Fetot Ratio Research Articles

Sorption and surface complexation modeling of 137Cs on Fe(II)-montmorillonite clay mineral relevant to nuclear waste disposal

Abstract Bentonite, a natural clay, is a promising candidate to be used as a buffer/backfill material in the geological disposal systems for management of high level waste (HLW). Fe released due to corrosion of canister/overpack may result in alteration of Na-montmorillonite (Na-Mt), present in bentonite, to Fe-montmorillonite (Fe-Mt) in a span of several years after the emplacement of vitrified HLW in deep geological repositories. For realistic performance assessment, it is essential to understand the sorption behavior of altered Mt with regard to the radionuclides present in HLW. Cs is one of the high-yield (137Cs t1/2 = 30.1 y, 6%) and long-lived (135Cs t1/2 = 2 × 106 y) fission products in spent fuels. The objective of present study is to understand the effect of various parameters, viz., time (0–48 h), pH (3.0–9.0), ionic strength (0.001–1 M) [Cs(I)] (10−10–10−3 M) and Fe dissolution on sorption behavior of Cs(I) on Fe(II)-Mt through batch sorption experiments. Fe(II)-Mt was synthesized by reducing Fe(III)-Mt using ascorbic acid as reducing agent in N2 atmosphere. The near-constancy in Cs(I) sorption on Fe(II)-Mt with pH (≥4), and decrease with increasing ionic strength, illustrate the ion exchange as dominant mode of Cs(I) sorption. Further, Cs(I) sorption isotherm on Fe(II)-Mt is found to be linear. The estimation of dissolved iron in the supernatant of Fe(II)-Mt suspensions demonstrated that dissolved Fe decreased with increase in pH and increased with increase in ionic strength. Moreover, the Fe2+/Fetotal ratio determined in all experiments was close to unity, thereby depicting that Fe(II) did not oxidize to Fe(III), except when suspension pH was ≥ 5.5. For comparison, Cs(I) sorption was also studied on Na(I)-Mt and compared with that on Fe(III)-Mt. Surface complexation modeling of Cs(I) sorption on the three clay minerals, viz., Na-Mt, Fe(II)-Mt and Fe(III)-Mt, has been successfully carried out.

Gamma (60Co) irradiation and thermal effect on redox behavior of interlayer iron in montmorillonite

Fe2+-montmorillonite (Mt) is expected to be one of the major altered clay minerals formed in the engineered barrier system proposed for deep geological disposal of high-level waste (HLW). The high energy radiations imparted by the radionuclides present in HLW as well as the temperature can impact the redox behavior of interlayer iron in clay. Therefore, we have studied the effect of γ- radiation (60Co) on the stability of Fe2+-Mt dispersed in water. Irradiation at a dose rate of 0.6 kGy/h for 18 h resulted in the oxidation of about 14% interlayer Fe2+ as indicated by Fe2+/Fetotal ratio. On further irradiation, these oxidized Fe3+ are reduced back to Fe2+. Such behavior was observed in inert, aerated, and N2O-saturated conditions.The effect of temperature was found to be similar to that of γ-irradiation, particularly on the reduction of interlayer Fe3+ from Fe3+-Mt. When the temperature was increased from 50 to 300 °C, > 90% reduction (Fe3+ → Fe2+) was observed, which is ascribed to the self-reduction mechanism. This sample is similar to that of Fe2+-Mt as evident from XRD, FTIR, TG and magnetic data anaysis. The increase in Fe2+/Fetotal ratio of the heat-treated samples enabled the solid-state reaction with o-phenanthroline (L) by in-situ complexation to form organo-modified Mt. (d001 = 18.5 Å) at room temperature (RT, 27 °C). Furthermore, the presence of Fe2+ in this heat-treated Mt. is evident from their reduction of aqueous CrVI. For instance, 55% of CrVI reduction occurred with Fe3+-Mt@200. On the other hand, the gradual oxidation of interlayer Fe2+ occurred when Fe2+-Mt was heated up to 300 °C. Only about 12% oxidation was observed when heated to 200 °C for 4 h. This is probably due to an equilibrium between oxidation and their self-reduction. In this heat treated sample, about 73% of CrVI reduction occurred, which is comparable to 80% reduction observed with freshly prepared Fe2+-Mt@RT. Thus, the redox behavior of interlayer iron was significantly influenced by γ-irradiation as well as thermal treatment. We believe the observations made here have significant implications on the plausible existence of clay minerals in the geothermal and extraterrestrial environment.

Magma water content of Pico Volcano (Azores Islands, Portugal): a clinopyroxene perspective

Clinopyroxenes from the Pico Volcano (Pico Island, Azores Archipelago) have been used as a proxy to define the water content of primitive magmas and the volcanological history of the erupted rocks. This very young volcano (53 ± 5 ka) is at a primordial stage of its evolution in comparison with the other volcanoes of the Azores. Clinopyroxenes from Pico Volcano underwent important dehydration processes and after annealing experiments under H2 gas flux, a pre-eruptive H2O content between 93 and 182 ppm was recovered. A moderately high cooling rate for the cpx-host lavas expressed by the clinopyroxene closure temperature (Tc = 755–928 °C ± 20 °C) correlates with the dehydration, suggesting that this process may have occurred during magma ponding at the Moho Transition Zone (17.3–17.7 km) and/or after the eruption. By applying an IVAl-dependent partition coefficient to the measured H amount in clinopyroxene, the pre-eruptive water content of the parental magma was calculated to vary between 0.71 and 1.20 (average of 1.0) wt%. Clinopyroxene geobarometry performed by combining X-ray diffraction with mineral chemistry points to a general crystallisation from the mantle lithosphere (~ 8–9 kbar) to the oceanic mantle/crust boundary (~ 4–5 kbar). The similar major and trace chemistry, water content and Fe3+/Fetot ratio of clinopyroxene, suggest similar conditions of oxygen fugacity, water content and fractional crystallisation of the magma from which clinopyroxene cores crystallised during the Pico Volcano central eruptions from 40 ka to historical times.

