PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2017GL075720
Key Points:• Pyrite-structured FeO2Hx is stable in
the Fe-H2O system at >78 GPa,
~2,000 K• Large amount of water might havebeen introduced into depths during
the accretion and separation of themetallic core
• Extensive reaction between water andiron might occur at the core-mantle
boundary

Chemical Reactions Between Fe and H2O up to Megabar
Pressures and Implications for Water Storage
in the Earth’s Mantle and Core
Liang Yuan1 , Eiji Ohtani1,2 , Daijo Ikuta1, Seiji Kamada1, Jun Tsuchiya3, Hirao Naohisa4
Yasuo Ohishi4, and Akio Suzuki1,

1

Department of Earth and Planetary Material Sciences, Tohoku University, Sendai, Japan, 2V.S. Sobolev Institute of Geology
and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia, 3Geodynamics Research Center, Ehime
University, Matsuyama, Japan, 4Japan Synchrotron Radiation Research Institute, Sayo, Japan

Abstract We investigated the phase relations of the Fe-H2O system at high pressures based on in situ
Supporting Information:• Supporting Information S1
Correspondence to:
L. Yuan,yuan.liang.s4@dc.tohoku.ac.jp

Citation:
Yuan, L., Ohtani, E., Ikuta, D., Kamada, S.,
Tsuchiya, J., Naohisa, H., … Suzuki, A.
(2018). Chemical reactions between Fe
and H2O up to megabar pressures andimplications for water storage in the
Earth’s mantle and core. GeophysicalResearch Letters, 45, 1330–1338. https://doi.org/10.1002/2017GL075720
Received 22 SEP 2017
Accepted 26 JAN 2018
Accepted article online 29 JAN 2018
Published online 8 FEB 2018©2018. American Geophysical Union.
All Rights Reserved.

YUAN ET AL.

X-ray diffraction experiments and ﬁrst-principles calculations and demonstrate that FeHx and FeO arepresent at pressures less than ~78 GPa. A recently reported pyrite-structured FeO2 was identiﬁed in the
Fe-H2O system at pressures greater than ~78 GPa after laser heating. The phase observed in this study has aunit cell volume 8%–11% larger than that of FeO2, produced in the Fe-O binary system reported previously,suggesting that hydrogen might be retained in a FeO2Hx crystal structure. Our observations indicate thatH2O is likely introduced into the deep Earth through reaction between iron and water during the accretionand separation of the metallic core. Additionally, reaction between Fe and H2O would occur at thecore-mantle boundary, given water released from hydrous subducting slabs that intersect with the metallic
core. Accumulation of volatile-bearing iron compounds may provide new insights into the enigmatic seismic
structures observed at the base of the lower mantle.

1. Introduction
The Earth’s core is mainly composed of iron. However, the density of the core, as revealed by seismologicalobservations, is lower than that of pure iron at the corresponding conditions (Birch, 1952). It is therefore
invoked that light elements are potentially important ingredients in the core. Among the light elements,hydrogen tends to be excluded as one of the important components in the core due to its high volatility
and low solubility in iron at ambient pressure, despite the fact that it is a cosmochemically abundant element.
Experiments and thermodynamic calculations suggest that signiﬁcant amount of hydrogen can be dissolved
into iron at high pressures (Antonov et al., 1998; Badding et al., 1991; Fukai, 1984). During accretion and coreformation, reaction between iron and water may produce FeHx, thus incorporating signiﬁcant amounts ofhydrogen into the core (Badding et al., 1992; Ohtani et al., 2005; Okuchi, 1997). Reaction between iron andwater would also be expected at the core-mantle boundary (CMB), as recent studies indicate that hydrous
phases may be stable at deepest lower mantle conditions (Ohira et al., 2014; Sano et al., 2008; Suzuki et al.,
2000; Walter et al., 2015).
Interest in both the Fe-H and Fe-O binary systems has been enhanced by the recent discoveries of new highpressure forms of iron hydrides and iron oxides. Experiments and ﬁrst-principles calculations have suggested
that a series of iron hydrides, for example, FeH (dhcp, hcp, and fcc) (Hirao et al., 2004; Isaev et al., 2007;
Narygina et al., 2011), FeH2 (tetragonal), FeH3 (cubic), and FeH5 (tetragonal) (Pépin et al., 2014, 2017) could
be stabilized at high pressures in the Fe-H system. Iron oxides with complex crystal structures develop in
the Fe-O binary system as well under extreme conditions. Beyond the three, well-known, simple ironoxides, that is, Fe1-xO wüstite, Fe3O4 magnetite, and Fe2O3 hematite, there is a growing number ofexperimental studies that have identiﬁed new high-pressure and high-temperature iron oxides, such as
h-Fe3O4 (Fei et al., 1999), Fe4O5 and Fe5O6 (Lavina et al., 2011; Lavina & Meng, 2015), and Fe7O9 (Sinmyo
et al., 2016) that could be stable at conditions relevant to the Earth’s interior. The iron peroxide FeO2, witha pyrite structure (space group Pa3), was found to be stable at high pressures (Weerasinghe et al., 2015) andwas recently synthesized through the reaction between hematite (Fe2O3) and oxygen (O2) (Hu et al., 2016).
Further studies suggest that goethite (α-FeOOH) would transform into a pyrite-type phase, a hydrogenbearing iron peroxide FeO2Hx (with x from 0 to 1), with losing certain amount of hydrogen (Hu et al., 2017;1330

Geophysical Research Letters10.1002/2017GL075720

Table 1
Experimental Conditions and Results of the Reaction Between Fe and H2O
Pressure (GPa)Run

Before heating

KA7_001
MEM_004
MEM_003a
CCTS_002aCCTS_005
TS2_002
TS3_003
TS4_004
ccts3_003
ccts3_002
ccts2_002
ccts2_003
ccts2_004
ccts2_005b
CCKN_004b
CCKN_006b
CCKN_005c
CCT_001

