Single-crystal elasticity of ringwoodite to high pressures and high temperatures: implications for 520 km seismic discontinuity

Abstract

The single-crystal elastic moduli of spinel-structured γ-(Mg 0.91Fe 0.09) 2SiO 4 (ringwoodite) were measured to 16 GPa at room temperature, and to 923 K at ambient pressure by Brillouin spectroscopy. A third-order finite-strain equation of state yields pressure derivatives of 4.1 (2) and 1.3 (1) for the adiabatic bulk modulus ( K S ) and shear modulus ( μ), respectively. The pressure derivatives of aggregate elastic moduli for all three polymorphs (α, β, and γ) of (Mg,Fe) 2SiO 4 in the Mg-rich end are equal within the mutual experimental uncertainties, with ( ∂K S / ∂P) T =4.1–4.3 and ( ∂μ/ ∂P) T =1.3–1.4. A linear decrease of the elastic moduli and sound velocities with temperature adequately describes the data. The temperature derivatives of K S and μ are −0.021 and −0.016 GPa/K, respectively. An extrapolation of our data to transition zone pressures and temperatures indicates that the shear and compressional impedance contrasts associated with β-(Mg,Fe) 2SiO 4→γ-(Mg,Fe) 2SiO 4 transition are sufficient to produce a visible discontinuity at 520 km depth even with only a moderate (30–50%) amount of olivine. Moreover, the partitioning of Fe between β-(Mg,Fe) 2SiO 4 and γ-(Mg,Fe) 2SiO 4 and majoritic garnet will produce a relatively sharp seismic discontinuity (in the order of 10–15 km) even when the latent heat of reaction is taken into account. The visibility of the discontinuity in regional and global studies could be inhibited by large topography that would accompany any local variations in temperature or Fe content. The high V P contrasts observed in some localities are difficult to explain by the β→γ transition and are suggestive of a chemical boundary. This is consistent with chemical heterogeneity in the transition zone, probably related to subduction. One of the possible explanations for high velocity contrasts near 520 km is a “buried Moho”—a boundary between garnetite (former oceanic crust) and γ-(Mg,Fe) 2SiO 4-enriched (former peridotite) layers.