Geodynamic-mineralogical predictions of mantle transition zone seismic structure

A main objective in geodynamics is to create models that provide quantitative information to other Earth science disciplines. In order to assess the validity of the underlying assumptions and chosen input parameters related to different geodynamic hypotheses, it is crucial to test these models against observations. In this, thermodynamic models of mantle mineralogy represent an essential tool. On the one hand, they enable the linking of temperature fields from mantle circulation models (MCMs) to seismic observations. On the other hand, they provide critical information on material behaviour in response to changing temperature and pressure conditions that occur over time within such mantle convection simulations. Some of the most interesting aspects in this context relate to mineral phase transitions and associated dynamic effects on mantle flow.

The mantle transition zone (TZ) in particular is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions as well as an increase in viscosity with depth. Still, neither its seismic structure nor the associated dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth (‘410’ and ‘660’) have classically been related to phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. The resulting topography of these discontinuities is seismically visible and can thus potentially provide insight on temperature and phase composition at depth. Besides the olivine phase changes, the disassociation of garnet may additionally impact the 660 at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified.

Here, we present hypothetical realizations of TZ seismic structure and major discontinuities based on the 3-D temperature field of a published MCM for a range of relevant mineralogies, including pyrolite and mechanical mixtures (MM). Systematic analysis of these models provides a framework for dynamically informed interpretations of seismic observations and gives insights into the potential dynamic behaviour of the TZ. Using our geodynamic-mineralogical approach we can identify which phase transitions induce specific topographic features of 410 and 660 and quantify their relative impact. Areal proportions of the garnet transition at the 660 are ∼3 and ∼1 per cent for pyrolite and MM, respectively. This proportion could be significantly higher (up to ∼39 per cent) in a hotter mantle for pyrolite, but remains low (< 2 per cent) for MM. In pyrolite, both slabs and plumes are found to depress the 660 —with average deflections of 14 and 6 km, respectively— due to the influence of garnet at high temperatures indicating its complex dynamic effects on mantle upwellings. Pronounced differences in model characteristics for pyrolite and MM, particularly their relative garnet proportions and associated topography features, could serve to discriminate between the two scenarios in Earth.