Ohira et al. Progress in Earth and Planetary Science (2016) 3:18
DOI 10.1186/s40645-016-0097-2

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Ultrahigh-pressure acoustic wave velocities
of SiO2-Al2O3 glasses up to 200 GPa
Itaru Ohira1*, Motohiko Murakami1, Shinji Kohara2,3, Koji Ohara2 and Eiji Ohtani1,4

Abstract
Extensive experimental studies on the structure and density of silicate glasses as laboratory analogs of natural
silicate melts have attempted to address the nature of dense silicate melts that may be present at the base of the
mantle. Previous ultrahigh-pressure experiments, however, have been performed on simple systems such as SiO2 or
MgSiO3, and experiments in more complex system have been conducted under relatively low-pressure conditions
below 60 GPa. The effect of other metal cations on structural changes that occur in dense silicate glasses underultrahigh pressures has been poorly understood. Here, we used a Brillouin scattering spectroscopic method up topressures of 196.9 GPa to conduct in situ high-pressure acoustic wave velocity measurements of SiO2-Al2O3 glasses
in order to understand the effect of Al2O3 on pressure-induced structural changes in the glasses as analogs ofaluminosilicate melts. From 10 to 40 GPa, the transverse acoustic wave velocity (VS) of Al2O3-rich glass (SiO2 + 20.
5 mol% Al2O3) was greater than that of Al2O3-poor glass (SiO2 + 3.9 mol% Al2O3). This result suggests that
SiO2-Al2O3 glasses with higher proportions of Al ions with large oxygen coordination numbers (5 and 6) become
elastically stiffer up to 40 GPa, depending on the Al2O3 content, but then soften above 40 GPa. At pressures from40 to ~100 GPa, the increase in VS with increasing pressure became less steep than below 40 GPa. Above~100 GPa, there were abrupt increases in the P-VS gradients (dVS/dP) at 130 GPa in Al2O3-poor glass and at 116 GPain Al2O3-rich glass. These changes resemble previous experimental results on SiO2 glass and MgSiO3 glass. Given
that changes of dVS/dP have commonly been related to changes in the Si-O coordination states in the glasses, ourresults, therefore, may indicate a drastic structural transformation in SiO2-Al2O3 glasses above 116 GPa, possibly
associated with an average Si-O coordination number change to higher than 6. Compared to previous acoustic
wave velocity data on SiO2 and MgSiO3 glasses, Al2O3 appears to promote a lowering of the pressure at which theabrupt increase of dVS/dP is observed. This suggests that the Al2O3 in silicate melts may help to stabilize thosemelts gravitationally in the lower mantle.
Keywords: Glass structure, Brillouin scattering, High pressure, Acoustic wave velocity, Heterogeneity of Earth’s
core-mantle boundary

Background
There are regions at the base of the mantle where anomalous reductions of seismic wave velocities occur (e.g.,
Garnero and Helmberger 1995; Mori and Helmberger
1995). These are known as ultralow velocity zones(ULVZs). They vary in thickness from 5 to 40 km justabove the core-mantle boundary (CMB) where there is a
~10–30 % seismic velocity reduction (e.g., Garnero et al.
1998). The possible presence of silicate melts resulting* Correspondence: i.ohira@dc.tohoku.ac.jp
1
Department of Earth Science, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan
Full list of author information is available at the end of the article

from partial melting of the lowermost mantle minerals(e.g., Williams and Garnero 1996) or remnants of thedense basal magma ocean (Labrosse et al. 2007) has
been proposed to explain these anomalous seismic reductions at the CMB. Silicate melts under lower mantle
conditions are not well understood because of the technical difficulties involved in conducting experiments on
silicate melts under relevant high-pressure conditions.
Although very recent in situ synchrotron X-ray diffraction measurements of basaltic melt were successful
under lower mantle pressures (Sanloup et al. 2013), thepressure conditions of the measurements were limited
to below ~60 GPa.

