ARTICLES
PUBLISHED ONLINE: 8 DECEMBER 2014 | DOI: 10.1038/NGEO2306
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
and Nobuyoshi Miyajima
The source rocks for basaltic lavas that form ocean islands are often inferred to have risen as part of a thermal plume from the
lower mantle. These rocks are water-rich compared with average upper-mantle rocks. However, experiments indicate that the
solubility of water in the dominant lower-mantle phases is very low, prompting suggestions that plumes may be sourced
from as-yet unidentified reservoirs of water-rich primordial material in the deep mantle. Here we perform high-pressure
experiments to show that Al2 SiO4 (OH)2 —the aluminium-rich endmember of dense, hydrous magnesium silicate phase D—
is stable at temperatures extending to over 2,000 ◦ C at 26 GPa. We find that under these conditions, Al-rich phase D is
stable within mafic rocks, which implies that subducted oceanic crust could be a significant long-term water reservoir in
the convecting lower mantle. We suggest that melts formed in the lower mantle by the dehydration of hydrous minerals in
dense ultramafic rocks will migrate into mafic lithologies and crystallize to form Al-rich phase D. When mantle rocks upwell,
water will be locally redistributed into nominally anhydrous minerals. This upwelling material provides a potential source for
ocean-island basalts without requiring reservoirs of water-rich primordial material in the deep mantle.
H
igh-pressure experiments indicate that nominally
anhydrous minerals at >200 km depth in the upper
mantle can host the H2 O contents inferred for basalt source
rocks (typically 70–700 wt ppm) as hydroxyl defects in their crystal
structures1,2 . The H2 O capacity of ultramafic rocks increases in the
transition zone, as recently confirmed by the discovery of hydrous
ringwoodite in diamond3 . In contrast, the lower-mantle phases
bridgmanite (magnesium silicate perovskite) and ferropericlase
seem to have a much lower H2 O solubility1,4,5 . It is therefore
problematic that ocean-island basalt (OIB) sources apparently
originating in the lower mantle have the highest H2 O contents6,7 .
Several dense hydrous magnesium silicates (DHMS phases) are
thermodynamically stable in peridotites within subducting lithosphere8,9 . In the lower mantle, the most important of these phases
are superhydrous phase B (also known as phase C, nominally
Mg10 Si3 O14 (OH)4 ; ref. 10), phase D (hereafter Mg-phase D; nominally MgSi2 O4 (OH)2 ; ref. 11) and the newly discovered phase H
(nominally MgSiO2 (OH)2 ; ref. 9). However, these 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 that can increase their thermal
stability in lower-mantle assemblages.
One potential stabilizing component in hydrous phases at
lower-mantle pressures is Al. Al-bearing Mg-rich phase D breaks
down at ∼1,600 ◦ C, about 200 ◦ C higher than the Mg-phase D
endmember12 . Phase H can also accept Al, forming a solid solution
with the similarly structured phase δ-AlOOH (ref. 13). In certain
compositions, this solid solution is stable even along typical mantle
geotherms at >40 GPa (ref. 14). However, Fe counteracts the
stabilizing effect of Al addition, such that Fe, Al-bearing phase D
in ultramafic compositions may not be any more stable than
the Mg-phase D endmember12 . 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
Al 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); ref. 15) in superdeep
diamond inclusions16 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 stable17 , 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 homogenized on the timescales of mantle convection
and will therefore persist as distinct lithologies18,19 . There is thus
a possibility that mafic components within a mechanically mixed
lower mantle could become a focus for H2 O, if a suitable host exists.
One untested candidate is superaluminous phase D, which has been
synthesized at 25 GPa and 1,500 ◦ C (ref. 20) in a bulk composition
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 stability field of Mg, Fe-free aluminous phase
D (hereafter Al-phase D; ideal formula Al2 SiO4 (OH)2 ) to examine
whether it could be a host for H2 O in the uppermost lower mantle.
The stability and composition of Al-phase D
Al-phase D was synthesized in multianvil experiments between
1,460 ◦ C and 2,100 ◦ C in the simplified Al2 O3 –SiO2 –H2 O system.
Two starting mixtures were employed with Al/Si ratios of 2:1
(mixture 1) and 1:1 (mixture 2) and H2 O contents of 13 and
19 wt% respectively (Supplementary Table 1). The resulting
phase assemblages were identified using X-ray diffraction (XRD),
electron probe microanalysis (EPMA), electron backscatter
detection (EBSD) and transmission electron microscopy (TEM;
Supplementary Table 2). Compositions determined by EPMA are
presented in Supplementary Table 3.
