Aluminous hydrous magnesium silicate as a lower-mantle hydrogen reservoir: a role as an agent for material transport

The potential for storage of a large quantity of water/hydrogen in the lower mantle has important implications for the dynamics and evolution of the Earth. A dense hydrous magnesium silicate called phase D is a potential candidate for such a hydrogen reservoir. Its MgO–SiO2–H2O form has been believed to be stable at lower-mantle pressures but only in low-temperature regimes such as subducting slabs because of decomposition below mantle geotherm. Meanwhile, the presence of Al was reported to be a key to enhancing the thermal stability of phase D; however, the detailed Al-incorporation effect on its stability remains unclear. Here we report on Al-bearing phase D (Al-phase D) synthesized from a bridgmanite composition, with Al content expected in bridgmanite formed from a representative mantle composition, under over-saturation of water. We find that the incorporation of Al, despite smaller amounts, into phase D increases its hydrogen content and moreover extends its stability field not only to higher temperatures but also presumably to higher pressures. This leads to that Al-phase D can be one of the most potential reservoirs for a large quantity of hydrogen in the lower mantle. Further, Al-phase D formed by reaction between bridgmanite and water could play an important role in material transport in the lower mantle.

it could be stable to ~ 130 GPa, corresponding to a pressure at the lowermost mantle, along a subducting slab geotherm. Thus, the presence of Al is a potential key-factor for enhancing the thermal stability of these DHMS phases at the lower-mantle pressures. The recent high-pressure experiments [16][17][18] demonstrated that Al ions are much more preferentially partitioned into hydrous phases (phase D or phase H) than anhydrous phases (bridgmanite or post-perovskite phase); this situation was observed even in Al-poor bulk-compositions 17 such as peridotitic (or pyrolitic) composition. This suggests that under the presence of suitable water amount, the aluminous hydrous phases with high stability could exist not only in Al-rich fields such as MORB of subducting slabs but also everywhere in the lower mantle. However, the crystal-chemical mechanism for the stability enhancement of these hydrous phases due to the incorporation of Al remains to be solved. Here we report on Al-bearing phase D (Al-phase D) synthesized from high-pressure experiments of a bridgmanite composition in the system MgO-SiO 2 -Al 2 O 3 , with Al content close to that reported for bridgmanite formed from a representative mantle composition, under over-saturation of water. We reveal the incorporation mechanism of Al and a large amount of hydrogen (H) into phase D and demonstrate the drastic enhancement in stability of phase D due to the incorporation of a relatively small amount of Al. We discuss the mechanism for such a high stability of Alphase D, in terms of crystal chemistry based on single-crystal X-ray diffraction. On the basis of these findings, we propose crucial implications for the recycle of water in the lower mantle.
We selected the starting composition of 0.92MgSiO 3 ·0.08Al 2 O 3 because it is close to the composition of bridgmanite, 0.94MgSiO 3 ·0.06Al 2 O 3 (Refs. 19,20 ), expected in a pyrolitic 21 lower-mantle. Our high-pressure experiments were conducted under the three different thermal histories. Their experimental conditions are summarized in Table 1. Reagent grade oxides and hydroxides were mixed in the required ratios and sealed in platinum (Pt) capsules together with amounts of liquid water suitable for over-saturation. The samples were compressed to 27 GPa (runs #1 and #2) or 26 GPa (run #3) and then heated to each target maximum temperature of 1600 °C (run #1) or 1900 °C (runs #2 and #3) using a Kawai-type multi-anvil apparatus 22 . After undergoing each thermal history, the samples were quenched at 1600 °C (runs #1 and #2) or 1300 °C (run #3) and recovered to ambient conditions. In all the runs, liquid water was seeping out of the Pt capsules when those were opened, which shows that the recovered samples were synthesized under over-saturation of water.
We measured the microfocus X-ray diffraction patterns for the recovered samples of the runs #1 and #2. A typical example of them is shown in Fig. 1. In the patterns, we observed the diffraction peaks corresponding to phase D, stishovite SiO 2 and brucite Mg(OH) 2 . The peaks that cannot be assigned to any known-phases were also observed; these are probably due to impurities precipitated, together with brucite, from fluid during quenching. Here, we determined the unit-cell parameters with trigonal symmetry by least-squares fits of the d spacings of 21 peaks assigned to phase D as follows: a = 4.8239(1) Å, c = 4.3134(2) Å, V = 86.924(4) Å 3 for the run #1; a = 4.8416(1) Å, c = 4.3236(2) Å, V = 87.771(4) Å 3 for the run #2. The calculated d values of these peaks are in good agreement with the observed ones (Table 2).
