The Fe/ΣFe ratios of MORB glasses and their implications for mantle melting     

Antoine Bézos  Eric Humler 《Geochimica et cosmochimica acta》2005,69(3):711-725

The Fe³⁺/ΣFe ratio of 104 MORB glasses from the Pacific, the Atlantic, the Indian, and the Red Sea spreading centers have been determined using wet chemical Fe²⁺ analyses and electron microprobe FeO_total measurements. The data provide a new estimate for the MORB oxygen fugacity (fO₂) of 0.41 ± 0.43 (1sigma, N = 100) log units below the fayalite-magnetite-quartz buffer (FMQ), equivalent to a Fe³⁺/ΣFe = 0.12 ± 0.02 (1sigma, N = 104). This new fO₂ estimate is 0.8 log units more oxidized than the average fO₂ proposed by Christie et al. (1986) (FMQ-1.20 ± 0.44; Fe³⁺/ΣFe = 0.07 ± 0.01; N = 87). This slight difference may be related in part to the 3.5% underestimation of the Fe²⁺ concentration determined by Christie et al. (1986) compared with this study. MORB oxygen fugacity does not display any significant difference between the three main oceanic domains, or between enriched and depleted MORB. Yet, the iron red-ox state ratio shows a broad increase during fractional crystallization. Detailed study of magmatic suites highlights the lack of systematic Fe³⁺/ΣFe ratio fractionation during differentiation. Despite the large variations of inferred partial melting degrees (from 5 to 20%), the present data set does not provide any evidence of Fe³⁺/ΣFe relationships with partial melting proxies such as Na_8.0.Based on the Fe³⁺ systematics during partial melting, it is suggested that the oxidation state of MORB reflects a “buffered mantle melting process” resulting in the apparent compatible behavior of Fe³⁺ during partial melting, and in the relatively constant Fe³⁺/ΣFe ratio irrespective of the extent of melting. This result implies that partial melting processes may be open relative to oxygen. We propose a model where the Fe³⁺/ΣFe ratio in the melt is buffered during partial melting. The MORB Fe₂O₃ systematics can be accounted for by using a fO₂ of FMQ-1 that is equivalent to the average fO₂ reported for abyssal peridotites.  相似文献   

Redox equilibria of iron and silicate melt structure: Implications for olivine/melt element partitioning     

Bjorn O. Mysen 《Geochimica et cosmochimica acta》2006,70(12):3121-3138

Olivine/melt partitioning of ΣFe, Fe²⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, and Ni²⁺ has been determined in the systems CaO-MgO-FeO-Fe₂O₃-SiO₂ (FD) and CaO-MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂ (FDA3) as a function of oxygen fugacity (f_O2) at 0.1 MPa pressure. Total iron oxide content of the starting materials was ∼20 wt%. The f_O2 was to used to control the Fe³⁺/ΣFe (ΣFe: total iron) of the melts. The Fe³⁺/ΣFe and structural roles of Fe²⁺ and Fe³⁺ were determined with ⁵⁷Fe resonant absorption Mössbauer spectroscopy. Changes in melt polymerization, NBO/T, as a function of f_O2 was estimated from the Mössbauer data and existing melt structure information. It varies by ∼100% in melts coexisting with olivine in the FDA3 system and by about 300% in the FD system in the Fe³⁺/ΣFe range of the experiments (0.805-0.092). The partition coefficients ( in olivine/wt% in melt) are systematic functions of f_O2 and, therefore, NBO/T of the melt. There is a -minimum in the FDA3 system at NBO/T-values corresponding to intermediate Fe³⁺/ΣFe (0.34-0.44). In the Al-free system, FD, where the NBO/T values of melts range between ∼1 and ∼2.9, the partition coefficients are positively correlated with NBO/T (decreasing Fe³⁺/ΣFe). These relationships are explained by consideration of solution behavior in the melts governed by Qⁿ-unit distribution and structural changes of the divalent cations in the melts (coordination number, complexing with Fe³⁺, and distortion of the polyhedra).  相似文献   

Mössbauer and ELNES spectroscopy of (Mg,Fe)(Si,Al)O3 perovskite: a highly oxidised component of the lower mantle     

S. Lauterbach  C. A. McCammon  P. van Aken  F. Langenhorst  F. Seifert 《Contributions to Mineralogy and Petrology》2000,138(1):17-26

(Mg,Fe)(Si,Al)O₃ perovskite samples with varying Fe and Al concentration were synthesised at high pressure and temperature at varying conditions of oxygen fugacity using a multianvil press, and were characterised using ex?situ X-ray diffraction, electron microprobe, Mössbauer spectroscopy and analytical transmission electron microscopy. The Fe³⁺/ΣFe ratio was determined from Mössbauer spectra recorded at 293 and 80?K, and shows a nearly linear dependence of Fe³⁺/ΣFe with Al composition of (Mg,Fe)(Si,Al)O₃ perovskite. The Fe³⁺/ΣFe values were obtained for selected samples of (Mg,Fe)(Si,Al)O₃ perovskite using electron energy-loss near-edge structure (ELNES) spectroscopy, and are in excellent agreement with Mössbauer data, demonstrating that Fe³⁺/ΣFe can be determined with a spatial resolution on the order of nm. Oxygen concentrations were determined by combining bulk chemical data with Fe³⁺/ΣFe data determined by Mössbauer spectroscopy, and show a significant concentration of oxygen vacancies in (Mg,Fe)(Si,Al)O₃ perovskite.  相似文献   