After heating

55
70
70
78
78
82
82
82
90
90
98
98
98
98
105
105
105
100

a

65
69
69
74
74
78
78
78
85
85
115
115
115
115
113
113
113
97

Temperature (K)d2,090 (260)
1,694 (100)Quench1,721 (100)Quench
1,700 (100)2,000 (100)Quench1,790 (100)Quench
1,700 (200)
2,000 (200)
2,400 (200)
Quench2,000 (160)
2,060 (160)
Quenchd2,000 (100)PhasesFeO (B1, B8), FeHx (hcp, dhcp) (Al2O3, δ-AlOOH)
FeO (B1, B8), FeHx (hcp, dhcp), H2O (Al2O3, W)
FeO (B1, B8), FeHx (hcp, dhcp), H2O (Al2O3, W)
FeO (B1, B8), FeHx (hcp, dhcp), H2O (W)FeO (B1, B8), FeHx (hcp, dhcp), H2O (W)FeO (B1, B8), FeO2Hx, FeHx (hcp, dhcp), H2O (Al2O3, W)
FeO (B1, B8), FeO2Hx, FeHx (hcp, dhcp), H2O (Al2O3, W)
FeO (B1, B8), FeO2Hx, FeHx (hcp, dhcp), H2O (Al2O3, W)
FeO (B1, B8), FeO2Hx, FeHx (dhcp), H2O (Al2O3, W)
FeO (B1, B8), FeO2Hx, FeHx (dhcp), H2O (Al2O3, W)
FeHx (hcp), FeO2Hx (Al2O3, W)
FeHx (hcp), FeO2Hx (Al2O3, W)
FeHx (hcp), FeO2Hx (Al2O3, W)
FeHx (hcp), FeO2Hx (Al2O3, W)
FeO2Hx, FeHx (hcp, dhcp), H2O, (W, WHx)
FeO2Hx, FeHx (hcp, dhcp), H2O, (W, WHx)
FeO2Hx, FeHx (hcp, dhcp), H2O, (W, WHx)
FeO2Hx, H2O, (W)bc

SiO2 loaded as thermal insulator was not detected during experiments. No thermal insulator was loaded. Startingdmaterials were hematite (Fe2O3) and water. Phases in parenthesis indicate materials (SiO2 or Al2O3) used as thermal
insulator and gasket identiﬁed by XRD.

Zhu et al., 2017) or without loss of any hydrogen (Nishi et al., 2017) at pressures >70–80 GPa upon laser
heating.
The Fe-H2O system was studied at pressures below 84 GPa and revealed that iron oxide FeO and iron hydride
FeHx are the stable run products (Ohtani et al., 2005). Recent studies reported that hydrogen-bearing ironperoxide FeO2Hx is also stable in the same system at higher pressures (Liu et al., 2017; Mao et al., 2017).
The diverse populations of high-pressure iron compounds (iron oxides, iron hydrides, hydrogen-bearing ironperoxides, etc.) make it desirable to constrain the stable-phase assemblages in the Fe-H2O system in an
expanded pressure range. Here by using in situ X-ray diffraction (XRD) observations and ﬁrst-principles calculations, we have examined the chemical reactions between iron and water in a wide pressure–temperature
range to clarify the phase relations in the Fe-H2O system.

2. Methods
Iron powder and liquid water were compressed to the target pressures using diamond anvil cells and thenlaser heated to target temperatures. In situ synchrotron powder XRD experiments were performed at
BL10XU of Spring-8, Japan (Ohishi et al., 2008). We also conducted ﬁrst-principles calculations to examine
the transition of stable run products from FeO + FeH to FeO2H + Fe. See supporting information for detailsof methods.

3. Results and Discussion
The phase relations of the Fe-H2O system were investigated at pressure-temperature conditions of 65–
115 GPa and 1,700–2,400 K. The experimental conditions and results are summarized in Table 1. In addition
to the phases formed from starting materials initially loaded into the sample chamber, most XRD patterns
contain diffraction lines from thermal insulators (if used), as well as contributions from tungsten gaskets.
At pressures <78 GPa, we found that FeO and FeHx were the stable run products of the reaction between ironand water, which is consistent with the results of Ohtani et al. (2005). At pressures between 78 GPa and
85 GPa, we found iron oxide FeO coexisting with the iron peroxide FeO2Hx (Hu et al., 2016). The iron peroxide
FeO2Hx adopts the crystal structure of pyrite FeS2 (space group Pa3) and its unit cell volumes are 8%–11%larger than that of anhydrous FeO2 synthesized from Fe2O3 + O2 (Hu et al., 2016). These features are consistent with hydrogen being dissolved in the crystal structure of FeO2 and FeO2Hx. Note that a more recent

YUAN ET AL.

1331

Geophysical Research Letters10.1002/2017GL075720

investigation showed that FeOOH, with a pyrite structure, could bestable in the lower mantle (Nishi et al., 2017). At pressures >85 GPa,FeO2Hx, and FeHx (hcp or dhcp) were readily indexed as the stable
run products. Our observations agree with the results of Mao et al.
(2017) and Liu et al. (2017), where FeO2Hx was synthesized from thereactions between iron/iron oxides (Fe, FeO, Fe2O3, etc.) and water(H2O). However, the FeO2Hx in our study has larger unit cell volumes
compared to that of previous studies (Figure 2a), which might be interpreted as the results of different starting materials and experimental
temperature conditions.
The in situ XRD patterns obtained at 113 GPa clearly showed diffraction
peaks that can be unambiguously assigned to a simple cubic symmetry
with a Pa3 space group (Figure 1a). All the symmetry-allowed spots forPa3-structured FeO2Hx were present. FeO2Hx developed sharp diffraction lines within 15 min, and its peaks grew over a 90 min laser heating.
The intensities of the diffraction lines from H2O (ice X), however, did not
decline signiﬁcantly. After temperature quenching, we checked thewhole sample chamber by scanning the X-ray beam, and FeO2Hx was
found as the dominant phase. While H2O (ice X) remained due to thelarge volume of water, as comparison to the initial iron, loaded into
the sample chamber. The experiments were kept at pressure for 2 weeks
and a second round of XRD analyses was conducted. Interestingly, we
found that the unit cell volume of the tungsten gasket obviously
expanded in certain area, which probably was induced by the incorporation of hydrogen. We speculate that free hydrogen was formed duringlaser heating and it dissolved into the tungsten gasket within this limited time duration; a Raman spectroscopy analysis on this sample could
not detect the existence of free hydrogen.
At pressures between 85 GPa and 78 GPa, the FeO2Hx displayed relatively weak diffraction lines compared to that at 113 GPa although the
ﬁtted unit cell volumes matched with that of recent experimental results
and theoretical calculations (Hu et al., 2017; Nishi et al., 2017). The diffraction lines developed by FeO2Hx completely disappeared at pressures 65 GPa, 69 GPa, and 74 GPa.
Figure 1. (a) XRD patterns at 113 GPa. (b) A two-dimensional XRD image at
113 GPa after laser-heating.

