Fe Oxidation State Research Articles

Nitrogen lifts iron to hexavalence

Inorganic Chemistry The myriad ways that iron can interact with oxygen have been amply studied in biochemical and geochemical contexts. More recently, chemists have explored the extent to which nitrogen can likewise stabilize iron in high oxidation states. Martinez et al. now report that an iron center coordinated by carbene ligands can react with an organic azide to form a pentavalent bis(imido) complex with two Fe=N bonds. One-electron oxidation then accessed the Fe(VI) oxidation state. Both compounds were sufficiently stable for crystallographic characterization. Science , this issue p. [356][1] [1]: /lookup/doi/10.1126/science.abd3054

Operando X-ray Absorption Spectroscopy (XAS) Observation of Photoinduced Oxidation in FeNi (Oxy)hydroxide Overlayers on Hematite (α-Fe2O3) Photoanodes for Solar Water Splitting.

An FeNi (oxy)hydroxide cocatalyst overlayer was photoelectrochemically deposited on a thin-film hematite (α-Fe2O3) photoanode, leading to a cathodic shift of ∼100 mV in the photocurrent onset potential. Operando X-ray absorption spectroscopy (XAS) at the Fe and Ni K-edges was used to study the changes in the overlayer with potential in the dark and under illumination conditions. Potential or illumination only had a minor effect on the Fe oxidation state, suggesting that Fe atoms do not accumulate significant amount of charge over the whole potential range. In contrast, the Ni K-edge spectra showed pronounced dependence on potential in the dark and under illumination. The effect of illumination is to shift the onset for the Ni oxidation because of the generated photovoltage and suggests that holes that are photogenerated in hematite are transferred mainly to the Ni atoms in the overlayer. The increase in the oxidation state of Ni proceeds at potentials corresponding to the redox wave of Ni, which occurs immediately prior to the onset of the oxygen evolution reaction (OER). Linear combination fitting analysis of the obtained spectra suggests that the overlayer does not have to be fully oxidized to promote oxygen evolution. Cathodic discharge measurements show that the photogenerated charge is stored almost exclusively in the Ni atoms within the volume of the overlayer.

Tuning the Co Oxidation State in Ba0.5Sr0.5Co0.8Fe0.2O3-δ by Flame Spray Synthesis Towards High Oxygen Evolution Reaction Activity

The perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is known as a highly active and stable oxygen evolution reaction (OER) electrocatalyst composited out of non-noble metals. The possibility of using the scalable flame spray synthesis (FSS) technique for the production of BSCF nanoparticles intensified the interest in this material for a future application in an alkaline water electrolyzer. A possible scale-up would require the optimization of the synthesis parameters to maximize the production rate. To further understand the influence of the synthesis parameters of the tunable FSS on the OER activity of BSCF, a systematic study was carried out by producing BSCF with different total metal concentrations (CTM), flow rates of the precursor solution (FRPS) and of the dispersion gas (FRDG). This study reveals that all three parameters have a direct impact on the OER activity of BSCF—measured in a rotating disc electrode (RDE) setup—due to the controllability of the initial Co and Fe oxidation state—indicated by X-ray absorption spectroscopy (XAS) measurements—and with that also of the oxygen vacancy concentration in the as-synthesized BSCF. This controllability enables the optimization of the OER activity of BSCF and emphasizes the importance of having Co in a lower initial oxidation state for reaching a high electrocatalytic performance.

Active Sites of Single-Atom Iron Catalyst for Electrochemical Hydrogen Evolution.

Electrochemical water splitting in alkaline media is an attractive way to produce the clear and renewable hydrogen fuel H2. In this work, we report a single-atom Fe1/NC catalyst, where the Fe-Nx moiety works as the active site, for high-efficiency alkaline hydrogen evolution reaction (HER). The Fe1/NC electrocatalyst exhibits a low overpotential of 111 mV at the current density of 10 mA cm-2, with a Tafel slope of 86.1 mV dec-1 in 1 M KOH solution. Operando X-ray absorption spectroscopy reveals that, under the working states, the Fe-support interaction weakened as the Fe-N coordination number and Fe oxidation state decreased. As such, the evolved single-atom Fe site with more d electrons provides a favorable structure for boosting HER performance. This work gives insight into the structural evolution of the active site under the alkaline HER and provides a strategy for the design of non-noble metal electrocatalysts.

