Lower mantle water reservoir implied by the

extreme stability of a hydrous aluminosilicate

Martha G. Pamato Robert Myhill

Tiziana Boffa Ballaran Daniel J. Frost

Florian Heidelbach Nobuyoshi Miyajima

October 1, 2014

Abstract

Ocean island basalt source rocks are often inferred to have risen as

part of a thermal plume from the lower mantle. These rocks appear

water-rich compared to average upper mantle, but experiments indicate

that H2O-solubilities in the dominant lower mantle phases are very low.

The implication is that if plumes do originate in the lower mantle, their

water must have been originally hosted in a melt or an as-yet unidenti-

fied solid phase. Here we report the results of high pressure experiments

showing that Al-phase D, the aluminosilicate analogue of dense hydrous

magnesium silicate phase D, has one of the highest thermal stabilities of

any hydroxide, extending to over 2000◦C at 26 GPa. We show that Al-

rich phase D is stable within mafic bulk compositions, and that recycled

oceanic crust could be a significant long-term water reservoir in the con-

vecting lower mantle. We suggest that lower mantle melts formed by the

dehydration of hydrous minerals in downwelling ultramafic rocks will mi-

grate into mafic lithologies, crystallising Al-rich phase D. During upwelling

1

out of the lower mantle, water will be locally redistributed into nominally

anhydrous minerals. This upwelling material provides a potential source

for ocean island basalts without requiring a water-rich primordial mantle.

High pressure experiments indicate that nominally anhydrous minerals at

>200 km depth in the upper mantle can host the H2O contents inferred for

basalt source rocks (typically 70–700 wt. ppm) as hydroxyl defects in their crys-

tal structures [1, 2]. The water capacity of ultramafic rocks increases in the

transition zone, as recently confirmed by the discovery of hydrous ringwoodite

in diamond [3]. In contrast, the lower mantle phases bridgmanite (magnesium

silicate perovskite) and ferropericlase appear to have a much lower H2O solubil-

ity [1, 4, 5]. It is therefore problematic that ocean island basalt (OIB) sources

apparently originating in the lower mantle have the highest H2O contents [6, 7].

Several dense hydrous magnesium silicates (DHMS phases) are thermody-

namically stable in peridotites within subducting lithosphere [8, 9]. In the

lower mantle, the most important of these phases are superhydrous phase B

(also known as phase C, nominally Mg10Si3O14(OH)4) [10], phase D (hereafter

Mg-phase D; nominally MgSi2O4(OH)2)[11] and the newly discovered phase H

(nominally MgSiO2(OH)2) [9]. However, the Mg-Si endmembers break down

at temperatures lower than those of typical mantle geotherms, and as a result

they cannot form long term water reservoirs in the lower mantle. An important

issue, therefore, is whether these phases have solid solutions which can increase

their thermal stability in lower mantle assemblages.

One potential stabilising component in hydrous phases at lower mantle pres-

sures is alumina. Al-bearing Mg-rich phase D breaks down at ∼1600◦C, about

200◦C higher than the Mg-phase D endmember [12]. Phase H can also accept

Al, forming a solid solution with the similarly structured phase δ-AlOOH [13].

In certain compositions, this solid solution is stable even along typical mantle

geotherms at >40 GPa [14]. However, Fe counteracts the stabilising effect of

2

Al addition, such that Al,Fe-bearing phase D in ultramafic compositions may

not be any more stable than the Mg-phase D endmember [12]. As a result, it is

unclear whether ultramafic rocks in the convecting lower mantle can contain any

hydrous phases. An intriguing possibility is that the relatively high aluminium

contents of recycled oceanic crust could yield greater hydrous phase stability

than observed in ultramafic systems. The presence of phase Egg (ideal formula

AlSiO3(OH)) [15] in superdeep diamond inclusions [16] implies the presence of at

least some hydrous recycled oceanic crust in the mantle transition zone. Even if

subduction effectively dehydrates crustal rocks before phase Egg becomes stable

[17], mafic lithologies could be rehydrated at greater depths by hydrous melts

released from surrounding ultramafics.

