ΣFe Values Research Articles

No detectable redox exchange between sulfur and iron during rapid cooling of basalts

The valence states of iron and sulfur in mantle-derived melts influence and respond to the composition of the mantle as well as the subsequent evolution of those melts in Earth's crust. Characterizing the valence states of iron and sulfur in silicate melts that have quenched to form glasses is therefore critical to understanding the composition of the mantle and the magmatic processes that take place before and during volcanic eruptions. Glasses quenched from silicate melts offer the opportunity to precisely measure sulfur and iron valence states, but whether the transition from melt to glass affects the valence state of sulfur and iron is uncertain. Here, we use the glassy margins of two pillow basalts, one from a mid-ocean ridge and one from a back-arc basin, as a natural experiment to test the effect of quench rate on sulfur and iron valence states. We report micro-X-ray Absorption Near Edge Structure (XANES) measurements of S6+/ΣS (S6+/[S6++S2−]) and Fe3+/ΣFe (Fe3+/[Fe2++Fe3+]) along transects from the rapidly quenched rims of the pillows down into the slowly cooled crystal-rich interiors. The range of average quench rates estimated for our selected analysis areas range from 1.2×105°C/sec at the pillow margin to 1.1°C/sec in the interior and span quench rates experienced by natural and experimental glasses. On average, S6+/ΣS = 0.09±0.01 and Fe3+/ΣFe = 0.147±0.002 in the mid-ocean ridge sample and S6+/ΣS = 0.21±0.01and Fe3+/ΣFe = 0.153±0.004 in the back-arc sample, where the quoted uncertainty is the 1σ standard deviation of n= 9-16 analyses. We find that the S6+/ΣS and Fe3+/ΣFe values along transects in each sample show no systematic changes with depth, and by proxy, no systematic changes with quench rate. In each pillow, all measured S6+/ΣS and Fe3+/ΣFe ratios are within the absolute uncertainty of each technique. We propose that these observations are consistent with either sluggish reaction kinetics between sulfur and iron, or an insignificant temperature effect on sulfur-iron redox exchange equilibria. These observations demonstrate that sulfur and iron valence states in glasses with varying quench rates can be directly compared, and that changes in sulfur and iron valence during quench are below the limits of detection with XANES.

Large iron isotope fractionation during mantle wedge serpentinization: Implications for iron isotopes of arc magmas

Arc magmas are usually more oxidized than lavas from intraplate settings and spreading ridges. However, iron isotopes of arc magmas are generally lighter, and the mechanism is still controversial. The mantle wedge serpentinite provides new insights to resolve this issue. Here we show that mantle wedge serpentinites/serpentinized peridotites from the Mariana forearc mantle wedge have both heavy Fe isotopes (δ56Fe=−0.01–0.15‰) and high Fe3+/ΣFe values (0.50–0.88), which are positively correlated. This indicates coupled iron oxidation and iron isotope fractionation induced by serpentinization in the mantle wedge. The high Fe3+/ΣFe values and high SO3 contents of the mature serpentinites further suggest that the forearc mantle wedge is highly oxidized. Here we present a new mechanism for iron isotope fractionation and iron oxidation during mantle wedge serpentinization. Because iron solubility is low in the resultant high-pH fluids at the early stage of serpentinization, ferrous iron liberated during olivine serpentinization preferentially precipitates as serpentine, Fe-rich brucite, and magnetite in the serpentinite. After the full consumption of olivine during the late-stage serpentinization, ferrous iron becomes soluble and mobile in low-pH chlorine/sulfate-rich penetrating fluids, and is therefore released to the fluid with light Fe isotopes during progressive serpentinization, leaving the residual serpentinite with heavy iron isotopes. This suggests that mature serpentinites at the base of mantle wedge can act as a filter that sequesters ferric iron with heavy iron isotopes and liberates ferrous iron with light iron isotopes to the penetrating slab fluid, which migrate up to the above mantle wedge and even the mantle melting zone. The highly oxidized forearc mantle wedge serpentinites can also be dragged down to the subarc depth, releasing oxidizing fluids by dehydration. These processes may oxidize the arc mantle wedge coupled with light Fe isotopes, the fluxed partial melting of which consequently produces oxidized arc magmas with light Fe isotopes.