Sulfur K-edge XANES analysis of natural and synthetic basaltic glasses: Implications for S speciation and S content as function of oxygen fugacity   总被引：9，自引：0，他引：9  

Pedro J. Jugo  Max Wilke 《Geochimica et cosmochimica acta》2010,74(20):5926-7631

XANES analyses at the sulfur K-edge were used to determine the oxidation state of S species in natural and synthetic basaltic glasses and to constrain the fO₂ conditions for the transition from sulfide (S²⁻) to sulfate (S⁶⁺) in silicate melts. XANES spectra of basaltic samples from the Galapagos spreading center, the Juan de Fuca ridge and the Lau Basin showed a dominant broad peak at 2476.8 eV, similar to the spectra obtained from synthetic sulfide-saturated basalts and pyrrhotite. An additional sharp peak at 2469.8 eV, similar to that of crystalline sulfides, was present in synthetic glasses quenched from hydrous melts but absent in anhydrous glasses and may indicate differences in sulfide species with hydration or presence of minute sulfide inclusions exsolved during quenching. The XANES spectra of a basalt from the 1991 eruption of Mount Pinatubo, Philippines, and absarokitic basalts from the Cascades Range, Oregon, USA, showed a sharp peak at 2482.8 eV, characteristic of synthetic sulfate-saturated basaltic glasses and crystalline sulfate-bearing minerals such as hauyne. Basaltic samples from the Lamont Seamount, the early submarine phase of Kilauea volcano and the Loihi Seamount showed unequivocal evidence of the coexistence of S²⁻ and S⁶⁺ species, emphasizing the relevance of S⁶⁺ to these systems. XANES spectra of basaltic glasses synthesized in internally-heated pressure vessels and equilibrated at fO₂ ranging from FMQ − 1.4 to FMQ + 2.7 showed systematic changes in the features related to S²⁻ and S⁶⁺ with changes in fO₂. No significant features related to sulfite (S⁴⁺) species were observed. These results were used to construct a function that allows estimates of S⁶⁺/ΣS from XANES data. Comparison of S⁶⁺/ΣS data obtained by S Kα shifts measured with electron probe microanalysis (EPMA), S⁶⁺/ΣS obtained from XANES spectra, and theoretical considerations show that data obtained from EPMA measurements underestimate S⁶⁺/ΣS in samples that are sulfate-dominated (most likely because of photo-reduction effects during analysis) whereas S⁶⁺/ΣS from XANES provide a close match to the expected theoretical values. The XANES-derived relationship for S⁶⁺/ΣS as a function of fO₂ indicates that the transition from S²⁻ to S⁶⁻ with increasing fO₂ occurs over a narrower interval than what is predicted by the EPMA-derived relationship. The implications for natural systems is that small variation of fO₂ above FMQ + 1 will have a large effect on S behavior in basaltic systems, in particular regarding the amount of S that can be transported by basaltic melts before sulfide saturation can occur.  相似文献   

Quantification of the ferric/ferrous iron ratio in silicates by scanning transmission X-ray microscopy at the Fe L2,3 edges     

Franck Bourdelle  Karim Benzerara  Olivier Beyssac  Julie Cosmidis  Daniel R. Neuville  Gordon E. Brown Jr.  Erwan Paineau 《Contributions to Mineralogy and Petrology》2013,166(2):423-434

Estimation of Fe³⁺/ΣFe ratios in materials at the submicrometre scale has been a long-standing challenge in the Earth and environmental sciences because of the usefulness of this ratio in estimating redox conditions as well as for geothermometry. To date, few quantitative methods with submicrometric resolution have been developed for this purpose, and most of them have used electron energy-loss spectroscopy carried out in the ultra-high vacuum environment of a transmission electron microscope (TEM). Scanning transmission X-ray microscopy (STXM) is a relatively new technique complementary to TEM and is increasingly being used in the Earth sciences. Here, we detail an analytical procedure to quantify the Fe³⁺/ΣFe ratio in silicates using Fe L_2,3-edge X-ray absorption near edge structure (XANES) spectra obtained by STXM, and we discuss its advantages and limitations. Two different methods for retrieving Fe³⁺/ΣFe ratios from XANES spectra are calibrated using reference samples with known Fe³⁺ content by independent approaches. The first method uses the intensity ratio of the two major peaks at the L₃-edge. This method allows mapping of Fe³⁺/ΣFe ratios at a spatial scale better than 50 nm by the acquisition of 5 images only. The second method employs a 2-eV-wide integration window centred on the L₂ maximum for Fe³⁺, which is compared to the total integral intensity of the Fe L₂-edge. These two approaches are applied to metapelites from the Glarus massif (Switzerland), containing micrometre-sized chlorite and illite grains and prepared as ultrathin foils by focused ion beam milling. Nanometre-scale mapping of iron redox in these samples is presented and shows evidence of compositional zonation. The existence of such zonation has crucial implications for geothermometry and illustrates the importance of being able to measure Fe³⁺/ΣFe ratios at the submicrometre scale in geological samples.  相似文献   

