Spectroscopic analysis (FTIR, Raman) of water in mafic and intermediate glasses and glass inclusions

Micro-Raman spectroscopy, even though a very promising technique, is not still routinely applied to analyse H₂O in silicate glasses. The accuracy of Raman water determinations critically depends on the capability to predict and take into account both the matrix effects (bulk glass composition) and the analytical conditions on band intensities. On the other hand, micro-Fourier transform infrared spectroscopy is commonly used to measure the hydrous absorbing species (e.g., hydroxyl OH⁻ and molecular H₂O) in natural glasses, but requires critical assumptions for the study of crystal-hosted glasses. Here, we quantify for the first time the matrix effect of Raman external calibration procedures for the quantification of the total H₂O content (H₂O_T = OH⁻ + H₂O_m) in natural silicate glasses. The procedures are based on the calibration of either the absolute (external calibration) or scaled (parameterisation) intensity of the 3550 cm⁻¹ band. A total of 67 mafic (basanite, basalt) and intermediate (andesite) glasses hosted in olivines, having between 0.2 and 4.8 wt% of H₂O, was analysed. Our new dataset demonstrates, for given water content, the height (intensity) of Raman H₂O_T band depends on glass density, reflectance and water environment. Hence this matrix effect must be considered in the quantification of H₂O by Raman spectroscopy irrespective of the procedure, whereas the parameterisation mainly helps to predict and verify the self-consistency of the Raman results. In addition, to validate the capability of the micro-Raman to accurately determine the H₂O content of multicomponent aluminosilicate glasses, a subset of 23 glasses was analysed by both micro-Raman and micro-FTIR spectroscopy using the band at 3550 cm⁻¹. We provide new FTIR absorptivity coefficients (ε₃₅₅₀) for basalt (62.80 ± 0.8 L mol⁻¹ cm⁻¹) and basanite (43.96 ± 0.6 L mol⁻¹ cm⁻¹). These values, together with an exhaustive review of literature data, confirm the non-linear decline of the FTIR absorptivity coefficient (ε₃₅₅₀) as the glass depolymerisation increases. We demonstrate the good agreement between micro-FTIR and micro-Raman determination of H₂O in silicate glasses when the matrix effects are properly considered.     

Influence of glass polymerisation and oxidation on micro-Raman water analysis in alumino-silicate glasses

The development of an accurate analytical procedure for determination of dissolved water in complex alumino-silicate glasses via micro-Raman analysis requires the assessment of the spectra topology dependence on glass composition. We report here a detailed study of the respective influence of bulk composition, iron oxidation state and total water content on the absolute and relative intensities of the main Raman bands related to glass network vibrations (LF: ∼490 cm⁻¹; HF: ∼960 cm⁻¹) and total water stretching (H₂O_T: ∼3550 cm⁻¹) in natural glasses. The evolution of spectra topology was examined in (i) 33 anhydrous glasses produced by the re-melting of natural rock samples, which span a very large range of polymerisation degree (NBO/T from 0.00 to 1.16), (ii) 2 sets of synthetic anhydrous basaltic glasses with variable iron oxidation state (Fe³⁺/Fe_T from 0.05 to 0.87), and (iii) 6 sets of natural hydrous glasses (C_H2OT from 0.4 to 7.0 wt%) with NBO/T varying from 0.01 to 0.76.In the explored domain of water concentration, external calibration procedure based on the H₂O_T band height is matrix-independent but its accuracy relies on precise control of the focusing depth and beam energy on the sample. Matrix-dependence strongly affects the internal calibrations based on H₂O_T height scaled to that of LF or HF bands but its effect decreases from acid (low NBO/T, SM) to basic (high NBO/T, SM) glasses. Structural parameters such as NBO/T (non-bridging oxygen per tetrahedron) and SM (sum of structural modifiers) describe the matrix-dependence better than simple compositional parameters (e.g. SiO₂, Na₂O + K₂O). Iron oxidation state has only a minor influence on band topology in basalts and is thus not expected to significantly affect the Raman determinations of water in mafic (e.g. low SiO₂, iron-rich) glasses. Modelling the evolution of the relative band height with polymerisation degree allows us to propose a general equation to predict the dissolved water content in natural glasses:

     

Temperature dependence of CO2 solubility in high pressure quenched glasses of diopside composition

