1 3 Contrib Mineral Petrol (2015) 170:9 DOI 10.1007/s00410-015-1163-2 ORIGINAL PAPER Water, iron, redox environment: effects on the wadsleyite– ringwoodite phase transition Maria Mrosko 1 · Monika Koch‑Müller 1 · Catherine McCammon 2 · Dieter Rhede 3 · Joseph R. Smyth 4 · Richard Wirth 1 Received: 16 October 2014 / Accepted: 24 June 2015 © Springer-Verlag Berlin Heidelberg 2015 concentration of coexisting phases. Mössbauer (MB) spec- troscopy and electron energy loss spectroscopy as well as single-crystal X-ray diffraction were applied to gain insight into the Fe 3+ content and incorporation mechanisms. Under hydrous and reducing conditions, the wadsleyite–ring- woodite 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 ampli- fied. Thus, the stability field of wadsleyite is extended to higher pressure, most likely due to the higher water and Fe 3+ content in the wadsleyite structure compared to ring- woodite. Based on results from MB spectroscopy and sin- gle-crystal X-ray diffraction, we infer that Fe 3+ in wadsley- ite is incorporated as a spinelloid component and stabilizes wadsleyite to higher pressures. Keywords Wadsleyite · Ringwoodite · Phase stability · Spinelloid · Electron microprobe · Nominally anhydrous minerals · Ferric iron · FTIR spectroscopy · Mössbauer spectroscopy · Electron energy loss spectroscopy · Single-crystal X-ray diffraction Introduction Seismic wave velocity profiles reveal the Earth’s lay- ered structure indicated by jumps in the wave velocities at certain depths. Discontinuities at 410, 520, and 660 km were assigned to phase transitions of olivine to wadsley- ite, wadsleyite to ringwoodite, and ringwoodite to bridg- manite plus periclase by combining seismic observations with experimental studies (e.g., Akaogi 2007). In recent years, improvement in the spatial resolution of seismic Abstract The transition zone of the Earth’s upper man- tle 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 ring- woodite [γ-(Mg, Fe) 2 SiO 4 ] (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 mul- titude 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 H 2 O < 2 wt%), and the redox envi- ronment [using different buffers: Fe/FeO (reducing), Re/ ReO 2 (oxidizing)]. Run products were investigated using electron microprobe and Fourier transform infrared spec- troscopy to obtain the composition including the hydroxyl Communicated by Max W. Schmidt. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1163-2) contains supplementary material, which is available to authorized users. * Monika Koch-Müller mkoch@gfz-potsdam.de 1 Sektion 3.3, Chemie und Physik der Geomaterialien, Deutsches GeoForschungsZentrum, Telegrafenberg, 14473 Potsdam, Germany 2 Bayerisches Geoinstitut Bayreuth, Bayreuth, Germany 3 Sektion 4.2, Anorganische und Isotopenchemie, Deutsches GeoForschungsZentrum, Telegrafenberg, 14473 Potsdam, Germany 4 Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA