Redox Budget Research Articles

Ferric iron stabilization at deep magma ocean conditions.

Fe2O3 produced in a deep magma ocean in equilibrium with core-destined alloy sets the early redox budget and atmospheric composition of terrestrial planets. Previous experiments (≤28 gigapascals) and first-principles calculations indicate that a deep terrestrial magma ocean produces appreciable Fe3+ but predict Fe3+/ΣFe ratios that conflict by an order of magnitude. We present Fe3+/ΣFe of glasses quenched from melts equilibrated with Fe alloy at 38 to 71 gigapascals, 3600 to 4400 kelvin, analyzed by synchrotron Mössbauer spectroscopy. These indicate Fe3+/ΣFe of 0.056 to 0.112 in a terrestrial magma ocean with mean alloy-silicate equilibration pressures of 28 to 53 gigapascals, producing sufficient Fe2O3 to account for the modern bulk silicate Earth redox budget and surficial conditions near or more oxidizing than the iron-wüstite buffer, which would stabilize a primitive CO- and H2O-rich atmosphere.

Concurrent Measurement of O2 Production and Isoprene Emission During Photosynthesis: Pros, Cons and Metabolic Implications of Responses to Light, CO2 and Temperature.

Traditional leaf gas exchange experiments have focused on net CO2 exchange (Anet). Here, using California poplar (Populus trichocarpa), we coupled measurements of net oxygen production (NOP), isoprene emissions and δ18O in O2 to traditional CO2/H2O gas exchange with chlorophyll fluorescence, and measured light, CO2 and temperature response curves. This allowed us to obtain a comprehensive picture of the photosynthetic redox budget including electron transport rate (ETR) and estimates of the mean assimilatory quotient (AQ = Anet/NOP). We found that Anet and NOP were linearly correlated across environmental gradients with similar observed AQ values during light (1.25 ± 0.05) and CO2 responses (1.23 ± 0.07). In contrast, AQ was suppressed during leaf temperature responses in the light (0.87 ± 0.28), potentially due to the acceleration of alternative ETR sinks like lipid synthesis. Anet and NOP had an optimum temperature (Topt) of 31°C, while ETR and δ18O in O2 (35°C) and isoprene emissions (39°C) had distinctly higher Topt. The results confirm a tight connection between water oxidation and ETR and support a view of light-dependent lipid synthesis primarily driven by photosynthetic ATP/NADPH not consumed by the Calvin-Benson cycle, as an important thermotolerance mechanism linked with high rates of (photo)respiration and CO2/O2 recycling.

Marine Fe cycling linked to dynamic redox variability, biological activity and post-depositional mineralization in the 1.1 Ga Mesoproterozoic Taoudeni Basin, Mauritania

The concentration of redox sensitive trace metals (RSTEs) and their isotopic composition preserved in Precambrian marine sediments, are critical for the reconstruction of ocean–atmosphere oxygenation history. Particularly, the concentration of Fe, its redox speciation, and isotopic distribution, have gained widespread use for inferring the biogeochemical processes that controlled Fe cycling in Precambrian oceans linked to the reconstruction of Earth surface redox budget. However, questions remain about the biotic and abiotic processes involved in Fe cycling in these ancient oceans, including the impact of post-depositional alterative processes on the reliability of the Fe redox proxy. Here we present a multi-proxy mineralogical and geochemical study of the ∼1.1 Ga Atar and El Mreiti strata of the Taoudeni Basin in Mauritania, to better constrain pathways involved in Fe cycling, linked to Fe mineralogy, redox speciation, isotopic ratios during this time and metamorphism. We compare unmetamorphosed sedimentary deposits with facies metamorphosed by dolerite sill intrusion. The results reveal the occurrence of diagenetic Fe minerals in the basal unmetamorphosed samples associated with light δ56Fe signatures, reflecting dominant anoxic conditions that promoted microbial dissimilatory Fe reduction. Notably, δ56Fe composition of these rocks reveal several fluctuations in evolving seawater redox state from oxic to anoxic/sulfidic conditions associated with changes in sea level stand and periods of full bottom water oxygenation and redox stratification. Overall, Ce anomalies suggest a general up sequence increase in seawater oxygen content. Metamorphosed rocks display heterogeneous δ56Fe distribution, consisting of light and heavy signatures associated with secondary Fe-bearing minerals produced by metamorphic and metasomatic overprinting of carbonated rocks by hot circulating fluids. The results thus indicate metamorphic overprinting of primary seawater δ56Fe promoted by increased mobility of reactive Fe during post-depositional metamorphic transformation. They show that post-depositional metamorphic/metasomatic overprinting complicates direct reconstruction of seawater biogeochemical Fe cycling and redox state using δ56Fe systematics.

CH4 Inclusions in High‐Pressure Metapelite: Revealing the Link Between Fluid Behavior and Redox Mechanisms for Subducting Sedimentary Carbon

Abstract(Ultra)high‐pressure metamorphic rocks provide valuable insights into the properties of slab‐derived fluids. Here, we report CH4‐rich fluid inclusions in garnet of a metapelite from the Zermatt‐Saas ophiolite, western Alps. Two types of metapelite, a CH4‐bearing pelitic schist and a calcareous pelitic schist, were investigated to unravel favorable P‐T‐fO2 conditions for preservation of CH4 in high‐pressure metapelite. In the CH4‐bearing pelitic schist, CH4‐rich fluid inclusions exclusively occur in the core of garnet (GrtI) rather than the rim (GrtII). GrtI records P‐T conditions of ∼2.85 GPa and ∼555°C, whereas GrtII records a prograde P‐T path from ∼1.75 GPa at 510°C to ∼2.0 GPa at 530°C. Compositional profile of garnet in the calcareous pelitic schist reflects a prograde metamorphic path from ∼1.9 GPa at 510°C to ∼2.12 GPa at 545°C. CH4‐rich fluid formation may primarily rise from graphite reduction at high‐pressure reduced conditions (ΔFMQ −3.5 to −4, 2.85 GPa, ∼550°C), while graphite and carbonates stabilize in a relatively oxidized environment (ΔFMQ ∼0, 2.12 GPa, 545°C). The initial redox budget of subducted sediments is primarily controlled by the amount of sedimentary carbonate and organic carbon, which plays the most important role in deciding the carbon speciation at different subduction depths. CH4 formation in COH fluids could primarily be attributed to the reduction of graphite. Subducted metasediments act as conduits for transporting non‐oxidized fluids to arc magmas, which provides crucial evidence to support the heterogeneity for slab‐derived COH fluids and offers new insights into the deep carbon cycle.

Marine Phosphate Level During the Archean Constrained by the Global Redox Budget

AbstractUnderstanding the oceanic phosphate concentration is critical for understanding marine productivity and oxygen evolution throughout Earth history. During the Archean, estimates of marine phosphate levels range from scarce to enriched conditions. However, biogeochemical conditions required for sustaining high phosphate concentrations while retaining an anoxic atmosphere during the Archean remain ambiguous. Here, we employ a biogeochemical model of the marine phosphate cycle to determine the conditions under which oceanic phosphate levels could have been higher than present‐day values during the Archean after the emergence of oxygenic photoautotrophs (OP). We show that, under the presence of OP, phosphate‐rich oceans require the limitation by factors other than phosphate, or a high outgassing rate of reducing gases. If these conditions were not met, the occurrence of oceanic phosphate levels higher than present‐day values during the Archean would require the absence of OP.

The deep Earth oxygen cycle: Mass balance considerations on the origin and evolution of mantle and surface oxidative reservoirs

