The Mantle Fe^3+/ΣFe Ratio Has Doubled Since the Early Archean

Domain: earth_science
Published: 2026-01-14
Editorial outcome: major_revision

Abstract
How mantle redox state developed, particularly the mantle source associated with mid-ocean ridge-like settings, remains a subject of ongoing debate. Here, we employ the thermodynamic-thermomechanical numerical simulations to explore the redox properties of melts formed under mid-ocean ridge-like settings in both Archean and modern conditions. By comparing these results with a global database of mid-ocean ridge-like rocks extending back to 3.8 Ga, we reconstruct the mantle’s redox evolution since the early Archean. Using the whole-rock Fe³⁺/ΣFe ratio as a robust redox proxy, derived from integrated numerical modeling and thermodynamic inversion, we find that the mantle’s average Fe³⁺/ΣFe ratio has approximately doubled since the early Archean. Our calculations further indicate that ultra-low-oxygen-fugacity mantle domains in modern oceanic lithosphere reflect an initially reduced origin rather than deeper or hotter melting. Our results suggest that Earth’s oxygenation and tectono-magmatic evolution may have been coupled.

Introduction
The redox state of Earth is a critical factor in determining its habitability, shaping the chemical composition of the surface and atmosphere while influencing biological activity and interior geological processes1–5. As the largest component of Earth’s mass, the mantle plays a pivotal role in regulating the planet’s redox balance2,4. This regulation is achieved through processes such as mantle convection, degassing, and the cycling of redox materials between Earth’s interior and surface4–7. These mechanisms not only govern the mantle chemical and physical properties but also have far-reaching effects on Earth’s surface environment, atmospheric composition, and conditions necessary for sustaining life4–10.

The mechanisms controlling the secular redox evolution of the MORB-source mantle have been debated for decades6,7,11–15. Notably, an increase in mantle oxygen fugacity (fO 2) exceeding 0.5 log units relative to the QFM (quartz-fayalite-magnetite) buffer could significantly influence atmospheric oxygen levels, potentially acting as a catalyst for the Great Oxidation Event (GOE)4. Some studies suggest that the mantle’s redox state has remained unchanged since the Archean or Hadean11,12. For example, the modern mantle’s fO 2 is considered to be comparable to Archean mantle based on similar V/Sc (vanadium/scandium) ratios in basalts12. However, growing petrological evidence from basalts, eclogites, komatiites, and ancient mantle6,7,13–15 indicates that the Archean mantle fO 2 was ~1–1.5 log units lower than QFM buffer, even 4–5 log units lower than modern levels.

Recent research, however, hypothesizes that the increasing fO 2 inferred from petrological records might be caused by the mantle gradual cooling11,15. During the Archean, a higher mantle potential temperature (Tp) likely resulted in deeper melt extraction depths11. The garnet-bearing mantle extends beyond ~8–10 GPa, but metal saturation at around these pressures begins to buffer the mantle fO₂, which generally decreases with increasing pressure. At pressures (P) < 3–3.5 GPa, the fO 2-depth profile is more complex, as demonstrated by phase equilibria and empirical models15–17. Previous studies have attempted to account for the pressure dependence of mantle redox conditions, but with differing approaches and limitations. A correction for the decrease in fO₂ with increasing pressure was explicitly applied, revealing significant differences between Archean and modern mantle-derived samples6. In contrast, a linear correction was applied to V/Ti-derived fO₂ values based on melt evolution during ascent11. However, since V/Ti reflects the redox state of the source condition rather than the melt after evolution, such a correction is inappropriate. Furthermore, both studies likely overestimated the fO₂ of Archean samples, which are typically derived from greater depths due to higher Tp, by extrapolating the fO₂–P relationship established for garnet peridotite to pressures down to 1 GPa6 or to surface conditions11. This extrapolation has since been shown to be invalid by phase equilibrium calculations17. It is also worth noting that the phase equilibrium is based on solid-solid relationships17, and the calculated results may also differ from those of melt-bearing systems15.

For a long time, the redox state of the mantle and mantle-derived melts has been characterized primarily using either oxygen fugacity6,7,11,13–15,18 or geochemical ratios such as V/Sc12. As mentioned above, although previous studies have attempted to correct for the effects of temperature (T) and P on fO₂6,11, different calibration approaches have led to significant discrepancies in the results. Fundamentally, these reliances have sparked controversy over whether the mantle’s redox state has evolved over time. Oxygen fugacity is inherently complex, being influenced by multidimensional factors such as P, T, whole-rock composition, and oxygen content (e.g., Fe3+/ΣFe)2,16.

Results
Modern and Archean MORB redox modeling
Basalt is suggested as a better proxy for the mantle’s redox state compared to komatiite, because its P-T estimates in natural records carry relatively small uncertainties11, and because its source characteristics and the melt extraction processes leading to MORB generation are relatively well constrained. Numerical simulations indicate that even when the mantle potential temperature reaches 1600°C—considered by some studies to be representative of the early Archean mantle19, though others argue this is too high20—MORB-like rocks still dominate the composition of the oceanic crust. We thus conducted a series of numerical experiments on mid-ocean ridge spreading for different Tp values representative of modern and Archean conditions (Supplementary Data 2). This forward model is based on simulations conducted within a coupled thermo-mechanical-thermodynamic framework, using prescribed geodynamic conditions and rock compositions. The modeling does not rely on V-based oxybarometers to determine the Fe3+/ΣFe ratio in basaltic melts. In reference to numerical experiments (half-spreading rate = 3 cm/yr; Fig. 2), the extraction/formation P-T conditions of newly formed basaltic melts were analyzed at ~6.5 Ma model time of mid-ocean ridge activity.

The results demonstrate that as the Tp increases, the P-T conditions for basaltic melt extraction also increase (Fig. 2). For modern Tp conditions, the average extraction P-T conditions are 1.1 GPa and 1344°C (Fig. 2F, G; Fig. S1A), consistent with petrological records18. In contrast, in Archean simulations, a Tp of 1500°C, which represents the previously estimated warm Archean mantle Tp20 and the lower bound of the hot Archean Tp range (1500–1600°C)19, indicates extraction conditions of 1475°C and 2.1 GPa. Further increases in Tp to 1550°C and 1600°C result in extraction P-T conditions of 1491°C and 2.3 GPa, and 1528°C and 2.6 GPa, respectively (Fig. 2G–I; Fig. S1B). This shows a positive feedback effect between Tp and the P-T conditions of melt extraction, consistent with previous suggestions11,15. The relatively lower melting extraction temperatures observed beneath mid-ocean ridges compared to Tp are primarily attributed to cooling effects induced by melting processes. This phenomenon generates a temperature-depth gradient that significantly exceeds the conventional value (0.5°C/km) in the partially melted regions. In Archean models, the presence of distinct cold drips and the subsequent mixing between the colder lithospheric mantle and asthenosphere enhances this cooling effect (Fig. 2; see also Fig. S6).

A key strength of our numerical model is its incorporation of redox thermodynamic iterative calculations within an open system (Methods). This approach effectively integrates the influence of geophysical parameters, the thermodynamic equilibrium among complex solution models, and the findings from experimental petrology. This allows for the calculation of the whole-rock Fe3+/ΣFe of basaltic melts based on their extraction P-T conditions and the given mantle composition (Fig. 2). In the reference experiment, two initial whole-rock Fe3+/ΣFe values were imposed for the mantle to assess their impact on the redox state of the derived basaltic melts. The first value = 0.04 falls within the reported modern mantle range (0.036–0.053)10,25,26, corresponding to source forming MORB with fO 2 near QFM at the related P-T conditions. The second value = 0.02 reflects a relatively reduced mantle state10.

The results indicate that at the modern Tp, the mantle composition with an initial Fe3+/ΣFe of 0.04 can produce basaltic melts with Fe3+/ΣFe ranging from 0.10 to 0.11 (Fig. 2F, G, K, L), aligning well with observed values of 0.10–0.1427–30. However, the reduced mantle composition with an initial Fe3+/ΣFe of 0.02 produces reduced basaltic melts with Fe3+/ΣFe ratios of 0.05–0.06 (Fig. 2F, G, K, L). At the Archean Tp, the basaltic melt exhibits a more reduced redox state. The mantle composition with an initial Fe3+/ΣFe of 0.04 yields basaltic melts with Fe3+/ΣFe ranging from 0.07 to 0.08 (Fig. 2H–J, M–O), while the reduced mantle (initial Fe3+/ΣFe = 0.02) produces basaltic melts with a low Fe3+/ΣFe of 0.04 (Fig. 2H–J, M–O). This low Fe3+/ΣFe of basaltic melts is roughly similar to that reconstructed for Archean eclogite xenoliths, which have been interpreted as metamorphosed oceanic crust derived from mid-ocean ridges7. Compared to modern results, these findings indicate that an increased Tp during the Archean promoted the reduction of basaltic melts. This is because the temperature of melt extraction is higher under the hotter Tp condition (Figs. 1 and 2; Supplementary Data 2).

The double Fe3+/ΣFe of modern mantle vs. Archean
Numerical forward modeling indicates that a higher Tp can indeed result in a lower Fe3+/ΣFe of basaltic melts within a certain range, which is primarily controlled by high temperature (Fig. 1A), even with the same whole-rock Fe3+/ΣFe of the mantle sources. However, does this phenomenon fully explain the differences in oxygen fugacity observed between Precambrian (particularly Archean) and modern basalts? We performed thermodynamic back-calculations on the compiled basalt samples (Methods, Fig. 3A; Supplementary Data 3). We first determined the P-T-fO 2 conditions during the melting extraction/formation of MORB-like basaltic rocks using V-based oxybarometers (Fig. 3B; Fig. S2A). This enabled us to apply our thermodynamic methods (alongside empirical methods)27,28 to back-calculate the whole-rock Fe3+/ΣFe of basaltic melts (Fig. 3C; Fig. S2B, C). Subsequently, thermodynamic modeling could estimate the oxygen fugacity and whole-rock Fe3+/ΣFe in the mantle sources (Fig. 3D, E; Fig. S2D–F).

For the Archean samples, our results indicate that the average V-Ti (titanium) oxygen fugacity13,18 recorded by basalt samples is −1.5±0.2 (Fig. 3B; Supplementary Data 3), under average P-T conditions of 2.2 GPa/1500°C that align with results obtained from numerical simulations at mantle potential temperatures of 1500–1600°C, with an average of 2.4 GPa/1502°C (Figs. S1, S2). The V-Sc oxygen fugacity13,18, based on a smaller number of samples than the V–Ti dataset, is reported as QFM-1.1±0.3 (Fig. S4; V-Ti oxygen fugacity of the same samples is QFM-1.4±0.2). Despite variations in filtering methods and data sources, the calculated primitive oxygen fugacity aligns with previous conclusions on basalts6,7,11,13, with values of QFM-1.2 at 1.9 GPa/1455°C and QFM-1.3 at 2 GPa/1469°C, both within error margins. This consistency reinforces the conclusion that the oxygen fugacity of Archean basaltic melts is relatively low.

The whole-rock Fe3+/ΣFe of basaltic rocks at primitive P-T-fO 2 conditions of melting extraction was calculated using thermodynamic simulations, yielding an average value of 0.04±0.01 (Fig. 3C; Fig. S2–S4), consistent with empirical methods (Fig. S2; 0.04). Additionally, our thermodynamic calculations for two previous basalt databases yielded a slightly higher whole-rock Fe3+/ΣFe of 0.05±0.0111,13 (Fig. S3; Supplementary Data 3). Thermodynamic back-calculation of all databases provided consistent results, showing that the Archean mantle had an average whole-rock Fe3+/ΣFe ratio of 0.02 (Fig. 3E; Fig. S3).

The back-calculation results closely match those of numerical simulations (Figs. 2–3; Supplementary Data 2). Specifically, if the Archean mantle had the same Fe3+/ΣFe ratio as the modern mantle, it would produce a basaltic melt with a Fe3+/ΣFe ratio of 0.07–0.08 with Tp = 1500–1600°C. However, actual records indicate an average Fe3+/ΣFe of 0.04–0.05, aligning with simulation results corresponding to melts with a mantle Fe3+/ΣFe ratio of 0.027. This indicates that the redox state of the Archean mantle as proxied by its Fe3+/ΣFe was significantly lower than the modern mantle (0.04–0.05).

We further evaluated the potential impact of valence state changes in carbon and sulfur on the mantle redox budget. In the modern mantle, carbon in MORB source is typically assumed to exist as C⁴⁺, serving as a reference state22. Thermodynamic calculations suggest that C⁴⁺ can remain stable down to QFM-1.5 to −2 buffer at 2–3 GPa23,24, corresponding to the oxygen fugacity of the Archean mantle. Furthermore, given the low abundance of C in typical MORB mantle sources22, its contribution to the redox budget is limited, even if valence transitions occur. Sulfur in the modern MORB mantle already exists in its reduced form as S²⁻, and it is unlikely to have been in a more reduced state in the Archean22. Thus, sulfur valence states are negligible in terms of their effect on the redox budget of the MORB mantle.

In summary, our findings suggest that the mantle Fe3+/ΣFe, as a proxy for redox state and budget, has doubled. A deeper onset of melting owing to higher Archean mantle Tp alone is insufficient to explain the differences in oxygen fugacity recorded in rocks. This stands in stark contrast to recent reports11,15 but is consistent with the conclusions of some previous studies6,7,13,14.

Ultra-reduction unattainable through deep and hot melting
Some highly reduced mantle domains (down to QFM-4 to -5, and spinel Fe3+/ΣFe ratios can be lower than 0.03) identified in present-day oceanic lithosphere are thought to represent residual material after partial melting of the Archean mantle15. These reduced mantle domains were interpreted as residues that had the same redox state as the modern mantle but underwent extensive partial melting (>20%) at high P-T conditions that could generate komatiites15.

A key implication of our study is that it does not support this inference. As shown in Fig. 4, we modeled the melt extraction from a modern oxidized mantle (Fe3+/ΣFe = 0.04) along an isobaric trajectory at 3–5 GPa until the melt fraction (F) exceeded 35%, then we calculated the Fe3+/ΣFe and other redox parameters (e.g., fO 2 and Fe3+ in minerals) of the residual material (Fig. 4A, B). After undergoing very high degrees of partial melting, the Fe3+/ΣFe in the residues decreases, but the degree is insufficient to match the observed values (Fig. 4A, B). For example, at 4 GPa and 1750°C, the mantle experiences high melting degrees (F = 38%). The residual mantle is harzburgite with a very high Cr# in spinel, consistent with observations15 (Fig. S5). However, the in-situ residual mantle oxygen fugacity is QFM-1.3 at this stage, and the Fe3+/ΣFe in spinel exceeds 0.2, both of which are much higher than the observed values15 (Fig. S5).

Therefore, the observed ultra-reduced mantle residues cannot be explained by high melting degrees at deeper depth during Archean. Such residue formation can only be attributed to a mantle source that initially had a very low Fe3+/ΣFe (even <0.01) before melting (Fig. 4B; Fig. S5B, D). This further suggests that Archean mantle was more reduced than the modern mantle.

The tortuous evolution of Mantle Fe3+/ΣFe ratio
Our study, in addition to comparing the redox states of the Archean and modern mantle, reveals the evolutionary trajectory of mantle Fe3+/ΣFe since the Hadean (Figs. 3 and 5), particularly for MORB-like mantle source regions, which is tortuous, not a monotonic process and likely reflects several major events in Earth’s geological history.

Our discussion is based on the trends reflected in the current database. However, it should be noted that, despite the application of stringent screening criteria (e.g., (Nb/La) ≥1) to ensure comparability with modern MORB-like environments, the precise provenance and tectonic setting of the basalt samples cannot be fully constrained. Moreover, preservation bias potentially induced by the supercontinent cycle and variations in tectonic regime strength may have influenced the spatiotemporal distribution of basaltic records (Fig. 3). In combination with limited statistical coverage in certain geological periods, these factors warrant caution when interpreting the compiled data. Future discoveries and reporting of additional rock records may help mitigate the underrepresentation of specific time intervals, such as the Mesoproterozoic and the Eoarchean.

Specifically, Hadean mantle-derived zircons suggest that the redox state of ~4.4 to 4.0 Ga upper mantle may have been close to or even exceeded modern levels31 (Fig. 5A, C). By the time a melt becomes saturated in zircon, its composition has typically been significantly modified or evolved from the original mantle-derived melt. Therefore, whether fO 2 recorded by such melts truly reflects mantle conditions remains questionable. Nevertheless, we still performed Fe³⁺/ΣFe estimations on the relevant samples for comparison with previously established redox trends. Our modeling suggests that these mantle source regions may have contained an average Fe³⁺/ΣFe of 0.07 (Methods). Some studies proposed that this Hadean-mantle redox state resulted from the redox disproportionation of Fe2+ into Fe3+ and Fe0 within a magma ocean stage, a process that may have extended to the top of lower mantle32,33. The high-density metallic iron would have sunk into the lower mantle or core2,33,34, leaving the upper mantle magma ocean increasingly oxidized (Fig. 5A).

Hadean zircon analyses further suggest that the mantle became progressively more reduced during the late Hadean31,32. This is consistent with findings from our basalt database and other studies6,11,13, which indicate that the Archean mantle exhibited highly reduced conditions (Fig. 5C). A plausible explanation for this reduction lies in a late accretion event before early Archean, during which chondritic meteorites comprising 0.5–1% of Earth’s mass delivered metallic iron to the mantle (Fig. 5A), triggering a reduction event32,33. This hypothesis suggests that the accretion of meteorites with negative tungsten (W) isotopic signatures (–190 ppm) lowered the W isotopic composition of the silicate mantle, which initially had a positive W isotope composition (e.g., 20–30 ppm), in the wake of core formation and evolution at high Hf/W35–38 (Fig. 5E). Mass balance calculations indicate that this impact event at the Archean–Hadean boundary could have significantly altered the mantle’s redox state into observational results (Fig. 5), even if the magma ocean initially had a high Fe³⁺/ΣFe ratio (>0.2)32. If this hypothesis is correct, the surface oxidizing environment was interrupted for over a billion years due to this meteorite impact event, hindering the early development of Earth’s habitability (Fig. 5B, C).

Following this event, the mantle redox state remained relatively reduced during the early Archean31, with sparse samples suggesting the persistence of single-lid tectonics (Figs. 3 and 5). It was not until 3.0–3.2 billion years ago that the mantle began to oxidize progressively (Fig. 5A, C). One hypothesis suggests that this oxidation is contributed from an Fe3+-rich-bridgmanite-bearing lower mantle layer through mantle plume activity9. From the perspective of mass balance and charge balance, this has been proven to be a reasonable assumption. However, the hypothesis still faces the following challenges:

The lower mantle is not necessarily more oxidized than the upper mantle. Recent thermodynamics simulations suggest that metallic Fe⁰, produced during the disproportionation reaction associated with the formation of Fe³⁺-rich bridgmanite, may have accumulated at the top of the lower mantle rather than sinking into the core34. This implies that the overall redox budget may not have changed.

Although recent studies suggest that a basal magma ocean is geodynamically plausible39, the melt fraction at the core–mantle boundary at that time would have been close to 100%, making it difficult for Fe³⁺-rich bridgmanite to crystallize and thus for the disproportionation reaction to proceed39. Therefore, from both thermodynamic and geodynamic perspectives, a new mechanism for generating Fe³⁺-rich bridgmanite may be required to further support the oxidized plume hypothesis.

Finally, considering the similarities to Earth, Fe³⁺-rich bridgmanite should also be present on Venus, and the planet exhibits vigorous mantle plume activity. However, it does not show any evidence for a mantle oxidation mechanism40. Nonetheless, the associated hypothesis presents an appealing explanation. Future progress in experimental petrology and planetary science may provide critical insights to test and potentially validate this model.

Another plausible mechanism is the onset of global mobile-lid tectonics from 3.0–3.2 billion years ago and continued into the Archean–Proterozoic boundary41–44, which initiated large-scale mantle oxidation (Fig. 5D–G). Multiple geological evidence supports this tectonic transition, including the increasing abundance of zircons, which suggest the growth of felsic rocks; changes in seawater strontium (Sr) isotopes and the proliferation of passive margins; and the emergence of bimodal metamorphism and eclogite-facies rocks, characteristic of subduction-driven tectonics41–43 (Fig. 5D–G). Recent data also indicate large-scale craton movement associated with mobile-lid tectonics at least at ~2.7 Ga44.

Our compiled database of arc rocks further highlight subduction processes and the associated development of arc magmatism since the late Archean (Fig. 5D; Supplementary Data 3). Subduction zones played a critical role in enhancing mantle oxidation by transporting accumulated surface redox budget into the mantle, which was generated through hydration reactions (e.g., formation of serpentinite and altered oceanic crust)45, thereby increasing mantle redox state.

Before the onset of global subduction (e.g., the early Archean), the oceanic lithosphere accumulated redox budget through oceanic hydration reactions and released reduced gases such as hydrogen. During the Archean, the escape rate of hydrogen was particularly high46, leaving behind a net redox budget in the oceanic lithosphere. In addition, these reduced gases likely acted as sinks for oxygen by reacting with O produced by photosynthesis after entering the atmosphere. Notably, prior to subduction, the redox budget stored in the oceanic lithosphere did not contribute oxygen to the atmosphere due to isolation by seawater.

With global subduction initiation at the Archean–Proterozoic boundary, a substantial portion of the redox budget was transferred into the mantle. This marked the emergence of new oxygen sources driven by subduction. On one hand, subduction generates oxidized arc magmas and their associated oxidized degassing. On the other hand, mantle oxidation induced by subduction can enhance the redox state of other tectonic degassing processes (e.g., mid-ocean ridges). These processes represent new mechanisms for transferring redox budget from Earth’s interior to the atmosphere.

Geological evidence indicates that most Archean komatiites exhibit elevated high Fe3+ contents as a result of serpentinization45, and the earliest records of serpentinization date back to the early Archean47. In addition, banded iron formations (BIFs), which are unique to the Precambrian, also exhibit very high ferric iron Fe3+ ratios, all of which indicate the early accumulation of oxidized materials in the lithosphere. Our latest quantitative results (D.W.Y., G.T., & Z.X., in preparation) reveal that the global subduction system transfers a net redox budget of 36±6×1012 mol/yr to the mantle. This estimate is conservative, as it does not account for the additional redox budget contributed by serpentinized komatiites, BIFs, and the altered oceanic crust unique to modern settings (e.g., Fe3+/ΣFe > 0.5), but only considers the contributions from previously observed Archean altered oceanic crust with relatively low Fe3+/ΣFe < 0.26. Mass-balance calculation suggest that this redox budget is sufficient to oxidize the whole mantle to the modern levels (D.W.Y., G.T., & Z.X., in preparation).