Redox state of iron in peralkaline rhyolitic glass/melt: X-ray absorption micro-spectroscopy experiments at high temperature     

N. Métrich  J. Susini  F. Farges  D. Massare  S. Lequien 《Chemical Geology》2006,231(4):350-363

Assessing the ferric-ferrous ratio in magmas prior to eruption remains a challenging task. X-ray absorption near-edge structure (μXANES) spectra were collected at the iron K-edge in water-rich peralkaline silicic melt/glass inclusions trapped in quartz. These experiments were carried out between 800 and 20 °C. The chemical environment of iron was also determined in the naturally quenched samples (glass inclusions and matrix glass) and in the peralkaline rhyolitic reference glasses, with variable [Fe³⁺ / ∑Fe] ratios.In the reference glasses, both the intensity of the pre-peaks (Fe²⁺, Fe³⁺) and site geometry of iron change as the oxidation state increases. Fourfold-coordinated Fe³⁺ prevails in highly oxidised peralkaline silicic glasses, using alkalis for charge balance. The position of the pre-edge centroid of the 1s-3d transition correlates with the Fe³⁺ / ΣFe ratios that allowed calibration of the redox state of iron of our natural samples.At high temperatures, Fe²⁺ dominates in the pre-edge structure of melt inclusions. Upon cooling down to 20 °C, the intensity of the Fe³⁺ peak increases, the centroid position of the pre-edge features shifts by nearly 0.5 eV and the main edge moves slightly towards higher energies. The slower the cooling rate, the higher the ferric iron contribution. Iterative μXANES experiments performed on the same samples show that the process is reversible. However, this apparent oxidation of iron upon cooling is an artefact of changes in Fe coordination. It implies that the [Fe³⁺ / ΣFe] ratio of glassy samples, measured at 20 °C, may be overestimated by a factor > 1.7, and that this ratio cannot be reliably retrieved by probing naturally cooled glass inclusions, and most silicate glasses. High temperature μXANES experiments led first to an assessment of the ferric-ferrous ratio in the water-rich peralkaline melt in pre-eruptive magmatic conditions and second to the determination of the corresponding oxygen fugacity at 740 °C.  相似文献   

The structural behavior of ferric and ferrous iron in aluminosilicate glass near meta-aluminosilicate joins     

Bjorn O. Mysen 《Geochimica et cosmochimica acta》2006,70(9):2337-2353

Iron-57 resonant absorption Mössbauer spectroscopy was used to describe the redox relations and structural roles of Fe³⁺ and Fe²⁺ in meta-aluminosilicate glasses. Melts were formed at 1500 °C in equilibrium with air and quenched to glass in liquid H₂O with quenching rates exceeding 200 °C/s. The aluminosilicate compositions were NaAlSi₂O₆, Ca_0.5AlSi₂O₆, and Mg_0.5AlSi₂O₆. Iron oxide was added in the form of Fe₂O₃, NaFeO₂, CaFe₂O₄, and MgFe₂O₄ with total iron oxide content in the range ∼0.9 to ∼5.6 mol% as Fe₂O₃. The Mössbauer spectra, which were deconvoluted by assuming Gaussian distributions of the hyperfine field, are consistent with one absorption doublet of Fe²⁺ and one of Fe³⁺. From the area ratios of the Fe²⁺ and Fe³⁺ absorption doublets, with corrections for differences in recoil-fractions of Fe³⁺ and Fe²⁺, the Fe³⁺/ΣFe is positively correlated with increasing total iron content and with decreasing ionization potential of the alkali and alkaline earth cation. There is a distribution of hyperfine parameters from the Mössbauer spectra of these glasses. The maximum in the isomer shift distribution function of Fe³⁺, δ_Fe³⁺, ranges from about 0.25 to 0.49 mm/s (at 298 K relative to Fe metal) with the quadrupole splitting maximum, Δ_Fe³⁺, ranging from ∼1.2 to ∼1.6 mm/s. Both δ_Fe³⁺ and δ_Fe²⁺ are negatively correlated with total iron oxide content and Fe³⁺/ΣFe. The dominant oxygen coordination number Fe³⁺ changes from 4 to 6 with decreasing Fe³⁺/ΣFe. The distortion of the Fe³⁺-O polyhedra of the quenched melts (glasses) decreases as the Fe³⁺/ΣFe increases. These polyhedra do, however, coexist with lesser proportions of polyhedra with different oxygen coordination numbers. The δ_Fe²⁺ and Δ_Fe²⁺ distribution maxima at 298 K range from ∼0.95 to 1.15 mm/s and 1.9 to 2.0 mm/s, respectively, and decrease with increasing Fe³⁺/ΣFe. We suggest that these hyperfine parameter values for the most part are more consistent with Fe²⁺ in a range of coordination states from 4- to 6-fold. The lower δ_Fe²⁺-values for the most oxidized melts are consistent with a larger proportion of Fe²⁺ in 4-fold coordination compared with more reduced glasses and melts.  相似文献   

Divergent T–ƒO<Subscript>2</Subscript> paths during crystallisation of H<Subscript>2</Subscript>O-rich and H<Subscript>2</Subscript>O-poor magmas as recorded by Ce and U in zircon,with implications for TitaniQ and TitaniZ geothermometry     

