A combined Fourier transform infrared and Cr K-edge X-ray absorption near-edge structure spectroscopy study of the substitution and diffusion of H in Cr-doped forsterite

International audienceSingle crystals of synthetic Cr-doped forsterite (Cr:Mg2SiO4) containing both Cr3+ and Cr4+ were partially hydroxylated in piston-cylinder apparatuses at 750-1300 degrees C and pressures from 0.5 to 2.5 GPa, with P(H2O) approximate to P-total. The oxygen fugacity (fO(2)) was buffered by graphite-water, Ni-NiO, Re-ReO2, Fe2O3-Fe3O4 or Ag-Ag2O, and the silica activity (a SiO2) was buffered by powdered forsterite plus either enstatite (Mg2Si2O6), periclase (MgO) or zircon-baddeleyite (ZrSiO4-ZrO2). Profiles of OH content versus distance from the crystal edge were determined using Fourier transform infrared (FTIR) spectroscopy, and profiles of the oxidation state and coordination geometry of Cr were obtained, at the same positions, using K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The techniques are complementary - FTIR spectroscopy images the concentration and nature of O-H bonds, where Cr K-edge XANES spectroscopy shows the effect of the added H on the speciation of Cr already present in the lattice. Profiles of defect-specific absorbance derived from FTIR spectra were fitted to solutions of Fick's second law to derive diffusion coefficients, which yield the Arrhenius relationship for H diffusion in forsterite: log(10)(D) over tilde ([001]) = -2.5 +/- 0.6 + -(224 +/- 12 + 4.0 +/- 2.0 P)/2.303 RT , where (D) over tilde is the measured diffusion coefficient in m(2) s(-1), valid for diffusion parallel to [001] and calibrated between 1000 and 750 degrees C, P and T are in GPa and K, and R is 0.008314 kJK(-1) mol(-1). Diffusivity parallel to [100] is around 1 order of magnitude lower. This is consistent with previous determinations of H diffusion associated with M-site vacancies. The FTIR spectra represent a variety of Cr-bearing hydrous defects, along with defects associated with the pure Mg-Si-O-H system. It is proposed that all of the defects can form by interaction between the dry lattice, including Cr3+ and Cr4+, and fully hydroxylated M-site vacancies. The initial diffusive wave of hydroxylation is associated with neither reduction nor oxidation of Cr but with Cr4+ changing from tetrahedral to octahedral coordination. Superimposed on the H diffusion and concomitant change in Cr4+ site occupancy, but at a slower rate, producing shorter profiles, is reduction of Cr4+ to Cr3+ and potentially of Cr4+ and Cr3+ to Cr2+. In addition, by comparing FTIR data to trace element contents measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), constraints can be placed on absorption coefficients used for converting absorbance to H2O contents - our data support either wavenumber- or defect-dependent values of absorption coefficients. We estimate absorption coefficients of between 60 200 and 68 200 L mol(-1) cm(-1) for OH- associated with octahedral Cr3+ and an M-site vacancy and 18 700 to 24 900 L mol(-1) cm(-1) for two OH- associated with octahedrally coordinated Cr4+ and a Si vacancy (i.e. a clinohumite-type point defect)

Spinel Harzburgite-Derived Silicate Melts Forming Sulfide-Bearing Orthopyroxenite in the Lithosphere. Part 1: Partition Coefficients and Volatile Evolution Accompanying Fluid- and Redox-Induced Sulfide Formation

We report abundances of major trace and volatile elements in an orthopyroxenite vein cutting a sub-arc, mantle-derived, spinel harzburgite xenolith from Kamchatka. The orthopyroxenite contains abundant sulfides and is characterized by the presence of glass (formerly melt) both interstitially and as inclusions in minerals, comparable with similar veins from the West Bismarck arc. The glass formed by quenching of residual melts following crystallization of abundant orthopyroxene, amphibole, and minor olivine and spinel. The interstitial glass has a low-Ti, high-Mg# andesite composition, with a wide range of H2O and S contents but more limited F and Cl variations. We calculate trace element partition coefficients using mineral and glass data, including those for halogens in amphibole, which agree with experimental results from the literature. Despite having a similar, high-Mg# andesite composition, the orthopyroxene-hosted glass inclusions usually contain much more H2O and S than the interstitial glass (4–7 wt% and ∼2,600 ppm, respectively). The initial vein-forming melts were oxidized, recording oxygen fugacity conditions up to ∼1.5 log units above the fayalite–magnetite–quartz oxygen buffer. They intruded the sub-arc mantle lithosphere at ≥1,300°C, where they partially crystallized to form high-Mg# andesitic derivative melts at ca. 1,050–1,100°C. Comparison with literature data on glass-free orthopyroxenite veins from Kamchatka and the glass-bearing ones from West Bismarck reveals fundamental similarities indicating common parental melts, which were originally produced by low-degree melting (≤5%) of spinel harzburgite at ≥1,360°C and ≤1.5 GPa. This harzburgite source likely contained ≤0.05 wt% H2O and a few ppm of halogens. Volatile evolution inferred from glass compositions shows that (i) redox exchange between S6+ in the original melt and Fe2+ in the host mantle minerals, together with (ii) the formation of an S-bearing, (H2O, Cl)-rich hydrothermal fluid from the original melt, provides the conditions for the formation of abundant sulfides in the orthopyroxenites during cooling. During this process, up to 85% of the original melt S content (∼2,600 ppm) is locally precipitated as magmatic and hydrothermal sulfides. As such, melts derived from spinel harzburgite sources can concentrate chalcophile and highly siderophile metals in orthopyroxenite dykes and sills in the lithosphere

