Tracing Mantle-Oxygen Fugacity Changes Through the Great Oxidation Event: Insights from Apatite Inclusions in Brazilian Igneous Zircons

Moreira, H., Storey, C., Bruand, E. et al. Sub-arc mantle fugacity shifted by sediment recycling across the Great Oxidation Event. Nat. Geosci. 16, 922–927 (2023).

Substantial accumulation of free oxygen in the atmosphere occurred between ~2.45 and 2.20 billion years ago , with permanent atmospheric oxygenation commencing between 2.3 and 2.2 Ga. This period is known as the Great Oxidation Event (GOE) and marks the most dramatic change in Earth’s surface chemistry and habitability. However, it remains unclear if these major atmospheric changes affected the amount of free or chemically available oxygen in the mantle and, consequently, the redox state of mantle-derived magmas. In the modern Earth, considerable amounts of surface-oxidized components infiltrate the mantle via slab fluids and subducted sediments, ultimately influencing the oxidation state of the mantle wedge and arc magmas.

Palaeoproterozoic magmatic transition.

a, Zircon U–Pb ages versus 176Hf/177Hf(t) ratios (expressed as ɛHf(t) values relative to chondrite at the time of crystallization t). Zircons from TTG magmas (n = 31) have significantly positive ɛHf(t), whereas zircons from the sanukitoid magmas (n = 33) are near the CHUR. A crustal evolution line links both suites of rocks to a DM melting event at ~2.5 Ga.
b, Zircon 18O/16O ratios (expressed as δ18O relative to Vienna Standard Mean Ocean Water) show that the basaltic crust was hydrothermally altered at high temperature (~4.5‰) before generating TTG magmas at 2.35 Ga and before remelting in the metasomatized mantle wedge at 2.13 Ga. The latter event generated sanukitoids that have zircons with heavier oxygen (~6.5‰). Individual error bars in a and b are shown at 2 standard errors.
c, Tectonic model for the generation of magmas in the Palaeoproterozic pre- and post-GOE peak. SCLM stands for subcontinental lithospheric mantle.

Mantle oxygen fugacity ( fO2) probably changed in the early Earth as a result of metallic Fe retention during core formation and further homogenization, but subsequent variations through time are debatable. The mantle fO2 is either described as largely unchanged or overall having a near-constant rate of increase through time.

Common explanations for the absence of identifiable fO2 change in the mantle are based on the ‘infinite reservoir’ argument, given its relatively larger size compared to surface reservoirs (that is, atmosphere, hydrosphere and crust). Nonetheless, mid-ocean ridge basalts show an increase in the Archaean potentially linked to early ‘whiffs’ of atmospheric oxygen, suggesting that the upper mantle can be affected by changes in the atmosphere. However, despite evidence for recycling of continental materials during at least the past 3.0 Gyr, it is unclear how much this has affected the redox state of the mantle, even across the GOE. Here we test the hypothesis of a change in the redox state of magmas in the sub-arc mantle region via recycling processes akin to subduction during the PalaeoProterozoic.

A better understanding of how magmas changed oxidation through time would clarify ocean–atmosphere influence on mantle redox potential and whether deep-ocean oxygenation is a feature restricted to the Phanerozoic.

Using a novel approach the authors use sulfur K-edge micro X-ray absorption near-edge structure spectroscopy to measure the relative abundances of S6+, S4+ and S2− state in apatite inclusions hosted in 2.4–2.1-billion-year-old igneous zircons from the Mineiro Belt, Brazil. The host magmas record intracrustal melting of juvenile crust and the involvement of recycled sediments in the sub-arc mantle wedge. Unaltered apatite inclusions reveal a change from reduced to more oxidized magmas from pre- to post-Great Oxidation Event during the early Proterozoic. The authors argue that this change is a direct result of deep subduction of oxidized sediments and thus evidence of mantle–atmosphere interaction across the Great Oxidation Event.

This suggests that the onset of sediment recycling in the Archaean provided atmospheric access to the mantle, and early ‘whiffs’ of oxygen may have already contributed to a localized increase of calc-alkaline magmatism and related ore deposits on Earth.  Although localized and with less magnitude than the GOE, Archaean ‘whiffs’ of oxygen may have similarly altered the nature of subducted sediments and started to modify the oxygen fugacity of mantle sources in the Archaean. It then suggests that similar mantle–surface interactions could have had a major influence on magmatism and metallogenic endowment since the Archaean and that the oxidation of the mantle was diachronous and related to the onset of global plate tectonics. The subsequent irreversible step-rise of oxygen in the Palaeoproterozoic may have led to the dominance of characteristically oxidized calc-alkaline magmas in the geological record.

Background and Methodology

The transition from tonalite-trondhjemite-granodiorite (TTG) to sanukitoid magmas in Earth’s history provides insights into surface-mantle interactions. This shift began in the Palaeoarchaean era and continued into the Palaeoproterozoic. TTGs form from partial melting of altered basaltic rocks at depth, while sanukitoids require interactions with TTG melts and sediments in a metasomatized mantle wedge, suggesting crustal recycling into the upper mantle. The tectonic settings of this transition are still debated, but they all involve the return of material from Earth’s surface to the mantle.

Sanukitoids are a type of high-Mg granitoid mainly found in convergent margin settings. Originally, the term referred to Archean plutonic rocks, but it now includes younger rocks with similar geochemical characteristics.   Sanukitoid rocks are characterized by orthopyroxene as the mafic mineral, andesine as the plagioclase, and a glassy groundmass. They have specific geochemical criteria, including SiO2 content between 55-60%, high Mg#, Ni, and Cr, enriched LREEs, and no or minor Eu anomalies. The term “sanukitoid suite” includes more evolved rocks derived from sanukitoids through fractional crystallization.  Sanukitoids and adakites share similarities in trace element compositions but differ in Mg and silica content. They are believed to form from the melting of metamorphosed mafic igneous rock protoliths in the mantle, possibly metasomatized by silicate melts from a subducting slab. These melts undergo changes in composition during melting and react with the mantle, resulting in high Sr, low Y, and high LREE/HREE ratios.  Sanukitoids are not likely to form in settings where thick crustal roots of island arcs melt because they cannot assimilate mantle wedge components. They are distinct from boninites, which have similar major element concentrations but are extremely depleted in incompatible trace elements, indicating a different mantle source and history.

This study investigates two suites of intermediate granitoids in the Mineiro Belt, Brazil, which are identified as a 2.35 Ga TTG and a 2.13 Ga sanukitoid. These suites record the youngest known transition from TTG to sanukitoid magmatism, occurring in the Palaeoproterozoic, shortly before a global “tectono-magmatic lull,” and during the time of the irreversible oxygenation of Earth’s atmosphere.

The study used X-ray absorption near-edge structure (μ-XANES) at the European Synchrotron Radiation Facility to determine the valence of sulfur in apatite. They found that crack-free apatite inclusions showed a core-to-rim zonation of sulfur concentration, suggesting sulfur incorporation and retention during early crystallization and reflecting primary characteristics of the crystallizing magma. This zonation is unlikely to result from sulfur exchange with the host zircon crystal, as zircon has minimal sulfur content and slow diffusion rates.

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