Ishii et al.
Progress in Earth and Planetary Science(2024) 11:10
https://doi.org/10.1186/s40645-024-00615-0

Progress in Earth andPlanetary Science

Open Access

RESEARCH ARTICLE

Hydrogen partitioning between stishoviteand hydrous phase δ: implications for water
cycle and distribution in the lower mantle
Takayuki Ishii1,2* , Giacomo Criniti2, Narangoo Purevjav2, Tomoo Katsura2,3 and Eiji Ohtani4

Abstract
Water is transported into the deep mantle by subducting slabs, playing important roles in mantle dynamics and evolution. An aluminous hydrous mineral, phase δ with a main component of AlOOH, has been considered an importantwater carrier in the lower mantle. Recent studies reported that ­SiO2 stishovite can accommodate weight percent
levels of water, indicating another important water carrier in the lower mantle. However, which mineral can mainly
carry water is not clear yet. Recent hydrous phase relation studies reported that stishovite is depleted in alumina
when coexisting with hydrous phase δ, in which water content of stishovite was not investigated. In this study,
we investigated hydrogen partitioning between stishovite and hydrous phase δ at 24–28 GPa and 1000–1200 °C
by means of Kawai-type multi-anvil press in combination with Fourier-transform infrared spectroscopy at ambientconditions on recovered samples. Fourier-transform infrared spectra of recovered stishovites showed that watercontents of stishovite coexisting with hydrous phase δ were limited to up to ~ 500 ppm. This indicates that coexisting hydrous phase δ causes not only depletion in alumina but also in hydrogen in stishovite and therefore mainlytransports water in a cold subducting slab. Once hydrous phase δ becomes thermally unstable, alumina and watercontents in silica minerals are increased by the chemical reaction between ­SiO2 and AlOOH, and aluminous silica minerals such as stishovite and ­CaCl2-type phase will be a main water carrier in the lower mantle. Presence of small-scale
seismic scatterers observed around 1900 km depth, which was considered to be caused by a transition from almost
pure ­SiO2 stishovite to ­CaCl2-type phase, might also be able to be explained by the phase transition of stishovite
coexisting with hydrous phase δ.
Keywords Water cycle, Mantle, Subducting slab, Hydrous mineral, Nominally anhydrous mineral, Multi-anvil press*Correspondence:
Takayuki Ishii
takayuki.ishii@okayama-u.ac.jp1
Institute for Planetary Materials, Okayama University, Misasa, Japan
2
Bayerisches Geoinstitut, University of Bayreuth, Universitätsstraße 30,
95447 Bayreuth, Germany
3
Center for High Pressure Science and Technology Advanced Research,
Beijing 100094, China
4
Department of Earth Science, Graduate School of Science, Tohoku
University, Sendai 980‑8578, Japan

1 Introduction
The transportation process of water into the deep mantle
is important for understanding the structure and dynamics and the chemical evolution of the Earth because thephysical and chemical properties such as melting phaserelations, rheology, electrical and thermal conductivity of minerals and rocks in water-bearing systems canbe significantly different from those of the dry system(e.g., Thompson 1992; Wang et al. 2006; Kohlstedt 2006;
Marzotto et al. 2020). The water solubility of nominallyanhydrous minerals and stability of hydrous minerals
are therefore essential to clarify the water transport process in the mantle. The discovery of water-rich minerals© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Ishii et al. Progress in Earth and Planetary Science(2024) 11:10

such as hydrous ringwoodite, superhydrous phase B, andhydrous phase egg as diamond inclusions suggest thatwater is transported at least into the mantle transitionzone (Wirth et al. 2007; Pearson et al. 2014; Tschauneret al. 2018). On the other hand, water transport and its
distribution in the lower mantle are poorly understood.
Oceanic island basalt contains more water than midocean ridge basalt, suggesting water-rich sources in thelower mantle (Hirschmann 2006). However, major minerals in lower-mantle peridotitic assemblages such as
bridgmanite and ferropericlase have generally limited
water solubilities less than 0.1 wt% (e.g., Bolfan-Casanova
et al. 2002; Fu et al. 2019).
High-pressure silica polymorphs of stishovite and­CaCl2-type phase are major constituents of basaltic
crusts, sediments, and continental crustal rocks in thelower mantle (e.g., Irifune et al. 1994; Hirose et al. 1999;
Ishii et al. 2012, 2019a, b, 2022a). Recent phase relation
studies in S
­ iO2–H2O systems reported that these silica
phases can accommodate water at the weight percent
levels (Lin et al. 2020; Nisr et al. 2020) although otherstudies showed limited water contents up to several
hundred ppm (Litasov et al. 2007; Purevjav et al. 2024).
Other studies examined their water storage capacities
in systems containing A­ l2O3, an important component
in the silica-rich rocks mentioned above, and foundthat the water incorporation mechanism is different
from that in an Al-free system and their water contents
increase with temperature. The increases are due to water
being stored as the AlOOH component, and the mixing entropy increasing its solubility with temperature(Litasov et al. 2007; Ishii et al. 2022b). Ishii et al. (2022b)
showed that the water content in ­CaCl2-type phasereaches 1.1 wt% at a temperature of 1900 °C, which iseven higher than the average mantle (Katsura 2022). The
stability of ­CaCl2-type phase at pressures up to ~ 120 GPa(Murakami et al. 2003) allows the silica-enriched rocks
that make up the upper layers of the subducting slab totransport water into the deep lower mantle.
Although aluminous silicas are important water hosts
in the lower mantle, subducting slabs can have lower
temperatures than the average mantle (Thompson 1992;
Ganguly et al. 2009), which may limit their water solubility. An aluminous hydrous mineral, hydrous phase δ withcompositions of (Al, Fe, Si, Mg)OOH, is considered to beanother lower-mantle water carrier because it is stable atpressures down to the bottom of the lower mantle (e.g.,
Sano et al. 2008; Duan et al. 2018; Strozewski et al. 2023).
A recent phase relation study of a hydrous basalt system
reported that stishovite coexists with hydrous phase δ at
25–26 GPa, the pressures at the top of the lower mantle, and at relatively low temperatures of 1000–1200 °C(Liu et al. 2019). Further studies reported that hydrous