To confirm the presence of phase D, we conducted the electron probe microanalyses for the recovered sample of the run #1. The analytical result showed the presence of products with a chemical composition of 23  Microfocus powder X-ray diffraction pattern of a recovered sample. As an example, that of the run #2 is given here. To assess why the presence of Al drastically enhances the stability of phase D, it is quite important to determine the detailed crystal structure of Al-phase D. The slow cooling from higher temperatures is effective to enhance crystal growth from melt, as in the case of Al-free MgSiO 3 bridgmanite 26 . The runs #2 and #3, with the slow cooling from 1900 °C, were thus conducted to try synthesis of Al-phase D single-crystals large enough for single-crystal X-ray diffraction. Numerous transparent and euhedral single-crystals ( Fig. 2), which possess a crystal habit implying a trigonal or a hexagonal symmetry, were found in the recovered samples. No intergrowth textures were observed under polarized microscope. A specimen for single-crystal X-ray diffraction was selected from the crystals produced in the run #3, with the slower cooling rate than in the run #2, because they were better in terms of size and crystallinity than those produced in the run #2. The electron probe microanalyses for the crystals produced in the run #3 showed a chemical composition of 28.46 mass% MgO, 48.69 mass% SiO 2 and 7.72 mass% Al 2 O 3 with a total of 84.87 mass% to give a chemical formula of Mg 1.25 Si 1.43 Al 0.27 O 6 H 2.97 by assigning the deficit from 100 mass% to H 2 O component. This composition differs somewhat from that in the run #1 shown above. This is probably due to the difference in thermal history and/or water fugacity, which can influence Mg/ Si ratio in fluid, between the two runs.
The crystal structure determined for the selected crystal (run #3) are shown in Fig. 3a-c, together with the residual electron density peak (Fig. 3d) assigned to a hydrogen (H) atom. The structure-analytical information and results are given in Supplementary Table S1, and Tables 3 and 4 Owing to the constraints of the space group, a pair of centrosymmetric H positions (Wyckoff position 6k) are present in close proximity (H···H = 1.07 Å) (Fig. 3c, Table 4). If an H position is occupied, its nearest H position must be unoccupied to avoid an H + -H + interaction (Fig. 3c). Thus, the maximum allowance for the number of H atoms contained in a unit cell is 3, corresponding to a half occupancy of the 6k-site. The H content of the Table 2. Microfocus X-ray diffraction data for Al-phase D in the recovered samples of the runs #1 and #2.   Fig. 4, possible phase relations for pure MgSiO 3 (dotted grey lines) and Al 2 O 3 -bearing MgSiO 3 (solid blue lines) under water-saturated conditions are shown together with the slow-cooling paths (solid red arrows) adopted for our crystal-growth experiments. As the liquidus temperature of MgSiO 3 at 27 GPa under water saturation is 1750 °C (Ref. 26 ), our charges would have been above liquidus when kept at 1900 °C in the crystalgrowth experiments, in spite of a slight increase of the liquidus due to the Al 2 O 3 component 28 . Thus, it is inferred that the single crystals of Al-phase D grew from melt. This view is supported by the fact that the crystals have a perfect euhedral-shape (see Fig. 2) and exhibit no intergrowth texture; these observations exclude the possibility that the crystals are product of reaction involving any other phases, such as Al-bearing MgSiO 3 bridgmanite (Al-Brg). It is therefore concluded that Al-phase D is stable up to temperatures substantially higher than the normal mantle-geotherm. Moreover, as Al-phase D crystallized from a starting material with the composition of Al-Brg, the former should be more stable than the latter under water-saturated conditions. Such a drastic change in stability relations between bridgmanite and phase D under water-saturated conditions by addition of a relatively small amount of Al 2 O 3 component is interpreted in terms of the difference in the coordination environments of Al ions between Al-Brg and Al-phase D. The Al ions in Al-Brg occupy both the eightfold-and sixfold-coordinated sites by the substitution VIII Mg 2+ + VI Si 4+ → VIII Al 3+ + VI Al 3+ (Ref. 29 ). Eightfold coordination is unsuitable for Al 3+ because of its small cationic size; indeed, no compound with eightfold-coordinated Al is known except for Al-Brg. Such an unusual eightfold-coordinated Al should enhance the cohesive energy of Al-Brg, and the repulsive interaction with adjacent Si 4+ or Al 3+ through the shared faces between (Si, Al)O 6 -octahedra and (Mg, Al)O 8 -polyhedra may especially tend to destabilize the structure. In contrast, the incorporation of Al into phase D will reduce the cation-cation repulsion across the shared edges between (Si, Al, Mg)O 6 -octahedra in S-site (Fig. 3a,b) by the substitution of Al 3+ for Si 4+ , owing to the lower charge and larger cationic size of Al 3+ . Thus, the incorporation of Al into phase D results in expansion of the stability field to much higher temperatures and presumably to much higher pressures.