The role of excess oxygen for modeling high-Mn, low-Ca garnets in metapelites from the northern Central Metasedimentary Belt of the Grenville Province, Ontario, Canada

The Bancroft terrane and the associated Central Metasedimentary Belt boundary thrust zone represent the northern part of the Central Metasedimentary Belt (CMB) of the Canadian Grenville Province. Only a few direct pressure and temperature calculations based on phase equilibrium petrology methods exist in the central Bancroft terrane, and this study applies thermodynamic approaches such as garnet isopleth geothermobarometry to fill this gap and investigate the metamorphic history of the northern CMB. Four metapelitic rock samples were collected in the vicinity of the enigmatic Bancroft shear zone, which approximates the border between the Bancroft terrane and the Elzevir terrane to the south. Garnet isopleths for these samples only intersect if a certain amount of excess oxygen is added to the bulk rock composition corresponding to a Fe3+/Fetot ratio of 0.33–0.38. The northernmost sample records metamorphic peak conditions of approximately 1 GPa and 780 °C, whereas the southernmost sample, which is located in the Elzevir terrane, records a peak metamorphic pressure of approximately 0.9 GPa at a temperature of 520 °C. The latter result contradicts previous pressure estimates of the region and the proposed metamorphic field gradient but is based on a poorly constrained sample in terms of thermodynamic modeling. Hence, we conclude that the metamorphic field gradient in the northern CMB conceals two different P–T trajectories. Such a scenario is commonly observed in crustal thickening models and suggests that the cold upper plate (Elzevir terrane) was thrust over the warm lower plate (Bancroft terrane) in a northwesterly direction.

A multi-proxy investigation of mantle oxygen fugacity along the Reykjanes Ridge

Mantle oxygen fugacity (fO2) governs the physico-chemical evolution of the Earth, however current estimates from commonly used basalt redox proxies are often in disagreement. In this study we compare three different potential basalt fO2 proxies: Fe3+/Fetot, V/Sc and V isotopes, determined on the same submarine lavas from a 700 km section of the Reykjanes Ridge, near Iceland. These samples provide a valuable test of the sensitivities of fO2 proxies to basalt petrogenesis, as they formed at different melting conditions and from a mantle that towards Iceland exhibits increasing long-term enrichment of incompatible elements. New trace element data were determined for 63 basalts with known Fe3+/Fetot. A subset of 19 lavas, covering the geographical spread of the ridge transect, was selected for vanadium isotope analyses.Vanadium is a multi-valence element whose isotopic fractionation is theoretically susceptible to redox conditions. Yet, the δ51VAA composition of basaltic glasses along the Reykjanes Ridge covers only a narrow range (δ51VAA = −1.09 to −0.86‰; 1SD = 0.02–0.09) and does not co-vary with fractionation-corrected Fe3+/Fetot (0.134–0.151; 1SD = 0.005) or V/Sc (6.6–8.5; 1SD = 0.1-1.3) ratios. However, on a global scale, basaltic δ51VAA may be controlled by the extent of melting. The V/Sc compositions of primitive (MgO > 7.5 wt%) basalts show no systematic change along the entire length of the Reykjanes Ridge. Typical peridotite melting models in which source Fe3+/Fetot is constant at 5% and that account for the increased mantle potential temperature nearer the plume center and the fO2 dependent partitioning of V, can reproduce the V/Sc data. However, while these melting models predict that basalt Fe3+/Fetot ratios should decrease with increasing mantle potential temperature towards Iceland, fractionation-corrected Fe3+/Fetot of Reykjanes Ridge lavas remain nearly constant over the ridge length. This discrepancy is explained by source heterogeneity, where an oxidized mantle pyroxenite component contributes to melting with increasing proximity to Iceland.Comparison of observed and modeled Fe3+/Fetot indicate that source variation in fO2 is present under the Reykjanes Ridge, with higher Fe3+/Fetot closer to Iceland. This source variability in fO2 cannot be resolved by V isotopes and redox-sensitive trace element ratios, which instead appear to record magmatic processes.

