Roland Stalder

Experimental approach to constrain second critical end points in fluid/silicate systems: Near-solidus fluids and melts in the system albite-H2O

Abstract Experimental investigations in the system albite-water have been carried out to evaluate the accuracy of a new procedure to determine major-element solubilities in fluid phases and water solubilities in melts. The system albite-water had been examined previously and some consensus has been achieved with respect to phase relations, solidi, and water solubilities in the melts. Above 15 kbar the system approaches a second critical end point, where a distinction between fluid and melt no longer can be made and the term solidus has to be reconsidered. All experimental runs were carried out at near-solidus conditions at 5-17 kbar and 625-775 °C. Experimental charges contained albite, water, and a layer of diamond crystals (grain size 50 mm). The pore space between the diamonds is preserved during the run indicating that the fluid was able to circulate throughout the entire capsule. During quenching, the material dissolved in the fluid precipitated between the diamond crystals and, as a result, could so be separated from the solid residue. The recovered capsules were directly embedded in epoxy and the diamond layers were analysed by laser ablation microprobe (LAM-ICP-MS). The new method allows the determination of a broad range of silicate/water ratios in aqueous fluids and hydrous melts. In some experimental runs at 15 and 17 kbar, fluids with approximately 40 wt% solute could be trapped leading to the conclusion that supercritical conditions were reached. Results furthermore indicate that albite dissolves non-stoichiometrically at 5 kbar producing a higher Na/Al and Si/Al in the solute than in the melt. Dissolution and melting at 10 kbar and higher pressures appears to be congruent within the limits of this method.

Influence of Fe, Cr and Al on hydrogen incorporation in orthopyroxene

Three series of enstatite crystals doped with different amounts of Fe, Cr or Al were synthesized under water-saturated conditions at 25 kbar. The OH-incorporation was studied by FTIR-spectroscopy on oriented crystal sections. Strongest absorptions (approximately 65 % of total absortion) occurred in all cases when n γ was parallel to the electric field vector E of the incident light, weakest absorptions were observed when n β was parallel to E. In addition to the O-H absortion bands of pure enstatite (at 3070 and 3361 cm −1 , respectively), IR spectra on orthopyroxenes containing trivalent cations reveal additional peaks between 3250 and 3350 cm −1 as well as above 3400 cm −1 , consistent with the generation of new hydrogen sites and a distortion of the polyhedral chains and cationic effects influencing the local electric field around the H bond. The amount of the incorporated OH correlates well with the concentration of trivalent cations in the crystal, consistent to the exchange mechanisms Fe 3+ + H + ⇔ 2 Mg 2+ and Cr 3+ + H + ⇔ 2 Mg 2+ . Comparison to natural samples from the Earth9s upper mantle strongly suggests that coupled substitutions involving Al 3+ dominate the H + incorporation.

The Mg(Fe)SiO3 orthoenstatite-clinoenstatite transitions at high pressures and temperatures determined by Raman-spectroscopy on quenched samples

Abstract The phase boundaries limiting the stability fields of the three different Mg(Fe)SiO3 phases [orthoenstatite (Oen), low-temperature clinoenstatite (LCen), and high-pressure clinoenstatite (HCen)] that occur at the pressure-temperature conditions prevailing in the Earth’s upper mantle are inferred from Raman spectroscopy on quenched samples from high P-T experiments. There are subtle but significant differences between the spectra of Oen and the spectra of samples quenched within the stability field of HCen or LCen. The most prominent differences are additional peaks at 369 and 431 cm-1 in the spectrum of Cen and a systematic shift of the peak at 236 cm-1 in the Oen spectrum to 243 cm-1 in the Cen spectrum. However, no distinction can be made between samples quenched from the HCen and LCen stability fields. This is consistent with the fact that the HCen phase is non-quench- able and that the pyroxene phase observed in the experimental products is LCen as verified by powder X-ray diffraction. Experiments performed in the pressure-temperature range 1.2-14 GPa and 750-1900 K have been used to constrain the Mg(Fe)SiO3 phase diagram. Pyroxene compositions cover the range from pure MgSiO3 to Mg0.9Fe0.1SiO3 with minor amounts of Al, Ca, Na, and Cr. The results are similar to previous determinations from X-ray and optical studies and tightly constrain the HCen-Oen phase boundary, which can be expressed by the equation P (GPa) = 0.00454 T (K) + 1.673. The LCen-Oen boundary is not as well constrained, but the data are sufficient to locate the invariant point where all three MgSiO3 phases coexist at 6.6 GPa and 820 °C.

