A poorly mixed mantle and its thermal state inferred from seismic waves

The Earth’s mantle transition zone is a complex region exhibiting mineralogical phase changes as revealed by sharp increases of seismic wave-speed between 410 and 660 km depths. Because of its potential in filtering chemical elements, the transition zone represents a key region for understanding how efficient is global mantle convection to mix and recycle geochemical heterogeneities. Global sampling of the transition zone is only possible with seismic methods, via the analysis of seismic waves generated by large, distant earthquakes and subsequently recorded by receivers located on the Earth’s surface. These waves propagate and illuminate the Earth’s deep internal interior, and provide critical constraints on the elastic structure. Seismologists and geophysicists have since the 90’s attempted to isolate the effects of temperature and composition on mantle elastic properties. However, a major issue is imperfect seismic data coverage that prevents from reconstructing the multiple length-scales of thermo-chemical heterogeneities. Seismic and laboratory-based data also suffer from large uncertainties, and the relationship between seismic observables and in situ thermo-chemical parameters remains questionable. I will describe collaborative efforts to overcome these limitations. We use mineral physics data and a partitioning approach to isolate the multi-scale effects of temperature and composition on the most comprehensive databases of seismic waves sensitive to the transition zone. I will illustrate the potential of these approaches in a two-case analysis. At regional scale, I will show that the complexity of the structure near the subduction zones of North and Central Honshu below the Japan (East) Sea --- single and double discontinuities ranging in depth from 650 to 730 km, as well as a low-velocity zone at the tip of the subducted Pacific plate --- can theoretically be explained by phase equilibria in a pyrolitic mantle composition. At global scale, we combine compressional, shear-wave reflection observations, and modeling, to build a new global thermal model for the transition zone. We demonstrate that imperfect chemical equilibration of a mantle with pyrolitic composition is able to explain the global absence of P-wave reflections underneath the 660-km discontinuity. We also show that the maximum required temperature at 660 km depth globally is 2000 K. Finally, we found broad regions of elevated temperatures beneath the Pacific that are demarcated by major hotspots, suggesting that the transition zone is a source of secondary mantle plumes, in which only the hottest material tends to rise through.

 

Benoît Tauzin
Assistant Professor, Lyon University
ANU Visiting Fellow
Marie Sklodowska-Curie Fellow, European Commission
benoit.tauzin@univ-lyon1.fr