The deep Earth oxygen cycle addresses the origin and long-term exchange of oxidized species between the mantle and exosphere and creates the geochemical context in which the interior and surface environment have evolved. The redox power of the bulk silicate Earth (BSE) can be measured in terms of its redox budget, RB, relative to a reference state of the predominant valences of redox-sensitive elements in the mantle. 82±4% (1.9±0.5 × 1023 moles) of the redox power resides in the mantle and 18±4% (4.2±0.5 × 1022 moles) in the exosphere. Vigorous outfluxes of oxidized species from the mantle (3.8±0.7 × 1013 mol/yr) can replenish the exosphere reservoir in 1.1±0.2 Ga, which requires efficient long-term recycling of redox power via subduction. Within uncertainties, recent (last 200 Ma) mantle RB outfluxes and subduction influxes (4.9±1.3 × 1013 mol/yr) are balanced, but outfluxes likely exceeded influxes earlier in Earth history. CO2 is 66±20% of the RB outfluxes but only 24±13% of the influxes. This is largely because of conjugate intervalence reactions; one in the shallow mantle creates carbonate at the expense of Fe2O3 and the other creates Fe2O3 from CO2 on the surface by a combination of organic carbon fixation and oxidative weathering.Scaling of mantle oxygen fugacity, fO2, to redox mass balance is approximately Δlog⁡(Fe3+Fe⊤)peridotite ≈14Δlog⁡(fO2)mantle . Consequently, inferences of secular evolution of mantle oxygen fugacity from the Archaean to the Proterozoic, amounting to about 1.5 log units in fO2, imply that the Archaean mantle had an Fe3+/FeT ratio <0.02, rather than the modern value of 0.04±0.01. Oceanic basalts derive from sources with greater redox budgets than mid-ocean ridges, and this is partly expressed as higher source Fe3+/FeT ratios, but more importantly, as greater source CO2 concentrations. In combination, these require that plumes in the deep upper mantle have Fe3+/FeT ratios significantly greater than the depleted mantle.The oxidative inventory of the BSE originated by a combination of H2O disproportionation in the atmosphere, leading to H2 escape, and FeO disproportionation in the deep mantle, with loss of Fe to the core. FeO disproportionation in a deep magma ocean inevitably produces a significant fraction of the BSE RB, but additional contributions are likely required. Further Fe loss could be from bridgemanite crystallization followed by Fe escape through a basal magma ocean. Gradual mixing of the resulting deep oxidized layer may account for secular oxidation of the mantle source regions of igneous rocks from 3 to 2 Ga. H2O disproportionation assisted in accumulation of the oxidized surface species, but was not a significant source of mantle oxidative power, as mechanisms of oxidative influx are quantitatively insufficient.

Fluid Redox Fingerprint of the CaCO3+Antigorite Dehydration Reaction in Subducted Metacarbonate Sediments

Antigorite dehydration is a process able to release, in comparison with other minerals, the highest amount of H2O from a subducting slab. The released fluid delivers critical elements (e.g., S, Cu, and REE) to the overlying subarc mantle, modifying the mantle source of arc magmas and related ore deposits. Whether antigorite breakdown produces oxidising or reducing fluids is debated. Whereas previous studies have investigated antigorite dehydration in serpentinites (i.e., in a (C)AMFS-H2O system), this contribution is devoted to the CMFS-COHS carbonate system, which is representative of the metacarbonate sediments (or carbonate-dominated ophicarbonate rocks) that sit atop the slab. Thermodynamic modelling is used to investigate the redox effect of the carbonate-buffered antigorite dehydration reactions (i.e., brucite breakdown and antigorite breakdown) on electrolytic fluid geochemistry as a function of P-T-fO2. The influence of P-T-fO2 conditions on the solubility of C and S, solute-bound H2 and O2, fluid pH, the average valence states of dissolved C and S, and the fluid redox budget indicates that, in metacarbonate sediments, the CaCO3+antigorite reaction tends to produce reducing fluids. However, the redox state of such fluids is buffered not only by the redox state of the system but also, most importantly, by concomitantly dissolving redox-sensitive minerals (i.e., carbonates, graphite, pyrite, and anhydrite). A qualitative correlation between the redox state of the system and the possible depth of fluid release into the mantle wedge is also derived.

Impact Induced Oxidation and Its Implications for Early Mars Climate

AbstractH2 in a CO2 atmosphere may serve as a potential solution to the early Mars climate paradox, but its unknown sources cast doubts on the proposed mechanism. Impact cratering is an energetic process that may modify the surface redox budget. Here, we investigate the potential influence of impact‐related melt oxidation and serpentinization on global climate conditions. We show that impact melt and the projectile's significant oxidizing potential during basin‐forming impacts (Basin size ≥1,250 km) result in sufficient H2 to raise the global mean temperature to above 273K, which lasts for up to 105 − 106 yr considering rate‐limited regime. Impact‐induced serpentinization has limited consequences on the global climate in comparison. Episodic warming after large impacts may have enabled the presence of liquid water for up to several million years in the Noachian, resulting in the chemical evolution of the planet's surface co‐evolving with the planetary atmosphere in an episodic manner.