Page 2 of 9phase δ is stable at relatively low temperatures up to
1400 °C and pressures of 24–28 GPa in ­SiO2–Al2O3–H2Oand ­MgSiO3–Al2O3–H2O systems (Ishii et al. 2022b,c). Although the presence of hydrous phase δ increases
the water storage capacity in the silica-enriched rocks,
it could limit the AlOOH component and therefore thewater storage capacities in the silica minerals. Thus, thesolubilities of the AlOOH component in the silica phases
coexisting with hydrous phase δ are needed to evaluate
the bulk water storage capacity in the silica-enrichedrocks.
In this study, we determined the water solubility instishovite coexisting with hydrous phase δ in a S­ iO2–
Al2O3–H2O system under water-undersaturated conditions using a multi-anvil press in combination withelectron probe microanalysis and Fourier transform
infrared spectroscopy (FTIR) after recovery.

2 Methods/experimental
2.1 Starting materials

To measure water content by FTIR, it is necessary togrow stishovite crystals at least to 50 microns due to thelimited spatial resolution of conventional FTIR spectrometers. However, it is very difficult to grow stishovite
crystals coexisting with hydrous phase δ because the synthesis temperature is relatively low and the coexisting
phase hampers grain growth. For this reason, the watercontents of stishovite coexisting with hydrous phase δ
were determined using pre-synthesized stishovite single
crystals annealed at a target pressure and temperature instishovite-hydrous phase δ matrix.
Our preliminary experiments showed that the alumina content in stishovite coexisting with hydrous phaseδ is 0.68–0.81 wt% at the same conditions as this study(Ishii et al. 2022b). Based on this result, single crystalsof aluminous stishovite were synthesized at 22 GPa and1900 °C for 2 h using a powdered mixture of reagentgrade ­SiO2 quartz and Al(OH)3 gibbsite at a molar ratioof 99:1 including ~ 0.85 wt% ­Al2O3. The recovered stishovite single crystals have dimensions up to 200–400 μm
and an alumina content of 0.64(3) wt% as determinedby electron microprobe analysis (Table 2 and Additional file 1: Table S1). The lower alumina content in stishovite may be due to presence of hydrous Al–Si melt
(Litasov et al. 2007). The water content was estimated to
be 994(44) ppm as an average value using 10 randomly
oriented crystals by unpolarized FTIR based on Paterson’s (1982) calibration. Single crystal X-ray diffraction of a recovered crystal gave the lattice parametersof a = 4.18077(7) Å, c = 2.66658(13) Å, and the unit cellvolume of V = 46.6086(27) Å3. Further details on the analytical methods are provided in the section below. As forthe matrix to be employed in the annealing experiments,

Ishii et al. Progress in Earth and Planetary Science(2024) 11:10

this was obtained by mixing reagent-grade S­ iO2 quartz
and α-AlOOH boehmite, which was then converted
to hydrous phase δ after high pressure–temperatureannealing.
2.2 High pressure–temperature experiments

All high-pressure experiments were performed using
Kawai-type multi-anvil presses installed at the Bayerisches Geoinstitut, University of Bayreuth. We conducteda pre-synthesis experiment of aluminous stishovite starting crystals at 22 GPa and 1900 °C for 2 h using a 10-MN
press with a split-sphere guide block. The pressure calibration was reported in Keppler and Frost (2005). The
cell assembly with 4 mm truncation tungsten carbide(WC) anvils and a 10 mm edge octahedron reported in
Kawazoe et al. (2017) was adopted. The powdered starting sample was packed in a welded Pt capsule.
The water partitioning experiments between stishovite and hydrous phase δ were carried out at pressuresof 24 and 28 GPa and temperatures of 1000–1200 °C for
24 h. The 10-MN press was also used for experiments
at 24 GPa, in which the pressure estimation by Keppler and Frost (2005) was also adopted. Experiments
at 28 GPa were conducted using the 15-MN press withan Osugi-type guide block, IRIS-15 (Ishii et al. 2016,2019b). Generated pressures against press load were
determined in Liu et al. (2017). A similar cell assembly design to that reported by Liu et al. (2020) was
adopted, except for the sample chamber (see Fig. 1) at
both conditions. A 7 mm C
­ r2O3-doped MgO pressure
medium was combined with tungsten carbide (WC)
anvils with 3 mm truncated edge lengths. A cylindrical ­L aCrO3 heater was inserted into the center of theoctahedron. ­L aCrO3 plugs were placed at both ends ofthe heater. Some single crystals of the pre-synthesizedaluminous stishovite were placed in a welded gold capsule, in which the single crystals were separated fromeach other by the S­ iO2–AlOOH powdered mixture. The

