Interpreting Seismic Constraints on 1-D Thermo-Chemical Structure of the Mantle Transition Zone: Implications for Mantle Dynamics

One-dimensional seismic reference models such as PREM and AK135 form the initial and background models for almost all inversions for three-dimensional seismic Earth structure. For quantitative interpretation in terms of physical parameters, i.e. temperature and composition, it is generally assumed that the mantle's seismic reference corresponds to an isochemical adiabatic structure produced by whole-mantle convection. However, tests have shown that the simplest mantle model - adiabatic pyrolite with a (MORB-formation consistent) potential temperature of 1300C - is not compatible with global seismic data sets. The discrepancy between the model and the seismic data could have resulted either from inappropriate use of mineral physics to generate the model, or because real average Earth structure is significantly different from this simple thermo-chemical structure. We test both these possibilities by generating a set of alternative 1-D thermal and chemical mantle models, and incorporating the effects of large uncertainties in both the elastic/anelastic parameters of the constituent minerals, and the thermodynamic procedures for calculating seismic velocities, into our computations. Although mineral physics uncertainties translate into substantial variation in seismic behaviour, there is subtle evidence to suggest that there are alternative thermo-chemical models which, either via a temporary shift to lower temperatures, and/or a change to a seismically faster chemical composition, in the transition zone and uppermost lower mantle, provide a significantly better fit to the seismic data than adiabatic pyrolite. This is compatible with average structures produced by thermo-chemical whole-mantle convection models from Tackley et al. (2005), with offset phase transition depths for the olivine and garnet components of the mantle's chemistry. In such models, average thermal structure is close to adiabatic, but average chemical structure departs substantially from pyrolite, as (seismically fast) basalt pools above 660 and (also seismically fast) harzburgite is enriched below. Our data show that such a complex physical background structure is likely. This needs to be considered when quantitatively interpreting models of seismic anomalies, especially when anomalies of different wave speeds are combined.