The influence of submarine hydrothermal systems on seawater sulfate

Seawater sulfate is a major carrier of oxidizing capacity and influences the redox budget of the Earth’s surface on geologic timescales. Records of oxygen and sulfur isotopes in seawater sulfate are used to track changes in sulfate cycling through geologic time. Interpretations of these records are typically based on models that describe the seawater sulfate reservoir as a mass balance between microbial, riverine and sedimentary sulfate fluxes. Here, we investigate the influence of hydrothermal sulfate cycling, which remains an unconstrained but potentially significant additional flux in this mass balance. We find that anhydrite (CaSO4) from eight submarine hydrothermal vent fields is consistently offset from seawater sulfate in δ18O but not Δ’17O or δ34S. Experiments at hydrothermal pressure–temperature conditions indicate that this δ18O offset is driven by oxygen isotope exchange between sulfate and hydrothermal H2O at high temperature. An updated isotope mass balance model shows that a flux of hydrothermal sulfate into seawater, derived from retrograde dissolution of hydrothermal anhydrite, could cause high-temperature oxygen isotope exchange to buffer seawater sulfate δ18O and Δ’17O by as much as 25%. Hydrothermal sulfate preserved in the Troodos Ophiolite (Cretaceous) also records a δ18O and Δ’17O signal of high-temperature oxygen isotope exchange, supporting the conclusion that geologic records of seawater sulfate oxygen isotopes may include a hydrothermal component.

Redox species and oxygen fugacity of slab-derived fluids: Implications for mantle oxidation and deep carbon-sulfur cycling

The generation and migration of slab-derived fluids modulate subduction zone seismicity, arc magmatism, and deep volatile cycling. However, the redox species and oxygen fugacity (fO2) (hereafter expressed as log units relative to the fayalite–magnetite–quartz buffer, △FMQ) of slab-derived fluids are highly debated. Here we conducted phase equilibria modeling on altered oceanic crust (AOC) and serpentinites along typical subduction geotherms in the C-S-bearing system over a pressure range of 0.5–6 GPa. With the averaged compositions of AOC and serpentinite, our calculated results show that oxidized carbon-sulfur species dominate slab-derived fluids during slab subduction. As a result, slab-derived fluids are highly oxidized and at or above the typical △FMQ values of arc magmas at forearc to subarc depths. The predicted oxidized carbon and sulfur species are compatible with natural observations in fluid inclusions from many oceanic HP metamorphic rocks. More importantly, it is revealed that, the redox state of slab-derived fluids is primarily controlled by the redox budget (RB) of the slab prior to subduction. Subduction-zone thermal structure, however, only exerts a minor influence on the slab-derived fluid fO2, which is supported by the similar fO2 ranges in arc lavas from cold and hot subduction zones. Our models further show that, if an open system is assumed, most of carbon (&amp;gt;70%) and sulfur (&amp;gt;50%) in cold subducted AOC and serpentinite would be lost at subarc depths. Small amounts of carbon and sulfur could be transported into the deeper mantle via closed-system subduction and open-system cold subduction, supplying the source materials for volatile-rich intraplate magmas and superdeep diamonds.

Oxidation Extent of the Upper Mantle by Subducted Slab and Possible Oxygen Budget in Deep Earth Inferred From Redox Kinetics of Olivine