Fig. 1 A schematic drawing of the cell assembly used for the waterpartitioning experiments

Page 3 of 9

sample capsule was placed at the center of the heater,from which it was electrically insulated using an MgO
sleeve and two plugs. Sample temperatures were measured at the surface of the capsule using a D-type thermocouple. Ceramic components of the cell assembly­(LaCrO3 and MgO parts and pressure media) were fired
at 1000 °C for at least 3 h. The sample was first compressed to a desired press load at room temperatureand then heated to the target temperature at a rate of
100 °C/min. The target conditions were kept for 24 hwith temperature fluctuations of less than ± 5 °C. After
keeping the target pressure–temperature conditions forthe desired time, the temperature was rapidly decreased
to room temperature by shutting off the electric power
supply to the heater, and then the cell assembly was
slowly decompressed to ambient pressure for 15 h.
2.3 Analysis of recovered samples

The phases present in the recovered matrix parts wereconfirmed using a micro-focused X-ray diffractometerwith a micro-focus source (IμS) of Co–Kα (Bruker, D8
Discover) in combination with a two-dimensional solidstate detector (VÅNTEC-500).
Single crystals of the starting stishovite were handpicked from the recovered Pt-capsule and observed
under a polarizing microscope. To estimate density ofthe single crystal stishovite for calculations of their watercontent based on their FTIR data, an inclusion-free grainwith ~ 50 μm in diameter displaying sharp optical extinction was then selected and mounted on glass needles for
single-crystal X-ray diffraction measurements. The unitcell lattice parameters of the single crystal were determined using a Eulerian-geometry Huber single-crystal
diffractometer equipped with a MoKα sealed tube operated at 40 kV and 50 mA and a point detector. The software program SINGLE (Angel and Finger 2011) was used
to drive the diffractometer and collect diffraction profiles of eight unique reflections, each centered using the
8-position method (King and Finger 1979). The symmetry-constrained lattice parameters were then calculated
via a vector least-square refinement.
Recovered crystals were double-side polished to
70–176 μm in thickness for unpolarized FTIR spectroscopy. The FTIR measurements were performed using a
Bruker IFS 120 high-resolution spectrometer in combination with a Bruker IR microscope and collected using
several randomly oriented crystals. A baseline correction
was applied by subtracting from each raw spectrum using
a spline function that was fitted outside the OH bands
wavenumber region. The water contents in recovered
aluminous stishovite were calculated based on the empirical calibration of Paterson (1982):

Ishii et al. Progress in Earth and Planetary Science

Xi
CH2 O = 3 × 150(2024) 11:10

K (υ)(3780−v) dv

Page 4 of 9(1)

where CH2O is the water content (wt ppm), K(ν) is theabsorption coefficient (­cm−1) at a given wave number ν,and Xi is a density factor given as Xi = 18/2d × ­106, whered is the mineral density (g/l) determined by single-crystal
X-ray diffraction and chemical-composition analysis. The
absorption coefficients were integrated in the wavenumber range of 2400–3700 ­cm−1.
The chemical compositions of the crystals were measured using an electron probe microanalyzer with wavelength-dispersive spectrometers (JEOL, JXA-8200) at
an accelerating voltage and probe current of 15 kV and
15 nA, respectively. Standard materials were quartz­(SiO2) and synthetic corundum ­(Al2O3) for Si and Al,
respectively.

3 Results and discussion
3.1 Water content of pre‑synthesized starting aluminous
stishovite

Unpolarized FTIR spectra of the pre-synthesized startingstishovite single crystals are shown in Fig. 2. The spectrum of the starting stishovite shows main OH-absorption bands at 2667, 3115, 3240, 3318, and 3403 ­cm−1.
These observations agree with those of stishovites
reported by Litasov et al. (2007). Although it is known
that stishovite is highly anisotropic (Litasov et al. 2007;
Ishii et al. 2022b), we observed limited differences in
the unpolarized FTIR profiles of 10 crystals with random crystal orientation, resulting in water contents