The temperature dependence of carbon dioxide solubility in glasses of diopside composition, quenched from 20 kbar, has been investigated using a combination of high-temperature mass spectrometry and Raman spectroscopy.CO₂-charged diopside glasses were synthesized in a piston-cylinder apparatus. Because of diffusion of hydrogen through the platinum capsules, significant amounts of H₂O, CH₄ and CO were detected along with CO: in the diopside glasses. All three carbon species show a bimodal release pattern in the mass pyrograms. The CO₂ solubility shows a linear and negative temperature dependence. We do not observe any maxima in the solubility curve as was reported previously (Mysen and virgo, 1980a).None of the additional bands observed in Raman spectra of CO₂-charged diopside glasses compared to those in the spectrum of diopside glass can be assigned to molecular CO₂. These bands are caused by CO^?2₃ ions and indicate that the physical solubility of molecular carbon dioxide is negligible. The bimodal release pattern observed for CO₂ in the mass pyrograms, is consistent with the Raman data which strongly suggests that CO^?2₃ ions are present in at least two distinct sites in the glass.     

Thermal expansion and glass transition temperatures of synthetic glasses of plagioclase-like compositions

The glass transition temperatures and the thermal expansions both below and above the glass transition temperature region of synthetic glasses of compositions close to those of plagioclases have been determined. The linear thermal expansion coefficient of the rigid glasses decreases on average from 7.4×10^–6/dgC for albite glass to 4.9×10^–6/dgC for glass close to anorthite composition. The glass transition temperature of the glasses initially decreases from 763° C for albite glass to 752°C for An_9.7 glass and then increases nearly linearly with further increasing anorthite content to 813° C for glass close to anorthite composition.—Measurements made for comparison on a glass prepared from Madagascar orthoclase yielded a linear thermal expansion coefficient of 6.1×10^–6/dgC and a glass transition temperature of 905° C.The variations in thermal expansion and glass transition temperature of the feldspar glasses with composition are discussed in terms of structural changes which are assumed to be associated with cation replacement.     

Quantification of water content and speciation in natural silicic glasses (phonolite, dacite, rhyolite) by confocal microRaman spectrometry

The determination of total water content (H₂O_T: 0.1-10 wt%) and water speciation (H₂O_molecular/OH) in volcanic products by confocal microRaman spectrometry are discussed for alkaline (phonolite) and calcalkaline (dacite and rhyolite) silicic glasses. Shape and spectral distribution of the total water band (H₂O_T) at ∼3550 cm⁻¹ show systematic evolution with glass H₂O_T, water speciation and NBO/T. In the studied set of silicic samples, calibrations based on internal normalization of the H₂O_T band to a band related to vibration of aluminosilicate network (TOT) at ∼490 cm⁻¹ vary with glass peraluminosity. An external calibration procedure using well-characterized glass standards is less composition-dependent and provides excellent linear correlation between total dissolved water content and height or area of the H₂O_T Raman band. Accuracy of deconvolution procedure of the H₂O_T band to quantify water speciation in water-rich and depolymerized glasses depends on the strength of OH hydrogen bonding. System confocal performance, scattering from embedding medium and glass microcrystallinity have a crucial influence on accuracy of Raman analyses of water content in glass-bearing rocks and melt inclusions in crystals.     

A composition-independent quantitative determination of the water content in silicate glasses and silicate melt inclusions by confocal Raman spectroscopy

A new approach was developed to measure the water content of silicate glasses using Raman spectroscopy, which is independent of the glass matrix composition and structure. Contrary to previous studies, the compositional range of our studied silicate glasses was not restricted to rhyolites, but included andesitic, basaltic and phonolitic glasses. We used 21 glasses with known water contents for calibration. To reduce the uncertainties caused by the baseline removal and correct for the influence of the glass composition on the spectra, we developed the following strategy: (1) application of a frequency-dependent intensity correction of the Raman spectra; (2) normalization of the water peak using the broad T–O and T–O–T vibration band at 850–1250 cm⁻¹ wavenumbers (instead of the low wavenumber T–O–T broad band, which appeared to be highly sensitive to the FeO content and the degree of polymerization of the melt); (3) normalization of the integrated Si-O band area by the total number of tetrahedral cations and the position of the band maximum. The calibration line shows a ±0.4 wt% uncertainty at one relative standard deviation in the range of 0.8–9.5 wt% water and a wide range of natural melt compositions. This method provides a simple, quick, broadly available and cost-effective way for a quantitative determination of the water content of silicate glasses. Application to silicate melt inclusions yielded data in good agreement with SIMS data.     

