Abstract
The transition zone of the Earth’s upper mantle is characterized by three discontinuities in seismic wave velocity profiles. One of these at about a depth of 520 km is assigned to the transformation of wadsleyite (β-) to ringwoodite [γ-(Mg, Fe)2SiO4] (e.g., Shearer in J Geophys Res 101:3053–3066, 1996). The exact location, width, and other properties of that discontinuity are affected by a multitude of parameters. The present study specifically focuses on the effect of water, iron content, and redox environment on the depth of the phase transition. We performed high-pressure experiments in a multi-anvil apparatus at 1200 °C with variation in Mg–Fe compositions (0.10 < x Fe < 0.24), water contents (0 < \(x_{{{\text{H}}_{2} {\text{O}}}}\) < 2 wt%), and the redox environment [using different buffers: Fe/FeO (reducing), Re/ReO2 (oxidizing)]. Run products were investigated using electron microprobe and Fourier transform infrared spectroscopy to obtain the composition including the hydroxyl concentration of coexisting phases. Mössbauer (MB) spectroscopy and electron energy loss spectroscopy as well as single-crystal X-ray diffraction were applied to gain insight into the Fe3+ content and incorporation mechanisms. Under hydrous and reducing conditions, the wadsleyite–ringwoodite boundary shifts by 0.5 GPa to higher pressures accompanied by a broadening of the region of coexisting wadsleyite and ringwoodite. In contrast, under hydrous and oxidizing conditions, the two-phase field gets narrower and the shift of the two-phase field to higher pressure is amplified. Thus, the stability field of wadsleyite is extended to higher pressure, most likely due to the higher water and Fe3+ content in the wadsleyite structure compared to ringwoodite. Based on results from MB spectroscopy and single-crystal X-ray diffraction, we infer that Fe3+ in wadsleyite is incorporated as a spinelloid component and stabilizes wadsleyite to higher pressures.
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Acknowledgments
We thank A. Schreiber, U. Dittmann, A. Ebert, R. Schulz, H.-P. Nabein for technical support and Wilhelm Heinrich for discussion. MM thanks Bastian Joachim for fruitful discussions. This study was supported by a Grant from Deutsche Forschungsgemeinschaft with the priority Program SPP1236 und Grant KO1260/11.
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Communicated by Max W. Schmidt.
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Mrosko, M., Koch-Müller, M., McCammon, C. et al. Water, iron, redox environment: effects on the wadsleyite–ringwoodite phase transition. Contrib Mineral Petrol 170, 9 (2015). https://doi.org/10.1007/s00410-015-1163-2
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DOI: https://doi.org/10.1007/s00410-015-1163-2