Abstract Natural witherite (Ba0.99Sr0.01CO3) has been studied by single-crystal X-ray diffraction... more Abstract Natural witherite (Ba0.99Sr0.01CO3) has been studied by single-crystal X-ray diffraction in the diamond anvil cell at eight pressures up to 8 GPa. At ambient pressure, cell dimensions are a = 5.3164(12) AĘ , b = 8.8921(19) AĘ , c = 6.4279(16) AĘ , and the structure was ...
ABSTRACT Hydrous olivines have been synthesized at 12 GPa at various conditions of temperature, s... more ABSTRACT Hydrous olivines have been synthesized at 12 GPa at various conditions of temperature, silica activity, and fayalite content. Samples were characterized by FTIR and Raman spectroscopy and single crystal X-ray diffraction. We observe a maximum water content of 8800 parts per million by weight H2O at 1250°C in both Fo90 and Fo100 compositions, with lower H2O contents at 1100°C and 1400°C. We observe similar amounts of hydration and FTIR spectra in samples coexisting with clinoenstatite and with clinohumite. We observe a systematic expansion of the unit cell volume with hydration so that an addition of 5000 ppmw H2O has the same effect on density as raising the temperature by 240°C. Compression experiments have been carried out in the diamond anvil cell to 8 GPa. We observe a decrease in the isothermal bulk modulus from about 128 GPa to about 120 GPa at 8000 ppmw H2O, which is similar in magnitude to the elastic softening of wadsleyite and ringwoodite with hydration. FTIR spectra and X-ray structure refinements indicate that the principal hydration mechanism is substitution of 2H+ for Mg2+ so that the geochemical compatibility of H appears to dramatically increase with pressure in the upper mantle near the 410 km discontinuity.
ABSTRACT The presence of water in nominally anhydrous minerals can dramatically affect the elasti... more ABSTRACT The presence of water in nominally anhydrous minerals can dramatically affect the elastic properties of mantle minerals (e.g. Mao et al., 2008). To determine the effect of water on the sound velocity of the upper mantle, we measured the single-crystal elasticity of forsterite, Mg2SiO4, with 0.9 wt.% H2O to 14 GPa by Brillouin scattering. At ambient conditions, the addition of this amount of H2O in forsterite decreases the aggregate bulk and shear moduli by 2.2-2.4% compared with the anhydrous phase (Jacobsen et al., 2008). However, the presence of 0.9 wt.% H2O increases the pressure derivatives of the bulk modulus of forsterite from 4.2 (2) (Zha et al., 1996) to 4.5(1), and the shear modulus from 1.4(1) to 1.8(1). As a result, the bulk and shear modulus of hydrous forsterite become greater than that of anhydrous forsterite at 14 GPa and 6 GPa, respectively. A similar crossover of the bulk modulus is observed in hydrous ringwoodite at around 12 GPa (Jacobsen and Smyth 2006). Using our measured elasticity of hydrous olivine, we examine the effect of water on the sound velocities of olivine with various water contents at typical mantle conditions. We assume a 1400°C adiabat, 10 mol% iron content, and that the temperature derivatives of the bulk and shear moduli of hydrous olivine are the same as those of the anhydrous phase. The compressional and shear velocities of hydrous olivine are offset to lower values at 0-km depth but become faster than those of the anhydrous phase beyond 150 km and 100 km, respectively. If mantle olivine is water saturated (~0.4 wt.% H2O), the corresponding compressional and shear velocities are 0.6% and 1.1% faster than those of anhydrous olivine at 400-km depth. A preliminary model based on the work of Karato (2006) has been constructed to account for effects of anelasticity due to temperature and H2O content. Our model indicates that the effect of anelasticity on the shear velocity of hydrous olivine with 0.9 wt% H2O may offset the increase in the observed anharmonic velocity components of hydrous olivine, and the net effect would be that seismic velocities in the hydrous olivine would be similar to those for dry olivine at 400 km.
