Isotopically enriched N-MORB: a new geochemical signature for plume-ridge interaction

The presence of E-MORB (mid-ocean ridge basalts with enrichments in light rare earth elements (LREE)) at MOR has long been considered as the necessary geochemical evidence for plume-ridge interaction, because plume materials are normally more enriched in both trace elements and isotopic compositions than the source of N-MORB (normal-MORB). However, global MORB compilation indicates that MORB erupted on the ridge segments within 200 km of hotspots do not always show obvious E-MORB signatures. Therefore, is it true that E-MORB is the only possible geochemical signature of such interaction? Alternatively, isotopic enriched N-MORB in this study, combined with MORB of such kind worldwide, would potentially provide a new perspective on our understanding on the geochemical expression of plume-ridge interactions. Interaction between the Southwest Indian Ridge (46°E and 52°20'E) and Crozet hotspot has been proposed by geophysical studies, but remains controversial mostly due to the lack of E-MORB. 47 new samples collected from this region during the RV Dayang Yihao Cruise, including 15 from the segment 27 centered at 50°28'E with 10-km thick crust, are all N-MORB and can be classified into two groups: a high-Al group only at 50°28'E and a Main group widespread. The former, with higher Al2O3 and lower TiO2 and SiO2, have slightly enriched Sr-Nd-Hf-Pb isotopic compositions. Considering almost twice the thickness of the segment 27 than nearby segments for at least 3 Ma, the enriched isotopic compositions call for the presence of a hot and isotopically enriched source with previous melt extraction to deplete the incompatible elements. Neither melting residue related to the Madagascar and Karoo flood basalts nor local mantle heterogeneity can meet such requirements. The Crozet plume melting residues, alternatively, are the likely source. The isotopically enriched N-MORB formed by re-melting of the Crozet hotspot with previous melt extraction likely during transportation towards the ridge. Isotopically enriched N-MORB are known elsewhere, mostly at slower-spreading ridges possibly influenced by plumes with large plume-ridge distances, suggesting the control of both spreading rate and plume-ridge distance on the geochemical expressions of plume-ridge interactions.