Evolving Views on the Scale and Nature of Mantle Convection

Abstract:

Since seminal studies of transition zone discontinuities in the 1960ies and the advent of seismic tomography a decade later much progress has been made with the understanding of the scale and nature of mantle convection. First order questions remain, however, about the fluxes between the canonical upper and lower parts of Earth’s mantle and the origin and nature of deep mantle heterogeneity. The first generation of tomographic models depicted fast shear wave propagation in the lowermost mantle beneath the circum-Pacific subduction zones and large low shear wspeed anomalies beneath Africa and the central Pacific. In P-wave models these structures are less apparent, and the anomalous Vp/Vs ratios and related variables are suggestive of chemical heterogeneity. Later tomographic studies revealed the pattern of subducted oceanic lithosphere in more detail and discovered that some slabs sink deep into the lower mantle whereas others remain, at least temporarily, in the transition zone. The complex flow trajectories and the evidence for compositional heterogeneity render simple end-member models of strict layering or unobstructed mantle flow untenable. Various seismic imaging methods have been used to map with increasing precision the variations in depth to the major mantle discontinuities, and also these results are not fully consistent with expectations for simple convection models. In addition, renewed scrutiny with more data and better methods suggest that the models of phase transitions around 410 and 660 km depth in the olivine component of a pyrolitic mantle composition are oversimplifications. Indeed, interfaces are also found at other depths, and many exceptions to the expected anti correlation of the interface topographies have been reported. Some of these observations can be explained with experimental and computational studies of the mineralogy and phase chemistry of deep mantle assemblages, but with such studies still restricted to fairly simple bulk compositions and with seismological probes continuing to discover hitherto unknown complexities, it is clear that the deep mantle still has many secrets to be uncovered and explained. This presentation will be a mix of review of published results and a presentation of preliminary results of seismic tomography and deep earth imaging.