Recycled sections of mafic oceanic crust are unlikely to be chemically ho-

mogenised on the time-scales of mantle convection and will therefore persist as

distinct lithologies [18, 19]. There is thus a possibility that mafic components

within a mechanically mixed lower mantle could become a focus for H2O, if a

suitable host exists. One currently untested candidate is superaluminous phase

D, which has been synthesised at 25 GPa and 1500◦C [20] in a bulk composi-

tion similar to portions of subducted oceanic crust. This phase D has high Al

and low Fe contents, suggesting that it could be significantly more stable than

the magnesium silicate endmember. In this study we investigate the stabil-

ity field of Mg,Fe-free aluminous phase D (hereafter Al-phase D; ideal formula

Al2SiO4(OH)2) to examine whether it could be a host for H2O in the uppermost

lower mantle.

The stability and composition of Al-phase D Al-phase D was synthesised

in multianvil experiments between 1460◦C and 2100◦C in the simplified Al2O3-

SiO2-H2O system. Two starting mixtures were employed with Al:Si ratios of 2:1

(Mixture 1) and 1:1 (Mixture 2) and H2O contents of 13 and 19 wt. % respec-

3

tively (Supplementary Table 1). The resulting phase assemblages were deter-

mined using X-ray diffraction, electron probe microanalysis, electron backscat-

ter detection and transmission electron microscopy (XRD, EPMA, EBSD and

TEM respectively; Supplementary Table 2). Compositions determined by elec-

tron probe are presented in Supplementary Table 3.

The chemical composition of Al-phase D was found to vary with bulk compo-

sition and with temperature. The Al2O3-rich composition (Mixture 1) produced

Al-phase D with an atomic Al:Si ratio of approximately 2.5:1. The SiO2-rich

composition (Mixture 2) produced Al-phase D with an Al:Si ratio of approx-

imately 1.5:1 buffered by the presence of stishovite. All electron probe mi-

croanalyses of Al-phase D yielded totals significantly below 100%. These large

deficiencies and their systematic variation with temperature imply that they can

be used to provide approximate estimates of H2O contents. In the more SiO2

rich composition the resulting chemical formula is Al1.54Si0.98O6H3.5 at 1460◦C

and Al1.73Si1.2O6H2 at 2100◦C, corresponding to a decrease in H2O content

from 18 to 10 wt. %. Raman analyses of Al-phase D synthesised at 2100◦C

(see Supplementary Figure 1) qualitatively confirm a significant hydroxyl con-

centration in the phase. The wavenumber of the hydroxyl peak indicates strong

hydrogen bonding [20].

A petrogenetic grid of the Al2O3-SiO2-H2O system at high pressure is pre-

sented in Figure 1. Constraints on pseudo-univariant reactions are provided

by the EBSD and TEM-determined experimental phase assemblages, and by

the results of previous studies [13, 21, 15]. Experimental run product assem-

blages are indicated by shaded regions in the sketched ternary diagrams. The

temperature-dependent mineral compositions are based on electron probe micro-

analyses. The P-T stabilities and relative positions of each reaction in Figure 1

are derived by applying Schreinemakers rules to the available experimental con-

4

straints. A degree of uncertainty arises from an incomplete knowledge of phase

composition. This is especially true of stishovite, phase Egg and δ-AlOOH,

which are nearly co-linear in composition space. Additionally, a degree of solid

solution in each phase means that, in reality, each reaction will be multivariant

and consequently of finite width. Despite these caveats, Schreinemakers analy-

sis provides good constraints on phase stability, and allows some extrapolation

to regions where experimental data are lacking.

The Si and Al-rich starting mixtures both produced assemblages of δ-AlOOH

and stishovite at temperatures at or below 1200◦C. δ-AlOOH is inferred to be

stable to 1460◦C, and found to contain up to 17 wt. % SiO2 in the more Al-rich

composition. As Si-free δ-AlOOH decomposes at 1200◦C at 26 GPa [21], we

infer that the addition of an SiO2 component increases its thermal stability.