Robert?R.?LoucksEmail authorView author&#;s OrcID profile  Marco?L.?Fiorentini  Bruce?D.?Rohrlach 《Contributions to Mineralogy and Petrology》2018,173(12):104

During solidification of magma chambers as systems closed to chemical exchange with environs, the residual siliceous melt may follow a trend of rising, constant, or decreasing oxidation state, relative to reference buffers such as nickel?+?nickel oxide (NNO) or fayalite?+?magnetite?+?quartz. Titanomagnetite–hemoilmenite thermometry and oxybarometry on quenched volcanic suites yield temperature versus oxygen fugacity arrays of varied positive and negative slopes, the validity of which has been disputed for several years. We resolve the controversy by introducing a new recorder of magmatic redox evolution employing temperature- and redox-sensitive trace-element abundances in zircon. The zircon/melt partition coefficients of cerium and uranium vary oppositely in response to variation of magma redox state, but vary in tandem as temperature varies. Plots of U/Pr versus Ce⁴⁺/Ce³⁺ in zircon provide a robust test for change in oxidation state of the melt during zircon crystallisation from cooling magma, and the plots discriminate thermally induced from redox-induced variation of Ce⁴⁺/Ce³⁺ in zircon. Temperature-dependent lattice strain causes Ce⁴⁺/Ce³⁺ in zircon to increase strongly as zircon crystallises from cooling magma at constant Ce⁴⁺/Ce³⁺ ratio in the melt. We examine 19 zircon populations from igneous complexes in varied tectonic settings. Variation of zircon Ce⁴⁺/Ce³⁺ due to minor variation in melt oxidation state during crystallisation is resolvable in 11 cases but very subordinate to temperature dependence. In many zircon populations described in published literature, there is no resolvable change in redox state of the melt during tenfold variation of Ce⁴⁺/Ce³⁺ in zircons. Varied magmatic redox trends indicated by different slopes on plots of zircon U/Pr versus Ce⁴⁺/Ce³⁺ are corroborated by Fe–Ti-oxide-based T–?O₂ trends of correspondingly varied slopes. Zircon and Fe–Ti-oxide compositions agree that exceptionally, H₂O-rich arc magmas tend to follow a trend of rising oxidation state of the melt during late stages of fluid-saturated magmatic differentiation at upper-crustal pressures. We suggest that H₂ and/or SO₃ and/or Fe²⁺ loss from the melt to segregating fluid is largely responsible. Conversely, zircon and Fe–Ti-oxide compositions agree in indicating that H₂O-poor magmas tend to follow a T–?O₂ trend of decreasing oxidation state of the melt during late stages of magmatic differentiation at upper-crustal pressures, because the precipitating mineral assemblage has higher Fe³⁺/Fe²⁺ than coexisting rhyolitic melt. We present new evidence showing that the Fe–Ti-oxide oxybarometer calibration by Ghiorso and Evans (Am J Sci 308(9):957–1039, 2008) retrieves experimentally imposed values of ?O₂ in laboratory syntheses of Fe–Ti-oxide pairs to a precision of ±?0.2 log unit, over a large experimental temperature range, without systematic bias up to at least log ?O₂?≈?NNO?+?4.4. Their titanomagnetite–hemoilmenite geothermometer calibration has large systematic errors in application to Ti-poor oxides that precipitate from very oxidised magmas. A key outcome is validation of Fe–Ti-oxide-based values of melt TiO₂ activity for use in Ti-in-zircon thermometry and Ti-in-quartz thermobarometry.  相似文献   

The influence of pressure and composition on the viscosity of andesitic melts     

Christian Liebske  Harald Behrens  Rebecca A. Lange 《Geochimica et cosmochimica acta》2003,67(3):473-485

The effect of pressure and composition on the viscosity of both anhydrous and hydrous andesitic melts was studied in the viscosity range of 10⁸ to 10^11.5 Pa · s using parallel plate viscometry. The pressure dependence of the viscosity of three synthetic, iron-free liquids (andesite analogs) containing 0.0, 1.06, and 1.96 wt.% H₂O, respectively, was measured from 100 to 300 MPa using a high-P-T viscometer. These results, combined with those from Richet et al. (1996), indicate that viscosities of anhydrous andesitic melts are independent of pressure, whereas viscosities of hydrous melts slightly increase with increasing pressure. This trend is consistent with an increased degree of depolymerization in the hydrous melts. Compositional effects on the viscosity were studied by comparing iron-free and iron-bearing compositions with similar degrees of depolymerization. During experiments at atmospheric and at elevated pressures (100 to 300 MPa), the viscosity of iron-bearing anhydrous melts preequilibrated in air continuously increased, and the samples became paramagnetic. Analysis of these samples by transmission electron microscopy showed a homogeneous distribution of crystals (probably magnetite) with sizes in the range of 10 to 50 nm. No significant difference in the volume fractions of crystals was found in samples after annealing for 170 to 830 min at temperatures ranging from 970 to 1122 K. An iron-bearing andesite containing 1.88 wt.% H₂O, which was synthesized at intrinsic fO₂ conditions in an internally heated pressure vessel, showed a similar viscosity behavior as the anhydrous melts. The continuous increase in viscosity at a constant temperature is attributed to changes of the melt structure due to exsolution of iron-rich phases. By extrapolating the time evolution of viscosity down to the time at which the run temperature was reached, for both the anhydrous (at 1055 K) and the hydrous (at 860 K) iron-bearing andesite, the viscosity is 0.7 log units lower than predicted by the model of Richet et al. (1996). This may be explained by differences in structural properties of Fe²⁺ and Fe³⁺ and their substitutes Mg²⁺, Ca²⁺, and Al³⁺, which were used in the analogue composition.The effect of iron redox state on the viscosity of anhydrous, synthetic andesite melts was studied at ambient pressure using a dilatometer. Reduced iron-bearing samples were produced by annealing melts in graphite crucibles in an Ar/CO atmosphere for different run times. In contrast to the oxidized sample, no variation of viscosity with time and no exsolution of iron oxide phases was observed for the most reduced glasses. This indicates that trivalent iron promotes the exsolution of iron oxide in supercooled melts. With decreasing Fe³⁺/ΣFe ratio from 0.58 to 0.34, the viscosity decreases by ∼1.6 log units in the investigated temperature range between 964 and 1098 K. A more reduced glass with Fe³⁺/ΣFe = 0.21 showed no additional decrease in viscosity. Our conclusion from these results is that the viscosity of natural melts may be largely overestimated when using data obtained from samples synthesized in air.  相似文献   

