Abstract Post-collisional ultrapotassic rocks (UPRs) in the Tibetan Plateau exhibit extreme enrichment in incompatible elements and radiogenic isotopes. Such enrichment is considered to be either inherited from a mantle source or developed during crustal evolution. In this study, to solve this debate we combined mineral textures and in situ geochemical composition of clinopyroxene phenocrysts in UPRs from southern Tibet to reveal their crustal evolution, enrichment cause and constrain metasomatism in their mantle source. Results show that the UPRs experienced an array of crustal processes, i.e., fractional crystallization, mixing, and assimilation. Fractional crystallization is indicated by decreases in Mg# and Ni and enrichment in incompatible elements (e.g. rare earth element (REE), Sr, Zr) toward the rims of normally zoned clinopyroxene phenocrysts (type-I). Magma mixing is evidenced by the presence of some clinopyroxene phenocrysts (type-II, -III) showing disequilibrium textures (e.g. reversed and overgrowth zoning), but in situ Sr isotope and trace element analysis of those disequilibrium zones indicate that late-stage recharged mafic magmas are depleted (87Sr/86Sr: 0.70659–0.71977) compared with the primitive ultrapotassic magmas (87Sr/86Sr: 0.70929–0.72553). Assimilation is revealed by the common presence of crustal xenoliths in southern Tibetan UPRs. Considering the much lower 87Sr/86Sr values (0.707759–0.709718) and incompatible element contents of these crustal xenoliths relative to their host UPRs, assimilation should have resulted in geochemical depletion of southern Tibetan UPRs rather than enrichment. The diluting impact of both assimilation and mixing is also supported by the modeling results based on the EC-E′RAχFC model combining the growth history of clinopyroxene. Trace elements ratios in clinopyroxenes also imply that the mantle source of southern Tibetan UPRs suffered an enriched and carbonatite-dominated metasomatism. Thus, we conclude that enrichment of southern Tibetan UPRs was inherited from the mantle source.
Abstract Oxygen fugacity ( f O 2 ) is a fundamental thermodynamic property governing redox potential in solid Earth systems. Analysis of magmatic f O 2 aids our understanding of the valence state and solubility of multivalent elements during magma evolution. Specialized software, Geo‐ f O 2 , was developed for calculating magmatic f O 2 on the basis of oxybarometers and thermobarometers for common minerals (amphibole, zircon, and biotite) in intermediate‐silicic magmas. With user‐friendly interfaces, it is easy to input files (.csv or Excel files), output data in Excel files, and plot results as binary diagrams that can be saved as vector graphics and modified using image‐processing software.
Abstract The redox state of Earth’s upper mantle in several tectonic settings, such as cratonic mantle, oceanic mantle, and mantle wedges beneath magmatic arcs, has been well documented. In contrast, oxygen fugacity () data of upper mantle under orogens worldwide are rare, and the mechanism responsible for the mantle condition under orogens is not well constrained. In this study, we investigated the of mantle xenoliths derived from the southern Tibetan lithospheric mantle beneath the Himalayan orogen, and that of postcollisional ultrapotassic volcanic rocks hosting the xenoliths. The of mantle xenoliths ranges from ΔFMQ = +0.5 to +1.2 (where ΔFMQ is the deviation of log from the fayalite-magnetite-quartz buffer), indicating that the southern Tibetan lithospheric mantle is more oxidized than cratonic and oceanic mantle, and it falls within the typical range of mantle wedge values. Mineralogical evidence suggests that water-rich fluids and sediment melts liberated from both the subducting Neo-Tethyan oceanic slab and perhaps the Indian continental plate could have oxidized the southern Tibetan lithospheric mantle. The conditions of ultrapotassic magmas show a shift toward more oxidized conditions during ascent (from ΔFMQ = +0.8 to +3.0). Crustal evolution processes (e.g., fractionation) could influence magmatic , and thus the redox state of mantle-derived magma may not simply represent its mantle source.
Geochemical data indicate that arc magmas are typically more oxidized than other voluminous magma types on Earth, such as mid-ocean ridge and intraplate basalts.In addition to having a significant impact on magmatic phase equilibrium, the higher oxidation state also leads to a variable fraction of sulfur being present in oxidized form in arc magmas, which has a significant impact on ore-forming process and volcanic degassing.Despite its significance, the processes controlling the redox state of arc magmas are poorly understood, and even the initial oxidation state of primitive magmas generated in arc settings is highly debated.Silicate melt inclusions (SMI) trapped in early crystalized minerals (e.g.olivine) allow direct determination of the undegassed composition and redox state of primitive arc magmas.In this study, we present data from the Trans-Mexican Volcanic Belt (TMVB), which is a unique continental arc because of the high abundance of monogenetic volcanoes delivering primitive mantle melts to the surface without significant interaction with the crust.The concentrations of volatile (S, Cl), major and trace elements, including the highly siderophile Au and Pt were determined in olivine-hosted melt inclusions by LA-ICP-MS to assess if magma oxidation is induced by slab-derived components.Results show that basaltic primitive melts in TMVB have high V/Sc ratio and fO 2 value, indicating an oxidized state.The S and Cl concentrations in SMI show wide variation and reach rather high values.Sulfur positively correlates with Cl, fluid-mobile elements (Ba, K) and Th/Yb ratio, suggesting a significant contribution to the total S budget from both slab-derived fluids and subducted sediments.Importantly, Cl increases with increasing V/Sc and fO 2 .This implies that the slab fluid is responsible for the oxidized state.Moreover, Pt and Au concentrations are higher than those in mid-ocean ridge basalts.In some cones, Pt and Au show positive correlation with V/Sc ratio and fO 2 value.This indicates that the Fe 3+ is an important component in slab fluids, because only Fe 3+ has capacity to oxidize the mantle wedge beyond the sulfide/sulfate buffer, a prerequisite for the release of Au and Pt into mantle melts.
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