Scientists found evidence that Earth’s mantle was interacting with (and being oxidized by) surface oxygen as early as ~3.4 billion years ago, roughly a billion years before the Great Oxidation Event permanently raised atmospheric oxygen levels around 2.3- 2.2 billion years ago.

This comes from a University of Portsmouth- led international study published in Nature Geoscience (DOI: 10.1038/s41561-026-02119-6; lead author Dr. Hugo Moreira). It builds on earlier 2023 work by the same group linking mantle chemistry changes to the Great Oxidation Event itself.

Key methods and findings

Researchers analyzed tiny “time capsules”: apatite mineral inclusions sealed inside durable zircon crystals from sanukitoid rocks (a distinctive type of magma linked to early subduction- like processes). Samples spanned more than 3 billion years and came from sites including the Pilbara Craton (Australia; ~3.42 and 2.9 Ga), Karelian and Kaapvaal Cratons (Finland and South Africa; ~2.7 Ga), São Francisco Craton (Brazil; ~2.13 Ga), and Scotland (~425 Ma).⁠

• Zircons were dated precisely (some to exactly 3.4 billion years old) using laser techniques at Portsmouth.
• Sulfur oxidation states (S⁶⁺, S⁴⁺, S²⁻) in the protected apatite inclusions were measured via sulfur K-edge micro- X- ray absorption near- edge structure (XANES) spectroscopy at the European Synchrotron Radiation Facility (ESRF) in France. These states act as a sensitive proxy for oxygen fugacity (how oxidizing the magma and mantle environment was).⁠

Results show a progressive increase in oxygen fugacity in the sub-arc mantle starting between ~3.4 and 3 billion years ago. The signature remained present across the studied interval and approached modern levels by ~2.1 billion years ago. This indicates that subduction was already recycling oxidized surface material (from early weathering and intermittent “whiffs” of oxygen) deep into the mantle long before permanent atmospheric oxygenation.⁠

The team ruled out alternative explanations (temperature and pressure variations, fractional crystallization, degassing) and noted that even small amounts of oxidized sulfur would have had a strong effect on the more reduced early mantle.

They propose feedback: early oxidative recycling raised sub-arc mantle oxidation → more oxidizing volcanic gases → more felsic crust and intensified surface weathering → further contribution to long- term atmospheric oxygen buildup.

This provides the most continuous timeline yet of how surface oxygen and deep- Earth processes co-evolved. It supports the idea that plate tectonics (or early analogs) connected Earth’s exterior and interior reservoirs far earlier than often assumed, helping create conditions for a habitable planet.

The work is curiosity- driven basic research into Earth’s origins and the emergence of life, rather than an immediate practical application.⁠

In short, the mantle was effectively “breathing” (exchanging and responding to) oxygen via recycling processes a billion years before free oxygen dominated the atmosphere.

Sanukitoid petrogenesis refers to the origin and formation processes of a distinctive suite of intermediate to felsic igneous rocks (mainly diorites, monzodiorites, granodiorites, and related rocks) that are characteristic of the late Archean to early Paleoproterozoic (~3.0- 2.5 Ga, with some earlier examples from ~3.4- 2.9 Ga).

They mark a transitional phase in Earth’s crustal evolution, bridging the dominant Archean tonalite- trondhjemite- granodiorite (TTG) magmatism and more modern- style calc- alkaline arc magmatism.

Geochemical Characteristics

Sanukitoids are defined by a dual “mantle and crust” signature:

• Mantle- compatible elements: High MgO (typically 1.5–9 wt%), Mg# (molecular Mg/ (Mg and Fe) ≈ 45–65 or higher in primitive members), Ni (>100 ppm in many definitions), Cr (>100- 200 ppm).
• Incompatible elements: Strong enrichment in large- ion lithophile elements (LILE), especially K₂O (1.5–5 wt%), Ba, and Sr (Ba and Sr often >1,000–1,400 ppm); high light rare- earth elements (LREE); relatively high K₂O/Na₂O compared with TTGs; fractionated REE patterns with low HREE and no or minor Eu anomalies.⁠

Classic strict definition (Stern et al., 1989) focused on SiO₂ 55–- 60 wt% with the above traits, but the broader “sanukitoid suite and series” includes more evolved members produced by fractional crystallization. High- Ti and low- Ti varieties are sometimes distinguished.

They resemble high- Mg andesites (sanukites) from the Setouchi volcanic belt in Japan, hence the name.

Petrogenetic Models

The consensus is that sanukitoids form by interaction between mantle peridotite and a recycled crustal component (typically TTG- like or slab- derived felsic melts, possibly with fluids and/or sediments).