S-rich apatite-hosted glass inclusions in xenoliths from La Palma: constraints on the volatile partitioning in evolved alkaline magmas

The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO₃ (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce₂O₃. Sulfur is incorporated in apatite via several independent exchange reactions involving (P⁵⁺, Ca²⁺) vs. (S⁶⁺, Si⁴⁺, Na⁺, and Ce³⁺). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from <0.07 to 0.65 wt% for F. The sulfur concentration in the glasses ranges from 0.06 to 0.23 wt% SO₃ (sulfate-saturated systems). The chlorine partition coefficients (D_Cl^{apatite/glass}) range from 0.4 to 1.3 (average D_Cl^{apatite/glass} = 0.8), in good agreement with the results of experimental data in mafic and rhyolitic system with low Cl concentrations. With increasing F in glass inclusions D_F^{apatite/glass} decreases from 35 to 3. However, most of our data display a high partition coefficient (~30) close to D_F^{apatite/glass} determined experimentally in felsic rock. D_S^{apatite/glass} decreases from 9.1 to 2.9 with increasing SO₃ in glass inclusions. The combination of natural and experimental data reveals that the S partition coefficient tends toward a value of 2 for high S content in the glass (>0.2 wt% SO₃). D_S^{apatite/glass} is only slightly dependent on the melt composition and can be expressed as: SO_{3 apatite} (wt%) = 0.157 * ln SO_{3 glass} (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and haüyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of haüyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.     

Composition dependence of elasticity in aluminosilicate glasses

The elastic properties of two types of aluminosilicate (basaltic and rhyolitic) glasses have been studied using both Brillouin and Raman spectroscopy at ambient conditions. It has been found that the elastic moduli of the basaltic glasses decrease with increasing SiO₂ concentration. The shear moduli displayed the least dependence on SiO₂ content. The bulk moduli of the basaltic glasses strongly depend on the sum of the Q ³ and Q ⁴ anionic units. Among the modifiers, iron cations showed the strongest effect on the elastic properties of the rhyolitic glasses. For the elastic moduli of rhyolitic glasses, the major effect of alkaline earth cations is on shear modulus; however, both iron and alkali cations showed stronger effects on bulk modulus and similar relative contribution between bulk and shear moduli (based on the equivalent M⁺ cation). The dependences of elastic moduli on bulk NBO/T observed in both types of glasses suggest that the elastic modulus of an aluminosilicate glass depends on the concentration of effective modifying cations rather than the apparent concentration of all non-network-forming cations. An analysis of data also indicated that the ideal molar mixing model is failed in prediction of the elastic properties of the present multicomponent glasses by using the known parameters.     

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

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.     

High temperature structural investigation of Na2O·0.5Fe2O3·3SiO2 and Na2O·FeO·3SiO2 melts and glasses

The structure of glasses and melts of Na₂O· 0.5Fe₂O₃·3SiO₂ and Na₂O·FeO·3SiO₂ compositions have been measured using high temperature Raman spectroscopy. For the oxidized sample it has been demonstrated that there is a close structural relationship between melt and glass. No coordination changes of Fe³⁺ with temperature and no new anionic species have been observed in the oxidized melt. The Raman spectra of the reduced sample clearly show a decrease in the degree of polymerization, as determined by the observation of the polarization character of the spectra and the details of the change of the Raman intensities during heating in hydrogen. Mössbauer spectra suggest that Fe³⁺ is tetrahedrally coordinated in the oxidized glass and part of the Fe²⁺ is tetrahedrally coordinated in the reduced glass.     