Changes in Sedimentary Phosphorus Burial Following Artificial Eutrophication of Lake 227, Experimental Lakes Area, Ontario, Canada

AbstractLake 227 of the Experimental Lakes Area (ELA) in Ontario, Canada, has been fertilized with phosphorus (P) since 1969, which resulted in a rapid transition from oligotrophic to eutrophic conditions. Sediment cores collected from the oxygenated epilimnion, and the mostly anoxic hypolimnion of this unique lake contain a historical record of the changes in sediment P speciation and burial rates across the trophic transition. To elucidate these changes, results of chemical extractions were combined with 210Pb sediment dating, and with 31P NMR, Mössbauer, and XANES spectroscopies. Prior to 1969, organic P (POrg) was the major sedimentary P sink in Lake 227. Eutrophication of the lake coincided with marked increases in the burial rate of total P (TP), as well as in the relative contribution of the NaHCO3‐extractable P pool (humic‐bound P, PHum). Together, PHum and POrg account for ≥70% of total P burial in the sediments deposited since artificial fertilization started. The PHum fraction likely comprises phosphate complexes with humic substances. The strong linear correlation between P and iron (Fe) extracted by NaHCO3 implies a close association of the two elements in the humic fraction. Mössbauer and XANES spectra further indicate that most Fe in the post‐1969 sediments remained in the Fe (III) oxidation state, which is attributed to the stabilization of reducible Fe by organic matter, in part via the formation of phosphate‐Fe (III)‐humic complexes. Importantly, our results show that the eutrophication experimentation of Lake 227 caused the accumulation of a large reservoir of reactive sediment P, which may continue to fuel internal P loading to the water column once artificial fertilization is terminated.

Spectroscopic study of volcanic ashes

Volcanic ashes particles are subjected to substantial modification during explosive eruptions. The mineralogical and compositional changes have important consequences on the environment and human health. Nevertheless, the relationship between the speciation of iron (Fe) and the mineralogical composition and particle granulometry of the ashes, along with their interaction with water, are largely unknown. In particular, the Fe oxidation state and the possible formation of new Fe-bearing phases in presence of S, Cl, and F in the plume are key points to assess the impact of the ashes.Fragmental material ejected during volcanic activity (tephra) in 2013, was collected on the Mt. Etna (Italy) and investigated using a multi-technique approach that included conventional Electron Paramagnetic Resonance (EPR), high field EPR (HFEPR), EchoEPR, and Fe K-edge X-ray Absorption Spectroscopy (XAS). These element-selective techniques allowed obtaining a detailed information on the oxidation state and coordination environment of Fe, and of its speciation in the ash samples as a function of the granulometry. A complex mineralogical assemblage, consisting of variable amounts of nanometric crystalline Fe inclusions in a glass matrix, and of Fe-oxides and Fe-sulfur phases was revealed. A risk assessment of the ashes is attempted.

A seawater throttle on H2 production in Precambrian serpentinizing systems

Since the initial discovery of low-temperature alkaline hydrothermal vents off the Mid-Atlantic Ridge axis nearly 20 y ago, the observation that serpentinizing systems produce abundant H2 has strongly influenced models of atmospheric evolution and geological scenarios for the origin of life. Nevertheless, the principal mechanisms that generate H2 in these systems, and how secular changes in seawater composition may have modified serpentinization-driven H2 fluxes, remain poorly constrained. Here, we demonstrate that the dominant mechanism for H2 production during low-temperature serpentinization is directly related to a Si deficiency in the serpentine structure, which itself is caused by low SiO2(aq) concentrations in serpentinizing fluids derived from modern seawater. Geochemical calculations explicitly incorporating this mechanism illustrate that H2 production is directly proportional to both the SiO2(aq) concentration and temperature of serpentinization. These results imply that, before the emergence of silica-secreting organisms, elevated SiO2(aq) concentrations in Precambrian seawater would have generated serpentinites that produced up to two orders of magnitude less H2 than their modern counterparts, consistent with Fe-oxidation states measured on ancient igneous rocks. A mechanistic link between the marine Si cycle and off-axis H2 production requires a reevaluation of the processes that supplied H2 to prebiotic and early microbial systems, as well as those that balanced ocean-atmosphere redox through time.

Promoting Active Electronic States in LaFeO3 Thin-Films Photocathodes via Alkaline-Earth Metal Substitution.