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany. †Present address: Department of Geology, University of Illinois
Urbana-Champaign, Illinois 61820, USA. *e-mail: [email protected]
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© 2014 Macmillan Publishers Limited. All rights reserved
75
NATURE GEOSCIENCE DOI: 10.1038/NGEO2306
ARTICLES
30
[egg]
H2O/2
L
D
stv D
δ cor
[D]
stv
egg
δ
egg
[egg]
cor
[L]
SiO2
δ
cor
stv
Al2O3/2
(D)
δ cor
L
P (GPa)
[egg]
26
S4517_2
S5050_2
D
S4523
cor
S4517_1 egg cor egg stv S5081
S5113
δ D
S5021_2
stv D
δ egg
[egg]
D
δ stv L
stv
δ L egg
[δ]
stv D
L
D egg
[stv]
δ L egg
[cor]
L
egg cor
stv
(egg)
δ cor
L
[D]
22
1,000
[D]
1,200
D L
cor
stv
1,400
1,600
[D]
1,800
2,000
2,200
Temperature (°C)
Figure 1 | Schreinemakers analysis of phase relations in the system
Al2 O3 –SiO2 –H2 O. Based on experiments performed between 22 and
26 GPa. Larger ternary diagrams show phase assemblages observed in this
study at 26 GPa. Smaller ternary diagrams show results from previous
studies or are deduced 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.
The chemical composition of Al-phase D was found to vary
with bulk composition and with temperature. The Al2 O3 -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 approximately 1.5:1
buffered by the presence of stishovite. All EPMA analyses 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 H2 O
contents. In the more SiO2 -rich composition the resulting chemical
formula is Al1.54 Si0.98 O6 H3.5 at 1,460 ◦ C and Al1.73 Si1.2 O6 H2 at
2,100 ◦ C, corresponding to a decrease in H2 O content from 18
to 10 wt%. Raman analyses of Al-phase D synthesized at 2,100 ◦ C
(Supplementary Fig. 1) qualitatively confirm a significant hydroxyl
concentration in the phase. The wavenumber of the hydroxyl peak
indicates strong hydrogen bonding20 .
A petrogenetic grid of the Al2 O3 –SiO2 –H2 O system at high
pressure is presented in Fig. 1. Constraints on pseudo-univariant
reactions are provided by the EBSD- and TEM-determined experimental phase assemblages, and by the results of previous studies13,15,21 . Experimental run product assemblages are indicated by
shaded regions in the sketched ternary diagrams. The temperaturedependent mineral compositions are based on EPMA analyses. The
pressure–temperature (P–T ) stabilities and relative positions of each
reaction in Fig. 1 are derived by applying Schreinemakers rules
to the available experimental constraints. 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 collinear in composition space. In addition, 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 analysis provides good constraints on phase stability, and allows some extrapolation to regions where experimental
data are lacking.
76
The Si- and Al-rich starting mixtures both produced assemblages
of δ-AlOOH and stishovite at temperatures at or below 1,200 ◦ C.
δ-AlOOH is inferred to be stable to 1,460 ◦ C, and found to contain
up to 17 wt% SiO2 in the more Al-rich composition. As Si-free
δ-AlOOH decomposes at 1,200 ◦ C at 26 GPa (ref. 21), we infer that
the addition of a SiO2 component increases its thermal stability.
A three-phase assemblage of stishovite, δ-AlOOH and phase Egg
crystallizes from the Si-rich mixture up to 1,460 ◦ C. The absence
of Al-phase D can be explained by its higher H2 O 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 1,200 ◦ C
and 1,460 ◦ C, which is approximately 5 GPa higher than previous
determinations15 . It is probable that the discrepancy in reported
stability fields reflects the near collinear compositions of stishovite,
δ-AlOOH and phase Egg; the solid solutions of all three phases
may cause the temperature of the reaction phase Egg → stishovite
+δ-AlOOH to be strongly dependent on bulk H2 O concentration.
Al-phase D appears in assemblages produced from both starting
mixtures after the breakdown of δ-AlOOH. Up to 1,600 ◦ 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 seems to fall within the compositional field of
Al-phase D, although in some cases minor stishovite was also
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 H2 O-rich liquid, is extremely unlikely.