Numerical simulations reveal that Archean-Paleoproterozoic subduction differed markedly from modern ones48, as higher Tp caused frequent slab break-off (Fig. 5A and Fig. S6). Due to the lack of negative buoyancy, some smaller slabs were retained in the mantle transition zone, where dense oceanic crust separated from lighter lithosphere48. Most subducted lithosphere returned to the upper mantle, driven by positive buoyancy, and recycling within it48 (Fig. 5A). This ancient cycle style can accelerate the upper (or shallow) mantle oxidation due to the smaller mass of mantle.

Some studies suggested that the mantle redox state during the Paleoproterozoic approached modern levels6,7,14. Our and other previous data do reveal the presence of such oxidized mantle compositions11, but also record relatively reduced compositions, likely reflecting mantle heterogeneity7. Thus, while our findings indicate further mantle oxidation during the Paleoproterozoic, the average redox state had not yet reached modern levels (Fig. 5C).

Whether plume-driven or subduction-driven mechanisms, the result is that mantle oxygen fugacity increased by more than 0.5 log units relative to the QFM buffer between 3 and 2.3 Ga (Fig. 3D), calculations of which suggest could still trigger GOE4. Limited Mesoproterozoic data from our and other studies6,7,11,14 may indicate a stagnation or even a decrease in mantle oxidation during this period. Geological evidence49,50, such as the decreasing convergent margins, an increase in metamorphic T/P ratios, and the decreasing metamorphic cooling and exhumation rates (Fig. 5), suggests a weakening or stagnation of mobile-lid tectonics. Subduction could no longer transfer redox budgets into mantle. However, due to limited data, we remain cautious about the trend during Mesoproterozoic period.

Mantle oxidation resumed from the Neoproterozoic to now13, overlapping with modern plate tectonics characterized by low-temperature metamorphism (e.g., blueschist; Fig. 5)43,50. During this period, accumulated surface redox budgets from the Mesoproterozoic could be rapidly transferred into the mantle via subduction (D.W.Y., G.T., & Z.X., in preparation). The simultaneous occurrence of Neoproterozoic oxidation events (NOE) suggests that these were likely not coincidental but driven by mantle oxidation processes (Fig. 5B, C), then further facilitated the emergence of complex life. ~4-billion-years accumulation of surface redox budgets47, over 2-billion-years global episodic subduction activities (Fig. 5) and mantle convection resulted in relatively uniform mantle redox conditions, leading to the more consistent redox states recorded in modern MORB27–30, compared to the early Earth (Figs. 3 and 5).

Our analysis highlights that shifts in the mantle redox state closely track major tectono-magmatic events, serving as key indicators of Earth’s dynamic evolution (Fig. 5). These redox changes, while primarily reflecting large-scale geodynamic reorganizations, also influence surface conditions by linking deep Earth processes to atmospheric oxygenation and planetary habitability. This underscores the dual role of mantle redox evolution, which both records and modulates the coupled geodynamic, geochemical, and biological systems that sustain Earth over geological time.

Methods
Basalt reflects the mantle redox state
During the early Archean, Tp was 150–300°C higher than that at present19,20. The preserved rocks from that period are exceedingly rare. These elevated temperatures have profound implications for the tectonic regimes, directly informing the selection of tectonic scenarios and rock types for subsequent redox simulations. Numerical simulation developed by ref. 51. can effectively reconstruct the tectonic regimes of that time. Here, we show a case of this model for the early Archean under the Tp condition of 1600°C (Fig. S6).

The early-Archean simulation indicates that the thick oceanic crust (~20–25 km), composed of cold and dense material, experienced subsidence due to negative buoyancy. This process triggered the formation of several mantle upwelling zones, where ascending, hotter, melt-bearing peridotite underwent additional decompression melting. This led to the generation of basaltic or picritic magma, which subsequently formed new hydrated basaltic lavas at the crust surface. The mantle became progressively depleted due to melt extraction.

A series of spreading centers formed, producing basaltic melts that generated a new basaltic crust (Fig. S6). Over time, this process facilitated the formation of oceanic slabs. A large spreading center emerged, with weak zones developing along its margins, eventually giving rise to Archean subduction events characterized by frequent slab break-off48.

Notably, basaltic melts at the spreading centers remained the primary product of mantle melting and the dominant component of oceanic crust, consistent with geological observations, despite the high Tp favored the frequent komatiite formation. In comparison, the P-T conditions for basalt formation are relatively constrained11, leading to smaller errors for redox-state calculation. Thus, MORB-like basalts are the more suitable candidates for tracking mantle redox states since the Archean, based on the simulation and observation. Under modern geodynamic regimes, if the upper and lower mantle can mix effectively and their composition becomes relatively homogeneous, basalts can represent the average state of the mantle.

It seems difficult to determine the redox properties of the lower mantle in the Archean. However, a reasonable hypothesis is that if magmatic redox states could only persist to the top of the lower mantle32, and meteorite impacts also affected the lower mantle35, then theoretically, the lower mantle in the Archean would have been reduced. The oxygen stagnation during the Mesoproterozoic might also indicate that more reducing materials from the lower mantle were brought into convection during mantle mixing (Fig. 5).

Thermomechanical modeling
In this study, we employed the two-dimensional (2D) thermo-mechanical simulation code I2VIS52 to solve the governing equations for momentum, mass continuity, and heat conservation. The numerical approach is based on the marker-in-cell method combined with a finite difference scheme, ensuring accurate spatial and temporal discretization52. The fundamental equations governing the system are expressed as follows:

∂σ0ij/∂xj - ∂P/∂xi + ρgi = 0 (1)
div (~v) = ∂vx/∂x + ∂vy/∂y = 0 (2)
ρCp DT/Dt = ∂/∂xj (κ ∂T/∂xj) + Ha + Hr + Hs (3)

where ∂σ0ij represents the deviatoric stress tensor, P denotes the pressure, ρ is the density, and gi corresponds to the gravitational acceleration. The velocity components in the horizontal and vertical directions are vx and vy, respectively. The parameter Cp stands for the isobaric heat capacity, while DT refers to the material derivative of temperature. The terms Ha, Hr, and Hs characterize the contributions from adiabatic, radiogenic, and shear heat production, respectively. The latent cooling and heating effects related to respectively melting and crystallization of rocks is taken into account implicitly by correcting Cp and Ha values in the temperature equation53.

Rheology. We adopt a visco-plastic rheology to describe rock deformation. The viscous creep of rocks is formulated based on deformation invariants and is influenced by temperature, pressure, and strain rate. Rocks exhibit both plastic and viscous behavior, transitioning into a slowly creeping fluid-like state over long timescales. The plastic rheology follows the Drucker–Prager yield criterion54, which defines the yield stress as:

σyield = C + Psin(φ) (4)

where C represents compressive strength (cohesion), P denotes pressure, and φ is the internal friction angle. For dry rocks, the friction angle is given by φdry, and the influence of pore fluid pressure is accounted for by the pore fluid pressure factor λ, modifying the frictional term as:

Psin(φ) = Psin(φdry(1-λ)) (5)

The plastic viscosity is determined using the relationship:

ηplastic = σyield / 2ε_II (6)

where ε_II represents the second invariant of the strain rate tensor. For the viscous rheology, the effective ductile viscosity follows a power-law dependence on stress:

ηductile = A-1/n σ(1-n)/n exp(Ea + PVa / RT) (7)

where: Ea is the activation energy, Va is the activation volume, n is the stress exponent, R is the universal gas constant, AD is a material constant. Finally, the effective viscosity is determined as the lower bound of plastic and ductile viscosities:

ηeff = min(ηplastic, ηductile) (8)

Melt. the melt fraction is assumed to follow a linear dependence on temperature. For a given rock type and pressure condition, the volumetric melt fraction M is determined using the following formulation:

M = 0, T ≤ Tsolidus
M = (T - Tsolidus) / (Tliquidus - Tsolidus), Tsolidus < T < Tliquidus (9)
M = 1, T ≥ Tliquidus

where Tsolidus and Tliquidus represent the solidus and liquidus temperatures for the specific lithology (see Supplementary Data 4). The effective density of partially molten rocks, ρeff, varies as a function of the melt fraction and pressure-temperature (P-T) conditions:

ρeff = ρsolid - M(ρsolid - ρmolten) (10)

where ρsolid and ρmolten denote the densities of the solid and molten phases, respectively. Additionally, the dependence of density on pressure and temperature is described as:

ρ(P,T) = ρ0 (1 - α(T - T0)) (1 + β(P - P0)) (11)

where ρ0 is the reference density at standard conditions (P0 = 0.1 MPa and T0 = 298 K), α is the thermal expansion coefficient, β is the compressibility coefficient. Once partial melting initiates (M>0), the effective heat capacity (Cpe) and thermal expansion coefficient (αe) are modified accordingly:

Cpe = Cp + QL (∂M/∂T)P=const (12)
αe = α + (ρQL / ρ) (∂M/∂P)T=const (13)

where QL represent the latent heat of fusion. The thermomechanical models simulate mid-ocean ridge extension experiments under different mantle potential temperatures, including modern conditions (1350°C and 1400°C) and Archean conditions (1500°C, 1550°C, and 1600°C)19,20, based on an area of 1000 km × 300 km (Figures. S7). The rectangular grid is composed of 501 × 151 nodes with a uniform resolution of 2 km × 2 km.

The model includes oceanic crust composed of basalts and gabbros, with thickness varying depending on the mantle potential temperature. The lithospheric depleted mantle and asthenospheric mantle are composed of dry olivine. In the modern models, oceanic crust and lithosphere follow the half-space cooling model, where temperature evolution as a function of time and depth follows the classical heat conduction equation. In the Archean, the mantle cooling and the melt extraction processes result in a thicker lithospheric mantle, but it is unknown whether the oceanic lithosphere follow the half-space cooling model. Based on simulation results and observations of cratonic lithospheric mantle thickness (up to 250 km)48,51, we thus assume that at Tp = 1600°C, the lithospheric mantle reaches a maximum thickness of approximately 250 km. A 50°C decrease in potential temperature corresponds to an approximate 20 km reduction in lithospheric mantle thickness. The thicknesses provided in the Supplementary Data 2. Detailed material physical properties can be found in Supplementary Data 4. In the reference model (Fig. 2), we present information after 6.5 Ma of mid-ocean ridge spreading, because mid-ocean ridge activity at this time has become mature under all Tp conditions. In addition, we simulated different extension rates (half-spreading rates of 1 and 5 cm/year) and the impact of melt extraction at different times on the melt extraction P-T conditions and Fe3+/ΣFe. According to reports, we also tested different melt extraction thresholds (2–10%)48,55. The results are displayed in the Supplementary Data 2. The results indicate that the Fe3+/ΣFe of the melt extracted has very little effect from these parameters of melt extraction thresholds and extension rates (<0.01).

Thermodynamic modeling
Thermodynamic modeling under the NaO-CaO-FeO-MgO-AlO -SiO -HO-O-CrO (NCFMASHOCr) system was carried out with the software GeoPS56, using the ds633 thermodynamic database57. Based on previous studies and numerical simulation results (Fig. S6)51,58, we use the reported dry depleted MORB mantle whole-rock composition58 as the initial source of MOR-basaltic melt (Supplementary Data 1). We also tested the effect of the evolution from a primitive mantle end-member58 to a depleted MORB mantle end-member on the Fe3+/ΣFe ratio in basaltic melts (Supplementary Data 1). The results indicate that the evolution of major element compositions from primitive to depleted mantle has a negligible impact on the modeling results (Fig. S8). The thermodynamic solution models employed including olivine (Ol), spinel (Sp), garnet (Gt), orthopyroxene (Opx), clinopyroxene (Cpx), and redox-sensitive melts (Melt) are from refs. 59,60. These updated models are applicable under conditions up to 6–8 GPa and align well with experimental petrology, particularly in predicting redox states, a capability that has been extensively validated in recent studies59–63. We also validated our approach against experimental petrology data64 before modeling. Specifically, previous studies have indicated that (Alpha-)Melt software tends to overestimate the oxygen fugacity (up to 1.2 orders of magnitude) of the residual system due to an unsuitable spinel model of the Melt software63 (Fig. S9). By comparing experimental petrology results64 for the relevant compositions with our thermodynamic outputs, we found that the predicted Fe3+ behavior in spinel matches experimental observations, indicating no bias in the model predictions. We further modeled the Fe3+/ΣFe-fO 2 relationship of the rock system, and that predicted by the thermodynamic model for mantle rocks under given P-T conditions aligns well with experimental observations26.

For the melt P-T conditions derived from thermomechanical numerical simulations, thermodynamic modeling will be called to conduct iterative modeling of depleted mantle compositions under open-system conditions with two initial whole-rock Fe3+/ΣFe values (0.04 and 0.02). 0.04 falls within the range of modern measured values, while 0.02 is used as a reduced end-member—essentially half of the modern value—for comparison experiments. The thermodynamic simulation adheres to the melt extraction thresholds. When the output P-T conditions surpass this threshold, the residual system after extracting melt is used for iterative calculations, and this process is repeated cyclically. The thermodynamic calculations will finally output the melt Fe3+/ΣFe corresponding to each data from the thermomechanical simulation.

To simulate the influence of melting degree on the redox evolution of residual refractory components at hot and deep conditions, we conducted isobaric melt fractionation simulations using an initial depleted mantle at 3–5 GPa. Results were output at 20°C intervals until the cumulative melt fraction (F) exceeded 35%. The results of initial Fe3+/ΣFe for the depleted mantle at 0.04 and 0.01 are shown, respectively. Because we conducted a series of calculations (ranging from 0.04 to 0.01), we found that only Fe3+/ΣFe as low as 0.01 are comparable to the observational data15.

Data collection and filtration
To calculate the mantle redox state since the Archean and facilitate comparison with previous studies11,13, we collected the oxygen fugacity (fO 2) using V/Ti and V/Sc redox proxies13,18 by compiling the Precambrian whole-rock composition dataset of Phanerozoic-MORB-like basalts since 3.8 Ga (Supplementary Data 3). The whole-rock geochemical data of the basalts were assembled from the EarthChem rock database and refs. 13,65–67. Some samples with imprecise age ranges have been corrected by tracing back to the original publications by refs. 66,67 and this study.

To precisely calculate the oxygen fugacity and P-T conditions of basalts, we first selected rocks with SiO contents ranging from 45 to 54 wt%. Then, rocks with MgO contents below 8 wt% were filtered out to avoid the potential influence of clinopyroxene and magnetite fractionation and contamination from continental material11,13. Similarly, komatiite samples with MgO contents exceeding 18 wt% were excluded, as discussed earlier, and highly altered samples (e.g., loss on ignition greater than 6 wt%) were also excluded although some studies suggest that V is not affected by lateral alteration or metamorphism.

Previous studies used (Nb/La)PM ≥ 0.75 as a criterion to identify MORB-like basalts11,13. We applied stricter filtering criteria of (Nb/La)PM ≥ 1 to enhance MORB-like characteristics and minimize the influence of continental contamination (the continental crust typically has low Nb concentrations). Furthermore, none of the samples exhibit significant Nb and Ta (if present) negative anomalies, which are commonly interpreted as signatures of subduction-related influence. Additionally, we examined other elements such as REEs; these samples display a slightly LREE-depleted or nearly flat pattern after chondrite normalization. This help to exclude samples derived from enriched mantle sources such as modern plume-related ocean island basalts, continental flood basalts, continental intraplate continental basalts, and continental rift basalts, characterized by light REE enrichment. Additionally, we compared the temperature and pressure conditions associated with mantle plumes from numerical models and modern observations68 to rule out plume-related influence (Fig. S10). The results show that, regardless of whether in the modern or Archean conditions, the extraction temperatures and pressures of plume-derived rocks are significantly higher than those recorded in our rock database (Fig. S10).

Finally, the V/Ti oxybarometer is specifically designed for peridotite-derived melts; therefore, we exclusively included lavas that satisfy geochemical criteria (e.g., CaO > 13.81–0.274*MgO) to ensure their formation from peridotite sources11. Additionally, we included a previously reported and rigorously filtered set of MORB-like metamorphic rock samples in the database6.

Other geological data were collected from the literature. Specifically, the arc magmatism frequency is based on refs. 66,67 the isotopic ratios of W were compiled from refs. 35–38 while zircon quantities were sourced from D.W.Y., G.T., & Z.X. (in preparation); the metamorphic T/P ratios and cold subduction rock types were sourced from ref. 69. Metamorphic cooling rates and folding rates were derived from ref. 49 data on the number of convergent margins, seawater Sr isotopes, and trends in biological evolution were summarized from refs. 50,70,71.

P-T-fO 2 calculation of Basalt
To accurately determine the whole-rock Fe3+/ΣFe in the basalt, it is essential to strictly constrain the P-T-fO 2 conditions59. Like the recent studies11,13,18, we estimated the oxygen fugacity of basaltic melts using the V/Ti and V/Sc redox proxies. The V/Ti method is advantageous because it is more sensitive to mantle redox conditions and is not influenced by residual garnet or volatile degassing11,13. Since the partition coefficients of V and Ti in silicate minerals are P-T-dependent, we calculated the melting (extraction/formation/in-situ) P-T conditions of MORB-like basalts using Fractionated-PT thermobarometer68 before determining final fO 2: olivine fractionation is corrected for by incrementally adding equilibrium olivine back into the magma until the magma is in equilibrium with olivine having a Mg# equivalent to that of the average mantle residuum; the Fo of the mantle source was set to 0.9, consistent with the original reference. 68. A key parameter affecting P-T condition estimation is the Fe³⁺/ΣFe ratio68. We used a bracketing approach (or squeeze theorem) to iteratively converge on the P-T-fO 2 conditions. Specifically, we first assigned an initial Fe³⁺/ΣFe ratio, from which the corresponding P-T-fO 2 conditions were derived. This allowed us to calculate a thermodynamically consistent Fe³⁺/ΣFe ratio. The new ratio was then used as input for the next iteration, and the process was repeated until the P-T-Fe³⁺/ΣFe values stabilized. The partition coefficients of V, Sc, and Ti for olivine, orthopyroxene, clinopyroxene, garnet, and spinel were also sourced from the literature11,13,18.

Given that sodium content is susceptible to alteration effects, titanium content is used as a proxy to constrain the degree of partial melting in the calculation11,13,18. During the calculation process, for samples with melting pressures within the stability range of spinel peridotite, we applied a spinel peridotite partial melting model to determine fO 2. For samples within the stability range of garnet peridotite, a garnet peridotite melting model was used (Fig. 1). Most of the samples fell on the spinel field, with only a small number of samples falling on the garnet field. As a result, Archean basalts are modeled and interpreted as highly reduced, which contrasts with a previous empirical model suggesting that many Archean rocks may not have been reduced if their melting primarily occurred within the spinel stability field15. The further details for calculating fO 2 can be found in refs. 11,13.

Additionally, the fO 2 values were also calculated using the V/Sc oxybarometer for comparison13, which is less affected by the melt fraction degree, especially under reduced conditions (Fig. 1). The oxygen fugacity values of melts from different time periods calculated by the two V-related methods are consistent within the range of uncertainty. For example, V-Sc oxygen fugacity of the Archean samples is QFM-1.1±0.3, while the V-Ti oxygen fugacity in the same samples is QFM-1.4±0.2; V-Sc oxygen fugacity of the Proterozoic samples is QFM-1±0.2, while the V-Ti oxygen fugacity in the same samples is QFM-1.2±0.2. All oxygen fugacity and Fe3+/ΣFe calculations are averaged at intervals of 250 Ma to display historical trends. Smaller intervals would result in more divisions with no samples. Periods without samples used dashed lines to transition to the average values of adjacent periods. We also conducted a binning sensitivity test, and the results show that regardless of whether the binning is based on the episodic mobile-lid or single-lid tectonic regime (Fig. 5), or on broader divisions as the Archean and Proterozoic, the conclusion that the mantle Fe³⁺/ΣFe ratio has doubled since the Archean remains unaffected.

Whole-Rock Fe3+/ΣFe calculation for Basalt
If the whole-rock composition of the melt, its formation P-T conditions, and the Fe3+/ΣFe are known, its oxygen fugacity can be easily calculated59, and vice versa, once the P-T-fO 2 conditions are strictly constrained, we can calculate the whole-rock Fe3+/ΣFe in the basaltic melts. Please note that the calculated Fe3+/ΣFe here refers to the initial value of melt formation, as V-related oxybarometers record the source redox conditions, unaffected by degassing11,13,18.

Since 1991, the empirical formula proposed by ref. 72. (Kress and Carmichael) has been widely used to relate the redox state of Fe to fO 2. However, recent studies have revealed that this method significantly overestimates the proportion of Fe3+/ΣFe in whole-rock samples27,28. Specifically, extensive datasets indicate that the oxygen fugacity of modern MORB is approximately at the QFM buffer, with a Fe3+/ΣFe ranging from 0.10 to 0.1426–29. The method from ref. 72, however, overestimates the Fe3+/ΣFe by up to 0.07. This overestimation is also observed in Precambrian samples in the database11 (Fig. S11).

Therefore, we do not rely on the method from ref. 72. to estimate the Fe3+/ΣFe in basalt. Instead, we use thermodynamic methods for calculation. A recently developed tool (fO 2melt) enables the calculation of the relationship between Fe3+/ΣFe and fO 2 under known P-T conditions59, embedding the latest thermodynamic model validated by experimental data.

Our computational approach utilizes fO 2melt to reverse-calculate iteratively the Fe3+/ΣFe in the melt under specific P-T-fO 2 values and record the results. Subsequently, the recorded whole-rock composition is forward-simulated using thermodynamic modeling, and the oxygen fugacity calculated is compared with the measured fO 2 values of the rocks. The results show that the two methods validate each other.

In addition, we applied the recently revised Fe-fO 2 empirical formula to calculate the Fe3+/ΣFe in the melt27,28. This formula has been extensively validated against previous datasets. The results obtained from the revised empirical formula and the thermodynamic method for are close to each other, but they are significantly lower than those obtained using the earlier method72 (Fig. S11). The whole-rock Fe3+/ΣFe corresponding to the P-T-fO 2 conditions recorded in the basalt database of previous studies11,13 was also calculated for comparison to our results. It is worth noting that we respected these previously established P-T-fO 2 results and calculated the Fe3+/ΣFe ratio based solely on these conditions. The variations in P-T-fO₂ estimates among different studies do not affect the overall conclusions.