Estimating ferric iron content in clinopyroxene using machine learning models

AbstractClinopyroxene ferric iron content is an important consideration for garnet-clinopyroxene geothermometry and estimations of water storage in the Earth’s interior but remains difficult and expensive to measure. Here, we develop seven classic algorithms and machine learning methods to estimate Fe3+/ΣFe in clinopyroxene using major element data from electron microprobe analyses. The models were first trained using a large data set of clinopyroxene Fe3+/ΣFe values determined by Mössbauer spectroscopy and spanning a wide compositional range, with major uncertainties ranging from 0.25 to 0.3 and root-mean-square errors on the test data set ranging from 0.071 to 0.089. After dividing the entire data set into three compositional sub-data sets, the machine learning models were trained and compared for each sub-data set. Our results suggest that ensemble learning algorithms (random forest and Extra-Trees) perform better than principal component analysis-based elastic net polynomial, artificial neural network, artificial neural network ensemble, decision trees, and linear regressions. Using a sub-data set excluding clinopyroxene in spinel peridotite and omphacite in eclogite, the new models achieved uncertainties of 0.15 to 0.2 and root-mean-square errors on the test data set ranging from 0.051 to 0.078, decreasing prediction errors by 30–40%. By incorporating compositional data on coexisting spinel, new models for clinopyroxene in spinel peridotite show improved performance, indicating the interaction between spinel and clinopyroxene in spinel peridotite. Feature importance analysis shows Na+, Ca2+, and Mg2+ to be the most important for predicting Fe3+ content, supporting the coupled substitution between Ca2+-M2+ and Na+-M3+ in natural clinopyroxenes. The application of our models to garnet-clinopyroxene geothermometry greatly improves temperature estimates, achieving uncertainties of ±50 °C, compared with uncertainties of ±250 °C using previous models assuming all Fe as Fe2+ or calculating Fe3+ by charge conservation. Differences in the ferric iron contents, as calculated using the machine learning models, of clinopyroxenes that did or did not experience hydrogen diffusion during their crystallization from basaltic magma support a redox-driven mechanism for hydrogen diffusion in clinopyroxene.

Improving the reliability of Fe- and S-XANES measurements in silicate glasses: Correcting beam damage and identifying Fe-oxide nanolites in hydrous and anhydrous melt inclusions

The redox state of silicate melts influences crystallization, element partitioning, and degassing behavior. Synchrotron-based micro-X-ray absorption near edge structure (μXANES) spectroscopy has emerged as a powerful tool for determining redox conditions through the direct measurement of speciation of multivalent elements such as iron and sulfur in silicate glasses. In particular, the high spatial resolution afforded by synchrotron μXANES makes it one of the few techniques available for determining redox conditions in melt inclusions, which can provide insights into pre-eruptive melt properties. However, the small size of melt inclusions, the deep penetration of X-rays, and irradiation-induced beam damage make μXANES measurements in melt inclusions challenging. Here we present data showing the rapid occurrence of Fe- and S-μXANES beam damage in experimental glasses, mid-ocean ridge basalt glasses, and olivine-hosted melt inclusions from the southern Cascade arc and Kīlauea Volcano and develop approaches to recognize and correct for beam damage through repeated rapid analyses. By applying a time-dependent correction to a series of rapid measurements (~82 s/scan) of Fe-μXANES pre-edge centroid positions, irradiation-induced photo-oxidation (Fe2+ to Fe3+) can be corrected back to undamaged initial Fe3+/ΣFe even in damage-susceptible hydrous glasses. Using this beam damage correction technique, hydrous basaltic melt inclusions from the southern Cascades have Fe3+/ΣFe values that are ~0.036 lower (corresponding to -0.5 log units lower oxygen fugacity) than would have been indicated by standard Fe-μXANES measurements. Repeated, rapid analyses (150–300 s/scan) were also used to identify S-μXANES beam damage (photo-reduction of S6+ to S4+), which was corrected with a peak fitting method to restore initial S6+/ΣS. We observe that S-μXANES beam damage can occur rapidly even in low-H2O mid-ocean ridge basaltic glasses and melt inclusions from Kīlauea Volcano, which are otherwise stable during even prolonged Fe-μXANES analyses. By mitigating and correcting for sulfur photo-reduction, we conclude that some mid-ocean ridge basaltic glasses contain 0.08–0.09 S6+/ΣS, which is more sulfate than might be expected based on the reduced oxidation state of these glasses (near the fayalite-magnetite-quartz oxygen buffer). Using beam damage identification and correction techniques, the valence states of iron and sulfur can be accurately measured even in beam damage-susceptible glasses and melt inclusions. Finally, using Fe-μXANES, we demonstrate the presence of Fe-oxide nanolites within otherwise glassy, naturally quenched melt inclusions, which can complicate determination of iron valence state in affected glasses.