A three-phase assemblage of stishovite, δ-AlOOH and phase Egg crystallises

from the Si-rich mixture up to 1460◦C. The absence of Al-phase D can be ex-

plained by its higher H2O content than either the bulk composition or δ-AlOOH

at these conditions. Two further experiments employing a stoichiometric phase

Egg bulk composition confirmed phase Egg stability at 26 GPa and at 1200◦C

and 1460◦C, which is approximately 5 GPa higher than previous determinations

[15]. It is probable that the discrepancy in reported stability fields reflects the

near co-linear compositions of stishovite, δ-AlOOH and phase Egg; the solid so-

lutions of all three phases may cause the temperature of the reaction phase Egg

→ stishovite + δ-AlOOH to be strongly dependent on bulk H2O concentration.

Al-phase D appears in assemblages produced from both starting mixtures

after the breakdown of δ-AlOOH. Up to 1600◦C, Al-phase D coexists with

stishovite and phase Egg in the Si-rich starting mixture, after which phase

Egg breaks down. The more Al-rich mixture appears to fall within the compo-

sitional field of Al-phase D, although in some cases minor stishovite was also

5

produced. It is assumed that Al-phase D is stable within the P-T stability field

of δ-AlOOH at 26 GPa. The lack of Al-phase D in runs conducted within this

field may reflect minor water loss from the capsules during the experiments. The

alternative, that decreasing temperature results in Al-phase D breaking down

to an assemblage of solid phases and H2O-rich liquid, is extremely unlikely.

Topological analysis indicates the presence of an invariant point in the system

located approximately at 1800◦C and 24 GPa ([δ] in Figure 1). This invariant

marks the pressure where Al-phase D takes over from phase Egg as the most

thermally stable hydrous phase in the system. The highest temperature experi-

ment was performed at 2100◦C where Al-phase D coexisted with stishovite and

an alumina-rich melt (Figure 2). Although thermal gradients inside this type

of multianvil assembly are expected to be ca. 200◦C/mm, the small distance

between the sample and the thermocouple (<0.5 mm) ensures that Al-phase D

must be stable to temperatures exceeding 2000◦C. This is one of the highest

known thermal stabilities of any hydroxide, being rivalled only by Mg-bearing

δ-AlOOH, as revealed in recent experiments [14]. The thermal stability of Al-

phase D probably reaches a maximum at a pressure above 26 GPa, and at

even higher pressures Si-bearing δ-AlOOH may replace phase D as the most

thermally stable hydrous phase in the Al2O3-SiO2-H2O system.

The relationship between phase D structure and stability The decom-

position temperature of Al-phase D at 26 GPa is at least 800◦C above that for

Mg-phase D. The results of single crystal structural refinements (details reported

as Supplementary Information) can illuminate the reasons why the replacement

of Mg and Si for Al has such a remarkable effect on the stability of this hydrous

structure. The crystal structure of Mg-phase D (Space Group # 162; P-31m)

[22] is based on a hexagonal close-packed array of oxygen atoms. The SiO6 and

MgO6 octahedra occur in two separated layers stacked along the c direction,

6

with Mg and Si in the 1a and 2d Wyckoff positions respectively (M1 and M2;

see Supplementary Figure 4). Three further octahedral sites corresponding to

the 2c and 1b Wyckoff positions (M3 and M4) remain vacant. However, when

Al replaces Mg in Al-phase D (Space Group # 193; P63/mcm), all six octahe-

dral sites become partially occupied by a random and disordered distribution

of Si and Al. M1 and M4 become equivalent, as do M2 and M3, resulting in

an increase in symmetry. One of the main differences between the structures

is that Al-Si disorder in Al-phase D results in essentially undistorted octahedra

of similar size, while in Mg-phase D large Mg-O distances cause the octahedra

to be strongly distorted. The lesser extent of octahedral distortion is likely to

stabilize Al-phase D relative to its Mg-bearing counterpart.