Whole-Rock fO 2 and Fe3+/ΣFe in mantle source
When the P-T conditions of the melt and the corresponding whole-rock Fe3+/ΣFe are obtained, we can readily determine the redox characteristics of its mantle source using thermodynamic modeling (e.g., Fig. 1A). This allows us to calculate both the oxygen fugacity and the whole-rock Fe3+/ΣFe of the mantle source. We iteratively assign different whole-rock Fe3+/ΣFe of the mantle source to generate corresponding melt recorded from P-T- Fe3+/ΣFe. The redox state of the mantle source corresponding to each basaltic sample is determined and systematically recorded. The Fe3+/ΣFe in the mantle source corresponding to zircons in mantle-derived melts of the Hadean can also be determined through thermodynamic modeling because their P-T-fO 2 conditions are already known (Supplementary Data 3).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors developed a novel approach to extract information on mantle ƒO2 through time, which combines numerical simulations and thermodynamic calculations using the latest models. This allows them to track Fe3+/Fe(T) in basalts as a function of pressure and temperature in the sub- and suprasolidus regions of peridotite phase relations and during decompression melting, which are ultimately convertible to ƒO2. I find this approach of forward modelling the most sophisticated yet, allowing to understand and quantify the complex relationships of these latter two parameters. In fact, the results are somewhat underexploited, as there are some interesting systematics (e.g. in Fig. 1A) that are not explored, though I understand that this is not the main thrust of the paper and there are limits to the length.\nOverall, I think this will be of interest to a wide readership and would be publishable after moderate to major revisions. Nevertheless, my list of comments is very long, but if I see something that might be improvable, I say something. I hope the authors find these useful in order to craft a stronger manuscript or otherwise can easily rebut them.\nMain concerns and suggestions:\n1. I can see that the authors extract Fe3+/Fe(T) of melts from the numerical experiments in Fig. 2F-J, and that the thermodynamic phase equilibria modelling can back-extract Fe3+ /Fe(T) from the V/Ti-based ƒO2 of natural samples. Maybe I’m being thick, but can the authors better expose how the numerical experiments link to the natural samples? In general, the modelling, in particular the numerical one, could be described in more detail. Is it correct that the authors used forward thermodynamic modelling using PT of melting of natural samples to find the associated Fe3+/Fe(T), from which they then extract ƒO2. That is, their ƒO2 estimate for the natural samples is completely unrelated to the V-based ƒO2 estimate? The onus is on the authors to ensure their manuscript becomes accessible to a wide and non-specialised readership.\n2. Related, the authors claim that their approach yields the most accurate and robust insights yet, but ultimately, constraints on mantle redox evolution rest with the natural samples and their shortcomings: (a) unclear tectonic setting (in fact, unlikely MOR-setting for the vast majority of Archaean basalts (Kamber 2015 Precambr Res; Puchtel and Arndt 2025 TOG; Brown+24 JGeolSoc) – this is important because we know that mantle ƒO2 varies as a function of tectonic setting (Cottrell+22 in Geophys Monogr 266); (b) uneven sampling through time, with some sparsely sampled periods; (c) uncertainties related to uncertainties in PT estimates from natural samples. Thus, the information from the modelling, however sophisticated, cannot be more accurate and precise than that derived from natural samples.\n3. There are quite a number of vague, sweeping or inaccurate statements, and some ad hoc statements, throughout the main part of the text that are not very informative and therefore leave more questions than answers. Instances are given below.\n4. The authors variably refer to redox state, oxygen fugacity, oxidation degree, oxidation budget, Fe3+/Fe(T) …. The mantle redox state reflects the sum of several multivalent elements, of which iron is certainly overall the most important one, but S, C and H are not negligible (Evans 2006 Geology). I suggest to make sure ƒO2, redox state (or redox budget), and Fe3+/Fe(T) are accurately used throughout the text, and to refrain from using additional terms (e.g. oxidation degree, oxidation budget etc).\n5. Given continued uncertainties regarding earliest Earth processes and the physicochemical-dynamic conditions attached to them, the authors are of course entitled to their preferred interpretation of why the ƒO2 of Earth’s mantle changed after the Hadean. However, the arguments against upward mixing of redox budget arising from disproportionated iron, and the arguments in favour of komatiite serpentinisation followed by subduction are substantially less clear-cut than presented, for reasons I detail below. This discussion could be a lot more balanced.\n6. Furthermore, the authors take evidence from short-lived radiogenic isotopes to argue for separate upper and lower mantle reservoirs, but this seems too simplistic, as early-generated heterogeneities survive to the present day (as sampled by OIBs) and are thought to reside in geographically restricted regions that are not identical to the lower mantle as a whole.\n7. The authors dismiss komatiites as “enigmatic”. This is a misrepresentation of the field, as komatiites have extremely well-constrained petrogeneses, which may be complicated by their derivation from thermochemically anomalous plumes, and it is true that their pressures of formation are difficult to estimate. I would accept this as an argument in favour of focusing on spreading ridge-derived samples (MORB-like basalts and picrites). By the same token, the authors should acknowledge that few if any Archaean basalts are truly spreading ridge-derived.\n8. The authors assume a modern subduction redox budget to estimate whether this could have oxidised the mantle to present-day levels. However, we know that the atmosphere was oxygenated to just 1% PAL as late as 2.4 Ga ago, and that oceanic bottom waters became oxygenated only in the Neoproterozoic (Stolper+Keller 18 Nature), before which time there would have been very little dissolved sulphate, which is the most potent oxidant (Tomkins+Evans 15 EPSL). The subducted redox budget surely must have been variable through time. Or maybe I misunderstood what the authors are aiming at. It also seems that they address this in some detail in a paper under consideration (ref. 38), this should be clarified.\n9. There are some sentences with awkward English that would benefit from polishing.\n10. The figures and their captions can be much improved. There is too much, in part easily overlooked, small font in the panels themselves. The captions are so terse as to be uninformative. IMO, readers should be able to understand what the diagrams show and what arguments they support without reading through the entire manuscript.\n11. Throughout, I suggest to use italics for state variables, such as T, P, f (fugacity) etc.\nIn-line comments and suggestions\nTitle: what is oxidation degree? fO2? Redox budget? Fe3+/Fe(T)? Please be more precise, also in the abstract and elsewhere\n11-13 It is clear what the authors want to express in the first sentence but the wording is inaccurate. What is oxygen-rich habilitability? The atmosphere is now O2-rich. What the mantle regulates is the exchange of redox budget, which ultimately enabled O2 to accumulate in the atmosphere, thereby affecting how life evolved\n13 our planet should be capitalised, also should be “Earth’s”\n14 remains a subject of\n16 they are not all basalts, but also picrites according to the supp table. As an aside, the mere fact that there are so many basalts implies that Archaean ambient mantle TP could not have been very hot. If the samples are basalts due to extensive differentiation, then they are not appropriate to estimate mantle fO2 from them\n16-17 when you are referring to a ridge setting specifically, there is no need to again mention \"geodynamic settings\". Perhaps \"modern conditions\"\n20 demonstrate\n20 clearly, measured Fe3+/Fe(T) are not reliable and if you estimated the values based on V/Sc or V/Ti, then your result cannot be more reliable than the information contained in the element ratios? Besides, Zhang et al. (2024, cited) already presented whole-rock Fe3+/Fe(T) thermodynamics-based estimates. If yours are significantly improved, or novel in the sense that you used a different approach, then this should be expressed differently – this relates to my main comment above\n24 please be more specific here: what kind of geological evidence? Below you also refer to biological evolution\n25-26 tectonic reorganisation - ok. But what do you mean by geological reorganisation?\n37-38 exert... influence, or simply “have… effects”. Effects are not exerted, I think.\n50 ref. 6 was updated in Aulbach+19 SciRep - and using both V/Sc and Fe3+/Fe(T)! - so you could cite this instead\n51 here and throughout: QFM units don’t exist. It is just a reference buffer. fO2 was lower by … orders of magnitude (or log units, if you will) – throughout manuscript\n56 “tends”. Also, please be more specific as we know it is only the garnet-bearing mantle down to metal saturation at ~8-10 GPa where fO2 decreases with increasing pressure. At P<3-3.5 GPa, the fO2-depth profile is more complex, as demonstrated from phase equilibria and empirical models (Stolper+20 AmMiner; Birner+24 Nature)\n58-61 This sentence does not reflect the state of the art:\n1. Aulbach+Stagno 16 (cited) did correct for the fO2 decrease with increasing pressure and did find significant differences between Archaean and modern samples.\n2. Zhang+24 (cited) wrongly corrected their basalt V/Ti-derived fO2 for the reduction of the melt with decreasing pressure after leaving the source. However, the V/Ti-derived fO2 is already that of the source.\n3. Both biased their Archaean, more deeply-derived samples (due to higher TP) to too-high fO2 by assuming the garnet-peridotite fO2-pressure trend could be extrapolated to 1 GPa, but we know since Stolper+20 that this is not the case.\n63 these are “elemental ratios”. Furthermore, this statement is blatantly incorrect: neither ref. (6) nor refs. (11,13,14) used direct geochemical ratios, as (6) accounted for P (T effect not known at the time), (14) for T and (11) accounted for PTX (though made a mistake), and (13) accounted for TX. It is true only for (12) that V/Sc was used directly. (16) should not be cited here - it’s a review paper and we were well aware of PTX effects\n68 “the” depleted mantle is understood by many to be the MORB source. What your fig. 1A seems to show is mantle that is residual from extraction of various extents of melting in the suprasolidus region as a function of distance from the solidus (we later learn in the methods that the bulk composition of the model corresponds to a depleted MORB mantle)\n69-70 This is not shown in Fig. 1? Qualitatively, if Fe3+ behaves like a moderately incompatible element (O’Neill+18, cited), one would expect the ratio to decrease with increasing melt F, which is a function of T, all other things being equal. This seems to be shown in Fig. 2.\n71 not just depleted mantle, but any mantle, or more specifically, peridotite\n73 perhaps refer redox budget sensu Evans (2006) and then stick with it\n72-76 I think you should refer to the phase eq. and empirical modelling of Stolper+20 (Am Miner) and Birner+24 (cited) here. Stolper highlighted how this decoupling is related to changing mineralogy across peridotite facies as a function of pressure\n79 please don’t refer to QFM units. Refer to fO2 and several orders of magnitude (or log units)\n82-84 I don’t see how this is more accurate? Melt fO2 also depends on PTX, and unlike elemental ratios, Fe3+/Fe(T) can be changed by redox interactions (due to redox melting, degassing etc). Besides, measured Fe3+/Fe(T) of ancient rocks are unreliable due to alteration. Therefore, ultimately, your F3+/Fe(T) modelling is novel and possibly more accurate, but it remains a model that, to be useful, you have to compare against a rock record\n89 even if this is explained as part of the Methods later, this reference to “thermodynamic and geochemical methods” is much too vague. A few more sentences are needed here, what these methods are, so we understand broadly what you did without having to consult the Methods\n93 yes, the modelling is supercool, novel and a major advance. But this does not make observations from the sparse rock record itself any more robust\n95 please be precise regarding what it is that has doubled. I think you mean Fe3+/Fe(T), which may be the most abundant multivalent element, but does not alone fix the redox budget (see Evans 2006 Geology)\n98-99 This needs much better justification. You could say that komatiites are plume-related and therefore may sample thermochemically anomalous sources, whereas basalts may sample an ambient mantle source representing a better-constrained, less variable system. You should also be honest and acknowledge in your manuscript that virtually no Archaean basalts are spreading ridge-derived (Puchtel and Arndt 2025 TOG; Brown+24 JGeolSoc). The samples you describe as “MORB-like” are actually basalts associated with melting in intraplate settings and erupted through attenuated continental mantle. The only true Archaean spreading-ridge derived material is now sampled as cratonic eclogite (no need to cite, but see Aulbach+Smart 23 AREPS if you want to know why)\n101 there is no representative TP for the Archaen period, as TP estimates vary by >200 oC (e.g. Herzberg+2010, cited vs. Aulbach+Arndt 19 EPSL)\n106 a totally expected result for all tectonic settings where decompression melting occurs…\n113 please mind language - what is melting T? T of solidus intersection? It is afterwards, when the melt takes with it heat, that cooling occurs. But this is not specific to MOR ridges, so I am not sure what you mean\n117 please explain here the presence of cold drips and the mixing, or refer to a text/Methods where we can read up on it - is this related to a pre-plate tectonic regime?\n122-123 not sure this sentence makes sense as written? What is an intricate “internal thermodynamic equilibrium”? Why internal? Aren’t the physical (not geophysical, presumably?) parameters (PT-fO2) dictating the equilibrium? Aren’t the thermodynamic calculations based on experimental petrology, first and foremost?\n126 imposed rather than provided?\n129 “MORB with fO2 near QFM”\n140-141 There is no rationale provided here why it is increased Tp rather than an intrinsically lower fO2 “promoted reduction of basaltic melts”? Maybe a few steps in the chain of arguments are missing here? (higher TP = deeper melting = lower ƒO2 in the garnet-bearing mantle source = lower ƒO2 in the melt which separated and reduced)\n144-146 Keeping your broad readership in mind, perhaps it would be better to say that a higher Tp results in a lower fO2 via its effect on the pressure of melting (which is deeper), at least in the garnet peridotite facies - at least this is what you suggest? Also, we’ve known this before, so this is not a new finding from the present study. This is why AS16 and Zhang+24 applied a P correction (though both wrongly extrapolating the garnet peridotite fO2-P profile to 1 GPa). AS16 actually estimated P of melting from their preferred TP evolution curve, so the link between TP-P-fO2 is clear. Finally, it is necessary to be very careful with the terminology: here you refer to a lower mantle redox state despite constant Fe3+/Fe(T) - I think you mean ƒO2\n150-154 1st sentence: keeping your broad readership in mind, perhaps it would be better to say that a higher Tp results in a lower fO2 via its effect on the pressure of melting (which is deeper), at least in the garnet peridotite facies - this is what you suggest? Also, we’ve known this before, so this is not a new finding from the present study. This is why AS16 and Zhang+24 applied a P correction (though wrongly extrapolating the garnet peridotite fO2-P profile to 1 GPa). AS16 actually estimated P of melting from their preferred TP evolution curve, so the link is clear. Finally, it is necessary to be very careful with the terminology: here you refer to a lower mantle redox state despite constant Fe3+/Fe(T) - I think you mean ƒO2\n157 looking at Stolper+20 AmMiner, it is clear that 2.4 GPa is below P where the simple relationship between pressure and fO2 in the garnet peridotite facies breaks down and fO2 varies significantly and non-linearly as a function of changing modal abundances\n162 “previous” - give reference. If Zhang+24, they overestimated Archaean fO2 in two ways, as I explain elsewhere in this review). So should we be worried about the consistency with your approach?\n164 what do you mean by in-situ? what do you mean by Mg-calibrated? Please note:\n(1) Zhang+24 made the same 2 mistakes when applying their corrections to the metabasalts reported in AS16 that they did with the basalt database, and their Archaean metabasalt-based fO2 estimates are also too high as a result\n(2) I happen to be familiar with ref. 6: AS16 did more than just look at MgO content. We used major and trace element constraints to understand the samples’ petrogenesis, which allowed us to exclude samples with cumulate protoliths, or those that had been metasomatised or that were LREE-enriched suggesting derivation from an enriched source or melting in the presence of lithospheric lid - this was done to ensure that the comparison to forward-modelled melt V/Sc (to estimate source fO2) and to modern MORB is justified. For many of the sample suites, AS16 used reported radiogenic isotope systematics to further demonstrate their derivation from a depleted mantle source, as is true for modern MORB, and given that at least mildly depleted sources existed in the Archaean. This was further corroborated in Aulbach+Arndt19 EPSL where many of the same eclogite suites for which AS16 estimated fO2 are shown to have initial 87Sr/86Sr indicative of a protolith derived from depleted mantle.\n(3) Your and other’s basalt database, on the other hand, includes in particular Archaean basalt samples that are not spreading ridge-derived, with (Nb/La)N down to 0.75, and the onus is on you to demonstrate that these samples record the fO2 of a convecting mantle source that can be usefully compared to the modern MORB source. That is, without complications that arise from mixing tectonic settings.\n176 any back-calculated result would match some result from the numerical simulations?\n181 redox state of the Archean mantle as proxied by its Fe3+/Fe(T)\n183 what is the oxidation degree and how do you double it?\n184-185 again, the effect of TP is indirect, and I think it would be better to make it more explicit. “Deeper onset of melting owing to higher Archean mantle TP” alone might be misinterpreted\n186 not only that, but it confirms earlier findings as per (6, 13, 14) – worth mentioning here\n188-189 sure, but this last statement is hanging in the air, as it is not followed by some statement as to whether this needs further investigation, or whether it conflicts with some evidence. Perhaps you can just leave it out.\n194 this is likely, but currently they are “thought to represent” Archaean - or were 187Os/188Os reported for these samples?\n199 in Figure 4\n201 then we calculated\n201 which other redox parameters?\n202 what kind of melting? Batch or fractional? It makes a difference for how concentrations of incompatible components in the residue evolve\n202 high degrees of partial melting\n212 suggests that Archean mantle was more reduced than the modern mantle\n214-217 Please add a statement/caveat regarding uncertain provenance/tectonic setting of basalts, a potential preservation bias linked to the supercontinent cycle and poor statistic coverage of certain eras.\n218-219 by the time a melt is able to saturate zircon, it is so far removed/processed from the original mantle-derived melt that it is questionable at best that the fO2 they record reflect the mantle rather than some later process\n232-233 I suggest to rewrite “… this hypothesis, as the accretion of meteorites with negative … lowered the W isotopic composition of the silicate mantle with an initial positive W isotope composition (+200…) in the wake of core formation and evolution at high Hf/W”.\n233 when did these meteorites arrive? Given how short-lived the system is, there must be an estimate for the timing\n235 should be ref 29? Might need to check neighbouring references, too\n235-237 please rewrite, this sentence is not well constructed\n240 how do you define habilitability? Life evolved even as the atmosphere contained <<1% PAL O2. So this change in mantle O2 would have affected the course of evolution of life, but not hindered life per se.\n246-249 this is much too simplistic an argument, and untenable. Even modern OIBs still contain components that must have been generated during earliest Earth evolution including W (e.g. Mundl+17 Science) and Nd (Horan+18 EPSL). The sources likely reside in long-lived heterogeneities (possibly atop the core) that cannot be framed in terms of upper vs. lower mantle\n247 “Archean rocks have W and Nd isotopic signatures…” (what was fractionated were the parent and daughter elements\n250 not today, but if iron disproportionation and extraction of metal to the core is real, then it would have been at the time of late accretion.\n254 what do you mean by “not show mantle oxidation mechanism”? This is awkward English. Is it correct to simply say it has a more reduced mantle than Earth? Despite having a metallic core, a lower mantle composed of perovskite and despite having had a (basal) magma ocean, which are pre-requisites for this mechanism to work?\n262 that’s 500 Myr later than what you call for at the beginning of the paragraph.\n263-275 please clarify overlaps with the manuscript under consideration as ref. 38\n266 they are transporting redox budget, not fluxes. Besides, this redox budget is then “missing” at the surface and unavailable to oxidise surficial reservoirs.\n268 ref. 38 is miscited in this place. They showed that komatiite serpentinisation produces H2 and that’s it. They never mention subduction or mantle redox. Furthermore, I question that the H2 so produced could be quantitatively lost from the Earth system, as hydrodynamic escape would have been highly inefficient by the time komatiites were emplaced\n271-274 (1) It is insufficient that ultra-oxidised oceanic crust occurs. What matters is its mass. E.g. Cratonic eclogite xenoliths undoubtedly represented subducted oceanic crust, including having fractionated O isotopic compositions pointing to seawater alteration, have consistently much lower Fe3+/Fe(T) than what you state here (no need to cite, I am just referencing in support of my statement: Aulbach+Smart 23 AREPS, Aulbach+22 JPet, Aulbach+19 SciRep). (2) It is not admissible to assume a modern subduction redox budget when we know that the atmosphere was oxygenated to just 1% PAL as late as 2.4 Ga ago, and that oceanic bottom waters became oxygenated only in the Neoproterozoic (Stolper+Keller 18 Nature), before which time there would have been very little dissolved sulphate, which is the most potent oxidant (Tomkins+Evans 15 EPSL). (3) I wonder where the idea came from that komatiites have relatively high Fe3+? If measured Fe3+/Fe(T) were reliable, someone would have exploited that before, but they are not. Berry+ 08 Nature were able to make an estimate on a melt inclusion in komatiite and obtained 0.10, similar to modern MORB.\n281 parent-daughter elements are fractionated not the isotopic compositions, which trace different sources with different isotope compositions owing to elemental fractionation at some time when the system was still alive. See my comment above regarding the inadmissibility of casting these different sources of ancient signatures in terms of upper vs. lower mantle.\n283-284 seems an ad hoc statement and requires a little more info\n287-288 this was already proposed by Aulbach+19 SciRep based on the eclogite and komatiite record.\n288-289 please see my comment above. This is worded peculiarly and moreover falsely diminishes how ref. 6 or 11 scrutinised their database (although 6 went a step further to demonstrate derivation from a depleted mantle source) – see similar comment above\n299 why would there have been separate upper and lower mantles? Where are isotopic anomalies from now extinct isotope systems in modern OIBs from when we know for sure we have whole-mantle convection? This paragraph is based on a series of highly uncertain findings and perhaps could be shortened to two sentences invoking the magmatic lull (which was not global, however) as a potential sign of sluggish tectonics that might explain the lack of mantle fO2 evolution during that period\n302 ref. 6 did not invoke a Neoproterozoic rise (though we were tempted)\n307 hm. This was surface driven (increased photosynthesis and organic carbon burial, plus more; e.g. Och+12 EarthSciRev), which ultimately caused oxygenation of oceanic bottom waters. This increased the subduction flux of redox budget, as seen by the advent of certain convergent-margin ore deposits (Tomkins+Evans 15 EPSL), then may have trickled down into the mantle\n313-326 last paragraph is too wordy, repetitive and vague. Could be shortened to a crisp and strong final statement.\n315 propose\n315 except for large impacts, nothing of what follows is abrupt.\n321 greater quantities of… fluxes makes no sense, the flux already comprises a quantity (referenced to a time unit). Please reword.