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Petrology, elasticity, and composition of the mantle transition zone

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1992, Journal of Geophysical Research

https://doi.org/10.1029/92JB00068
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Abstract

We compare the predictions of compositional models of the mantle transition zone to observed seismic properties by constructing phase diagrams in the MgO-FeO-CaO-AI203-SiO 2 system and estimating the elasticity of the relevant minerals. Mie-Grfineisen and Birch-Murnaghan finite strain theory are combined with ideal solution theory to extrapolate experimental measurements of thermal and elastic properties to high pressures and temperatures. The resulting thermodynamic potentials are combined with the estimated phase diagrams to predict the density, seismic parameter, and mantle adiabats for a given compositional model. We find that the properties of pyrolite agree well with the observed density and bulk sound velocity of the upper mantle and transition zone. Piclogite significantly underestimates the magnitude of the 400-km velocity discontinuity and overestimates the velocity gradient in the transition zone. Substantially enriching piclogite in AI provides an acceptable fit to the observations. Invoking a chemical boundary layer between the uppermost mantle and transition zone leads to poor agreement with observed seismic properties for the compositions considered. Within the transition zone, the dissolution of garnet to Ca-perovskite near 18 GPa may explain the proposed 520-km seismic discontinuity. Below 700 km depth, all compositions disagree with observed bulk sound velocities, implying that the lower mantle is chemically distinct from the upper mantle.

FAQs

sparkles

AI

What explains the discrepancies in seismic observations of pyrolite and piclogite compositions?add

The study reveals that pyrolite aligns closely with seismic data across the mantle, while piclogite significantly underestimates velocity and density between 400 and 500 km depth.

How did phase transitions affect velocity gradients in the transition zone?add

Anomalous velocity gradients near 18 GPa are attributed to the transformation of garnet to Ca-perovskite, impacting bulk sound velocities across all studied compositions.

When did the analysis of mantle transition zone composition first recognize pressure-induced phase transformations?add

Pressure-induced phase transformations as explanations for seismic velocity gradients in the transition zone were first articulated in the early 1990s, leading to critical insights.

What methodologies were used to synthesize experimental petrologic data for the transition zone?add

The researchers employed a semiempirical thermodynamic potential formalism to derive density and bulk modulus across pressure-temperature regimes, ensuring consistent comparisons with mineral properties.

Why is chemical layering in the upper mantle considered unlikely according to the findings?add

The results indicate that seismic and mineral physics data cannot confirm multiple layers, suggesting that the upper mantle remains chemically distinct from the lower mantle.

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    Friedemann Wenzel

    Journal of Geophysical Research, 1998

    Short-period, three-component recordings of peaceful nuclear explosions (PNEs) in northern Eurasia are used to constrain the P wave velocity structure of the mantle transition zone. The properties of the upper mantle discontinuities play an important role in understanding the nature of mantle processes. Data from several P NE seismic sounding profiles reveal reflections and refractions from upper mantle discontinuities at 410 and 660 kin depth. The amplitude and the sharpness of these velocity discontinuities contain important information to assess models of upper mantle phase changes and chemical layering. The absence of strong critical and precritical reflections from the 660 km discontinuity is characteristic for all the data in northern Eurasia. By studying the P wave reflections and refractions from the 660 km discontinuity, several velocity models were derived.

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    Chemical and Clapeyron-induced buoyancy at the 660 km discontinuity
    Yanbin Wang

    Journal of Geophysical Research, 1998

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    Sound velocities in MgSiO 3 -garnet to 8 GPa
    Robert Liebermann, Ganglin Chen

    Geophysical Research Letters, 1998

    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 '= (3Ks/3P)T = 6.7 + 0.4 and Go '= (3G/3P)T = 1.9 + 0.1. Substitution of Si for Mg and A1 in these garnets increase Ko' by 26%, and Go' by 16%, from Mg3A12Si30•2pyrope 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). GWANMESIA ET AL.' SOUND VELOCITIES IN MgSiO3-GARNET TO GPA J 2456, 1990.

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    Thermal equation of state and thermodynamic properties of molybdenum at high pressures
    Pavel Gavryushkin, Anton Shatskiy

    Journal of Applied Physics, 2013

    1] Resent experimental and theoretical studies suggested preferential stability of Fe 3 C over Fe 7 C 3 at the condition of the Earth's inner core. Previous studies showed that Fe 3 C remains in an orthorhombic structure with the space group Pnma to 250 GPa, but it undergoes ferromagnetic (FM) to paramagnetic (PM) and PM to nonmagnetic (NM) phase transitions at 6-8 and 55-60 GPa, respectively. These transitions cause uncertainties in the calculation of the thermoelastic and thermodynamic parameters of Fe 3 C at core conditions. In this work we determined P-V-T equation of state of Fe 3 C using the multianvil technique and synchrotron radiation at pressures up to 31 GPa and temperatures up to 1473 K. A fit of our P-V-T data to a Mie-Gruneisen-Debye equation of state produce the following thermoelastic parameters for the PM-phase of Fe 3 C: V 0 = 154.6 (1) Å 3 , K T0 = 192 (3) GPa, K T ′ = 4.5 (1), γ 0 = 2.09 (4), θ 0 = 490 (120) К, and q = À0.1 (3). Optimization of the P-V-T data for the PM phase along with existing reference data for thermal expansion and heat capacity using a Kunc-Einstein equation of state yielded the following parameters: V 0 = 2.327 cm 3 /mol (154.56 Å 3 ), K T0 = 190.8 GPa, K T ′ = 4.68, Θ E10 = 305 K (which corresponds to θ 0 = 407 K), γ 0 = 2.10, e 0 = 9.2 × 10 À5 K À1 , m = 4.3, and g = 0.66 with fixed parameters m E1 = 3n = 12, γ ∞ = 0, β = 0.3, and a 0 = 0. This formulation allows for calculations of any thermodynamic functions of Fe 3 C versus T and V or versus T and P. Assuming carbon as the sole light element in the inner core, extrapolation of our equation of state of the NM phase of Fe 3 C suggests that 3.3 ± 0.9 wt % С at 5000 К and 2.3 ± 0.8 wt % С at 7000 К matches the density at the inner core boundary.

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    Seismic structure of the upper mantle in a central Pacific corridor
    Lind Gee, J. Gaherty

    Journal of Geophysical Research, 1996

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