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Effects on the Wadsleyite–Ringwoodite Phase Transition

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Effects on the Wadsleyite–Ringwoodite Phase Transition 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 Mrosko1 · Monika Koch‑Müller1 · Catherine McCammon2 · Dieter Rhede3 · Joseph R. Smyth4 · Richard Wirth1 Received: 16 October 2014 / Accepted: 24 June 2015 © Springer-Verlag Berlin Heidelberg 2015 Abstract The transition zone of the Earth’s upper man- concentration of coexisting phases. Mössbauer (MB) spec- tle is characterized by three discontinuities in seismic wave troscopy and electron energy loss spectroscopy as well as velocity profiles. One of these at about a depth of 520 km single-crystal X-ray diffraction were applied to gain insight 3 is assigned to the transformation of wadsleyite (β-) to ring- into the Fe + content and incorporation mechanisms. Under woodite [γ-(Mg, Fe)2SiO4] (e.g., Shearer in J Geophys hydrous and reducing conditions, the wadsleyite–ring- Res 101:3053–3066, 1996). The exact location, width, and woodite boundary shifts by 0.5 GPa to higher pressures other properties of that discontinuity are affected by a mul- accompanied by a broadening of the region of coexisting titude of parameters. The present study specifically focuses wadsleyite and ringwoodite. In contrast, under hydrous and on the effect of water, iron content, and redox environment oxidizing conditions, the two-phase field gets narrower and on the depth of the phase transition. We performed high- the shift of the two-phase field to higher pressure is ampli- pressure experiments in a multi-anvil apparatus at 1200 °C fied. Thus, the stability field of wadsleyite is extended to with variation in Mg–Fe compositions (0.10 < xFe < 0.24), higher pressure, most likely due to the higher water and 3 water contents (0 < xH2O < 2 wt%), and the redox envi- Fe + content in the wadsleyite structure compared to ring- ronment [using different buffers: Fe/FeO (reducing), Re/ woodite. Based on results from MB spectroscopy and sin- 3 ReO2 (oxidizing)]. Run products were investigated using gle-crystal X-ray diffraction, we infer that Fe + in wadsley- electron microprobe and Fourier transform infrared spec- ite is incorporated as a spinelloid component and stabilizes troscopy to obtain the composition including the hydroxyl wadsleyite to higher pressures. Keywords Wadsleyite · Ringwoodite · Phase stability · Communicated by Max W. Schmidt. Spinelloid · Electron microprobe · Nominally anhydrous minerals · Ferric iron · FTIR spectroscopy · Mössbauer Electronic supplementary material The online version of this spectroscopy · Electron energy loss spectroscopy · article (doi:10.1007/s00410-015-1163-2) contains supplementary Single-crystal X-ray diffraction material, which is available to authorized users. * Monika Koch‑Müller mkoch@gfz‑potsdam.de Introduction 1 Sektion 3.3, Chemie und Physik der Geomaterialien, Deutsches GeoForschungsZentrum, Telegrafenberg, Seismic wave velocity profiles reveal the Earth’s lay- 14473 Potsdam, Germany ered structure indicated by jumps in the wave velocities 2 Bayerisches Geoinstitut Bayreuth, Bayreuth, Germany at certain depths. Discontinuities at 410, 520, and 660 km were assigned to phase transitions of olivine to wadsley- 3 Sektion 4.2, Anorganische und Isotopenchemie, Deutsches GeoForschungsZentrum, Telegrafenberg, 14473 Potsdam, ite, wadsleyite to ringwoodite, and ringwoodite to bridg- Germany manite plus periclase by combining seismic observations 4 Department of Geological Sciences, University of Colorado, with experimental studies (e.g., Akaogi 2007). In recent Boulder, CO 80309, USA years, improvement in the spatial resolution of seismic 1 3 9 Page 2 of 12 Contrib Mineral Petrol (2015) 170:9 measurements have shown that the depths of the afore- In this study, we investigate the effect of water and oxy- mentioned discontinuities are not constant, but can vary gen fugacity on the phase stability of wadsleyite and ring- by about 100 km on a global scale (e.g., Deuss and Wood- woodite at 1200 °C and pressures between 15 and 20 GPa. house 2001; Van der Meijde et al. 2005; Schmerr and Gar- We show that in the presence of water and under reducing nero 2007; Cao et al. 2010). While some depth variations conditions (Fe/FeO), the wadsleyite–ringwoodite phase can be explained by temperature and chemical gradients, boundary is shifted by 0.5 GPa to higher pressures accom- there is still controversy over whether the incorporation of panied by a broadening of the region of coexisting wads- water has an effect on the position of the discontinuities leyite and ringwoodite. In contrast, under hydrous and oxi- (e.g., Litasov and Ohtani 2007). dizing conditions (Re/ReO2), the two-phase field narrows Olivine, wadsleyite, and ringwoodite belong to the and the shift of the phase transition to higher pressures is group of nominally anhydrous minerals (NAM), which can amplified. incorporate hydrogen as hydroxyl groups via point defects. Expressed as wt% H2O, olivine can incorporate hydrogen up to about 1 