ABSTRACT Water may be incorporated as hydroxyl into both the hydrous and nominally anhydrous sili... more ABSTRACT Water may be incorporated as hydroxyl into both the hydrous and nominally anhydrous silicates of the upper mantle and transition zone. Incorporation of hydroxyl has significant effects on the elastic properties of the major and minor phases of this region such that it may be possible to detect the presence of water from seismic studies. We have been measuring the effects of hydration on the equation of state and elastic properties of hydrous and anhydrous forsterite, wadsleyite, ringwoodite. We have used these new studies together with existing data to model the velocity structure of the upper mantle and transition zone, based on a thermodynamic and empirical model of mantle mineralogy. We find that it is difficult to model the velocity structure of PREM or other seismic velocity models through the transition zone with an anhydrous pyrolite composition, whereas it is straightforward to match the spherical velocity model structures with the addition of a few thousand parts per million H2O by weight. Further, it is difficult to dehydrate a serpentinized subducting slab. Phase A and 10Å phase both result from partial dehydration of serpentine and are stable to depths in excess of 250km where olivine begins to incorporate significant amounts of hydroxyl. At 360 km depth olivine is capable of incorporating nearly one percent water by weight.
ABSTRACT The elastic properties of wadsleyite, thought to be the dominant phase from 410 to 520-k... more ABSTRACT The elastic properties of wadsleyite, thought to be the dominant phase from 410 to 520-km depth in the mantle, are essential to interpret the seismic images and profiles in the transition zone. Our previous experimental measurements showed that elasticity of Mg2SiO4 wadsleyite can be significantly reduced by hydration at high pressures (e.g. Mao et al., 2008a,b). These results provide the first constraints on the effect of hydration on the high-pressure sound velocities of wadsleyite, and are significantly important for identifying the potential hydrogen rich region in the Earth’s transition zone. Since mantle wadsleyite contains ~10 mol.% Fe, it is more important to investigate the combined effect of Fe and hydration on the elastic properties of wadsleyite. Here, we measured the single-crystal elasticity of wadsleyite with 1.0 wt.% H2O, Mg1.73Fe0.19SiO4H0.16, up to 12 GPa using Brillouin scattering. At ambient conditions, the aggregate bulk modulus, KS0, and shear modulus, G0, are 158.4(5) GPa and 99.2(3) GPa, respectively. Including the results of current and previous studies, we find that the elasticity of wadsleyite decreases linearly with Fe and H2O content according to relations (in GPa): KS0 = 171(3)-13.0(8)CH2O, G0 = 112(2)-8.8(3)CH2O-40(10)XFe, where CH2O is the concentration of hydrogen expressed as weight percent H2O, and XFe is the Fe molar fraction (XFe = Fe/(Mg+Fe)). Further high-pressure measurements showed that the presence of 1 wt.% H2O in Fe-bearing wadsleyite increases the pressure derivative of the shear modulus from 1.5(1) to 1.9(1). But Fe-bearing wadsleyite with this amount of H2O might have a similar pressure derivative of the bulk modulus (4.8(1)) similar to the corresponding anhydrous phase. Using our results, we computed the sound velocities of wadsleyite with 1 wt.% H2O up to 12 GPa at 300 K. Compared to Fe-bearing anhydrous wadsleyite, 1 wt.% H2O causes a 1.5(4)% reduction in the compressional velocity at 12 GPa, and a 1.2(6)% reduction in the shear velocity. The reduction of velocity produced by this amount of H2O is smaller than previously estimated from Mg2SiO4 wadsleyite. These results are also applied to evaluate the effect of hydration on the velocity contrast between wadsleyite and ringwoodite at 520-km depth. We found that hydration increases the compressional velocity contrast, but decreases the shear contrast. The shear velocity contrast will be absent when ~1.4% H2O is present at 12 GPa in a pyrolite mantle. This amount of H2O will increase the compressional velocity contrast from 1.2(5)% to 2.0(4)%. These results may help to identify possible regions of hydration in the transition zone.
