Elasticity of single-crystal iron-bearing pyrope up to 20 GPa and 750 K

Geophysical Research Letters

2014

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We have measured acoustic V P and V S velocities of (Fe,Al)-bearing MgSiO 3 silicate glasses and an Icelandic basalt glass up to 25 GPa. The velocity profiles of the (Fe,Al)-bearing and basaltic silicate glasses display decreased V P and V S with minima at approximately 5 and 2 GPa, respectively, which could be explained by the mode softening in the aluminosilicate networks. Our results represent the first observation of such velocity softening extending into the chemically complex basaltic glass at a relatively low transition pressure, which is likely due to its degree of polymerization, while the Fe and Al substitutions reduce sound velocities in MgSiO 3 glass. If the velocity softening in the basaltic and silicate glasses can be used as analogs for understanding melts in Earth's interior, these observations suggest that the melt fraction needed to account for the velocity reduction in the upper mantle low-velocity zone may be smaller than previously thought.

Section: Methodsmentioning

confidence: 99%

Abnormal acoustic wave velocities in basaltic and (Fe,Al)‐bearing silicate glasses at high pressures

Liu

Geophysical Research Letters

2014

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“…In our modeling, we have taken into account the phase relations as a function of depth as well as the partitioning of Fe between olivine polymorphs and between olivine and residual basaltic components at the phase equilibria using previous experimental and theoretical results (Akaogi et al, 1989;Angel et al, 1992;Irifune, 1987;Irifune and Isshiki, 1998;Irifune and Ringwood, 1987;Katsura and Ito, 1989;Pacalo and Gasparik, 1990;Stixrude and Lithgow-Bertelloni, 2005;Xu et al, 2008). After considering the variation in phases and compositions with depth, the velocity of pyrolite has been constructed using the elasticity of olivine in this study and elasticity of other mantle phases in previous studies (Table 1) (Chai et al, 1997;Finger and Ohashi, 1976;Isaak et al, 2010;Jackson et al, 2003Jackson et al, , 2007Jiang et al, 2004b;Lu et al, 2013;Sang and Bass, 2014;Bass, 2000, 2002;Sinogeikin et al, 1998Sinogeikin et al, , 2001Wang et al, 2014;Zhao et al, 1997;Zou et al, 2012). The computed velocity model for a pyrolitic composition is compared to literature seismic profiles (Fig.…”

Section: The Velocity Contrast At the 410-km Depthmentioning

confidence: 98%

Elasticity of single-crystal olivine at high pressures and temperatures

Mao

Fan

Earth and Planetary Science Letters

et al. 2015

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Available online xxxx Editor: J. BrodholtKeywords: elasticity of single crystal San Carlos olivine high pressure and temperature metastable olivine wedge mantle seismic anisotropy Elasticity of single-crystal San Carlos olivine has been derived from sound velocity and density measurements at simultaneous high pressure-temperature conditions up to 20 GPa and 900 K using in situ Brillouin spectroscopy and single-crystal X-ray diffraction in externally-heated diamond anvil cells. These experimental results are used to evaluate the combined effect of pressure and temperature on full elastic constants of single-crystal olivine to better understand its velocity profiles and anisotropies in the deep mantle. Analysis of the results shows that the shear moduli display strong concave behaviors as a function of pressure at a given high temperature, while the longitudinal modulus, C 11 , and the off-diagonal moduli, C 12 and C 13 , exhibit greater temperature dependence at higher pressures than at relatively lower pressures. Using a finite-strain theory and thermal equation of state modeling for a pyrolitic mantle composition along an expected mantle geotherm, our results show that the magnitude of the V P and V S jumps at the 410-km depth are 6% and 6.4%, respectively, which are greater than that found in seismic observations, suggesting a mantle olivine content of 40-50 vol%, which is less than what is expected for the pyrolite model. Our modeled velocity profiles for a metastable olivine wedge in the subduction slabs along a representative cold slab geotherm are 6% and 10% lower than those of wadsleyite and ringwoodite, respectively, at corresponding depths of the normal mantle. Our modeled results also show that metastable olivine in the cold slabs could have strong V P and V S anisotropies. The maximum V P anisotropy is estimated to be 19-22% at transition zone depth, whereas the maximum V S splitting is 13-23% and increases with depth. As a result, the presence of a metastable olivine wedge at the transition zone depth would exhibit a seismic signature of low velocity and strong seismic anisotropy which are consistent with recent seismic observations for various locations of the slabs and can be used as mineral physics constraints for future seismic detections of the metastable olivine wedges in the deep mantle.

“…To understand the consequences of Fp Fe 2+ spin transition and Fe partitioning on the seismic profiles of the Earth's lower mantle, we have also modeled the mass density (ρ) and bulk sound velocity (V Φ ) profiles of a representative pyrolitic lower mantle composition along an expected mantle geotherm [Brown and Shankland, 1981] (calculation details are shown in SI section 4.2 [Tange et al, 2012[Tange et al, , 2009Dubrovinsky et al, 2000;Lu et al, 2013;Murakami et al, 2012;Ricolleau et al, 2010;Shukla et al, 2015;van Westrenen et al, 2005;Wang et al, 2015;Wu and Wentzcovitch, 2014]) and compared them with the density and bulk velocity without the Fe 2+ spin transition in Fp (Figure 2b). Here the bulk sound velocity is defined as V Φ = (K S /ρ) 1/2 , where K S is the adiabatic bulk modulus and values are taken along the geotherm.…”

Section: 1002/2016jb013543mentioning

confidence: 99%

Iron partitioning between ferropericlase and bridgmanite in the Earth's lower mantle

Morgan

2017

JGR Solid Earth

Earth's lower mantle is generally believed to be seismically and chemically homogeneous because most of the key seismic parameters can be explained using a simplified mineralogical model at expected pressure‐temperature conditions. However, recent high‐resolution tomographic images have revealed seismic and chemical stratification in the middle‐to‐lower parts of the lower mantle. Thus far, the mechanism for the compositional stratification and seismic inhomogeneity, especially their relationship with the speciation of iron in the lower mantle, remains poorly understood. We have built a complete and integrated thermodynamic model of iron and aluminum chemistry for lower mantle conditions and from this model has emerged a stratified picture of the valence, spin, and composition profile in the lower mantle. Within this picture the lower mantle has an upper region with Fe3+‐enriched bridgmanite with high‐spin ferropericlase and metallic Fe and a lower region with low‐spin, iron‐enriched ferropericlase coexisting with iron‐depleted bridgmanite and almost no metallic Fe. The transition between the regions occurs at a depth of around 1600 km and is driven by the spin transition in ferropericlase, which significantly changes the iron partitioning and speciation to one that favors Fe2+ in ferropericlase and suppresses Fe3+ and metallic iron formation. These changes lead to lowered bulk sound velocity by 3–4% around the middle‐lower mantle and enhanced density by ~1% toward the lowermost mantle. The predicted chemically and seismically stratified lower mantle differs dramatically from the traditional homogeneous model.