Hydrogen incorporation in enstatite

A series of synthetic enstatites containing up to 1000 ppm (wt) Al were analysed with FTIR-spectroscopy. Pure enstatite exhibits only two essential O-H absorption bands at 3070 and 3362 cm −1 . As Al is incorporated in the crystal the absorption band at 3362 cm-1 is continuously split into two other peaks. In addition several new bands above 3400 cm −1 progressively evolve with Al incorporation. The intensities of these peaks correlate well with the Al-content of the sample. The observations are consistent with a model, where hydrogen in pure, Al-free enstatite is linked to the oxygen atoms O(3A) and O(3B), which are located at the SiO 4 -tetrahedron faces pointing towards the cavity between two M2-positions along the crystallographic b-axis. In Al-bearing samples additional water is mainly incorporated between the \(oxygen\ atoms\ O(2A)\ {\mbox{--}}\ O(1\ A)\ and\ O(2B)\ {\mbox{--}}\ O(1B)\) .

Water in upper mantle pyroxene megacrysts and xenocrysts – a survey study

Abstract Water content, mineral chemistry, and oxidation state of clino- and orthopyroxene xenocrysts and megacrysts were investigated by Fourier transform infrared (FTIR) spectroscopy-including focal plane array (FPA) detector mapping, Mössbauer spectroscopy, and electron microprobe. Various tectonic settings, ages, and modes of emplacement are represented by 23 samples from 6 areas (Massif Central, France; Letseng, Lesotho; Colorado, U.S.A.; Kakanui, New Zealand; Oahu, Hawaii; and New South Wales, Australia). The xenocrysts are from both garnet and spinel peridotites-including lherzolite and harzburgite varieties-and one sample of clinopyroxenite. Water contents vary between ~10 and 600 wt ppm. Samples from Massif Central, Colorado, Kakanui, and Hawaii have high water contents: 180-600 wt ppm. The samples from Lesotho and New South Wales have considerably lower amounts: ~10-300 wt ppm. Water contents of xenocrysts and megacrysts from New South Wales vary within a narrow range (clinopyroxene: ~50 wt ppm, orthopyroxene: 15-20 wt ppm), whereas the water content of the Lesothian samples scatter considerably. No significant correlations are observed between water content, mineral chemistry, or oxidation state of the samples. FPA mapping reveals homogenous distribution of water in the pyroxene lattice. The results are compared to available literature and research on water diffusion in natural mantle pyroxene. Altogether, the data suggest that water in pyroxene found in fresh peridotite xenoliths partly reflects the water content of the mantle source region. On the other hand, variable mineral chemistry and water contents of megacryst pyroxenes indicate processes such as magmatic equilibration, magma mixing, and contamination.

OH incorporation in synthetic diopside

Single crystals of diopside doped with different amounts of Fe, Cr, Al and Na were synthesized under silica- and water-saturated conditions at 20 kbar. The OH-incorporation was studied by FTIR-spectroscopy and secondary ion mass spectrometry. Pure diopside exhibits one OH-absorption band at 3357 cm −1 , whereas additional absorption bands appear for Al-, Fe- and Cr-doped diopsides at 3462, 3443 and 3432 cm −1 , respectively. The observed absorption bands are interpreted to be associated with M-site vacancies, and the absence of high wavenumber absorption bands is suggested to be diagnostic for a high silica activity. Incorporation of Na in a Cr-doped diopside caused a decrease of OH-defects, consistent with a simple increase of the kosmochlor component.

Intermediate states on the way to edge-sharing BO4 tetrahedra in M6B22O39·H2O (M=Fe, Co).