\n334 please specify somewhere the composition of the mantle that you modelled (Fe3+/Fe(T) is not enough to characterise the system).\n337 remove first comma\n337 how thick in km?\n340 what do you mean by “melt-bearing peridotite”? The melt was similarly dense as the peridotite and failed to separate? Or simply the peridotite being above its solidus T?\n341 basaltic or picritic?\n341 why hydrated?\n344-348 Are we still in Fig. S6? Can you refer to panels, additionally?\n354-357 It is fine to not consider komatiites in your work and to say that the petrogenesis of spreading-ridge derived basalts, but this is an inexact summary of the state of the art, and a weak way to dismiss the komatiite record. With the exception of a single komatiite (Commondale), where subduction MAY be a possibility, there is no evidence for komatiite in subduction zones, or from particularly wet sources. No one says komatiites form at the core-mantle boundary. Please duly distinguish between mantle sources and melts. Please refer to Waterton+Arndt 25 (In: Hofmann+ eds The Archaean Earth: Tempos and Events 2nd Edition of The Precambrian Earth), Puchtel+22 ChemGeol, Puchtel+Arndt 25 TOG, Herzberg 16 JPet etc. By the same token, why not acknowledge that virtually no Archaean basalt is strictly comparable to modern MORB (and hence mantle sources) because they were emplaced on continental margins?\n364 where is the evidence for that? Sources such as Early Enriched Reservoir and similar that were sluggishly mixed throughout the mantle, are though to reside in geographically isolated parts of the (lower) mantle and do not require separate UM and LM compositions\n365 please don’t refer to komatiites as highly enigmatic. They are not.\n369 how did accreted meteorites get into the lower mantle, if not through the upper mantle? Wouldn’t they have equilibrated with an upper magma ocean more efficiently than with a partially solidified lower mantle?\n370-372 In the discussion, you say that the mantles were already mixed and that plate tectonics are weak. Here, you want to bring up deep reduced mantle. Can you have it both ways?\n433 ref. 55: this is a different citation from the two you referenced above when talking about the spread of Archaean TP\n447-448 fix this sentence.\n455 not crystal-clear what “these” refers to\n469 up to 1.2 orders of magnitude? Average 1.2? Surely this must depend on the modelled conditions??\n480 please explain what motivates this choice of 0.02 and 0.04\n483 and are these increments then aggregated?\n489 why 0.04 and 0.01 here, but 0.04 and 0.02 above?\n496 what kind of age correction? Why?\n500 fix materia\n502 “low degree of alteration”. How was this assessed? Did you use LOI?\n518 how many of how many is “most”?\n520 what are zircon quantities?\n528-529 Maybe, but a recent study finds that V/Sc is less sensitive to mantle source and melt fraction (Liu+25 GPL). For the P you report, garnet-present melting is subordinate, anyways (as per your next paragraph).\n539 what is the P interval? Is it itself T-dependent?\n544 more specifically a previous empirical model\n545 awkward sentence, please rewrite\n545 following ref. 11 is bad news because they calculated fO2 based on V/Ti and V/Sc, which records source fO2 directly (forward-modelling of peridotite melts) rather than low-P crystallisation, but then applied a correction assuming that the melt self-reduces during decompression after separating from the source. This reduction is relevant to olivine-melt V partitioning, and to Fe3+/Fe(T) recorded in the emplaced melt, but not to forward-modelled V/Sc or V/Ti. I gather from a later paragraph that you did not apply this wrong correction.\n562-563 See comment to line 545. The statement itself is inaccurate because olivine-melt V partitioning is a V-based oxybarometer that does not record ƒO2 of melt formation (in the mantle source), but of low-pressure saturation of liquidus olivine when the melt may have self-reduced.\n570 last sentence is a bit strange. Perhaps say it is also observed in the database assembled in this study, or similar\n597 the (inherited) zircons, but not the melts (or lavas) are in the record. The melts were likely SiO2-rich and not mantle-derived, but either remelted or strongly differentiated from a mantle-derived melt, so as to be able to saturate in zircon. Either would imply that zircon fO2 does not record that of the mantle-derived melt and much less that of the mantle source.\nI attach my comments to figures and captions in the main text and in the supplement directly in the pdfs.", "reviewer_2": "This manuscript explores the redox evolution of Earth's mantle since the early Archean, employing thermodynamic-thermomechanical numerical simulations and a comprehensive database of mid-ocean ridge basalt-like (MORB-like) samples. The authors perform advanced numerical simulations to calculate the redox state of basaltic melts under varying mantle potential temperatures and mantle Fe3+/FeTotal ratios. The study suggests that the mantle's average oxidation degree has approximately doubled since the early Archean, reflecting significant geological and tectonic reorganization events.\nThe main innovation lies in the integrated approach combining empirical observations with high-resolution numerical simulations, highlighting the intrinsic coupling between Earth's oxygen-rich habitability, biological evolution, and tectono-magmatic processes.\nHowever, this study has some shortcomings in the selection and organization of MORB-like basalt samples, which undermine the robustness of its conclusions. These issues include: (1) There are many repeated samples with consistent chemical compositions in the dataset, but they display different ages, P-T-fO2 conditions and Fe3+/FeTotal values. (2) The machine learning (ML) method used in this study may have several drawbacks, leading to some uncertainties in MORB-like sample filtration. (3) The quantity of MORB-like samples (if they are) varies significantly over time and is insufficient overall. Therefore, the trend of basalt oxygen fugacity presented in Figure 3 may have some problems. As the following discussions are all dependent on this trend, I suggest that the authors should deal with the above-mentioned comments first.\n1. The descriptions of Figure 1 are overly simplistic and lack the necessary details to fully convey the key information (After reading the whole manuscript, most of the figure captions have similar problems). How could you obtain this P-T diagram? What is the meaning of the five-pointed stars in Figure 1A? How about the white dotted lines? All of these should require detailed introductions in both manuscript and figure captions.\n2. Data Collection and Filtration: The authors used a ML method published by Liu et al. (2024) to filter Archean MORB-like basalt data. However, the training dataset listed in Liu et al. (2024) was subdivided into subduction and non-subduction groups, respectively. The latter contains not only many MORBs, but also a lot of oceanic island basalts (OIBs), oceanic plateau basalts (OPBs), continental flood basalts (CFBs), continental intraplate basalts (CIBs) and continental rift basalts (CRBs) and so on. Therefore, samples with the prediction results > 90% do not necessarily originate from the mid-oceanic ridge magmatism. My main concern is how the authors determine that these Archean basalts are MORB-like, rather than OIB-like, within plate-like or others.\n3. After detailed examination of the listed dataset in Liu et al. (2024), the ML method may have some drawbacks: (1) Nearly two thousand lines of basalt data are older than Cenozoic (65 Ma), which might not be suitable for using as training data; (2) The generation of CFBs, CIBs and CRBs was commonly accompanied by various degrees of continental contamination. However, the influences of continental contamination were not precluded. Obviously, it is confusing to use samples that are strongly affected by continental materials to discuss their origins and tectonic settings; (3) The listed arc basalts are composed of many samples from back-arc basin areas. However, the simultaneous influences of back-arc spreading and mantle plume were recently proposed for the tectonic evolution of the West Philippine Basin. In essence, the chemical compositions of back-arc basin basalts (BABBs) vary in a large scale, including MORBs, normal arc basalts, and OIBs and so on, suggesting that the generation of BABBs may be complicated. Therefore, labeling the BABBs simply as subduction origins may be unreasonable; and (4) The elements of U, Pb, Ba, Sr and Rb (and the ratios made up by these elements) were also applied to the ML model. But these elements can be easily affected by post-magmatic alterations. In fact, I am concerned about the impact of element mobility on the predicted results of Archean basalts, which were mainly implemented in the ‘Black Box’. Therefore, I suggest that the authors should apply a more reasonable method to screen the Archean MORB-like samples.\n4. P-T-fO2 Calculation of Basalt: The calculated parameters for Fractionated-PT thermobarometer should be introduced. I notice that Table S2 contains many repeat samples (e.g., samples Z2967-49 and S z-1) in lists of ‘Data in Zhang et al. (2024)’ and ‘Data in Gao et al. (2022)’. However, the same sample displays distinct P-T-fO2 conditions and calculated Fe3+/FeTotal values in each data list. In addition, many repeat samples in list of‘MORB-like samples in this study’have different ages. Therefore, I consider that the organization of basalt dataset in this manuscript should be significantly improved.\n5. The quantity of MORB-like samples fluctuates dramatically over time, and the overall sample quantity is insufficient. With these points in mind, I am skeptical about the authenticity of the trend shown in Figure 3. As the following discussions in this manuscript are all dependent on this trend, I suggest that the authors should deal with the above-mentioned comments first.", "reviewer_3": "Review on “The Mantle Oxidation Degree Has Doubled Since the Early Archean” by Zhu et al.\nThe secular redox evolution of the mantle attracts broad interests because it may influence the long-term evolution of the atmosphere. The redox evolution of the mantle could be dictated by oxygen fugacity (fO2) or redox capacity (Fe3+/∑Fe). Whether the fO2 has increased in Archean remains hotly disputed (Zhang et al., 2024; Li and Lee, 2004; Aulbach and Stagno, 2016). In this study, the authors inferred the redox evolution of the mantle from the perspective of redox capacity (Fe3+/∑Fe). They estimated circuitously the Fe3+/∑Fe of the mantle from calculated T, P and fO2. Despite that the authors performed arduous thermodynamic-thermomechanical simulations, the results are not novel, and even be wrong for the following reasons.\n1. In estimatation of Fe3+/∑Fe in the mantle, the authors used fO2 estimated by Zhang et al. (2024) and Gao et al. (2022). Both of the studies show that the fO2 of mantle-derived basalt increased around 2.5 Ga. Therefore, it’s foreseeable that the estimated mantle Fe3+/∑Fe will increase. I wonder what the result will be if the authors use a constant fO2 (as proposed by Li and Lee, 2004) in their calculations.\n2. If the increase of mantle Fe3+/∑Fe at stage 4 (3.2-1.8 Ga) was caused by subduction of oxidized surficial materials into the deep mantle, then from the point of mass-balance, we should expect a decrease in O2 of the atmosphere. However, in the contrary, the O2 content of the atmosphere increased at this stage. In addition, if the mantle was remoulded in Fe3+/∑Fe, the bulk composition of the mantle should also change due to subduction. That means the authors cannot use a constant mantle composition in the simulations.\n3. The estimated mantle Fe3+/∑Fe has a large uncertainty, which may lead to misleading rather than clarity.\nMinor points:\nThe title is pompous, the word “oxidation degree” is unclear. If the authors mean “Fe3+/∑Fe ratio, they should use Fe3+/∑Fe.\nLines 11-13 The expression is vague, how mantle’s redox properties enables cyclying of redox sensitive materials? Or the redox state of the mantle regulated by material cyclying? What’s the defination of “redox-sensitive materials”\nLines 33-34: Yes, the mass of the mantle is large, however, the MORBs are derived from the very shallow part of the mantle, how could the authors use MORB represent the whole mantle?\nLine 39-41: It’s very abrupt to compare the redox state of Earth with other terrestrial bodies, what’s the point here?\nLine 62: change “the mantle and derived-melt redox state” to “the redox state of the mantle and mantle-derived melts”\nLine 70: According to which study, the authors alleged “The Fe³⁺/Fetot in melts decreases with increasing temperature”?\nLine 78-79: That’s also the reason why Aulbach and Stagno (2016) normalized the V/Sc ratio to constant P of 1 GPa."}, "author_response": "Responses to Reviewers’ Comments Point by Point\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors developed a novel approach to extract information on mantle ƒO2 through time, which\ncombines numerical simulations and thermodynamic calculations using the latest models. This allows them to\ntrack Fe3+/Fe(T) in basalts as a function of pressure and temperature in the sub- and suprasolidus regions of\nperidotite phase relations and during decompression melting, which are ultimately convertible to ƒO2. I find\nthis approach of forward modelling the most sophisticated yet, allowing to understand and quantify the complex\nrelationships of these latter two parameters. In fact, the results are somewhat underexploited, as there are some\ninteresting systematics (e.g. in Fig. 1A) that are not explored, though I understand that this is not the main thrust\nof the paper and there are limits to the length.\nAnswer: Thank you for your detailed and constructive comment. We greatly appreciate your recognition\nof the complexity and sophistication of our forward modeling approach, which indeed provides valuable insights\ninto the intricate relationship between Fe³⁺/ΣFe and fO₂. The systematic trends you pointed out (e.g., as shown\nin Fig. 1A) are certainly worth further investigation, and we have expanded on this aspect within the space\nallowed by Nature Communications -- please see Lines 85-112 for details.\nOverall, I think this will be of interest to a wide readership and would be publishable after moderate to major\nrevisions. Nevertheless, my list of comments is very long, but if I see something that might be improvable, I\nsay something. I hope the authors find these useful in order to craft a stronger manuscript or otherwise can easily\nrebut them.\nAnswer: We would like to once again express our sincere gratitude for your detailed and insightful\nsuggestions. They have been extremely helpful in improving our manuscript, particularly in clarifying some\npreviously ambiguous sections. We have carefully addressed all of your comments and made corresponding\nrevisions. Please find our detailed responses below.\nMain concerns and suggestions:\n1. I can see that the authors extract Fe3+/Fe(T) of melts from the numerical experiments in Fig. 2F-J, and that\nthe thermodynamic phase equilibria modelling can back-extract Fe3+ /Fe(T) from the V/Ti-based ƒO2 of natural\nsamples. Maybe I’m being thick, but can the authors better expose how the numerical experiments link to the\nnatural samples? In general, the modelling, in particular the numerical one, could be described in more detail.\nIs it correct that the authors used forward thermodynamic modelling using PT of melting of natural samples to\nfind the associated Fe3+/Fe(T), from which they then extract ƒO2. That is, their ƒO2 estimate for the natural\nsamples is completely unrelated to the V-based ƒO2 estimate? The onus is on the authors to ensure their\nmanuscript becomes accessible to a wide and non-specialised readership.\nAnswer: In the natural sample workflow, fO₂ is estimated using V-based oxybarometers (e.g., V/Ti or V/Sc\nratios; Fig. 3). In contrast, the numerical model (Fig. 2) derives melt Fe³⁺/ΣFe, fO₂, and other redox parameters\nsolely from known P-T conditions and initial mantle compositions, without the need for V-based constraints.\nThe numerical experiments presented in Figure 2 are fundamentally independent of the calculations based\non natural samples; however, they serve as mutual validation of each other. The modeling results in the figure\nare derived from forward simulations based on prescribed geodynamic conditions and rock compositions within\na coupled thermomechanical-thermodynamic framework. These simulations do not require V-based\noxybarometers to obtain the Fe³⁺/ΣFe ratio in basaltic melts.\nIn contrast, the calculations for natural samples are inversions. When pressure, temperature, fO₂, and bulk\ncomposition are known, thermodynamic modeling can be used to back-calculate the Fe³⁺/ΣFe ratio at the time\nof melt formation, which can in turn be used to estimate the Fe³⁺/ΣFe ratio of the mantle source. Therefore, the\nnumerical and natural-sample-based approaches are conceptually and methodologically independent. The only\ncommonality is their reliance on thermodynamic calculations.\nWe have expanded the relevant sections of the manuscript to better clarify these distinctions and to improve\naccessibility for a broader, non-specialist readership (e.g., Lines 173-176; 228-233).\n2. Related, the authors claim that their approach yields the most accurate and robust insights yet, but ultimately,\nconstraints on mantle redox evolution rest with the natural samples and their shortcomings: (a) unclear tectonic\nsetting (in fact, unlikely MOR-setting for the vast majority of Archaean basalts (Kamber 2015 Precambr Res;\nPuchtel and Arndt 2025 TOG; Brown+24 JGeolSoc) – this is important because we know that mantle ƒO2 varies\nas a function of tectonic setting (Cottrell+22 in Geophys Monogr 266); (b) uneven sampling through time, with\nsome sparsely sampled periods; (c) uncertainties related to uncertainties in PT estimates from natural samples.\nThus, the information from the modelling, however sophisticated, cannot be more accurate and precise than that\nderived from natural samples.\nAnswer: We agree that this section may benefit from clearer wording and it has been done. Our original\nintent was to emphasize that oxygen fugacity is inherently complex -- it reflects redox conditions in P-T-X\nspace\nand is often relative in nature. To address this, we transform fO₂ into a redox budget (expressed as Fe³⁺/ΣFe),\nwhich is governed by mass and charge balance and therefore independent of pressure, temperature, and\ncomposition. This transformation enables a broader audience to understand:\nHow varying mantle compositions, along with their geodynamic and thermodynamic contexts,\nquantitatively control the redox evolution of melts (as shown in Figure 2); How we can better quantify the\noxidation state of the mantle in a thermodynamically consistent way. The integration of both numerical\nmodeling and natural sample analysis is thus essential for quantitatively constraining the redox budget of the\nmantle over time. This dual approach marks a methodological advancement over previous studies.\nWe fully acknowledge the limitations of natural samples:\n(1) Uncertain tectonic settings- We agree that the majority of Archaean basalts may not be from MOR-\nsettings. This is precisely why, among the hundreds of thousands of entries in the database, only a few dozen\nare usable. We are well aware of this issue and have taken steps to address it. In particular, we have applied\nmultiple geochemical screening methods to identify samples most analogous to modern MORBs. Trace element\npatterns in these samples closely resemble those of present-day MORB. In the revised version, we have followed\nthe suggestions from you and other reviewers, such as using a threshold of (Nb/La) ≥1 to minimize crustal\nPM\ncontamination. In addition, we included new calculations using MORB-derived metamorphic rocks (from\nAulbach and Stagno, 2016) and their mantle source regions to reinforce and validate our previous interpretations.\n(2) Uneven temporal distribution-this is a well-recognized challenge not only in our study but across nearly\nall research dealing with long-term geochemical evolution (e.g., Herzberg et al. 2010; Zhang et al., 2024;\nAulbach and Stagno, 2016; Aulbach and Arnd 2019). The particularly sparse sampling in the early Archean and\nMesoproterozoic may reflect a global reduction in tectonic activity. While the number of preserved samples is\nbeyond our control, we do not believe this justifies halting efforts to understand early Earth processes.\nFortunately, the available datasets from both the whole Archean and modern times are sufficiently abundant to\nsupport the key conclusions of our study. We have also noted in the manuscript that these trends should be\napproached with caution. In addition, we conducted two sets of sensitivity tests on the relationship between time\nand mantle Fe³⁺/ΣFe, and the results indicate that our conclusions are robust (see our response to Reviewer 2’\nComment No. 5 for details).\n(3) Uncertainty in P-T conditions of natural samples-we have taken deliberate steps to minimize related\nerrors. For example, we focus on basalts instead of komatiites, which are associated with greater uncertainties.\nWhen estimating P-T conditions, Fe³⁺/ΣFe itself becomes a key variable influencing the outcome. To resolve\nthis, we applied an iterative bracketing strategy (e.g., bracketing criterion): we start by assigning an initial\nFe³⁺/ΣFe within a reasonable range, use this to calculate P-T, derive fO₂ and a new Fe³⁺/ΣFe, then repeat the\nprocess until convergence is achieved-i.e., until both P-T and Fe³⁺/ΣFe stabilize. This method ensures reliable\nestimates of the thermodynamic conditions.\nFinally, we completely agree that no matter how sophisticated the modeling is, its accuracy and precision\ncan never exceed the empirical constraints provided by natural samples. This is precisely why we adopted a\ndual approach: to cross-validate our findings. Numerical forward modeling is not constrained by empirical\nrecords, allowing us to explore how thermodynamic and thermomechanical conditions influence the Fe³⁺/ΣFe\nratio in the mantle. In contrast, natural samples provide direct geological records. We firmly believe that\nmodeling is the extension of observation, and observation is the foundation of modeling.\n3. There are quite a number of vague, sweeping or inaccurate statements, and some ad hoc statements,\nthroughout the main part of the text that are not very informative and therefore leave more questions than\nanswers. Instances are given below.\nAnswer: Thank you for pointing that out. Due to word count limitations in the initial draft, many parts\nwere not explained as clearly as we would have liked. However, within the word limit allowed by Nature\nCommunications, we have now expanded these sections to help readers better understand our logic and content.\n4. The authors variably refer to redox state, oxygen fugacity, oxidation degree, oxidation budget, Fe3+/Fe(T) ….\nThe mantle redox state reflects the sum of several multivalent elements, of which iron is certainly overall the\nmost important one, but S, C and H are not negligible (Evans 2006 Geology). I suggest to make sure ƒO2,\nredox state (or redox budget), and Fe3+/Fe(T) are accurately used throughout the text, and to refrain from using\nadditional terms (e.g. oxidation degree, oxidation budget etc).\nAnswer: Overall, what we refer to as the “redox state” essentially corresponds to the redox budget (Evans,\n2006), which we simply express using the Fe³⁺/ΣFe ratio. As you rightly point out, in addition to iron, other\nmultivalent elements such as C, H, and S also contribute to the mantle’s redox state at MORB formation depths.\nHowever, their concentrations are generally low, and in typical MORB-source mantle, their valence states are\noften assumed to be relatively constant. For example, sulfur is mainly present as S²⁻, hydrogen as H⁺, and carbon\nas C⁴⁺ under these conditions (Evans 2006, 2012). By the way, although at greater depths, carbon may exist as\nC⁰. Upon ascent, the transformation of C⁰ to C⁴⁺ would consume Fe³⁺ (i.e., cause reduction), but the overall\nredox budget remains conserved in MORB mantle source.\nThus, by assuming fixed valence states for C-H-S species under MORB-source conditions, changes in the\nFe³⁺/ΣFe ratio can be used as a reliable proxy for changes in the mantle redox state. Furthermore, variations in\noxygen fugacity from QFM-2 to 0 likely reflect changes in the oxidation states of both Fe and S, but not the C,\nespecially under shallow mantle conditions as C4+ can be stable down to QFM-1.5 to QFM-2 at 2-3 GPa (Maffeis\net al., 2024; Walters et al., 2020).\nTherefore, we accept your suggestion and will standardize our terminology throughout the manuscript,\nlimiting ourselves to terms such as redox state, redox budget, and oxygen fugacity, and avoid less well-defined\nexpressions like “oxidation degree” or “oxidation budget”. We also discussed the influence of other redox-\nsensitive elements (such as carbon and sulfur); please see Lines 118-132; 260-276.\n5. Given continued uncertainties regarding earliest Earth processes and the physicochemical-dynamic\nconditions attached to them, the authors are of course entitled to their preferred interpretation of why the ƒO2\nof Earth’s mantle changed after the Hadean. However, the arguments against upward mixing of redox budget\narising from disproportionated iron, and the arguments in favour of komatiite serpentinisation followed by\nsubduction are substantially less clear-cut than presented, for reasons I detail below. This discussion could be a\nlot more balanced.