Ultrahigh-pressure acoustic wave velocities of SiO2-Al2O3 glasses up to 200 GPa
© The Author(s). 2016
Received: 6 January 2016
Accepted: 14 June 2016
Published: 29 June 2016
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 under ultrahigh pressures has been poorly understood. Here, we used a Brillouin scattering spectroscopic method up to pressures 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 of aluminosilicate melts. From 10 to 40 GPa, the transverse acoustic wave velocity (V S ) 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 from 40 to ~100 GPa, the increase in V S with increasing pressure became less steep than below 40 GPa. Above ~100 GPa, there were abrupt increases in the P-V S gradients (dV S /dP) at 130 GPa in Al2O3-poor glass and at 116 GPa in Al2O3-rich glass. These changes resemble previous experimental results on SiO2 glass and MgSiO3 glass. Given that changes of dV S /dP have commonly been related to changes in the Si-O coordination states in the glasses, our results, 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 the abrupt increase of dV S /dP is observed. This suggests that the Al2O3 in silicate melts may help to stabilize those melts gravitationally in the lower mantle.
KeywordsGlass structure Brillouin scattering High pressure Acoustic wave velocity Heterogeneity of Earth’s core-mantle boundary
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 just above 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 from partial melting of the lowermost mantle minerals (e.g., Williams and Garnero 1996) or remnants of the dense 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), the pressure conditions of the measurements were limited to below ~60 GPa.
Silicate glasses have been used as possible analogs for silicate 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 structural changes 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 structural change that silicate glasses undergo to the changes in silicate 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 high-pressure Brillouin scattering spectroscopy as a way to monitor structural changes in silicate melts under ultrahigh-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, inferred from in situ synchrotron X-ray diffraction measurements up to 100 GPa (Sato and Funamori 2010). This indicates that 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-V S gradient (dV S /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 change such 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, and therefore 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 in nature. 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 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; Linh and Hoang 2007).
Previous high-pressure experiments using NMR spectroscopic techniques showed that the proportion of five- and sixfold coordination of Al in aluminosilicate glasses quenched from high pressures and temperatures corresponding to the upper mantle is positively correlated with pressure (e.g., Yarger et al. 1995; Lee et al. 2004; Allwardt et al. 2005, 2007), suggesting that the increase in Al-O coordination number occurs at lower pressures than 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 of silicate 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. Based on the results, we discuss the effect of Al2O3 on the densification mechanism of aluminosilicate glasses at the lowermost mantle pressure conditions and consider possible implications for the dynamics of dense silicate melts at the base of the mantle.
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, both contamination of the molten samples and heterogeneous nucleation by crucible surfaces were avoided. Blocks of silicate 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.
where λ is the laser wavelength of the incident laser beam and θ is the external scattering angle. Δω indicates the 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-pass tandem 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 symmetric scattering geometry.
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 nearest-neighbor 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; Meade et al. 1992) and those of SiO2 + 25.0 mol% Al2O3 glass (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.
Variation in acoustic wave velocities with pressure was divided 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 ultrahigh-pressure region above 100 GPa, with an anomalously increasing V S , which trend is highly consistent with that of previous works on SiO2 and MgSiO3 glasses (Murakami and Bass 2010, 2011).
Figure 5a shows the P-V S profiles of SA1 and SA2 up to 40 GPa as functions of pressure together with those of SiO2 glass (Murakami and Bass 2010) and MgSiO3 glass (Murakami and Bass 2011). The values of V S 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-V S plots of SA1 fitted to a fourth degree polynomial function and SA2 fitted to a quadratic function. The P-V S profile of SA1 up to 40 GPa exhibited an increasing V S with an inflection point at ~26 GPa. Such a trend was also observed in the P-V S profile of SiO2 glass (Murakami and Bass 2010). In contrast, the P-V S 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 to that of MgSiO3 glass (Murakami and Bass 2011). The P-V S profile of SA2 below 40 GPa seems to be that of SA1 just shifted to lower pressures (Fig. 5). The difference in V S in the pressure range from 10 to 40 GPa cannot be explained simply by density differences caused by the differing 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 we apply 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 were equivalent, V S would be expected to decrease as Al2O3 content increases in SiO2-Al2O3 glasses because density is a term in the denominator in the formula of an acoustic wave velocity. However, the V S of the “Al2O3-rich” SA2 is higher than that of the “Al2O3-poor” SA1 at pressures between 10 and 40 GPa. This P-V S 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 Si-O 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-fold coordination with oxygen even at ambient pressure. This can serve as a network modifier, at least if the Al2O3 content is lower than 20.5 mol% (the composition of SA2). 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, as inferred from MD simulations for liquid SiO2-Al2O3 (Poe et al. 1992a, b). Therefore, the proportion of Al ions with large oxygen coordination numbers (five- and sixfold Al) and the value of V S in SA2 are always greater than in SA1 at isobaric conditions below 40 GPa. The increase of average Al-O coordination number of SA2 with increasing pressure may reflect its V S increase. The V S 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-V S profile of SA2 seems, consequently, to be that of SA1 shifted to lower pressures.