In-situ high-temperature Raman spectroscopic studies of aluminosilicate liquids

We have measured in-situ Raman spectra of aluminosilicate glasses and liquids with albite (NaAlSi₃ O₈) and anorthite (CaAl₂Si₂O₈) compositions at high temperatures, through their glass transition range up to 1700 and 2000 K, respectively. For these experiments, we have used a wire-loop heating device coupled with micro-Raman spectroscopy, in order to achieve effective spatial filtering of the extraneous thermal radiation. A major concern in this work is the development of methodology for reliably extracting the first and second order contributions to the Raman scattering spectra of aluminosilicate glasses and liquids from the high temperature experimental data, and analyzing these in terms of vibrational (anharmonic) and configurational changes. The changes in the first order Raman spectra with temperature are subtle. The principal low frequency band remains nearly constant with increasing temperature, indicating little change in the T-O-T angle, and that the angle bending vibration is quite harmonic. This is in contrast to vitreous SiO₂, studied previously. Above Tg, intensity changes in the 560–590 cm^?1 regions of both sets of spectra indicate configurational changes in the supercooled liquids, associated with formation of additional Al-O-Al linkages, or 3-membered (Al, Si)-containing rings. Additional intensity at 800 cm^?1 reflects also some rearrangement of the Si-O-Al network.     

Polymerization of silicate and aluminate tetrahedra in glasses,melts, and aqueous solutions—III. Local silicon environments and internal nucleation in silicate glasses

The local, up to second nearest neighbor, around Si atoms in alkali and alkaline earth-silicate glasses has been characterized by SiKβ X-ray emission spectroscopy. Principally two types of Si atoms can be distinguished. These are Si atoms with only other Si atoms as second nearest neighbours, and those with one or more alkali or alkaline earth atoms in their second coordination sphere. The spectroscopic results indicate that the lower molecular weight alkali and alkaline earth-silicate glasses tend towards a bimodal distribution of local Si environments, which is designated Q₄-Q₀ following the assignment by Engelhardtet al. (1975) for silica species in aqueous solutions. From a different perspective the outcome of these experiments suggests that, though the concentration of bridging oxygens (O(br)) and non-bridging oxygens (O(nbr)) is fixed by the stoichiometry of the glass, the distribution of O(nbr) in the glass varies according to the kind of alkali or alkaline earth atom present. From observed nucleation data on R₂O-SiO₂ (R = Li, Na, K) glasses it is inferred that a bimodal Q distribution and in particular the presence of Q₀ species dominates the internal nucleation process in the alkali and alkaline earth-silicate glasses studied. Using this inference rationalizations can readily be found to explain the observed resistance to thermal shock and devitrification rates of these glasses.     

Solubility mechanisms of fluorine in peralkaline and meta-aluminous silicate glasses and in melts to magmatic temperatures

Structural interaction between dissolved fluorine and silicate glass (25°C) and melt (to 1400°C) has been examined with ¹⁹F and ²⁹Si MAS NMR and with Raman spectroscopy in the system Na₂O-Al₂O₃-SiO₂ as a function of Al₂O₃ content. Approximately 3 mol.% F calculated as NaF dissolved in these glasses and melts. From ¹⁹F NMR spectroscopy, four different fluoride complexes were identified. These are (1) Na-F complexes (NF), (2) Na-Al-F complexes with Al in 4-fold coordination (NAF), (3) Na-Al-F complexes with Al in 6-fold coordination with F (CF), and (4) Al-F complexes with Al in 6-fold, and possibly also 4-fold coordination (TF). The latter three types of complexes may be linked to the aluminosilicate network via Al-O-Si bridges.The abundance of sodium fluoride complexes (NF) decreases with increasing Al/(Al + Si) of the glasses and melts. The NF complexes were not detected in meta-aluminosilicate glasses and melts. The NAF, CF, and TF complexes coexist in peralkaline and meta-aluminosilicate glasses and melts.From ²⁹Si-NMR spectra of glasses and Raman spectra of glasses and melts, the silicate structure of Al-free and Al-poor compositions becomes polymerized by dissolution of F because NF complexes scavenge network-modifying Na from the silicate. Solution of F in Al-rich peralkaline and meta-aluminous glasses and melts results in Al-F bonding and aluminosilicate depolymerization.Temperature (above that of the glass transition) affects the Qⁿ-speciation reaction in the melts, 2Q³ ⇔ Q⁴ + Q², in a manner similar to other alkali silicate and alkali aluminosilicate melts. Dissolved F at the concentration level used in this study does not affect the temperature-dependence of this speciation reaction.     