Effect of controlled crystallization on polaronic transport in phosphate‐based glass‐ceramics

AbstractThe effect of induced crystallization on changes in electrical transport of two types of glass‐ceramics, pure polaron conductive 40Fe2O3‐60P2O5 (F40) (mol%) and predominantly polaronic 5Li2O‐5ZnO‐40P2O5‐50WO3 (Li‐50W) (mol%) was investigated. F40 glass‐ceramics produced at low heat‐treatment temperatures contain single‐phase Fe3(P2O7)2 whereas at higher temperatures two more phases Fe4(P2O7)3 and Fe(PO3)3 are formed. Structural modifications strongly depend on the crystallization temperature and time. The appearance of crystalline phases studied by Mössbauer spectroscopy exhibits changes in Fe2+/Fetot ratio in crystalline/glassy phases. The detailed analysis of different iron sites allows their correlation with changes in electrical conductivity as crystallization progresses. Depending on the course of crystallization, the contribution of each phase to the overall conductivity is determined by the frequency dependence of Zʺ(ω) and Mʺ(ω). DC conductivity shows a sharp decrease as Fe3(P2O7)2 phase appears and consequently glass matrix remains impoverished in Fe2+‐Fe3+ pairs. In the multiphase systems prepared at higher crystallization temperatures, the overall electrical conductivity increases although the continuous grain boundaries along different crystalline grains play a limiting factor. In contrast, the slight conductivity change in Li‐50W glass‐ceramics upon crystallization is a result of remaining W5+‐W6+ pairs in the residual glassy phase. Independence of electrical transport on Li+ ions confirms predominantly polaronic transport in Li‐50W glass‐ceramics.

Meteoroid atmospheric entry investigated with plasma flow experiments: Petrography and geochemistry of the recovered material

Melting experiments attempting to reproduce some of the processes affecting asteroidal and cometary material during atmospheric entry have been performed in a high enthalpy facility. For the first time with the specific experimental setup, the resulting material has been recovered, studied, and compared with natural analogues, focusing on the thermal and redox reactions triggered by interaction between the melt and the atmospheric gases under high temperature and low pressure conditions. Experimental conditions were tested across a range of parameters, such as heat flux, experiment duration, and pressure, using two types of sample holders materials, namely cork and graphite. A basalt served as asteroidal analog and to calibrate the experiments, before melting a H5 ordinary chondrite meteorite. The quenched melt recovered after the experiments has been analyzed by μ-XRF, EDS-SEM, EMPA, LA-ICP-MS, and XANES spectroscopy.The glass formed from the basalt is fairly homogeneous, depleted in highly volatile elements (e.g., Na, K), relatively enriched in moderately siderophile elements (e.g., Co, Ni), and has reached an equilibrium redox state with a lower Fe3+/Fetot ratio than that in the starting material. Spherical objects, enriched in SiO2, Na2O and K2O, were observed, inferring condensation from the vaporized material. Despite instantaneous quenching, the melt formed from the ordinary chondrite shows extensive crystallization of mostly olivine and magnetite, the latter indicative of oxygen fugacity compatible with presence of both Fe2+ and Fe3+. Similar features have been observed in natural meteorite fusion crusts and in micrometeorites, implying that, at least in terms of maximum temperature reached and chemical reactions, the experiments have successfully reproduced the conditions likely encountered by extraterrestrial material following atmospheric entry.

Crystal structure model development for soil clay minerals – I. Hydroxy-interlayered smectite (HIS) synthesized from bentonite. A multi-analytical study

During the acidic weathering of soils, aluminum (Al) is dissolved from primary silicates such as feldspars or chlorites and intercalated in layer silicates. Part of this Al polymerizes in the interlayer of former smectites or vermiculites and the resulting phases are called hydroxy-interlayered minerals. These phases are known for a long time but are still insufficiently understood in terms of chemical composition and crystallographical structure. In the present study a sample set consisting of a continuous series of hydroxy-interlayered smectite samples with different degrees of hydroxy-interlayering ranging from 0 to 96% mol/mol smectite/HIS was prepared from a bentonite sample by adding different amounts of a 0.1 mol/L AlCl3 solution and different reaction times. The products were investigated by a multi-analytical approach using X-ray diffraction, Infrared spectroscopy, X-ray Fluorescence spectrometry, Fe2+:Fetotal determination, Nuclear Magnetic Resonance spectroscopy, Simultaneous Thermal Analysis coupled with mass spectrometry and Cation Exchange Capacity to provide an in-depth characterization. X-ray diffraction indicated that the turbostratic disorder typical for smectite did not change during hydroxy-interlayering. Nuclear Magnetic Resonance spectroscopy proved the exclusive six-fold coordination of interlayer Al and based on chemical calculations the average size and charge of the interlayer structures were characterized. Hydroxy-interlayers evolved in-situ in the smectite interlayers by polymerization. These interlayer “polymers” were small oligomers, the average size of which increased from monomers to pentamers with increasing degree of hydroxy-interlayering while the average net charge of each oligomer decreased. An average structural formula of each interstratified smectite-hydroxy-interlayered smectite was calculated from X-ray Fluorescence data including Fe2+:Fetotal ratios. If all exchangeable cations are attributed to smectite layers and all hydroxy-interlayers are attributed to hydroxy-interlayered smectite layers, the degree of hydroxy-interlayering ranged from 0 to 96% mol/mol smectite/HIS. The analysis provides an improved structural model of hydroxy-interlayered smectite suitable for Rietveld refinement which can be used to quantify the amounts of hydroxy-interlayered smectite in natural samples.

Accurate Fe3+/Fetot ratio from XAS spectra at the Fe K-edge

A new approach to extract accurate information on the Fe oxidation state has been tested on three different Fe-rich amphiboles annealed at 194–520 °C. The method is based on the analysis of the pre-edge structure of the X-ray absorption spectra at the Fe-K edge. The pre-edge band is deconvolved into four Gaussian components where the integrated intensities of the pairs of lower- and higher-energy bands are related to the ferrous to ferric ion ratio. These Fe3+/Fetot ratios have been compared with those obtained with independent techniques (X-ray diffraction and Mossbauer spectroscopy) on the same samples. The experimental data allowed establishing a calibration curve that is in good agreement with a recent statistical analysis based on spectroscopic data for the same type of minerals. The method could be extended to the analysis of other complex systems containing both Fe3+ and Fe2+ in octahedral coordination or to study the dynamics of iron under non-ambient conditions.