The effects of alkaline-earth metal cation (AMC; Mg2+, Ca2+, Sr2+, and Ba2+) substitution on the photoelectrochemical properties of phase-pure LaFeO3 (LFO) thin-films are elucidated by X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), diffuse reflectance, and electrochemical impedance spectroscopy (EIS). XRD confirms the formation of single-phase cubic LFO thin films with a rather complex dependence on the nature of the AMC and extent of substitution. Interestingly, subtle trends in lattice constant variations observed in XRD are closely correlated with shifts in the binding energies of Fe 2p3/2 and O 1s orbitals associated with the perovskite lattice. We establish a scaling factor between these two photoemission peaks, unveiling key correlation between Fe oxidation state and Fe-O covalency. Diffuse reflectance shows that optical transitions are little affected by AMC substitution below 10%, which are dominated by a direct bandgap transition close to 2.72 eV. Differential capacitance data obtained from EIS confirm the p-type characteristic of pristine LFO thin-films, revealing the presence of sub-bandgap electronic state (A-states) close to the valence band edge. The density of A-states is decreased upon AMC substitution, while the overall capacitance increases (increase in dopant level) and the apparent flat-band potential shifts toward more positive potentials. This behavior is consistent with the change in the valence band photoemission edge. In addition, capacitance data of cation-substituted films show the emergence of deeper states centered around 0.6 eV above the valence band edge (B-states). Photoelectrochemical responses toward the hydrogen evolution and oxygen reduction reactions in alkaline solutions show a complex dependence on alkaline-earth metal incorporation, reaching incident-photon-to-current conversion efficiency close to 20% in oxygen saturated solutions. We rationalize the photoresponses of the LFO films in terms of the effect sub-bandgap states on majority carrier mobility, charge transfer, and recombination kinetics.

Past endolithic life in metamorphic ocean crust

The known deep subsurface biosphere on Earth persists in diversified habitats, including deep within igneous rocks of the oceanic crust. Here, we extend the range of the deep subsurface biosphere to metamorphic ocean crust of a subduction zone. We report fossilised life in zeolite facies rocks, which formed by low grade metamorphism, from the southern Mariana trench. Dense carbonaceous spheroids, filaments, and Frutexites-like structures are preserved in these rocks, which are enriched in organic carbon but depleted in 13C. The distinct difference in the GDGT-0 vs. crenarchaeol and the branched vs. isoprenoid tetraether values between the inner and outer portions of these rocks indicate the in situ production of organic carbon. We demonstrate that these structures may result from the past activity of potential chemolithoautotrophs within the metamorphic crust, as implied by their morphologies, Raman spectra, carbon isotopes, and biomarker signatures, as well as the Fe oxidation state within whole rocks. We propose that fluid-rock reactions at temperatures within the tolerance of life during low grade metamorphism contributed to microbial subsistence within the biotope. The low grade metamorphic ocean crust of the subduction zone likely represents Earth’s deepest, and one of its largest, microbial ecosystems, which may potentially influence the deep carbon cycle.

Micro- and nano-scale mineralogical characterization of Fe(II)-oxidizing bacterial stalks.

Neutrophilic, microaerobic Fe(II)-oxidizing bacteria (FeOB) from marine and freshwater environments are known to generate twisted ribbon-like organo-mineral stalks. These structures, which are extracellularly precipitated, are susceptible to chemical influences in the environment once synthesized. In this paper, we characterize the minerals associated with freshwater FeOB stalks in order to evaluate key organo-mineral mechanisms involved in biomineral formation. Micro-Raman spectroscopy and Field Emission Scanning Electron Microscopy revealed that FeOB isolated from drinking water wells in Sweden produced stalks with ferrihydrite, lepidocrocite and goethite as main mineral components. Based on our observations made by micro-Raman Spectroscopy, field emission scanning electron microscopy and scanning transmission electron microscope combined with electron energy-loss spectroscopy, we propose a model that describes the crystal-growth mechanism, the Fe-oxidation state, and the mineralogical state of the stalks, as well as the biogenic contribution to these features. Our study suggests that the main crystal-growth mechanism in stalks includes nanoparticle aggregation and dissolution/re-precipitation reactions, which are dominant near the organic exopolymeric material produced by the microorganism and in the peripheral region of the stalk, respectively.

Radical SAM Enzyme HydE Generates Adenosylated Fe(I) Intermediates En Route to the [FeFe]-Hydrogenase Catalytic H-Cluster.