Two main (not mutually exclusive) pathways are debated:

1. Two- stage (metasomatism and later melting) process (most widely invoked)
   • Stage 1: Subduction- related fluids and or silicate melts (from partial melting of subducted oceanic crust and metabasite, often eclogite- facies, ± sediments) metasomatize the mantle wedge or subcontinental lithospheric mantle. This creates enriched domains (e.g., amphibole- or phlogopite- bearing veins).
   • Stage 2: Later thermal perturbation (slab break-off, mantle plume, or continued subduction heating) causes partial melting of this metasomatized peridotite, generating the sanukitoid parental magma.⁠
2. One- stage (assimilation and reaction) process
   • TTG- like or hydrous eclogite melts directly react with and assimilate mantle peridotite during ascent, producing hybrid melts with the characteristic dual geochemistry.⁠

Recent Ti- isotope studies (δ⁴⁹Ti) of primitive sanukitoids show elevated values requiring a hydrous metabasite (eclogite) melt component formed in equilibrium with residual rutile. This favors fluid- fluxed melting of eclogite in a subduction- like setting and indicates parental magmas were water- rich and relatively oxidized, comparable to modern arc magmas.⁠

Melting conditions are typically high- pressure (>1- 2.5 GPa, corresponding to ~35- 80 and km depth), hydrous, and oxidized. Differentiation often involves amphibole ± plagioclase ± orthopyroxene fractionation, sometimes at lower pressures where a distinct cotectic may control the series.

Geodynamic Significance

Sanukitoids appear mainly near the Archean- Proterozoic transition and are linked to the progressive cooling of Earth and the evolution of subduction:

• Early Archean: High heat production favored extensive slab melting → pure TTG magmas with limited mantle interaction.
• Late Archean: Reduced slab melting → more interaction with the mantle wedge → sanukitoids (hybrids).
• Post ~2.5 Ga: Predominantly fluid- fluxed melting of the mantle wedge (modern- style arcs), with slab melts rarer.⁠

They are therefore key evidence for the onset or maturation of plate- tectonic (or proto- subduction) processes, deep crust- mantle recycling, and the changing nature of continental crust growth.

In the context of the recent Nature Geoscience study on mantle oxidation, sanukitoids serve as excellent monitors of sub- arc mantle redox state because their sources record metasomatism by recycled surface materials.

Modern Analogues and Distribution

Phanerozoic high- Mg diorites, vaugnerites, appinites, and some post-collisional I- type granitoids share similarities and may represent analogues. Archean examples occur worldwide (e.g., Superior Province, Pilbara, Kaapvaal, Karelian, North China, Amazonian/Carajás, Yilgarn, and São Francisco cratons).

In summary, sanukitoid petrogenesis records hybridization between depleted mantle and recycled crustal melts and fluids in a subduction- related setting under hotter Archean conditions.

This dual signature makes them invaluable for tracking early Earth geodynamics, mantle metasomatism, and the co- evolution of crust and mantle.

Progressive oxidative weathering raised sub-arc mantle fugacity on the Archaean Earth

Abstract (summary)

In the modern Earth, sub- arc mantle oxygen fugacity is linked to addition of sulfur species by slab fluids. It has been unclear to what extent volatile recycling controlled redox changes in the early Earth.

Using sulfur K -edge micro- X- ray absorption near- edge structure (μ-XANES) spectroscopy, the authors measured the relative abundances of S⁶⁺, S⁴⁺, and S²⁻ in apatite inclusions hosted in geochemically characterized and precisely dated zircons from sub- arc sanukitoid magmas spanning 3.4 billion years of Earth history.

Key results:

• The sub- arc mantle shows a marked increase in oxygen fugacity (fO₂) between 3.4 and 3.0 Ga.
• It is substantially more oxidized than the unaltered ambient upper mantle.
• This cannot be explained by fractional crystallization, degassing, or intrinsic mantle processes.

The authors propose that progressive oxidative weathering increased the proportion of oxidized sulfur recycled into mantle- wedge sources.

This established a feedback between subduction of surface materials and progressively more oxidized arc magmatism.

Early subduction involving inorganic oxidation and intermittent “whiffs” of oxygen between 3.4 and 2.7 Ga may have driven an early rise in sub-arc mantle fugacity before the Great Oxidation Event (GOE, ~2.3-2.2 Ga), reaching conditions comparable to the modern Earth by ~2.1 Ga.

Methods highlights

• Samples: Geochemically coherent sanukitoids (metasomatized- mantle- derived magmas) from the Pilbara Craton (Australia; 3.42 and 2.9 Ga), Karelian Craton (Finland; 2.7 Ga), Kaapvaal Craton (South Africa; 2.7 Ga), São Francisco Craton (Brazil; 2.13 Ga), and Northern Highland Terrane (Scotland; 0.425 Ga).
• Protected apatite inclusions inside zircon crystals acted as unaltered “time capsules”.
• 337 analyses on 39 apatite inclusions + 213 analyses on 65 matrix apatites, performed at the European Synchrotron Radiation Facility (ESRF, ID21 beamline).
• S⁶⁺/S_total ratios converted to ΔFMQ (deviation from the fayalite- magnetite- quartz buffer) using an experimental oxybarometer.
• Supporting U- Pb dating, trace- element, and oxygen- isotope data on the host zircons.

Broader implication

This provides the first continuous record showing that surface oxidative weathering products were already being recycled into the deep Earth via early subduction- like processes more than a billion years before permanent atmospheric oxygenation. The mantle was not a passive spectator; it participated in a feedback loop that helped shape the conditions leading to a habitable, oxygenated Earth.