© 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
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Ohira et al. Progress in Earth and Planetary Science (2016) 3:18

Silicate glasses have been used as possible analogs forsilicate melts to understand the nature of dense magmas
in the lower mantle (e.g., Murakami and Bass 2010,2011; Petitgirard et al. 2015). Although the structuralchanges that silicate glasses undergo cannot be assumed
to be a completely valid model for silicate melts, previous experimental works have shown that the Si-O coordination numbers of SiO2 glass and a molten basalt
appear to change in generally similar ways (Meade et al.
1992; Lin et al. 2007; Sato and Funamori 2008, 2010;
Benmore et al. 2010; Zeidler et al. 2014; Sanloup et al.
2013). This suggests that the analogy of the structuralchange that silicate glasses undergo to the changes insilicate melts appears to work well for Si-O coordination
number and probably for most structural aspects at least
above 10 GPa (Sanloup et al. 2013).
The behavior of silicate glasses under ultrahigh pressures above 100 GPa has recently been explored by acoustic wave velocity measurements using in situ highpressure Brillouin scattering spectroscopy as a way tomonitor structural changes in silicate melts underultrahigh-pressure conditions (Murakami and Bass 2010,2011). The results for SiO2 glass (Murakami and Bass
2010) showed that the changes in acoustic wave velocity
occur at the same pressure as the changes in the Si-O coordination number of SiO2 glass from 4 to 6, inferredfrom in situ synchrotron X-ray diffraction measurements
up to 100 GPa (Sato and Funamori 2010). This indicatesthat acoustic wave velocity can be a good indicator for
tracking the pressure evolution of the Si-O coordination
number of silicate glasses. Murakami and Bass (2010) also
reported an abrupt increase of P-VS gradient (dVS/dP) in
SiO2 glass above ~140 GPa, which was also observed in
MgSiO3 glass at ~133 GPa (Murakami and Bass 2011)and proposed the possibility of a further structural changesuch as a gradual increase of the average Si-O coordination number from 6 to 6+. Such a structural change may
increase the densities of silicate melts significantly, andtherefore can constrain the dynamics of silicate melts that
may exist at the base of the mantle.
Acoustic wave velocity measurements of silicate
glasses under ultrahigh-pressure conditions have so far
been conducted only in very simple systems such as
SiO2 and MgSiO3 (Murakami and Bass 2010, 2011), and
it is still unclear whether the densification mechanisms
inferred from the velocity measurements of SiO2 or
MgSiO3 glasses are directly applicable to the silicate
glasses/melts in more realistic compositional systems innature. To address this issue, we focus on the effect of
Al2O3, the second-most abundant oxide in natural
magma, on structural changes in silicate glasses. The
SiO2-Al2O3 system is the simplest from among the aluminous silicate systems. A number of previous studies
have reported structural features at ambient pressure, as

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inferred by Raman scattering (Kato 1976; McMillan and
Piriou 1982), infrared absorption (Poe et al. 1992a;
Okuno et al. 2005), X-ray diffraction (Morikawa et al.
1982; Okuno et al. 2005), nuclear magnetic resonance(NMR) spectroscopic techniques (Risbud et al. 1987;
Sato et al. 1991; Poe et al. 1992a, b; Sen and Youngman
2004; Weber et al. 2008), and by molecular dynamics(MD) computer simulations (Poe et al. 1992a, b; Linhand Hoang 2007).
Previous high-pressure experiments using NMR spectroscopic techniques showed that the proportion of fiveand sixfold coordination of Al in aluminosilicate glasses
quenched from high pressures and temperatures corresponding to the upper mantle is positively correlatedwith pressure (e.g., Yarger et al. 1995; Lee et al. 2004;
Allwardt et al. 2005, 2007), suggesting that the increasein Al-O coordination number occurs at lower pressuresthan that in Si-O. In situ XRD measurement for CaAl2Si2O8 glass supports the conclusions inferred from those
NMR studies (Drewitt et al. 2015). However, the role
Al2O3 plays in the structure and physical properties ofsilicate glasses and melts under lower mantle pressures
is still unknown.
To advance our understanding on the effect the presence of Al2O3 has on the structures of aluminosilicate
glasses at lower mantle pressures, we performed in situ
high-pressure Brillouin scattering measurements of
SiO2-Al2O3 glasses up to pressures of 196.9 GPa. Basedon the results, we discuss the effect of Al2O3 on thedensification mechanism of aluminosilicate glasses at thelowermost mantle pressure conditions and consider possible implications for the dynamics of dense silicate
melts at the base of the mantle.

Methods
Two aluminosilicate glass samples, SiO2 + 3.9 mol%
Al2O3 glass (SA1) and SiO2 + 20.5 mol% Al2O3 glass(SA2) were synthesized by means of a containerless levitation heating method at the SPring-8 BL04B2 beamline
at the SPring-8 synchrotron radiation facility in Hyogo,
Japan (Kohara et al. 2007). By using this method, bothcontamination of the molten samples and heterogeneous
nucleation by crucible surfaces were avoided. Blocks ofsilicate powders weighing 1–2 mg were levitated in air
using an aerodynamic levitator and heated by a 100 W
CO2 laser beam. After melting the samples, small, rapidly cooled vitreous spheroids (~0.5–1 mm in diameter)
were obtained by blocking the CO2 laser.
The X-ray diffraction data for the SA1 and SA2 werecorrected for polarization, absorption, and background,and the contribution of Compton scattering was subtracted using standard analysis procedures (see Kohara
et al. 2007 for the technical details). The corrected data
sets were normalized to give the Faber-Ziman total

