Abstract
We performed multi-anvil experiments in the system MgO-SiO2 ± H2O at 13.0–13.7 GPa and 1,025–1,300°C and in the system MgO-FeO-SiO2 ± H2O, under reducing conditions, at 11.0–12.7 GPa and 1,200°C, to depict the effect of H2O on the P-T-x coordinates of the 410-km discontinuity, i.e. the olivine–wadsleyite phase boundary. The charges were investigated with Electron Microprobe (EMP), Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), Secondary Ion Mass Spectrometry (SIMS) and Electron Energy Loss Spectroscopy (EELS). We observe in the MgO-SiO2-H2O system at 1,200°C a 0.6 GPa shift of the phase boundary to lower pressure compared to dry conditions, due to the stronger water fractionation into wadsleyite (wad) rather than in olivine (ol). In the MgO-FeO-SiO2-H2O system, we reproduced the triple point, i.e. observed coexisting hydrous ol, wad and ringwoodite (ring). SIMS H quantifications provided partitioning coefficients for water: \( D_{\text{wad/ol}}^{\text{water}} \) ~ 3.7(5) and \( D_{\text{ring/ol}}^{\text{water}} \) ~ 1.5(2) and \( D_{\text{wad/ring}}^{\text{water}} \) ~ 2.5(5). For a bulk composition of x Fe = 0.1, our data indicate only a slight difference in the width of the loop of the two phase field ol–wad under hydrous conditions compared to dry conditions, i.e. no broadening with respect to composition but a shift to lower pressures. For bulk compositions of x Fe > 0.2, i.e. in regions where wad–ring and ol–ring coexist, we observe, however, an unexpected broadening of the loops with a shift to higher iron contents. In total, the stability field of hydrous wad expands in both directions, to lower and higher pressures. Fe3+ concentrations as determined by EELS are very low and are expected to play no role in the broadening of the loops.
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Notes
Further on expressed either with wt ppm H2O or with the general term “water”.
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Acknowledgments
We thank Reiner Schulz for his important help and technical support during the multi-anvil press experiments and Julia Pohlenz. We are also very grateful to Dr. Michael Wiedenbeck for the precious help during the SIMS measurements. Further, we greatly appreciated the help of Oona Appelt (EMPA), Ilona Schäpan (SIMS) and the precious work of Anja Schreiber who prepared the FIB foils that were analyzed at TEM. We also thank Hans-Peter Nabein for his help in collecting the powder XRD-diffraction pattern. Last but not least, we are very grateful to Stefan Gehrmann and Gerhard Berger who prepared the samples for the EMPA and SIMS. The quality of this manuscript was improved, thanks to the suggestion and comments by an anonymous reviewer and of the editor Jochen Hoefs. This study was supported by a grant from Deutsche Forschungsgemeinschaft within the priority program SPP1236 under grant KO1260/7-1.
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Deon, F., Koch-Müller, M., Rhede, D. et al. Water and Iron effect on the P-T-x coordinates of the 410-km discontinuity in the Earth upper mantle. Contrib Mineral Petrol 161, 653–666 (2011). https://doi.org/10.1007/s00410-010-0555-6
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DOI: https://doi.org/10.1007/s00410-010-0555-6