Another factor influencing the dehydration temperatures of phase D is the

strength of the OH bond. One way to compare the OH bond strength in each

structure is through Pauling bond-strength sums [23]. These can be calculated

by assuming the hydrogen position and site occupancy in Al-phase D are iden-

tical to those determined for Mg-phase D through neutron diffraction measure-

ments [24]. Due to the difference in cation distribution and disorder between

the two structures the protonated O site in Mg-phase D has an effective Pauling

bond strength of +1.67 compared to +1.42 for Al-phase D. This implies that the

O site in Al-phase D is more underbonded than in Mg-phase D, resulting in a

stronger O-H bond and potentially strengthening hydrogen bonds with adjacent

O sites of the same type.

An important factor in the charge distribution in Al-phase D described above

is the large degree of Al-Si disorder. While the contribution to the configura-

tional entropy due to Al-Si disorder has a stabilizing influence on low pressure

minerals such as feldspar, the preference of Al for octahedral rather than tetra-

hedral coordination at pressures of a few GPa means that this effect plays only

7

a minor role in much of the upper mantle and transition zone. Such disorder

appears to become important again in the lower mantle, however, where Si and

Al both exist exclusively in octahedral coordination. Al-phase D may be the

first member of a new class of completely disordered hydrous aluminosilicates

that consequently have high thermal stabilities. The high thermal stabilities

recently reported for the solid solution between phase H and δ-AlOOH above

40 GPa [14] may also indicate Al-Si disorder.

The composition and stability of hydrous phases in the lower mantle

Phase D contains roughly equal atomic proportions of Fe and Al when it coex-

ists with Fe-Al-bearing bridgmanite with compositions similar to those existing

in lower mantle ultramafic rocks [25, 26, 12]. Fe cancels out the stabilising effect

of Al [12], such that phase D in ultramafic compositions is likely to break down

at similar temperatures to the Mg-phase D endmember. In other words, phase

D in ultramafic rocks is only likely to be stable within subducting slabs. To

investigate the partitioning behaviour in mafic compositions, we performed an

additional experiment (S4253 [20]) at 25 GPa and ∼1500◦C using a hydrated

bulk composition fabricated from 13.6 wt. % Al2O3, 21.6% Fe2O3, 33% SiO2

and 31.8% Mg(OH)2. Recovered bridgmanite (Figure 2b) has the approximate

chemical formula Mg0.63Fe0.37Al0.37Si0.63O3, similar to compositions expected

within subducted basalts [27]. Coexisting Al-phase D has the approximate com-

position Mg0.24Fe0.16Al1.83H1.5SiO6. The Al:Fe ratio in this phase D is much

higher than that observed in Al-Fe-poor compositions (Figure 3). We suggest

that in hydrous lower mantle ultramafic rocks, phase D and bridgmanite accom-

modate Fe3+ and Al3+ in roughly equal proportions. As Al contents increase,

the strong preference for the coupled substitution (Fe3+Al3+)↔ (Mg2+Si4+) in

bridgmanite results in high Al:Fe ratios in phase D. As increasing Al:Fe in phase

D expands its thermal stability, subducted crustal rocks with high Al contents

8

could even host Al-rich phase D at temperatures well above typical lower mantle

isentropes.

Deep geochemical processing by hydrous phases in the mantle The

remarkable stability of Al-phase D has major implications for the hydrogen

budget of the lower mantle. Within subducting slabs in the deep upper mantle

and transition zone, it is believed that Al-poor ultramafic lithologies host the

majority of subducted water in the form of hydrous phases including phases A,

superhydrous B, D and brucite [8, 28]. Due to the low thermal stability of Al-

poor hydrous phases and the low H2O solubility of nominally anhydrous minerals

in the lower mantle [1, 8, 9], ultramafic rocks descending through the upper-

lower mantle boundary will become supersaturated with H2O, releasing hydrous

melts as they heat up. The resulting melts will migrate through ultramafic

rocks but will form Al-phase D within Al-rich mafic crustal rocks, as shown in

Figure 4. Subducted mafic units have been proposed to be present throughout

the mantle [18, 19], due to the extreme timescales required for homogenization

through chemical diffusion [29]. Rocks containing Al-rich phase D in the lower

mantle would still be able to host ∼1000 wt. ppm H2O in majoritic garnet and

clinopyroxene after upwelling into the upper mantle [1, 30].