Multiresponsive Spin Crossover Driven by Rotation of Tetraphenylborate Anion in an Iron(III) Complex

Dynamic molecular materials, which involve mechanical motions in crystals, usually exhibit tunable properties related to conformational polymorphs. Herein, we describe a solvent-free molecular crys...

Processes and Conditions of the Origin for Fe3+-Bearing Magnesiowüstite under Lithospheric Mantle Pressures and Temperatures

An experimental study, implicated in the revealing of the conditions for the origin for Fe3+-bearing magnesiowüstite in the lithospheric mantle, was performed using Mössbauer spectroscopy of pre-synthesized samples. Experiments were carried out using a multi-anvil high-pressure split-sphere apparatus at 6.3–7.5 GPa, in the range of 1100–1650 °C in carbonate-metal, carbonate–oxide-metal, carbonate-oxide, carbide-oxide and carbonate–metal- sulphur systems. In three experimental series, oxygen fugacity gradient in the samples was created, which enabled the study of the processes of magnesiowüstite formation under oxidizing and reducing conditions (ΔlogfO2 (FMQ) values from −1 to −5). It was established that Fe3+-bearing magnesiowüstite can form both in assemblage with oxidized phases, such as carbonate or with reduced ones—metal, carbides, sulphides, graphite and diamond. According to the Mössbauer spectroscopy, the composition of synthesized magnesiowüstite varied within a range of Fe3+/ΣFe values from 0 to 0.3, with IV and VI coordination of Fe3+ depending on P, T, fO2, x-parameters. It was established that Fe3+-bearing magnesiowüstite formation processes under upper mantle P,T-conditions include redox reactions, with magnesiowüstite being (1) reductant or (2) product of interaction, (3) crystallization processes of magnesiowüstite from an oxidized melt, where magnesiowüstite acts as a sink for ferric iron and (4) iron disproportionation.

Oxidation States of Fe in Constituent Minerals of a Spinel Lherzolite Xenolith from the Tariat Depression, Mongolia: The Significance of Fe3+ in Olivine

The oxidation states of Fe within olivine, orthopyroxene, clinopyroxene, and spinel in a spinel lherzolite xenolith from the Tariat Depression, Mongolia were investigated using 57Fe Mössbauer spectroscopy to evaluate the redox condition of the upper mantle from which the Tariat spinel lherzolite xenolith was derived. The purity of separated minerals for the Mössbauer spectroscopic analysis was examined using X-ray powder diffraction, Raman spectroscopy, and transmission electron microscopy. Average Fo and Fe contents of olivine at the core part of the xenolith are 89.9(4) mol % and 0.195(3) atoms per formula unit, respectively. The Fe3+/ΣFe values of the olivine, orthopyroxene, clinopyroxene, and spinel, determined by Mössbauer spectroscopic analysis, are 0.027(2), 0.15(1), 0.26(3), and 0.34(5), respectively. The Mössbauer spectrum of olivine consists of two doublets assigned to Fe2+ at the octahedral sites and one doublet, with I.S. of 0.40(2) mm/s and Q.S. of 0.69(3) mm/s assigned to Fe3+ at the octahedral site. Since the Tariat spinel lherzolite xenolith in this study shows no evidence of metasomatism or thermal alteration, the existence of a small amount of Fe3+ in olivine and the fairly high Fe3+ contents of clinopyroxene, orthopyroxene, and spinel imply that the upper mantle under the Tariat area was in a rather oxidized condition.