Crystal chemistry of ferric iron in (Mg,Fe)(Si,Al)O<Subscript>3</Subscript> majorite with implications for the transition zone     

C. A. McCammon  N. L. Ross 《Physics and Chemistry of Minerals》2003,30(4):206-216

Fifteen samples of (Mg,Fe)SiO₃ majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O₃ majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe³⁺ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe²⁺–Fe³⁺ charge transfer, where band intensity increases with increasing Fe³⁺ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe³⁺ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO₃ majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe³⁺ concentration of (Mg,Fe)(Si,Al)O₃ majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O₃ perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.  相似文献   

Influence of oxygen fugacity and temperature on the redox state of iron in natural silicic aluminosilicate melts     

M. V. Volovetskii  O. A. Lukanin  V. S. Rusakov  A. A. Kargal’tsev 《Geochemistry International》2012,50(4):330-343

The influence of oxygen fugacity (fO₂) and temperature on the valence and structural state of iron was experimentally studied in glasses quenched from natural aluminosilicate melts of granite and pantellerite compositions exposed to various T-fO₂ conditions (1100–1420°C and 10^?12–10^?0.68 bar) at a total pressure of 1 atm. The quenched glasses were investigated by Mössbauer spectroscopy. It was shown that the effect of oxygen fugacity on the redox state of iron at 1320–1420°C can be described by the equation log(Fe³⁺/Fe²⁺) = k log(fO₂) + q, where k and q are constants depending on melt composition and temperature. The Fe³⁺/Fe²⁺ ratio decreases with decreasing fO₂ (T = const) and increasing temperature (fO₂ = const). The structural state of Fe³⁺ depends on the degree of iron oxidation. With increasing Fe³⁺/Fe²⁺ ≥ 1, the dominant coordination of Fe³⁺ changes from octahedral to tetrahedral. Ferrous iron ions occur in octahedral (and/or five-coordinated) sites independent of Fe³⁺/Fe²⁺.  相似文献   

Redox evolution of differentiating hydrous basaltic magmas recorded by zircon and apatites in mafic cumulates: The case of the Malayer Plutonic Complex,Western Iran     

《Chemie der Erde / Geochemistry》2023,83(1):125946

Mafic-ultramafic cumulates can provide records of basaltic magma chambers' conditions and processes, which are often difficult to determine in areas dominated by crustal-derived felsic intrusions, such as the Malayer Plutonic Complex (MPC), Western Iran. New U-Pb zircon ages for mafic cumulates in the MPC confirm the presence of isolated magma chambers of contrasting compositions during Middle Jurassic. Mafic cumulates found in seven separate zones across the MPC vary from olivine gabbro to anorthosite. While the mineralogical, textural, and geochemical lines of evidence recorded in mafic cumulates indicate pH₂O controls on the liquidus phases, the estimated oxygen fugacity (logfO₂) using zircon and apatite chemistry suggests a smoothly rising redox state during the fractionation process, consistent with the trend expected for late-stages differentiation of hydrous arc magmas. This trend is further confirmed by sulfur speciation in apatites determined from microbeam sulfur K-edge X-ray absorption near edge structure (μ-XANES) spectra (S⁶⁺/∑S = 0.93–0.98 ~ FMQ + 2 to 0.99 ~ FMQ + 3, where ∑S = S⁶⁺+S⁴⁺+S²⁻). The low S content and increasing redox state of the fractionating basaltic melts most likely resulted from preferential removal of sulfur en-route to the magma chambers along with effective assimilation of oxidizing crustal components. The reduced condition in the early basaltic melt is also evidenced by the presence of pyrite and magnetite inclusions in olivines in mafic cumulates. The shift in the prevailing fO₂ from sulfide-saturated to sulfate-bearing recorded by MPC mafic cumulates, similar to that in other magmatic arcs, is accompanied by changes in the differentiation path from transitional tholeiitic to calc-alkaline.  相似文献   

