrees and 15 degrees S, with maximum values for (La/Sm)(N) = 1.95, Sr-87/Sr-86 = 0.703526 and Pb-206/Pb-204 = 18.7558, are more enriched than those from the southern segments (16 degrees to 20 degrees S) known to be influenced by the Reunion mantle component. The new trace element and isotopic compositions of MORB suggest that three mantle end-members are required to explain the geochemical variations shown between 14 degrees and 16 degrees S: the depleted Indian-type MORB mantle, Reunion Plume (RP), and Seychelles/Madagascar-like continental crust components. Moreover, our mixing model suggests that the differences in enriched MORB signature from 14 degrees to 20 degrees S are due to variable proportions of continental material previously mixed with the RP. Our study implies that a continental component interacted with the plume into the asthenosphere, possibly beneath Madagascar or below Mauritius Island and the Mascarene plateau.

Extreme gradients in topography related to variations in magma supply are observed on the Southern Explorer Ridge (SER), part of the northern Juan de Fuca ridge system. We report radiogenic isotope (Pb, Sr, Nd, Hf) and geochemical data for twenty-four basalt whole-rock and glass samples collected from the length of the SER and from Explorer Deep, a rift to the north of the SER. Lavas from the SER form a north-south geochemical gradient, dominated by E-MORB at the northern axial high, and range from T-MORB to N-MORB towards the southern deepest part of the ridge. Linear relationships between incompatible element ratios and isotopic ratios in MORB along the ridge are consistent with mixing of magmas beneath the ridge to generate the geographic gradient from E- to N-MORB. The E-MORB have high Sr and Pb, and low Nd and Hf isotopic ratios, typical of enriched mantle that includes a FOZO or HIMU isotopic component. The West Valley and Endeavour segments of the northern Juan de Fuca ridge also include this isotopic component, but the proportion of the FOZO or HIMU component is more extreme in the SER basalts. The FOZO or HIMU component may be garnet-bearing peridotite, or a garnet pyroxenite embedded in peridotite. Recycled garnet pyroxenite better explains the very shallow SER axial high, high Nb/La and La/Sm, and the “enriched” isotopic compositions.

Extreme gradients in topography related to variations in magma supply are observed on the Southern Explorer Ridge (SER), part of the northern Juan de Fuca ridge system. We report radiogenic isotope (Pb, Sr, Nd, Hf) and geochemical data for twenty-four basalt whole-rock and glass samples collected from the length of the SER and from Explorer Deep, a rift to the north of the SER. Lavas from the SER form a north-south geochemical gradient, dominated by E-MORB at the northern axial high, and range from T-MORB to N-MORB towards the southern deepest part of the ridge. Linear relationships between incompatible element ratios and isotopic ratios in MORB along the ridge are consistent with mixing of magmas beneath the ridge to generate the geographic gradient from E- to N-MORB. The E-MORB have high Sr and Pb, and low Nd and Hf isotopic ratios, typical of enriched mantle that includes a FOZO or HIMU isotopic component. The West Valley and Endeavour segments of the northern Juan de Fuca ridge also include this isotopic component, but the proportion of the FOZO or HIMU component is more extreme in the SER basalts. The FOZO or HIMU component may be garnet-bearing peridotite, or a garnet pyroxenite embedded in peridotite. Recycled garnet pyroxenite better explains the very shallow SER axial high, high Nb/La and La/Sm, and the “enriched” isotopic compositions.