It follows from this discussion that phase D could be stable along the normal lower-mantle geotherm up to much higher pressures if it contains some amount of Al 2 O 3 component (cf. Fig. 4). Bridgmanite in the lower mantle probably contains about 6 mol% Al 2 O 3 component 19,20 , and Al-phase D in the lower mantle could form by reaction between bridgmanite and free water. Given that the representative formula of Al-phase D is MgSi 2-x Al x O 6 H 2+x , the reaction can be expressed as follows: Bolfan-Casanova et al. 30 demonstrated that the major lower-mantle constituents, bridgmanite and ferropericlase (magnesiowüstite), can accommodate very little water, and completely denied the previous result 31 32,33 . This Al 2 O 3 content is higher than that (about 8 mass%) in the bridgmanite composition employed in the present high-pressure experiments. Therefore, phase D in slabs would contain a large amount of Al 2 O 3 component, and this content would be higher than that in the present Al-phase D. Indeed, super-aluminous phase D (simplified formula Al 2 SiO 6 H 2 ), which could be stable over 2000 °C at 26 GPa (Ref. 9 ), was produced from the similar Al-rich bulk composition 10 . If the incorporation of Al into phase D extends its stability field to higher pressure following the above crystal-chemical prediction, then Al-phase D in slabs can carry hydrogen much deeper in the lower mantle than previously estimated 7,34 . This speculation is consistent with the phase relation reported in the simplified system such as MgO-Al 2 O 3 -SiO 2 -H 2 O (Ref. 17 ), but is inconsistent with that reported recently in the hydrous MORB system 35 . It was reported that in the former systems 17 Al-phase D was stable up to ~ 55 GPa and Al-phase H was the stable hydrous phase at higher pressures, whereas in the latter system 35 the stable region of Al-phase D was drastically reduced to ~ 25 GPa. This discrepancy in the stability relation of the hydrous phases may be attributed to the difference in the staring compositions including water contents, but the details remain to be solved. Even if Al-phase D decomposes into Al-phase H at lower pressures in actual subducting-slabs as suggested in the recent study 35 , hydrogen should be transported still deeper in the lower mantle by Al-phase H, stable up to much higher pressures 16 . The ultimately released water would be hardly absorbed into the surrounding lower-mantle constituents and be stored as hydroxyl groups in Al-phase D in the upper region of the lower mantle according to the reaction (1), although Al-phase H produced by reaction with bridgmanite might intervene depending on depth as will discussed below.  [36][37][38][39] are only a little higher than that of the present Al-phase D. These ρ 0 values, including the present data, are all considerably lower than the representative value of the lower-mantle ρ 0 = 4.15 g/cm 3 (Ref. 40 ). Therefore, a "wet-metasomatized" region containing Al-phase D would move upward owing to pronounced buoyancy even if it contains some amount of FeO/Fe 2 O 3 component. Similarly, the ρ 0 value of (Al, Fe)-free phase H is 3.38 g/cm 3 (Ref. 41 ), very close to that (3.43 g/cm 3 ) 24 of (Al, Fe)-free phase D; containing of some amount of Al 2 O 3 and/or FeO/Fe 2 O 3 components will not yield a significant increase in ρ 0 of phase H as well [cf. ρ 0 = 3.54 g/cm 3 for δ-AlOOH (Refs. 42,43 ); ρ 0 = 4.45 g/cm 3 for ε-FeOOH (Ref. 43 ), isostructural with δ-AlOOH]. Thus, the same situation due to pronounced buoyancy would also occur in a "wet-metasomatized" region containing Al-phase H. This implies that Al-phase D and Al-phase H could be important agents for material transport in the lower mantle. These aluminous DHMS phases might have played an important role in extraction of water from the solid Earth to form the oceans.