Ohira et al. Progress in Earth and Planetary Science (2016) 3:18

structure factor S(Q). The radial distribution function
RDF(r) was obtained by a Fourier transformation of
S(Q) (Fig. 1). The composition and homogeneity of
those glasses were confirmed by scanning electron
microscope with energy-dispersive X-ray spectrometer(SEM-EDS). The compositional errors in the Al2O3
mol% were ±0.4 % in SA1 and ±0.5 % in SA2, estimated from the standard deviation of 10 analyzed
points in SA1 and SA2.
In situ high-pressure acoustic wave velocity measurements were carried out at room temperature using ahigh-pressure Brillouin scattering spectroscopic system
combined with a symmetric-type diamond anvil cell apparatus at Tohoku University (see also Mashino et al.
2016). The acoustic wave velocities, i.e., a longitudinal
wave velocity (VP) and a transverse wave velocity (VS), ina symmetric geometry are calculated from the relation(Whitfield et al. 1976):
V ¼

Δωλ2 sinðθ=2Þ

where λ is the laser wavelength of the incident laser
beam and θ is the external scattering angle. Δω indicatesthe Brillouin frequency shifts of longitudinal or transverse acoustic modes. An argon ion laser with a wavelength of 532 nm was used as the probe beam. The
incident laser beam was focused to a spot size of~20 μm. The scattered light was analyzed by a six-passtandem Fabry-Pérot interferometer. In all Brillouin experiments, we used a symmetric scattering geometry
with a 50° external scattering angle. A borosilicate crown
optical glass (BK7) was used to calibrate the symmetricscattering geometry.
The SiO2-Al2O3 glass powders were loaded without apressure-transmitting medium into a 50 or 70 μm hole

Page 3 of 10

drilled in a rhenium gasket. The samples were compressed with 100 and 130 μm beveled diamond anvils
and a 300 μm non-beveled diamond anvil. Pressure wasdetermined using the Raman T2g mode of the diamondanvil (Akahama and Kawamura 2004) by measuring 3–5
points at the center of the incident laser beam and surrounding points, yielding uncertainties typically within
2 GPa (see Additional file 1: Table S1 and Additional file
2: Table S2). Representative Raman spectra of the surface of the diamond anvils directly in contact with thesample and their differential spectra dI∕dν (I intensity, ν:wave number) from the different points are shown in
Fig. 2. The high-frequency edge of the Raman band was
defined as a minimum of dI∕dν, which is determined bycurve fitting using a Gaussian function. As shown in
Fig. 2, we obtained very sharp peaks for the dI∕dν function at each pressure point, even above 100 GPa. Brillouin spectra were collected at pressure conditions from11 to 169 GPa in 1–13 GPa increments for SA1 andfrom 10 to 197 GPa in 2–15 GPa increments for SA2.
The collecting time for each Brillouin scattering measurement was from 3 to 96 h. At each pressure, the raw
Brillouin spectra of Stokes and anti-Stokes peaks werefitted with Lorentzian functions to obtain the peak
positions.

Results and discussion
The RDFs showed that the first peaks observed in SA1
and SA2 were at 1.62 Å and at 1.65 Å, respectively(Fig. 1). As the Al-O bond length in the tetrahedral site
is 0.13 Å longer than the Si-O bond length in the tetrahedral site (Shannon 1976), the average first nearestneighbor T-O (T = Si, Al) lengths in SiO2-Al2O3 glasses
are longer than the Si-O length. These lengths for SA1
and SA2 were between that of SiO2 glass (1.59 Å; Meadeet al. 1992) and those of SiO2 + 25.0 mol% Al2O3 glass

Fig. 1 a Total structure factors S(Q) and b radial distribution functions of SA1 and SA2. The first neighbor T-O (T = Si, Al) lengths in SA1 and SA2
were 1.62 and 1.65 Å, respectively. The measurements were carried out in step-scanning mode with 0.05°/2θ in the range 0.3° < 2θ < 10.0°, 0.10°/
2θ in the range 10.0° < 2θ < 19.9°, 0.15°/2θ in the range 19.9° < 2θ < 40.0°, and 0.20°/2θ in the range 40.0° < 2θ < 58.0° in transmission geometry.
Incident X-ray energy was 61.55 keV and scattering vector (Q) changes were from 0.2 Å−1 to 30.2 Å−1Ohira et al. Progress in Earth and Planetary Science (2016) 3:18

