Redox condition is key to understanding crust formation and differentiation processes. We measured Eu anomalies in zircon in the Neoarchean tonalite-trondhjemite-granodiorite rocks (TTG) from the North China Craton to infer the Eu systematics in TTG melts, which allows us to constrain the minimum melt oxidation states. We find that those TTG samples formed at medium to high pressures have less Eu depletions (high Eu/Eu*) that are comparable to those of Phanerozoic porphyry copper deposits. To generate the zircon Eu signatures in the medium to high pressure TTGs, the melt oxygen fugacity needed to be at least 0–1 log units above the Ni-NiO buffer based on our crystallization modeling. The more oxidized compositions of medium to high pressure TTGs than those of low pressure TTGs are best explained as a consequence of endogenic oxidation due to garnet retention in their sources. Our findings further support that the redox condition of felsic crustal magmas is connected to differentiation pressure or crustal thickness, and this connection holds over much of the Earth’s crustal differentiation history. In the late Archean, continuous formation of such oxidized crust may have profoundly influenced volatile output from Earth’s interior and oxidative weathering processes at the surface.

中文翻译：

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太古代 TTG 中的锆石 Eu/Eu* 及其对太古代地壳分化中内源氧化作用的影响


氧化还原条件是理解地壳形成和分化过程的关键。我们测量了华北克拉通新太古代英闪长岩-长辉长岩-花岗闪长岩（TTG）中锆石中的Eu异常，以推断TTG熔体中的Eu系统学，这使我们能够限制最小熔体氧化态。我们发现，那些在中高压下形成的 TTG 样品具有较少的 Eu 消耗（高 Eu/Eu*），与显生宙斑岩铜矿床的样品相当。为了在中压至高压 TTG 中生成锆石 Eu 特征，根据我们的结晶模型，熔体氧逸度需要比 Ni-NiO 缓冲液至少高 0-1 个对数单位。中高压 TTG 的氧化成分比低压 TTG 的氧化成分更多，最好的解释是由于石榴石保留在其来源中而导致内源性氧化的结果。我们的研究结果进一步支持长英质地壳岩浆的氧化还原条件与分异压力或地壳厚度有关，并且这种联系在地球地壳分异历史的大部分时期都成立。在太古代晚期，这种氧化壳的持续形成可能深刻地影响了地球内部的挥发性输出和地表的氧化风化过程。