Meanwhile, Al-phase D with much higher Al-content, such as super-aluminous phase D 9,10 , would be stable even in slabs subducting still deeper, but this would also decompose finally into Al-phase H, i.e. solid solutions between δ-AlOOH and MgSiO 4 H 2 (phase H), with much higher Al-content to transport hydrogen presumably to the bottom of lower mantle 16 . In addition to such a super-aluminous phase H, recently discovered pyrite-type FeOOH x (x ≤ 1) [44][45][46] , which could be produced in subducted banded-iron-formations, may also be a key to yielding the "wet-metasomatized" region under water-saturated conditions. This iron hydroxide is also promising as a hydrous phase stable at the bottom of lower mantle [44][45][46] and probably forms solid solutions containing AlOOH and/or MgSiO 4 H 2 components in deep subducted slabs 46 . A portion of water released by decomposition of the super-aluminous phase H and pyrite-type FeOOH x due to heating at the core-mantle boundary may be spent on incorporating hydrogen into the outer core through the production of iron hydride FeH x . The remainder moving upward may contribute to yielding the water-saturated region in the lower mantle, and may migrate to the surface via Al-phase D from Al-phase H produced by reaction with bridgmanite. (In this process, Al-phase D is implicitly assumed to be formed in the upper region by reaction between Al-phase H and a high-pressure SiO 2 polymorph.) Okuchi 47 reported that hydrogen is highly-siderophile at high pressure conditions, and suggested that the core-materials (iron ponds) could incorporate a huge amount of hydrogen included in the magma  www.nature.com/scientificreports/ ocean and the incorporated hydrogen never returns to the silicate Earth. If the Earth's core has been saturated with hydrogen, it would have been released from the outer core over geologic time. However, the geophysical observations combined with the mineral physics data suggested that the core is undersaturated with hydrogen 48 . At present, hydrogen is thus unlikely to be provided from the outer core. In future, however, water continuously released from super-aluminous phase H and pyrite-type FeOOH x might saturate the core with hydrogen. If so, hydrogen might come to be released from the outer core and this released hydrogen might also come to migrate to the surface via Al-phase D from Al-phase H.

Methods
High-pressure experiments. The high-pressure experiments were conducted using a 5000-ton Kawaitype multi-anvil apparatus 22 installed at the Institute for Planetary Materials, Okayama University. The experimental procedures and techniques are essentially the same as those described in our previous studies 49-52 as follows. We employed a 6 mm regular octahedron of sintered MgO containing 5% of Cr 2 O 3 as a pressure-transmitting medium and a LaCrO 3 as a heating material. The three runs reported here were performed under the different conditions shown in Table 1. The mixture of the starting materials, including an amount of liquid water suitable for over-saturation, was placed in a Pt capsule and sealed by arc-welding the capsule ends. In particular, liquid water was carefully injected into the capsule using a microsyringe. During arc-welding, the capsule was cooled by wrapping in water-soaked absorbent cotton to prevent evaporation of injected water. The Pt capsule was inserted into the LaCrO 3 heater and electrically insulated from the heater by a MgO spacer. The heater was surrounded with ZrO 2 thermal insulator, and then was put into the MgO octahedron. This cell assembly was set in the anvil assembly of tungsten carbide cubes with truncated edge lengths of 2 mm, and then was compressed www.nature.com/scientificreports/ up to the target pressure (26 or 27 GPa) at room temperature. The temperature was then raised to the target maximum temperature (1600 or 1900 °C) in each run at a rate of 35 °C/min. The temperature was controlled with a W97%Re3%-W75%Re25% thermocouple, whose junction was put at the midpoint of the outer surface of the Pt capsule. No correction was made for the pressure effect on emf. After being exposed to the different thermal history in each run (Table 1), the products were quenched at 1300 or 1600 °C by shutting off the electric power supply. The pressure was released slowly and the products were recovered at ambient conditions. The recovered samples were mounted with epoxy and polished for the chemical analyses using a JEOL JCMA-733II electron probe microanalyzer. For the analyses, the irradiated electron beam was focused to 5 μm in diameter, sufficiently smaller than area sizes of analyzed crystals, under operation conditions of a 15 kV acceleration voltage and a 10 nA beam current. No contamination from the cell assembly materials into the products was detected from qualitative electron probe microanalyses. For the phase identification, the polished samples were also characterized by a Rigaku RINT RAPID-R microfocus X-ray diffractometer with Cu Kα radiation (λ = 1.54184 Å) operated at 40 kV and 200 mA.