AbstractRedox input by subducted slab into mantle is important for deep cycle and isotopic evolution of volatile elements, whose stable forms are controlled by redox state. Given reduced condition in lower part of the upper mantle, taking redox budget from lithospheric mantle into consideration is crucial in redefining redox state there. To constrain to which extent subducted slab modified redox state of the uppermost mantle and how much oxygen budget slab carried into deep Earth, we investigated redox kinetics of olivine adopting diffusion couple method at 1 GPa and 1,373–1573 K in a piston cylinder apparatus. It is found that redox process in olivine is diffusion‐controlled, and diffusing on the order of 10−12 m2/s at 1473 K. Oxidation process in reduced part is oxygen fugacity (fO2)‐independent with activation enthalpy of 235 ± 56 kJ/mol, while reduction process in oxidized part is fO2‐dependent with an fO2 exponent of 2/5. Diffusion profile analysis reveals that for magnetite‐free couple, redox process is controlled by oxygen grain boundary diffusion (GBD) below ΔFMQ + 1, and rate‐limited by faster species which might be hydrogen related Mg vacancy above ΔFMQ + 1. However, for magnetite‐bearing couple, oxygen GBD dominates redox process across wide fO2 range. The extremely slow rate limits the homogenization of the slab with surrounding mantle so that redox state of the uppermost mantle remains unchanged in the past 3.5 Gyrs. A highly underestimated oxygen reservoir may have formed in deep Earth, when subducted slab transports oxidized components to region deeper than the mantle transition zone.

Ferruginous oceans during OAE1a and collapse of the marine sulfate pool

Seawater sulfate is one of the largest oxidant pools at Earth's surface today and its concentration in the oceans is generally assumed to have varied between 5 and 28 mM since the early Phanerozoic Eon. Intermittent and potentially global Oceanic Anoxic Events (OAEs) are accompanied by changes in seawater sulfate concentrations and signal perturbations in the Earth system associated with major climatic anomalies and biological crises. Ferruginous (Fe-rich) ocean conditions developed transiently during multiple OAEs, implying strong variability in seawater chemistry and global biogeochemical cycles. The precise evolution of seawater sulfate concentrations during OAEs, however, is uncertain and thus models that aim to mechanistically link oceanic anoxia to broad-scale disruptions in the Earth system remain incomplete. We use analyses of Fe-speciation and redox sensitive trace metals in sediments deposited in the Tethys and Pacific oceans to constrain seawater sulfate concentrations and underlying dynamics in marine chemistry during OAE1a, ∼120 Ma. We find that large parts of the global oceans were anoxic and ferruginous for more than 1 million years. Calculations show that the development of ferruginous conditions requires that seawater sulfate concentrations drop below 600 μM and possibly below 100 μM, which is an order of magnitude lower than previous minimum estimates. Such a collapse of the seawater sulfate pool in less than one-hundred thousand years is a key and previously unrecognized feature of Phanerozoic Earth surface redox budgets. This sensitivity of the Earth system to changes in seawater sulfate concentrations illustrates potential for dramatically altered global biogeochemical cycles with corresponding climate impacts on remarkably short timescales.

Sedimentary pyrite sulfur isotopes track the local dynamics of the Peruvian oxygen minimum zone

Sulfur cycling is ubiquitous in sedimentary environments, where it mediates organic carbon remineralization, impacting both local and global redox budgets, and leaving an imprint in pyrite sulfur isotope ratios (δ34Spyr). It is unclear to what extent stratigraphic δ34Spyr variations reflect local aspects of the depositional environment or microbial activity versus global sulfur-cycle variations. Here, we couple carbon-nitrogen-sulfur concentrations and stable isotopes to identify clear influences on δ34Spyr of local environmental changes along the Peru margin. Stratigraphically coherent glacial-interglacial δ34Spyr fluctuations (>30‰) were mediated by Oxygen Minimum Zone intensification/expansion and local enhancement of organic matter deposition. The higher resulting microbial sulfate reduction rates led to more effective drawdown and 34S-enrichment of residual porewater sulfate and sulfide produced from it, some of which is preserved in pyrite. We identify organic carbon loading as a major influence on δ34Spyr, adding to the growing body of evidence highlighting the local controls on these records.

Textural and Geochemical Evidence for Magnetite Production upon Antigorite Breakdown During Subduction

AbstractMagnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in subducted ultramafic rocks. Here, we present a detailed petrological and geochemical study of magnetite in hydrous ultramafic rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite–hematite solid solution minerals, and sulfides coexist in both antigorite-serpentinite and chlorite-harzburgite at c. 670 °C and 1·6 GPa, displaying successive crystallization stages, each characterized by specific mineral compositions. In antigorite-serpentinite, magnetite inherited from seafloor hydration and recrystallized during subduction has moderate Cr (Cr2O3 &amp;lt; 10 wt%) and low Al and V concentrations. In chlorite-harzburgite, polygonal magnetite is in textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, and ilmenite–hematite solid solution minerals. The Cr2O3 contents of this magnetite are up to 19 wt%, higher than any magnetite data obtained for antigorite-serpentinite, along with higher Al and V, derived from antigorite breakdown, and lower Mn concentrations. This polygonal magnetite displays conspicuous core to rim zoning as recognized on elemental maps. Cr–V–Al–Fe3+ mass-balance calculations, assuming conservative behavior of total Fe3+ and Al, were employed to model magnetite compositions and modes in the partially dehydrated product chlorite-harzburgite starting from antigorite-serpentinite, as well as in the serpentinite protolith starting from the chlorite-harzburgite. The model results disagree with measured Cr and V compositions in magnetite from antigorite-serpentinites and chlorite-harzburgites. This indicates that these two rock types had different initial bulk compositions and thus cannot be directly compared. Our mass-balance analysis also reveals that new magnetite formation is required across the antigorite-breakdown reaction to account for the mass conservation of fluid-immobile elements such as Cr–V–Al–Fe3+. Complete recrystallization and formation of new magnetite in equilibrium with peak olivine (Mg# 89–91), chlorite (Mg# ∼95), orthopyroxene (Mg# 90–91), and pentlandite buffer the released fluid to redox conditions of ∼1 log unit above the quartz–fayalite–magnetite buffer. This is consistent with the observation that the Fe–Ti solid solution minerals (hemo-ilmenite and ilmeno-hematite) crystallized as homogeneous phases and exsolved upon exhumation and cooling. We conclude that antigorite-dehydration reaction fluids carry only a moderate redox budget and therefore may not be the only reason why the magmas are comparatively oxidized.

Deserpentinization in Subduction Zones as a Source of Oxidation in Arcs: a Reality Check

AbstractPrevious studies have concluded that dehydration of serpentinites in subduction zones produces oxidizing fluids that are the cause of oxidized arc magmas. Here, observations of natural samples and settings are combined with thermodynamic models to explore some of the factors that complicate interpretation of the observations that form the basis of this conclusion. These factors include the variability of serpentinite protoliths, the roles of carbon and sulfur in serpentinite evolution, variability in serpentinization in different tectonic settings, changes in the bulk compositions of ultramafic rocks during serpentinization, fundamental differences between serpentinization and deserpentinization, and the absence of precise geothermobarometers for ultramafic rocks. The capacity of serpentinite-derived fluids to oxidize sub-arc magma is also examined. These fluids can transport redox budget as carbon-, sulfur-, and iron-bearing species. Iron- and carbon-bearing species might be present in sufficient concentrations to transport redox budget deep within subduction zones, but are not viable transporters of redox budget at the temperatures of antigorite breakdown, which produces the largest proportion of fluid released by serpentinite dehydration. Sulfur-bearing species can carry significant redox budget, and calculations using the Deep Earth Water (DEW) model show that these species might be stable during antigorite breakdown. However, oxygen fugacities of ∼ΔFMQ + 3 (where FMQ refers to the fayalite–magnetite–quartz buffer, and ΔFMQ is log fO2 – log fO2, FMQ), which is close to, or above, the hematite–magnetite buffer at the conditions of interest, are required to stabilize oxidized sulfur-bearing species. Pseudosection calculations indicate that these conditions might be attained at the conditions of antigorite breakdown if the starting serpentinites are sufficiently oxidized, but further work is required to assess the variability of serpentinite protoliths, metamorphic pressures and temperatures, and to confirm the relative positions of the mineral buffers with relation to changes in fluid speciation.

Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnet–epidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet δ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones. Lawsonite dehydration and release of oxidizing fluids could play an important role in sub-arc mantle oxidation in subduction zones, suggest measurements of changing oxygen fugacity in zoned garnets from Sifnos, Greece.