The new borates Fe(II)(6)B(22)O(39)·H(2)O (colourless) and Co(II)(6)B(22)O(39)·H(2)O (dichroic: red/bluish) were synthesised under the high-pressure/high-temperature conditions of 6 GPa and 880 °C (Fe)/950 °C (Co) in a Walker-type multi-anvil apparatus. The compounds crystallise in the orthorhombic space group Pmn2(1) (Z=2) with the lattice parameters a=771.9(2), b=823.4(2), c=1768.0(4) pm, V=1.1237(4) nm(3), R(1)=0.0476, wR(2)=0.0902 (all data) for Fe(6)B(22)O(39)·H(2)O and a=770.1(2), b=817.6(2), c=1746.9(4) pm, V=1.0999(4) nm(3), R(1)=0.0513, wR(2)=0.0939 (all data) for Co(6)B(22)O(39)·H(2)O. The new structure type of M(6)B(22)O(39)·H(2)O (M=Fe, Co) is built up from corner-sharing BO(4) tetrahedra and BO(3) groups, the latter being distorted and close to BO(4) tetrahedra if additional oxygen atoms of the neighbouring BO(4) tetrahedra are considered in the coordination sphere. This situation can be regarded as an intermediate state in the formation of edge-sharing tetrahedra. The structure consists of corrugated multiple layers interconnected by BO(3)/BO(4) groups to form Z-shaped channels. Inside these channels, iron and cobalt show octahedral (M1, M3, M4, M5) and strongly distorted tetrahedral (M2, M6) coordination by oxygen atoms. Co(II)(6)B(22)O(39)·H(2)O is dichroic and the low symmetry of the chromophore [Co(II)O(4)] is reflected by the polarised absorption spectra (Δ(t)=4650 cm(-1), B=878 cm(-1)).

Dehydration-hydration mechanisms in synthetic Fe-poor diopside

Small amounts (ppm) of OH in nominally anhydrous minerals (NAMs) can have a dramatic effect on the physical properties of the upper mantle. The pyroxenes of the upper mantle have been shown to inco ...

Influence of Fe on hydrogen diffusivity in orthopyroxene

The presence of iron has important consequences for hydrogen diffusion in many minerals, allowing charge compensation during hydrogen extraction by a redox mechanism. Here we present new experimentally determined diffusion data for hydrogen in synthetic orthopyroxenes in the system MgSiO 3 – FeSiO 3 derived from hydrogen extraction experiments. Results confirm the previously made assumption that hydrogen extraction is rate limited by hydrogen self diffusion, if a threshold value for x Fe is exceeded. Our best estimate for this threshold value is x Fe = 0.12 (±0.02). Natural orthopyroxenes from ultramafic rocks exhibit small, but significantly higher diffusivities than the synthetic samples, leading to the conclusion that other factors such as the concentration of M-site vacancies or the presence of other cations also may influence the rate of hydrogen extraction.

Dehydration mechanisms in synthetic Fe-bearing enstatite

In this study, we have investigated the relation between hydrogen extraction and Fe valence states by step-wise heating experiments performed on synthetic orthopyroxene. Samples doped with 57Fe, as well as 57Fe and Al, were synthesised at 25 kbar and 1400–1150°C, and were subsequently subjected to heating experiments in air or H2 at 700–1000°C and 1 atmosphere pressure. The change in OH content and Fe valence state was traced by infrared and Mosbauer spectra obtained on the same single crystals after the heat treatments. After heat treatments in air, the infrared spectra for Fe-doped samples show a progressive decrease in band intensities coupled to a corresponding increase in the Fe3+ doublet in Mosbauer spectra, indicating that the dehydration follows the redox reaction \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(OH^{{-}}\ +\ Fe^{2+}\ =\ O^{2{-}}\ +\ Fe^{3+}\ +\ \frac{1}{2}\ H_{2}.\) \end{document} However, the sample with the maximum Al-content showed a different behaviour, where the dehydration is coupled to a decrease in Fe2+ contents associated with hematite precipitation, as shown by Mosbauer spectra. For samples annealed in H2 atmosphere (700–1000°C) a correlation of the decrease of the amount of Fe3+ and the intensity of specific IR bands associated with Fe3+, that decrease faster than other bands, was observed. The results indicate that at least two different dehydration steps are active in dehydration processes in orthopyroxene, i.e. one reaction creating a Fe3+-specific OH-defect and one reaction consuming a Fe3+-specific OH-defect. Both dehydration steps exhibit different kinetic characteristics and the previous observation is confirmed that dehydration kinetics in pyroxenes depends on Fe content. For one sample the spatial distribution of the different OH-defects was mapped using synchrotron FTIR micro-spectroscopy, confirming the different stability of Fe-related and Fe-unrelated OH-defects.