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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 87Sr/86Sr 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 207Pb/204Pb versus 206Pb/204Pb. 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. Plain Language Summary: The upper mantle of our Earth is highly heterogeneous. Early crystallized minerals and their melt inclusions are powerful tools to investigate the mantle source. In this study, we developed a new in situ Pb analytical protocol for Pb‐poor materials (sub‐ppm level) and apply it to the plagioclase‐hosted melt‐inclusions of E‐MORBs from IODP Expedition 349 Site U1433, South China Sea (SCS). We also analyzed the major and trace elements and Sr isotopes of plagioclase phenocrysts, as well as undertaking element mapping. We found that unlike the low and uniform whole‐rock 87Sr/86Sr ratios, the plagioclase phenocrysts record extreme Sr isotopic heterogeneity. Strontium isotopic heterogeneity is observed between crystals even in a single thin section. The plagioclase‐hosted melt inclusions show highly variable Pb isotopic ratios, which exhibit a typical linear variation in 207Pb/204Pb versus 206Pb/204Pb. Such a mixing trend indicates an end‐member with highly unradiogenic Pb isotopes. The enrichment of LREE and negative correlations between La/Sm and Pb isotopes melt inclusions, suggest that ancient cratonic lower continent crustal materials are likely present in the sub‐ridge mantle of the southwest Sub‐basin, SCS, and that they are the highly unradiogenic component. Key Points: A new in situ Pb analytical protocol (20xPb/204Pb ratios) is developed for Pb‐poor materials (sub‐ppm level)Crystal‐scale Sr‐Pb heterogeneity is recorded by the plagioclase phenocrysts and melt inclusions from E‐MORB at IODP Site U1433Ancient cratonic lower continental crustal material is involved in the E‐MORB source in the Southwest Sub‐basin of the South China Sea [ABSTRACT FROM AUTHOR]

The multi‐valent manganese is ubiquitous in various marine environments and is involved in many biogeochemical processes with electron transfers between different oxidation states. The involvement of Mn (IV) in the anaerobic oxidation of methane (AOM) in cold seeps has long been proposed by experimental simulation and was suggested to serve as an important sink of methane in the paleo‐oceanic environment. Nonetheless, metal‐dependent AOM processes still lack explicit geological evidence. Meanwhile, research on micronodules gained momentum in recent years, but the authigenic manganese micronodules in active cold seeps were yet not well studied. In this study, Mn‐rich sediments were discovered at the active Haima cold seeps area in the South China Sea, from which Mn‐micronodules were then extracted. These micronodules are irregular or subspheroidal with rough and porous surfaces and are mainly composed of 7 Å and 10 Å phyllomanganates. They are enriched in Mn, and deficient in Fe, Co, Ni, Cu, and REYs. Characteristic microbial‐like mineralization structures are also observed. Based on these mineralogical/geochemical features and the abnormally high growth rate (calculated by the cobalt chronometer algorithm), these authigenic manganese micronodules were probably formed rapidly via biomineralization and chemical precipitation and were supplied with sufficient Mn by the metal‐rich seepage fluids. The occurrence of these micronodules demonstrates manganese bacterial activities, and indirectly reflects metal‐dependent AOM. Plain Language Summary: Cold seeps have received extensive attention due to their close association with gas hydrate, petroleum, seepage biological communities, and the global carbon cycle. Sulfate reduction (SR) and anaerobic oxidation of methane (AOM) are important redox processes in seepages. Sulfate has long been regarded as the main oxidant of AOM, while manganese oxides were proposed to involve in AOM a decade ago. Manganese, one of the most abundant transition metals in the Earth's crust, can precipitate and accumulate to form aggregations, like micronodules, in various environments. With the development of analysis methods, micronodules research gain momentum in recent years, but authigenic micronodules from cold seeps have not been well studied yet. In this paper, abundant Mn‐micronodules were identified in the surface sediments from the active Haima cold seeps of the South China Sea. Comprehensive geochemical and mineralogical analyses have been conducted on these micronodules to characterize their chemical and mineralogical features (e.g., enriched in Mn, deficient in Fe, Co, Ni, Cu, and REYs, and microbial‐like mineralization structures). Based on these characteristics, the occurrence of these micronodules demonstrates manganese bacterial activities, and indirectly reflects metal‐dependent AOM. Overall, this study deepens the understanding of authigenic Mn minerals and the Mn geochemistry in active seepages. Key Points: Authigenic Mn‐micronodules are found in the Mn‐rich sediments from the active Haima cold seeps of the South China SeaThese micronodules are precipitated rapidly from the seepage fluids and formed via microbial‐mediated and chemical precipitationThe cold seeps are sites where Mn‐bacteria thrive and Mn‐dependent anaerobic oxidation of methane occurs [ABSTRACT FROM AUTHOR]