Thermochemical interpretation of one-dimensional seismic reference models for the upper mantle: evidence for bias due to heterogeneity

A 1-D reference model for the mantle that is physically meaningful would be invaluable both in geodynamic modelling and for an accurate interpretation of 3-D seismic tomography. However, previous studies have shown that it is difficult to reconcile the simplest possible 1-D physical model-1300 degrees C adiabatic pyrolite-with seismic observations. We therefore generate a set of alternative 1-D thermal and chemical mantle models, down to 900 km depth, and compare their properties with seismic data. We use several different body and surface wave data sets that provide complementary constraints on mantle structure. To assess the agreement between our models and seismic data, we take into account the large uncertainties in both the elastic/anelastic parameters of the constituent minerals, and the thermodynamic procedures for calculating seismic velocities. These uncertainties translate into substantial differences in seismic structure. However, in spite of such differences, subtle trends remain. We find that models which attain (1) higher velocity gradients between 250 and 350 km; (2) higher velocity gradients in the lower transition zone; and (3) higher average velocities immediately beneath the 660-discontinuity, than 1300 degrees C adiabatic pyrolite-either via a temporary shift to lower temperatures, and/or a change to a seismically faster chemical composition-provide a significantly better fit to the seismic data than adiabatic pyrolite. This is compatible with recent thermochemical dynamic models by Tackley et al. in which average thermal structure is smooth and monotonous, but average chemical structure deviates substantially from pyrolite above, in, and below the transition zone. Our results suggest that 1-D seismic reference models are being systematically biased by a complex 3-D chemical structure. This bias should be taken into account when attempting quantitative interpretation of seismic anomalies, since those very anomalies contribute to the 1-D average signal.