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Fig. 2 Representative Raman spectra of different pressure conditions. Raman spectra were obtained from SA1 (SiO2 + 3.9 mol% Al2O3) and SA2(SiO2 + 20.5 mol% Al2O3), respectively. They were measured from reflected light of the incident laser beam three times (a1, a2, a3, b1, b2, b3).
Their differential spectra dI∕dv are also shown (a4, a5, a6, b4, b5, b6). The high-frequency edge of the Raman band was defined as the minimumof the dI∕dv. The red lines represent the fitting curves using Gaussian functions to determine the minimum position. Numbers in parentheses represent uncertainties in the last digit for both wave number and pressure. Uncertainties for the wave number of the minimum position of dI∕dv
in each spectrum were derived from the errors of the curve fitting. The total uncertainties for pressure were finally estimated from the standard
deviation (1σ) of each determined pressure value at each point(1.66 Å; Okuno et al. 2005) and SiO2 + 28.2 mol% Al2O3
glass (1.75 Å; Morikawa et al. 1982), showing that our
results were in agreement with previous studies about
T-O lengths at ambient conditions.
Brillouin peaks from the transverse acoustic modesof both SA1 and SA2 were observed over the entirepressure range up to 169.3 and 196.9 GPa, respectively (Figs. 3, 4, and 5, Additional file 1: Table S1,
Additional file 2: Table S2). The peaks from the longitudinal acoustic modes of those glasses were also
observed at pressures below 45 GPa (a and b in
Fig. 3). The peaks from the longitudinal acoustic
modes were masked by those of the diamond transverse acoustic modes above 45 GPa (c and d in Fig. 3).
As shown in Fig. 3, no significant peak broadening
was observed with increasing pressure, indicating thatthe hydrostaticity in the sample chamber did notchange significantly as the pressure changed (see also

full width at half maximum (FWHM) in Additional
file 1: Table S1 and Additional file 2: Table S2). The
velocity uncertainties based on the difference between
the peak positions of the Brillouin spectra of Stokes
and anti-Stokes peaks were 0.8 % in SA1 and 0.3 %in SA2 on average throughout the pressure range
measured (Additional file 1: Table S1 and Additional
file 2: Table S2).
Variation in acoustic wave velocities with pressure wasdivided into three distinct pressure regions (Fig. 4): (i)the lowest pressure region where acoustic wave velocities increased rapidly up to 40 GPa; (ii) a high-pressure
region from 40 to 100 GPa with a gentle gradient along
a progressively flattening trend, and (iii) an ultrahighpressure region above 100 GPa, with an anomalously increasing VS, which trend is highly consistent with that ofprevious works on SiO2 and MgSiO3 glasses (Murakami
and Bass 2010, 2011).

Ohira et al. Progress in Earth and Planetary Science (2016) 3:18

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Fig. 3 In situ high-pressure Brillouin spectra. Spectra of a SA1 at 20 GPa, b SA2 at 16 GPa, c SA1 at 169 GPa, and d SA2 at 189 GPa. TA and LA indicate transverse and longitudinal acoustic modes of the Brillouin shift, respectively. Ghost peaks are artifacts of the interferometry method. LA
peaks are masked above around 45 GPa because of the overlap of the diamond’s TA peaks

Fig. 4 P-VS profiles up to 200 GPa. SA1 (blue squares, this study), SA2(light blue diamonds, this study), SiO2 glass (red circles, Murakami andBass 2010), and MgSiO3 glass (green triangles, Murakami and Bass
2011). The curves show the fourth order polynomial fitting of SA1(R2 = 0.9960), SA2 (R2 = 0.9978), SiO2 glass (R2 = 0.9964), and MgSiO3
glass (R2 = 0.9985). The pressure conditions at which the increase ofdVS/dP occurs (the inflection pressure) are indicated by thesemi-transparent dark-blue box (SA1) and the semi-transparent
light-blue box (SA2)

