Low-pressure polymorphs of AlOOH and FeOOH are common natural oxyhydroxides at the Earth's surface, which may transport hydrogen to the deep mantle via subduction. At elevated pressures, the low-pressure polymorphs transform into δ-AlOOH and ε-FeOOH with CaCl2-type structure, which form a solid solution above 18 GPa. Nevertheless, few studies have examined the solid solution behavior of this binary system in detail. In this study, we ascertain the phase relations in the AlOOH–FeOOH binary system at 15–25 GPa and 700–1200 °C. X-ray diffraction (XRD) measurements of quenched samples show that δ-AlOOH and ε-FeOOH partly form solid solutions over wide pressure and temperature ranges. Our results demonstrate that a binary eutectic diagram is formed without dehydration or melting below 1200 °C at 20 GPa. We also observe that the maximum solubilities of Al and Fe in the solid solutions are more strongly influenced by temperature than pressure. Our results suggest that the CaCl2-type hydroxides subducted into the deep mantle form a solid solution over a wide composition range. As AlOOH and FeOOH are present in hydrous crust, these phases may be subducted into the deep interior, transporting a significant amount of hydrogen to deeper regions. Therefore, a better understanding of this binary system may help elucidate the model geodynamic processes associated with the deep water cycling in the Earth.

H2O, which is present as H+ or OH in minerals, plays an important role in geological processes in the Earth's interior, affecting the physical and chemical properties of its surroundings such as melting temperature, melt composition, and phase relations (e.g., Inoue 1994; Chang et al. 2017; Hwang et al. 2017; Myhill et al. 2017). Water may be transported into the deep Earth via subducting slabs within hydrous phase. Previous studies have been evaluated the stability regions of various hydrous phases, especially dense hydrous magnesium silicates (DHMSs), which have been suggested as potential water carriers to the transition zone and even to the lower mantle in cold subducting slabs (e.g., Frost and Fei 1998; Litasov and Ohtani 2003; Komabayashi et al. 2005).

The oxyhydroxides goethite (α-FeOOH) and AlOOH are common hydrous minerals in the sediments. δ-AlOOH is a high-pressure polymorph of diaspore (α-AlOOH) and boehmite (γ-AlOOH) (e.g., Suzuki et al. 2000; Xue and Kanzaki 2007) that has been shown to be stable up to 170 GPa (e.g., Sano et al. 2008; Tsuchiya and Tsuchiya 2011; Ohira et al. 2014). The other potential water carrier, ε-FeOOH, which is a high-pressure polymorph of goethite (α-FeOOH), akaganeite (β-FeOOH), and lepidocrocite (γ-FeOOH), has a CaCl2-type structure as δ-AlOOH (e.g., Suzuki 2010). ε-FeOOH has been known to have a stability field in the lower mantle (e.g., Nishi et al. 2017). To investigate the physical properties of these CaCl2-type hydrous phases, numerous experiments and calculations have been performed particularly addressing the stability, compressibility, and crystallography of the end-members δ-AlOOH, ε-FeOOH, and phase H (MgSiO4H2) (e.g., Tsuchiya et al. 2002; Panero and Stixrude 2004; Xue and Kanzaki 2007; Suzuki 2010; Komatsu et al. 2011; Kuribayashi et al. 2013; Nishi et al. 2014; Bindi et al. 2014; Zhong et al. 2016; Thompson et al. 2017). Earlier results have also shown that phase H and δ-AlOOH form a solid solution because of their similar structures (e.g., Ohira et al. 2014; Panero and Caracas 2017). This AlOOH-MgSiO4H2 solid solution can coexist with bridgmanite above 40 GPa along the slab geotherm (e.g., Nishi et al. 2014; Ohira et al. 2014; Nishi 2015; Bindi et al. 2015; Walter et al. 2015; Panero and Caracas 2017).

The discovery of hydrous aluminum silicate phase Egg (AlSiO3OH) as inclusions in diamond implies that the crustal hydrous material may have been subducted into the transition zone, at least locally (e.g., Wirth et al. 2007). More recently, hydrous ringwoodite and ice-VII were also found as inclusions in diamond, suggesting their presence in a partially hydrated mantle transition zone and the lower mantle (e.g., Pearson et al. 2014; Tschauner et al. 2018). Because AlOOH and FeOOH are common oxyhydroxides at Earth's surface, these solid solutions in the deep Earth play an important role for water transportation in subducted slabs (e.g., Nishi et al. 2017). Nevertheless, the solubility behavior of this binary system is not well constrained. A better understanding of this system will also help elucidate the water cycle of iron-rich martian mantle. Therefore, this study was conducted to determine the solubility behavior of AlOOHFeOOH binary system at 15–25 GPa and 700–1200 °C.

