Raman Observations of the oh Stretching Region in Hydrous ß-Mg₂SiO₄ (Wadsleyite) at High Hydrostatic Pressures

Annette Kleppe (annettek@earth.ox.ac.uk)¹, Andrew P. Jephcoat (andrew@earth.ox.ac.uk)¹, Helmut Olijnyk¹, Audrey E. Slesinger², Bernard J. Wood² & Simon C. Kohn²

¹ Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK

² Department of Earth Sciences, Wills Memorial Building, Bristol BS8 1RJ, UK

The presence of water as OH in nominally anhydrous minerals such as ß-Mg₂SiO₄ (wadsleyite) could significantly affect phase relations and the bulk elastic properties in the mantle transition zone. Much recent work including IR, Raman and NMR spectroscopies as well as bond-strength systematics has provided evidence for protonation at the O1a,b sites, although O2 protonation has also been suggested. The pressure dependence of the Raman spectrum has not been well characterized above 21 GPa. We report high-pressure Raman observations on single crystals (polished to 0.050x0.030 mm) in the diamond-anvil cell in the range 200-4000 cm^-1 including the OH stretch region. The hydrous wadsleyite was synthesized at 16 GPa and 1300°C in a 1000-ton multi-anvil press from mixtures of MgO, Mg(OH)₂ and SiO₂ in welded platinum capsules, and well characterized by NMR techniques, giving a water content of up to a maximum of 2.5wt%. The 1-bar Raman spectra showed a broad asymmetric doublet comprising two components at 3331 and 3376 cm^-1 and a single mode at 3586 cm^-1. Change in the orientation of the crystal correlated with a relative change in the intensity of these components and may be associated with differences in the H environment at this high water content. Previous observations in the OH region have usually reported a single mode, but with a frequency corresponding to either one of the above components. The experiments in the DAC were performed with a helium pressure medium and 514.5- and 647.1-nm excitation. A baseline structural constraint was obtained from Rietveld refinement of powder diffraction data obtained with image-plate techniques at beamline ID9 of the European Synchrotron Radiation Facility and is consistent with orthorhombic symmetry (S.G. Imma) of previous studies.

Dehydration Melting Reaction of Amphibole: A Comparative Study of Melting and Crystallization Experiments

Kevin Klimm (k.klimm@mineralogie.uni-hannover.de), Wilhelm Johannes & Jürgen Koepke

Institut für Mineralogie, Universität Hannover, Welfengarten 1, 30167 Hannover, Germany

Numerous melting experiments with plagioclase(plag)-amphibole(hbl)-bearing assemblages were performed to elucidate the formation of felsic magmas from amphibolites. Chemical equilibrium in such experiments is difficult to attain. Therefore, in this study crystallization experiments with melts of same composition as the solid starting materials have been performed in addition to melting experiments to clarify the reaction kinetics in both types of experiments. The runs were conducted in a piston-cylinder apparatus at 1000°C, 1.2 GPa, for 2 to 36 days. Starting materials were (1) a mixture of 65wt% hbl and 35 wt% plag (An₆₀) for melting experiments and (2) a glass containing 1.25 wt% H₂O for crystallization experiments. Hbl and plag starting minerals were separated in fractions with grain sizes: < 3, 5-10, 10-50 µm. In melting experiments, the products were plag, clinopyroxene, garnet and melt. Only plag with grain size < 3 µm reacted completely after 8 days or more. No complete equilibrium could be attained in runs with grain sizes above 5 µm and duration up to 36 days. The composition of glasses changed significantly with run duration as long as the plag has not completely reacted. In crystallization experiments hbl was observed in addition to plag, cpx, garnet, and melt. The composition of these phases do not change with time. When compared to melting experiments in which plag reacted completely, the plag and melt composition are similar, even after 2 day runs. Thus, equilibrium conditions can be reached more rapidly with crystallization experiments. A possible explanation for the presence of hbl is the lower Fe²⁺/Fe³⁺ ratio in the glass when compared to the mineral mixture, which may shift the stability field of hbl towards higher temperatures. Experiments with identical Fe²⁺/Fe³⁺ need to be conducted to compare adequately the melting and crystallization products.

Cordierite as a Monitor of Fluid and Melt Sodium Activity in Metapelites, Migmatites and Granites: Constraints from Incorporation Experiments

Erich Knop (erich.knop@uibk.ac.at) & Peter W. Mirwald (peter.mirwald@uibk.ac.at)

Leopold-Franzens-Universität, Institut für Mineralogie und Petrographie, Innrain 52, A-6020 Innsbruck, Austria

Many analyses of natural cordierites show significant sodium contents up to 1.1 wt% Na₂O (0.22 apfu). In order to investigate the poorly constrained Na incorporation mechanisms, we performed piston cylinder experiments in the system Na₂O-MgO-Al₂O₃-SiO₂-CO₂-H₂O (NMASCH) from 2 to 8 kbars and 500 to 850°C. We used mixtures of pre-synthesised orthorhombic low-cordierite + albite-glass + H₂O or H₂O/CO₂-mixtures as starting materials. The experiments using Crd + Ab + H₂O are complicated since an eutectic occurs at about Ab₉₀-Crd₁₀ leading to the formation of small amounts of melt at temperatures as low as 680°C at 5 kbar.

Results of incorporation experiments under subsolidus conditions suggest a nearly linear decrease of the Na content between 500 and 700°C. If CO₂ is present in the experiments the amount of incorporated Na significantly decreases, compared with runs with pure H₂O. Experiments performed between 700 and 850°C, i.e. above the observed eutectic, showed a significantly higher Na content due to the formation of partial melt. However, the data still maintain an inverse correlation. The experimental data show that Na incorporation into cordierite is (1) a function of temperature and (2) a function of activity of Na (a_Na) in the coexisting fluid or melt. The data suggest that a_Na in the coexisting fluid phase is a function of the physical state of the fluid itself (vapour or melt) as well as a function of the CO₂ content of the vapour. Melt formation leads to an increase in a_Na, and introduction of CO₂ into the vapour phase leads to a decrease in a_Na.

Implications of the experimental results for the Na content of natural cordierites will be discussed.