Figure 5a shows the P-VS profiles of SA1 and SA2 up
to 40 GPa as functions of pressure together with thoseof SiO2 glass (Murakami and Bass 2010) and MgSiO3
glass (Murakami and Bass 2011). The values of VS of the
SiO2-Al2O3 glasses increase rapidly from 10 to 40 GPa,with no discontinuous changes in slope. Figure 5b shows
the regression curves of P-VS plots of SA1 fitted to afourth degree polynomial function and SA2 fitted to aquadratic function. The P-VS profile of SA1 up to
40 GPa exhibited an increasing VS with an inflection
point at ~26 GPa. Such a trend was also observed in the
P-VS profile of SiO2 glass (Murakami and Bass 2010). In
contrast, the P-VS profile of SA2 to 40 GPa can be fitted
by a low-degree polynomial function such as a concave
quadratic function (Fig. 5b), which is quite similar tothat of MgSiO3 glass (Murakami and Bass 2011). The PVS profile of SA2 below 40 GPa seems to be that of SA1
just shifted to lower pressures (Fig. 5). The difference in
VS in the pressure range from 10 to 40 GPa cannot be
explained simply by density differences caused by thediffering Al2O3 contents in SA1 and SA2. Previous studies on SiO2-Al2O3 glasses at ambient conditions reported that the density of SiO2-Al2O3 glasses increases
with increasing Al2O3 content (Okuno et al. 2005). If weapply this density relationship in the SiO2-Al2O3 system,
the densities of SA1 and SA2 at ambient pressure can be
estimated to be 2.24 and 2.40 g/cm3, respectively, which
are denser than pure SiO2 glass (2.20 g/cm3) (e.g.,
Brückner 1970). If the shear moduli for the two glasses

Ohira et al. Progress in Earth and Planetary Science (2016) 3:18

Page 6 of 10

Fig. 5 Pressure-transverse acoustic wave velocity (P-VS) profiles up to 40 GPa. a SA1 (blue squares, this study), SA2 (light blue diamonds, this study),
SiO2 glass (red circles, Murakami and Bass 2010), and MgSiO3 glass (green triangles, Murakami and Bass 2011) up to 40 GPa. b Regression curves of
SA1 and SA2 fitted by polynomial functions. P-VS plots of SA1 from 11 to 60 GPa are fitted with a fourth order polynomial (R2 = 0.9933) and P-VS
plots of SA2 from 10 to 59 GPa are fitted with a quadratic function (R2 = 0.9957)

were equivalent, VS would be expected to decrease as
Al2O3 content increases in SiO2-Al2O3 glasses because
density is a term in the denominator in the formula ofan acoustic wave velocity. However, the VS of the“Al2O3-rich” SA2 is higher than that of the “Al2O3-poor”
SA1 at pressures between 10 and 40 GPa. This P-VS relation is thus likely to be due to differences in the elasticity/elastic moduli of SA1 and SA2, which appear in the
numerator in the formula for acoustic wave velocity.
One possibility for explaining such an elasticity change
is that the proportion of Al ions with large oxygen coordination numbers could depend on the Al2O3 content.
A number of studies have shown that the average Al-O
coordination numbers of aluminosilicate glasses always
reach five and six at lower pressures than the average SiO coordination number of SiO2 glass and aluminosilicate glasses (e.g., Yarger et al. 1995; Lee et al. 2004; Allwardt et al. 2005; Drewitt et al. 2015; Benmore et al.
2010; Sato and Funamori 2010; Mysen and Richet 2005,
and see also Additional file 3: Figure S1). In the present
study, SA2 has more Al ion than SA1. In addition, some
Al ions in SiO2-Al2O3 glasses may be in 5- and 6-foldcoordination with oxygen even at ambient pressure. Thiscan serve as a network modifier, at least if the Al2O3
content is lower than 20.5 mol% (the composition ofSA2). The Al ions in SA2 may be mainly in fourfold coordination with oxygen and serve as a network former,
but about 40 % of the Al ions may exist in five- and sixfold coordination with oxygen at ambient pressure, asinferred from MD simulations for liquid SiO2-Al2O3(Poe et al. 1992a, b). Therefore, the proportion of Alions with large oxygen coordination numbers (five- andsixfold Al) and the value of VS in SA2 are always greaterthan in SA1 at isobaric conditions below 40 GPa. The
increase of average Al-O coordination number of SA2