Cooling rate variation in natural volcanic glasses from Tenerife,Canary Islands

 Silicate melts form glasses in a variety of geological environments. The relaxation (equilibration) of the frozen glass structure provides a means of investigating the quench rates of natural glasses, and this cooling history provides an important constraint for models of melt dynamics. Phonolite glasses from the central volcanic edifice of Tenerife, Canary Islands indicate a range of five orders of magnitude cooling rate, determined by modeling the relaxation of the structure-dependent property, enthalpy (H) across the glass transition. The relaxation of enthalpy is determined by heat capacity (c _p = ΔH/ΔT) measurement of natural glass samples by differential scanning calorimetry (DSC). Upon heating, the heat capacity curve in the vicinity of the glass transition has a geometry characteristic of the previous cooling rate. A series of thermal treatments applied to each individual sample results in a set of sample-specific parameters which are used to model the heat capacity curve of the naturally cooled glass. The cooling rate is then derived. The equivalence of shear and enthalpic relaxation enables the relaxation of enthalpy for these volcanic samples to be described by a general term for the evolution of fictive temperature. Quench rates for thirty-one glasses are calculated to be within the range 10°C s^–1 to 7°C per day. The cooling rates quoted are linear approximations across the glass transition. Within different volcanic facies cooling rates depend on several factors. The most rapidly cooled glasses occur where samples lose heat by radiation from the surface. Our analyses indicate that in certain environments, a natural annealing process results in slow quench rates. This is interpreted as either a slow initial cooling process or the reheating of a glass to an annealing temperature within the glass transition interval. The latter results in relaxation to a lower temperature structure. Controls on these processes include the initial temperature and dissipation of thermal energy from the volcanic body. Our results are consistent with an influence of volatiles on quench rates in volcanic bombs where glass adjacent to vesicular layers is relatively rapidly quenched. We interpret this as a rapid quench rate frozen into the glass resulting from a change in viscosity due to volatile degassing. In lava flows, the conduction of heat from the hot flow interior controls the cooling process and diminishes the effect of volatile exsolution. Relaxation geospeedometry can be applied to glass samples from a variety of geological environments where cooling rates cannot be measured directly. Such measurements provide a means of determining cooling rates for a variety of volcanic processes, an independent calibration for existing temperature and time data and a means for testing cooling-rate-dependent models. Received: 9 January 1996 / Accepted: 13 May 1996     

Vanadium K edge XANES of synthetic and natural basaltic glasses and application to microscale oxygen barometry

A new microscale oxybarometer for solar system basaltic glasses, based on vanadium K-edge X-ray absorption near edge structure (XANES) spectroscopy, is described. Vanadium is unique among abundant elements in siliceous materials in that it can potentially occur in nature in four valence states: V²⁺, V³⁺, V⁴⁺ and V⁵⁺. Consequently, the vanadium redox system is a robust oxybarometer covering at least six orders of magnitude in buffer-relative oxygen fugacity. The method was calibrated using synthetic glass standards produced under known fO₂ and temperature conditions. Correction for temperature differences among standards and unknowns was quantified using microXANES data for isobaric synthetic glass couples. Application of the method to lunar, martian, and terrestrial glasses yielded fO₂ estimates from 1.6 log units more reduced than the iron-wüstite (IW) buffer (IW-1.6) for lunar glasses, to IW + 4.0 for terrestrial glass inclusions. The martian and terrestrial results are in good agreement with previous estimates by other methods. The inferred fO₂ values for lunar pyroclastic glasses are ∼0.5 log unit more reduced than previous estimates, but the differences are comparable to analytical uncertainties. Micro-extended X-ray absorption fine structure spectra were consistent with the valence states determined by microXANES and provided additional constraints on vanadium site geometry. These results demonstrate the value of this new oxybarometer, which can be applied nondestructively to individual grains in conventional thin sections with ∼ micrometer resolution and ∼100 ppm elemental sensitivity.     

Sulfur speciation and network structural changes in sodium silicate glasses: Constraints from NMR and Raman spectroscopy