Topological analysis indicates the presence of an invariant point
in the system located approximately at 1,800 ◦ C and 24 GPa ([δ] in
Fig. 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 experiment was performed
at 2,100 ◦ C where Al-phase D coexisted with stishovite and an
alumina-rich melt (Fig. 2). Although thermal gradients inside this
type of multianvil assembly are expected to be about 200 ◦ C mm−1 ,
the small distance between the sample and the thermocouple
(<0.5 mm) ensures that Al-phase D must be stable to temperatures
exceeding 2,000 ◦ 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 experiments14 . 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
Al2 O3 –SiO2 –H2 O system.
The relationship between phase D structure and stability
The decomposition temperature of Al-phase D at 26 GPa is at
least 800 ◦ C above that for Mg-phase D. The results of singlecrystal structural refinements (details reported as Supplementary
Information) can illuminate the reasons why the replacement of
Mg and Si with Al has such a remarkable effect on the stability of
this hydrous structure. The crystal structure of Mg-phase D (space
group no. 162; P 3̄1m; ref. 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, with Mg and
Si in the 1a and 2d Wyckoff positions respectively (M1 and M2;
Supplementary Fig. 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 no. 193;
P63 /mcm), all six octahedral 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
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2306
a
ARTICLES
feldspar, the preference of Al for octahedral rather than tetrahedral
coordination at pressures of a few gigapascals means that this effect
plays only a minor role in much of the upper mantle and transition
zone. Such disorder seems 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 (ref. 14) may also indicate Al–Si disorder.
The composition of phase D in the lower mantle
JEOL COMP
15.0 kV
×110
100 µm WD11 mm
b
20 µm
Figure 2 | Scanning electron micrographs of recovered phase D-bearing
samples. a, Backscattered electron image of experiment S5113 recovered
from 26 GPa and 2,100 ◦ 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 grains of Al-phase (dark) and smaller grains of
stishovite (light). b, A backscattered image of a sample recovered from
25 GPa and 1,500 ◦ C (S4253) in the system H–Fe–Mg–Al–Si–O2 . Fe- and
Al-bearing bridgmanite (light grey) coexists with grains of Al-phase D
(darker grey). Pockets of hydrous melt can be seen at grain boundaries.
that Al–Si disorder in Al-phase D results in essentially undistorted
octahedra of similar size, whereas 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
sums23 . These can be calculated by assuming that the hydrogen
position and site occupancy in Al-phase D are identical to those
determined for Mg-phase D through neutron diffraction measurements24 . Owing 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 with +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. Although
the contribution to the configurational entropy due to Al–Si
disorder has a stabilizing influence on low-pressure minerals such as
Phase D contains roughly equal atomic proportions of Fe and Al
when it coexists with Fe, Al-bearing bridgmanite with compositions
similar to those existing in lower-mantle ultramafic rocks12,25,26 .
Fe cancels out the stabilizing effect of Al (ref. 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 experiment20 (S4253)
at 25 GPa and ∼1,500 ◦ C using a hydrated bulk composition fabricated from 13.6 wt% Al2 O3 , 21.6% Fe2 O3 , 33% SiO2 and 31.8%
Mg(OH)2 . Recovered bridgmanite (Fig. 2b) has the approximate
chemical formula Mg0.63 Fe0.37 Al0.37 Si0.63 O3 , similar to compositions
expected within subducted basalts27 . Coexisting Al-phase D has
the approximate composition Mg0.24 Fe0.16 Al1.83 H1.5 SiO6 . The Al/Fe
ratio in this phase D is much higher than that observed in Fe,
Al-poor compositions (Fig. 3). We suggest that in hydrous lowermantle ultramafic rocks, phase D and bridgmanite accommodate
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 could even host Al-rich phase D
at temperatures well above typical lower-mantle isentropes.
Deep geochemical processing 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 most subducted water in the
form of hydrous phases including phases A, superhydrous B, D and
brucite8,28 . Owing to the low thermal stability of Al-poor hydrous
phases and the low H2 O solubility of nominally anhydrous minerals
in the lower mantle1,8,9 , ultramafic rocks descending through the
upper–lower-mantle boundary will become supersaturated with
H2 O, 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 Fig. 4. Subducted
mafic units have been proposed to be present throughout
the mantle18,19 , owing to the extreme timescales required for
homogenization through chemical diffusion29 . Rocks containing
Al-rich phase D in the lower mantle would still be able to host
∼1,000 wt ppm H2 O in majoritic garnet and clinopyroxene after
upwelling into the upper mantle1,30 .