The process described above implies that H2O in the lower mantle will be-

come preferentially concentrated in mafic rocks. One prediction of this premise

is that magmas produced from lower mantle sources that display geochemical

evidence for the presence of recycled material should also be more H2O rich.

Although there seems to be a general agreement that OIB mantle sources con-

tain more H2O than the depleted mantle, there is some question as to whether

this arises from the recycling of hydrated lithosphere or reflects H2O in a pri-

mordial source. Several studies have demonstrated a negative correlation be-

tween H2O concentrations of OIB mantle and enriched mantle components

9

(e.g. EM1) defined, for example, by extremely radiogenic Sr isotopic ratios

and considered to result from subducted sediments [6, 31]. This has been in-

terpreted to indicate that the lithosphere is efficiently dehydrated during the

subduction process [6]. However, subducted sediments are not representative

of subducted oceanic crust. Instead, recycled oceanic crust has been linked

to a “focus zone” or FOZO component with moderately radiogenic Pb and Sr

ratios (206Pb/204Pb∼20, 87Sr/86Sr∼0.703) [32]. H2O/Ce values increase from

EM1-influenced basalts towards those with FOZO-like isotopic characteristics

[6], implying high H2O concentrations in the FOZO source. The progressive

hydration of recycled mafic crust in the lower mantle due to crystallisation of

Al-phase D, and potentially other hydrous alumino-silicate phases stable at even

higher pressures[9, 14] would explain the relationship between higher H2O con-

tents and the FOZO mantle component without needing to invoke the presence

of a wet primordial lower mantle source.

Methods

Starting compositions were based on analyses of crystals of Al-phase D pre-

viously reported [20], which demonstrate a compositional range in the Al:Si

ratio. Two starting mixtures were fabricated from SiO2, Al2O3 and Al(OH)3

with Al:Si ratios of 2:1 and 1:1 and H2O contents of 13 and 19 wt. % respec-

tively (Supplementary Table 1). In two further experiments a stoichiometric

phase Egg composition (AlSiO3OH) was employed. These powders were sealed

inside 1 mm diameter welded Pt capsules. In most experiments two capsules,

each with a different composition, were placed symmetrically either side of a

horizontally inserted thermocouple within the multianvil assembly. Multianvil

experiments were performed at pressures between 22 and 26 GPa and at tem-

peratures between 1050 and 2100◦C. Further experimental details are reported

10

as Supplementary Materials. Recovered samples were analyzed using powder

x-ray diffraction (XRD), electron probe microanalysis (EPMA), transmission

electron microscopy (TEM) and electron back-scattered diffraction (EBSD). A

single crystal of Al-phase D with dimensions of 30x15x15 µm was recovered

from a sample synthesised at 1460◦C for data collection using single crystal

x-ray diffraction.

Supplementary materials associated with this article include a full descrip-

tion of the structural refinement, further experimental details and analytical

information. Run conditions and averaged electron probe microanalyses are

also provided as a separate excel file.