Fig. 2 Representative unpolarized FTIR spectra of recoveredstishovite single crystalsof 940–1059 ppm. Thus, the average water content of
993 ppm was obtained with one standard deviation of themean of 44 ppm. The aluminum and hydrogen contentsof the starting stishovite were estimated to be 0.75(4)mol% and 0.66(3) mol%, respectively, indicating that the
Al/H ratio is close to 1. Previous studies reported Al/H
ratios between 2 and 7 (Litasov et al. 2007; Zhang et al.
2022; Ishii et al. 2022b), indicating a high concentrationof oxygen vacancy. Thus, the present starting stishovite
has a higher Al/H ratio than previously reported. Thisis probably because our stishovite was synthesized at
a higher temperature of 1900 °C than those in previous
studies (1200–1800 °C) (Litasov et al. 2007; Ishii et al.
2022b). Ishii et al. (2022b) reported a positive temperature dependence of water content in aluminous silica
phases coupled with an increase in the alumina concentration close to or under alumina saturated conditions.
On the other hand, since our stishovite with the limited
alumina content was synthesized quite far from the alumina saturated conditions, the present experimental
condition is not the same as the previous conditions withmuch more alumina. However, it would be possible toincorporate more hydrogen by a possible positive temperature dependence of the AlOOH content instead ofoxygen vacancy formation. This hypothesis is supported
by a previous ab initio calculation study (Panero andStixrude 2004).
3.2 Water contents of aluminous stishovite coexistingwith δ‑AlOOH

We conducted three water partitioning experiments at
24–28 GPa and 1000–1200 °C (Table 1). Figure 3 showsa representative cross section of a recovered sample capsule including a stishovite single crystal and the matrix
part. Powder X-ray diffraction and electron microprobe
analysis confirmed that the matrix portion became stishovite with ~ 0.7 wt% ­Al2O3 and hydrous phase δ with
~ 4.0 wt% ­SiO2 after annealing, showing negligible temperature dependence of the major compositions in thepresent temperature range. The annealed crystals show
the same OH-absorption bands as those of the startingstishovite (Fig. 2). On the other hand, the OH bands ofthe annealed crystals have lower integrated intensities
than those of the starting crystal, indicating a decrease in
the water content during annealing. The water contents ofaluminous stishovite after annealing have been estimatedto be 456–535 ppm corresponding to 0.30–0.35 mol%of hydrogen (Table 2). Thus, the Al/H ratio became ~ 2,which is close to the trend previously reported (Fig. 4)(Litasov et al. 2007; Zhang et al. 2022; Ishii et al. 2022b).
We confirmed that the recovered single crystals werechemically homogeneous based on EPMA and FTIR
analyses at different places (Table 2 and Additional

Ishii et al. Progress in Earth and Planetary Science(2024) 11:10

Page 5 of 9

Table 1 Experimental conditions and average water contents of recovered stishovite
Run #

P

T

D

GPa

°C

h

Starting sample

Phase

CH2O

Al2O3 content*

ppm

wt%

H5276

22

1900

3

SiO2:Al(OH)3 = 99:1

sSt

994(44)0.64(3)

H5324

24

1000

24

sSt/SiO2:AlOOH = 8:2

St/St + δ456(32)0.63(4)

H5365

24

1200

24

sSt/SiO2:AlOOH = 8:2

St/St + δ

491(41)0.64(2)

I1130

28

1200

24

sSt/SiO2:AlOOH = 8:2

St/St + δ534(25)0.62(2)sSt, Starting stishovite single crystal; St, stishovite; δ, hydrous phase δ*Average compositions from several grains (see details in Table 2 and Additional file 1: Table S1)Table 2 Water and alumina contents of recovered stishovites
Run #Grain #

Water content (ppm)

Al2O3 content* (wt%)

H5276

1

972

0.59(4)2960

0.62(5)

90

3

1036

0.63(5)

105

4

940

0.63(5)

80

5

1059

0.62(6)

70

6

944

0.63(5)

80

7

1046

0.65(5)

70

8

1011

0.65(3)

140

9

1004

0.66(4)

75

10

964

0.68(4)

Average994(44)§0.64(3)14260.64(3)

140

2

483

0.64(3)

120

3

489

0.62(3)

120

4

463

0.64(4)

100

5

421

0.60(4)

Average456(32)§0.63(4)1

540

0.66(1)

103

2

475

0.64(2)

150

3

481

0.66(2)

150

4

458

0.63(2)

150

5

545

0.62(3)

120

6

448

0.64(2)Average491(41)§0.64(2)1504

0.65(6)

176

2

511

0.59(4)

176

3

538

0.63(2)

154

4

558

0.62(5)

154

5

558

0.64(5)

Average534(25)§0.62(2)

H5324

H5365

I1130

§

H content (mol%)Thickness (μm)
701050.66(3)100
0.30(2)150
0.33(2)154
0.35(2)The errors are expressed as one standard deviation of the mean

*Detailed chemical compositions of recovered stishovite single crystals were shown in Additional file 1: Table S1

file 1: Table S1). The annealing time in this study would
be enough to reach equilibrium for hydrogen based on
previously determined H diffusivities of minerals suchas olivine, its polymorphs, and T­ iO2 rutile that has thesame structure as stishovite (Ingrin and Blanchard 2006;
Chakraborty 2010). We note that alumina contents instishovite single crystals were consistent within error
before/after the partitioning experiments (Table 1, 2 andIshii et al. Progress in Earth and Planetary Science(2024) 11:10