\nAnswer: We have provided a more balanced discussion on this part, including the possibilities and\nweaknesses of both scenarios, and a more detailed explanation of the calculation process. Please see Lines 353-\n443 and our response below.\n6. Furthermore, the authors take evidence from short-lived radiogenic isotopes to argue for separate upper and\nlower mantle reservoirs, but this seems too simplistic, as early-generated heterogeneities survive to the present\nday (as sampled by OIBs) and are thought to reside in geographically restricted regions that are not identical to\nthe lower mantle as a whole.\nAnswer: Some of the more uncertain discussions have been removed, as they are not directly relevant to\nthe main content. A detailed response to this issue is provided below (see comments on Lines 246–249); here,\nwe briefly summarize the key points:\nWe agree that the mantle should not be oversimplified into distinct “upper” and “lower” compartments.\nA more accurate view is that mantle reservoirs exhibiting different short-lived isotope signatures have largely\nmixed into an approximately homogeneous state over prolonged geological timescales. That said, we fully\nacknowledge that the mantle is not perfectly homogeneous even today and retains localized anomalous domains,\nsuch as those sampled by ocean island basalts (OIBs).\nRecent studies combining W–Ru isotopic systematics (Fischer-Gödde et al., 2020; Messling et al., 2025)\nprovide strong support for the incorporation of late accreted material, as different mantle reservoirs and\nprocesses carry distinct isotopic fingerprints:\n1. Core interaction (sampled by modern OIBs): negative 182W and positive 100Ru\n2. Late accreted material: negative 182W and negative 100Ru\n3. Archean mantle: positive 182W and positive 100Ru\nThe transition from the positive isotopic signature of the Archean mantle to the near-zero values observed\nin the modern mantle cannot be explained by ongoing core–mantle interaction alone. Instead, it necessitates the\naddition and mixing of late accreted material. This indicates that core–mantle interaction, as inferred from OIB\ncompositions, is unlikely to be the primary control on Archean mantle isotopic characteristics.\nRegarding mantle mixing, the progressive decline in mantle ¹⁸²W after the Archean still requires a reservoir\nwith negative ¹⁸²W that was not sampled during the Archean—most likely residing in the deep mantle. However,\nthe detailed mixing mechanisms remain difficult to constrain, due to the lack of information on the scale and\ndistribution of this negative ¹⁸²W reservoir, as well as sparse data coverage in the ¹⁸²W record—particularly after\n~2.5 Ga.\nFor these reasons, we have retained the discussion of late accretion in the manuscript, as it is consistent\nwith both W–Ru isotopic system constraints and the oxidation state inferred in this study. However, we have\nremoved the broader discussion of time-dependent mantle mixing due to the greater associated uncertainties.\nBelow is Figure 3a from Messling et al. (2025), illustrating the effects of late accretion and core–mantle\ninteraction:\n7. The authors dismiss komatiites as “enigmatic”. This is a misrepresentation of the field, as komatiites have\nextremely well-constrained petrogeneses, which may be complicated by their derivation from thermochemically\nanomalous plumes, and it is true that their pressures of formation are difficult to estimate. I would accept this\nas an argument in favour of focusing on spreading ridge-derived samples (MORB-like basalts and picrites). By\nthe same token, the authors should acknowledge that few if any Archaean basalts are truly spreading ridge-\nderived.\nAnswer: The related statements have been removed. We now only retain the point that the formation\npressures of komatiites are indeed difficult to constrain, which may result in larger uncertainties in the derived\nresults. Additionally, we have included a discussion acknowledging that few Archaean basalts are truly derived\nfrom spreading ridge environments.\n8. The authors assume a modern subduction redox budget to estimate whether this could have oxidised the\nmantle to present-day levels. However, we know that the atmosphere was oxygenated to just 1% PAL as late as\n2.4 Ga ago, and that oceanic bottom waters became oxygenated only in the Neoproterozoic (Stolper+Keller 18\nNature), before which time there would have been very little dissolved sulphate, which is the most potent oxidant\n(Tomkins+Evans 15 EPSL). The subducted redox budget surely must have been variable through time. Or\nmaybe I misunderstood what the authors are aiming at. It also seems that they address this in some detail in a\npaper under consideration (ref. 38), this should be clarified.\nAnswer: The redox budget we used in our calculations represents a very conservative estimate. It is\nconstrained by petrological records, ocean drilling data, and seismic observations since ~3 Ga. Specifically, we\ndid not include the highly oxidized lithologies unique to the Phanerozoic, such as those with Fe³⁺/ΣFe ratios\nexceeding 0.5 even up to 0.8 as reported in Stolper and Keller (2018). Instead, we based our calculations on\naltered rocks with Fe³⁺/ΣFe values of ~0.20 for serpentinites, ~0.21 for gabbros, and ~0.26 for basalts -- values\nconsistent with observations from the Precambrian (Fig. a below; Precambrian data from Gard et al., 2019,\nStolper and Keller 2018).\nIt is worth noting that we also did not account for the additional redox budget carried by highly oxidized\nlithologies such as serpentinized komatiitic rocks (Fig. b below; data from Tamblyn and Hermann 2023) or\nbanded iron formations (BIFs). Therefore, we consider our calculation to be conservative.\nThese moderately oxidized lithologies are sufficient to carry a redox budget capable of oxidizing the mantle\nto present-day levels. It is also important to emphasize that none of the rocks we considered contain oxidants\nlike seawater-derived sulphate, which are characteristic of the Phanerozoic (Tomkins and Evans 2015). In our\nmodel, sulphide minerals (e.g., FeS ) within subducted lithologies can be oxidized to sulphate in situ via internal\nx\nredox reactions, driven by the reduction of Fe³⁺ within the rock (S2- + 8Fe3+ = S6+ + 8Fe2+). Part of S6+ goes into\nfluids (Duan et al., 2024). The released oxidized fluids then contribute to the oxidation of the mantle wedge,\nwhile the remaining Fe³⁺ is transported into the deep mantle, incrementally raising its redox budget.\nIt is also worth noting that the redox fluxes we report represent net fluxes, after accounting for the oxygen\nsinks associated with MORB, arc, and OIB melting.\nOur goal in presenting these calculations is to provide a simulation- and observation-based perspective on\nthe long-standing debate over whether subduction can oxidize the mantle to its present-day state. We have\nexpanded the relevant discussion in the revised manuscript to clarify this reasoning.\n9. There are some sentences with awkward English that would benefit from polishing.\nAnswer: We have carefully revised the manuscript to improve clarity and fluency throughout. Sentences\nwith awkward or unclear phrasing have been reworded to ensure smoother and more precise English expression.\n10. The figures and their captions can be much improved. There is too much, in part easily overlooked,\nsmall font in the panels themselves. The captions are so terse as to be uninformative. IMO, readers should be able to\nunderstand what the diagrams show and what arguments they support without reading through the entire\nmanuscript.\nAnswer: We have revised the figures and their captions accordingly. Specifically, we have: Increased the\nfont size within figure panels to improve readability; Simplified and clarified visual elements to reduce clutter;\nExpanded the captions to provide sufficient context, including clear descriptions of what each figure shows and\nhow it supports the main arguments of the manuscript. Our goal is to ensure that readers can grasp the key\nmessages of the figures independently, without needing to refer back to the main text.\n11. Throughout, I suggest to use italics for state variables, such as T, P, f (fugacity) etc.\nAnswer: We have carefully revised the manuscript to ensure that all state variables such as T, P, and f\n(fugacity) are consistently formatted in italics throughout the text.\nIn-line comments and suggestions\nTitle: what is oxidation degree? fO2? Redox budget? Fe3+/Fe(T)? Please be more precise, also in the abstract\nand elsewhere\nAnswer: We have revised that as suggested to use Fe³⁺/ΣFe, and have provided a detailed explanation in both\nthe abstract and the main text. The oxidation degree we refer to represents changes in the redox budget of the\nsystem. In other words, it reflects shifts in the distribution of elements among different valence states (e.g., Fe²⁺\nvs. Fe³⁺), see also Lines 119-133; 260-266.\n11-13 It is clear what the authors want to express in the first sentence but the wording is inaccurate. What is\noxygen-rich habilitability? The atmosphere is now O2-rich. What the mantle regulates is the exchange of redox\nbudget, which ultimately enabled O2 to accumulate in the atmosphere, thereby affecting how life evolved\nAnswer: We have rewritten the sentence.\n13 our planet should be capitalised, also should be “Earth’s”\nAnswer: Yes, has been changed.\n14 remains a subject of\nAnswer: Has been revised.\n16 they are not all basalts, but also picrites according to the supp table. As an aside, the mere fact that there are\nso many basalts implies that Archaean ambient mantle TP could not have been very hot. If the samples are\nbasalts due to extensive differentiation, then they are not appropriate to estimate mantle fO2 from them.\nAnswer: The basaltic melt here includes both traditional basalt and picritic basalt. In the simulation, the\ncomposition depends on the temperature and pressure conditions. In the initial version, we did not specifically\ndistinguish picritic melts. It has already been indicated in the relevant parts of the table and text.\nWe have conducted a series of mantle potential temperature (Tp) experiments that cover both lower Tp\nvalues (1350-1400 °C) and higher Tp values (1500-1600 °C), encompassing the range predicted by both\nHerzberg et al. (2010; 1500-1600 °C) and Aulbach and Arnd (2019; 1500°C). Notably, our numerical models\nshow that even under very high Tp conditions (up to 1600 °C), Archaean-style lithospheric extension, basaltic\nrocks generation, and subduction can still occur. This may support the plausibility of MORB-like rock formation\nunder high Tp in the Archaean. We agree that basalts produced through extensive differentiation are not suitable\nfor estimating mantle fO₂. Therefore, we have made efforts to focus our study on basalts that are as close to\nprimary melts as possible (see Methods).\n16-17 when you are referring to a ridge setting specifically, there is no need to again mention \"geodynamic\nsettings\". Perhaps \"modern conditions\"\nAnswer: Yes, has been revised.\n20 demonstrate\nAnswer: Yes, has been revised.\n20 clearly, measured Fe3+/Fe(T) are not reliable and if you estimated the values based on V/Sc or V/Ti,\nthen your result cannot be more reliable than the information contained in the element ratios? Besides, Zhang et al.\n(2024, cited) already presented whole-rock Fe3+/Fe(T) thermodynamics-based estimates. If yours are\nsignificantly improved, or novel in the sense that you used a different approach, then this should be expressed\ndifferently – this relates to my main comment above\nAnswer: Yes, we agree that the measurements are not entirely accurate. We also did not use the measured ferric\niron (Fe³⁺) values. However, the thermodynamic approach we refer to here involves cross-validation between\nforward numerical modeling and inverse estimation of Fe³⁺ proportions based on recorded P-T-V/Ti conditions.\nOur method incorporates internal thermodynamic consistency as well as geodynamic context, representing the\nfirst such attempt in related research. Although Zhang et al. also inferred Fe³⁺/ΣFe ratios in rocks, they primarily\nrelied on the Kress and Carmichael (1991, K91) model, which tends to overestimate ferric iron content.\nMoreover, their correction for oxygen fugacity appears to be flawed, likely introducing additional estimation\nerrors. Moreover, Zhang et al. (2024) did not calculate the ferric iron (Fe³⁺) proportion in the mantle. We have\nrevised the phrasing to better highlight the novelty and uniqueness of our approach.\n24 please be more specific here: what kind of geological evidence? Below you also refer to biological evolution.\nAnswer: We have revised the sentence to clarify that the observed changes in molar volume are broadly coupled\nwith variations in the redox state of the mantle.\n25-26 tectonic reorganisation - ok. But what do you mean by geological reorganisation?\nAnswer: Perhaps the wording was inappropriate; it has been revised.\ncomment above\nAnswer: Has been revised. But please see our response to “Line 240 how do you define habilitability?” for\nmore detail.\nAnswer: Has been revised.\n37-38 exert... influence, or simply “have… effects”. Effects are not exerted, I think.\nAnswer: Has been revised.\n50 ref. 6 was updated in Aulbach+19 SciRep - and using both V/Sc and Fe3+/Fe(T)! - so you could cite this\ninstead\nAnswer: Has been cited.\n51 here and throughout: QFM units don’t exist. It is just a reference buffer. fO2 was lower by … orders of\nmagnitude (or log units, if you will) – throughout manuscript\nAnswer: Has been revised.\n56 “tends”. Also, please be more specific as we know it is only the garnet-bearing mantle down to metal\nsaturation at ~8-10 GPa where fO2 decreases with increasing pressure. At P<3-3.5 GPa, the fO2-depth profile\nis more complex, as demonstrated from phase equilibria and empirical models (Stolper+20 AmMiner;\nBirner+24 Nature)\nAnswer: Has been revised.\n58-61 This sentence does not reflect the state of the art:\n1. Aulbach+Stagno 16 (cited) did correct for the fO2 decrease with increasing pressure and did find significant\ndifferences between Archaean and modern samples.\n2. Zhang+24 (cited) wrongly corrected their basalt V/Ti-derived fO2 for the reduction of the melt with\ndecreasing pressure after leaving the source. However, the V/Ti-derived fO2 is already that of the source.\n3. Both biased their Archaean, more deeply-derived samples (due to higher TP) to too-high fO2 by assuming\nthe garnet-peridotite fO2-pressure trend could be extrapolated to 1 GPa, but we know since Stolper+20 that this\nis not the case.\nAnswer: Yes, perhaps the meaning was not clearly conveyed. Here, we intended to say that the use of direct\nelemental ratios follows the approach used in reference 12. We have tried to expand the sentence to more\naccurately reflect the current state of research. Has been revised. See Lines 59-74.\n63 these are “elemental ratios”. Furthermore, this statement is blatantly incorrect: neither ref. (6) nor refs.\n(11,13,14) used direct geochemical ratios, as (6) accounted for P (T effect not known at the time), (14) for T and\n(11) accounted for PTX (though made a mistake), and (13) accounted for TX. It is true only for (12) that V/Sc\nwas used directly. (16) should not be cited here - it’s a review paper and we were well aware of PTX effects\nAnswer: Yes, perhaps the meaning was not clearly conveyed. Here, we intended to say that the use of direct\nelemental ratios follows the approach used in reference 12. We have tried to expand the sentence to more\naccurately reflect the current state of research.\n68 “the” depleted mantle is understood by many to be the MORB source. What your fig. 1A seems to show is\nmantle that is residual from extraction of various extents of melting in the suprasolidus region as a function of\ndistance from the solidus (we later learn in the methods that the bulk composition of the model corresponds to\na depleted MORB mantle)\nAnswer: Yes, the initial bulk composition corresponds to a depleted MORB mantle, not the residual material\nitself. This is consistent with the view held by many that the source of MORB-like basalts is a depleted mantle,\nand this is also reflected in the numerical simulations shown in the supplementary figures. In the revised version,\nwe have also included the bulk composition of the primitive mantle. We demonstrate that the transition in whole-\nrock composition from the primitive mantle to a depleted mantle does not affect the simulation results (see\nanswer and figure for Reviewer 3 comments “In addition, if the mantle was remoulded in Fe3+/∑Fe…..”).\ncoupled vs. decoupled?\nAnswer: This is described in detail in the manuscript (please see lines 85-112).\n69-70 This is not shown in Fig. 1? Qualitatively, if Fe3+ behaves like a moderately incompatible element\n(O’Neill+18, cited), one would expect the ratio to decrease with increasing melt F, which is a function of T,\nall other things being equal. This seems to be shown in Fig. 2.\nAnswer: It is shown -- the decreasing trend of Fe³⁺/ΣFe is indicated by the different colors. In a closed system,\nthe decrease in Fe³⁺/ΣFe is correlated with the degree of partial melting (F). However, in an open system,\ndue\nto batch melt extraction, the solidus of the residual mantle shifts toward higher temperatures. When the residue\nundergoes subsequent melting, F starts from zero again, but the Fe³⁺/ΣFe in the melt at a given temperature\nremains unaffected. Therefore, we would argue that Fe³⁺/ΣFe is more a function of temperature or total\ncumulative melting, rather than the melt fraction (F) after batch extraction in an open system.\n71 not just depleted mantle, but any mantle, or more specifically, peridotite\nAnswer: Yes, it has been revised.\n73 perhaps refer redox budget sensu Evans (2006) and then stick with it\nAnswer: It has been revised and added.\n72-76 I think you should refer to the phase eq. and empirical modelling of Stolper+20 (Am Miner) and\nBirner+24 (cited) here. Stolper highlighted how this decoupling is related to changing mineralogy across\nperidotite facies as a function of pressure\nAnswer: Yes, it has already been cited.\n79 please don’t refer to QFM units. Refer to fO2 and several orders of magnitude (or log units)\nAnswer: Yes, it has been changed.\ntime” or similar\nAnswer: Yes, it has been changed.\n82-84 I don’t see how this is more accurate? Melt fO2 also depends on PTX, and unlike elemental ratios,\nFe3+/Fe(T) can be changed by redox interactions (due to redox melting, degassing etc). Besides, measured\nFe3+/Fe(T) of ancient rocks are unreliable due to alteration. Therefore, ultimately, your F3+/Fe(T) modelling is\nnovel and possibly more accurate, but it remains a model that, to be useful, you have to compare against a rock\nrecord\nAnswer: We reworded that. What we meant by saying our method is “more accurate” does not imply that\nexisting fO₂ estimates are inherently inaccurate. Rather, the issue lies in the complexity of calibrating data across\nsamples from different geological times, which can introduce significant uncertainties. For example, this has\nbeen discussed in the work of Zhang et al. Such calibration challenges may lead to the misleading impression\nthat the redox state of the mantle has remained unchanged over time.\nIn our model, Fe³⁺/ΣFe directly reflects the mantle redox budget. This is because, under the P-T conditions\ncorresponding to MORB-like source regions, the reference state of C-S-H is well-constrained. As a result, in\nour simplified modeling framework, the Fe³⁺/ΣFe of the melt at the time of generation is not influenced by\ncomplex external thermodynamic variables, nor does it require complicated calibration procedures.\nWe fully agree that measured Fe³⁺/ΣFe in ancient rocks is often unreliable due to alteration. Fe³⁺/ΣFe can\nindeed be affected by redox processes such as redox melting and degassing. However, these processes are not\nincluded in our modeling approach. As you know, the Fe³⁺/ΣFe values in petrological databases are also inferred\nfrom elemental ratios and P-T conditions, rather than measured directly, and thus are likewise not affected by\nalteration or degassing.\nTherefore, our aim is to define a redox state that is simple, intuitive, and constant, which helps make\ncomparisons of redox evolution across geological time more accessible to a broader audience.\n89 even if this is explained as part of the Methods later, this reference to “thermodynamic and geochemical\nmethods” is much too vague. A few more sentences are needed here, what these methods are, so we understand\nbroadly what you did without having to consult the Methods\nAnswer: Yes, we have added some sentences, please see Lines 136-147.\n93 yes, the modelling is supercool, novel and a major advance. But this does not make observations from the\nsparse rock record itself any more robust\nAnswer: Yes, the rock record is what it is -- we cannot make it more precise. However, it is worth noting that\nour calculations on rock samples incorporate a relatively novel thermodynamic approach, which brings the\nresults closer to the actual record -- something that had not been achieved in previous studies. Moreover, earlier\nempirical methods for estimating ferric iron content often involve substantial uncertainties. See discussion for\ndetails.\n95 please be precise regarding what it is that has doubled. I think you mean Fe3+/Fe(T), which may be the most\nabundant multivalent element, but does not alone fix the redox budget (see Evans 2006 Geology)\nAnswer: This point warrants further clarification: in our study, the Fe³⁺/ΣFe ratio effectively represents the\nredox budget. This is because other redox-sensitive elements are either present in very low abundances or have\nreference states that are unlikely to change significantly in a typical depleted mantle setting. For example, sulfur\ngenerally exists as S²⁻, hydrogen as H⁺, and carbon -- with an abundance of only about 50 ppm (Evans, 2006,\n2012) -- occurs predominantly as CO₂ < ~2-3 GPa, or potentially as elemental carbon at greater depths.\nImportantly, as deep-sourced materials ascend, any carbon that transforms into CO₂ draws its redox budget\nfrom Fe³⁺/ΣFe, ultimately yielding a Fe³⁺/ΣFe ratio consistent with that of the shallow mantle that is the main\nfocus of this study. Therefore, based on the definitions and reference states proposed by Evans (2012), Fe³⁺/ΣFe\ncan be used as a reliable proxy for tracking redox state variations in all our calculations. We will add a few\nsentences to the manuscript to clarify this relationship.\nconditions (what you call thermodynamic conditions)\nAnswer: Yes, it has been changed.\n98-99 This needs much better justification. You could say that komatiites are plume-related and therefore may\nsample thermochemically anomalous sources, whereas basalts may sample an ambient mantle source\nrepresenting a better-constrained, less variable system. You should also be honest and acknowledge in your\nmanuscript that virtually no Archaean basalts are spreading ridge-derived (Puchtel and Arndt 2025 TOG;\nBrown+24 JGeolSoc). The samples you describe as “MORB-like” are actually basalts associated with melting\nin intraplate settings and erupted through attenuated continental mantle. The only true Archaean spreading-ridge\nderived material is now sampled as cratonic eclogite (no need to cite, but see Aulbach+Smart 23 AREPS if you\nwant to know why).\nAnswer: Regarding komatiites, we have decided to accept your suggestion -- we will remove the lengthy and\npotentially inaccurate discussion, and retain only the explanation regarding the large P-T uncertainties.\nAs for basalts, we are well aware that identifying definitively representative MORB samples is indeed\nchallenging, and we also acknowledge the associated debates, as you pointed out. This is precisely why we use\nthe term \"MORB-like\" similar to how some other studies refer to \"no-arc\" samples.\nHowever, what we can confidently confirm is that these samples do not originate from subduction settings\nor from sources similar to the modern enriched mantle. This distinction does not affect the validity of using\nthem to assess the mantle redox state.\nWe have added a few sentences to clarify this point.\n101 there is no representative TP for the Archaen period, as TP estimates vary by >200 oC (e.g. Herzberg+2010,\ncited vs. Aulbach+Arndt 19 EPSL)\nAnswer: We have carefully reviewed both papers and found that our simulations cover the range and average\nvalues of Archean samples reported in the two studies. Collectively, the Herzberg et al. (2010) suggests that the\nArchean mantle potential temperature (Tp) may have been around 1500-1600 °C, while Aulbach and Arndt\n(2019) suggests a warm Tp of 1500°C. We have added a sentence addressing the differences in these estimates\naccordingly.\n106 a totally expected result for all tectonic settings where decompression melting occurs…\nAnswer: This is merely a descriptive statement.\nmodelling approach?\nAnswer: We appreciate the comment and agree with your assessment. The text has been revised to clarify that\nthe result serves to validate our model, rather than to highlight a new finding.\n113 please mind language - what is melting T? T of solidus intersection? It is afterwards, when the melt takes\nwith it heat, that cooling occurs. But this is not specific to MOR ridges, so I am not sure what you mean\nAnswer: Has been done. It is melting extracted temperature.\n117 please explain here the presence of cold drips and the mixing, or refer to a text/Methods where we can read\nup on it - is this related to a pre-plate tectonic regime?\nAnswer: Has been done. Please see also Fig. S6 and related refs.