In one further experiment, we compressed hematite (Fe2O3) and waterto 97 GPa and laser heated it to ~2,000 K for ~30 min. XRD showed thatthe pyrite-structured FeO2Hx formed, indicating the following
reaction: Fe2O3 + H2O → FeO2Hx.

Hu et al. (2017) found that the volumes of pyrite-structured FeO2 produced from the dehydration of goethite
FeOOH are larger than that of pure FeO2 synthesized via the reaction between Fe2O3 and O2 in the pressure
range of 71–133 GPa. An obvious scattering of the compression data was attributed to the variable nonstoichiometric composition. The volume of FeO2Hx in this study is also 8%–11% larger than that of pure FeO2
grown from the Fe2O3 + O2 system (Hu et al., 2016) (Figure 2a). Assuming that the volume of FeO2Hx expands
linearly in proportion to the amount of hydrogen dissolved in interstitial sites, the hydrogen concentration inthis iron compound can be estimated, based on the following formula (Hu et al., 2017): 

x ¼ V x; exp  V 0; exp = V 1; cal  V 0; cal(1)where Vx, exp is the observed volume of FeO2Hx, V0, exp is the volume of pure FeO2 synthesized in the
Fe2O3-O2 system (Hu et al., 2016), and V1, cal, and V0, cal denote the volume of FeO2H and FeO2 calculated fromﬁrst-principles methods (Nishi et al., 2017). Our experimentally determined volumes for FeO2Hx are bracketedby the calculated values given by Nishi et al. (2017) and are comparable to the experimental results from

YUAN ET AL.

1332

Geophysical Research Letters10.1002/2017GL075720

previous studies (Hu et al., 2017; Nishi et al., 2017). Using the twobaselines of FeO2 and FeO2H (Nishi et al., 2017), the mole fractionof hydrogen for FeO2Hx in our study is in the range of 0.79–1.00. It
was suggested that up to ~50% hydrogen would be released during
the phase transition of ε-FeOOH to pyrite-structured FeO2H (Zhu
et al., 2017). The mechanism of dehydrogenation was ascribed tothe breakdown of symmetrized hydrogen bonds during the structuraltransition. Whereas Nishi et al. (2017) suggested that ε-FeOOH directlytransforms into pyrite-structured FeO2H without losing hydrogen,indicating the hydrogen concentration x is likely to be unity for
FeO2Hx. Our calculated hydrogen contents in FeO2Hx are systematically higher than that in Liu et al. (2017), where x is in the range of
0.50–0.70. Most experimental data, for example, unit cell volumes
and sound velocities, related to FeO2Hx in previous studies werebased on experiments where iron oxides (FeO, Fe2O3, etc.) and water(H2O) were used as starting materials (Hu et al., 2017; Liu et al., 2017;
Mao et al., 2017). These systems are more oxidized compared to the
Fe-H2O system explored in this study. We speculate that differencesof oxygen fugacity may contribute to the various hydrogen contents
in FeO2Hx among different studies.

Figure 2. (a) The pressure-volume relations for FeO2Hx of this study (redcircles). Experimental data of pyrite FeO2Hx are from Hu et al. (2017) (pinkcircles) and Nishi et al. (2017) (green diamonds), pyrite FeO2 from Hu et al.
(2016) (black solid line), ε-FeOOH from Gleason et al. (2013) (blue solid line).
Theoretical data of FeO2Hx are from Nishi et al. (2017) (red and green dashedlines correspond to generalized gradient approximation (GGA) and local
density approximation (LDA) results), FeO2 from Nishi et al. (2017) (black
dashed line). (b) The pressure-volume relations for hcp FeHx (circles) anddhcp FeHx (squares) of this study. Compressibility curves of pure iron(Dewaele et al., 2006) and FeH (Pépin et al., 2014) were used as baselines toevaluate water contents. All the experimental data are collected at 300 K.

Our volume-pressure data of dhcp FeHx and hcp FeHx are plotted in
Figure 2b. A coexistence of hcp FeHx and dhcp FeHx in the Fe-H systemat high pressures was also suggested by Fukai (1992). Water concentrations in iron hydrides have been investigated according to the formula:x = [V(FeHx)  V(Fe)]/ΔV(H) (Ohtani et al., 2009; Terasaki et al., 2012).
However, uncertainty exists as to the value of ΔV(H) under high pressures. Here we use the volume difference between the Fe and FeH undervarious pressures for the full range of x from 0 to 1. The amount ofhydrogen x can thus be expressed as follows:x ¼ ½V ðFeH; x Þ  V ðFe; 0Þ=½V ðFeH; 1Þ  V ðFe; 0Þ(2)where V(FeH, x), V(Fe,0), and V(FeH,1) are the observed volume of hcp
FeHx or dhcp FeHx, the volume of pure hcp Fe from Dewaele et al.
(2006) and the volume of dhcp FeH from Pépin et al. (2014). The amountof hydrogen dissolved in the hcp FeHx and dhcp FeHx is in the range of
0.13–0.32 and 0.41–0.90, respectively.
The effect of pressure on the transition of stable products from
FeO + FeH → FeO2H + FeH can be evaluated based on the volume
change of the reaction:
2FeO þ FeH → FeO2 H þ 2Fe(3)

