Constraints on the mantle lithology of enriched components: evidence from E-MORB of Southwest Indian Ridge

Abstract

Mid-oceanic ridge basalts (MORBs) are frequently grouped as depleted (D-MORBs), normal (N-MORBs), or enriched (E-MORBs) based on the abundances of highly incompatible elements. E-MORBs, characterized by enriched in incompatible elements, were first noted near mantle plumes, so that enrichment of highly incompatible elements was initially understood as a result of infiltration of plume-related melts into the MORB plumbing system. But for E-MORBs far from plumes, it is still controversial of the lithology of enriched components of these E-MORBs. Two major explanations are proposed that the enriched components are generated by the melting of entrained recycled crust (pyroxenite) beneath ridges or by the melting of refertilized peridotites from subducted slabs. The reason for this problem is that melting of the ambient refractory peridotite along with the enriched component will dilute the signals recorded in MORB. Therefore, the in-situ analyses of minerals and/or melt inclusions will shed new light on this question. Here high-precision in-situ analyses were conducted on olivine and plagioclase in the studied samples. According to the major and minor element contents of olivine phenocrysts, we found they have similar Ni, Mn and Ni/(Mg/Fe) contents with those from N-MORB at a given Fo, indicating a peridotitic source. Furthermore, in-situ Sr isotope in plagioclase phenocrysts and in-situ Pb isotope of plagioclase-hosted melt inclusions are also reported to constrain origin of parental magma. The isotopic results show that unlike the uniform whole-rock 87Sr/ 86Sr and 206Pb/204Pb ratios, the plagioclase phenocrysts record highly Sr and Pb isotopic heterogeneity. Strontium isotopic heterogeneity is observed between crystals even in a single thin section. Based on the high An contents of plagioclase phenocrysts and chemical disequilibrium between plagioclase phenocrysts and groundmass, we propose they crystallized early in the magma chamber and were most likely formed in different batches of mantle-derived melts. The enrichment of LREE and negative correlations between La/Sm and Pb isotopes melt inclusions, suggest that ancient continent lithosphere materials are likely present in the sub-ridge mantle of the east of the Melville FZ, SWIR. Collectively, we proposed that the continent lithosphere materials were slabbed into the upper mantle under MOR as refertilized peridotites.