Page 6 of 9that Al may have not completely reached chemical equilibrium during the experiments. In the latter case, for an
Al/H ratio of 2 (Fig. 4), the water content would increaseby only a few tens ppm at the Al content in the matrix
part, which is anyway within two standard deviations of
the analytical error on the ­H2O concentration. Thus, sucha small difference in alumina content does not change
our conclusions. This result suggests strong water partitioning into hydrous phase δ and is consistent withwater partitioning between nominally anhydrous minerals and hydrous minerals in other systems such as bridgmanite and hydrous phase δ/D and olivine polymorphs
and hydrous minerals (Ishii and Ohtani 2021, Ishii et al.
2022c).
3.3 Water transport in the lower mantle
Fig. 3 A representative photo of recovered stishovite after the waterpartitioning experiment (H5324)

Fig. 4 H+ contents of stishovite as a function of ­Al3+ content.
ST, starting stishovite pre-synthesized; AA, stishovite coexistingwith hydrous phase δ after annealing. Some of stishovites in previous
studies coexist with liquid phase or hydrous phase egg. Errorsof the data from this study are within the size of the symbol

Additional file 1: Table S1). Alumina contents of stishovite in the matrix part (~ 0.7 wt%) appear to be slightly
higher than those of the single crystals (0.59–0.67 wt%).
While these values are often in agreement with eachother within their mutual uncertainties, it is also possible

Here, we discuss water transport in the mantle. Down
to the mantle transition zone, subducted slabs transportwater mainly as dense hydrous magnesium silicates suchas phase A, superhydrous phase B, and phase D (Litasov
and Ohtani 2003; Liu et al. 2019). On the other hand, inthe lower mantle, a solid solution between hydrous phaseδ and H may crystallize in basaltic and peridotitic layers
when the slab temperature is relatively lower than the
average mantle (Liu et al. 2019; Nishi et al. 2015; Ishii
et al. 2022c). As mentioned earlier, although the nominally anhydrous minerals in the lower mantle, such as
bridgmanite and ferropericlase, are almost dry evenunder water-saturated conditions (e.g., Bolfan-Casanova
et al. 2002; Fu et al. 2019), the presence of hydrous phaseδ further facilitates dehydration of these minerals and stishovite (Ishii et al. 2022c; this study) and can destabilize
alkali element- and alumina-rich minerals, such as thealuminous phases of calcium ferrite phase and hexagonalphase (Ishii et al. 2023), due to the strong alumina andwater partitioning to hydrous phase δ.
Recent experimental and theoretical studies suggested
that the water solubility of davemaoite is up to a few
weight percent, although the unquenchability prevents a
definitive conclusion (Chen et al. 2020; Shim et al. 2022).
Chen et al. (2020) demonstrated that davemaoite may
be hydrous even when coexisting with hydrous phase δ
based on the crystal structure change in davemaoite fromcubic to tetragonal and the volume reduction relative tothe dry sample by a laser-heated diamond anvil cell. The
strong alumina and water partitioning to hydrous phaseδ against bridgmanite and stishovite suggests that davemaoite is also alumina- and water-depleted at least when
coexisting with hydrous phase δ, although further experiments are needed to investigate the water solubility of
davemaoite in the ­CaSiO3-AlOOH system.
Therefore, we suggest that a main water carrier would
be hydrous phase δ-H solid solution in a relatively low

Ishii et al. Progress in Earth and Planetary Science(2024) 11:10

temperature slab. Because slab temperature should
be gradually increasing, hydrous phases are likely to
decompose at some depths when the slab temperatureapproaches that of the average lower mantle (Katsura2022). After dehydration, the released water will reactwith aluminous silica phases, resulting in the water carrier being replaced by hydrous aluminous silicas suchas stishovite and ­CaCl2-type phase due to their positive
temperature dependence of alumina and water contents(Ishii et al. 2022b).

Page 7 of 9solubility of nominally anhydrous lower-mantle minerals suggest that hydrous phase δ is a main water carrierin the lower mantle when a subducting slab is at relatively low temperatures compared with the average mantle. Once the subducting slab becomes warm enough totrigger the dehydration of hydrous minerals, aluminous
silica minerals such as stishovite and ­CaCl2-type phasewill become the main water carriers. Seismic scatterers
observed around 1900 km depth may be explained by thetransition of Al, H-poor stishovite to ­CaCl2-type phasecoexisting with hydrous phase δ at low temperatures.