Our ﬁrst-principles calculations indicate that the pyrite-structured FeO2H + 2Fe has lower enthalpies compared to 2FeO + FeH throughout the explored pressure range of 50–150 GPa (Figure S1 in the supporting
information). Previous calculations suggest that ε-FeOOH transforms into the pyrite-structured FeO2H ataround 70 GPa (Nishi et al., 2017). Within the stability ﬁeld of pyrite-structured FeO2H, instead of
FeO + FeH, the pyrite-structured FeO2H would become one of the stable run products of Fe + H2O.
Whereas the calculated enthalpy of FeOOH +2Fe is lower than that of 2FeO + FeH at pressures less than
78 GPa. This inconsistency is most likely induced by the exchange-correlation functional used in thecalculations. The standard generalized gradient approximation or local density approximation often fails to
reproduce the stability of iron oxide systems (e.g., Otte et al., 2011). In addition, uncertainty of pressure measurement, stress in the sample chamber, and kinetics of the reaction may also contribute to the inconsistency
between experimental and computational results.

YUAN ET AL.

1333

Geophysical Research Letters10.1002/2017GL075720

At 78 GPa, the molar volume for FeO is 9.04 cm3/mol (Fischer et al.,2011), FeH is 6.25 cm3/mol (Pépin et al., 2014), and Fe is 5.28 cm3/mol(Dewaele et al., 2006). Using the measured volume of 13.60 cm3/mol
for FeO2H in this work, the reaction has a volume reduction of
0.7%. Pressure lowers the Gibbs free energy of the reaction by
ΔG = ∫ ΔVdP, thus a transition for the stable run products from
2FeO + FeH → FeO2H + 2Fe is likely to occur at high pressures within
the stability ﬁeld of FeO2H. This volume change, however, tends tobecome positive as FeO2H becomes less compressible relative to
FeO + FeH at pressures higher than ~100 GPa. The crossover of thevolume change, as reproduced by our ﬁrst-principles calculations(Figure S1), could potentially stabilize FeO + FeH under the deep
core conditions.
Phase relations for Fe-H2O are summarized in the P-T phase diagram(Figure 3a). Our experimental results demonstrate that iron and water
would react along geotherms of a typical mantle and that for a cold subducting slab (Brown & Shankland, 1981; Kirby et al., 1996). We conclude
that FeO + FeH is the stable assemblage <78 GPa, whereas FeO2H + FeH
becomes stable >78 GPa.

4. Geophysical Implications
Experimental and theoretical studies of FeHx suggest that such an iron
hydride can satisfy the density and sound velocity of the Earth’s core
(Hirao et al., 2004; Mao et al., 2004; Sakamaki et al., 2016; Shibazaki
et al., 2012; Tagawa et al., 2016). A primary mechanism for including
Figure 3. (a) P-T phase diagram showing the phases relations of Fe + H2O.
hydrogen into the Earth’s core might be through reaction between iron
Geotherms for the normal mantle (Brown & Shankland, 1981) and theand water during core formation. FeHx has a much lower melting temcold subduction slabs (Kirby et al., 1996) are depicted by thick solid lines.
perature than pure iron by several hundreds of degrees (Sakamaki(b) Comparison of bulk sound velocities of FeO2Hx, (Mg0.61Fe0.39)O, δ
et al., 2009). The dissolution of hydrogen into iron can greatly lower its
AlOOH, and MgSiO3 bridgmanite. The experimental sound velocity for
FeO2Hx (Liu et al., 2017) is indicated by the diamond symbol. The FeO2Hx has
melting temperature, which might be a prerequisite for efﬁciently segreslower bulk sound velocities compared to the PREM (Dziewonski & Anderson,
gating heavy metal from surrounding silicates (Stevenson, 1990). In the1981). Compression data refer to Fei et al. (2007) for (Mg0.61Fe0.39)O,
shallow mantle, FeO is absorbed into the surrounding silicates, while
Mashino et al. (2016) for δ AlOOH, Kiefer et al. (2002) and Stixrude andFeHx is considered to be unreactive with surrounding silicates (Ohtani
Lithgow-Bertelloni (2005) for MgSiO3 bridgmanite, and Nishi et al. (2017) andet al., 2005). Due to the strong Fe-H bonding, FeHx is unlikely to decomthe present study for FeO2Hx.
pose and can exist in a molten state at increasing temperatures(Badding et al., 1992; Nishi et al., 2017; Okuchi, 1997). Therefore, hydrogen can be stably transported by FeHx to great depths in the Earth. With its notably high density (~6.897 g/
cm3 at 113 GPa) in comparison to the density (~5.357 g/cm3) of surrounding silicates, FeO2Hx would normallysink into the base of the mantle. However, whether pyrite-structured FeO2Hx can deliver hydrogen into thedeep of the proto-Earth depends on its thermal stability. The present experiment implies a coexistence ofFeHx and FeO2Hx at pressure greater than ~80 GPa, which begins to appear in a proto-Earth at a diameter
greater than 4,500 km. The unit cell volumes for FeO2Hx obtained in this study are consistent with thatreported by Nishi et al. (2017) and this pyrite phase may be stoichiometric in composition, that is, x ~ 1, whichindicates very small amount of hydrogen would be released during its transportation to the base of
the mantle.
Silicates such as pyroxene, garnet, and Al-bearing bridgmanite at high pressures can incorporate ferric iron
(Frost et al., 2004; McCammon, 1997; Rohrbach et al., 2007). Metallic iron could be saturated in the deep
mantle because of charge disproportionation where Fe2+ (silicates) → Fe3+ (silicates) + Fe0 (metal).
Experimental evidence is in line with the discovery of metallic iron phases included in natural diamonds presumably originated from the mantle transition zone or uppermost lower mantle (Smith et al., 2016). Note thatrecently reported aluminous bridgmanite has an unexpected drop of Fe3+ content in the midmantle of 1,100
to 1,700 km depths (Shim et al., 2017). The oxidation state change of Fe in bridgmanite will lead to a lower

YUAN ET AL.