Weathering processes and crystal chemistry of Ni-bearing minerals in saprock horizons of New Caledonia ophiolite

This paper proposes a detailed study of the formation of the main Ni ore that is today mined in the saprock horizon of New Caledonia ophiolite. More specifically, this study aims at, i) characterizing the nature and the crystal chemistry of the main Ni-bearing phases, and ii) better understanding the weathering processes that lead to such nickel enrichment in these horizons. The study focuses on a boulder (~20 cm in diameter) sampled in the saprock facies of the Koniambo massif, reflecting the early stages of peridotite bedrock weathering. This boulder is explored with a large panel of techniques, from the centimeter to the nanometer scale, including micro-imaging and X-ray absorption spectroscopy at the Ni and Fe K-edges. A characterization along the cross section reveals important heterogeneities in textures, and a large concentration of nickel in the outer part through an intense and connected Ni-phyllosilicate network that develop at the expense of the primary lizardite network. The inner part is preserved from the weathering by supergene fluids. Primary lizardite initially contains about 0.4 wt% NiO, similarly to surrounding olivine and pyroxene. In the outer part, the main Ni-bearing phases are identified as polygonal serpentine (up to ~5 wt% NiO) and two successive generations of talc-like minerals that contain ~20 to 36 wt% NiO. In these two minerals, nickel is very likely to form nano-clusters in the octahedral layers. The Fe3+/Fetotal ratios in serpentine minerals increases regularly from ~0.5 in primary lizardite, to ~0.95 in polygonal serpentine, traducing increasing local oxygen fugacity with increasing weathering degree.From molecular modeling, we propose that the large excess of water systematically observed for these Ni-rich minerals can be explained by nanometer sized crystals, which potentially require many additional hydrogen bonds (i.e., mainly silanol groups) to passivate crystal edges and preserve electro-neutrality. On the other hand, the main driving force leading to important nickel accumulation in the saprock horizon is likely to be the strong pH gradient present at all scales in the regolith profile, including at the boulder scale, which have major effects on the hydroxide solubility. As the pH decreases in the phyllosilicate network, we observe, i) the weathering and oxidation of the primary lizardite network, ii) the formation of a newly formed lizardite, iii) the formation of Ni-rich polygonal serpentine, iv) the formation of successive generations of Ni-rich talc-like minerals.

High‐P metamorphism of rodingites during serpentinite dehydration (Cerro del Almirez, Southern Spain): Implications for the redox state in subduction zones

AbstractThe transition between antigorite‐serpentinite and chlorite‐harzburgite at Cerro del Almirez (Betic Cordillera, Southern Spain) exceptionally marks in the field the front of antigorite breakdown at high pressure (~16–19 kbar) and temperature (~650°C) in a paleosubducted serpentinite. These ultramafic lithologies enclose three types of metarodingite boudins of variable size surrounded by metasomatic reaction rims. Type 1 Grandite‐metarodingite (garnet+chlorite+diopside+titanite±magnetite±ilmenite) mainly crops out in the antigorite‐serpentinite domain and has three generations of garnet. Grossular‐rich Grt‐1 formed during rodingitization at the seafloor (<2 kbar, ~150–325°C, ~FMQ buffer). During subduction, the alternating growth of Grt‐2b (richer in andradite and pyralspite components than Grt‐1) and Grt‐3 (very rich in andradite component) reflects the change from internally buffered metamorphic conditions (>10 kbar, ~350–650°C, ~FMQ buffer) to influx events of oxidizing fluids (fO2 ~HM buffer) released by brucite breakdown in the host antigorite‐serpentinite. Type 2 Epidote‐metarodingite (epidote+diopside+titanite±garnet) derives from Type 1 and is the most abundant metarodingite type enclosed in dehydrated chlorite‐harzburgite. Type 2 formed by increasing μSiO2 (from −884 to −860 kJ/mol) and decreasing μCaO (from −708 to −725 kJ/mol) triggered by the flux of high amounts of oxidizing fluids during the high‐P antigorite breakdown in serpentinite. The growth of Grt‐4, with low‐grandite and high‐pyralspite components, in Type 2 metarodingite accounts for progressive reequilibration of garnet with changing intensive variables. Type 3 Pyralspite‐metarodingite (garnet+epidote+amphibole+chlorite±diopside+rutile) crops out in the chlorite‐harzburgite domain and formed at peak metamorphic conditions (16–19 kbar, 660–684°C) from Type 2 metarodingite. This transformation caused the growth of a last generation of pyralspite‐rich garnet (Grt‐5) and the recrystallization of diopside into tremolitic amphibole at decreasing fO2 and μCaO (from −726 to −735 kJ/mol) and increasing μMgO (from −630 to −626 kJ/mol) due to chemical mixing between the metarodingite and the reaction rims. The different bulk Fe3+/FeTotal ratios of antigorite‐serpentinite and chlorite‐harzburgite, and of the three metarodingite types, reflect the highly heterogeneous oxidation state of the subducting slab and likely point to the transfer of localized oxidized reservoirs, such as metarodingites, into the deep mantle.