3.4 Seismic scatterers in the lower mantle

Seismological studies found ~ 10 km thick seismic scattering bodies with lower shear-wave velocities and higher
densities than the surrounding mantle between 700 and1900 km depths (e.g., Kaneshima 2019 and references
therein). These observations have been explained by thesecond-order ferroelastic transition from stishovite to­CaCl2-type phase in subducted basaltic crusts because
the transition pressure decreases with increasing alumina
and water content (e.g., Lakshtanov et al. 2007; Zhang
et al. 2022; Ishii et al. 2022b; Criniti et al. 2023). Thus,
the deepest seismic scatterers around 1900 km depth
were interpreted by the transition in dry and almost pure
­SiO2 system (e.g., Nomura et al. 2010; Fischer et al. 2018),
implying that subducted basaltic crusts at this depth
could be dry because stishovite in a dry basaltic crust isalumina depleted (Irifune et al. 1994; Ishii et al. 2019a,b, 2022a). The present study suggests that even lowtemperature hydrous basaltic crust with hydrous phaseδ may be compatible with these observations as alumina
and water will be strongly partitioned into hydrous phaseδ. We note that stishovite coexisting with hydrous phaseδ has a small amount of alumina and water as describedabove and even such small amounts of alumina and water
may affect the transition pressure. Since previous studies demonstrated a reduction in the transition pressureby H and Al incorporation in relatively high aluminaand water-bearing samples with more than 3 wt% and0.25 wt%, respectively (e.g., Lakshtanov et al. 2007; Zhang
et al. 2022; Criniti et al. 2023), further studies are needed
to clarify the effect of low H and Al incorporation.

4 Conclusions
We have determined water contents of aluminous stishovite coexisting with hydrous phase δ at 24–28 GPa
and 1000–1200 °C using a Kawai-type multi-anvil press
in combination with FTIR measurements of recoveredstishovites at ambient pressure. Aluminous stishovite
coexisting with hydrous phase δ is depleted in alumina(< 1 wt%) and has ~ 500 ppm water, showing a strong
water and alumina partitioning to hydrous phase δ. The
present results in addition to previous results of water

Abbreviation
FTIR	Fourier transform infrared spectroscopy

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​024-​00615-0.
Additional file 1: Table S1. Chemical compositions for major components of recovered stishovite single crystals.
Acknowledgements
We thank H. Fischer and R. Njul for the preparation of cell assemblies and thinsection samples, respectively. We thank the Editor of Craig R. Bina and two
anonymous reviewers for their constructive comments.
Author contributions
TI and EO designed the research topic. TI and NP discussed the preliminary
results at the initial stage of this project. TI carried out the high-pressure
experiments and measured and analyzed FTIR spectra and micro-focus XRD
data of recovered samples. GC measured and analyzed the single crystal XRD
data. TI wrote the original draft and revised it by including inputs from GC, NP,
TK, and EO. All authors read and approved the final manuscript.
Funding
This work was funded by the European Research Council (ERC) under theEuropean Union’s Horizon 2020 research and innovation programme (Proposal
No. 787,527) to T.K. and a DFG grant (IS350/1-1) to T.I. The National Natural
Science Foundation of China (NSFC) also supported this work (grant no.:92158206 to R. Tao and T.I.). E.O. and T.I. were supported by the Kakenhi Grant
Number JP20H00187 and JP23K19067, respectively. E.O. was also supportedby the research award from the Alexander von Humboldt foundation.
Availability of data and materials
FTIR data that support the findings of this study are available on Zenodo(https://​doi.​org/​10.​5281/​zenodo.​83026​93).

Declarations
Competing interests
The authors declare that they have no competing interest.
Received: 5 September 2023 Accepted: 21 February 2024

References
Angel RJ, Finger LW (2011) SINGLE: a program to control single-crystal diffractometers. J Appl Cryst 44:247–251