1334

Geophysical Research Letters10.1002/2017GL075720

metallic iron content in this region. Metallic iron may play the role of storing large amount of water in the
deep mantle as hydrogen was suggested to be favorably partitioned into metallic iron rather than the coexisting silicate mineral (Ohtani et al., 2009). At the top of the lower mantle, dehydration melting would occur
when mantle transition zone material is swept down to the lower mantle by mantle convection due to a large
contrast of water partitioning between ringwoodite and bridgmanite (Schmandt et al., 2014). Additionally,
dense hydrous magnesium phases, for example, superhydrous phase B and phase D, in subducting slabs thatare stagnating near 660 km seismic discontinuity might go through hydrous melting (Ghosh & Schmidt, 2014;
Ohtani et al., 2003). Dehydration melting of phase δ-H solid solutions would be expected in peridotite system
at 60–70 GPa (Walter et al., 2015). The produced hydrous melts would react with the saturated metallic ironproducing assemblages of FeHx and iron oxides. The aluminous hydrous phase, δ AlOOH, was suggested to
be a stable water reservoir in the lower mantle (Sano et al., 2008). At pressures greater than ~78 GPa, δ AlOOH
would react with metallic iron to form FeHx and FeO2Hx. However, FeO2Hx may not exist independently; it ismore likely stabilized as a solid solution with δ AlOOH under deep mantle conditions, as revealed by theexperiments of Nishi et al. (2017).
The CMB might be a critical location where water can meet with iron and extensive reactions might occur
between them (Mao et al., 2017). Postperovskite containing 1–2 wt.% H2O at D″ region was suggested to
be stable based on ab initio calculations (Townsend et al., 2015). In the Al-bearing systems, hydrogen favors
postperovskite over perovskite with water partition coefﬁcients ranging from 3:1 to 8:1 (Townsend et al.,2016), although the results need to be conﬁrmed experimentally. Hydrous melts or ﬂuids might be generated
if hydrous postperovskite is swept by mantle convection through the postperovskite-bridgmanite phaseboundary due to potentially high solubility of water in postperovskite and the large contrast in water
portioning coefﬁcient between postperovskite and bridgmanite. The hydrous minerals δ AlOOH, phase H
MgSiO2(OH)2, and their solid solutions can be stable up to CMB conditions (Ohira et al., 2014; Sano et al.,2008; Suzuki et al., 2000; Walter et al., 2015). Therefore, water could be transported to the deep lower mantle
and core by hydrous phases in cold subducting slabs. However, these hydrous phases are thermally unstable
when they are stagnant at the CMB due to the large geothermal gradient on the mantle side of this boundary.
The released water (or hydrous melts) can react with the core and likely form the assemblage of
FeO2Hx + FeHx. The thermal stability of FeO2Hx is as yet unknown. FeO2Hx might be less favored as one ofthe products of the reaction between δ AlOOH and iron because δ AlOOH has such a wide stability ﬁeld
(Sano et al., 2008) and the dehydration temperatures of δ AlOOH very likely exceed the thermal stability of
FeO2Hx. The δ AlOOH may incorporate signiﬁcant amounts of Si4+ and Mg2+ in the lower mantle (Ohiraet al., 2014), which may reduce its thermal stability, and therefore, it can dehydrate at much lower temperatures where FeO2Hx would be stable.
Sound velocity of FeO2Hx may provide new insights into seismic structure of the ultralow velocity zone. The
sound velocities for FeO2Hx have been measured at 133 GPa, 300 K with P waves velocity Vp = 11.32 km/s andS waves velocity Vs = 4.50 km/s (Liu et al., 2017), which are relatively slower than that in the ambient silicate
mantle. In view of the scarcity of experimental elastic data, here we also estimate the bulk sound velocity VΦ
of FeO2Hx according to the following equation:V Φ ¼ ðK S =ρÞ1=2(4)

where KS is the adiabatic bulk modulus and ρ is the density. Ks is related to the isothermal bulk modulus, KT,
by KS = KT(1 + αγT) ≈ KT at room temperature conditions. KT and ρ were obtained using the compressibility
data of FeO2Hx from the present study and the data of experiments and ﬁrst-principles calculations (Nishi
et al., 2017). At ~133 GPa, the bulk sound velocity of FeO2Hx is estimated to be 10.37 km/s, close to
10.06 km/s calculated from the experimental data of Liu et al. (2017). A difference of the bulk sound velocity
can be attributed to the different hydrogen contents as incorporation of hydrogen increases VP whereas
decreases VS of FeO2Hx (Liu et al., 2017). Oxygen fugacity may control the hydrogen contents in
FeO2Hx, which in turn affect the sound velocities of this phase. The reaction of iron and water explored
here might better simulate that occurs at the CMB where metallic iron from the core reacts with ﬂuids
released from the slabs at the bottom of the lower mantle. It might become critical to explore the properties of FeO2Hx under oxidation conditions applicable to the CMB. The bulk sound velocities of FeO2Hx are
slower than that of bridgmanite (MgSiO3) and the preliminary reference Earth model values (Dziewonski &Anderson, 1981) at the base of the lower mantle (Figure 3b). FeO2Hx thus could be a candidate for the

YUAN ET AL.

1335

Geophysical Research Letters10.1002/2017GL075720

distinct ultralow velocity zone. Furthermore, FeO2Hx produced at the CMB could be linked to the global
scale events, such as the Great Oxidation Event and geodynamic instability of the mantle materials (Maoet al., 2017). Testing these hypotheses must await more studies on its physical and chemical properties, forexample, high-temperature sound velocities and thermal stability.

5. Conclusion
We investigated the reaction of H2O and Fe at deep Earth conditions using in situ X-ray diffraction experiments and ﬁrst-principles calculations. We found that the stable run products are FeO and FeHx at pressures<78 GPa, and FeO2Hx and FeHx at pressures >78 GPa. An 8%–11% larger volume than pure FeO2, indicatesthat hydrogen is retained in the crystal structure of FeO2Hx. We suggest that large amount of water might
have been introduced into depths during the accretion and separation of the metallic core via iron-water
reaction. Reaction between iron and water may also provide new insights into the enigmatic seismic structures at the CMB.

