Gabriel D. Gwanmesia

Sound velocities of olivine and beta polymorphs of Mg2SiO4 at Earth's transition zone pressures

Sound velocities of olivine (α) and beta (β) polymorphs of Mg2SiO4 are measured to P > 12 GPa at room temperature on polycrystalline samples in a multi-anvil apparatus using ultrasonic interferometry. The new velocity data for olivine are lower by 1% for P waves and 2% for S wave at transition zone pressures than extrapolations of low pressure (<1 GPa) data. However, the elastic bulk (KS) and shear (G) moduli for the polycrystalline olivine exhibit good agreement to 10 GPa with recent Brillouin scattering and ultrasonic data for single crystals. The velocity data for the polycrystalline beta phase at 12 GPa agree within 1% with Eulerian finite strain extrapolations of previous data by Gwanmesia et al. (1990b) for similar specimens. The pressure derivatives of the elastic moduli calculated by linear fittings to KS and G vs. pressure yield K0′ = 4.4 and G0′ = 1.3 for the olivine and K0′ = 4.2 and G0′ = 1.5 for the beta phases. The isothermal velocity contrast at room temperature between these two phases, (Vβ-Vαrpar;/Vα decreases systematically with increasing pressure, reaching 8.0% for P wave and 11.3% for S wave at pressures equivalent to 410 km depth.

Pressure Dependence of Elastic Wave Velocity for β-Mg2SiO4 and the Composition of the Earth's Mantle

The pressure dependence of the elastic wave velocities for hot-pressed, elastically isotropic polycrystals of the β (modified spinel) phase of magnesium orthosilicate (Mg2SiO4) has been determined at room temperature to 3 gigapascals (GPa) by ultrasonic pulse interferometry. Pressure derivatives of the bulk (dK/dP = 4.8) and shear (dG/dP = 1.7) moduli derived from the travel times of the compressional (P) and shear (S) waves clearly demonstrate that the velocity contrast between the olivine and β phases of Mg2SiO4 decreases with increasing pressure. When combined with plausible values for the (as yet unmeasured) temperature derivatives, these new data can be used to calculate the contrast in P and S wave velocities across an olivine-β phase transformation occuaring at pressure-temperature conditions corresponding to about 400 kilometers depth in the earth. The seismologically observed contrasts ΔV in both P and S wave velocities constrain the percentage of orthosilicate in a model mantle of uniform chemical composition for appropriate relative magnitudes of the temperature (T) derivatives of the bulk and shear moduli for the β phase. Allowed combinations of orthosilicate content (percent), dK/dT, and dG/dT (both in gigapascals per Kelvin) for a pair of recent seismological models with ΔVp = ΔVs 4.6% include (65, -0.018, -0.020), (55, -0.015, -0.018), and (45, -0.012, -0.016).

Hydrostatic compression of ?-Mg2SiO4 to mantle pressures and 700 K: Thermal equation of state and related thermoelastic properties

P-V-T equations of state for the γ phase of Mg2SiO4 have been fitted to unit cell volumes measured under simultaneous high pressure (up 30 GPa) and high temperature (up to 700 K) conditions. The measurements were conducted in an externally heated diamond anvil cell using synchrotron x-ray diffraction. Neon was used as a pressure medium to provide a more hydrostatic pressure environment. The P-V-T data include 300 K-isothermal compression to 30 GPa, 700 K-compression to 25 GPa and some additional data in P-T space in the region 15 to 30 GPa and 300 to 700 K. The isothermal bulk modulus and its pressure derivative, determined from the isothermal compression data, are 182(3) GPa and 4.2(0.3) at T=300 K, and 171(4) GPa and 4.4(0.5) at T=700 K. Fitting all the P-V-T data to a high-temperature Murnaghan equation of state yields: KTO=182(3.0) GPa, KTO=4.0(0.3), ∂KT/∂T)0=−2.7(0.5)×10−2 GPa/K and (∂2KT/∂P∂T)0=5.5(5.2)×10−4/K at the ambient condition.

Elastic wave velocities of a pyrope-majorite garnet to 3 GPa

Abstract The pressure dependence of the elastic wave velocities for a polycrystalline specimen (100% theoretical density of majorite-pyrope garnet (Py 62 Mj 38 )) has been determined at room temperature to 3 GPa by ultrasonic phase comparison interferometry. Pressure derivatives of the adiabatic bulk modulus ( dK S dP= 5.3 ) and shear modulus ( dGdP= 2.0 ) are derived from the travel times of the compressional (P) and shear (S) waves. These derivatives clearly demonstrate that pressure has a more pronounced effect on the elasticity of the majorite-rich garnet than on the pure pyrope end-member, presumably in response to the coupled substitution of Si and Mg for Al on the octahedral site in the garnet structure. When combined with plausible values for the (as yet unmeasured) temperature derivatives, these new data can be used to calculate velocity-depth profiles along a 1400°C adiabat for the range of depths in which majorite is expected to be stable in the transition zone of the Earths mantle (450–600 km). For shear waves, these majorite gradients match those from regional seismic models much better than those for a pure olivine mantle. More reliable calculations of elastic wave velocities at mantle temperatures and pressures await measurement of the temperature derivatives of elastic modulus.

Elastic wave velocities at Mg 3 Al 2 Si 3 O 13 -pyrope garnet to 10 GPa

Abstract Elastic wave velocities of Mg3Al2Si3O12 pyrope garnet were measured to 10 GPa at ambient temperature, using ultrasonic interferometry in a 1000 ton split-cylinder, multi- anvil apparatus (USCA-1000). The sample used in the ultrasonic measurements was a polycrystalline specimen hot-pressed at 5 GPa and 1350 °C in a 2000 ton uniaxial split- sphere apparatus (USSA-2000) from a homogeneous glass of pyrope composition. Special P-T paths used during synthesis minimized effects of decompressing and thermal cracking; the bulk density of the sample was indistinguishable from the X-ray density. The elastic wave velocities measured at the ambient condition agree with the Hashin-Shtrikman averages of the single crystal values within the mutual uncertainties. The high-pressure experiments yielded the elastic moduli and their pressure derivatives (finite strain fit) for the shear modulus G0 = 92 ± 1 GPa, G′0 = (∂G/∂P)T = 1.6 ± 0.2 and for the longitudinal modulus L0 = 294 ± 1 GPa, L′0 = (∂L/∂P)T = 1A ± 0.5, (L = Ks + 4/3G), from which the bulk modulus K0 = 171 ±2 GPa, K′0 = (∂K/∂P)t = 5.3 ± 0.4 was calculated. The pressure derivative for the shear modulus of pyrope did not differ from those of natural pyrope-almandine-grossular garnets. For the bulk modulus, the pressure derivative for pyrope agreed with that for pyrope-almandine but was substantially higher (25%) than that for the Ca-bearing garnet. In the pyrope-majorite series, K′0 remained constant, whereas G′0 increased by 25 for 38% majorite content.

Sound velocities in MgSiO3‐garnet to 8 GPa

Velocities of acoustic compressional (P) and shear (S) waves were measured to 8 GPa at room temperature for dense (99.7% of theoretical density), isotropic polycrystalline specimens of MgSiO3-majorite garnet using the phase comparison method of ultrasonic interferometry in a 1000-ton split-cylinder apparatus (USCA-1000). The pressure derivatives of the adiabatic bulk modulus (KS) and the shear modulus (G) are Ko′ = (∂Ks/∂P)T =6.7±0.4 and Go′ = (∂G/∂P)T =1.9±0.1. Substitution of Si for Mg and Al in these garnets increase Ko′ by 26%, and Go′ by 16%, from Mg3Al2Si3O12-pyrope to majorite. The high values of Ko′ and Go′ from this study lead to much steeper velocity-depth profiles in the transition zone (400–700 km) than those calculated from the Ko′ and Go′ estimates from elasticity systematics by Duffy and Anderson (1989).

An olivine to beta phase transformation mechanism Mg2SiO4

Fine-grained powder samples of Mg2SiO4 forsterite (α phase) have been partially transformed to the beta phase in a 2000-ton uniaxial split-sphere apparatus (USSA-2000) at P = 15 GPa and T = 1000 °C in runs of 5 minutes duration. This technique allowed us to catch the first steps of the α --> β transformation in Mg2SiO4. A TEM study of the run products showed that a structurally disordered phase is an intermediate of the reaction. The oxygen sublattice of this intermediate phase is well ordered, but a very large density of planar faults on (010) planes destroys the long range order in the cation sublattices. A two-steps mechanism for the α --> β transformation is therefore probable, involving first the nucleation of the structurally disordered phase, then the ordering of this phase. These two steps may be associated with low energy activation barriers and could thus explain the fast kinetics of the α --> β transformation.

