Upper mantle shear-wave velocity conversions to temperature and density: open-source V2RhoT_gibbs sheds light into challenges and possibilities

Variations of seismic wave speed, in particular shear-wave velocity (v_s), are largely temperature controlled in the Earth’s mantle. Seismic tomography models thus provide us with insights into upper mantle temperature variations, the rheological configuration of the deep lithosphere and, thus, zones susceptible to strain localization within tectonic plates. With this contribution, we introduce V2RhoT_gibbs, an open-source Python tool for converting v_s from upper mantle seismic tomography models to temperature and density in a self-consistent thermodynamic manner.

Our conversion approach utilizes an open-source Gibbs-free energy minimization algorithm (Perple_X), which computes the thermodynamically stable phase and mineral assemblages for a given mantle chemical composition (in terms of major oxides of the (Na2O-)CaO-FeO-MgO-Al2O3-SiO2 system) and a wide range of pressures and temperatures. Users of our tool can choose from different thermodynamic databases to constrain the Gibbs-free energy minimization and thus produce lookup tables for upper mantle pressure-temperature conditions and the associated variations in modal composition and simultaneously calculated bulk rock physical properties (e.g., seismic velocities and thermal conductivities). V2RhoT_gibbs is developed to analyze these lookup tables and hence consider the complex, non-linear relations between v_s, temperature and mechanical properties. The tool corrects the pre-calculated anharmonic seismic velocities for anelastic attenuation effects and partial melts, and finally allocates thermodynamically consistent values for temperature and density to the v_s‑depth-pairs in upper mantle tomographic models.

We will illustrate and discuss the differences between different chemical compositions, representing various degrees of upper mantle depletion, with respect to their effects on the v_s-converted temperature and density fields. In addition, we will show V2RhoT_gibbs conversion results to discuss derived depth variations of the thermal lithosphere-asthenosphere boundary for different tectonic settings (convergent and divergent plate boundaries). Finally, we will discuss challenges and possible solution strategies regarding the interpretation of v_s variations in the shallowest upper mantle.