Highly Unradiogenic Pb Isotopes Revealed by Plagioclase-Hosted Melt Inclusions in E-MORB From the South China Sea

Mid-ocean ridge basalts (MORBs) can record heterogeneity of the upper mantle. Early crystallized minerals and their melt inclusions commonly record greater isotopic variability than whole-rocks and are powerful tools to investigate the mantle source. In this study, we develop a new in situ Pb analytical protocol for Pb-poor materials (sub-ppm level) and apply it to the plagioclase-hosted melt-inclusions from IODP Expedition 349 Site U1433, South China Sea (SCS). Together with element mapping, in situ major and trace element analyzes and in situ Sr isotopes of plagioclase phenocrysts, we show that unlike the low and uniform whole-rock Sr-87/Sr-86 ratios, the plagioclase phenocrysts record extreme Sr 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 high Mg# values of their melt inclusions, we propose they crystallized early in the magma chamber and were most likely formed in different batches of mantle-derived melts. The plagioclase-hosted melt inclusions show highly variable Pb isotopes, exhibiting a typical linear variation in Pb-207/Pb-204 versus Pb-206/Pb-204. Such a mixing trend indicates an end-member with highly unradiogenic Pb isotopes. The LREE enrichment and negative correlations between La/Sm and Pb isotopes of the melt inclusions, indicate that ancient cratonic lower continent crustal materials, with low time-integrated U/Pb ratios, are likely present in the sub-ridge mantle of the southwest Sub-basin, SCS.