The redox budget of the Mariana subduction zone

Assessing the efficiency of material recycling at convergent margins is critical to constraining the impact of plate tectonic processes on the composition of surface and deep mantle reservoirs on geologic timescales. In particular, oceanic lithosphere bearing oxidized phases such as Fe-oxy-hydroxides, Fe-oxides, serpentine, carbonate, and sulfate minerals, subduct at convergent margins and the infiltration of aqueous fluids and sediment melts from the subducting slab into the mantle beneath arc volcanoes may thus carry oxidized forms of multi-valent elements (e.g., S, Fe, C) and lead to the generation of primitive arc melts that record elevated oxygen fugacities relative to mid-ocean ridge primitive melts. It is unclear, however, how efficiently aqueous fluids and silicate melts transport the oxidized signatures of any given subducting slab into the mantle wedge in a single subduction zone, and how much, if any, of these oxidized phases may be recycled into the deeper mantle. We present a mass balance of Fe3+, S2−, S6+, and C4+, as well as the O2 associated with these species, through the Mariana subduction zone to assess the efficiency of recycling oxidized materials in an end-member type subduction zone, where old oceanic lithosphere and a thick sediment package is subducted. To do this, we report Fe3+/ΣFe ratios of bulk sediments and altered oceanic crust recovered from ODP Site 801 in the western Pacific in order to constrain the bulk Fe3+/ΣFe ratio of the Pacific plate prior to subduction in the Mariana convergent margin. Site 801 sediments have Fe3+/ΣFe ratios >0.69 and the altered oceanic crust (801 Super Composite) has Fe3+/ΣFe of 0.51. Bulk Fe3+/ΣFe ratios of altered oceanic crust at Site 801 increase from 0.14 (pristine Jurassic-aged MORB glass) to 0.78 with increasing extent of alteration. We find that 68–95% of the O2 added to the subducting crust by sedimentation, in situ alteration of basaltic crust on the seafloor, and serpentinization of the mantle lithosphere is not output by Mariana arc or back-arc magmas. This result demonstrates that significant amounts of oxidized materials from Earth's surface are transported into the deeper mantle beyond subduction zones, despite the production of oxidized arc and back-arc basalts, and may contribute to elevated oxygen fugacities recorded by ocean island lavas such as Iceland and Hawaii. This oxygen cycle is likely to have been operating at least for the past 400–800 million years, and potentially for the duration of plate tectonics.

Redox processes and the role of carbon-bearing volatiles from the slab–mantle interface to the mantle wedge

The valence of carbon is governed by the oxidation state of the host system. The subducted oceanic lithosphere contains considerable amounts of iron so that Fe 3+ /Fe 2+ equilibria in mineral assemblages are able to buffer the (intensive) f O 2 and the valence of carbon. Alternatively, carbon itself can be a carrier of (extensive) ‘excess oxygen’ when transferred from the slab to the mantle, prompting the oxidation of the sub-arc mantle. Therefore, the correct use of intensive and extensive variables to define the slab-to-mantle oxidation by C-bearing fluids is of primary importance when considering different fluid/rock ratios. Fluid-mediated processes at the slab–mantle interface can also be investigated experimentally. The presence of CO 2 (or CH 4 at highly reduced conditions) in aqueous COH fluids in peridotitic systems affects the positions of carbonation or decarbonation reactions and of the solidus. Some methods to produce and analyse COH fluid-saturated experiments in model systems are introduced, together with the measurement of experimental COH fluids composition in terms of volatiles and dissolved solutes. The role of COH fluids in the stability of hydrous and carbonate minerals is discussed comparing experimental results with thermodynamic models and the message of nature.