The H-cluster of [FeFe]-hydrogenase consists of a [4Fe-4S]H-subcluster linked by a cysteinyl bridge to a unique organometallic [2Fe]H-subcluster assigned as the site of interconversion between protons and molecular hydrogen. This [2Fe]H-subcluster is assembled by a set of Fe-S maturase enzymes HydG, HydE and HydF. Here we show that the HydG product [FeII(Cys)(CO)2(CN)] synthon is the substrate of the radical SAM enzyme HydE, with the generated 5'-deoxyadenosyl radical attacking the cysteine S to form a C5'-S bond concomitant with reduction of the central low-spin Fe(II) to the Fe(I) oxidation state. This leads to the cleavage of the cysteine C3-S bond, producing a mononuclear [FeI(CO)2(CN)S] species that serves as the precursor to the dinuclear Fe(I)Fe(I) center of the [2Fe]H-subcluster. This work unveils the role played by HydE in the enzymatic assembly of the H-cluster and expands the scope of radical SAM enzyme chemistry.

Electronic Structure of the Hieber Anion [Fe(CO)3(NO)]− Revisited by X-ray Emission and Absorption Spectroscopy

While the Hieber anion [Fe(CO)3(NO)]- has been reincarnated in the last years as an active catalyst in organic synthesis, there is still a debate about the oxidation state of the central Fe atom and the resulting charge of the NO ligand. To shed new light on this question and to understand the Fe-NO interaction in the Hieber anion, it is investigated in comparison to the formal 3d8 reference Fe(CO)5 and the formal 3d10 reference [Fe(CO)4]2- by the combination of valence-to-core X-ray emission spectroscopy (VtC-XES), X-ray absorption near-edge structure spectroscopy (XANES), and high-energy-resolution fluorescence-detected XANES. In order to extract information about the electronic structure, time-dependent density functional theory and ground-state density functional theory calculations are applied. This combination of experimental and computational methods reveals that the electron density at the Fe center of the Hieber resembles that of the isoelectronic [Fe(CO)4]2-. These observations challenge recent descriptions of the Hieber anion and reopen the debate about the experimentally and computationally determined Fe oxidation state and charge on the NO ligand.

Comparative Fe and Sr isotope study of nephrite deposits hosted in dolomitic marbles and serpentinites from the Sudetes, SW Poland: Implications for Fe-As-Au-bearing skarn formation and post-obduction evolution of the oceanic lithosphere

Nephrites belonging to both the dolomite-related and serpentinite-related genetic types, from the Złoty Stok deposit, and Jordanów Śląski and Nasławice prospects in the Ślęża Ophiolite, respectively, were analyzed for Fe and Sr isotope compositions. These deposits in the Central Sudetes (SW Poland), are unusually situated relatively close to each other. This study is a first attempt to perform combined Fe and Sr isotope study of nephrites. Our results show that Fe in the Fe-As-Au-bearing skarn samples from Złoty Stok (2.77–7.00 wt% Fe, δ56Fe +0.07 to +0.14‰, and δ57Fe +0.12 to +0.21‰), to which the dolomite-related nephrite belongs, is not inherited from the host dolomitic marbles (ca. 0.4 wt% Fe; δ56Fe −0.08 to −0.07‰, and δ57Fe −0.12 to −0.09‰), but derived from the nearby, ca. 340 Ma granite intrusion (2.64–5.64 wt% Fe). In contrast, iron in the serpentinite-related nephrites (2.06–3.88 wt% Fe, δ56Fe +0.41 to +0.45‰, and δ57Fe +0.59 to +0.66‰) was likely inherited from host serpentinites (5.27–4.81 wt% Fe), rather than metasomatically derived from adjacent granite intrusions (0.13–0.45 wt% Fe). Moreover, both the marble transformation into a dolomite-related nephrite (and Fe-As-Au-bearing skarn), and the abyssal serpentinite – through ophiolite – towards serpentinite-related nephrite transformation, are accompanied by a progressive increase of δ56Fe and δ57Fe. This increase is regardless of changes in the bulk Fe content, increasing during the dolomite-related nephrite formation, and decreasing during serpentinite-related nephrite formation. This phenomena may be caused by one of the below listed factors, or combinations: a) preferential incorporation of heavy iron by tremolite and diopside; b) preferential incorporation of the heavy iron in the Fe3+-bearing phases, crystallizing during nephrite formation; c) preferential incorporation of the heavy Fe from migrating granite-derived fluids (positive fractionation between aqueous Fe-bearing fluid, and tremolite/diopside); d) positive isotope fractionation between dolomite/antigorite precursor, and newly formed amphibole/clinopyroxene; e) change of the Fe oxidation state. Furthermore, Fe in the rock-forming tremolite from the serpentinite-related nephrites is isotopically heavier than Fe in tremolite from the dolomite-related ones. In contrast to the iron, the isotopic composition of strontium, combined with its content and structural evidence, suggest multiple sources for this element. In the case of the dolomite-related nephrite (ca. 3–4 ppm Sr; initial 87Sr/86Sr = 0.7085), Sr inheritance from the host-dolomitic marble (ca. 90 ppm Sr; initial 87Sr/86Sr = 0.7081), as well as its derivation from intrusions of both the syn- to late-tectonic, ca. 340 Ma granites (ca. 400–440 ppm Sr), and post-tectonic ~305 Ma granites (86.4 ppm Sr), is suggested. In the case of the serpentinite-related nephrite (6.3–61.7 ppm Sr; initial 87Sr/86Sr 0.7037 to 0.7081), the situation is similar, i.e., Sr in the most primitive samples tends to be sourced from ophiolitic rocks (up to 2.4 ppm Sr in serpentinite), whereas in the more evolved types, it was likely delivered from the adjacent, ca. 340 Ma partially rodingitized granites (8.8–102.6 ppm Sr), as well as ~305 Ma granites (ca. 80 ppm Sr). Moreover, these granites caused the fluid-flow, responsible for Sr-bearing clinozoisite vein formation; these veins are likely related with nephrite formation, and with a metallic mineralization.