\n122-123 not sure this sentence makes sense as written? What is an intricate “internal thermodynamic\nequilibrium”? Why internal? Aren’t the physical (not geophysical, presumably?) parameters (PT-fO2) dictating\nthe equilibrium? Aren’t the thermodynamic calculations based on experimental petrology, first and foremost?\nAnswer: This reflects the thermodynamic equilibrium among complex mineral solid solution models, which is\na key strength of our approach, as it captures realistic mineral behavior under varying P-T-fO₂ conditions.\n126 imposed rather than provided?\nAnswer: Have revised.\n129 “MORB with fO2 near QFM”\nAnswer: Have revised.\nyour choice of Fe3+/Fe(T) of 0.02 - that he suggested that this is the maximum ratio in case there was a mantle\nredox evolution since the Archaean\nAnswer: Yes, we have already cited this reference here.\neclogite xenoliths interpreted as metamorphosed subducted MOR-derived oceanic crust\nAnswer: We have cited this paper at this point.\n140-141 There is no rationale provided here why it is increased Tp rather than an intrinsically lower fO2\n“promoted reduction of basaltic melts”? Maybe a few steps in the chain of arguments are missing here? (higher\nTP = deeper melting = lower ƒO2 in the garnet-bearing mantle source = lower ƒO2 in the melt which separated\nand reduced)\nAnswer: We have expanded the relevant explanation. However, our results show that the effect is largely\ntemperature-dependent -- even at equal extraction pressures, similar Fe³⁺/ΣFe ratios are observed.\nAnswer: We have revised the sub-title. Strictly speaking, the mantle redox state should be represented by the\ncombined contributions of Fe³⁺, C⁴⁺, S⁶⁺, and H⁺. However, as discussed earlier, the valence states of C-O-H\ncomponents remain largely unchanged in the normal, shallow, depleted MORB mantle, and their overall\nconcentrations are very low. Therefore, we simplify the redox state representation to Fe³⁺/ΣFe. We have provided\na more detailed explanation in the manuscript regarding why a doubling of the ferric iron ratio can be interpreted\nas indicating at least a double increase in oxidation of the MORB mantle source. Please see Lines 119-133; 260-\n276.\n144-146 Keeping your broad readership in mind, perhaps it would be better to say that a higher Tp results in a\nlower fO2 via its effect on the pressure of melting (which is deeper), at least in the garnet peridotite facies - at\nleast this is what you suggest? Also, we’ve known this before, so this is not a new finding from the present study.\nThis is why AS16 and Zhang+24 applied a P correction (though both wrongly extrapolating the garnet peridotite\nfO2-P profile to 1 GPa). AS16 actually estimated P of melting from their preferred TP evolution curve, so the\nlink between TP-P-fO2 is clear. Finally, it is necessary to be very careful with the terminology: here you refer\nto a lower mantle redox state despite constant Fe3+/Fe(T) - I think you mean ƒO2\nAnswer: Yes, we have revised the text. However, what we intended to convey here is not the oxygen fugacity\n(fO₂) per se, but rather the Fe³⁺ proportion in the melt. As shown in Fig. 1a, this is primarily controlled by\ntemperature, not pressure -- even at the same pressure, higher temperatures result in lower Fe³⁺/ΣFe ratios in the\nmelt.\nWhen we referred to a \"constant Fe³⁺/ΣFe,\" we meant in the initial mantle source itself. The lower redox\nstate we describe refers specifically to the Fe³⁺/ΣFe ratio in the basaltic melt, which decreases with increasing\nTp.\n150-154 1st sentence: keeping your broad readership in mind, perhaps it would be better to say that a higher Tp\nresults in a lower fO2 via its effect on the pressure of melting (which is deeper), at least in the garnet peridotite\nfacies - this is what you suggest? Also, we’ve known this before, so this is not a new finding from the present\nstudy. This is why AS16 and Zhang+24 applied a P correction (though wrongly extrapolating the garnet\nperidotite fO2-P profile to 1 GPa). AS16 actually estimated P of melting from their preferred TP evolution curve,\nso the link is clear. Finally, it is necessary to be very careful with the terminology: here you refer to a lower\nmantle redox state despite constant Fe3+/Fe(T) - I think you mean ƒO2\nAnswer: See above answer.\n157 looking at Stolper+20 AmMiner, it is clear that 2.4 GPa is below P where the simple relationship between\npressure and fO2 in the garnet peridotite facies breaks down and fO2 varies significantly and non-linearly as\na function of changing modal abundances\nAnswer: Stolper et al. (2020) is indeed an excellent reference. We cited this. However, it is important to note\nthat their model is based on solid-solid phase relationships. Once the melt phase is taken into account, the\noxygen fugacity and corresponding values can shift, even under identical P-T conditions and bulk compositions.\n162 “previous” - give reference. If Zhang+24, they overestimated Archaean fO2 in two ways, as I explain\nelsewhere in this review). So should we be worried about the consistency with your approach?\nAnswer: This has been clarified. Please note that we did not apply any corrections to the obtained fO₂ values-\nthey directly reflect the source region’s oxygen fugacity. Furthermore, all values we cite refer to source fO₂,\nnot\nto corrected values such as the “potential oxygen fugacity” used in Zhang et al. (2024).\n164 what do you mean by in-situ? what do you mean by Mg-calibrated? Please note:\nAnswer: Thank you for point this out. We have deleted the relevant discussion. Since rocks in Aulbach and\nStagno (2016) can indeed represent MORB sources, we have also included the relevant rocks in the new version.\n(1)Zhang+24 made the same 2 mistakes when applying their corrections to the metabasalts reported in AS16\nthat they did with the basalt database, and their Archaean metabasalt-based fO2 estimates are also too high as a\nresult.\n(2) I happen to be familiar with ref. 6: AS16 did more than just look at MgO content. We used major and\ntrace element constraints to understand the samples’ petrogenesis, which allowed us to exclude samples with\ncumulate protoliths, or those that had been metasomatised or that were LREE-enriched suggesting derivation\nfrom an enriched source or melting in the presence of lithospheric lid - this was done to ensure that the\ncomparison to forward-modelled melt V/Sc (to estimate source fO2) and to modern MORB is justified. For\nmany of the sample suites, AS16 used reported radiogenic isotope systematics to further demonstrate their\nderivation from a depleted mantle source, as is true for modern MORB, and given that at least mildly depleted\nsources existed in the Archaean. This was further corroborated in Aulbach+Arndt19 EPSL where many of the\nsame eclogite suites for which AS16 estimated fO2 are shown to have initial 87Sr/86Sr indicative of a protolith\nderived from depleted mantle.\nAnswer: We fully acknowledge that some samples in your study are of MORB origin. Therefore, in the new\nversion, we have also included your sample as part of the database. We have made a concerted effort to\ndemonstrate the MORB-like nature of the samples in our database. These samples exhibit characteristics of\nderivation from a depleted mantle source and show no evidence of arc-related influence. We also narrowed the\n(Nb/La) range (≥1) to further minimize the possibility of crustal contamination.\nPM\n(3) Your and other’s basalt database, on the other hand, includes in particular Archaean basalt samples that are\nnot spreading ridge-derived, with (Nb/La)N down to 0.75, and the onus is on you to demonstrate that these\nsamples record the fO2 of a convecting mantle source that can be usefully compared to the modern MORB\nsource. That is, without complications that arise from mixing tectonic settings.\nAnswer: Yes, in the revised version, we have further refined our basalt database by narrowing the sample\nselection criteria -- for example, by requiring (Nb/La) ≥1 -- to minimize the influence of crustal\nPM\ncontamination and non-MORB-like geochemical signatures.\n176 any back-calculated result would match some result from the numerical simulations?\nAnswer: Here we are talking about the average value, but since numerical simulations can produce thousands\nor even tens of thousands of results, we can always find some that are very similar among them.\n181 redox state of the Archean mantle as proxied by its Fe3+/Fe(T)\nAnswer: Have done.\n183 what is the oxidation degree and how do you double it?\nAnswer: In this article, the oxidation state is simplified to the proportion of ferric iron (Fe³⁺). Therefore,\nassuming other elements remain in their reference valence states, the oxidation state of the mantle has doubled\ncompared to before. See also answer for comments of Line 143.\n184-185 again, the effect of TP is indirect, and I think it would be better to make it more explicit. “Deeper onset\nof melting owing to higher Archean mantle TP” alone might be misinterpreted\nAnswer: We have rewritten this sentence.\n186 not only that, but it confirms earlier findings as per (6, 13, 14) – worth mentioning here\nAnswer: Yes, has been mentioned.\nAnswer: Have done.\n188-189 sure, but this last statement is hanging in the air, as it is not followed by some statement as to whether\nthis needs further investigation, or whether it conflicts with some evidence. Perhaps you can just leave it out.\nAnswer: Yes, it has been deleted.\n194 this is likely, but currently they are “thought to represent” Archaean - or were 187Os/188Os reported for\nthese samples?\nAnswer: To our knowledge, 187Os/188Os ratios were not reported for those samples; instead, the inference\nwas made based on estimated degrees of melting. Accordingly, our interpretation in that section follows their\nassumption to reconstruct the nature of the corresponding initial mantle source.\n199 in Figure 4\nAnswer: Have done.\n201 then we calculated\nAnswer: Have done.\n201 which other redox parameters?\nAnswer: FO2, redox state of minerals, etc.\n202 what kind of melting? Batch or fractional? It makes a difference for how concentrations of incompatible\ncomponents in the residue evolve\nAnswer: It is from mafic to komatiitic melts, depending on pressure-temperature conditions. Melting is batch\nrather than fractional. The melt fraction (F) refers to the proportion of melting relative to the initial source\ncomposition, not the incremental fraction after each melting step. The different data points represent values\nsampled at equal temperature intervals.\n202 high degrees of partial melting\nAnswer: Have done.\n212 suggests that Archean mantle was more reduced than the modern mantle\nAnswer: Have done.\n214-217 Please add a statement/caveat regarding uncertain provenance/tectonic setting of basalts, a potential\npreservation bias linked to the supercontinent cycle and poor statistic coverage of certain eras.\nAnswer: Have done. Please see Lines 313-323\n218-219 by the time a melt is able to saturate zircon, it is so far removed/processed from the original mantle-\nderived melt that it is questionable at best that the fO2 they record reflect the mantle rather than some later\nprocess\nAnswer: Yes, we have added some sentences to clarify the reliability of the relevant records.\n232-233 I suggest to rewrite “… this hypothesis, as the accretion of meteorites with negative … lowered the W\nisotopic composition of the silicate mantle with an initial positive W isotope composition (+200…) in the wake\nof core formation and evolution at high Hf/W”.\nAnswer: Have done.\n233 when did these meteorites arrive? Given how short-lived the system is, there must be an estimate for the\ntiming\nAnswer: At the Archean-Hadean boundary. Have done.\n235 should be ref 29? Might need to check neighbouring references, too\nAnswer: Have done.\n235-237 please rewrite, this sentence is not well constructed\nAnswer: Have done.\n240 how do you define habilitability? Life evolved even as the atmosphere contained <<1% PAL O2. So this\nchange in mantle O2 would have affected the course of evolution of life, but not hindered life per se.\nAnswer: Yes, \"habitability\" is a multidisciplinary concept, and we think its specific definition may vary\ndepending on the research context. In our view, habitability is primarily determined by energy and matter.\nWe\nthink the central question is whether an environment can support the existence and sustained evolution of life.\nIn our view, supporting the existence of life does not necessarily require an oxygen-rich environment, as\ncyanobacteria existed long before the Great Oxidation Event. However, when it comes to the sustained evolution\nof life -- especially complex, carbon-based life as we know it -- oxygen-rich conditions are essential. Therefore,\nin our manuscript, we will add “oxygen-rich” before “habitability”, to refer specifically to environments capable\nof sustaining the long-term evolution of complex life. In the revised version, we have removed it.\n246-249 this is much too simplistic an argument, and untenable. Even modern OIBs still contain components\nthat must have been generated during earliest Earth evolution including W (e.g. Mundl+17 Science) and Nd\n(Horan+18 EPSL). The sources likely reside in long-lived heterogeneities (possibly atop the core) that cannot\nbe framed in terms of upper vs. lower mantle\nAnswer: Indeed, this part of the content may not have been clearly expressed in the original version. We agree\nthat the original ‘upper/lower mantle’ argument was oversimplified, and the evidence based solely on tungsten\n(W) and neodymium (Nd) isotopes is relatively weak. We have substantially revised this section to address these\npoints.\nFor discussion about isotopes, in the revised version, we removed the discussion of neodymium isotope\nbecause discussion parts involving Nd are very limited. We instead incorporated recent findings (Messling\net\nal., 2025) on ruthenium (Ru) isotopes to support the addition of late accretion material in the mantle in Archean\nin this answer. The key insight from (Messling et al., 2025) is that the 182W anomalies observed in modern\nocean island basalts (OIB) and early Earth rocks are likely originate from different mechanisms: modern OIB\n(which represents core leakage) show positive 100Ru and negative 182W, whereas Archean samples show both\npositive in 100Ru and 182W. Therefore, continuous core interaction couldn’t fully explain the evolution of\n182W and 100Ru in Archean, and this observation could be only explained by the incorporation of meteoritic\nmaterial with negative isotopic signatures into the mantle (Fig. 5 and Fischer-Gödde et al. 2020).\nBelow is Figure 3a from Messling et al 2025, showing the effect of late accretion and core:\nTherefore, for mantle mixing, if continuous core-input cannot explain the transition of 182W from positive\nArchean values to around zero for modern MORB (otherwise the constraints from 100Ru would be invalid, as\ncore input cannot decrease the mantle 100Ru to present day value), a complementary negative 182W reservoir\nthat participates in later mantle mixing would still be required, which is likely located in the deeper mantle.\nOn the other hand, when discussing the dynamics of this mixing process, we agree that it is important to\navoid overemphasizing a rigid “upper mantle versus lower mantle” division. However, we would like to\nemphasize that this view is not only based on isotopes but also based on our numerical modeling results (see\nfigure and text in green box below), which show that Archean subduction differs significantly from modern\nsubduction. This difference is due to phase transitions at 410km and 660km, thus causes differences in the\nupper/lower mantle.\nA subduction model with a mantle potential temperature at Tp=1500°C demonstrates that oceanic plates\ncould still undergo subduction. However, the subducting slab experiences frequent break-off episodes due to\nthe high Tp, which significantly reduces both plate strength and mantle resistance to slab penetration.\nAdditionally, large slab-pull forces produced by a thick and dense crust further contribute to slab break-off. Cold\ndetached slab fragments accumulate in the bottom of the mantle transition zone due to the negative Clapeyron\nslope of the perovskite transition, which precludes their penetration into the lower mantle. Subsequently, the\npositively buoyant highly depleted mantle lithosphere of the slab fragments separates from the dense eclogitized\ncrust and returns to the upper mantle. In this scenario, most oxidized materials can only cycle in the upper\nmantle. This type of subduction has also been documented in our previous studies and those of others (Perchuk,\net al. 2020; van Hunen and van den Berg, 2008). This is precisely why we previously emphasized the most\nintuitive evidence for the distinction between the upper and lower mantle. At the same time, this shallow-level\nmaterial recycling model can also explain why both the mantle and atmospheric oxygen became oxidized before\n2.4-2.7 Ga, while the characteristic positive 182W signatures from the early Archean to Paleoproterozoic could\nstill be preserved until this time.\nWe also should not assume that the mantle has become fully homogenized. As previous studies have\npointed out, even today, some anomalous components still exist in the mantle. Therefore, we prefer to describe\nthe mantle as having undergone an “approximately homogeneous mixing” process -- meaning that although not\nperfectly uniform, it has reached a broadly equilibrated state over billions of years of geological time. This has\nbeen demonstrated in several studies.\nTherefore, we kept the discussion about late accretion in the text as it’s consistent with the constraints from\nW-Ru isotopic system and the inferred oxidation states in our model. However, for mantle mixing, we have\nremoved the detailed discussion based on 182W, given the uncertainties with mantle mixing due to unknown\nextent of the negative 182W reservoir and lack of data during Proterozoic. A detailed discussion lies outside the\nscope of this study.\n247 “Archean rocks have W and Nd isotopic signatures…” (what was fractionated were the parent and daughter\nelements\nAnswer: Have done.\n250 not today, but if iron disproportionation and extraction of metal to the core is real, then it would have been\nat the time of late accretion.\nAnswer: Have done.\nconvinced that the last word has been spoken on whether or not metal formed by disproportionation can be\nextracted to the core. Hirschman 23 EPSL (cited, though not in this context), invoked a basal magma ocean to\nexplain Earth’s redox evolution, and recent work (Boukaré+25 Nature) suggests that this would have been\ninescapable on Earth. It seems difficult to argue that metal formed by Fe2+ disproportionation on top of the core\n(expected at this pressure) would not have been able to be efficiently extracted to the core even if Boukaré+\nwere not concerned with this particular aspect.\nAnswer: We agree that the mantle plume model is a reasonable explanation for redox budgets, and we think we\nboth agree that Fe²⁺ disproportionation requires the presence of a mineral, such as perovskite, that can\naccommodate Fe³⁺. However, recent thermodynamic calculations (Stixrude and Lithgow-Bertelloni 2024)\nsuggest that the Fe disproportionation reaction occurs only at the top of the lower mantle (near mantle transition\nzone), which was recently observed in natural samples (Pan et al., 2025). This suggests that iron can remain in\nplace at least. If the thermodynamic results and this observational evidence are correct, then it implies that the\nmantle redox budget has not changed. This implies that FeO in the BMO (basal magma ocean) does not undergo\ndisproportionation. The reason that this reaction only occurs at the top of the lower mantle is not due to the\ndepth of the magma ocean, but is instead determined by the thermodynamic properties of the minerals.\nFor Boukaré et al., (2025): setting aside the thermodynamic results (i.e., Stixrude and Lithgow-Bertelloni\n2024), if we assume that Fe2+ disproportionation forming perovskite can occur under the lowermost mantle\nconditions, then in Boukare paper, the melt fraction in the BMO region all reaches 100% (their Fig. 1j and\nSupplementary Video 1). If that is the case, it would be difficult for perovskite to crystallize from a fully molten\nBMO. Without crystallized Fe³⁺ perovskite, my understanding is that the disproportionation reaction may also\nhardly occur.\nAnyway, we acknowledge that the mantle plume model is an appealing explanation, and we have revised\nthe relevant discussion for both models of BMO-related mantle plume model and subduction model.\n254 what do you mean by “not show mantle oxidation mechanism”? This is awkward English. Is it correct to\nsimply say it has a more reduced mantle than Earth? Despite having a metallic core, a lower mantle composed\nof perovskite and despite having had a (basal) magma ocean, which are pre-requisites for this mechanism to\nwork?\nAnswer: Have modified.\npermanently sequester reducing power, which you can do by extracting metal to the core or having\nhydrodynamic escape of H2 which surely would have been much more efficient during earliest planetary\nevolution (even if it still occurs at an exceedingly low level today). Taking oxidising power from the ocean-atmosphere system and sticking it into the mantle means you are reducing the surface reservoirs, thereby\ninhibiting the advent of the GOE.\nAnswer: We have revised the wording from “more plausible” to “another”. In order to achieve net oxidation of\nEarth's surface environment, the reducing flux must either be lost to space, sequestered into the core, or\nconsumed by biological processes. The plate tectonic (or subduction-related) mechanism we now propose\nemphasizes the need to couple hydrogen escape and biological consumption over geological timescales.\nHowever, as we noted in our previous discussion, when considering hydrogen escape (and biological\nconsumption) as an oxidation mechanism, it is crucial to remember that the oxidation flux is not solely\ndetermined by the escape rate (and consumed rate), but also by the duration over which escape occurs. That is,\nthe related redox budget = H escape flux (+ H₂ consumed by microbes) = hydrogen escape rate × escape\n2\nduration (+H₂ consumed rate × consumed duration) (i.e., the accumulation timescale of surface oxidation flux).\nFor example, during the Archean, the redox budget driven by hydrogen escape could reach ~7 × 10¹² mol\nO₂/yr (Catling et al., 2001). Over a hydrogen escape timescale of 1.0-1.5 billion years (from ~3.9 to 2.4 Ga),\nthe total accumulated redox budget -- presumably stored in the oceanic lithosphere -- is sufficient to explain a global\noxidation transition. These oxidation reservoirs could have been eventually recycled into the mantle during a\nsingle Wilson cycle (~200 Myrs) via subduction.\nFurthermore, photosynthesis clearly existed well before the Great Oxidation Event (GOE), which suggests\nthat a significant portion of the reduced flux was already being consumed biologically. The net long-term\noxidative storage is likely preserved in hydrated, oxidized lithospheric rocks. Our mechanism highlights the\nquick features of global redox budgets input mantle via subduction.\nBesides, this subducted material and redox budget would eventually resurface (we see it in OIBs and even\nMORBs), resulting in no net change. To the extent that mantle fO2 affects the fO2 of magmatic gases, you\nwould, after the onset of subduction of oxidised material, still have a shallow reduced mantle degassing reduced\nvolatile species that are O2 sinks. In fact, before the oxygenation of ocean bottom waters in the\nNeoproterozoic, the redox budget of what subducts might not have been hugely positive (see Stolper+Keller 18\nNature; Tomkins+Evans 15 EPSL), and you would need to subduct a great volume to achieve an effect on the mantle,\nwhich then must be reconciled with other evidence for the maximum mass of subducted material in the mantle.\nAnswer: Yes, we agree -- if you are referring to those Phanerozoic highly oxidized rocks (e.g., Fe³⁺/ΣFe > 0.5)\nsuch as those shown in the figure from Stolper and Keller (2018), they are indeed not suitable for past flux\nestimations. We've taken that into account. To avoid overestimation, we based our flux calculations on\nmoderately oxidized rocks, specifically those with Fe³⁺/ΣFe < 0.26, which is consistent with observation. For\nfurther details, please refer to our response to comment NO. 8.\n262 that’s 500 Myr later than what you call for at the beginning of the paragraph.\nAnswer: Yes, the timing of mobile-lid tectonics spans a broad interval, with many researchers suggesting it\noccurred between 3.2 and 2.5 Ga. Therefore, the event mentioned here at ~2.7 Ga falls within that timeframe.\nWe have revised the wording.\n263-275 please clarify overlaps with the manuscript under consideration as ref. 38\nAnswer: Only the calculation results from this reference are cited here; there is no duplication involved.\n266 they are transporting redox budget, not fluxes. Besides, this redox budget is then “missing” at the surface\nand unavailable to oxidise surficial reservoirs.\nAnswer: We have already revised \"flux\" to \"budget,\" and the net redox budget we refer to has already accounted\nfor the portion that is missing from the surface.\n268 ref. 38 is miscited in this place. They showed that komatiite serpentinisation produces H2 and that’s it.