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This study reports the first geochemical and Pb isotopic data for mantle xenoliths from beneath the Neoproterozoic Ribeira Belt, southeastern Brazil. The cm-sized spinel peridotite xenoliths are hosted by a Cretaceous lamprophyre dike that intruded high-grade metamorphic rocks. Major- and trace-element compositions of the main minerals indicate that the xenoliths derive from a shallow fertile mantle that has undergone a low degree of melt extraction (2–9% partial melting). On the basis of modeled isochemical phase diagrams for lherzolites, pressure and temperature conditions are inferred to vary from 1300–1350 °C and 17–19 kbar (fertile composition) to 1330–1430 °C and 17–23 kbar (relatively depleted composition), which correspond to high geothermal gradients of 65–80 mW/m2. Temperatures of last equilibration calculated based on the average REE content of pyroxenes for the same lherzolite samples vary from 1233 ± 56 °C to 1085 ± 42 °C, while conventional thermometry (TBKN) yields average values of 807 and 755 °C, indicating re-equilibration at lower temperatures. Pb isotope ratios of clinopyroxene define a mixing line that intercepts the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 200 Ma. Linear regressions yield two errorchrons of 56 ± 75 Ma and 571 ± 99 Ma (95% confidence level). These results combined with the ages and tectonic settings of host rocks are suggestive of an overprint of a younger tectono-thermal event, most likely related to the opening of the South Atlantic Ocean, over a mantle previously equilibrated during the Precambrian development of the Ribeira Belt. [ABSTRACT FROM AUTHOR]

A long-standing debate on the genesis of magmatic-hydrothermal Cu-Au deposits is whether the elevated oxygen fugacity (f O 2) and metal endowment of their parental magmas are inherited from the mantle source or acquired during fractionation within the crust. This debate is built mainly on the composition of magmas emplaced into the upper crust that underwent varying degrees of differentiation and assimilation, which inevitably obscure their relationship to mantle processes. Here, we approach this debate from a new perspective by studying melt generation directly in mantle samples. This opportunity is provided by xenoliths from Lake Bullen Merri (Australia), which record partial melting of a pre-existing metasomatic assemblage. Melting occurred when the xenoliths were incorporated in hot basalts and carried towards the surface, where quenching upon eruption froze-in the melt-restite relationships. These samples preserve patches and interconnected networks of silicate glass along grain boundaries, caught in the act of undergoing melting and melt extraction. Melting consumed mainly metasomatic hornblende, phlogopite, and clinopyroxene, generating shoshonitic compositions that can be sulfide-rich, or have negligible sulfide content. We use four independent approaches to estimate the f O 2 of glass, including sulfide content, oxybarometry based on spinel-olivine pairs, Fe3+/Fe tot of glass determined by synchrotron X-ray absorption near-edge structure (XANES) spectroscopy, and Cu/Fe ratio of the sulfide assemblage. Sulfide-rich melt had higher f O 2 compared to the sulfide-poor melt, and these characteristics were inherited from the metasomatic assemblage; the oxidation state of the melts controlled their sulfide content. Subduction-modified mantle can thus store elevated oxidation state, sulfur, and metal content for hundreds of millions of years before low-degree partial melting generates oxidised, sulfur- and metal-enriched high-K/shoshonitic melts in post-collisional settings. • Xenoliths from Lake Bullen Merri contain glass produced by partial melting of metasomatic minerals. • Partial melts inherited oxidation state from the metasomatic assemblage. • Oxidised glass has high Cu-rich sulfide content, in reduced glass sulfide is negligible. • f O 2 , Cu and S stored in metasomatised mantle for hundreds of millions of years. • S and Cu content of melts controlled by mantle oxidation. [ABSTRACT FROM AUTHOR]