Potassic-magnesio-arfvedsonite, KNa2(MgFe2+Fe3+)5Si8O22(OH)2: mineral description and crystal chemistry

AbstractThe complete mineral description of potassic-magnesio-arfvedsonite, a recently approved (IMA2016-083) new species of the amphibole supergroup is provided using electron microprobe analysis (EMPA), laser ablation inductively coupled plasma mass spectrometry, single-crystal structure refinement, Mössbauer and Raman spectroscopy, as well as measurement of optical and physical properties. The holotype material was found in syenitic and granitic dyke rocks in association with quartz, potassium feldspar and aegirine–augite from the Buhovo–Seslavtsi pluton, Bulgaria. Potassic-magnesio-arfvedsonite is monoclinic C2/m, with unit-cell parameters: a = 9.9804(11), b = 18.0127(19), c = 5.2971(6) Å, β = 104.341(2)° and V = 922.61 Å3. In transmitted plane-polarised light (λ = 590 cm–1), potassic-magnesio-arfvedsonite is pleochroic: X = yellow pale-green, Y = green and Z = dark-violet brown. It is biaxial (–), α = 1.645(2), β = 1.655(2), γ = 1.660(2) and 2Vmeas. = 60° and 2Vcalc. = 70°. The empirical unit formula obtained from EMPA and structure refinement is A(K0.86Na0.0.08)0.94B(Na1.74Ca0.25 Mn2+0.01)2.00C(Mg2.67Fe2+1.42Fe3+0.76Ti0.12Mn2+0.03)5.00TSi8O22W(OH1.58F0.22O0.20)2.00. The Fe3+/Fetot ratio (0.35) is consistent with both the Mössbauer spectra and the single-crystal structure refinement. The 10 strongest X-ray powder reflections [d values (in A°), I, (hkl)] are: 8.519, 80.5, (110); 3.402, 67.3, (131); 3.295, 41.0, (240); 3.173, 65.0, (310); 2.752, 35.6, ($\bar{3}$31); 2.715, 100.0 (151); 2.591, 44.1, (061); 2.542, 73.2, ($\bar{2}$02); 2.348, 38.5, ($\bar{3}$51); 2.174, 42.0, (261). Potassic-magnesio-arfvedsonite is the product of strongly peralkaline and potassic (perpotassic) magma compositions. Trace-element analysis shows that this amphibole did not exert significant control on trace-element distribution in the crystallising peralkaline magma.

Iron release in aqueous environment by fresh volcanic ash from Mount Etna (Italy) and Popocatépetl (Mexico) volcanoes

In this study, we performed leaching experiments for timescales of hours-to-months in deionized water on fresh volcanic ash from Mt. Etna (Italy) and Popocatepetl (Mexico) volcanos to monitor Fe release as a function of ash mineral chemistry and size, with the aim of clarifying Fe release mechanisms and eventually evaluating the impact of volcanic ash on marine and lacustrine environments. To define sample mineralogy and Fe speciation, inclusive characterization was obtained by means of XRF, SEM, XRPD, EELS and Mossbauer spectroscopies. For Etna and Popocatepetl samples, glass proportions were quantified at 73 and 40%, Fe2O3 total contents at 11.6–13.2 and 5.8 wt%, and Fe3+/FeTot ratios at 0.33 and 0.23, respectively. Leaching experiments showed that significant amounts of iron, ~ 30 to 150 and ~ 750 nmol g−1 l−1 for pristine Etna and Popocatepetl samples, respectively, are released within the first 30 min as a function of decreasing particle size (from 1 to 0.125 mm). The Popocatepetl sample showed a very sustained Fe release (up to 10 times Etna samples) all along the first week, with lowest values never below 400 nmol g−1 l−1 and a maximum of 1672 nmol g−1 l−1 recorded after 5 days. This sample, being composed of very small particles (average particle size 0.125 mm) with large surface area, likely accumulated large quantities of Fe-bearing sublimates that quickly dissolved during leaching tests, determining high Fe release and local pH decrease (that contributed to release more Fe from the glass) at short timescale (hours-to-days). The fractional Fe solubility (FeS) was 0.004–0.011 and 0.23% for Etna and Popocatepetl samples, respectively, but no correlation was found between Fe released in solution and either ash Fe content, glass/mineral ratio or mineral assemblage. Results obtained suggest that volcanic ash chemistry, mineralogy and particle size assume a relevant role on Fe release mostly in the medium-to-long timescale, while Fe release in the short timescale is dominated by dissolution of surface sublimates (formed by physicochemical processes occurring within the eruption plume and volcanic cloud) and the effects of such a dissolution on the local pH conditions. For all samples, a moderate to sustained Fe release occurred for leaching times comparable with their residence time within the euphotic zone of marine and lacustrine environments (variable from few minutes to few hours), revealing their possible contribution to increase Fe bioavailability.