Information about the state of sulfur in silicate melts and glasses is important in both earth sciences and materials sciences. Because of its variety of valence states from S²⁻ (sulfide) to S⁶⁺ (sulfate), the speciation of sulfur dissolved in silicate melts and glasses is expected to be highly dependent on the oxygen fugacity. To place new constraint on this issue, we have synthesized sulfur-bearing sodium silicate glasses (quenched melts) from starting materials containing sulfur of different valence states (Na₂SO₄, Na₂SO₃, Na₂S₂O₃ and native S) using an internally heated gas pressure vessel, and have applied electron-induced SKα X-ray fluorescence, micro-Raman and NMR spectroscopic techniques to probe their structure. The wavelength shift of SKα X-rays revealed that the differences in the valence state of sulfur in the starting compounds are largely retained in the synthesized sulfur-bearing glasses, with a small reduction for more oxidized samples. The ²⁹Si MAS NMR spectra of all the glasses contain no peaks attributable to the SiO_4-nS_n (with n > 0) linkages. The Raman spectra are consistent with the coexistence of sodium sulfate (Na₂SO₄) species and one or more types of more reduced sulfur species containing S-S linkages in all the sulfur-bearing silicate glasses, with the former dominant in glasses produced from Na₂SO₄-doped starting materials, and the latter more abundant in more reduced glasses. The ²⁹Si MAS NMR and Raman spectra also revealed changes in the silicate network structure of the sulfur-bearing glasses, which can be interpreted in terms of changes in the chemical composition and sulfur speciation.     

Metasomatism in mantle xenoliths from Gees,West Eifel,Germany: evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment

 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO₂ when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO₂), characterized by high Al₂O₃ (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO₂ and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO₂ is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995     

Secondary Ion Mass Spectrometry Bias on Isotope Ratios in Dolomite–Ankerite,Part II: δ13C Matrix Effects

This study is Part II of a series that documents the development of a suite of calibration reference materials for in situ SIMS analysis of stable isotope ratios in Ca‐Mg‐Fe carbonates. Part I explored the effects of Fe²⁺ substitution on SIMS δ¹⁸O bias measured from the dolomite–ankerite solid solution series [CaMg(CO₃)₂–CaFe(CO₃)₂], whereas this complementary work explores the compositional dependence of SIMS δ¹³C bias (calibrated range: Fe# = 0.004–0.789, where Fe# = molar Fe/(Mg+Fe)). Under routine operating conditions for carbonate δ¹³C analysis at WiscSIMS (CAMECA IMS 1280), the magnitude of instrumental bias increased exponentially by 2.5–5.5‰ (session‐specific) with increasing Fe‐content in the dolomite structure, but appeared insensitive to minor Mn substitution [< 2.6 mole % Mn/(Ca+Mg+Fe+Mn)]. The compositional dependence of bias (i.e., the matrix effect) was expressed using the Hill equation, yielding calibration residual values ≤ 0.3‰ relative to CRM NBS‐19 for eleven carbonate reference materials (6‐μm‐diameter spot size measurements). Based on the spot‐to‐spot repeatability of a drift monitor material that ‘bracketed’ each set of ten sample‐spot analyses, the analytical precision was ± 0.6–1.2‰ (2s, standard deviations). The analytical uncertainty for individual sample analyses was approximated by combining the precision and calibration residual values (propagated in quadrature), suggesting an uncertainty of ± 1.0–1.5‰ (2s).     

A micro-XAS/XRF and thermodynamic study of Ce speciation after long-term aqueous alteration of simulated nuclear waste glass: Relevance for predicting Pu behavior?

In the present study, the dissolution and mobilization of Ce introduced in a simulated nuclear waste glass (MW) as a surrogate of Pu was investigated after leaching in pure water over 12 a at 90 °C and pH ∼ 9.6. The microscopic distribution and oxidation state of Ce in the altered glass were studied using micro-X-ray fluorescence (micro-XRF) mapping techniques and micro-X-ray near-edge absorption spectroscopy (micro-XANES). Distribution maps of Ce^III and Ce^IV were obtained by recording the L_α fluorescence emission at two different incident X-ray energies, coinciding with the maximum contrast between Ce^III and Ce^IV fluorescence intensities. The micro-XRF maps revealed that Ce was dominantly present as oxidized species (Ce^IV) in the original glass. After dissolution from the glass matrix, Ce^IV was partly reduced and re-immobilized as Ce^III at grain boundaries or in the interstitial spaces between the glass particles. The concentration of Ce^III was found to correlate with the spatial distribution of secondary Mg-clay formed during the aqueous corrosion as the main glass alteration product. Micro-XANES spectra collected at locations representative of both altered and non-altered glass domains confirmed the findings obtained by the redox mapping. Because redox-sensitive elements in the pristine MW glass (Fe, Cr, Se) occur almost exclusively as oxidized species, reduction of Ce^IV was probably mediated by an external source of reductants, such as Fe(0) from the steel reaction vessel.