Acknowledgments
We thank T. Sakamaki, F. Maeda, and
I. Ohira for their experimental assistance. We thank H. K. Mao for the
information on his recent works.
Comments from H. K. Mao and an
anonymous reviewer have improved
this manuscript. This study was supported by the JSPS KAKENHI (grants
JP15H05748 to E. O., JP15H05828 to
A. S., and JP15H05834 to J. T.). J. T.
was also supported in part by MEXT as“Exploratory Challenge on Post-K
computer” (Frontiers of Basic Science:
Challenging the Limits). E. O. was also
partly supported by the Ministry of
Education and Science of the RussianFederation (Project 14B25.31.0032).
S. K. was partly supported by KAKENHI
grants 26247089, 15H05831, and16K13902. The synchrotron radiation
experiments were performed at
SPring-8 with the approval of theJapan Synchrotron Radiation ResearchInstitute (Proposals 2016B1494,
2016B1214, 2017A1474, 2017A1251,
2017A1673, 2017B1514, and2017B169). L. Y. is supported by theInternational Joint Graduate Programin Earth and Environmental Science(GP-EES), Tohoku University.
Experimental data has been shown in
Figures 1–3, S1–S3, and Tables 1,
S1 and S2.

YUAN ET AL.

References
Antonov, V. E., Cornell, K., Fedotov, V. K., Kolesnikov, A. I., Ponyatovsky, E. G., Shiryaev, V. I., & Wipf, H. (1998). Neutron diffraction investigationof the dhcp and hcp iron hydrides and deuterides. Journal of Alloys and Compounds, 264(1-2), 214–222. https://doi.org/10.1016/
S0925-8388(97)00298-3
Badding, J., Hemley, R., & Mao, H. (1991). High-pressure chemistry of hydrogen in metals: In situ study of iron hydride. Science, 253(5018),
421–424. https://doi.org/10.1126/science.253.5018.421
Badding, J. V., Mao, H.-K., & Hemley, R. J. (1992). High-pressure crystal structure and equation of state of iron hydride: Implications for the
Earth’s core. In Y. Syono & M. H. Manghnani (Eds.), High-pressure research: application to Earth and planetary sciences (pp. 363–371).
Washington, DC: American Geophysical Union.
Birch, F. (1952). Elasticity and constitution of the Earth’s interior. Journal of Geophysical Research, 57(2), 227–286. https://doi.org/10.1029/
JZ057i002p00227
Brown, J., & Shankland, T. (1981). Thermodynamic parameters in the Earth as determined from seismic proﬁles. Geophysical Journal of theRoyal Astronomical Society, 66(3), 579–596. https://doi.org/10.1111/j.1365-246X.1981.tb04891.x
Dewaele, A., Loubeyre, P., Occelli, F., Mezouar, M., Dorogokupets, P. I., & Torrent, M. (2006). Quasihydrostatic equation of state of iron above 2
Mbar. Physical Review Letters, 97(21), 215504. https://doi.org/10.1103/PhysRevLett.97.215504
Dziewonski, A. M., & Anderson, D. L. (1981). Preliminary reference Earth model. Physics of the Earth and Planetary Interiors, 25(4), 297–356.
https://doi.org/10.1016/0031-9201(81)90046-7
Fei, Y., Frost, D. J., Mao, H.-K., Prewitt, C. T., & Haeusermann, D. (1999). In situ structure determination of the high-pressure phase of Fe3O4.
American Mineralogist, 84(1-2), 203–206. https://doi.org/10.2138/am-1999-1-222
Fei, Y., Zhang, L., Corgne, A., Watson, H., Ricolleau, A., Meng, Y., & Prakapenka, V. (2007). Spin transition and equations of state of (Mg, Fe)O
solid solutions. Geophysical Research Letters, 34, L17307. https://doi.org/10.1029/2007GL030712
Fischer, R. A., Campbell, A. J., Shofner, G. A., Lord, O. T., Dera, P., & Prakapenka, V. B. (2011). Equation of state and phase diagram of FeO.
Earth and Planetary Science Letters, 304(3-4), 496–502. https://doi.org/10.1016/j.epsl.2011.02.025
Frost, D. J., Liebske, C., Langenhorst, F., & McCammon, C. A. (2004). Experimental evidence for the existence of iron-rich metal in the Earth’s
lower mantle. Nature, 428(6981), 409–412. https://doi.org/10.1038/nature02413
Fukai, Y. (1984). The iron-water reaction and the evolution of the Earth. Nature, 308(5955), 174–175. https://doi.org/10.1038/308174a0
Fukai, Y. (1992). Some properties of the Fe-H system at high pressures and temperatures, and their implications for the Earth’s core. In
High-Pressure Research: Application to Earth and Planetary Sciences (Vol. 67, pp. 373–385). Washington, DC: TERRAPUB.
Ghosh, S., & Schmidt, M. W. (2014). Melting of phase D in the lower mantle and implications for recycling and storage of H2O in the deep
mantle. Geochimica et Cosmochimica Acta, 145, 72–88. https://doi.org/10.1016/j.gca.2014.06.025
Gleason, A., Quiroga, C., Suzuki, A., Pentcheva, R., & Mao, W. (2013). Symmetrization driven spin transition in ε-FeOOH at high pressure.
Earth and Planetary Science Letters, 379, 49–55. https://doi.org/10.1016/j.epsl.2013.08.012
Hirao, N., Kondo, T., Ohtani, E., Takemura, K., & Kikegawa, T. (2004). Compression of iron hydride to 80 GPa and hydrogen in the Earth’s innercore. Geophysical Research Letters, 31, L06616. https://doi.org/10.1029/2003GL019380
Hu, Q., Kim, D. Y., Liu, J., Meng, Y., Yang, L., Zhang, D., et al. (2017). Dehydrogenation of goethite in Earth’s deep lower mantle.
Proceedings of the National Academy of Sciences of the United States of America, 114(7), 1498–1501. https://doi.org/10.1073/
pnas.1620644114
Hu, Q., Kim, D. Y., Yang, W., Yang, L., Meng, Y., Zhang, L., & Mao, H.-K. (2016). FeO2 and FeOOH under deep lower-mantle conditions andEarth’s oxygen-hydrogen cycles. Nature, 534(7606), 241–244. https://doi.org/10.1038/nature18018
Isaev, E. I., Skorodumova, N. V., Ahuja, R., Vekilov, Y. K., & Johansson, B. (2007). Dynamical stability of Fe-H in the Earth’s mantle and coreregions. Proceedings of the National Academy of Sciences of the United States of America, 104(22), 9168–9171. https://doi.org/10.1073/
pnas.0609701104
Kiefer, B., Stixrude, L., & Wentzcovitch, R. M. (2002). Elasticity of (Mg,Fe)SiO3-perovskite at lower mantle conditions. Geophysical Research
Letters, 29(11), 1539. https://doi.org/10.1029/2002GL014683
Kirby, S. H., Stein, S., Okal, E. A., & Rubie, D. C. (1996). Metastable mantle phase transformations and deep earthquakes in subducting oceanic
lithosphere. Reviews of Geophysics, 34(2), 261–306. https://doi.org/10.1029/96RG01050
Lavina, B., Dera, P., Kim, E., Meng, Y., Downs, R. T., Weck, P. F., et al. (2011). Discovery of the recoverable high-pressure iron oxide Fe4O5.
Proceedings of the National Academy of Sciences of the United States of America, 108(42), 17,281–17,285. https://doi.org/10.1073/
pnas.1107573108
Lavina, B., & Meng, Y. (2015). Unraveling the complexity of iron oxides at high pressure and temperature: Synthesis of Fe5O6.
Science Advances, 1(5), e1400260. https://doi.org/10.1126/sciadv.1400260