Carbonation of mantle peridotite by CO2-rich fluids: the formation of listvenites in the Advocate ophiolite complex (Newfoundland, Canada)

The mantle section of the Advocate ophiolite (Newfoundland, Canada) contains unique outcrops of listvenite (magnesite-quartz), antigorite- and quartz-bearing talc-magnesite rock, and carbonated antigorite-serpentinite. This lithological sequence records the sequential carbonation of serpentinite by CO2-rich hydrothermal fluids. High Cr and Ni contents and preservation of Cr-spinel with a composition similar to that of Atg-serpentinite (molar Mg/Mg + Fe = 0.50–0.65; Cr/Cr + Al = 0.50–0.70), show that the Advocate listvenite and talc-magnesite rocks formed by carbonation of variably serpentinized mantle harzburgite. Replacement of lizardite by magnesite coeval with the breakdown of lizardite to antigorite + brucite and the lack of prograde olivine and magnetite in antigorite serpentinite and talc-magnesite rocks constrain the temperature of carbonation between c. 280 °C and 420 °C. Thermodynamic modelling of carbonation of serpentinite at 300 °C and 0.2–0.5 GPa accounts for the sequence of carbonated rocks in the Advocate complex. Phase relations and petrological observations indicate that the aqueous aSiO2 and aCO2 of the infiltrating CO2-rich fluid were buffered at the Atg-Tlc-Mgs and Qtz-Tlc-Mgs pseudo-invariant points, forming dominantly three-phase rocks by variable extents of carbonation at these pseudo-invariant points. Listvenites formed at large fluid-rock ratio when quartz became saturated in the fluid and precipitated along magnesite grain boundaries and in variably sized tensional veins.The whole rock Fe3+/Fetotal ratio of the Advocate carbonate-bearing sequence decreases with increasing whole rock carbon content, from 0.65–0.80 in brucite-bearing antigorite serpentinite to 0.10–0.30 in talc-magnesite rocks and listvenite. The whole rock iron reduction is associated with an increase in the ferrous iron content of magnesite and the formation of hematite and goethite, indicating a concomitant increase of the fluid oxygen fugacity. The sequence of carbonation reactions is uniquely preserved in three main growth zones characteristic of listvenite magnesite: (i) an inner zone of magnetite-bearing, Fe-poor, Mn-bearing magnesite formed by carbonation of lizardite, brucite and olivine from Atg-serpentinite; (ii) an outer zone of Fe-rich magnesite formed by carbonation of antigorite and in equilibrium with Fe-poor talc; and (iii) an outermost rim of Fe-poor magnesite formed by carbonation of talc.We propose that carbonation of the Advocate serpentinized mantle harzburgite occurred in a supra-subduction upper plate ophiolite by fluxing of slab-derived, CO2-rich fluids channelled along deep faults at the onset of accretion of the forearc basin (c. 300 °C, <0.5 GPa). The rather constant δ18O (11.0–14.4‰ V-SMOW) and relatively low δ13C (−8.9 to −5.0‰ V-PDB) of magnesite throughout the sequence of carbonated rocks in the Advocate complex is consistent with CO2-rich fluids derived from decarbonation or dissolution of organic carbon- and carbonate-bearing meta-sediments, such as those occurring in the underlying Birchy complex — the partially subducted continental margin of Laurentia. Carbonation of serpentinized oceanic or continental mantle lithosphere by reactive percolation of CO2-rich fluids derived from the slab in forearc settings may represent a significant carbon reservoir for the deep carbon cycle.

Dietary magnesium-to-iron intake ratios and risk of impaired fasting glucose in Chinese adults: The prospective Jiangsu Nutrition Study (JIN)

BackgroundStudies have consistently shown that risk of type 2 diabetes (T2D) is positively associated with dietary haem iron intake and inversely associated with dietary magnesium intake in a dose-response way. However, interaction effects of these two clinically important nutrients on T2DM risk in a prospective setting is unknown. ObjectiveTo determine the five-year risk of developing impaired fasting glucose (IFG) associated with dietary magnesium-to-iron intake ratios (Mg/Fe), including Mg/total Fe, Mg/haem-Fe and Mg/non-haem Fe. DesignA cohort study of 1056 participants recruited into the Jiangsu Nutrition Study (JIN) from 2002 to 2007, aged at least 20 years and without known diabetes and IFG at baseline were followed up for five years. Dietary magnesium and iron intake at baseline was assessed by 3-day weighed food records. Fasting plasma glucose was measured both at baseline and follow up. Logistic regression models were performed to determine the associations between quartiles (using bottom quartiles as referent categories) of magnesium to iron (including total Fe, haem-Fe and non-haem Fe) ratio and the risk of IFG (>5.6 mmol/L) adjusted for covariates: age, gender, body mass index (BMI), hypertension, serum ferritin, haemoglobin and family history of diabetes. ResultsThe mean (SD) intake of total Fe and magnesium was 25.0 (9.2) mg/d and 323 (125) mg/d. The incidence of IFG during 5-year follow up was 11.8%. Inverse associations were found between quartiles (Q) of Mg/haem-Fe and the risk of IFG in the fully adjusted model: odds ratios (OR) were 1.00, 0.59 (95%CI 0.35, 0.98), 0.49 (95%CI 0.28, 0.84), and 0.28 (95% CI 0.14, 0.55) (Q4), respectively. Weaker associations were found for quartiles of Mg/total Fe and no association was found between Mg/non-haem Fe and IFG risk. ConclusionsLow Mg/haem-Fe ratio is an independent risk factor for developing IFG in Chinese adults. Future research to determine the added predictive value of assessment of low dietary Mg/haem-Fe ratio beyond current T2D risk models in specific populations is justified.