A re-assessment of the oxidation state of iron in MORB glasses

The oxidation state of Fe, Fe/3+ΣFe (where ΣFe=Fe2++Fe3+), in glass samples of mid-ocean ridge basalt (MORB), from a wide range of localities, was determined by XANES spectroscopy to be 0.10(2) (n=42). This value is lower than that reported previously by XANES, 0.16(1) (n=103), but consistent with the most recent value determined by redox titrations, 0.11(2) (n=104), all for similar sets of samples. We attribute the anomalously high XANES value of 0.16 to a calibration error resulting from the interpretation of Mössbauer spectra and the resulting Fe/3+ΣFe values of the standards. Our alternative interpretation removes the problem of resolving Fe/3+ΣFe values <∼0.1 in basaltic glasses, produces isomer shift and quadrupole splitting values for Fe3+ that are independent of Fe3+/ΣFe (as is the case for Fe2+), and gives Fe/3+ΣFe values that are consistent with the thermodynamically expected dependence on oxygen fugacity (fO2). Fe/3+Fe2+ is related to fO2 for our synthetic MORB composition by the temperature independent expression ΔQFM=4log⁡(Fe3+/Fe2+)+4.23(5), where ΔQFM is the fO2 in log units relative to the quartz–fayalite–magnetite buffer. The average fO2 of natural MORB was estimated to be QFM+0.1.

Iron K-edge X-ray absorption near-edge structure spectroscopy of aerodynamically levitated silicate melts and glasses

The local structure about Fe(II) and Fe(III) in silicate melts was investigated in-situ using iron K-edge X-ray absorption near-edge structure (XANES) spectroscopy. An aerodynamic levitation and laser heating system was used to allow access to high temperatures without contamination, and was combined with a chamber and gas mixing system to allow the iron oxidation state, Fe3+/ΣFe, to be varied by systematic control of the atmospheric oxygen fugacity. Eleven alkali-free, mostly iron-rich and depolymerized base compositions were chosen for the experiments, including pure oxide FeO, olivines (Fe,Mg)2SiO4, pyroxenes (Fe,Mg)SiO3, calcic FeO-CaSiO3, and a calcium aluminosilicate composition, where total iron content is denoted by FeO for convenience. Melt temperatures varied between 1410 and 2160K and oxygen fugacities between FMQ – 2.3(3) to FMQ+9.1(3) log units (uncertainties in parentheses) relative to the fayalite-magnetite-β-quartz (FMQ) buffer. Remarkably, XANES pre-edge peak areas imply mean Fe-O coordination numbers (nFeO) close to 5 in all cases, with only a slight tendency toward higher values in the most iron rich melts, suggesting an intermediate role for both Fe(II) and Fe(III) in terms of network formation. End member coordination numbers for Fe(II)-O and Fe(III)-O are estimated to be similar, having means (and standard deviations) of 5.0(2) and 4.9(1), respectively. As such, the preference for ferric iron to occupy lower coordination sites than ferrous is weak, in contrast to published behavior in some alkali-rich systems, which may explain the larger published viscosity variations with Fe3+/ΣFe in alkali-, compared to alkaline earth-iron silicates. Temperature effects on nFeO are inferred to be small based on the melt data, as well as by comparison to glasses formed on quenching. Positive shifts of the pre-edge peak centroids observed in many cases on quenching are attributed to rapid oxidation enabled by the stirring of the melt droplets by the levitation gas jet. Fe3+/ΣFe values were estimated from XANES pre-edge peaks using published calibrations and compared to semi-empirical thermodynamic model calculations and Mössbauer measurements on quench products. Whilst showing positive correlation, the comparisons highlight the limitations involved in applying XANES calibrations and models for Fe3+/ΣFe derived from measurements on glasses, to high temperature basic melts. Fe3+/ΣFe varies from approximately zero up to about 65% in the high temperature melts and 75% in the glasses.