ABSTRACT The presence of water in the upper mantle and transition zone has the potential to expla... more ABSTRACT The presence of water in the upper mantle and transition zone has the potential to explain various phenomena such as shear velocity anomalies or uplift and broadening of the 410-km discontinuity. The presence of H2O in the transition zone has also been frequently invoked to reconcile laboratory elasticity data on olivine polymorphs with seismic data for the amplitude of the 410-km discontinuity (Li et al., 2001; Chambers et al., 2005). Recently, we have measured the single-crystal elastic properties of hydrous olivine (Jacobsen et al., 2006) and a suite of hydrous wadsleyites (Mao et al., 2007a) at ambient conditions and one hydrous wadsleyite composition (0.84 wt% H2O) up to 12 GPa (Mao et al., 2007b). These data provide new constraints on elastic moduli and their pressure derivatives for hydrous olivine and wadsleyite. Using this data, we first examine the effect of H2O on bulk sound velocities under transition zone conditions because anelastic effects can be neglected in this case. At 410 km depth (~13.8 GPa, along a 1400°C adiabat), the bulk sound velocity of wadsleyite with 1 wt% H2O is 3.1% lower than for dry wadsleyite. Comparison of the seismic velocity jump across the 410-km discontinuity with the measured velocity contrast between wadsleyite and olivine provides a means to estimate the olivine abundance at 410-km depth. For mantle wadsleyite with 0.1-0.2 wt% H2O (Huang et al., 2005) and using experimentally determined olivine- wadsleyite H2O partition coefficients, the olivine abundance is found to be 40%, much lower than a pyrolite model. In order for a pyrolite composition to satisfy the seismic data, 1.2 wt. % H2O is needed in wadsleyite- a value greater than its maximum solubility under these conditions. The anomalously steep seismic gradient in the transition zone has been another feature of the region that has long defied explanation. We show that the seismic gradient can be matched if there is a gradient in H2O concentration across the transition zone such that the H2O content drops, for example, from 0.3 wt% at 410 km to 0.1 wt% at 520 km dpeth. For compressional and shear wave velocities, 0.1 wt% H2O in wadsleyite would lead to 0.3% and 0.4% reductions in VP and VS, respectively, neglecting any anelasticity. If the water content of wadsleyite was instead 1.0 wt. %, then the corresponding velocity reductions would be 3.3% and 3.6%. Following the work of Karato and Jung (1998), we have implemented a preliminary model accounting for the effect of H2O on anelasticity. This model indicates that 0.1 wt% H2O in wadsleyite could be responsible for reductions in shear velocities up to 1.0%.
Wadsleyite (beta-Mg2SiO4) is the high-pressure polymorph of olivine that is expected to be a domi... more Wadsleyite (beta-Mg2SiO4) is the high-pressure polymorph of olivine that is expected to be a dominant mineral in the transition zone from 410 km to 520 km depth in the mantle. The elasticity of wadsleyite is crucial to constrain the mineralogy of the transition zone. Previous studies show wadsleyite can incorporate variable amounts of water up to 3.3 wt% of water
Abstract Natural witherite (Ba0.99Sr0.01CO3) has been studied by single-crystal X-ray diffraction... more Abstract Natural witherite (Ba0.99Sr0.01CO3) has been studied by single-crystal X-ray diffraction in the diamond anvil cell at eight pressures up to 8 GPa. At ambient pressure, cell dimensions are a = 5.3164(12) AĘ , b = 8.8921(19) AĘ , c = 6.4279(16) AĘ , and the structure was ...
ABSTRACT Hydrous olivines have been synthesized at 12 GPa at various conditions of temperature, s... more ABSTRACT Hydrous olivines have been synthesized at 12 GPa at various conditions of temperature, silica activity, and fayalite content. Samples were characterized by FTIR and Raman spectroscopy and single crystal X-ray diffraction. We observe a maximum water content of 8800 parts per million by weight H2O at 1250°C in both Fo90 and Fo100 compositions, with lower H2O contents at 1100°C and 1400°C. We observe similar amounts of hydration and FTIR spectra in samples coexisting with clinoenstatite and with clinohumite. We observe a systematic expansion of the unit cell volume with hydration so that an addition of 5000 ppmw H2O has the same effect on density as raising the temperature by 240°C. Compression experiments have been carried out in the diamond anvil cell to 8 GPa. We observe a decrease in the isothermal bulk modulus from about 128 GPa to about 120 GPa at 8000 ppmw H2O, which is similar in magnitude to the elastic softening of wadsleyite and ringwoodite with hydration. FTIR spectra and X-ray structure refinements indicate that the principal hydration mechanism is substitution of 2H+ for Mg2+ so that the geochemical compatibility of H appears to dramatically increase with pressure in the upper mantle near the 410 km discontinuity.