Ishii et al. Progress in Earth and Planetary Science(2024) 11:10

Bolfan-Casanova N, Mackwell S, Keppler H, McCammon C, Rubie DC (2002)
Pressure dependence of H solubility in magnesiowüstite up to 25
GPa: implications for the storage of water in the Earth’s lower mantle.
Geophys Res Lett 29:89–91
Chakraborty S (2010) Diffusion coefficients in olivine, wadsleyite and ringwoodite. Rev Mineral Geochem 72:603–639
Chen H, Leinenweber K, Prakapenka V, Prescher C, Meng Y, Bechtel H, Kunz
M, Shim SH (2020) Possible H­ 2O storage in the crystal structure of­CaSiO3 perovskite. Phys Earth Planet Inter 299:106412
Criniti G, Ishii T, Kurnosov A, Glazyrin K, Boffa Ballaran T (2023) High-pressure
phase transition and equation of state of hydrous Al-bearing silica. Am
MineralDuan Y, Sun N, Wang S, Li X, Guo X, Ni H, Prakapenka VB, Mao Z (2018) Phasestability and thermal equation of state of δ-AlOOH: implication forwater transportation to the deep lower mantle. Earth Planet Sci Lett
494:92–98
Fischer RA, Campbell AJ, Chidester BA, Reaman DM, Thompson EC, Pigott JS,
Prakapenka VB, Smith JS (2018) Equations of state and phase boundary
for stishovite and ­CaCl2-type ­SiO2. Am Mineral 103:792–802
Fu S, Yang J, Karato S, Vasiliev A, Presniakov MY, Gavriliuk AG (2019) Water
concentration in single-crystal (Al, Fe)-bearing bridgmanite grownfrom the hydrous melt: Implications for dehydration melting at the
topmost lower mantle. Geophys Res Lett 46:10346–10357
Ganguly J, Freed AM, Saxena SK (2009) Density profiles of oceanic slabs andsurrounding mantle: integrated thermodynamic and thermal modeling, and implications for the fate of slabs at the 660 km discontinuity.
Phys Earth Planet Inter 172:257–267
Hirose K, Fei Y, Ma Y, Mao HK (1999) The fate of subducted basaltic crust in
the Earth’s lower mantle. Nature 397:53
Hirschmann MM (2006) Water, melting, and the deep Earth H­ 2O cycle. Annu
Rev Earth Planet Sci 34:629–653
Ingrin J, Blanchard M (2006) Diffusion of hydrogen in minerals. Rev Mineral
Geochem 62:291–320
Irifune T, Ringwood AE, Hibberson WO (1994) Subduction of continental
crust and terrigenous and pelagic sediments: an experimental study.
Earth Planet Sci Lett 126:351–368
Ishii T, Ohtani E (2021) Dry metastable olivine and slab deformation in a wet
subducting slab. Nat Geosci 14:526–530
Ishii T, Kojitani H, Akaogi M (2012) High-pressure phase transitions andsubduction behavior of continental crust at pressure–temperature
conditions up to the upper part of the lower mantle. Earth Planet Sci
Lett 357:31–41
Ishii T, Shi L, Huang R, Tsujino N, Druzhbin D, Myhill R, Li Y, Wang L, Yamamoto T, Miyajima N, Kawazoe T, Nishiyama N, Higo Y, Tange Y, Katsura
T (2016) Generation of pressures over 40 GPa using Kawai-type multianvil press with tungsten carbide anvils. Rev Sci Instrum 87:024501
Ishii T, Kojitani H, Akaogi M (2019a) Phase relations of harzburgite and MORB
up to the uppermost lower mantle conditions: precise comparison
with pyrolite by multisample cell high-pressure experiments with
implication to dynamics of subducted slabs. J Geophys Res Solid Earth
124:3491–3507
Ishii T, Liu Z, Katsura T (2019b) A breakthrough in pressure generation by a
Kawai-type multi-anvil apparatus with tungsten carbide anvils. Engineering 5:434–440
Ishii T, Miyajima N, Criniti G, Hu Q, Glazyrin K, Katsura T (2022a) High pressure-temperature phase relations of basaltic crust up to mid-mantle
conditions. Earth Planet Sci Lett 584:117472
Ishii T, Criniti G, Ohtani E, Purevjav N, Fei H, Katsura T, Mao HK (2022b)
Superhydrous aluminous silica phases as major water hosts in hightemperature lower mantle. Proc Natl Acad Sci 119:e2211243119
Ishii T, Ohtani E, Shatskiy A (2022c) Aluminum and hydrogen partitioning
between bridgmanite and high-pressure hydrous phases: Implications
for water storage in the lower mantle. Earth Planet Sci Lett 583:117441
Ishii T, Criniti G, Wang X, Glazyrin K, Ballaran TB (2023) Synthesis
and structural analysis of ­CaFe2O4-type single crystals in the­NaAlSiO4-MgAl2O4-Fe3O4 system. Am Mineral 108:217–221
Kaneshima S (2019) Seismic scatterers in the lower mantle near subduction
zones. Geophys J Int 219:S2–S20
Katsura T (2022) A revised adiabatic temperature profile for the mantle. J
Geophys Res Solid Earth 127:e2021JB023562