La spectrométrie Raman, un outil de choix pour étudier les volatils dissous dans un verre ou un silicate fondu : le cas de l’eau

L’eau dissoute dans un verre ou un silicate fondu affecte fortement ses propriétés rhéologiques, thermodynamiques et sa structure. Pour certaines applications des sciences de la Terre ou industrielles, la quantification de cette eau est nécessaire. À l’aide de la nouvelle méthode que nous avons développée, la Spectrométrie Raman permet de le faire sans préparation de l’échantillon, de manière rapide, sans aucun standard, avec une résolution spatiale micrométrique. Cette calibration est, de plus, indépendante de la chimie du verre étudié. Elle offre des perspectives intéressantes pour l’étude de la spéciation et la diffusion de l’eau dans les verres et silicates fondus

Water speciation in silicate melts: an high temperature Raman spectroscopy study.

In addition to temperature, pressure and main chemical components, volatiles exert a strong influence on the physical properties of magmas. In particular, water plays a fundamental role in the dynamics and evolution of magmas in the deep interior and during volcano eruption. However, water speciation in silicate melts is not fully understood. Infrared spectroscopy had provided some valuable information about the H2O/OHspeciation. We know that this speciation is a function of temperature and water contents of melts. It can be very interesting to know it from in situ experiments. This can be done using Raman spectroscopy. Raman spectra are composed of i) a low-wavenumber region which corresponds to vibrations of the silicate network (0-1500 cm-1), and ii) a high-wavenumber region which correspond to the OH- stretching vibration and H2O molecular vibration (3100-3750 cm-1). We have performed a first set of in situ experiments using a micro-furnace at ambient pressure. We have observed an evolution of the high-wavenumber region as a function of time and temperature. New Raman peaks can be distinguished, particularly near 3650-3700 cm-1. In this communication, we will present our first results on this subject and then discuss them in terms of relation between water and the silicate network

Water speciation in silicate melts investigated by Raman spectroscopy: Implication for volcanic processes

In addition to temperature, pressure and main chemical components, volatiles exert a strong influence on the physical properties of magmas. In particular, water plays a fundamental role in the dynamics and evolution of magmas in the deep interior and during volcano eruptions. However, water speciation in silicate melts is not fully understood. Infrared and NMR spectroscopy had provided some valuable informations about the H2O/OH- speciation. Nevertheless, some issues still remain unsolved about the OH-/H2O linkage to the silicate network. Raman spectroscopy already allows quantifying the proportion of water dissolved in an aluminosilicate melt. Raman spectra are composed of i) a low wave number region that corresponds to vibrations of the silicate network (0-1500 cm-1), and ii) a high wave-number region, which corresponds to the OH- stretching vibrations and H2O molecular vibrations (3100-3750 cm-1). We have performed a first set of in situ experiments using a micro-furnace at ambient atmosphere. An evolution of the high wave-number region in function of the time and temperature is observed. New Raman peaks can be distinguished, particularly near 3650-3700 cm-1. In this communication, we will present and discuss these observations between water and the silicate network in melts. Raman spectroscopy provides valuable informations to build a general model of water speciation, distinguishing the OH bonds between tetrahedral species and network modifiers for example. Results from this new model confirmed the amphoteric behavior of the water previously reported from polymeric modelling for instance, and opens new ways to study water in melts

Magmas are the largest repositories and carriers of earth's redox processes

Magma is the most important chemical transport agent throughout our planet. This paper provides an overview of the interplay between magma redox, major element chemistry, and crystal and volatile content, and of the influence of redox on the factors that drive igneous system dynamics. Given the almost infinite combinations of temperature, pressure, and chemical compositions relevant to igneous petrology, we focus on the concepts and methods that redox geochemistry provides to understand magma formation, ascent, evolution and crystallization. Particular attention is paid to the strong and complex interplay between melt structure and chemistry, and to the influence that redox conditions have on melt properties, crystallization mechanisms and the solubility of volatile components