Hot Pressing of Polycrystals of High-pressure Phases of Mantle Minerals in Multi-anvil Apparatus

In the 1960s, E. Schreiber and his colleagues pioneered the use of hot-pressed polycrystalline aggregates for studies of the pressure and temperature dependence of the elastic wave velocities in minerals. We have extended this work to the high-pressure polymorphs of mantle minerals by developing techniques to fabricate large polycrystalline specimens in a 2000-ton uniaxial split-sphere apparatus. A new cell assembly has been developed to extend this capability to pressures of 20 GPa and temperatures of 1700°C. Key elements in the new experimental design include: a telescopic LaCrO3 forT>1200°C; Toshiba Tungaloy grade F tungsten carbide anvils; and the use of homogeneous glasses or seeded powder mixtures as starting material to enhance reactivity and maximize densities. Cell temperatures are linearly related to electrical power to 1700°C and uniform throughout the 3 mm specimens. Pressure calibrations at 25°C and 1700°C are identical to 15 GPa. Cylindrical specimens of the beta and spinel phases of Mg2SiO4, stishovite (SiO2-rutile), and majorite-pyrope garnets have been synthesized within their stability fields in runs of 1–4 hr duration and recovered at ambient conditions by simultaneously decompressing and cooling along a computer-controlledP-T path designed to preserve the high-pressure phase and to relax intergranualar stress in the polycrystalline aggregate. These specimens are single-phased, fine-grained (<5 micron), free of microcracks and preferred orientation, and have bulk densities greater than 99% of X-ray density. The successful fabrication of these high-quality polycrystalline specimens has made possible experiments to determine the pressure dependence of acoustic velocities in the ultrasonics laboratory of S. M. Rigden and I. Jackson at the Australian National University.

The symmetry of garnets on the pyrope (Mg3Al2Si3O12)-majorite (MgSiO3) join

Garnets with compositions between majorite and pyrope, Mj38, Mj48, Mj75 and Mj79 were synthesized at high pressures and temperatures in a 2000-ton uniaxial split-sphere apparatus (USSA-2000) and investigated using high resolution synchrotron X-ray powder diffraction and transmission electron microscopy. The results from both techniques are consistent with the tetragonal field for these garnets extending to a majorite composition just below Mj75. The cubic-tetragonal structural phase transition in garnet along the majorite-pyrope join is sensitive to both composition and temperature and is expected to result in anomalous behavior in elastic shear moduli. This phase transition may occur in the transition zone of the earths mantle and will have important effects on the elastic and rheological properties of this region where these garnets are stable phases.

Elastic wave velocities of pyrope–majorite garnets (Py62Mj38 and Py50Mj50) to 9 GPa

Abstract Polycrystalline specimens of two pyrope–majorite garnets 2 (Py 62 Mj 38 and Py 62 Mj 38 ) were synthesized in a 2000-ton uniaxial split-sphere apparatus (USSA-2000) at pressures of 16 GPa and temperatures of 1670 K for run durations of about 2 h using homogeneous glasses as starting materials. Ultrasonic interferometric measurements on these specimens were conducted at pressures up to 9 GPa at room temperature using a 1000-ton uniaxial split-cylinder apparatus (USCA-1000) with Bi as an internal pressure marker. From the measurements of the travel times for compressional (P) and shear (S) waves, the velocities and elastic moduli are calculated using length/density changes determined from the travel-time data. Third-order Eulerian finite strain analysis of these data yield the longitudinal ( L ) and shear ( G ) moduli and their pressure derivatives [ M 0 ′=(∂ M /∂ P ) T ]: for Py 62 Mj 38 , L 0 =291±6 GPa and L 0 ′=9.1±0.6, G 0 =90±1 GPa and G 0 ′=1.9±0.2, from which we calculate the values for the adiabatic bulk modulus ( K = L −4/3 G ) to be K 0 =171±5 GPa and K 0 ′=6.2±0.5. These new data for the shear modulus and its pressure derivative are in agreement with the previous values for the same composition (Rigden, S.M., Gwanmesia, G.D., Liebermann, R.C., 1994. Elastic wave velocities of a pyrope–majorite garnet to 3 GPa. Phys. Earth Planet. Inter. 86, 35–44.), but the P wave velocity, the bulk modulus and its pressure derivative are somewhat higher. For the Py 50 Mj 50 composition, these values are L 0 =289±6 GPa, L 0 ′=9.2±0.7, G 0 =89±1 GPa, G 0 ′=2.1±0.2, K 0 =170±5 GPa, and K 0 ′=6.4±0.5. The high-pressure derivatives of K and G for these Py–Mj garnets may be important in explaining the high velocity gradients observed in seismic models of the transition zone of the Earths mantle.