The influence of oceanic oxidation on serpentinite dehydration during subduction

Serpentinites are central to carry water and fluid-mobile elements down subduction zones. The breakdown of antigorite represents the most prominent aqueous fluid release, boosting fluid-mediated element cycling from the slab to the mantle wedge. At Cerro del Almirez, Spain, an antigorite dehydration reaction front is preserved in subducted serpentinites. Bulk rock and mineral major element chemistry linked to detailed petrography reveals that silicate mineral Mg# (100⁎[Mg/(Mg+Fe)]molar) are higher than bulk rocks due to the presence of magnetite, and olivine Mg# are lower in Chl-harzburgite than in Atg-serpentinite. The amount of magnetite is lower in Chl-harzburgite (1.4 vol%) than in Atg-serpentinite (2.8 vol%), resulting in reactive bulk rock compositions with Mg# of 92.7 and 96.0, respectively. Pseudosection modelling employing these reactive bulk compositions yields a small temperature field at 670 °C, 1.6 GPa where Atg-serpentinite and Chl-harzburgite coexist at the same metamorphic conditions. Thus, the antigorite dehydration front represents a compositional boundary rather than a thermal front (isograd). We suggest that this compositional boundary represents an oxidation front established upon serpentinisation at the ocean floor. Previous studies have shown that with increasing extent of serpentinisation increasing amounts of magnetite are formed concomitant with an increase in the Mg# of coexisting silicates. During subduction of such heterogeneous ultramafic rocks antigorite dehydration will be a continuous reaction that can occur over up to ∼40 °C, controlled by variations in Mg# of the reactant silicates. The difference in the amount of magnetite between Atg-serpentinite and Chl-harzburgite is thus not related to a change in redox budget of subducting serpentinites at Almirez imposed by the antigorite dehydration reaction. Rather, mass balance considerations suggest that ferric iron from antigorite may actually be the most prominent contribution to the redox budget of the antigorite dehydration reaction. Our findings also imply that direct comparison between Atg-serpentinites and Chl-harzburgites to infer geochemical changes associated with prograde dehydration reactions may lead to erroneous conclusions, including estimates on element loss mediated by aqueous fluid escape and associated changes in redox budget based on Fe3+/Fetot.

The effects of metamorphism on iron mineralogy and the iron speciation redox proxy

As the most abundant transition metal in the Earth’s crust, iron is a key player in the planetary redox budget. Observations of iron minerals in the sedimentary record have been used to describe atmospheric and aqueous redox environments over the evolution of our planet; the most common method applied is iron speciation, a geochemical sequential extraction method in which proportions of different iron minerals are compared to calibrations from modern sediments to determine water-column redox state. Less is known about how this proxy records information through post-depositional processes, including diagenesis and metamorphism. To get insight into this, we examined how the iron mineral groups/pools (silicates, oxides, sulfides, etc.) and paleoredox proxy interpretations can be affected by known metamorphic processes. Well-known metamorphic reactions occurring in sub-chlorite to kyanite rocks are able to move iron between different iron pools along a range of proxy vectors, potentially affecting paleoredox results. To quantify the effect strength of these reactions, we examined mineralogical and geochemical data from two classic localities where Silurian-Devonian shales, sandstones, and carbonates deposited in a marine sedimentary basin with oxygenated seawater (based on global and local biological constraints) have been regionally metamorphosed from lower-greenschist facies to granulite facies: Waits River and Gile Mountain Formations, Vermont, USA and the Waterville and Sangerville-Vassalboro Formations, Maine, USA. Plotting iron speciation ratios determined for samples from these localities revealed apparent paleoredox conditions of the depositional water column spanning the entire range from oxic to ferruginous (anoxic) to euxinic (anoxic and sulfidic). Pyrrhotite formation in samples highlighted problems within the proxy as iron pool assignment required assumptions about metamorphic reactions and pyrrhotite’s identification depended on the extraction techniques utilized. The presence of diagenetic iron carbonates in many samples severely affected the proxy even at low grade, engendering an interpretation of ferruginous conditions in all lithologies, but particularly in carbonate-bearing rocks. Increasing metamorphic grades transformed iron in carbonates into iron in silicate minerals, which when combined with a slight increase in the amount of pyrrhotite, drove the proxy toward more oxic and more euxinic conditions. Broad-classes of metamorphic reactions (e.g. decarbonation, silicate formation) occurred at distinct temperatures-pressures in carbonates versus siliciclastics, and could be either abrupt between metamorphic facies or more gradual in nature. Notably, these analyses highlighted the importance of trace iron in phases like calcite, which otherwise might not be included in iron-focused research i.e. ore-system petrogenesis, metamorphic evolution, or normative calculations of mineral abundance. The observations show that iron is mobile and reactive during diagenesis and metamorphism, and these post-depositional processes can readily overprint primary redox information held by iron speciation. However, in principle, additional mineralogical and petrographic approaches can be combined with iron speciation data to help untangle many of these post-depositional processes and arrive at more accurate estimates of paleoenvironmental redox conditions and processes, even for metamorphosed samples.