with increasing pressure may reflect its VS increase. The
VS difference between SA1 and SA2 up to 40 GPa implies that aluminosilicate glasses become elastically stiffer as the proportion of five- and sixfold Al ion
increases. In the present study, SA2 is elastically stiffer
than SA1 at isobaric conditions between 10 and 40 GPa,and the P-VS profile of SA2 seems, consequently, to bethat of SA1 shifted to lower pressures.
The gradient of VS with pressure becomes less steep
above 40 GPa in contrast to its trend at lower pressures,
which appears similar to the behavior of SiO2 glass(Murakami and Bass 2010) and MgSiO3 glass (Murakami and Bass 2011) (Fig. 4). Previous high-pressure experimental studies on SiO2 glass showed that the Si-O
coordination number of SiO2 glass most likely changes
from four to six up to ~40 GPa (Meade et al. 1992; Linet al. 2007; Sato and Funamori 2008, 2010; Benmore
et al. 2010; Zeidler et al. 2014) and that the sixfold coordination state may be preserved up to at least~100 GPa (Sato and Funamori 2010). This behavior offers a reasonable explanation for the high-pressure
acoustic wave velocity profile of SiO2 glass (Murakami
and Bass 2010). Because the trend of the P-VS profile ofSA1 is nearly identical to that of SiO2 glass, SA1 alsopreserves the sixfold coordination state of Si, at leastfrom 40 to 100 GPa. In contrast, the P-VS profile of SA2
is convex upward with a gentle gradient from 10 toaround 100 GPa, which is similar to that observed in
MgSiO3 glass (Murakami and Bass 2011). This gradually
increasing VS for SA2, similar to that of MgSiO3 glass
up to at least 100 GPa, also suggests that the average SiO coordination number increases gradually from four to
six in this pressure region.
The values of VS for SA1 and SA2 increase more rapidly as pressure grows past ~100 GPa (Fig. 4), which wasOhira et al. Progress in Earth and Planetary Science (2016) 3:18

also observed in SiO2 glass (Murakami and Bass 2010)
and MgSiO3 glass (Murakami and Bass 2011). Such dVS/
dP changes observed above ~100 GPa have previously
been interpreted to reflect the onset of the average Si-O
coordination number changing from just 6 to 6+ (Murakami and Bass 2010, 2011). This conclusion subsequently found support in the results from MD computer
simulations (Brazhkin et al. 2011; Wu et al. 2012). To
determine the pressure conditions at which this abruptchange of dV S/dP occurs, we adopted the same criteria
as Murakami and Bass (2010, 2011). Each P-VS profile
was fitted to fourth and fifth polynomial functions ofpressure with an adjusted R-square of above 0.995. The
pressure conditions at the onset of the rapid VS increasein the P-VS profiles of SA1 and SA2 were determined by
calculating the point at which the second derivative of
the fitted polynomial functions with respect to pressure(d2VS∕dP2) is a maximum. The pressure conditions at
which the increase of dVS/dP occur (hereafter referred
to as “the inflection pressure”) are 130 ± 5 GPa in SA1
and 116 ± 9 GPa in SA2. The inflection pressure of SA1
is 10 GPa lower than that of SiO2 glass (Murakami andBass 2010) and 3 GPa lower than that of MgSiO3 glass(Murakami and Bass 2011). The inflection pressure ofSA2 is 24 GPa lower than that of SiO2 glass (Murakami
and Bass 2010) and 17 GPa lower than that of MgSiO3
glass (Murakami and Bass 2011).
Such a change of dVS/dP may indicate the onset offurther structural change in silicate glasses resulting
from an average Si-O coordination number increase
from 6 to 6+. Our results may suggest that the inflection
pressure does not strongly depend on the amount ofmetal ions because the M/Si (M = Mg, Al) values forPage 7 of 10

SA1 and SA2 are less than for MgSiO3 glass, which has
much higher inflection pressure than SA1 and SA2(Fig. 6). This indicates that Al2O3 is much more effective
than MgO in reducing the pressure at which the average
Si-O coordination number starts to increase from 6 to 6+
. Another explanation of the lowered inflection pressureis that the average Al-O coordination numbers increase
from 6 to 6+ at a lower pressure than do those of Si-O,
similar to the transition of average Al-O coordination
number from 4 to 6 that occurs at lower pressures than
the transition of Si-O. However, no study has yet indicated the different pressures at which the average Al-O
and Si-O coordination numbers exceed 6. In addition,
the P-VS profiles of SA1 and SA2 have only one inflection point, at the onset of the average Si-O coordination
number’s increase from 6 to 6+ (Brazhkin et al. 2011;
Wu et al. 2012; Murakami and Bass 2010, 2011). Therefore, the inflection points under ultrahigh pressure may
primarily reflect a change of the average Si-O coordination number from 6 to 6+.
Density increases caused by structural transitions accompanied by Si-O coordination number growth occur
continuously in glasses and melts and occur discontinuously in crystalline materials (e.g., Loerting et al. 2009).
Although it is not obvious whether the analogy between
silicate glasses and silicate melts works well as a modelof their structural or density changes with pressure, experimental results on the evolution of average Si-O coordination number of SiO2 glass (Meade et al. 1992; Linet al. 2007; Sato and Funamori 2008, 2010; Benmore
et al. 2010; Zeidler et al. 2014) shows good agreementwith that of SiO2 melt calculated by MD simulations(Karki et al. 2007). A recent experimental study