ABSTRACT The presence of water in nominally anhydrous minerals can dramatically affect the elasti... more ABSTRACT The presence of water in nominally anhydrous minerals can dramatically affect the elastic properties of mantle minerals (e.g. Mao et al., 2008). To determine the effect of water on the sound velocity of the upper mantle, we measured the single-crystal elasticity of forsterite, Mg2SiO4, with 0.9 wt.% H2O to 14 GPa by Brillouin scattering. At ambient conditions, the addition of this amount of H2O in forsterite decreases the aggregate bulk and shear moduli by 2.2-2.4% compared with the anhydrous phase (Jacobsen et al., 2008). However, the presence of 0.9 wt.% H2O increases the pressure derivatives of the bulk modulus of forsterite from 4.2 (2) (Zha et al., 1996) to 4.5(1), and the shear modulus from 1.4(1) to 1.8(1). As a result, the bulk and shear modulus of hydrous forsterite become greater than that of anhydrous forsterite at 14 GPa and 6 GPa, respectively. A similar crossover of the bulk modulus is observed in hydrous ringwoodite at around 12 GPa (Jacobsen and Smyth 2006). Using our measured elasticity of hydrous olivine, we examine the effect of water on the sound velocities of olivine with various water contents at typical mantle conditions. We assume a 1400°C adiabat, 10 mol% iron content, and that the temperature derivatives of the bulk and shear moduli of hydrous olivine are the same as those of the anhydrous phase. The compressional and shear velocities of hydrous olivine are offset to lower values at 0-km depth but become faster than those of the anhydrous phase beyond 150 km and 100 km, respectively. If mantle olivine is water saturated (~0.4 wt.% H2O), the corresponding compressional and shear velocities are 0.6% and 1.1% faster than those of anhydrous olivine at 400-km depth. A preliminary model based on the work of Karato (2006) has been constructed to account for effects of anelasticity due to temperature and H2O content. Our model indicates that the effect of anelasticity on the shear velocity of hydrous olivine with 0.9 wt% H2O may offset the increase in the observed anharmonic velocity components of hydrous olivine, and the net effect would be that seismic velocities in the hydrous olivine would be similar to those for dry olivine at 400 km.
ABSTRACT Water may be incorporated as hydroxyl into both the hydrous and nominally anhydrous sili... more ABSTRACT Water may be incorporated as hydroxyl into both the hydrous and nominally anhydrous silicates of the upper mantle and transition zone. Incorporation of hydroxyl has significant effects on the elastic properties of the major and minor phases of this region such that it may be possible to detect the presence of water from seismic studies. We have been measuring the effects of hydration on the equation of state and elastic properties of hydrous and anhydrous forsterite, wadsleyite, ringwoodite. We have used these new studies together with existing data to model the velocity structure of the upper mantle and transition zone, based on a thermodynamic and empirical model of mantle mineralogy. We find that it is difficult to model the velocity structure of PREM or other seismic velocity models through the transition zone with an anhydrous pyrolite composition, whereas it is straightforward to match the spherical velocity model structures with the addition of a few thousand parts per million H2O by weight. Further, it is difficult to dehydrate a serpentinized subducting slab. Phase A and 10Å phase both result from partial dehydration of serpentine and are stable to depths in excess of 250km where olivine begins to incorporate significant amounts of hydroxyl. At 360 km depth olivine is capable of incorporating nearly one percent water by weight.
ABSTRACT The elastic properties of wadsleyite, thought to be the dominant phase from 410 to 520-k... more ABSTRACT The elastic properties of wadsleyite, thought to be the dominant phase from 410 to 520-km depth in the mantle, are essential to interpret the seismic images and profiles in the transition zone. Our previous experimental measurements showed that elasticity of Mg2SiO4 wadsleyite can be significantly reduced by hydration at high pressures (e.g. Mao et al., 2008a,b). These results provide the first constraints on the effect of hydration on the high-pressure sound velocities of wadsleyite, and are significantly important for identifying the potential hydrogen rich region in the Earth’s transition zone. Since mantle wadsleyite contains ~10 mol.% Fe, it is more important to investigate the combined effect of Fe and hydration on the elastic properties of wadsleyite. Here, we measured the single-crystal elasticity of wadsleyite with 1.0 wt.