Modern rather than Mesoarchaean oxidative weathering responsible for the heavy stable Cr isotopic signatures of the 2.95 Ga old Ijzermijn iron formation (South Africa)

Previously reported stable Cr isotopic fractionation in Archaean paleosols and iron formations (IFs) have been interpreted as a signature of oxidative weathering of Cr(III) to Cr(VI) in soils, and delivery of isotopically heavy Cr(VI) into the oceans. One of the oldest reported fingerprints of this process is isotopically heavy Cr preserved in the 2.95 Ga old Ijzermijn IF, Sinqeni Formation of the Mozaan Group (Pongola Supergroup), South Africa and could suggest that atmospheric free oxygen was present ca. 600 million years earlier than the Great Oxidation Event (GOE). However, fractionated stable Cr isotopic signatures have only been found to date in surface outcrop samples of the White Mfolozi Inlier exposed along the White Mfolozi River Gorge. In this study, the latter outcrop was resampled along with two drill cores of the Ijzermijn IF and a drill core of the Scotts Hill IF to represent multiple exposures of Mozaan Group IFs with different states of preservation. A detailed geochemical comparison on bulk samples of different units was undertaken using stable Cr isotopes coupled with trace and major elements. Outcrop iron-rich mudstones (Fe - lutites) show very low LOI [wt] %, and very low Fe(II)/Fetot ratios, and lower Ca and Mg relative to equivalent facies in drill cores, indicating the effects that oxidative recent surface weathering had on Fe/Mn-rich carbonate minerals of the IF. Overall rare earth element and yttrium (REE + Y) mixing models agree well with previous studies, confirming that they were minimally disturbed by weathering and are consistent with a high magnitude of continental solutes delivered in a near–shore depositional environment, with a minor contribution of hydrothermally derived fluids that upwelled into shallower depositional setting. Importantly, all drill core samples of this study revealed δ53/52Cr values within the igneous inventory, despite variable amounts of detrital Cr input that includes nearly detritus-free, chert/jasper-rich units. By contrast, a specific group of Fe-lutite samples near the base of White Mfolozi River outcrop bear fractionated Cr isotopic signatures with δ53/52Cr values up to 0.418‰. These outcrop samples also display unusually high U/Th ratios (max. 12.6) as well as enrichments of other elements (W, Tl, As, MREE) that far exceed that observed in correlative drill core units. These observations together with the lack of Cr isotopic fractionation in drill core samples lead us to propose that the heavy δ53/52Cr values of Fe-lutites from outcrop Ijzermijn IF samples reported here and in a previous study are the product of modern oxidative weathering rather than an indicator for Mesoarchaean oxidative weathering at ca. 2.95 Ga.

Effects of oxalate and persulfate addition to Electrofenton and Electrofenton-Fenton processes for oxidation of Ketoprofen: Determination of reactive species and mass balance analysis

Electrofenton (EF) has been widely used to remove contaminants from aqueous solution, while Electrofenton-fenton (EFF), is a two stage process that employs partial electrolysis, needs further study. In addition, a combination of oxalate and persulfate with these systems (EF-Oxalate, EFF-Oxalate, EF-Persulfate, and EFF-Persulfate) could improve the oxidation capacity. This study assessed the applicability of these systems for degradation of Ketoprofen as an emerging contaminant. Under batch experiments, results showed that the order of oxidation capacity and mineralization is EF-Persulfate > EFF-Persulfate > EF-Oxalate > EFF-Oxalate > EFF > EF. Radical scavenger experiments indicated that OH is the dominant radical species in EF, EFF, EF-Oxalate, and EFF-Oxalate systems. However, both OH and 1O2 participated in oxidation of Ketoprofen in both EF-Persulfate and EFF-Persulfate processes. Fe(II)/Total Fe ratios in both solution and sludge samples decreased as oxidation capacity enhanced. Solution and sludge analyses revealed that the amount of Ketoprofen and oxidation byproducts increased in sludge over time. Carbon mass balance calculations showed that EF-Persulfate leaves the lowest amount of oxidation by-products in the system. BOD5/COD ratio analyses demonstrated that all studied systems improved biodegradability of the raw wastewater. EFF and EF-Persulfate systems had the lowest and highest operational costs, respectively. Competitive maps were developed to compare all six processes by Ketoprofen removal efficiency, mineralization capacity, and operational costs. Results confirmed that addition of oxalate and persulfate to EF and EFF systems provide good solutions to decrease the load and risk posed by Ketoprofen to water systems.

Experimental insight into redox transfer by iron- and sulfur-bearing serpentinite dehydration in subduction zones