We conducted high-pressure high-temperature experiments using a Kawai-type 1000-ton multi-anvil apparatus at the Geodynamics Research Center (GRC), Ehime University. Pressures were calibrated at room temperature by diagnostic changes in the electrical resistances of ZnTe (9.6 and 12.0 GPa), ZnS (15.5 GPa), GaAs (18.3 GPa), and GaP (23.0 GPa) induced by the semiconductor–metal phase transitions at high pressures. Tungsten carbide cubes with a truncation edge length (TEL) of 4 mm were used in combination with Co-doped MgO-octahedra of 10 mm edge length (10/4 assemblage). Preformed pyrophyllite gaskets were used between the cubes, LaCrO3 was used as the heater, and a gold sample capsule was used in the cell assemblage. The temperature was monitored using a W97Re3-W75Re25 thermocouple, and the electromotive force (EMF) was not corrected for the effects of pressure. The sample was compressed to a desired pressure and then the oil pressure was held constant. Subsequently, the AC power was supplied to the heater in the furnace assemblage. After heating at high pressure, the power was stopped by shutting off the electric power supply. Samples were recovered after releasing pressure slowly for 12 h.

The starting material, with a chemical composition of (Al0.5Fe0.5)OOH, was prepared by mixing oxides Al(OH)3, Al2O3, and Fe2O3 in appropriate proportions. The recovered run products were mounted in epoxy resin and were polished to perform phase identification and chemical composition analysis. The phase assemblages were characterized using a micro-focus X-ray diffractometer (MicroMax-007HF; Rigaku Corp.) with CuKα radiation. The obtained data were processed by 2PD software, which can display and process two-dimensional data, including smoothing, background correction and 2D to 1D conversion. The micro-textures and composition were obtained using a field emission scanning electron microscope (FESEM, JSM7000F; JEOL) combined with an energy-dispersive X-ray spectrometer (EDS, X-MaxN; Oxford Instruments). Working parameters of 15 kV, 1 nA and collection times of 30–50 s were used. The chemical composition was analyzed using EDS. We used software (Aztec ver. 2.4; Oxford Instruments Nanotechnology Tools Ltd.) to process EDS data. Stoichiometric composition of (FeX,Al1–X)OOH was assumed, where X is the Fe/(Fe+Al) ratio. The total contents of the obtained (FeX,Al1–X)OOH are approximately 85 wt%.

Table 1 presents the experimental conditions and chemical compositions of the synthesized samples determined by EDS. In all runs, the backscattered electron (BSE) images show that the recovered samples consist mainly of two phases except for the highest temperature run (1200 °C), which was likely above the solidus (Fig. 1). X-ray diffraction (XRD) patterns of the samples indicate that these phases are ε-FeOOH and δ-AlOOH (Fig. 2), although their peak positions are shifted from those of the ideal end-members, as discussed later.

At 20 GPa and 900 °C, a BSE image (Fig. 1a) shows the presence of two phases in bright and dark regions with respective chemical compositions of ε-Fe0.78Al0.22OOH and δ-Al0.91Fe0.09OOH. These chemical compositions can be explained to be the result of forming solid solutions because of the similar crystal structures of the two end-members. The compositions and grain sizes of both phases changed at elevated temperatures up to 1100 °C (Fig. 1b). With increasing temperature, the Al/Fe ratio decreases in Al-rich δ-phase, whereas it increases in Fe-rich ε-phase (Table 1). At 1200 °C, Al-rich δ-phase was found to coexist with iron oxide (bright regions) and melt (Fig. 1c), which means Fe-rich ε-phase totally decomposed at this condition. Some amount of H2O escaped when the capsule was opened. At a lower pressure of 15 GPa at 1000 °C, the crystal shape of Al-rich δ-phase seemed to transform into an elongated shape (Fig. 1d), which might occur because of the phase change, as indicated by XRD (described below).

Selected room-temperature XRD patterns of the recovered samples are summarized in Figure 2. Al-rich δ-phase, Fe-rich ε-phase and low-pressure Al-rich α-phase are observed. All the peaks are consistent with the earlier studies of δ-AlOOH and ε-FeOOH (e.g., Suzuki et al. 2000; Suzuki 2010; Kuduvalli et al. 2008; Mashino et al. 2016). Our results suggest that the solid solutions are formed in the δ-AlOOH–ε-FeOOH binary systems over large compositional ranges. The XRD peaks of Fe-rich ε-phase and Al-rich δ-phase are shifted to toward higher and lower angles, respectively, at 1100 °C and 20 GPa compared to 900 °C and 20 GPa. These observations are consistent with SEM-EDS results, which indicate that Al contents increase in Fe-rich ε-phase, whereas Fe contents increase in Al-rich δ-phase at higher temperatures (Table 1). At 15 GPa and 1000 °C, probably because of the low pressure, a diffraction peak of α-AlOOH instead of δ-AlOOH is present (Fig. 2c).