% H2O, Mg1.73Fe0.19SiO4H0.16, up to 12 GPa using Brillouin scattering. At ambient conditions, the aggregate bulk modulus, KS0, and shear modulus, G0, are 158.4(5) GPa and 99.2(3) GPa, respectively. Including the results of current and previous studies, we find that the elasticity of wadsleyite decreases linearly with Fe and H2O content according to relations (in GPa): KS0 = 171(3)-13.0(8)CH2O, G0 = 112(2)-8.8(3)CH2O-40(10)XFe, where CH2O is the concentration of hydrogen expressed as weight percent H2O, and XFe is the Fe molar fraction (XFe = Fe/(Mg+Fe)). Further high-pressure measurements showed that the presence of 1 wt.% H2O in Fe-bearing wadsleyite increases the pressure derivative of the shear modulus from 1.5(1) to 1.9(1). But Fe-bearing wadsleyite with this amount of H2O might have a similar pressure derivative of the bulk modulus (4.8(1)) similar to the corresponding anhydrous phase. Using our results, we computed the sound velocities of wadsleyite with 1 wt.% H2O up to 12 GPa at 300 K. Compared to Fe-bearing anhydrous wadsleyite, 1 wt.% H2O causes a 1.5(4)% reduction in the compressional velocity at 12 GPa, and a 1.2(6)% reduction in the shear velocity. The reduction of velocity produced by this amount of H2O is smaller than previously estimated from Mg2SiO4 wadsleyite. These results are also applied to evaluate the effect of hydration on the velocity contrast between wadsleyite and ringwoodite at 520-km depth. We found that hydration increases the compressional velocity contrast, but decreases the shear contrast. The shear velocity contrast will be absent when ~1.4% H2O is present at 12 GPa in a pyrolite mantle. This amount of H2O will increase the compressional velocity contrast from 1.2(5)% to 2.0(4)%. These results may help to identify possible regions of hydration in the transition zone.
ABSTRACT The presence of water in the upper mantle and transition zone has the potential to expla... more ABSTRACT The presence of water in the upper mantle and transition zone has the potential to explain various phenomena such as shear velocity anomalies or uplift and broadening of the 410-km discontinuity. The presence of H2O in the transition zone has also been frequently invoked to reconcile laboratory elasticity data on olivine polymorphs with seismic data for the amplitude of the 410-km discontinuity (Li et al., 2001; Chambers et al., 2005). Recently, we have measured the single-crystal elastic properties of hydrous olivine (Jacobsen et al., 2006) and a suite of hydrous wadsleyites (Mao et al., 2007a) at ambient conditions and one hydrous wadsleyite composition (0.84 wt% H2O) up to 12 GPa (Mao et al., 2007b). These data provide new constraints on elastic moduli and their pressure derivatives for hydrous olivine and wadsleyite. Using this data, we first examine the effect of H2O on bulk sound velocities under transition zone conditions because anelastic effects can be neglected in this case. At 410 km depth (~13.8 GPa, along a 1400°C adiabat), the bulk sound velocity of wadsleyite with 1 wt% H2O is 3.1% lower than for dry wadsleyite. Comparison of the seismic velocity jump across the 410-km discontinuity with the measured velocity contrast between wadsleyite and olivine provides a means to estimate the olivine abundance at 410-km depth. For mantle wadsleyite with 0.1-0.2 wt% H2O (Huang et al., 2005) and using experimentally determined olivine- wadsleyite H2O partition coefficients, the olivine abundance is found to be 40%, much lower than a pyrolite model. In order for a pyrolite composition to satisfy the seismic data, 1.2 wt. % H2O is needed in wadsleyite- a value greater than its maximum solubility under these conditions. The anomalously steep seismic gradient in the transition zone has been another feature of the region that has long defied explanation. We show that the seismic gradient can be matched if there is a gradient in H2O concentration across the transition zone such that the H2O content drops, for example, from 0.3 wt% at 410 km to 0.1 wt% at 520 km dpeth. For compressional and shear wave velocities, 0.1 wt% H2O in wadsleyite would lead to 0.3% and 0.4% reductions in VP and VS, respectively, neglecting any anelasticity. If the water content of wadsleyite was instead 1.0 wt. %, then the corresponding velocity reductions would be 3.3% and 3.6%. Following the work of Karato and Jung (1998), we have implemented a preliminary model accounting for the effect of H2O on anelasticity. This model indicates that 0.1 wt% H2O in wadsleyite could be responsible for reductions in shear velocities up to 1.0%.
Wadsleyite (beta-Mg2SiO4) is the high-pressure polymorph of olivine that is expected to be a domi... more Wadsleyite (beta-Mg2SiO4) is the high-pressure polymorph of olivine that is expected to be a dominant mineral in the transition zone from 410 km to 520 km depth in the mantle. The elasticity of wadsleyite is crucial to constrain the mineralogy of the transition zone. Previous studies show wadsleyite can incorporate variable amounts of water up to 3.3 wt% of water
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