Calculated acoustic velocities compared with reference models

In Figure 6.3-1 Vs and Vp calculated for the three bulk compositions are compared with the seismic reference models PREM and AK135.

Figure 6.3-1. Sound velocities for pyrolite (red) harzburgite (green) and MORB (blue) compositions in the transition zone and uppermost lower mantle. The solid and dashed black curves show PREM and AK135 seismic reference models, respectively.

While the pyrolite model is in agreement at least with AK135 in terms of V_p, there is a consistent negative deviation for Vs, by ~0.2 km/s, between pyrolite and both reference models over the 150 km of the base of the transition zone. Garnet elastic properties are the main reason for this deviation as both ringwoodite and CaSiO3-perovskite display velocities which are above both reference models at these conditions. Only garnet brings the bulk velocities below the seismic models. While some deviation from reference models may be expected in detail, on average one would expect negative deviations to be balanced by local positive deviations (Cammarano et al., 2005) if the mineral model is correct. The good agreement between pyrolite and both reference models in the lower mantle means that negative deviations at the base of the transition zone are not balanced by positive deviations in the lower mantle.

Irifune et al. (2008) also proposed that estimated velocities for pyrolite would be lower than reference models at approximately 575 km, but suggested that the subsequent exsolution of CaSiO₃-rich perovskite from the garnet would alleviate this discrepancy towards the base of the transition zone. Sinogeikin and Bass (2002a) similarly argued that such exsolution would raise the velocity gradient. In this study, the gradual formation of CaSiO₃-rich perovskite and the compositional variations in garnet were taken into account. A very slight increase in the velocity gradient for the pyrolite model occurs above 570 km due to CaSiO₃ exsolution.

harzburgite. Although shear wave properties of CaSiO3 perovskite are poorly constrained, the Go used in the current model (Karki and Crain, 1998) is at the very high limit of recent theoretical studies and is significantly higher than all experimental estimates (Kudo et al., 2012). Therefore it seems unlikely that the Vs for CaSiO3 perovskite is significantly underestimated, in fact the contrary seems more likely. It would therefore seem that pyrolite along a 1673 K adiabat provides a poor match to seismic reference models at the base of the transition zone.

While other bulk compositions have been proposed for the transition zone, such as piclogite (Bass and Anderson, 1984; Anderson and Bass, 1986) it can be seen that increasing the basaltic component of a composition cannot raise V_s to levels compatible with the reference models. For the MORB composition the exsolution of CaSiO₃ perovskite can be clearly seen to cause an increase in gradient above 550 km. While the resulting gradient is closer to the reference models, the absolute values remain at least 0.3 km/s below both reference models throughout the base of the transition zone.

A number of other effects that could in principal cause differences between mineral and seismic models are also likely to only lower calculated mineral velocities further. Anelastic effects that cause dispersion and potential significant variations of mineral velocities as a function of acoustic wavelength, would only lower velocities of mineral models that accounted for this effect. Similarly the presence of minor defects such as those caused by the presence of dissolved OH^- in minerals should also only lower velocities (Jacobsen, 2006).

If subducted material were to accumulate at the base of the transition zone it is possible that a significant portion of the material in this regions is composed of melt depleted harzbugite. As shown in Figure 6.3-1, however, depleting pyrolite in this way and increasing the component of the (Fe,Mg)2SiO4 phase raises velocities but they still fall below the reference models.

One of the few remaining plausible explanations for the deviation between mineral and seismic models at the base of the transition zone would be if the average mantle temperature over this depth interval was below the 1673 K adiabat. While mantle adiabatic temperatures determined from erupted basalt melt compositions vary by approximately ±150°, (Lee et al., 2009) in order for the pyrolite model to match the seismic model at the base of the transition

zone, it can be estimated that temperatures would need to be 600° lower. This would place mantle temperatures far outside of the range of adiabatic temperature estimates from the surface or from temperature estimates based on the depth of the 410 km discontinuity (Frost, 2008).

Saikia et al. (2008) noted, however, that if the 520 km seismic discontinuity is associated with the wadsleyite to ringwoodite transformation, then it also occurs at a depth that implies lower than expected average mantle temperatures. This can be seen in Figure 6.3-1, where the transformation along a 1673 K adiabat occurs at 550 km, and only if temperatures were ~300 K lower would the transition occur at 520 km. Saikia et al. (2008) proposed that the observation might be explained if subducting slabs stagnate at the base of the transition zone and flatten out to form significant lateral cold heterogeneities. Some tomographic models (e.g. Kárason and van der Hilst, 2000) appear to clearly indicate that such heterogeneities exist. As temperatures in the center of such slabs could be easily 600 K below the average mantle, then they could drag down average mantle temperatures at these depths, if the lateral anomalies were large enough. Furthermore it is possible that a significant proportion of this material will be of near harzburgite composition. It can be estimated that for harzburgite mineral model velocities to match seismic reference models at the base of the transition zone, temperatures would have to be only 200 K below the 1673 K adiabat. If such global horizontal anomalies exist, this average reduction in temperature may be plausible and would be also consistent with the 520 km seismic discontinuity being cause by the wadsleyite to ringwoodite transformation.

A further issue that needs to be investigated is that the mineral akimotoite would be expected to form in both harzburgite and pyrolite compositions if temperatures at the base of the transition zone were several hundred degrees lower than the 1673 K adiabat. The elastic properties of akimotoite are poorly explored and no single crystal data exist on the elastic tensor. Similarly only theoretical calculations exist on the pressure and temperature dependent properties. The shear modulus of akimotoite is expected to be greater than that of garnet but possibly below that of perovskite (Stixrude and Lithgow-Bertelloni, 2011), and its presence may well help to explain the discrepancy in velocities at the base of the transition zone.