Fig. 6 Inflection pressures in the P-VS profiles of the silicate glasses. SA1 (blue square, this study), SA2 (light blue diamond, this study), SiO2 glass(red circle, Murakami and Bass 2010), and MgSiO3 glass (green triangle, Murakami and Bass 2011) are shown in the left graph. This is regarded as
the onset of the average Si-O coordination number increasing from 6 to 6+ (“6+-fold (Si)” in this figure) in each glass (see Murakami and Bass
2010, 2011). The solid black line shows the possible transitional boundary at which the average Si-O coordination number increases from 6 to 6+
in SiO2-Al2O3 glasses, calculated using a linear leasts squares method based on the plots of SiO2 glass, SA1, and SA2. The pressure-intercept isfixed at 140 GPa, derived from the inflection pressure of SiO2 glass reported by Murakami and Bass (2010). Assuming an Al2O3 content of
13 mol% in an SiO2-Al2O3 glass, the average Si-O coordination number is expected to exceed 6 at 124 GPa (purple lines), which corresponds to adepth of 2690 km, much shallower than the CMB. The dashed gray line shows the boundary at which the average Si-O coordination number
increases from just 6 to 6+ in MgSiO3 glass. The picture on the right is a schematic image of the accumulation of “dense” partial melts having
structures governed by 6+-fold Si, generated from MORBs

Ohira et al. Progress in Earth and Planetary Science (2016) 3:18

suggested that the Si-O coordination state in molten
basalt is roughly consistent with that in SiO2 glass andmelt up to 60 GPa (Sanloup et al. 2013). If this is thecase, the pressure-induced acoustic wave velocitychanges of the aluminosilicate glasses explored in thepresent study may offer possible implications for the dynamics of dense magmas that potentially exist at thebase of the mantle. The possible existence of dense silicate melts at or near the CMB has been proposed (e.g.,
Ohtani 1983; Ohtani and Maeda 2001; Williams andGarnero 1996; Labrosse et al. 2007) to explain theanomalous reduction of the seismic wave velocities justabove the CMB (e.g., Garnero and Helmberger 1995;
Mori and Helmberger 1995). Recent high-pressure melting experiments indicated that such dense silicate melts
at the CMB can be generated by partial melting of midocean ridge basalts (MORBs) (Andrault et al. 2014;
Pradhan et al. 2015). The densification in SA1 and SA2,
which probably reflects an average Si-O coordination
number in excess of 6, occurs at lower pressures than inpure MgSiO3 glass (Fig. 6). Figure 6 shows the inflection
pressure as a function of Al2O3 contents in the SiO2Al2O3 system, indicating a linear relationship. If we assume that an Al2O3 content of ~13 mol% may possibly
be included in partial melts of MORBs generated ataround 100 GPa (Pradhan et al. 2015), the inflection
pressure is expected to be 124 GPa corresponding to adepth of ~2690 km. The silicate melts generated by thepartial melting of MORBs might thus undergo a structural transformation involving possible densification
changes related to Si-O coordination states at depths ofaround ~2690 km, which is clearly shallower than the
CMB.
Although the possibility of densification processes in thesilicate melts occurring due to the addition of heavy elements such as Fe cannot be ruled out, the increases indensity associated with structural changes in Si-O coordination states that possibly take place at pressures within
the lower mantle might also play an important role in the
dynamics of silicate melts at the base of the mantle.

Conclusions
The transverse acoustic wave velocities of SiO2-Al2O3
glasses were measured at pressures up to near 200 GPa.
At pressures from 10 to 40 GPa, the VS of the Al2O3rich SA2 is higher than that of the Al2O3-poor SA1,
which is likely related to differences in the elasticity/elastic moduli between SA1 and SA2. Such a difference inthe elasticity may be derived from the different proportions of five- and sixfold Al ions in those glasses. The
dVS/dP gradients of both SA1 and SA2 became lesssteep at pressures from 40 to 100 GPa. In this pressure
range, SA1 had a P-VS profile nearly identical to that of
SiO2 glass, indicating that the sixfold coordinated

Page 8 of 10

structure is preserved. SA2, however, had a P-VS profile
similar to that of MgSiO3 glass, suggesting a gradually
increasing average Si-O coordination number from 4 to
6 up to around 100 GPa. dV S/dP also increases at 130 ±5 GPa in SA1 and at 116 ± 9 GPa in SA2, which can beinterpreted as the onset of the average Si-O coordination
number change from 6 to 6+. The inflection pressuresfor SA1 and SA2 are much lower than in MgSiO3 glass,suggesting that Al2O3 promotes the reduction of pressure conditions of inflection caused by an increase of theaverage Si-O coordination number from 6 to 6+ much
more strongly than does MgO. Such a densification
mechanism associated with the Si-O coordination number change may play a key role in the gravitational
stabilization of the partial melting of MORBs within thelowermost mantle deeper than 2690 km.