Figure 3 plots the unit-cell volumes of solid solutions between δ-AlOOH and ε-FeOOH as a function of FeOOH content. As shown in this figure, partial solid solutions were formed at 20–25 GPa and 700–1200 °C. The volume of Fe-rich ε-phase increases with increasing the Fe content, likely because of the large size of VIFe3+ ion (0.645 Å) relative to VIAl3+ ion (0.535 Å). This is consistent with previous experimental studies of a similar (Al,Fe)OOH bulk composition conducted at 21 and 40 GPa (e.g., Fig. 4; Kawazoe et al. 2017; Nishi et al. 2017).

The phase relations of the δ-AlOOH–ε-FeOOH binary system are presented in Figure 4. The dashed line in Figure 4 shows the dehydration temperature of δ-AlOOH (approximately 1200 °C) at 20 GPa (e.g., Ohtani et al. 2001). Similar thermal stability limit for δ-AlOOH was also reported in the previous studies that show the decomposition of the phase Egg into δ-AlOOH + SiO2 at ~1200 °C at transition-zone pressures (e.g., Sano et al. 2004; Fukuyama et al. 2017). In this study, a binary eutectic diagram is formed without dehydration or melting below the dashed line in Figure 4 from 700 to 1100 °C at 20 GPa, and at pressures of 15–25 GPa at 1000 °C (except for the stability field of α-AlOOH). AlOOH and FeOOH solid solutions are observed over wide temperature and pressure ranges. Although an earlier study described the dehydration of δ-AlOOH at 1200 °C (e.g., Ohtani et al. 2001), our data show that Al-rich δ-phase persists at 1200 °C, whereas the Fe-rich ε-phase decomposes into Fe2O3 + H2O (Fig. 2d) under the same P–T conditions. Al-rich δ-phase was observed at 1200 °C in contrast to previous studies (e.g., Ohtani et al. 2001). This suggests that a thermal gradient may have existed in our cell but could not be conclusively identified, because we placed the thermocouple at the center of the heater, where the highest temperature should be expected. To the best of our knowledge, no previous study has determined the melting temperature of ε-FeOOH at the transition zone pressure conditions. Our results suggest that ε-FeOOH has a lower dehydration temperature than that of δ-AlOOH. We also found that the Al-content in FeOOH is larger than the Fe-content of AlOOH. Because of the smaller ionic radius of Al3+ than Fe3+, on the other hand, Al3+ is more compatible with the octahedral site in ε-FeOOH than that of Fe3+ in δ-AlOOH, leading to the higher Al-content in ε-FeOOH than the Fe-content of δ-AlOOH. It should be pointed out that EDS cannot precisely measure chemical compostion of the recovered sample at 700 °C due to its small crystal size. Therefore, chemical compositions of the two phases at 700 °C (dashed circles in Fig. 4) were estimated from the relation between unit-cell volume and composition from Figure 3.

The chemical compositions and their temperature dependences of δ-phase and ε-phase at 20 GPa in this work are consistent with those found at 40 GPa in an earlier study (e.g., Fig. 4; Nishi et a. 2017). Consequently, compared with pressure, temperature seems to play a more significant role in the partitioning of Al and Fe between δ-AlOOH and ε-FeOOH at pressures up to 40 GPa. For example, although the samples were recovered from different pressures, the similar Al/Fe ratios of hydroxides were observed at 1000 °C. This means that temperature has a more profound influence on compositional stability than pressure in the AlOOH-FeOOH binary system. As a result, a series of intermediate compositions were identified in the temperature range 700–1250 °C. On the other hand, the dehydration temperatures of both δ-phase and ε-phase are likely to increase concomitantly with increasing pressure, because these phases were observed at 1240 °C and 40 GPa in an earlier study (Fig. 4). Therefore, AlOOH and FeOOH may form a complete solid solution near the solidus at higher pressures, although they do not at 20 GPa.

The CaCl2-type FeOOH phase has been observed to remain stable at 76 GPa and 2150 K (e.g., Hu et al. 2016). The FeOOH and AlOOH end-members form solid solutions at elevated temperatures and pressures in the subducting plate. As such, they are expected to transport water (H+) into the deep lower mantle, even undergoing change in their respective chemical compositions depending on the temperature, as shown in this study. Moreover, δ-AlOOH reportedly forms a solid solution with phase H (MgSiO4H2) and coexists with bridgmanite along the pressure and temperature path corresponding to the subduction (e.g., Nishi et al. 2014; Ohira et al. 2014; Panero and Caracas 2017). Consequently, it is important to examine the influence of the addition of phase H (MgSiO4H2) to the stability of CaCl2-type hydrous phases in the δ-AlOOH–ε-FeOOH phase H ternary system, which is expected to form over a wide composition range, and may carry a significant amount of hydrogen into the lower mantle.

This work was supported by JSPS KAKENHI Grant Numbers 18J12511 to C.X. and 26247073, 15H05828, 18H03740 to T.I. C.X. was supported by Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists (DC2).

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