Linking early Earth’s internal and external reservoirs: a change in oxygen fugacity of sub-arc magmas across the Great Oxidation Event
Hugo MoreiraCraig StoreyEmilie BruandJames DarlingMike FowlerMarine CotteEdgar E. Villalobos-PortilloFleurice ParatLuís SeixasPascal PhilippotBruno Dhuime
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Abstract:
Plate tectonics exerts a first-order control on the interaction between Earth’s reservoirs. Atmospherically-altered surface materials are recycled to the mantle via subduction, while volatiles from the mantle are liberated to the atmosphere via volcanism. This cycle regulates much of Earth’s climate, ocean levels and metallogenetic processes within the continental crust. However, the interplay between Earth’s atmospheric changes and the geochemical evolution of mantle-derived magmas has remained obscure for the ancient geological history. This has led to multiple conflicting models for the crustal evolution in the early Earth. A time-integrated evolution of the mantle-crust-atmosphere-hydrosphere interaction is yet to be fully established. For instance, secular change of the ocean and atmosphere system is evident from several proxies but the feedback of these changes to magmatic and geochemical processes in the lithosphere remain unclear. Moreover, no clear consensus has been reached on the timing of modern-style plate tectonic initiation and the evolution of net growth of the continental crust. To explain overt and cryptic global trends in the geochemistry of magmatic rocks, a better understanding of mineral reactions and how these control trace element evolution in magmas at the lithosphere-scale is paramount. For example, the elemental and isotopic composition of apatite inclusions hosted by zircon offers a way to better understand the evolution of magmas and, to some extent, the nature of magma sources. These proxies rely on the robust data acquisition of other isotope systems with different geochemical behaviour, such as U-Pb and Lu-Hf analyses in the host zircon crystal. A combination of methods and proxies including the elemental composition of apatite via EPMA and the oxygen fugacity based on sulphur speciation via μ-XANES of apatite inclusions was applied to ancient sub-arc magmas formed in regions akin to modern subduction zones. These magmas share a common mantle source but crystallised more than 200 million years apart (at 2.35 and 2.13 billion years ago). Importantly, they bracket the Great Oxidation Event, when atmospheric oxygen levels increased by five orders of magnitude, causing a permanent and dramatic change in Earth’s surface chemistry. As such, these sub-arc magmas were investigated as potential tracers of the interaction between Earth’s atmosphere and the mantle. The information from several inclusions from co-magmatic rocks can then be interpreted in the light of U-Pb, Lu-Hf, trace elements and oxygen isotope analyses of the host zircon grains. Altogether, the results show a shift in oxygen fugacity of sub-arc magmas across the Great Oxidation Event. The change in oxygen fugacity is thought to be caused by recycling into the mantle of sediments that had been geochemically altered at the surface by the increase in atmospheric oxygen levels. This study opens a wide window of opportunities for the time-integrated investigation of the interaction between atmosphere and oceans with the evolving terrestrial mantle.Keywords:
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The T-oxygen fugacity equilibrium for the system has been developed for a semi-quantitative T-oxygen fugacity petrogenic grid applicable to both basic igneous rocks and to meteorites. Olivine-silica-pyroxene-metallic iron-liquid exist at 1305 degrees C and 10 (super -11.8) atm (1 atm total), and in the system Fe-O-SiO ~2~ at 17.5 kb, 1280 degrees C, and 10 (super -10.8) atm oxygen fugacity. Subsolidus Mg enrichment of the silicate phases occurs with decreasing T and increasing oxygen fugacity in the magnetite-bearing assemblages at 1 atm. For iron-bearing assemblages, a T increase and oxygen fugacity decrease leads to Mg enrichment of the silicate phase. These phases can be divided by a region in which neither Mg nor metallic iron are stable, and transition between the two is unlikely.
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The ore-forming metallogenic specialization of felsic rocks is closely related to oxygen fugacity. In this paper, however, oxygen fugacity is considered to be one of the necessary but not sufficient conditions for ore formation, and the analytical thinking concerned is partly based on the authors' geological field work. The metallogenic significance of oxygen fugacity is divided into metallogenic specialization and ore-bearing potential. Two aspects of understanding have been otained: 1 Porphyry Cu-Au deposits derived from the reduced I-type granitoid should have a high oxygen-fugacity origin; different sub-regions within the covariogram of oxygen fugacity versus other geochemical parameters, such as degree of magmatic evolution, temperature, pressure, pH, sulfur fugacity and rock-type, correspond to different metals' geochemical behavior or mineralization, respectively, and oxygen fugacity versus rock-type seems to be of significant specialization; 2 The evolution from metallogenic specialization to real metallogenesis is the evolution from magma to hydrothermal solution and then from hydrothermal solution to ore;during such a process, oxygen fugacity is also an essential controlling factor for elemental geochemical behaviors,whose influence on mineral precipitation, however, should not be overemphasized, because some kinds of precipitation might have had nothing to do with oxygen fugacity.
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The kinetics of rock/water interactions are sufficiently rapid that most hydrothermal systems in nature will be in equilibrium with the adjacent rock mass. The bulk rock chemistry buffers the fugacity of oxygen, which in turn fixes the fugacities of water and of hydrogen for a given pressure and temperature. Systems in which only water, oxygen, and hydrogen are present as fluid phases are considered here. Variations in the fugacity of oxygen by several orders of magnitude are possible locally, controlled by variations in local rock chemistry; these lead to relative small variations in the fugacity of water. Incorporation of a hydrogen defect that is capable of acting as an acceptor into silicates leads to a strong dependence of point defect chemistry upon the fugacities of both water and oxygen. The strong dependence on the fugacity of water is capable of explaining the hydrolytic weakening effect, but in view of the strong dependence on oxygen fugacity, the question should also be raised whether it is an oxygen effect that is observed in the classical hydrolytic weakening process or solely a dependence on changes in the fugacity of water. Examples are given for impure natural quartz, olivine, and albite with trace amounts of calcium.
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