Additional files
Additional file 1: Table S1. Data of Raman and Brillouin scattering
spectra for SA1. Peak position of the Raman differential spectrum (dI∕dv)
for pressure determination (R peak), pressure (P) and transverse acoustic
wave velocity (VS) of SA1 (SiO2 + 3.9 mol% Al2O3), and full width at half
maximum (FWHM) of the Brillouin peak from TA mode. (PDF 125 kb)
Additional file 2: Table S2. Data of Raman and Brillouin scattering
spectra for SA2. Peak position of the Raman differential spectrum (dI∕dv)
for pressure determination (R peak), pressure (P) and transverse acoustic
wave velocity (VS) of SA2 (SiO2 + 3.9 mol% Al2O3), and full width at half
maximum (FWHM) of the Brillouin peak from TA mode. (PDF 129 kb)
Additional file 3: Figure S1. Summary of the pressure dependence of
average coordination numbers (CNs) of Al-O in aluminosilicate glasses
and average CNs of Si-O in SiO2 glass. The results of aluminosilicate
glasses from NMR spectroscopy and in situ XRD results are indicated by
red symbols. Pressure-recovered glasses are used for NMR studies. The insitu XRD results of pure SiO2 are also indicated by open symbols. Detailsof references are as follows: Ab50NTS50 (red square, Y95) shows data for(NaAlSi3O8)0.5(Na2Si4O9)0.5 glass using NMR spectroscopy by Yarger et al.
(1995); NAS150560 (red circle, L04) and NAS6 (red diamond, L04) show
data for (Na2O)0.75(Al2O3)0.253SiO2 glass and NaAlSi3O8 glass using NMR
spectroscopy by Lee et al. (2004); KAS (red triangle, A05), NAS (redinversed triangle, A05), and CAS (red pentagon, A05) show data for
K3AlSi3O9 glass, Na3AlSi3O9 glass, and Ca3Al2Si6O18 glass using NMR
spectroscopy by Allwardt et al. (2005). An glass (red star, D15) shows data
for CaAl2Si2O8 glass using in situ XRD results by Drewitt et al. (2015); SiO2
glass (blue square, B10) shows data for pure SiO2 glass using in situ XRD
by Benmore et al. (2010); SiO2 glass (blue circle, S10) shows data for pure
SiO2 glass using in situ XRD by Sato and Funamori (2010). (TIF 221 kb)
Abbreviations
CMB, core-mantle boundary; MD, molecular dynamics; MORBs, mid-ocean
ridge basalts; NMR, nuclear magnetic resonance; RDF, radial distributionfunction; SA1, SiO2 + 3.9 mol% Al2O3 glass; SA2, SiO2 + 20.5 mol% Al2O3
glass; SEM-EDS, scanning electron microscope with energy-dispersive Xray spectrometer; ULVZs, ultralow velocity zones; VP, longitudinal acoustic
wave velocity; VS, transverse acoustic wave velocity.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
IO and MM performed the Brillouin scattering measurements. IO, MM, SK,
and KO synthesized and analyzed the glassy starting materials. IO, MM, andKO wrote the paper. All authors discussed the results and commented on
the manuscript. All authors read and approved the final manuscript.

Ohira et al. Progress in Earth and Planetary Science (2016) 3:18

Acknowledgements
The authors thank T. Sakamaki for his useful discussions. The authors also
thank T. Nishimoto for his technical support on the Brillouin scattering
measurements. This work was supported by JSPS KAKENHI grant numbers
22684028 and 21654075 awarded to MM and 15H05748 awarded to EO. The
synchrotron radiation experiments were performed at the BL04B2 unit of theSPring-8 facility with the approval of the Japan Synchrotron Radiation
Research Institute (JASRI) (proposal no. 2011B1159).
Author details
1
Department of Earth Science, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan. 2Research and Utilization Division, Japan
Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan.
3
Present address: Synchrotron X-ray Group, Quantum Beam Unit, Advanced
Key Technologies Division, National Institute for Materials Science (NIMS),
Sayo, Hyogo 679-5148, Japan. 4V.S. Sobolev Institute of Geology andMineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk
630090, Russia.
Received: 6 January 2016 Accepted: 14 June 2016

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