The mantle transition zone (TZ) is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions and an increase in viscosity with depth. Still, neither its seismic structure nor its dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth (‘410’ and ‘660’) are classically associated with phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. Resulting discontinuity topography 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. This study presents hypothetical realizations of TZ seismic structure and major discontinuities based on the temperature field of a published 3-D mantle circulation model 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 percent for pyrolite and MM, respectively. This proportion could be significantly higher (up to ∼39 percent) in a hotter mantle for pyrolite, but remains low (< 2 percent) 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.
<p>Mantle convection is primarily driven by gravitational forces acting on thermally buoyant structures in Earth's interior. The associated vertical stresses generate phases of uplift and subsidence of the surface, leaving observable traces in the geologic record. Utilizing new data assimilation techniques, geodynamic inverse models of mantle flow can provide theoretical estimates of these surface processes, which can be tested against geologic observations. These so-called mantle flow retrodictions are emerging as powerful tools that have the potential to allow for tighter constraints on the inherent physical parameters.</p> <p>To contain meaningful information, the inverse models require an estimate of the present-day buoyancy distribution within the mantle, which can be derived from seismic observations. By using thermodynamically self-consistent models of mantle mineralogy, it is possible to convert the seismic structure of global tomographic models to temperature. However, both seismic and mineralogical models are significantly affected by different sources of uncertainty and often require subjective modelling choices, which can lead to different estimated properties. In addition, due to the complexity of the mineralogical models, the relation between temperature and seismic velocities is highly nonlinear and not strictly bijective: In the presence of phase transitions, different temperatures can result in the same seismic velocity, further complicating the conversion between the two parameters.</p> <p>&#160;</p> <p>Using a synthetic closed-loop experiment, we investigate the theoretical ability to estimate the present-day thermal state of Earth's mantle based on tomographic models. The temperature distribution from a 3-D mantle circulation model with earth-like convective vigour serves as a representation of the "true" temperature field, which we aim to recover after a set of processing steps. These steps include the &#8220;forward and inverse&#8221; mineralogical mapping between temperatures and seismic velocities, using a thermodynamic model for pyrolite composition, as well as applying a tomographic filter to mimic the limited resolution and uneven data coverage of the underlying tomographic model. Owing to imperfect knowledge of the parameters governing mineral anelasticity, we test the effects of changes to the anelastic correction applied in forward and inverse mineralogical mapping. The mismatch between the recovered and the initial temperature field carries a strong imprint of the tomographic filter. Additionally, we observe systematic errors in the recovered temperature field in the vicinity of phase transitions. Our results highlight that, given the current limits of tomographic models and the incomplete knowledge of mantle mineralogy, amplitudes and spatial scales of a temperature field obtained through global seismic models will deviate significantly from the true state. Strategies to recover the present-day buoyancy field must be carefully selected in order to minimize additional uncertainties.</p>
<p>The mantle transition zone (TZ) is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions and an increase in viscosity with depth. Still, neither its seismic structure nor its dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth ('410' and '660') are classically associated with phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. Resulting discontinuity topography 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.</p> <p>This study presents hypothetical realizations of TZ seismic structure and major discontinuities based on the temperature field of a published 3-D mantle circulation model 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 &#8764;3 and &#8764;1 per cent for pyrolite and MM, respectively. This proportion could be significantly higher (up to &#8764;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 &#8212;with average deflections of 14 and 6 km, respectively&#8212; 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.</p>
<p>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 (&#8216;410&#8217; and &#8216;660&#8217;). 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 &#8216;660&#8217; at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified.</p><p>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.</p><p>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 &#8216;410&#8217; and &#8216;660&#8217; 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.</p><p>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 &#8216;660&#8217; in a pyrolite mantle and ~ 1 % in MM. Absolute base temperatures could however be higher (or lower) than predicted by the MCM&#8217;s geotherm. For different plausible background temperature fields the garnet proportion at the &#8216;660&#8217; 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 &#8216;660&#8217; might be even more complex than previously assumed.</p>
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