Influence of phase transformations on lateral heterogeneity and dynamics in Earth's mantle
Introduction
Below the lithosphere, Earth structure is approximately spherically symmetric and physical properties depend most strongly on depth. Deviations from radial structure, that is variations in physical properties with latitude and longitude at a given depth, while small, are disproportionately important in our understanding of Earth's dynamics and evolution. For example, lateral variations in density drive mantle flow and plate motions and produce dynamic topography and the non-hydrostatic part of the geoid and gravity field. Seismic wave velocities vary laterally and the results of seismic tomography are providing us with an increasingly clear and robust view of three-dimensional mantle structure.
With advances in seismic tomography and mineral physics, particularly knowledge of the elastic properties of mantle phases at high pressure and temperature, has come an increasing interest in the origins of lateral heterogeneity. Much of the lateral variations seen in seismic tomography are likely to be related to lateral variations in temperature. Quantifying this relationship has proved difficult, in part because of the non-uniqueness of tomographic models (e.g. damping) and remaining uncertainties in key mineralogical properties. Because the magnitude of lateral variations is small, consideration of second-order effects such as attenuation and dispersion are important, if still ill-constrained experimentally, particularly at mantle pressures. Lateral variations in chemical composition almost certainly contribute as well, for example associated with depletion of the continental lithospheric mantle. In the deep mantle, the relationship between lateral variations in S- and P-wave velocities cannot be explained by lateral variations in temperature alone, although what other causes might be responsible is still a matter of debate.
Here we demonstrate another important contributor to lateral heterogeneity that has so far received little attention: that due to lateral variations in phase proportions and compositions (Anderson, 1987). We will argue that the contribution from phase transformations is reasonably well constrained experimentally, is comparable in magnitude to that of temperature alone in a monophase aggregate, and should be included in future analyses of three-dimensional structure. We first outline the thermodynamic theory and illustrate the effect of phase transformations with examples. We then explore the influence of phase transformations on the temperature variation of velocity and density in a realistic mantle composition. Finally we draw some conclusions regarding the likely importance of phase transformations in the interpretation of mantle structure.
Section snippets
Theory
Consider the seismic wave velocity in the vicinity of a phase transformation (Fig. 1). Because the mantle is a multi-component system, phase transformations generally occur over a finite depth interval, ΔP. Assume that the velocity contrast between the two phases is ΔV. It is generally assumed that the passage of the seismic wave is sufficiently rapid and the stresses generated sufficiently small, that no phase transformation is induced. In this case, the velocity increases monotonically with
Method
In order to investigate the likely effects of phase transformations on laterally varying structure in a realistic mantle composition, we use the method described in detail in our previous publications (Stixrude and Lithgow-Bertelloni, 2005a, Stixrude and Lithgow-Bertelloni, 2005b). Briefly, this is a thermodynamically self-consistent method, based on the concept of fundamental thermodynamic relations, that captures phase equilibria and physical properties, including the full elastic constant
Results
The computed phase assemblage along the reference adiabat is in good agreement with our previous results and many previous studies (Ita and Stixrude, 1992, Cammarano et al., 2003, Stixrude and Lithgow-Bertelloni, 2005b) (Fig. 2). As in our previous work, which extended to a depth of 500 km (Stixrude and Lithgow-Bertelloni, 2005b), the amount of cpx appears to be underestimated as compared with experiment and this is likely due to the neglect of the jadeite component of cpx in our study. The
Discussion
The highly non-linear dependence of the velocity–temperature scaling with depth has important consequences for understanding the structure of the mantle (Fig. 10). In the vicinity of sharp phase transformations, the sensitivity of seismic wave velocities to temperature changes suddenly over a narrow range of depth. This means that the magnitude of lateral variations in either i) temperature or ii) seismic wave velocities must depend strongly on depth. For example, in the vicinity of 400 km
Conclusions
There are three potential causes of laterally varying structure: lateral variations in temperature, chemical composition, and phase. The first two of these have received most attention. Lateral variations in phase are likely to be important as well, particularly in the upper 800 km of the mantle, where phase transformations are ubiquitous, and possibly in the lowermost few hundred km of the mantle. The contributions of phase transformations to the scaling of velocity to temperature or density
Acknowledgements
We thank Alex Forte and an anonymous reviewer for thoughtful comments that improved the manuscript. This work supported by the U.S. National Science Foundation under grants EAR-0635815 and EAR-0456112.
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Now at: Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, United Kingdom.