CO<inf>2</inf>-Undersaturated Melt Inclusions From the South West Indian Ridge Record Surprisingly Uniform Redox Conditions

Publication status: PublishedFunder: Konstantin IgnatyevAbstractThe behavior of Fe3+ during mantle partial melting strongly influences the oxidation state of the resulting magmas, with implications for the evolution of the atmosphere's oxidation state. Here, we challenge a prevailing view that low‐degree partial melts are more oxidized due to the incompatible behavior of Fe3+. Our study is based on measurements of Fe3+/∑Fe along with major, minor, trace and volatile elements in olivine‐ and plagioclase‐hosted melt inclusions of CO2 undersaturated mantle melts in South West Indian Ridge lava. These inclusions record minimum entrapment pressures equivalent to depths up to 10 km below the seafloor, record magma ascent rates of 0.03–0.19 m/s, and display exceptionally high CO2/Ba, CO2/Rb, and CO2/Nb ratios, indicative of a CO2‐rich mantle source. Accounting for fractional crystallization, we find a uniform melt oxidation state (with an Fe3+/ΣFe at 0.140 ± 0.005 at MgO = 10 wt.%) that displays no systematic variation with major, minor, volatile or trace element contents, thus providing no evidence for a relationship between the degree of partial melting and Fe3+/ΣFe. This can be explained by efficient buffering of Fe3+/∑Fe and fO2 of mid‐ocean ridge basalt melts by their surrounding mantle and/or a decrease in the bulk peridotite‐melt Fe2O3 partition coefficient with increasing partial melting. We conclude that changes in the Earth's upper mantle temperature over geological time need not have affected the oxidation state of volcanic products or of the atmosphere.</jats:p

Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust

International audienceBasaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials1,2. However, the residence time of these subducted materials in the mantle is uncertain and model-dependent3, and compelling evidence for their return to the surface in regions of mantle upwelling beneath hotspots is lacking. Here we report anomalous sulphur isotope signatures indicating mass-independent fractionation (MIF) in olivine-hosted sulphides from 20-million-year-old ocean island basalts from Mangaia, Cook Islands (Polynesia), which have been suggested to sample recycled oceanic crust3,4. Terrestrial MIF sulphur isotope signatures (in which the amount of fractionation does not scale in proportion with the difference in the masses of the isotopes) were generated exclusively through atmospheric photochemical reactions until about 2.45 billion years ago5-7. Therefore, the discovery of MIF sulphur in these young plume lavas suggests that sulphur--probably derived from hydrothermally altered oceanic crust--was subducted into the mantle before 2.45 billion years ago and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with negative D33S values, in the mantle source for Mangaia lavas. Our data also complement evidence for recycling of the sulphur content of ancient sedimentary materials to the subcontinental lithospheric mantle that has been identified in diamond-hosted sulphide inclusions8,9. This Archaean age for recycled oceanic crust also provides key constraints on the length of time that subducted crustal material can survive in the mantle, and on the timescales of mantle convection from subduction to upwelling beneath hotspots

Mantle source heterogeneity in subduction zones: constraints from elemental and isotope (Sr, Nd, and Pb) data on Vulcano Island, Aeolian Archipelago, Italy

Vulcano is part of the Aeolian volcanic arc in the southern Tyrrhenian Sea. Its products were emplaced through multiple episodes of edifice building and collapse since about 120 ka B.P. to present. A major discontinuity in the activity occurred after about 28 ka, while the focus of volcanism moved from SE to NW. The older products are basalts to shoshonites, and have lower K2O than the younger ones, shoshonites to rhyolites. Between these two groups, Lentia latites-rhyolites, Spiaggia Lunga basalts, and Quadrara shoshonite-trachytes, erupted along the western side of Vulcano Island. The Spiaggia Lunga basalts (i) are the most primitive magmas erupted at Vulcano after 28 ka (ii) mark the change between older and younger phases, and (iii) overlap geochemically with a monzogabbroic intrusion of similar age. This work focuses on the Spiaggia Lunga products, discussed within a large dataset of geochemical and radiogenic isotope analyses on the entire Vulcano sequence. Older products have more primitive geochemical and isotope characteristics, and lower incompatible element contents, than younger ones. The Spiaggia Lunga basalts exhibit intermediate geochemical characteristics between the older and the younger groups, and can likely be regarded as a third magmatic phase, which represents a distinct mantle reservoir active during the magmatic history of Vulcano. Significant variations of Sr, Nd, and Pb isotope ratios, and isotopic disequilibrium between phenocrysts and groundmass, are present among the Vulcano products. This variability suggests crustal assimilation in shallow-level magma chambers, which also accounts for the formation of evolved products by combined assimilation and fractional crystallization, particularly in the younger series. Considering only the mafic products, incompatible element patterns with high LILE/HFSE and enriched signatures of Sr, Nd, and Pb isotope ratios, indicate enriched mantle sources. Besides, chemical and isotope variability among older, younger, and Spiaggia Lunga mafic rocks, suggests an origin from geochemically diverse primary melts, derived from distinct mantle reservoirs. Their parent magmas, based on geochemical and isotope patterns, were from both MORB- and OIB-type mantle sources, subject to variable degrees of metasomatism by subducted sediments