Oxidised phase relations of a primitive basalt from Grenada,Lesser Antilles     

C.?C.?StamperEmail author  E.?Melekhova  J.?D.?Blundy  R.?J.?Arculus  M.?C.?S.?Humphreys  R.?A.?Brooker 《Contributions to Mineralogy and Petrology》2014,167(1):954

A series of liquidus determinations is reported for a primitive arc basalt (15.4 wt % MgO, 45.5 wt % SiO₂) from Grenada, Lesser Antilles, at anhydrous, H₂O-undersaturated and H₂O-saturated conditions in the pressure range 1 atm to 1.7 GPa. \(\hbox{Fe}^{3+}/\Upsigma\hbox{Fe}\) of high-pressure experimental glasses as measured by μXANES ranges from 0.44 to 0.86, corresponding to oxygen fugacities (fO₂) between 3.2 and 7.8 log units above the nickel–nickel oxide redox buffer (NNO). 1-atm experiments conducted from NNO ? 2.5 to + 3.8 show that increasing fO₂ mainly increases the forsterite content (Fo) of olivine and has little effect on phase relations. The crystallisation sequence at lower crustal pressures for all water contents is forsteritic olivine + Cr-rich spinel followed by clinopyroxene. The anhydrous liquidus is depressed by 100 and 120 °C in the presence of 2.9 and 3.8 wt % H₂O, respectively. H₂O-undersaturated experiments at NNO + 3.2 to + 4.5 produce olivine of equivalent composition to the most primitive olivine phenocrysts in Grenadan picrites (Fo_91.4). We conclude that direct mantle melts originating beneath Grenada could be as oxidised as ~NNO + 3, consistent with the uppermost estimates from olivine–spinel oxybarometry of high Mg basalts. μXANES analyses of olivine-bearing experimental glasses are used to develop a semi-empirical oxybarometer based on the value of \({{K}_{D}}_{\rm ol-melt}^{\rm Fe-Mg}\) when all Fe is assumed to be in the Fe²⁺ state (\({K}_{D}^{{\rm Fe}_T}\)). The oxybarometer is tested on an independent data set and is able to reproduce experimental fO₂ to ≤1.2 log units. Experiments also show that the geochemically and petrographically distinct M- and C-series lavas on the island can be produced from hydrous melting of a common picritic source. Low pressures expand the olivine stability field at the expense of clinopyroxene, enriching an evolving melt in CaO and forcing differentiation to take place along a C-series liquid line of descent. Higher pressure conditions allow early and abundant clinopyroxene crystallisation, rapidly depleting the melt in both CaO and MgO, and thus creating the M-series.  相似文献   

Experimental study of magmatic melt oxidation by CO2     

A. G. Simakin  T. P. Salova  G. V. Bondarenko 《Petrology》2012,20(7):593-606

The process of CO₂ flashing through hydrous albite-hedenbergite melt was experimentally examined at a temperature of 1100°C and a pressure of 2 kbar. Carbon dioxide was generated when the melt interacted with calcite, and wollastonite was the predominant synthesized phase. Mafic components were introduced into the hydrous albite melt via the dissolution of natural hedenbergite. Raman spectroscopic data on bubbles of the fluid phase in the quench glass indicate that the CO₂/H₂O proportions of the bubbles vary. IR spectroscopic data on the glass prove that the water concentration after CO₂ flashing decreased from 5.5 to approximately 3 wt %. The comparison of the composition of the recrystallized clinopyroxene in contact with melt (with and without CO₂ blowing) indicates that CO₂ oxidizes Fe in the melt. The redox effect of CO₂ is quantified by the empirical clinopyroxene tool for metering oxygen fugacity (oxometer), which was calibrated based on experimental data. The oxygen fugacity in our experiments with CO₂ flashing (estimated by the clinopyroxene oxometer) was NNO + (3.0?C3.5). Our estimates with the application of the clinopyroxene oxometer indicate that the maximum oxygen fugacity in the magmatic chambers of Vesuvius and Stromboli volcanoes (which are bubbled with CO₂) is also close to NNO + (3.5 ± 0.5).  相似文献   

The experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt     

Jan A. Schuessler  Ronny Schoenberg  Harald Behrens  Friedhelm von Blanckenburg 《Geochimica et cosmochimica acta》2007,71(2):417-433