1336

Geophysical Research Letters10.1002/2017GL075720

Liu, J., Hu, Q., Kim, D. Y., Wu, Z., Wang, W., Xiao, Y., et al. (2017). Hydrogen-bearing iron peroxide and the origin of ultralow-velocity zones.
Nature, 551(7681), 494–497. https://doi.org/10.1038/nature24461
Mao, H. K., Hu, Q., Yang, L., Liu, J., Kim, D. Y., Meng, Y., et al. (2017). When water meets iron at Earth’s core-mantle boundary. National Science
Review, nwx109. https://doi.org/10.1093/nsr/nwx109
Mao, W. L., Sturhahn, W., Heinz, D. L., Mao, H. K., Shu, J., & Hemley, R. J. (2004). Nuclear resonant X-ray scattering of iron hydride at highpressure. Geophysical Research Letters, 31, L15618. https://doi.org/10.1029/2004GL020541
Mashino, I., Murakami, M., & Ohtani, E. (2016). Sound velocities of δ-AlOOH up to core-mantle boundary pressures with implications forthe seismic anomalies in the deep mantle. Journal of Geophysical Research: Solid Earth, 121, 595–609. https://doi.org/10.1002/
2015JB012477
McCammon, C. (1997). Perovskite as a possible sink for ferric iron in the lower mantle. Nature, 387(6634), 694–696. https://doi.org/10.1038/
42685
Narygina, O., Dubrovinsky, L. S., McCammon, C. A., Kurnosov, A., Kantor, I. Y., Prakapenka, V. B., & Dubrovinskaia, N. A. (2011). X-ray diffraction
and Mössbauer spectroscopy study of fcc iron hydride FeH at high pressures and implications for the composition of the Earth’s core.
Earth and Planetary Science Letters, 307(3-4), 409–414. https://doi.org/10.1016/j.epsl.2011.05.015
Nishi, M., Kuwayama, Y., Tsuchiya, J., & Tsuchiya, T. (2017). The pyrite-type high-pressure form of FeOOH. Nature, 547(7662), 205–208.
https://doi.org/10.1038/nature22823
Ohira, I., Ohtani, E., Sakai, T., Miyahara, M., Hirao, N., Ohishi, Y., & Nishijima, M. (2014). Stability of a hydrous δ-phase, AlOOH–MgSiO2(OH)2, anda mechanism for water transport Into the base of lower mantle. Earth and Planetary Science Letters, 401, 12–17. https://doi.org/10.1016/
j.epsl.2014.05.059
Ohishi, Y., Hirao, N., Sata, N., Hirose, K., & Takata, M. (2008). Highly intense monochromatic X-ray diffraction facility for high-pressure research
at SPring-8. High Pressure Research, 28(3), 163–173. https://doi.org/10.1080/08957950802208910
Ohtani, E., Hirao, N., Kondo, T., Ito, M., & Kikegawa, T. (2005). Iron-water reaction at high pressure and temperature, and hydrogen transport
Into the core. Physics and Chemistry of Minerals, 32(1), 77–82. https://doi.org/10.1007/s00269-004-0443-6
Ohtani, E., Toma, M., Kubo, T., Kondo, T., & Kikegawa, T. (2003). In situ X-ray observation of decomposition of superhydrous phase B at highpressure and temperature. Geophysical Research Letters, 30(2), 1029. https://doi.org/10.1029/2002GL015549
Okuchi, T. (1997). Hydrogen partitioning into molten iron at high pressure: Implications for Earth’s core. Science, 278(5344), 1781–1784.
https://doi.org/10.1126/science.278.5344.1781
Otte, K., Pentcheva, R., Schmahl, W. W., & Rustad, J. R. (2011). Pressure-induced structural and electronic transitions in FeOOH form ﬁrst
principles. Physical Review B, 80, 205116
Pépin, C., Geneste, G., Dewaele, A., Mezouar, M., & Loubeyre, P. (2017). Synthesis of FeH5: A layered structure with atomic hydrogen slabs.
Science, 357(6349), 382–385. https://doi.org/10.1126/science.aan0961
Pépin, C. M., Dewaele, A., Geneste, G., Loubeyre, P., & Mezouar, M. (2014). New iron hydrides under high pressure. Physical Review Letters,
113(26), 265504. https://doi.org/10.1103/PhysRevLett.113.265504
Rohrbach, A., Ballhaus, C., Golla-Schindler, U., Ulmer, P., Kamenetsky, V. S., & Kuzmin, D. V. (2007). Metal saturation in the upper mantle.
Nature, 449(7161), 456–458. https://doi.org/10.1038/nature06183
Sakamaki, K., Takahashi, E., Nakajima, Y., Nishihara, Y., Funakoshi, K., Suzuki, T., & Fukai, Y. (2009). Melting phase relation of FeHx up to 20 GPa:
Implication for the temperature of the Earth’s core. Physics of the Earth and Planetary Interiors, 174(1-4), 192–201. https://doi.org/10.1016/
j.pepi.2008.05.017
Sakamaki, T., Ohtani, E., Fukui, H., Kamada, S., Takahashi, S., Sakairi, T., et al. (2016). Constraints on Earth’s inner core composition inferredfrom measurements of the sound velocity of hcp-iron in extreme conditions. Science Advances, 2(2), e1500802. https://doi.org/10.1126/