In situ measurement of ferric iron in lunar glass beads using Fe-XAS

Through use of a new X-ray Absorption Spectroscopy (XAS) calibration for Fe3+ analysis in silicate glasses, the first direct measurements of ferric iron in natural lunar picritic glasses are presented. Lunar glass beads from the Apollo sample collection contain up to 60.0% Fe3+. No correlation with melt chemical properties, such as Mg# or weight % TiO2, or physical properties, such as bead diameter, was observed. Fe3+/ΣFe is negatively correlated with NBO/T. These elevated Fe3+/ΣFe values reflect eruption and post-eruption oxidation due to magmatic degassing of H or OH. Glass beads observed to be zoned to lower Fe3+/ΣFe rims may represent a subsequent reduction in the lunar vacuum prior to cooling through the glass transition temperature.▪

Ferric saponite and serpentine in the nakhlite martian meteorites

Transmission electron microscopy and Fe-K X-ray absorption spectroscopy have been used to determine structure and ferric content of the secondary phase mineral assemblages in the nakhlite martian meteorites, NWA 998, Lafayette, Nakhla, GV, Y 000593, Y 000749, MIL 03346, NWA 817, and NWA 5790. The secondary phases are a rapidly cooled, metastable assemblage that has preserved Mg# and Ca fractionation related to distance from the fluid source, for most of the nakhlites, though one, NWA 5790, appears not to have experienced a fluid pathway. All nine nakhlite samples have also been analysed with scanning electron microscopy, electron probe micro analysis, Bright Field high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction. By measuring the energy position of the Fe-K XANES 1s→3d pre-edge transition centroid we calculate the ferric content of the minerals within the nakhlite meteorites. The crystalline phyllosilicates and amorphous silicate of the hydrothermal deposits filling the olivine fractures are found to have variable Fe3+/ΣFe values ranging from 0.4 to 0.9. In Lafayette, the central silicate gel parts of the veins are more ferric than the phyllosilicates around it, showing that the fluid became increasingly oxidised. The mesostasis of Lafayette and NWA 817 also have phyllosilicate, which have a higher ferric content than the olivine fracture deposits, with Fe3+/ΣFe values of up to 1.0. Further study, via TEM analyses, reveal the Lafayette and NWA 817 olivine phyllosilicates to have 2:1 T–O–T lattice structure with a the d001-spacing of 0.96nm, whereas the Lafayette mesostasis phyllosilicates have 1:1 T–O structure with d001-spacings of 0.7nm. Based on our analyses, the phyllosilicate found within the Lafayette olivine fractures is trioctahedral ferric saponite (Ca0.2K0.1)∑0.3(Mg2.6Fe2+1.3Fe3+1.7Mn0.1)∑5.7[(Si6.7AlIV0.9Fe3+0.4)∑8.0O20](OH)4·nH2O, and that found in the mesostasis fractures is an Fe-serpentine (Ca0.1Mg0.7Fe3+1.0AlVI0.4)∑2.2[Si2O5]OH4, with a ferric gel of similar composition in Lafayette and found as fracture fills throughout the other nakhlites.

Iron oxidation state in garnet from a subduction setting: a micro-XANES and electron microprobe (“flank method”) comparative study

The variation of the Fe-redox state across a strongly zoned garnet crystal occurring in a FeTi-oxide metagabbro from a subduction setting has been investigated comparing micro-XANES and electron microprobe (“flank method”) results. This sample represents an ideal monitor of the redox processes associated with devolatilization reactions occurring in the subducting slab and likely involved in the oxidation of the mantle wedge, as well as a challenge for the analytical methods. Since in the studied garnet the Fe3+/ΣFe variations are small and occur on a micrometric scale, we exploited the X-ray microprobe available at the ESRF ID22 beamline to reach a spot size of 1.7 μm × 5.3 μm. Our study represents a first attempt to measure the Fe3+/ΣFe variations across a single crystal of garnet, ca. 400 μm in diameter, using the micro-XANES technique. The absolute Fe3+/ΣFe values obtained with micro-XANES and “flank method” are perfectly comparable, ranging from a maximum of about 0.1 in the garnet core to a minimum of 0.0 in the garnet rim.

Differences in Fe-redox for asbestiform and nonasbestiform amphiboles from the former vermiculite mine, near Libby, Montana, U.S.A.