The process described above implies that H2 O in the lower
mantle will become preferentially concentrated in mafic rocks. One
prediction of this premise is that magmas produced from lowermantle sources that exhibit geochemical evidence for the presence
of recycled material should also be more H2 O-rich. Although there
seems to be a general agreement that OIB mantle sources contain
more H2 O than the depleted mantle, there is some question as
to whether this arises from the recycling of hydrated lithosphere
or reflects H2 O in a primordial source. Several studies have
demonstrated a negative correlation between H2 O concentrations
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77
NATURE GEOSCIENCE DOI: 10.1038/NGEO2306
ARTICLES
Al-rich phase D
MORB
Peridotite
10.0
Al/Fephase D
crust. Instead, recycled oceanic crust has been linked to a ‘focus
zone’ or FOZO component with moderately radiogenic Pb and Sr
ratios (206 Pb/204 Pb ∼ 20, 87 Sr/86 Sr ∼ 0.703; ref. 32). H2 O/Ce values
increase from EM1-influenced basalts towards those with FOZOlike isotopic characteristics6 , implying high H2 O concentrations
in the FOZO source. The progressive hydration of recycled mafic
crust in the lower mantle due to crystallization of Al-phase D,
and potentially other hydrous aluminosilicate phases stable at even
higher pressures9,14 , would explain the relationship between higher
H2 O contents and the FOZO mantle component without needing to
invoke the presence of a wet primordial lower-mantle source.
Mg-rich phase D
1.0
Methods
Ref. 12
Ref. 26
This study
0.1
0.0
0.1
0.2
0.3
0.4
Al3+ + Fe3+pv
Figure 3 | Al/Fe ratio in phase D as a function of bridgmanite composition.
Cation concentrations based on EPMA reveal that Al/Fe ratios in phase D
increase sharply as a function of increasing trivalent cation content in
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
bridgmanite compositions in peridotitic25 and mid-ocean-ridge basalt27
(MORB) bulk compositions are shown as grey bars.
Upper mantle
Received 13 June 2014; accepted 4 November 2014;
published online 8 December 2014
Al-rich D
°C
0
,60
1
°C
∼
00
,1 2
∼
Melt
Al-poor D
atg
law A
∼410 km
Transition zone
D + br/shB
Lower mantle
∼670 km
Figure 4 | Potential mechanisms of deep hydrogen transport between
hydrous phases and melts in a subduction zone. The convecting mantle is
shown as a heterogeneous mixture of mantle and recycled crust. The
crustal layer on top of the subducting slab (dark grey) becomes nominally
anhydrous at 300 km depth, after the breakdown of lawsonite (law).
Hydrated lithospheric mantle can carry water to greater depths, but
hydrous phases become unstable at high temperatures, releasing melts. In
the lower mantle, recycled oceanic crust can become rehydrated by these
melts, forming Al-rich phase D that remains stable even at high
temperatures. Mineral abbreviations are as follows: atg, antigorite; A, phase
A; D, phase D; br, brucite; shB, superhydrous phase B; NAMs, nominally
anhydrous minerals.
of OIB mantle and enriched mantle components (for example,
EM1) defined, for example, by extremely radiogenic Sr isotopic
ratios and considered to result from subducted sediments6,31 .
This has been interpreted to indicate that the lithosphere is
efficiently dehydrated during the subduction process6 . However,
subducted sediments are not representative of subducted oceanic
78
Starting compositions were based on analyses of crystals of Al-phase D previously
reported20 , which demonstrate a compositional range in the Al/Si ratio. Two
starting mixtures were fabricated from SiO2 , Al2 O3 and Al(OH)3 with Al/Si ratios
of 2:1 and 1:1 and H2 O contents of 13 and 19 wt% respectively (Supplementary
Table 1). In two further experiments a stoichiometric phase Egg composition
(AlSiO3 OH) 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 temperatures between
1,050 and 2,100 ◦ C. Further experimental details are reported as Supplementary
Methods. Recovered samples were analysed using powder XRD, EPMA, TEM and
EBSD. A single crystal of Al-phase D with dimensions of 30 × 15 × 15 µm was
recovered from a sample synthesized at 1,460 ◦ C for data collection using
single-crystal XRD.
The Supplementary Information includes a full description of the structural
refinement, further experimental details and analytical information. Run
conditions and averaged EPMA analyses are also provided as a separate Excel file.
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Acknowledgements
The authors would like to thank G. Gollner, H. Fischer, S. Übelhack, 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.
Author contributions
T.B.B. and D.J.F. designed the study; M.G.P. performed 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.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to R.M.
Competing financial interests
The authors declare no competing financial interests.
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