Page 8 of 9

Kawazoe T, Ohira I, Ishii T, Boffa Ballaran T, McCammon C, Suzuki A,
Ohtani E (2017) Single crystal synthesis of δ-(Al, Fe)OOH. Am Mineral
102:1953–1956
Keppler H, Frost DJ (2005) Introduction to minerals under extreme conditions. In: Miletich R (ed) Mineral behavior at extreme conditions. Eötvös
University Press, Budapest
King HE, Finger LW (1979) Diffracted beam crystal centering and its application to high-pressure crystallography. J Appl Cryst 12:374–378
Kohlstedt DL (2006) The role of water in high-temperature rock deformation.
Rev Mineral Geochem 62:377–396
Lakshtanov DL, Sinogeikin SV, Litasov KD, Prakapenka VB, Hellwig H, Wang
J, Sanches-Valle C, Perrillat JP, Chen B, Somayazulu M, Li J, Ohtani E,
Bass DJ (2007) The post-stishovite phase transition in hydrous aluminabearing ­SiO2 in the lower mantle of the earth. Proc Natl Acad Sci
104:13588–13590
Lin Y, Hu Q, Meng Y, Walter M, Mao HK (2020) Evidence for the stability ofultrahydrous stishovite in Earth’s lower mantle. Proc Natl Acad Sci
117:184–189
Litasov K, Ohtani E (2003) Stability of various hydrous phases in CMAS pyroliteH2O system up to 25 GPa. Phys Chem Miner 30:147–156
Litasov KD, Kagi H, Shatskiy A, Ohtani E, Lakshtanov DL, Bass JD, Ito E (2007)
High hydrogen solubility in Al-rich stishovite and water transport in thelower mantle. Earth Planet Sci Lett 262:620–634
Liu Z, Nishi M, Ishii T, Fei H, Miyajima N, Ballaran TB, Ohfuji H, Sakai T, Wang L,
Shcheka S, Arimoto T, Tange Y, Higo Y, Irifune T, Katsura T (2017) Phase
relations in the system ­MgSiO3-Al2O3 up to 2300 K at lower mantle pressures. J Geophys Res Solid Earth 122:7775–7788
Liu X, Matsukage KN, Nishihara Y, Suzuki T, Takahashi E (2019) Stability of thehydrous phases of Al-rich phase D and Al-rich phase H in deep subducted oceanic crust. Am Mineral 104:64–72
Liu Z, McCammon C, Wang B, Dubrovinsky L, Ishii T, Bondar D, Nakajima A,
Tange Y, Higo Y, Cui T (2020) Stability and solubility of the ­FeAlO3 component in bridgmanite at uppermost lower mantle conditions. J Geophys
Res Solid Earth 125:e2019JB018447
Marzotto E, Hsieh WP, Ishii T, Chao KH, Golabek GJ, Thielmann M, Ohtani E(2020) Effect of water on lattice thermal conductivity of ringwoodite andits implications for the thermal evolution of descending slabs. Geophys
Res Lett 47:e2020GL087607
Murakami M, Hirose K, Ono S, Ohishi Y (2003) Stability of ­CaCl2-type and
α-PbO2-type ­SiO2 at high pressure and temperature determined by insitu X-ray measurements. Geophys Res Lett 30:11
Nishi M, Irifune T, Gréaux S, Tange Y, Higo Y (2015) Phase transitions of serpentine in the lower mantle. Phys Earth Planet Inter 245:52–58
Nisr C, Chen H, Leinenweber K, Chizmeshya A, Prakapenka VB, Prescher C,
Tkachev SN, Meng Y, Liu Z, Shim SH (2020) Large H­ 2O solubility in dense
silica and its implications for the interiors of water-rich planets. Proc Natl
Acad Sci 117:9747–9754
Nomura R, Hirose K, Sata N, Ohishi Y (2010) Precise determination of poststishovite phase transition boundary and implications for seismic heterogeneities in the mid-lower mantle. Phys Earth Planet Inter 183:104–109
Panero WR, Stixrude LP (2004) Hydrogen incorporation in stishovite at highpressure and symmetric hydrogen bonding in δ-AlOOH. Earth Planet Sci
Lett 221:421–431
Paterson M (1982) The determination of hydroxyl by infrared absorption inquartz, silicate glasses and similar materials. Bull Mineral 105:20–29
Pearson D, Brenker F, Nestola F, McNeill J, Nasdala L, Hutchison M, Matveev S,
Mather K, Silversmit G, Schmitz S (2014) Hydrous mantle transition zone
indicated by ringwoodite included within diamond. Nature 507:221–224
Purevjav N, Fei H, Ishii T, Criniti G, Lin Y, Mao HK, Katsura T (2024) Temperaturedependence of ­H2O solubility in Al-free stishovite. Geophys Res Lett
51:e2023GL104029
Sano A, Ohtani E, Kondo T, Hirao N, Sakai T, Sata N, Ohishi Y, Kikegawa T (2008)
Aluminous hydrous mineral δ-AlOOH as a carrier of hydrogen into thecore-mantle boundary. Geophys Res Lett 35:L03303
Shim SH, Chizmeshya A, Leinenweber K (2022) Water in the crystal structure of­CaSiO3 perovskite. Am Mineral 107:631–641
Strozewski B, Buchen J, Sturhahn W, Ishii T, Ohira I, Chariton S, Lavina B, Zhao J,
Toellner TS, Jackson JM (2023) Equation of state and spin crossover of (Al,Fe)-phase H. J Geophys Res Solid Earth 128:e2022JB026291
Thompson AB (1992) Water in the Earth’s upper mantle. Nature 358:295–302

Ishii et al. Progress in Earth and Planetary Science(2024) 11:10

Tschauner O, Huang S, Greenberg E, Prakapenka V, Ma C, Rossman GR, Shen
AH, Zhang D, Newville M, Lanzirotti A, Tait K (2018) Ice-VII inclusions in
diamonds: evidence for aqueous fluid in Earth’s deep mantle. Science
359:1136–1139
Wang D, Mookherjee M, Xu Y, Karato SI (2006) The effect of water on theelectrical conductivity of olivine. Nature 443:977–980
Wirth R, Vollmer C, Brenker F, Matsyuk S, Kaminsky F (2007) Inclusions of
nanocrystalline hydrous aluminium silicate “Phase Egg” in superdeep
diamonds from Juina (Mato Grosso State, Brazil). Earth Planet Sci Lett
259:384–399
Zhang Y, Fu S, Karato SI, Okuchi T, Chariton S, Prakapenka VB, Lin JF (2022) Elasticity of hydrated Al-bearing stishovite and post-stishovite: Implications
for understanding regional seismic VS anomalies along subducting slabs
in the lower mantle. J Geophys Res Solid Earth 127:e2021JB023170

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