A synthesis and meta-analysis of the Fe chemistry of serpentinites and serpentine minerals.

The iron chemistry of serpentinites and serpentine group minerals is often invoked as a record of the setting and conditions of serpentinization because Fe behaviour is influenced by reaction conditions. Iron can be partitioned into a variety of secondary mineral phases and undergo variable extents of oxidation and/or reduction during serpentinization. This behaviour influences geophysical, geochemical and biological aspects of serpentinizing systems and, more broadly, earth systems. Iron chemistry of serpentinites and serpentines is frequently analysed and reported for single systems. Interpretations of the controls on, and the implications of, Fe behaviour drawn from a single system are often widely extrapolated. There is a wealth of serpentinite/serpentine chemical composition data available in the literature. Consequently, compilation of a database including potential predictors of Fe behaviour and measures of Fe chemistry enables systematic investigation of trends in Fe behaviour across a variety of systems and conditions. The database presented here contains approximately 2000 individual data points including both bulk rock and serpentine mineral geochemical data which are paired whenever possible. Measures of total Fe and Fe oxidation state, which are more limited, are compiled with characteristics of the systems from which they were sampled. Observations of trends in Fe chemistry in serpentinites and serpentines across the variety of geologic systems and parameters will aid in verifying and strengthening interpretations made on the basis of Fe chemistry. This article is part of a discussion meeting issue 'Serpentinite in the Earth system'.

Ultrafast pump-probe spectroscopy of neutral FenOm clusters (n, m < 16).

Neutral iron oxide clusters (FenOm, n, m ≤ 16) are produced in a laser vaporization source using O2 gas seeded in He. The neutral clusters are ionized with a sequence of femtosecond laser pulses and detected using time-of-flight mass spectrometry. Small clusters are confirmed to be most prominent in the stoichiometric (n = m) distribution, with m = n + 1 clusters observed above n = 4. Pump-probe spectroscopy is employed to study the dynamics of an electron transfer from an oxygen orbital to an iron nonbonding orbital of iron oxide clusters that is driven by absorption of a 400 nm photon. A bifurcation of the initial wavepacket occurs, where a femtosecond component is attributed to electron relaxation assisted through internuclear vibrational relaxation and high density of states, and a slow relaxation shows the formation of a bound excited state. The lifetime and relative ratio of the two pathways depend on both the cluster size and iron oxidation state. The femtosecond lifetime decreases with increased cluster size until a saturation timescale is achieved at n > 5. The relative population of the long-lived excited state decreases with cluster size and suggests that the excited electron remains on the Fe atom for >20 ps.

DFT study of α-Keggin, lacunary Keggin, and ironII-VI substituted Keggin polyoxometalates: the effect of oxidation state and axial ligand on geometry, electronic structures and oxygen transfer.

