Geodynamic predictions of seismic structure and discontinuity topography of the mantle transition zone

The mantle transition zone (TZ) is expected to influence convective flow, but neither its structural characteristics nor dynamic effects have been conclusively constrained. Lateral temperature variations modulate the topography of associated seismic discontinuities at approximately 410 and 660 km depth (‘410’ and ‘660’). These discontinuities are related to mineral phase transitions and thus also sensitive to composition. Consequently, discontinuity topography can potentially provide insight on temperature and even phase composition at depth. It has been recognized that, in addition to phase transitions in olivine polymorphs, the transition of garnet to lower mantle minerals may impact particularly the ‘660’ at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified.

We address this question by predicting synthetic seismic structure and discontinuity topography of the TZ based on the temperature field of a 3-D mantle circulation model (MCM) for a range of relevant bulk compositions and associated mineralogy models. The models differ in complexity in terms of the number of incorporated oxide-components and include pyrolite, depleted mantle and mechanical mixing (MM) models. We thus create a suite of relevant hypothetical realizations of TZ seismic structure and major discontinuities.

Our theoretical approach allows us to systematically investigate the effects of varying mineralogy, in combination with a dynamically constrained temperature field, on TZ structure. We explicitly relate major phase transitions as given by the mineralogical tables to specific topographic features of the ‘410’ and ‘660’ and quantify the relative impact of the different phases. Analyzing a number of statistical measures for our synthetic discontinuity topographies provides theoretical predictions on possible distribution and magnitude of real-world depth variations. Our study thus provides a framework for dynamically informed interpretations of seismically derived TZ structure in terms of mantle temperature and composition. It moreover gives insights on the potential dynamic behavior of the TZ by constraining the importance of garnet in our theoretical models.

We find that garnet only occurs in regions with excess temperatures above 150 - 300 K, depending on phase composition. This leads to ~ 3 % garnet at the ‘660’ in a pyrolite mantle and ~ 1 % in MM. Absolute base temperatures could however be higher (or lower) than predicted by the MCM’s geotherm. For different plausible background temperature fields the garnet proportion at the ‘660’ could vary between ~ 1 and 39 % in pyrolite, while remaining largely unaffected in MM. Since not all warmer than average but only the hottest mantle regions see the garnet transition, dynamic effects of the ‘660’ might be even more complex than previously assumed.