sciadv.1500802
Sano, A., Ohtani, E., Kondo, T., Hirao, N., Sakai, T., Sata, N., et al. (2008). Aluminous hydrous mineral δ-AlOOH as a carrier of hydrogen into thecore-mantle boundary. Geophysical Research Letters, 35, L03303. https://doi.org/10.1029/2007GL031718
Schmandt, B., Jacobsen, S. D., Becker, T. W., Liu, Z., & Dueker, K. G. (2014). Dehydration melting at the top of the lower mantle. Science,
344(6189), 1265–1268. https://doi.org/10.1126/science.1253358
Shibazaki, Y., Ohtani, E., Fukui, H., Sakai, T., Kamada, S., Ishikawa, D., et al. (2012). Sound velocity measurements in dhcp-FeH up to 70 GPa withinelastic X-ray scattering: Implications for the composition of the Earth’s core. Earth and Planetary Science Letters, 313, 79–85.
Shibazaki, Y., Ohtani, E., Terasaki, H., Suzuki, A., & Funakoshi, K.-i. (2009). Hydrogen partitioning between iron and ringwoodite:
Implications for water transport into the Martian core. Earth and Planetary Science Letters, 287(3–4), 463–470. https://doi.org/10.1016/
j.epsl.2009.08.034
Shim, S.-H., Grocholski, B., Ye, Y., Alp, E. E., Xu, S., Morgan, D., et al. (2017). Stability of ferrous-iron-rich bridgmanite under reducing midmantle
conditions. Proceedings of the National Academy of Sciences of the United States of America, 114(25), 6468–6473. https://doi.org/10.1073/
pnas.1614036114
Sinmyo, R., Bykova, E., Ovsyannikov, S. V., McCammon, C., Kupenko, I., Ismailova, L., & Dubrovinsky, L. (2016). Discovery of Fe7O9: A new ironoxide with a complex monoclinic structure. Scientiﬁc Reports, 6(1), 32852. https://doi.org/10.1038/srep32852
Smith, E. M., Shirey, S. B., Nestola, F., Bullock, E. S., Wang, J., Richardson, S. H., & Wang, W. (2016). Large gem diamonds from metallic liquid in
Earth’s deep mantle. Science, 354(6318), 1403–1405. https://doi.org/10.1126/science.aal1303
Stevenson, D. J. (1990). Fluid dynamics of core formation. In H. E. Newsom & J. E. Jones (Eds.), Origin of the Earth (pp. 231–250). New York:
Oxford University Press.
Stixrude, L., & Lithgow-Bertelloni, C. (2005). Thermodynamics of mantle minerals—I. Physical properties. Geophysical Journal International,
162(2), 610–632. https://doi.org/10.1111/j.1365-246X.2005.02642.x
Suzuki, A., Ohtani, E., & Kamada, T. (2000). A new hydrous phase δ-AlOOH synthesized at 21 GPa and 1000 C. Physics and Chemistry of Minerals,27(10), 689–693. https://doi.org/10.1007/s002690000120
Tagawa, S., Ohta, K., Hirose, K., Kato, C., & Ohishi, Y. (2016). Compression of Fe–Si–H alloys to core pressures. Geophysical Research Letters, 43,
3686–3692. https://doi.org/10.1002/2016GL068848
Terasaki, H., Ohtani, E., Sakai, T., Kamada, S., Asanuma, H., Shibazaki, Y., et al. (2012). Stability of Fe–Ni hydride after the reaction between
Fe–Ni alloy and hydrous phase (δ-AlOOH) up to 1.2 Mbar: Possibility of H contribution to the core density deﬁcit. Physics of the Earth andPlanetary Interiors, 194, 18–24.
Townsend, J. P., Tsuchiya, J., Bina, C. R., & Jacobsen, S. D. (2015). First-principles investigation of hydrous post-perovskite. Physics of the Earthand Planetary Interiors, 244, 42–48. https://doi.org/10.1016/j.pepi.2015.03.010
Townsend, J. P., Tsuchiya, J., Bina, C. R., & Jacobsen, S. D. (2016). Water partitioning between bridgmanite and postperovskite in thelowermost mantle. Earth and Planetary Science Letters, 454, 20–27. https://doi.org/10.1016/j.epsl.2016.08.009

YUAN ET AL.

1337

Geophysical Research Letters10.1002/2017GL075720

Walter, M., Thomson, A., Wang, W., Lord, O., Ross, J., McMahon, S., et al. (2015). The stability of hydrous silicates in Earth’s lower mantle:
Experimental constraints from the systems MgO–SiO2–H2O and MgO–Al2O3–SiO2–H2O. Chemical Geology, 418, 16–29. https://doi.org/
10.1016/j.chemgeo.2015.05.001
Weerasinghe, G. L., Pickard, C. J., & Needs, R. (2015). Computational searches for iron oxides at high pressures. Journal of Physics
Condensed Matter, 27(45), 455501. https://doi.org/10.1088/0953-8984/27/45/455501
Zhu, S. C., Hu, Q., Mao, W. L., Mao, H. K., & Sheng, H. (2017). Hydrogen-bond symmetrization breakdown and dehydrogenation mechanism of
FeO2H at high pressure. Journal of the American Chemical Society, 139(35), 12,129–12,132. https://doi.org/10.1021/jacs.7b06528

YUAN ET AL.

1338