\nThey never mention subduction or mantle redox. Furthermore, I question that the H2 so produced could be\nquantitatively lost from the Earth system, as hydrodynamic escape would have been highly inefficient by the\ntime komatiites were emplaced\nAnswer: Please note that our calculations (36±6×1012 mol/year) do not include the redox budget from\nserpentinization of komatiites. We only consider it as a potential additional budget. Moreover, since\nserpentinization readily occurs underwater, most serpentinized komatiites -- being part of the Archean oceanic\ncrust -- were likely subducted long ago (former Ref. 40; Tamblyn and Hermann, 2023). In fact, another paper\n(Tamblyn et al., 2023) by the same authors of former Ref. 40 also discusses how the subduction of these\nserpentinized komatiites contributed to the formation of TTGs, indicating that they too recognize these rocks\nwould have been subducted.\nAs for hydrogen escape during the Archean, as mentioned in our previous response, the escape rate could\nhave been substantial. Furthermore, the total hydrogen escape flux is a function of both the escape rate and the\nduration over which it occurred.\n271-274 (1) It is insufficient that ultra-oxidised oceanic crust occurs. What matters is its mass. E.g. Cratonic\neclogite xenoliths undoubtedly represented subducted oceanic crust, including having fractionated O isotopic\ncompositions pointing to seawater alteration, have consistently much lower Fe3+/Fe(T) than what you state here\n(no need to cite, I am just referencing in support of my statement: Aulbach+Smart 23 AREPS, Aulbach+22 JPet,\nAulbach+19 SciRep). (2) It is not admissible to assume a modern subduction redox budget when we know that\nthe atmosphere was oxygenated to just 1% PAL as late as 2.4 Ga ago, and that oceanic bottom waters became\noxygenated only in the Neoproterozoic (Stolper+Keller 18 Nature), before which time there would have been\nvery little dissolved sulphate, which is the most potent oxidant (Tomkins+Evans 15 EPSL). (3) I wonder where\nthe idea came from that komatiites have relatively high Fe3+? If measured Fe3+/Fe(T) were reliable, someone\nwould have exploited that before, but they are not. Berry+ 08 Nature were able to make an estimate on a melt\ninclusion in komatiite and obtained 0.10, similar to modern MORB.\nAnswer: Regarding points (1) and (2), please note that we did not use highly oxidized AOC rocks with Fe³⁺/ΣFe >\n0.26 even >0.5, which are characteristic of the Phanerozoic. The Fe³⁺/ΣFe values we used are 0.20 for\nserpentinite, 0.21 for gabbro, and 0.26 for basalt (see our response to comment NO. 8). At the same time, the\nArchean oceanic crust was thick (20–25 km), and we did not assume that the entire crust was altered. We only\nsuggest that alteration occurred in the upper few kilometers, which would be sufficient to carry redox budget\n(RB). From this perspective, the estimate is conservative.\nWe are not sure whether the Fe³⁺/ΣFe values in the cratonic eclogites mentioned here refer to the original\nmagma composition or the post-alteration state. The redox budget we refer to is based on the altered (i.e.,\nhydrated or serpentinized) state. Moreover, it is worth noting that eclogites likely underwent dehydration.\nAssuming the altered rocks originally had Fe³⁺/ΣFe ratios of 0.2–0.3 (Figure in response to comment NO. 8), a\nportion of their redox budget (RB) could have been released into the sub-arc mantle during subduction, through\nreactions involving sulfur (see Duan et al., 2024). This implies that the RB remaining in exhumed eclogites may\nnot reflect the total redox budget acquired during seafloor alteration prior to subduction.\nThe redox budget we calculated already subtracts the oxygen sinks associated with MORB, arc, and OIB\ncomponents. It also does not consider the oceanic bottom-water sulfate unique to the Phanerozoic. Therefore,\ninvoking a Great Oxidation Event or requiring widespread oxidation of ocean water is not necessary -- only\nwater-induced alteration reactions (i.e., hydration) are needed.\nRegarding point (3) The relatively high Fe³⁺ content in komatiites does not refer to their original (primary)\nferric iron content, but rather to the Fe³⁺/ΣFe ratio after serpentinization alteration (Fig. b in comment NO. 8).\nLarge datasets indicate that most komatiites have undergone serpentinization, and these serpentinized komatiites\ncommonly exhibit elevated Fe³⁺ proportions.\n281 parent-daughter elements are fractionated not the isotopic compositions, which trace different sources with\ndifferent isotope compositions owing to elemental fractionation at some time when the system was still alive.\nSee my comment above regarding the inadmissibility of casting these different sources of ancient signatures in\nterms of upper vs. lower mantle.\nAnswer: The isotope-related discussion has been removed; our analysis is now based exclusively on numerical\nsimulations.\n283-284 seems an ad hoc statement and requires a little more info\nAnswer: The isotope-related discussion has been removed; our analysis is now based exclusively on numerical\nsimulations.\n287-288 this was already proposed by Aulbach+19 SciRep based on the eclogite and komatiite record.\nAnswer: It has already been cited.\n288-289 please see my comment above. This is worded peculiarly and moreover falsely diminishes how ref. 6\nor 11 scrutinised their database (although 6 went a step further to demonstrate derivation from a depleted mantle\nsource) – see similar comment above\nAnswer: We have removed the relevant statement to avoid any misunderstanding.\n299 why would there have been separate upper and lower mantles? Where are isotopic anomalies from now\nextinct isotope systems in modern OIBs from when we know for sure we have whole-mantle convection? This\nparagraph is based on a series of highly uncertain findings and perhaps could be shortened to two sentences\ninvoking the magmatic lull (which was not global, however) as a potential sign of sluggish tectonics that might\nexplain the lack of mantle fO2 evolution during that period\nAnswer: As noted above, the anomalies observed in modern OIBs result from mechanisms clearly distinct from\nthose in the Archean. Recent studies have attributed OIB signatures to possible core leakage. While present-day\nobservations support whole-mantle convection, we agree that discussions related to the early Archean and\nMesoproterozoic are uncertain and not central to the focus of this study. Therefore, we accept the suggestion to\nshorten the discussion, retaining only the point that the magmatic lull may reflect sluggish tectonic activity,\nwhich could help explain the apparent stagnation in mantle redox evolution during that period.\n302 ref. 6 did not invoke a Neoproterozoic rise (though we were tempted)\nAnswer: Have modified.\n307 hm. This was surface driven (increased photosynthesis and organic carbon burial, plus more; e.g.\nOch+12 EarthSciRev), which ultimately caused oxygenation of oceanic bottom waters. This increased the\nsubduction flux of redox budget, as seen by the advent of certain convergent-margin ore deposits (Tomkins+Evans\nAnswer: We agree. However, as we've emphasized, even the net subduction budget of moderately oxidized\nrocks alone is sufficient to drive mantle oxidation. Please see answer for comment NO. 8.\n313-326 last paragraph is too wordy, repetitive and vague. Could be shortened to a crisp and strong final\nstatement.\nAnswer: We have condensed it into a clear and concise concluding statement.\n315 propose\nAnswer: Have done.\n315 except for large impacts, nothing of what follows is abrupt.\nAnswer: Have done.\n321 greater quantities of… fluxes makes no sense, the flux already comprises a quantity (referenced to a time\nunit). Please reword.\nAnswer: Have done.\nfrom basalts around 2.9 Ga that support a warm rather than hot Archaean ambient mantle (e.g. Zhang+24\nNatcomms, cited), continue to be ignored. But that’s a different story…\nAnswer: Following your suggestion, we have shortened this subsection to highlight that basalts are more\nsuitable mainly due to their smaller P-T uncertainties. We have read the Aulbach and Arndt (2019) paper, which\nconstrains a lower temperature range, and we have incorporated those constraints into our earlier discussion.\nHowever, we focus on the higher-end estimates in this paragraph because if similar spreading centers and\nArchean-style subduction could occur even under higher mantle potential temperatures, then such tectonic\nactivity would likely persist as the mantle continued to cool.\n334 please specify somewhere the composition of the mantle that you modelled (Fe3+/Fe(T) is not enough to\ncharacterise the system).\nAnswer: We have added this information; please refer to the citation below.\n337 remove first comma\nAnswer: Have done.\n337 how thick in km?\nAnswer: ~20-25km.\n340 what do you mean by “melt-bearing peridotite”? The melt was similarly dense as the peridotite and failed\nto separate? Or simply the peridotite being above its solidus T?\nAnswer: The term \"melt-bearing peridotite\" refers to a state in which the original solid peridotite transitions,\nupon exceeding a temperature threshold, into a mixture of peridotite and partial melt. It is simply the peridotite\nbeing above its solidus T.\n341 basaltic or picritic?\nAnswer: Basaltic and picritic. It depends on pressure-temperature conditions and petrological definitions.\n341 why hydrated?\nAnswer: This represents a secondary alteration process, in which the rock was hydrated through reaction with\nseawater at the surface.\n344-348 Are we still in Fig. S6? Can you refer to panels, additionally?\nAnswer: Yes, have done.\nbasins are made up of picrites, so there is no conflict at all. Syntax in last part of sentence needs fixing\nAnswer: Yes, have done.\n354-357 It is fine to not consider komatiites in your work and to say that the petrogenesis of spreading-ridge\nderived basalts, but this is an inexact summary of the state of the art, and a weak way to dismiss the komatiite\nrecord. With the exception of a single komatiite (Commondale), where subduction MAY be a possibility, there\nis no evidence for komatiite in subduction zones, or from particularly wet sources. No one says komatiites form\nat the core-mantle boundary. Please duly distinguish between mantle sources and melts. Please refer to\nWaterton+Arndt 25 (In: Hofmann+ eds The Archaean Earth: Tempos and Events 2nd Edition of The\nPrecambrian Earth), Puchtel+22 ChemGeol, Puchtel+Arndt 25 TOG, Herzberg 16 JPet etc. By the same token,\nwhy not acknowledge that virtually no Archaean basalt is strictly comparable to modern MORB (and hence\nmantle sources) because they were emplaced on continental margins?\nAnswer: We have removed the potentially inaccurate summary regarding komatiites. For basalts, we have\napplied more stringent selection criteria to ensure that the selected samples most likely originate from MORB-\nlike source regions. We agree that identifying true MORB samples is challenging -- this is precisely why, out of\ntens of thousands of samples, only a few dozen meet the criteria. In addition, we have corrected our previous\noversight by including the metamorphosed MORB samples from Aulbach and Stagno (2016) in our dataset.\n364 where is the evidence for that? Sources such as Early Enriched Reservoir and similar that were sluggishly\nmixed throughout the mantle, are though to reside in geographically isolated parts of the (lower) mantle and do\nnot require separate UM and LM compositions\nAnswer: Have deleted.\n365 please don’t refer to komatiites as highly enigmatic. They are not.\nAnswer: Have deleted.\n369 how did accreted meteorites get into the lower mantle, if not through the upper mantle? Wouldn’t they have\nequilibrated with an upper magma ocean more efficiently than with a partially solidified lower mantle?\nAnswer: Meteorites, being rich in iron and thus having higher density, tend to continue sinking into the lower\nmantle rather than remaining in the upper mantle.\n370-372 In the discussion, you say that the mantles were already mixed and that plate tectonics are weak. Here,\nyou want to bring up deep reduced mantle. Can you have it both ways?\nAnswer: Yes, we believe both can be true. Since the mantle was largely mixed (while not excluding the\nexistence of chemically anomalous domains), a relatively reduced lower mantle would allow the mixed mantle\nto effectively suppress oxidation in the upper mantle -- or even reduce previously oxidized upper mantle regions.\nThis is supported by rock records. Unless there was a continuous influx of redox budgets into the mantle during\nthe Mesoproterozoic, further oxidation could not be sustained. However, due to weakened plate tectonics,\nsuch\ninput would have been limited.\nWe suggest that, if applicable, isotopic data can only provide an upper limit for the timing of mantle mixing,\nrather than a lower limit. This is because, as more data become available, the lower limit may continue to shift\ndownward. In addition, numerical modeling evidence indicates that subduction styles characteristic of the\nArchean-Proterozoic boundary -- namely, recycling of subducted materials within the upper mantle -- may have\npersisted until before the Neoproterozoic. Therefore, we propose that the final equilibration between the upper\nand lower mantle (excluding chemically anomalous domains) may not have been achieved until the\nNeoproterozoic.\nTherefore, we propose that a reduced lower mantle, ongoing mantle mixing, and a decrease in redox budget\ninput caused by diminished tectonic activity collectively contributed to the stalling of oxidation during the\nMesoproterozoic.\n433 ref. 55: this is a different citation from the two you referenced above when talking about the spread of\nArchaean TP\nAnswer: We have added the previous references. The temperature of 1500 °C can encompass the Archean\npotential temperatures (Tp) reported both in Herzberg et al. (2010; 1500-1600 °C) and Aulbach and Arnd (2019;\n1500°C).\nrequire excess TP), but please note that these are present-day thicknesses (for intact cratons). Ancient diamond\nformation supports that cratons already reached thicknesses of 150-180 km.\nAnswer: Yes, it is indeed.\n447-448 fix this sentence.\nAnswer: Have done.\n455 not crystal-clear what “these” refers to\nAnswer: Have done.\ntoo).\nAnswer: Yes, have done.\n469 up to 1.2 orders of magnitude? Average 1.2? Surely this must depend on the modelled conditions??\nAnswer: Up to 1.2 orders of magnitude.\n480 please explain what motivates this choice of 0.02 and 0.04\nAnswer: Yes, have done.\n483 and are these increments then aggregated?\nAnswer: No, it is the actual value representing each temperature-pressure point.\n489 why 0.04 and 0.01 here, but 0.04 and 0.02 above?\nAnswer: Yes, Has already explained.\n496 what kind of age correction? Why?\nAnswer: Has already explained.\n500 fix materia\nAnswer: Yes, have done.\n502 “low degree of alteration”. How was this assessed? Did you use LOI?\nAnswer: Highly altered samples with a loss on ignition higher than 6 wt%.\nformation in convergent margins\nAnswer: To ensure minimal crustal contamination, we further filtered the dataset using a stricter (Nb/La) ≥\nPM\n1 threshold (see original text for details).\nexpressed wrongly?\nAnswer: The relevant criterion is no longer used due to concerns raised by Reviewer 2.\n518 how many of how many is “most”?\nAnswer: The relevant criterion is no longer used due to concerns raised by Reviewer 2.\n520 what are zircon quantities?\nAnswer: Eight grains are considered to be mantle-derived, based on their oxygen isotope characteristics.\n528-529 Maybe, but a recent study finds that V/Sc is less sensitive to mantle source and melt fraction (Liu+25\nGPL). For the P you report, garnet-present melting is subordinate, anyways (as per your next paragraph).\nAnswer: While their approach appears to rely on large-scale data analysis, our study emphasizes the\nthermodynamic behavior of elements.\n539 what is the P interval? Is it itself T-dependent?\nAnswer: Basically, its interpretation is based on correlations, as seen in the garnet appearance line in Figure 1,\nrather than on a fixed pressure or temperature.\n544 more specifically a previous empirical model\nAnswer: Have done.\n545 awkward sentence, please rewrite\nAnswer: Have done.\n545 following ref. 11 is bad news because they calculated fO2 based on V/Ti and V/Sc, which records source\nfO2 directly (forward-modelling of peridotite melts) rather than low-P crystallisation, but then applied a\ncorrection assuming that the melt self-reduces during decompression after separating from the source. This\nreduction is relevant to olivine-melt V partitioning, and to Fe3+/Fe(T) recorded in the emplaced melt, but not\nto forward-modelled V/Sc or V/Ti. I gather from a later paragraph that you did not apply this wrong correction.\nAnswer: Yes, we did not used. We have consistently emphasized that our fO₂ values represent the redox state\nof the mantle source, and that the Fe³⁺/ΣFe ratios also reflect the source characteristics.\nwhich will have contributions from uncertainties in PT estimates and on source composition\nAnswer: Has done.\n562-563 See comment to line 545. The statement itself is inaccurate because olivine-melt V partitioning is a V-\nbased oxybarometer that does not record ƒO2 of melt formation (in the mantle source), but of low-pressure\nsaturation of liquidus olivine when the melt may have self-reduced.\nAnswer: Already corrected.\n570 last sentence is a bit strange. Perhaps say it is also observed in the database assembled in this study, or\nsimilar\nAnswer: Already corrected.\ncomparison of sample-derived V/Ti-based fO2 to modelling results. You only have one independent Fe3+/Fe(T)\nestimate (from the model). What am I missing? Please make it more explicit.\nAnswer: The former approach uses the measured fO₂ to reverse-calculate the Fe³⁺/ΣFe ratio and does not rely\non the whole-rock composition. In contrast, the latter is a forward-modeling method that requires the input of\nthe initial whole-rock composition and the Fe³⁺/ΣFe ratio (obtained from the reverse calculation) to compute the\nfO₂ under the corresponding conditions. Although both methods utilize thermodynamic calculations, they differ\nslightly in approach.\n597 the (inherited) zircons, but not the melts (or lavas) are in the record. The melts were likely SiO2-rich and\nnot mantle-derived, but either remelted or strongly differentiated from a mantle-derived melt, so as to be able\nto saturate in zircon. Either would imply that zircon fO2 does not record that of the mantle-derived melt and\nmuch less that of the mantle source.\nAnswer: We agree, and we have already acknowledged this possibility.\nI attach my comments to figures and captions in the main text and in the supplement directly in the pdfs.\nAnswer: We have responded to or revised all the points raised. Finally, we would like to express our sincere\ngratitude for your thorough and thoughtful comments, which have significantly improved the quality of our\nmanuscript!\nReviewer #2 (Remarks to the Author):\nThis is a much improved version of the original submission of this manuscript, and all of my previous comments\nhave been addressed satisfactorily. I am happy to recommend this manuscript for publication in its current form.\nAnswer: We sincerely thank you for the positive evaluation and recommendation for publication. We greatly\nappreciate your previous insightful comments on our data, which have helped us improve the accuracy and\nstrengthen the reliability of our results.\nReviewer #3 (Remarks to the Author):\nI have read the rebuttal letter and the revised manuscript carefully. My major concerns are: 1) A close check of\nFig. S3b and Fig. S4c shows that the Fe3+/ΣFe for both the peridotitic mantle and the basalt whole rocks are\nhighly variable. I suspect the alleged \"doubled Fe3+/ΣFe\" of the mantle by the authors. It shows that no clear\nsecular evolution of Fe3+/ΣFe of the mantle. 2) If exists, the explanations for the tortuous evolution of the mantle\nare mainly qualitative. No figures were used to explain the possible causal relation between the elevated mantle\nFe3+/ΣFe and tectonic parameters.\nAnswer: We sincerely thank you for the careful evaluation of our rebuttal letter and the revised manuscript.\n1) Regarding Figures 3, S3b, and S4c, we would like to emphasize that the “doubling” we refer to\nrepresents the average state of the MORB-source mantle and the majority of samples, rather than individual\nextreme values. For example, approximately 90% of basalts derived from the Archean mantle have Fe³⁺/ΣFe\nratios below 0.06 (Table S3), whereas in the modern, MORB Fe³⁺/ΣFe ratios are around 0.10–0.14. The few\nArchean basalts with higher Fe³⁺/ΣFe ratios (0.06–0.10) likely reflect localized, more oxidized mantle domains,\nwhich may have acquired additional redox budget through specific processes such as subduction-related inputs,\nas we discussed. Therefore, our conclusion regarding a doubling of the mantle’s Fe³⁺/ΣFe ratio remains robust.\nIt is also important to note that long-term secular trends are not always visually apparent in scatter plots, because\nseveral extreme values in the Precambrian dataset can obscure the overall pattern. However, a direct comparison\nbetween Archean and modern samples clearly (Table S3) reveals the increase in mantle and basalt oxidation\nstate through time.\n2) As we emphasized earlier, the primary objective and conclusion of this study are to determine whether\na long-term evolution occurred in the mantle Fe³⁺/ΣFe. Our conclusion is that the mantle experienced a doubling\nprocess in Fe³⁺/ΣFe. However, we believe that it remains necessary to further discuss the possible mechanisms\nresponsible for this result, rather than leaving the topic unresolved. In the previous revised version, both mantle\nplume and subduction-related models were discussed as potential explanations, but these were presented as\ninterpretative discussions, not definitive conclusions.\nMinor points:\n1) The authors used many vague nouns, such as \"oxidized characteristics, L.44\" and \"redox characteristics,\nL752.\"\nAnswer: The term “oxidized characteristics” was used intentionally in the introduction as a broad and general\ndescription to introduce the long-standing debate on the oxidation state of the mantle. In the main text, however,\nwe have replaced such general expressions with more precise terms, including “redox budget” and “Fe³⁺/ΣFe\nratio”, to ensure quantitative clarity throughout the manuscript.\n2) L.45 \"the evolution trends of mantle that forms mid ocean ridge basalt\" could be \"the secular redox evolution\nof the MORB mantle\"\nAnswer: We have revised the sentence as suggested.\n3) L.47 what's the meaning of \"exceeding 0.5 log units than QFM\", do you mean \"0.5 log unit increase in fO2\"?\nthe same for L.435\nAnswer: Following Reviewer #1’s comment, we have revised the expression to “log units relative to the QFM.”\n4) The last paragraph of the main text is really confusing, is the redox state an indicator or a driving force?\nIn the first sentence, the authors claimed that \"shifts in mantle redox state track major tectono-magmatic\nevents\", but in the last sentence, they said \"this underscores the fundamental role of mantle redox evolution\nin driving the coupled ...\"\nAnswer: The redox evolution of the mantle plays a dual role in Earth’s system. It primarily responds to major\ntectono-magmatic reorganizations (thus serving as an indicator of deep geodynamic changes), but it also feeds\nback on surface environments by influencing oxygen fugacity and atmospheric evolution. To clarify this logic,\nwe have slightly revised the last paragraph to emphasize this dual role more clearly.\nHope the above suggestions will be helpful to the authors.\nWe sincerely thank all three reviewers for their constructive comments and thoughtful suggestions. Their\nfeedback has greatly helped us improve the clarity, robustness, and overall quality of our data interpretation and\nconclusions."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have read the authors’ rebuttal letter and the manuscript version with tracked changes. I am impressed with the detailed, careful and mostly well-argued responses they have given. Although I don’t agree with all of the authors’ reasonings, this concerns fine details that have no significant influence on the main outcomes of the study, which is novel and robust as far as I can tell.\nHere, I first discuss remaining instances where I am not in complete agreement with the authors with respect to their rebuttal, the consideration of some of which is at the authors’ discretion. Then, I make detailed comments on the revised text, many of which do require the authors’ attention.\n1. For example, to my main point #4, they say that “although at greater depths, carbon may exist as C⁰. Upon ascent, the transformation of C⁰ to C⁴⁺ would consume Fe³⁺ (i.e., cause reduction), but the overall redox budget remains conserved in MORB mantle source”. I think this is only true so long as mantle remains subsolidus, but if you generate CO2, then you have redox melting, in which case the redox budget in the residue should decrease, CO2 being basically perfectly incompatible, Fe3+ mildly incompatible.\n2. They further say a few lines later that “oxygen fugacity from QFM-2 to 0 likely reflect changes in the oxidation states of both Fe and S”, whereas I would say that there is no appreciable S4 at those conditions.\n3. In their reply to my comment #8, they write that “sulphide minerals (e.g., FeSx) within subducted lithologies can be oxidized to sulphate in situ via internal redox reactions, driven by the reduction of Fe³⁺ within the rock (S2- + 8Fe3+ = S6+ + 8Fe2+).” I only know of iron-reducing sulphate-producing bacteria. Where do you have fO2 favouring appreciable S6+ over Fe3+ in subduction zones? I can envision that highly oxidised surficial material (e.g. sediments) releases oxidising fluids and S6+ upon subduction that are then reduced by the mantle (via oxidation primarily of Fe), but not that the reaction is shifted to the right. E.g. Tomkins and Evans 2015 EPSL model early anhydrite breakdown and later pyrite breakdown releasing H2S, while Maffeis et al. 2024 SciAdv suggest that pyrite might disproportionate to generate also sulphate, using modelling relevant to metacarbonate sediments.