We obtained oriented Fe-XANES spectra on three amphibole bundles of fiber (i.e., asbestiform morphology), two single-crystal fragments (i.e., nonasbestiform morphology), and one aggregate of thousands of smaller particles (termed herein a “puffball”) by use of a spindle stage mounted on beamline X26A, National Synchrotron Light Source, Brookhaven National Lab. These particles of differing morphology were obtained from a single rock sample collected at the former vermiculite mine near Libby, Montana, U.S.A. The XANES spectra vary as a function of orientation for the singlecrystal fragments and bundles of fibers and were the same regardless of orientation for the puffball. Differences in the spectra for the single-crystal fragments and bundles of fibers indicate variations in Fe oxidation states between the two differing morphologies. Comparison to a calibration line based on amphibole standards allowed determination of Fe 3+ /ΣFe values of 0.69 for the bundles of fibers and 0.60 for the single-crystal fragments, while the puffball had a Fe 3+ /ΣFe value of 0.66. These values agree well with our earlier Mossbauer data on a bulk sample of this same material, with Fe 3+

Microanalysis of the iron oxidation state in (Mg,Fe)O and application to the study of microscale processes

We report application of the flank method using the electron microprobe to a suite of twelve (Mg,Fe)O samples with composition 2–47 wt% Fe and Fe3+/ΣFe = 1 to 11%, where Fe3+/ΣFe was determined independently using Mossbauer spectroscopy on the same grains used for the flank method measurements. A calibration curve of the form Fe2+ = A + B × (ΣFe)2 + C × (Lβ/Lα) was fit to the data and gave excellent agreement between Fe3+/ΣFe calculated from the flank method and Fe3+/ΣFe determined using Mossbauer spectroscopy. We found the method to be sufficiently sensitive to determine meaningful variations in Fe3+/ΣFe for geophysically relevant compositions of (Mg,Fe)O (<25 wt% Fe), and calibration parameters remained constant within experimental uncertainty over the course of the entire study (20 months). Flank method measurements on an inhomogeneous sample of synthetic (Mg,Fe)O showed evidence of diffusion processes resulting from rupture of the capsule during the high-pressure experiment and the possibility to measure Lβ/Lα variations with a spatial resolution of a few microns. We detected the presence of exsolved magnesioferrite in a suite of (Mg,Fe)O single crystals using transmission electron microscopy and Mossbauer spectroscopy. Flank method measurements on the same suite of single crystals showed enhanced Fe3+/ΣFe values, consistent with the presence of magnesioferrite even though the grains were too small to be resolved by conventional electron microprobe measurements.

Crystal chemistry of Fe32+Cr2Si3O12-Fe32+ Fe23+Si3O12 garnet solid solutions and related spinels

Garnet solid solutions have been synthesized across the skiagite–Fe-knorringite (Fe 3 2+ Fe 2 3+ Si 3 O 12 –Fe 3 Cr 2 Si 3 O 12 ) binary join. Such compositions reflect a simple Cr-Fe 3+ exchange on the octahedral sites, with Fe 2+ occupying the adjacent dodecahedral sites. Solid solution is complete across the join and the correct stoichiometry was verified by Mossbauer spectroscopy. A symmetric fit to the molar volume data yields 119.77(2) cm 3 /mol [unit-cell parameter a o =11.6736(7) A] for the Fe-knorringite end-member and a small negative excess volume, W V = −0.76(15) cm 3 /mol. Combining this result with literature data reveals that binary joins involving octahedral site substitutions exhibit significantly different behavior than those where substitution occurs on the dodecahedral sites. In the former case, W V is usually negative, whereas the latter joins have positive deviations from ideal behavior. Therefore, we conclude that the garnet structure responds in a fundamentally different way when accommodating different cations on the dodecahedral or octahedral sites. Mossbauer spectra of skiagite–Fe-knorringite garnets do not exhibit any significant asymmetry in the [8] Fe 2+ doublet. Fe 3+ /ΣFe values determined at room temperature and 80 K confirm the general applicability of the recoil-free fraction correction factors reported by Woodland and Ross (1994) for mixed-valence garnets. Coexisting spinels in some samples are either binary Fe 3 O 4 –Fe 2 SiO 4 or Fe 3 O 4 –FeCr 2 O 4 solid solutions. Very little mutual solubility is apparent suggesting a significant solvus may exist between the silicate and Cr-bearing spinel series.

Nanometer-scale measurements of Fe3+/ΣFe by electron energy-loss spectroscopy: A cautionary note

The effects of electron-beam damage on the Fe 3+ /ΣFe (total iron) ratio were measured by electron energy-loss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra were acquired from crushed and ion-beam-thinned cronstedtite. For fluences below 1 × 10 4 e/A 2 , the Fe 3+ /ΣFe values from crushed grains range between 0.43 and 0.49, consistent with undamaged material. These measurements were acquired from flakes 180 to 1000 A thick. With increase in fluence, samples 3+ /ΣFe values >0.5. The critical fluence for radiation damage by 100 kV electrons as defined by Fe 3+ /ΣFe 4 e/A 2 . The absorbed dose to the speciman during acquisition of a typical EELS spectrum is large, with values around 2.2 × 10 10 Gy (J/kg), equivalent to the deposition of 620 eV/A 3 . Cooling to liquid N 2 temperature did not significantly slow the damage process. Ion-beam thinning produces an amorphous layer on crystal surfaces. Spectra from the thinnest regions, which are amorphous, exhibit Fe 3+ /ΣFe >0.7. With increase in sample thickness, the Fe 3+ /ΣFe values decrease to a minimum, consistent with data from the undamaged material. The increase of Fe 3+ /ΣFe with respect to electron-beam irradiation is likely caused by loss of H. At low fluences, the loss of H is negligible, thus allowing consistent Fe 3+ /ΣFe values to be measured. The cronstedtite study illustrates the care required when using EELS to measure Fe 3+ /ΣFe values. Similar damage effects occur for a range of high-valence and mixed-oxidation state metals in minerals. EELS is the only spectroscopic method that can be used routinely to determine mixed-valence ratios at the nanometer scale, but care is required when measuring these data. Consideration needs to be given to the incident beam current, fluence, fluence rate, and sample thickness.

A Mössbauer study of the oxidation state of Fe in silicate melts

Fe 3 + /ΣFe ratios were determined from Mossbauer spectra recorded for a series of 17 anorthite-diopside eutectic glasses containing 1 wt% 5 7 Fe 2 O 3 quenched from melts equilibrated over a range of oxygen fugacities from f O 2 ∼ 10 5 bars (Fe 3 + /ΣFe = 1) to 10 - 1 3 bars (Fe 3 + /ΣFe = 0) at 1682 K. Fe 3 + /Fe 2 + was found to be proportional to f O 2 to the power of 0.245 ′ 0.004, in excellent agreement with the theoretical value of 0.25 expected from the stoichiometry of the reaction Fe 2 + O + 0.25 O 2 = Fe 3 + O 1 . 5 . The uncertainty in the Fe 3 + /ΣFe ratios determined by Mossbauer spectroscopy was estimated as ′ 0.01 (1σ) from the fit of the data to the theoretical expression, which is significantly less than that quoted for previous measurements on silicate glasses; this results from fitting the spectra of a large number of systematically varying samples, which allows many of the ambiguities associated with the fitting procedure to be minimized. Fe 3 + /ΣFe ratios were then determined for samples of the anorthite-diopside eutectic composition equilibrated at selected values of f O 2 , to which up to 30 wt% Fe 2 O 3 had been added. Fe 3 + /ΣFe was found to vary with ΣFe (or FeO T ), but both the 1 wt% and high FeO T data could be satisfactorily fit assuming the ideal stoichiometry (i.e., Fe 3 + /Fe 2 + f O 2 1 / 4 ) by the inclusion of a Margules term describing Fe 2 + -Fe 3 + interactions. The large negative value of this term indicates a tendency toward the formation of Fe 2 + -Fe 3 + complexes in the melt. The resulting expression, using the ideal exponent of 0.25. gave a fit to 289 Fe 3 + /ΣFe values, compiled from various literature sources, of similar quality as previous empirical models which found an exponent of ∼0.20. Although the empirical models reproduce Fe 3 + /ΣFe values of glasses with high FeO T reasonably well, they describe the data for 1 wt% FeO T poorly. The non-ideal values of the exponent describing the dependence of Fe 3 + /ΣFe on f O 2 at high FeO T are an artifact of models that did not include a term explicitly to describe the Fe 2 + -Fe 3 + interactions. An alternative model in which Fe in the silicate melt is described in terms of three species, Fe 2 + O, Fe 3 + O 1 . 5 , and the non-integral valence species Fe 2 . 6 + O 1 , 3 , was also tested with promising results. However, at present there is no model that fits the data within the assessed accuracy of the experimental measurements.