A first experimental study was conducted to determine the equilibrium iron isotope fractionation between pyrrhotite and silicate melt at magmatic conditions. Experiments were performed in an internally heated gas pressure vessel at 500 MPa and temperatures between 840 and 1000 °C for 120-168 h. Three different types of experiments were conducted and after phase separation the iron isotope composition of the run products was measured by MC-ICP-MS. (i) Kinetic experiments using ⁵⁷Fe-enriched glass and natural pyrrhotite revealed that a close approach to equilibrium is attained already after 48 h. (ii) Isotope exchange experiments—using mixtures of hydrous peralkaline rhyolitic glass powder (∼4 wt% H₂O) and natural pyrrhotites (Fe_1 − xS) as starting materials— and (iii) crystallisation experiments, in which pyrrhotite was formed by reaction between elemental sulphur and rhyolitic melt, consistently showed that pyrrhotite preferentially incorporates light iron. No temperature dependence of the fractionation factor was found between 840 and 1000 °C, within experimental and analytical precision. An average fractionation factor of Δ ⁵⁶Fe/⁵⁴Fe_{pyrrhotite-melt} = −0. 35 ± 0.04‰ (2SE, n = 13) was determined for this temperature range. Predictions of Fe isotope fractionation between FeS and ferric iron-dominated silicate minerals are consistent with our experimental results, indicating that the marked contrast in both ligand and redox state of iron control the isotope fractionation between pyrrhotite and silicate melt. Consequently, the fractionation factor determined in this study is representative for the specific Fe²⁺/ΣFe ratio of our peralkaline rhyolitic melt of 0.38 ± 0.02. At higher Fe²⁺/ΣFe ratios a smaller fractionation factor is expected. Further investigation on Fe isotope fractionation between other mineral phases and silicate melts is needed, but the presented experimental results already suggest that even at high temperatures resolvable variations in the Fe isotope composition can be generated by equilibrium isotope fractionation in natural magmatic systems.  相似文献   

Iron isotopic systematics of UHP eclogites respond to oxidizing fluid during exhumation   总被引：1，自引：0，他引：1       下载免费PDF全文

D.‐Y. Li  Y. L. Xiao  W.‐Y. Li  X. Zhu  H. M. Williams  Y.‐L. Li 《Journal of Metamorphic Geology》2016,34(9):987-997

The iron stable isotope compositions (δ⁵⁶Fe) and iron valence states of ultrahigh‐pressure eclogites from Bixiling in the Dabie orogen belt, China, were measured to trace the changes of geochemical conditions during vertical transportation of earth materials, for example, oxygen fugacity. The bulk Fe³⁺/ΣFe ratios of retrograde eclogites, determined by Mössbauer spectroscopy, are consistently higher than those of fresh eclogites, suggesting oxidation during retrograde metamorphism and fluid infiltration. The studied eclogites (five samples) display limited mid‐ocean ridge basalts (MORB)‐like (~0.10‰) δ⁵⁶Fe values, which are indistinguishable from their protoliths, that is, gabbro cumulates formed through differentiation of mantle‐derived basaltic magma. This suggests that Fe isotope fractionation during continental subduction is limited. Garnet separates display limited δ⁵⁶Fe variation ranging from ?0.08 ± 0.07 ‰ to 0.02 ± 0.07‰, whereas coexisting omphacite displays a large variation of δ⁵⁶Fe values from 0.15 ± 0.07‰ to 0.47 ± 0.07‰. Omphacite also has highly variable Fe³⁺/ΣFe ratios from 0.367 ± 0.025 to 0.598 ± 0.024, indicating modification after peak metamorphism. Omphacite from retrograde eclogites has elevated Fe³⁺/ΣFe ratios (0.54–0.60) compared to that from fresh eclogites (~0.37), whereas garnet displays a narrow range of ferric iron content with Fe³⁺/ΣFe ratios from 0.039 ± 0.013 to 0.065 ± 0.022. The homogenous δ⁵⁶Fe values and Fe³⁺/ΣFe ratios of garnet suggest that it survived the retrograde metamorphism and preserved its Fe‐isotopic features and ferric contents of peak metamorphism. Because of similar diffusion rates of Fe and Mg in garnet and omphacite, and constant Δ²⁶Mg_{omphacite‐garnet} values (1.14 ± 0.04‰), equilibrium iron isotope fractionation between garnet and omphacite was probably achieved during peak metamorphism. Elevated Fe³⁺/ΣFe ratios of omphacite from retrograde eclogites and variant Δ⁵⁶Fe_{omphacite‐garnet} values of the studied eclogites (0.13 ± 0.10‰ to 0.48 ± 0.10‰) indicate that oxidized geofluid infiltration resulted in the elevation of δ⁵⁶Fe values of omphacite during retrograde metamorphism.  相似文献   

Mössbauer spectroscopy studies of the valence state of iron in chromite from the Luobusa massif of Tibet: implications for a highly reduced deep mantle     

T. RUSKOV  I. SPIROV  M. GEORGIEVA  S. YAMAMOTO  H. W. GREEN  C. A. McCAMMON  L. F. DOBRZHINETSKAYA 《Journal of Metamorphic Geology》2010,28(5):551-560

Mössbauer spectroscopy was applied to study the valence state of iron in chromite from massive, nodular and disseminated podiform chromitite ores of the Luobasa ophiolite massif of Tibet. The results show that Fe³⁺/ΣFe = 0.42 in chromite from massive ore, and Fe³⁺/ΣFe = 0.22 in chromite from nodular and disseminated ores. The massive ore records traces of ultra high pressure mineralogical assemblages, such as diamond inclusions in OsIr alloys, exsolution lamellae of coesite and diopside in chromite, inclusions of metal‐nitrides, native iron and others, which suggests a strongly reducing environment. In contrast, chromite from nodular and disseminated ore contains abundant low‐pressure OH‐bearing mineral inclusions whose formation requires a more oxidizing environment. The high value of Fe³⁺/ΣFe in the ‘reduced’ massive ore is explained by crystallographic stabilization of Fe³⁺ in a high‐pressure polymorph of chromite deep in the upper mantle despite low ambient fO₂ conditions. The presence of high‐pressure phases within the massive chromitite ore requires that the latter, together with its host peridotite, was transported in the solid state from a highly reduced deep mantle environment to shallow depths beneath an ocean spreading centre. It is suggested that in the low‐pressure environment of the spreading centre, the deep‐seated, reduced, massive chromitites partially reacted with their host peridotite in the presence of hydrous melt, yielding the nodular and disseminated chromitite ores. The preponderance of evidence suggests that the latter interaction involved boninitic melts in a supra‐subduction zone environment as proposed previously.  相似文献   