\n4. Reply to my comment to L271-274: The authors “We are not sure whether the Fe³⁺/ΣFe values in the cratonic eclogites mentioned here refer to the original\nmagma composition or the post-alteration state … it is worth noting that eclogites likely underwent dehydration. Assuming the altered rocks originally had Fe³⁺/ΣFe ratios of 0.2–0.3 … This implies that the RB remaining in exhumed eclogites may not reflect the total redox budget acquired during seafloor alteration prior to subduction.” Sure, but dehydration and subduction-related reduction of the redox budget would also apply to the ancient oceanic crust that the authors infer subducted and increased the mantle redox.\n5. Reply to Reviewer 3 comment on L78-79: I did not point out that applying a constant pressure of 1 GPa to V/Sc ratios is invalid, but that the V/Sc-derived ƒO2 must be corrected to a common pressure (we chose 1 GPa) in order to make valid comparisons between melts generated at different average pressures, because differences in ƒO2 may arise from the “intrinsic” (at constant bulk Fe3+/ Fe) change in mantle ƒO2 as a function of pressure alone. Our correction (assuming an increase of ƒO2 by 0.4 orders of magnitude per GPa) was wrong because the complex redox profile of the mantle below 3 GPa was not known in 2016, but that’s a different issue.\n6. Regarding a top-down oxidation of the mantle:\nI am still not convinced that subduction of positive redox budget could have brought about the observed increase in mantle ƒO2, for the same reason I explained in the original review (also given by Reviewer 3). Here, the authors argue that “seafloor oxidized materials formed through oceanic hydration reactions subsequently accumulate in the oceanic lithosphere and remain isolated from the atmosphere by seawater”. However, the ocean-atmosphere-uppermost crust form part of the exosphere and in reservoir modelling, also those concerned with volatiles including oxygen, and are considered as an entity (e.g. Hirschmann 23 EPSL) that is interacting with the mantle on geological timescales. Taking redox budget out of the deep ocean water therefore means removing redox budget from the exosphere, thereby acting against oxygenation of the atmosphere. To then argue that “after global subduction (During Stage 4: ~3.2–1.8 billion years ago), through processes like arcs volcanic degassing, these subducted oxidized oceanic rocks are capable to deliver redox budget to the mantle and then to the atmosphere by volcanic degassing” illustrates that subduction of positive redox budget generates a short-circuit that diverts at least a fraction of it back to the atmosphere.\nIf the subduction mechanism worked, the question also arises as to why the mantle ƒO2 has started to increase before 3.2 Ga (in the single-lid phase according to Fig. 5) and why it has not continually increased after the onset of global subduction and before the Proterozoic tectonic lull. Instead, the Fe3+/ Fe evolution ends several 100 Ma before the tectonic lull. I suggest this shows that after the transition to plate tectonics the exosphere and mantle are in some type of equilibrium on geologic scales, with the subducted redox budget being more or less balanced by release of redox budget via magmatism.\nIt should be noted that the increase in the mantle’s Fe3+/ Fe also carries petrologic consequences, as investigated by Asimow (2022 in Book: Magma Redox Geochemistry) with respect to the argument on the modern MORB Fe3+/ Fe:\n“Oxidizing conditions predict cold, low-MgO primary aggregate magmas that have difficulty crystallizing the most magnesian olivine phenocryst compositions found in MORB, that imply potential temperatures too cold to generate the traditionally assumed typical thickness of oceanic crust, and that are so close to the erupted basalt composition that it becomes difficult to explain the origin of a thick, cumulate lower crust as the complementary product of fractional crystallization”.\n7. Regarding the inferred role of komatiite:\nI am not sure that komatiitic oceanic crust could have subducted in the modern sense. Once metamorphosed, this lithology should convert to dense pyroxenite that is prone to delamination, as discussed by Foley et al 2003 Nature who also emphasised that continental crust production would have required partial melting of mafic, not ultramafic oceanic crust.\nRegarding measured Fe3+/ Fe in komatiites, how do you distinguish oceanic, quasi syn-volcanic serpentinisation from post-emplacement (near)surface weathering-related serpentinisation?\nComments on the manuscript with tracked changes:\nThere are some formatting issues related to Fe3+/ Fe (superscript missing in L25, bold font in L33) – other instances later in text\nL28 “especially when” – ambiguous, “given that” or “assuming that”\nL37 “tectonic activity”, “tectonic reorganization events” – vague, perhaps “Earth’s evolving tectonic regime” and “a transition from single-lid to mobile-lid tectonics” or something more concrete as per your Fig. 5. I’m not sure about the abstract word count, but you could specifically mention the Proterozoic tectonic lull or refer to the relatively well-known “boring billion”\nL58 “log units than QFM” should be “log units relative to the QFM”\nL71 sentence needs rewriting. The garnet-bearing mantle extends down further than 8-10 GPa, but metal saturation buffers the mantle ƒO2 at higher pressure. The word “where” could be misunderstood to imply that it is above 8-10 GPa that ƒO2 decreases\nL86-87 Birner et al. (ref 15) should be quoted here\nI note that the f in ƒO2 is not italicized consistently, other state variables (P, T) also not italicized\nL94 ƒO2 is not measured directly in any of the studies. It is related to Fe3+/ Fe and some thermodynamic equilibrium; not sure what you mean by “corrected”\nL98 reference 18 is Canil et al. 1994 and is not recent. Ref 17 would be appropriate – please check all your references! [I later see that the references in the ms with tracked changes don’t agree with the clean ms so I am no longer checking]\nL98-99 not sure what you mean by “using direct ratios… even greater discrepancies” – Ref 17 showed specifically the great impact of temperature (while also considering variations in source compositions, with minor effects on V-derived ƒO2 estimates). The preceding lines already establish that other effects, such as pressure, were considered, so direct ratios were not used.\n114 “under high-pressure conditions” – give the pressure, as high pressure means different things to different readers\nL129 which source pressure is given here? There is no single source pressure…The average pressure of melt extraction?\nL129 it is not the Archean melt that is extrapolated, but some parameter – formatting off in this and following sentence\nL159 “under uppermost mantle conditions” – the mantle is vast and most carbon is not in the 4+ state…\nL188 it would be better to list some attributes that you used to identify MORB-like basalts (e.g. REE patterns, Nb/La)\nL196 “mutually” is redundant with “each other”\nL213 and because its source and the melt extraction processes leading to MORB generation are relatively well-constrained\nL276 what is “melt-extracted temperature”?\nL301 unclear what you mean by “within the fewer samples”?\nL303 what is “primitive ƒO2”? That of un/little differentiated melts?\nL333-335 this begs the question how the alternative assumption affects the result. Here, I am particularly interested in the effect of redox melting, occurring as shallow as 120 km for a more reduced Archaean uppermost mantle source (Aulbach and Stagno 16, cited)? My feeling is that even at higher TP, the onset of partial melting in Archaean ridges was (dominantly) shallower than 120 km, so that no complication from redox melting is expected.\nL340 the reference to subarc depths comes a bit out of the blue, as up until here you seem mostly concerned with ridge settings and decompression melting\nL342-344, L350 I strongly disagree. For the solid-melt system combined, redox melting has of course no consequence, but if you oxidise C to C4+, you induce redox melting, whereby CO2 behaves like a highly incompatible component that is extracted. At the same time, you have a residue the Fe3+/ Fe of which was reduced to make C4+. Moreover, the remaining Fe3+ behaves like a moderately incompatible component. Combined, this implies a reduction of redox budget in the residue. However, as I note in the comment to L333, it may not be an issue for melting even of Archaean ambient mantle.\nL353-355 this is an odd way to end this section. Either delete, or give your estimate for the Archaean mantle for comparison.\nL388 “redox state”\nL388-440 this section still fails to convince, there are too many unknowns and no rock record. Given the uncertainties regarding disproportionation, the comparison to Venus is also not warranted. This is really a topic for a different paper in my opinion and distracts from the main points in the present manuscript.\nL457-460 what do you mean by “making it difficult for bridgmanite to crystallize” – obviously, the magma ocean does not still exist?\nL474 it is not highly plausible for reasons I explain in point 6 above. At the very least, please moderate the language.\nL490-491 clearly, alteration/weak metamorphism has had a major effect on the Fe3+/ Fe of komatiites, the compositions of which we can measure today, such that e.g. fluid-mobile elements are typically not used to constrain komatiite petrogenesis.\nL529 typo\nL1101-1102 syntax off – needs fixing (“indicating that”)\nL1154 I would delete “historical” as this adjective is typically used in the context of human history. The word “trend” already implies the evolution", "reviewer_2": "This is a much improved version of the original submission of this manuscript, and all of my previous comments have been addressed satisfactorily. I am happy to recommend this manuscript for publication in its current form.", "reviewer_3": "I have read the rebuttal letter and the revised manuscript carefully. My major concerns are: 1) A close check of Fig. S3b and Fig. S4c shows that the Fe3+/ΣFe for both the peridotitic mantle and the basalt whole rocks are highly variable. I suspect the alleged \"doubled Fe3+/ΣFe\" of the mantle by the authors. It shows that no clear secular evolution of Fe3+/ΣFe of the mantle. 2) If exists, the explanations for the tortuous evolution of the mantle are mainly qualitative. No figures were used to explain the possible causal relation between the elevated mantle Fe3+/ΣFe and tectonic parameters.\nMinor points:\n1) The authors used many vague nouns, such as \"oxidized characteristics, L.44\" and \"redox characteristics, L752.\"\n2) L.45 \"the evolution trends of mantle that forms mid ocean ridge basalt\" could be \"the secular redox evolution of the MORB mantle\"\n3) L.47 what's the meaning of \"exceeding 0.5 log units than QFM\", do you mean \"0.5 log unit increase in fO2\"? the same for L.435\n4) The last paragraph of the main text is really confusing, is the redox state an indicator or a driving force? In the first sentence, the authors claimed that \"shifts in mantle redox state track major tectono-magmatic events\", but in the last sentence, they said \"this underscores the fundamental role of mantle redox evolution in driving the coupled ...\"\nHope the above suggestions will be helpful to the authors."}, "author_response": "Responses to Reviewers’ Comments Point by Point\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nI have read the authors’ rebuttal letter and the manuscript version with tracked changes. I am impressed with the detailed, careful and mostly well-argued responses they have given. Although I don’t agree with all of the authors’ reasonings, this concerns fine details that have no significant influence on the main outcomes of the study, which is novel and robust as far as I can tell.\nHere, I first discuss remaining instances where I am not in complete agreement with the authors with respect to their rebuttal, the consideration of some of which is at the authors’ discretion. Then, I make detailed comments on the revised text, many of which do require the authors’ attention.\nAnswer: We sincerely thank you for the careful reading of our revised manuscript and the rebuttal letter, and for the constructive comments and thoughtful suggestions. We have carefully considered all remaining points raised and have revised the manuscript accordingly, as detailed below.\n1. For example, to my main point #4, they say that “although at greater depths, carbon may exist as C⁰. Upon ascent, the transformation of C⁰ to C⁴⁺ would consume Fe³⁺ (i.e., cause reduction), but the overall redox budget remains conserved in MORB mantle source”. I think this is only true so long as mantle remains subsolidus, but if you generate CO2, then you have redox melting, in which case the redox budget in the residue should decrease, CO2 being basically perfectly incompatible, Fe3+ mildly incompatible.\n2. They further say a few lines later that “oxygen fugacity from QFM-2 to 0 likely reflect changes in the oxidation states of both Fe and S”, whereas I would say that there is no appreciable S4 at those conditions.\n3. In their reply to my comment #8, they write that “sulphide minerals (e.g., FeSx) within subducted lithologies can be oxidized to sulphate in situ via internal redox reactions, driven by the reduction of Fe³⁺ within the rock (S2- + 8Fe3+ = S6+ + 8Fe2+).” I only know of iron-reducing sulphate-producing bacteria. Where do you have fO2 favouring appreciable S6+ over Fe3+ in subduction zones? I can envision that highly oxidised surficial material (e.g. sediments) releases oxidising fluids and S6+ upon subduction that are then reduced by the mantle (via oxidation primarily of Fe), but not that the reaction is shifted to the right. E.g. Tomkins and Evans 2015 EPSL model early anhydrite breakdown and later pyrite breakdown releasing H2S, while Maffeis et al. 2024 SciAdv suggest that pyrite might disproportionate to generate also sulphate, using modelling relevant to metacarbonate sediments.\n4. Reply to my comment to L271-274: The authors “We are not sure whether the Fe³⁺/ΣFe values in the cratonic eclogites mentioned here refer to the original magma composition or the post-alteration state … it is worth noting that eclogites likely underwent dehydration. Assuming the altered rocks originally had Fe³⁺/ΣFe ratios of 0.2–0.3 … This implies that the RB remaining in exhumed eclogites may not reflect the total redox budget acquired during seafloor alteration prior to subduction.” Sure, but dehydration and subduction-related reduction of the redox budget would also apply to the ancient oceanic crust that the authors infer subducted and increased the mantle redox.\n5. Reply to Reviewer 3 comment on L78-79: I did not point out that applying a constant pressure of 1 GPa to V/Sc ratios is invalid, but that the V/Sc-derived ƒO2 must be corrected to a common pressure (we chose 1 GPa) in order to make valid comparisons between melts generated at different average pressures, because differences in ƒO2 may arise from the “intrinsic” (at constant bulk Fe3+/ Fe) change in mantle ƒO2 as a function of pressure alone. Our correction (assuming an increase of ƒO2 by 0.4 orders of magnitude per GPa) was wrong because the complex redox profile of the mantle below 3 GPa was not known in 2016, but that’s a different issue.\n6. Regarding a top-down oxidation of the mantle:\nI am still not convinced that subduction of positive redox budget could have brought about the observed increase in mantle ƒO2, for the same reason I explained in the original review (also given by Reviewer 3). Here, the authors argue that “seafloor oxidized materials formed through oceanic hydration reactions subsequently accumulate in the oceanic lithosphere and remain isolated from the atmosphere by seawater”. However, the ocean-atmosphere-uppermost crust form part of the exosphere and in reservoir modelling, also those concerned with volatiles including oxygen, and are considered as an entity (e.g. Hirschmann 23 EPSL) that is interacting with the mantle on geological timescales. Taking redox budget out of the deep ocean water therefore means removing redox budget from the exosphere, thereby acting against oxygenation of the atmosphere. To then argue that “after global subduction (During Stage 4: ~3.2–1.8 billion years ago), through processes like arcs volcanic degassing, these subducted oxidized oceanic rocks are capable to deliver redox budget to the mantle and then to the atmosphere by volcanic degassing” illustrates that subduction of positive redox budget generates a short-circuit that diverts at least a fraction of it back to the atmosphere.\nIf the subduction mechanism worked, the question also arises as to why the mantle ƒO2 has started to increase before 3.2 Ga (in the single-lid phase according to Fig. 5) and why it has not continually increased after the onset of global subduction and before the Proterozoic tectonic lull. Instead, the Fe3+/ Fe evolution ends several 100 Ma before the tectonic lull. I suggest this shows that after the transition to plate tectonics the exosphere and mantle are in some type of equilibrium on geologic scales, with the subducted redox budget being more or less balanced by release of redox budget via magmatism.\nIt should be noted that the increase in the mantle’s Fe3+/ Fe also carries petrologic consequences, as investigated by Asimow (2022 in Book: Magma Redox Geochemistry) with respect to the argument on the modern MORB Fe3+/ Fe:\n“Oxidizing conditions predict cold, low-MgO primary aggregate magmas that have difficulty crystallizing the most magnesian olivine phenocryst compositions found in MORB, that imply potential temperatures too cold to generate the traditionally assumed typical thickness of oceanic crust, and that are so close to the erupted basalt composition that it becomes difficult to explain the origin of a thick, cumulate lower crust as the complementary product of fractional crystallization”.\n7. Regarding the inferred role of komatiite:\nI am not sure that komatiitic oceanic crust could have subducted in the modern sense. Once metamorphosed, this lithology should convert to dense pyroxenite that is prone to delamination, as discussed by Foley et al 2003 Nature who also emphasised that continental crust production would have required partial melting of mafic, not ultramafic oceanic crust.\nRegarding measured Fe3+/ Fe in komatiites, how do you distinguish oceanic, quasi syn-volcanic serpentinisation from post-emplacement (near)surface weathering-related serpentinisation?\nComments on the manuscript with tracked changes:\nThere are some formatting issues related to Fe3+/ Fe (superscript missing in L25, bold font in L33) – other instances later in text\nL28 “especially when” – ambiguous, “given that” or “assuming that”\nL37 “tectonic activity”, “tectonic reorganization events” – vague, perhaps “Earth’s evolving tectonic regime” and “a transition from single-lid to mobile-lid tectonics” or something more concrete as per your Fig. 5. I’m not sure about the abstract word count, but you could specifically mention the Proterozoic tectonic lull or refer to the relatively well-known “boring billion”\nL58 “log units than QFM” should be “log units relative to the QFM”\nL71 sentence needs rewriting. The garnet-bearing mantle extends down further than 8-10 GPa, but metal saturation buffers the mantle ƒO2 at higher pressure. The word “where” could be misunderstood to imply that it is above 8-10 GPa that ƒO2 decreases\nL86-87 Birner et al. (ref 15) should be quoted here\nI note that the f in ƒO2 is not italicized consistently, other state variables (P, T) also not italicized\nL94 ƒO2 is not measured directly in any of the studies. It is related to Fe3+/ Fe and some thermodynamic equilibrium; not sure what you mean by “corrected”\nL98 reference 18 is Canil et al. 1994 and is not recent. Ref 17 would be appropriate – please check all your references! [I later see that the references in the ms with tracked changes don’t agree with the clean ms so I am no longer checking]\nL98-99 not sure what you mean by “using direct ratios… even greater discrepancies” – Ref 17 showed specifically the great impact of temperature (while also considering variations in source compositions, with minor effects on V-derived ƒO2 estimates). The preceding lines already establish that other effects, such as pressure, were considered, so direct ratios were not used.\n114 “under high-pressure conditions” – give the pressure, as high pressure means different things to different readers\nL129 which source pressure is given here? There is no single source pressure…The average pressure of melt extraction?\nL129 it is not the Archean melt that is extrapolated, but some parameter – formatting off in this and following sentence\nL159 “under uppermost mantle conditions” – the mantle is vast and most carbon is not in the 4+ state…\nL188 it would be better to list some attributes that you used to identify MORB-like basalts (e.g. REE patterns, Nb/La)\nL196 “mutually” is redundant with “each other”\nL213 and because its source and the melt extraction processes leading to MORB generation are relatively well-constrained\nL276 what is “melt-extracted temperature”?\nL301 unclear what you mean by “within the fewer samples”?\nL303 what is “primitive ƒO2”? That of un/little differentiated melts?\nL333-335 this begs the question how the alternative assumption affects the result. Here, I am particularly interested in the effect of redox melting, occurring as shallow as 120 km for a more reduced Archaean uppermost mantle source (Aulbach and Stagno 16, cited)? My feeling is that even at higher TP, the onset of partial melting in Archaean ridges was (dominantly) shallower than 120 km, so that no complication from redox melting is expected.\nL340 the reference to subarc depths comes a bit out of the blue, as up until here you seem mostly concerned with ridge settings and decompression melting\nL342-344, L350 I strongly disagree. For the solid-melt system combined, redox melting has of course no consequence, but if you oxidise C to C4+, you induce redox melting, whereby CO2 behaves like a highly incompatible component that is extracted. At the same time, you have a residue the Fe3+/ Fe of which was reduced to make C4+. Moreover, the remaining Fe3+ behaves like a moderately incompatible component. Combined, this implies a reduction of redox budget in the residue. However, as I note in the comment to L333, it may not be an issue for melting even of Archaean ambient mantle.\nL353-355 this is an odd way to end this section. Either delete, or give your estimate for the Archaean mantle for comparison.\nL388 “redox state”\nL388-440 this section still fails to convince, there are too many unknowns and no rock record. Given the uncertainties regarding disproportionation, the comparison to Venus is also not warranted. This is really a topic for a different paper in my opinion and distracts from the main points in the present manuscript.\nL457-460 what do you mean by “making it difficult for bridgmanite to crystallize” – obviously, the magma ocean does not still exist?\nL474 it is not highly plausible for reasons I explain in point 6 above. At the very least, please moderate the language.\nL490-491 clearly, alteration/weak metamorphism has had a major effect on the Fe3+/ Fe of komatiites, the compositions of which we can measure today, such that e.g. fluid-mobile elements are typically not used to constrain komatiite petrogenesis.\nL529 typo\nL1101-1102 syntax off – needs fixing (“indicating that”)\nL1154 I would delete “historical” as this adjective is typically used in the context of human history. The word “trend” already implies the evolution\nReviewer #2 (Remarks to the Author):\nThis is a much improved version of the original submission of this manuscript, and all of my previous comments have been addressed satisfactorily. I am happy to recommend this manuscript for publication in its current form.\nReviewer #3 (Remarks to the Author):\nI have read the rebuttal letter and the revised manuscript carefully. My major concerns are: 1) A close check of Fig. S3b and Fig. S4c shows that the Fe3+/ΣFe for both the peridotitic mantle and the basalt whole rocks are highly variable. I suspect the alleged \"doubled Fe3+/ΣFe\" of the mantle by the authors. It shows that no clear secular evolution of Fe3+/ΣFe of the mantle. 2) If exists, the explanations for the tortuous evolution of the mantle are mainly qualitative. No figures were used to explain the possible causal relation between the elevated mantle Fe3+/ΣFe and tectonic parameters.\nMinor points:\n1) The authors used many vague nouns, such as \"oxidized characteristics, L.44\" and \"redox characteristics, L752.\"\n2) L.45 \"the evolution trends of mantle that forms mid ocean ridge basalt\" could be \"the secular redox evolution of the MORB mantle\"\n3) L.47 what's the meaning of \"exceeding 0.5 log units than QFM\", do you mean \"0.5 log unit increase in fO2\"? the same for L.435\n4) The last paragraph of the main text is really confusing, is the redox state an indicator or a driving force? In the first sentence, the authors claimed that \"shifts in mantle redox state track major tectono-magmatic events\", but in the last sentence, they said \"this underscores the fundamental role of mantle redox evolution in driving the coupled ...\"\nHope the above suggestions will be helpful to the authors."}]

Decision letter