The gradient of V S 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; Lin et 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-V S profile of SA1 is nearly identical to that of SiO2 glass, SA1 also preserves the sixfold coordination state of Si, at least from 40 to 100 GPa. In contrast, the P-V S profile of SA2 is convex upward with a gentle gradient from 10 to around 100 GPa, which is similar to that observed in MgSiO3 glass (Murakami and Bass 2011). This gradually increasing V S for SA2, similar to that of MgSiO3 glass up to at least 100 GPa, also suggests that the average Si-O coordination number increases gradually from four to six in this pressure region.
The values of V S for SA1 and SA2 increase more rapidly as pressure grows past ~100 GPa (Fig. 4), which was also observed in SiO2 glass (Murakami and Bass 2010) and MgSiO3 glass (Murakami and Bass 2011). Such dV S /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 abrupt change of dV S /dP occurs, we adopted the same criteria as Murakami and Bass (2010, 2011). Each P-V S profile was fitted to fourth and fifth polynomial functions of pressure with an adjusted R-square of above 0.995. The pressure conditions at the onset of the rapid V S increase in the P-V S 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 (d 2 V S ∕dP 2 ) is a maximum. The pressure conditions at which the increase of dV S /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 and Bass 2010) and 3 GPa lower than that of MgSiO3 glass (Murakami and Bass 2011). The inflection pressure of SA2 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).
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 model of 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; Lin et al. 2007; Sato and Funamori 2008, 2010; Benmore et al. 2010; Zeidler et al. 2014) shows good agreement with that of SiO2 melt calculated by MD simulations (Karki et al. 2007). A recent experimental study suggested that the Si-O coordination state in molten basalt is roughly consistent with that in SiO2 glass and melt up to 60 GPa (Sanloup et al. 2013). If this is the case, the pressure-induced acoustic wave velocity changes of the aluminosilicate glasses explored in the present study may offer possible implications for the dynamics of dense magmas that potentially exist at the base 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 and Garnero 1996; Labrosse et al. 2007) to explain the anomalous reduction of the seismic wave velocities just above 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 mid-ocean 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 in pure MgSiO3 glass (Fig. 6). Figure 6 shows the inflection pressure as a function of Al2O3 contents in the SiO2-Al2O3 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 at around 100 GPa (Pradhan et al. 2015), the inflection pressure is expected to be 124 GPa corresponding to a depth of ~2690 km. The silicate melts generated by the partial melting of MORBs might thus undergo a structural transformation involving possible densification changes related to Si-O coordination states at depths of around ~2690 km, which is clearly shallower than the CMB.
Although the possibility of densification processes in the silicate melts occurring due to the addition of heavy elements such as Fe cannot be ruled out, the increases in density 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.
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 V S of the Al2O3-rich 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 in the elasticity may be derived from the different proportions of five- and sixfold Al ions in those glasses. The dV S /dP gradients of both SA1 and SA2 became less steep at pressures from 40 to 100 GPa. In this pressure range, SA1 had a P-V S profile nearly identical to that of SiO2 glass, indicating that the sixfold coordinated structure is preserved. SA2, however, had a P-V S 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 be interpreted as the onset of the average Si-O coordination number change from 6 to 6+. The inflection pressures for 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 the average 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 the lowermost mantle deeper than 2690 km.
CMB, core-mantle boundary; MD, molecular dynamics; MORBs, mid-ocean ridge basalts; NMR, nuclear magnetic resonance; RDF, radial distribution function; SA1, SiO2 + 3.9 mol% Al2O3 glass; SA2, SiO2 + 20.5 mol% Al2O3 glass; SEM-EDS, scanning electron microscope with energy-dispersive X-ray spectrometer; ULVZs, ultralow velocity zones; V P , longitudinal acoustic wave velocity; V S , transverse acoustic wave velocity.
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 the SPring-8 facility with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2011B1159).
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