Single-crystal X-ray diffraction intensity measurements and structure refinements. The single-crystal X-ray diffraction intensity measurements, data processing and structure refinements were conducted according to essentially the same procedures and techniques as those described in our previous studies 49-58 as follows. A single crystal with a size of 75 × 45 × 20 μm 3 produced in the run #3 was selected and then mounted on the tip of a glass fiber for X-ray diffraction intensity measurements using a graphite-monochromatized Mo Kα radiation (λ = 0.71069 Å). The measurements were conducted at room temperature (296 K) using a Rigaku AFC-7R four-circle diffractometer operated at 60 kV and 250 mA. The unit-cell parameters were determined by the least-squares method from a set of 27 reflections within the range of 38° ≤ 2θ ≤ 50°. The intensity data of a total of 1961 reflections within 2° ≤ 2θ ≤ 100° were collected using the continuous ω-2θ scan mode and corrected for Lorentz-polarization factors and absorption effects (ψ-scan method). The unit-cell parameters were calculated as a = 4.8372 (8) [51][52][53][54][55][56] . Finally, 167 unique reflections were used in the present refinements. The crystal structure was determined by the direct method using the program SIR97 (Ref. 59 ) and refined by minimizing the function σ −2 F (|F o | − |F c |) 2 using the full matrix least-squares program RADY 60 . Among the space groups subjected to Laue symmetry 3 1m, the possible ones are P31m, P312 and P31m because no systematic absences were observed. We selected the centrosymmetric space group P31m, adopted in Al-free phase D 23-25 , because the structure refinements assuming the remaining two space groups resulted in unsuccessful convergence with larger reliability indices. Indeed, in the difference Fourier synthesis after the final refinement assuming P3 1m, no significant residual electron densities were observed around the M, S and O sites; thus, site-splitting due to symmetry reduction to non-centrosymmetric subgroup P31m or P312 is most unlikely. H atom was excluded from the structure refinements because of its low X-ray scattering power. Scattering factors of Mg 2+ , Al 3+ , Si 4+ (  61 . Several correction models for the secondary extinction effects were attempted during the refinements, and the isotropic correction of Type I 63,64 with a Gaussian mosaic spread distribution model yielded the best fit. In super-aluminous phase D 9,10 (simplified formula Al 2 SiO 6 H 2 ), the following three 10 or four 9 symmetrically distinct octahedral-sites are partially occupied by a disordered distribution of Al and Si: M-site (Wyckoff position 1a) and S-site (2d), which are also occupied in Al-free phase D 23-25 (simplified formula MgSi 2 O 6 H 2 ), and the one (2c) 10 or two (2c, 1b) 9 additional octahedral sites, which are vacant in Al-free phase D. The difference Fourier synthesis for the present Al-phase D, however, showed that no significant residual electron density peak is detected on these additional octahedral sites, which indicates that cations are distributed only on M-and S-sites as in Al-free phase D [23][24][25] . The structure refinements were therefore performed by varying P( M Mg) as the only valuable site occupancy parameter under the following constraints to keep the chemical composition from the electron probe microanalyses: P( S Si) ≡ 0.715 (fix), P( S Al) ≡ 0.135 (fix), P( S Mg) ≡ 0.625 − 0.5 × P( M Mg), where the superscripts M and S represent the occupied sites of the cations. The final structure refinement converged smoothly to R = 0.0320 and wR = 0.0319 with anisotropic displacement parameters. The resulting P( M Mg) is 0.979 (8), indicating that M-and S-sites both are almost full occupied. In the final difference Fourier synthesis, the residual electron density peaks with a height of 0.36 eÅ −3 (Fig. 3d) were observed at equivalent positions of the coordinates (0.495, 0, 0.124), located 0.91 Å and 1.80 Å away from adjacent O atoms. These distances are reasonable as H-O (donor) and H···O (acceptor) bond lengths, respectively, which was also confirmed from the bond valence calculations 27 . We therefore assigned the peaks to H atoms.
The summary of crystallographic data, data-collection and refinement parameters is given in Supplementary  Table S1. The refined structural parameters and the selected interatomic distances and angles are listed in Tables 3  and 4