Ti K-edge XANES study on the coordination number and oxidation state of Titanium in pyroxene,olivine, armalcolite,ilmenite, and silicate glass during mare basalt petrogenesis     

F.?P.?LeitzkeEmail authorView author&#;s OrcID profile  R.?O.?C.?Fonseca  J.?G?ttlicher  R.?Steininger  S.?Jahn  C.?Prescher  M.?Lagos 《Contributions to Mineralogy and Petrology》2018,173(12):103

Lunar mare basalts are a product of partial melting of the lunar mantle under more reducing conditions when compared to those expected for the Earth’s upper mantle. Alongside Fe, Ti can be a major redox sensitive element in lunar magmas, and it can be enriched by up to a factor of ten in lunar basaltic glasses when compared to their terrestrial counterparts. Therefore, to better constrain the oxidation state of Ti and its coordination chemistry during lunar magmatic processes, we report new X-ray absorption near edge structure (XANES) spectroscopy measurements for a wide range of minerals (pyroxene, olivine, Fe–Ti oxides) and basaltic melt compositions involved in partial melting of the lunar mantle. Experiments were conducted in 1 bar gas-mixing furnaces at temperatures between 1100 and 1300 °C and oxygen fugacities (fO₂) that ranged from air to two orders of magnitude below the Fe–FeO redox equilibrium. Run products were analysed via electron microprobe and XANES Ti K-edge. Typical run products had large (>?100 µm) crystals in equilibrium with quenched silicate glass. Ti K-edge XANES spectra show a clear shift in energy of the absorption edge features from oxidizing to reducing conditions and yield an average valence for Fe–Ti oxides (armalcolite and ilmenite) of 3.6, i.e., a 40% of the overall Ti is Ti³⁺ under fO₂ conditions relevant to lunar magmatism (IW ??1.5 to ??1.8). Pyroxenes and olivine have average Ti valence of 3.75 (i.e., 25% of the overall Ti is trivalent), while in silicate glasses Ti is exclusively tetravalent. Pre-edge peak intensities also indicate that the coordination number of Ti varies from an average V-fold in silicate glass to VI-fold in the Fe–Ti oxides and a mixture between IV and VI-fold coordination in the pyroxenes and olivine, with up to 82% ^[IV]Ti⁴⁺ in the pyroxene. In addition, our results can help to better constrain the Ti³⁺/∑Ti of the lunar mantle phases during magmatic processes and are applied to provide first insights into the mechanisms that may control Ti mass-dependent equilibrium isotope fractionation in lunar mare basalts.  相似文献   

Iron-bearing silicate melts: Relations between pressure and redox equilibria     

Bjorn O. Mysen  David Virgo 《Physics and Chemistry of Minerals》1985,12(4):191-200

Mossbauer spectroscopy has been used to determine the redox equilibria of iron and structure of quenched melts on the composition join Na₂Si₂O₅-Fe₂O₃ to 40 kbar pressure at 1400° C. The Fe³⁺/ΣFe decreases with increasing pressure. The ferric iron appears to undergo a gradual coordination transformation from a network-former at 1 bar to a network-modifier at higher (≧10 kbar) pressure. Ferrous iron is a network-modifier in all quenched melts. Reduction of Fe³⁺ to Fe²⁺ and coordination transformation of remaining Fe³⁺ result in depolymerization of the silicate melts (the ratio of nonbridging oxygens per tetrahedral cations, NBO/T, increases). It is suggested that this pressure-induced depolymerization of iron-bearing silicate liquids results in increasing NBO/T of the liquidus minerals. Furthermore, this depolymerization results in a more rapid pressure-induced decrease in viscosity and activation energy of viscous flow of iron-bearing silicate melts than would be expected for iron-free silicate melts with similar NBO/T.  相似文献   

The Ti4+/Ti3+ ratio of magmatic melts: Application to the problem of the reduction of lunar basalts     

A. A. Borisov 《Petrology》2012,20(4):391-398

Using published experimental data an expression was derived for the Ti⁴⁺/Ti³⁺ ratio as a function of temperature, oxygen fugacity, and melt composition. The equation can be used to estimate Ti³⁺ content in lunar basaltic melts. It was shown that the Ti³⁺ content in melts is probably no higher than the Fe3+ content even under the reduced conditions typical of lunar magmas. Trivalent Ti can lead to some decrease in $f_{O_2 }$ during melt cooling under closed-system conditions, but it cannot reduce Fe²⁺ in melt to metal, because it will be completely consumed by Fe³⁺ reduction to Fe²⁺. The presence of additional reducers, such as Cr²⁺, can be favorable for the formation of metal during melt cooling.  相似文献