wt% H2O (Smyth et al. 2006), while both Experimental and analytical methods wadsleyite and ringwoodite can take up to 3 wt% H2O as hydroxyl (e.g., Inoue et al. 1995). Beyond that, it is known High-pressure experiments that both wadsleyite and ringwoodite may incorporate vari- able amounts of ferric iron in their structures (e.g., McCam- The experimental runs were performed in a multi-anvil mon et al. 2004). Frost and McCammon (2009) determined apparatus similar to that of Walker (1991), but with a spe- experimentally that the stability field of wadsleyite expands cial tool that allows alternatively a continuous 360° rota- with respect to the stability fields of olivine and ringwood- tion or a 180° rocking motion of the Walker high-pressure 1 ite under more oxidizing conditions. Bolfan-Casanova module with 5° s− in order to avoid separation of the et al. (2012) investigated the ferric iron and H content in fluid and solid during the run and thus ensure a homogene- wadsleyite synthesized at 12–14 GPa and 1400 °C under ous starting material (see Schmidt and Ulmer 2004; Deon oxidizing and hydrous conditions. They used Mössbauer et al. 2011). We applied the 180° rocking mode to all of our (MB) spectroscopy as well as X-ray absorption spectros- hydrous runs. We used a 10/5-assembly (octahedron length/ copy to measure the ferric iron content and secondary ion truncation length) with an MgO-based octahedron serving mass spectroscopy to determine the water concentration. as pressure-transmitting medium, a stepped graphite heater, They observed that the ferric iron content increases with and pyrophyllite gaskets. One experiment (MA-286) was 3 total iron content reaching a maximum of 30 % Fe +/ΣFe performed with a stepped LaCrO3 heater. Starting materials at x 0.4, while the water concentration ranged from 740 were placed in platinum capsules, which were 2 mm long Fe = to 1650 wt ppm H2O. Smyth et al. 2014 determined the and had a 1.4 mm outer diameter. The temperature was con- crystal structure of ferrous and ferric iron-bearing hydrous trolled by a W5 %Re–W26 %Re thermocouple with emf wadsleyite of the former study and found up to 11 % iron values (electromotive force) uncorrected for pressure. The (presumably ferric) in the tetrahedral site. Bolfan-Casanova estimated uncertainty in temperature is 25 °C. In each run, ± et al. (2012) and Smyth et al. (2014) speculate that both we recorded power–temperature profiles to produce a cali- oxidizing and hydrous conditions are necessary to enlarge bration for experiments, in which the thermocouple did not the stability field of wadsleyite relative to ringwoodite. survive the pressure increase. The temperature uncertainty Evidence for water incorporation into natural ringwood- based on electrical power output is estimated to be 50 °C. ± ite is given in a recent study (Pearson et al. 2014), which The 10/5-assemblies (both with graphite and LaCrO3 heat- reports 1.4 wt% H2O in ringwoodite occurring as inclusion ers) were calibrated using the following phase transitions: in diamond. coesite–stishovite (Akaogi et al. 1995), α–β Mg2SiO4 Experiments simulating the conditions at the 410-km (Morishima et al. 1994), β–γ Mg2SiO4 (Inoue et al. 2006), discontinuity already showed that the presence of water and enstatite–β-Mg2SiO4–stishovite (Gasparik 1989). For extends the stability field of wadsleyite to lower pressures the 10/5-assembly, 17 calibration runs were performed and (Frost and Dolejs 2007; Deon et al. 2011) due to high frac- we estimated the uncertainty in pressure to 0.3 GPa. ± tionation of water into wadsleyite. Former experimental In the Mg-endmember system, we used synthetic for- studies, which simulate the conditions at the 520-km dis- sterite as starting material either with or without water continuity in a peridotite system, proposed that there is a in excess (Table 1). This allows us to determine whether significant shift of the wadsleyite to ringwoodite transition the presence of water affects the transformation pressure of about 2 GPa to higher pressures in the presence of water of wadsleyite to ringwoodite at 1200 °C compared to the (Litasov and Ohtani 2003; Kawamoto 2004). However, this dry system (19 GPa, 1200 °C, Inoue et al. 2006). In run needs further investigation. MA-289, D2O (plus H2O) was added instead of pure H2O 1 3 Contrib Mineral Petrol (2015) 170:9 Page 3 of 12 9 to ensure that the broad absorption bands in ringwoodite filled into the capsule to the other runs before addition of are solely related to hydrogen vibrations. the starting material. The filled capsules were cold-sealed. In all but one run in the (Mg, Fe)2SiO4 system, stoichio- We used different buffers to control the oxygen fugacity metric oxide mixtures of FeO, MgO, and SiO2 with differ- during the runs. Oxidizing conditions (runs MA-311, -326, ent FeO/MgO ratios served as starting materials (Table 2). -328) were set by adding Re/ReO2 to the starting material, 57 In run MA-326, iron was added as Fe-enriched Fe2O3 for while runs MA-309 and MA-323 were performed under MB spectroscopy.
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