Oxidation state of iron in hydrous mantle phases: implications for subduction and mantle oxygen fugacity

Samples of olivine, wadsleyite, ringwoodite and majorite with Mg/(Mg+Fe)=0.85–0.90 were synthesised at high pressure and temperature using a multianvil press under nominally anhydrous conditions as well as hydrous conditions with varying conditions of oxygen fugacity, and examined ex situ using Mössbauer spectroscopy to determine Fe 3+ /ΣFe, and either Fourier Transform Infrared Spectroscopy or Secondary Ion Mass Spectrometry to determine H concentration. Under oxidising conditions, the relative concentration of Fe 3+ increases with water content in olivine, wadselyite and ringwoodite. Majorite does not incorporate significant water, but shows high Fe 3+ /ΣFe values when synthesised under oxidising conditions in the presence of water. Fe 3+ /ΣFe is strongly influenced by oxygen fugacity for all phases studied, and the mechanism of hydration is likely coupled to Fe 3+ incorporation in the crystal structure. Enhanced Fe 3+ /ΣFe in mantle-derived olivine and orthopyroxene has a negligible effect on oxygen fugacities estimated using olivine–orthopyroxene–garnet oxybarometers, while increased Fe 3+ /ΣFe in garnet due to dehydrogenation could lead to an overestimation of oxygen fugacity in exhumed samples. Subduction zones are likely oxidised relative to the rest of the mantle, and hence represent regions where not only water is concentrated, but also Fe 3+ .

Oxidation state of iron in hydrous mantle phases: implications for subduction and mantle oxygen fugacity

Samples of olivine, wadsleyite, ringwoodite and majorite with Mg/(Mg+Fe)=0.85–0.90 were synthesised at high pressure and temperature using a multianvil press under nominally anhydrous conditions as well as hydrous conditions with varying conditions of oxygen fugacity, and examined ex situ using Mössbauer spectroscopy to determine Fe 3+/ΣFe, and either Fourier Transform Infrared Spectroscopy or Secondary Ion Mass Spectrometry to determine H concentration. Under oxidising conditions, the relative concentration of Fe 3+ increases with water content in olivine, wadselyite and ringwoodite. Majorite does not incorporate significant water, but shows high Fe 3+/ΣFe values when synthesised under oxidising conditions in the presence of water. Fe 3+/ΣFe is strongly influenced by oxygen fugacity for all phases studied, and the mechanism of hydration is likely coupled to Fe 3+ incorporation in the crystal structure. Enhanced Fe 3+/ΣFe in mantle-derived olivine and orthopyroxene has a negligible effect on oxygen fugacities estimated using olivine–orthopyroxene–garnet oxybarometers, while increased Fe 3+/ΣFe in garnet due to dehydrogenation could lead to an overestimation of oxygen fugacity in exhumed samples. Subduction zones are likely oxidised relative to the rest of the mantle, and hence represent regions where not only water is concentrated, but also Fe 3+.

XANES calibrations for the oxidation state of iron in a silicate glass

Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities at 1409 °C. The Fe3+/ΣFe ratios were determined previously by 57Fe Mössbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mössbauer results as a reference, various methods for extracting Fe3+/ΣFe ratios from XANES spectra were investigated. The energy of the 1s → 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s → 4s and crest (1s → 4p) transitions. The Fe3+/ΣFe ratios determined from linear combinations of end-member spectra (Fe3+/ΣFe ~0 and ~1) were found to deviate significantly from the Mössbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in Fe2+ or Fe3+ coordination with the Fe3+/ΣFe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe3+/ΣFe values may be obtained from XANES spectra, with an accuracy comparable to Mössbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.