Herein, the geometry, electronic structure, Fe–ligand bonding nature and simulated IR spectrum of α-Keggin, lacunary Keggin, iron(ii/iii)-substituted and the important oxidized high-valent iron derivatives of Keggin type polyoxometalates have been studied using the density functional theory (DFT/OPTX-PBE) method and natural bond orbital (NBO) analysis. The effects of different Fe oxidation states (ii–vi) and H2O/OH−/O2− ligand interactions have been addressed concerning their geometry and electronic structures. It has been revealed that the d-atomic orbitals of Fe and 2p orbitals of polyoxometalate's oxygen-atoms contribute in ligand binding. Compared with other high valent species, the considered polyoxometalate system of [PW11O39(FeVO)]4−, possesses a high reactivity for oxygen transfer.

Synthesis, structure and magnetic behavior of iron arsenites with hierarchical magnetic units

Iron-arsenites may cover multiple Fe oxidation states, while the versatile (AsO3)2− condensation offer ideal spacers between magnetic sub-units with various dimensionalities (D). We show here four novel compounds covering the 0–3D magnetic panorama.

Iterative energy self-calibration of Fe XANES spectra.

Determining the oxidation state of Fe through parameterization of X-ray absorption near-edge structure (XANES) spectral features is highly dependent on accurate and repeatable energy calibration between spectra. Small errors in energy calibration can lead to vastly different interpretations. While simultaneous measurement of a reference foil is often undertaken on X-ray spectroscopy beamlines, other beamlines measure XANES spectra without a reference foil and therefore lack a method for correcting energy drift. Here a method is proposed that combines two measures of Fe oxidation state taken from different parts of the spectrum to iteratively correct for an unknown energy offset between spectra, showing successful iterative self-calibration not only during individual beam time but also across different beamlines.

Evolution of chromium, manganese and iron oxidation state during conversion of nuclear waste melter feed to molten glass

Oxidation state and chemical conversion of three surrogate nuclear high-level waste melter feeds heat treated to temperatures between 500 and 1200 °C were studied using X-ray near-edge structure, feed expansion tests, X-ray diffraction, and evolved gas analyses. For the high-chromium feed (3.00 mass% Cr2O3 in glass), the average Cr oxidation state was predominately Cr3+ at the lowest and highest heat-treatment temperatures and Cr6+ at intermediate temperatures while the Fe coordination shifted from octahedral to non-centrosymmetric tetrahedral at higher temperatures with Fe oxidation state unchanged. For the high-manganese-high-iron feed (3.23 mass% MnO and 16.01 mass% Fe2O3 in glass), the average Mn oxidation state decreased from 4 to ~2 while the Fe oxidation state shifted towards increasing ferrous content and changed from five-fold coordination to tetrahedral. For the high-alumina feed (24.02 mass% Al2O3 in glass), average Fe coordination did not change, but Fe oxidation state shifted towards higher ferrous content with increasing temperature.

Chasing the mantle: Deciphering cryptic mantle signals through Earth's thickest continental magmatic arc

Understanding the magmatic processes occurring in the upper mantle and lowermost crust above subduction zones is paramount if we hope to understand the primary mechanisms driving arc magmatism. In the Central Volcanic Zone of the Andes (CVZ), extreme crustal thicknesses and the prominence of silicic magmatism in the Neogene have prevented a clear understanding of the primitive magma input into what is arguably the Earth's archetypal active continental margin. However, we propose that phase chemistry and equilibria from mafic mineral assemblages in basaltic andesite enclaves from the Purico-Chascon Volcanic Complex reveal rare constraints on the character of primitive mantle-derived magmas feeding the CVZ arc. We interpret olivine, Cr-spinel, and olivine-hosted melt inclusion compositions from these mafic assemblages to record the partial melting of wet, oxidized mantle peridotite and the production of high-Mg basalt in the upper mantle. In turn, clinopyroxene and spinel-free olivine are interpreted to reflect magma modification in the lower crust and the adiabatic ascent of differentiated basaltic magma up to ∼20 km. Interestingly, olivine-hosted Cr-spinel compositions suggest that magmas in the Purico-Chascon system ascend from the upper mantle to surface with no significant change in Fe-oxidation state despite obvious differentiation and crustal interactions. Finally, comparison of high Mg-basalt in melt inclusions from Purico-Chascon (CVZ arc) and CVZ back-arc lavas show that the magmas may share a common peridotite source. However, the peridotite beneath the arc appears to be more hydrous and oxidized than that beneath the back-arc, confirming that back-arc lava compositions may not adequately represent mafic magma input into the arc. We posit that the relationships between H2O-content, oxidation conditions, and mafic phase equilibria discussed in this study are directly linked to subduction zone geometry, and thus may be applicable to arcs globally.