Dehydration of antigorite in subduction zones releases a large amount of aqueous fluid and volatile elements, which can potentially oxidize the mantle wedge. The redox capacity of three synthetic serpentinites with variable Fetotal, Fe3+ and S− contents is investigated using XANES spectroscopy at both, Fe and S K-edges. Experiments are performed between 450 and 900 °C, at 2 GPa and fO2∼QFM-2; conditions similar to those encountered in subduction zones.Redox reactions in the synthetic serpentinites, which involve Fe and S can be summarized as follows: 1) the reduction of (S−)-pyrite into (S2−)-pyrrhotite (∼450 °C), with ∼4.4 mg/g of the sulfur degassed most likely as H2S, 2) the consumption of magnetite that reacts with antigorite to form Fe-rich olivine (<500 °C), 3) the reduction of (Fe3+)-antigorite into (Fe2+)-antigorite (∼580 °C), occurring about 100 °C below the temperature of antigorite breakdown, 4) the main (Fe2+)-antigorite breakdown that forms olivine and enstatite (∼675 °C), and 5) the decomposition of minor amounts of (Fe2+/3+)-clinochlore (∼800 °C). The bulk Fe3+/Fetotal ratio is found to decrease with run temperature from ∼0.82–0.97 depending on the hydrous starting material, down to 0.1–0.2 in the high-temperature anhydrous assemblages.The evolution of mineral modes and Fe3+/Fetotal with temperature in our synthetic samples shows similar trends to what has been reported in serpentinite rocks collected, for example, along a metamorphic transect in the western Alps. We show that a large amount of O2-equivalent – up to 10 mol/kg of rock – can be generated at temperature around 450 °C due to the presence of oxides and sulfides such as magnetite and pyrite. Owing to the poor capacity of aqueous fluid to transfer redox conditions, we surmise that this O2-equivalent is “consumed” at the scale of the lithospheric-mantle top which is partially serpentinized and therefore bear strong redox gradients.

The effect of oxygen fugacity and Na/(Na+K) ratio on iron speciation in pantelleritic glasses

Reduction experiments were conducted to study the structural role (redox state and coordination) of iron in peralkaline rhyolitic glasses as a function of oxygen fugacity and alkali atomic ratio ([Na/(Na+K)]=0.0, 0.69 and 1.0). The glasses investigated, analogues of natural pantellerites of the Kenya Rift Valley, have been synthesized at ambient pressure, 1250°C and equilibrated under controlled redox conditions. Oxygen fugacities, ranging from NNO+7 to NNO- 6.8 (relative to Ni/NiO assemblage buffer), were adjusted by different gases and/or gas mixtures (air, O2, CO2, Ar/H2/H2O, Ar/H2) in a horizontal gas-mixing furnace.Iron oxidation state and coordination geometry have been inferred by Fe K-edge X-ray Absorption Near-Edge Structure (XANES) spectroscopy, which suggested the presence of Fe3+ in four-fold coordination and Fe2+ having an average coordination number intermediate between four- and five-fold in all the glasses.A colorimetric method has been employed to determine the Fe2+/Fetot ratios for all the samples. We verified in the samples a continuous increase of divalent iron for more reducing conditions (e.g. Fe2+/Fetot ranging from 0.09 in air to 0.93 in Ar/H2 atmosphere) but a non-linear relation between [Na/(Na+K)] molar ratio and iron redox was observed.The weak dependence of log (Fe3+/Fe2+) on log (fO2/bar) indicates a strong deviation from ideal mixing of iron species in pantelleritic compositions which suggests a different behaviour of alkali dominated peralkaline melts compared to other aluminosilicate melts in terms of fO2 and alkalis effects.

Investigations on FCAM-III (Ca2.38Mg2.09Fe3+10.61Fe2+1.59Al9.33O36): A new homologue of the aenigmatite structure-type in the system CaO-MgO-Fe2O3-Al2O3

In the course of a systematic study of a part of the quaternary system Fe2O3-CaO-Al2O3-MgO (FCAM) the previously unknown compound Ca2.38Mg2.09Fe3+10.61Fe2+1.59Al9.33O36 (FCAM-III) has been synthesized. By analogy with the so-called SFCA series [1–5], our investigation in the system of FCAM shows the existence of a stoichiometric homologous series M14+6nO20+8n, where M = Fe, Ca, Al, Mg and n = 1 or 2.In air, we can prove the formation of coexisting FCAM-III and FCAM-I solid solutions at 1400°C. By increasing the temperature up to 1425°C FCAM-I disappears completely and FCAM-III co-exists with magnesiumferrite and a variety of calcium iron oxides. At 1450°C FCAM-III breaks down to a mixture of FCAM-I again as well as magnesioferrite and melt. Small single-crystals of FCAM-III up to 35µm in size could be retrieved from the 1425°C experiment and were subsequently characterized using electron microprobe analysis and synchroton X-ray single-crystal diffraction. Finally the Fe2+/Fetot ratio was calculated from the total iron content based on the crystal-chemical formula obtained from EMPA measurements and charge balance considerations. FCAM-III or Ca2.38Mg2.09Fe3+10.61Fe2+1.59Al9.33O36 has a triclinic crystal structure (space group P1̅). The basic crystallographic data are: a = 10.223(22) Å, b = 10.316(21) Å, c = 14.203(15) Å, α = 93.473(50)°, β = 107.418(67)°, γ = 109.646(60)°, V = 1323.85(2) ų, Z = 1.Using Schreinemaker's technique to analyze the phase relations in the system Fe2O3-CaO-Al2O3-MgO it was possible to obtain the semi-quantitative stability relations between the participating phases and construct a topologically correct phase sequence as a function of T and fO2. The analysis shows that Ca2Al0.5Fe1.5O5 (C2A0.25F0.75) and CaAl1.5Fe2.5O7 (CA0.75F1.25) with higher calculated Fe2+ contents are preferably formed at lower oxygen fugacity and react to CaAl0.5Fe1.5O4 (CA0.25F0.75) by increasing fO2. Spinel-type magnesium-aluminium-ferrite (Mg,Fe2+)Fe3+1.25Al0.75O4 or (MA0.375F0.625) is the typical phase which occurs at high temperature (1400°C).