11

Figures

26

P (G

Pa)

1000 1200 1400 1600 1800 2000 2200Temperature (°C)

30

22

S5021_2

S4517_2

S4517_1

S4523S5081S5113

S4517_1

S5050_2S5050_2

[D]

[L]

[stv]

[cor]

[egg]

[egg]

[D]

[egg]

[δ]

[D]

[egg]

[D]

D

cor

L

stv

eggδ

H2O/2

SiO2 Al2O3/2

egg

stv Dδ

coreggDδ

δ cor(D)

L

egg corD

stv

Dcor

L

stv

δ cor(egg)

L

Deggδ L

stv Degg L

egg corL

stv

Dstvδ L

eggLδ

stv

egg

stvδ

cor

corstv

δD

Figure 1: Schreinemakers analysis of phase relations in the systemAl2O3-SiO2-H2O. Based on experiments performed between 22 and 26 GPa.Larger ternary diagrams show phase assemblages observed in this study at 26GPa. Smaller ternary diagrams show results from previous studies or are de-duced from known phase relations. Phase abbreviations are as follows:- δ: δ-AlOOH, stv: stishovite, D: Al-phase D, egg: phase Egg, cor: corundum, L:hydrous liquid/melt.

12

a

b

Figure 2: SEM images of recovered phase D-bearing samples. a)Backscattered electron image of experiment S5113 recovered from 26 GPa and2100◦C. Dark quenched melt can be seen in the lower part of the Pt capsule,overlain by a layer of stishovite. The bulk of the assemblage comprises grainsof Al-phase (dark) and smaller grains of stishovite (light). b) A backscatteredimage of a sample recovered from 25 GPa and 1500◦C (S4253) in the systemH-Fe-Mg-Al-Si-O2. Fe and Al-bearing bridgmanite (light grey) coexists withgrains of Al-phase D (darker grey). Pockets of hydrous melt can be seen atgrain boundaries.

13

MO

RB

PE

RID

OT

ITE

0.1

1

10

Al/F

ephase D

0.0 0.1 0.2 0.3 0.4

Al3+ + Fe3+pv

Mg−rich phase D

Al−rich phase D

Ghosh and Schmidt (2014)

Litasov et al. (2007)

This study

Figure 3: Al:Fe ratio in phase D as a function of bridgmanite com-position. Cation concentrations based on EPMA reveal that Al:Fe ratios inphase D increase sharply as a function of increasing trivalent cation contentin bridgmanite. Fe and Al in bridgmanite are reported on a one-cation basis,and all Fe is assumed to be ferric in both phases. Representative bridgmanitecompositions in peridotitic [25] and MORB [27] bulk compositions are shownas grey bars.

atg

A

D + br/shB

law

transition zone

upper mantle

~410 km

~670 kmlower mantle

melt

~1200

°C~1600°CAl-rich D

Al-poor D

Figure 4: Potential mechanisms of deep hydrogen transport betweenhydrous phases and melts in a subduction zone. The convecting mantleis shown as a heterogeneous mixture of mantle and recycled crust. The crustallayer on top of the subducting slab (dark grey) becomes nominally anhydrous at300 km depth, after the breakdown of lawsonite (law). Hydrated lithosphericmantle can carry water to greater depths, but hydrous phases become unstableat high temperatures, releasing melts. In the lower mantle, recycled oceaniccrust can become rehydrated by these melts, forming Al-rich phase D whichremains stable even at high temperatures. Mineral abbreviations are as follows:-atg: antigorite, law: lawsonite, A: phase A, D: phase D, br: brucite, shB:superhydrous phase B, NAMs: nominally anhydrous minerals.

14

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Correspondence and requests for materials should be addressed to

R.M. (bob.myhill@uni-bayreuth.de).

Acknowledgements The authors would like to thank G. Gollner, H. Fischer,

S. Ubelhack, G. Manthilake, H. Schulze, U. Dittman and D. Krauße. This work

was funded through the support of European Research Council (ERC) Advanced

Grant “DEEP” (#227893). R.M. is supported by an Alexander von Humboldt

Postdoctoral Fellowship.

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Author Contributions T.B.B. and D.J.F. designed the study; M.G.P. per-

formed the experiments and processed the analytical data with assistance from

F.H. (EBSD) and N.M. (TEM); T.B.B. performed the structural refinement;

R.M., T.B.B. and D.J.F. interpreted the analytical data; R.M., M.G.P. and

D.J.F. wrote the paper. All the authors discussed the results and implications

and commented on the manuscript at all stages.

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