Frédéric Deschamps

Pdiff and SHdiff waves, diffracted along the core‐mantle boundary (CMB), are highly sensitive to the structures of the lowermost mantle (LMM). Using an automated procedure, we measure the absolute traveltime residuals (, ) from cross‐correlation ( cc ) between observations and synthetics for 2.02 million Pdiff + P and 1.96 million SHdiff + SH waves in multiple period bands. The gridded mean and , and , exhibit a degree two pattern consistent with the Large Low Shear‐Velocity Provinces (LLSVPs) observed in global seismic tomography models beneath Africa and the Pacific. Furthermore, the are 2–3 times stronger than , and under the African LLSVP are stronger than under the Pacific LLSVP by a factor of 2–3. The distribution of the ratio shows a broad, dispersive pattern, with the majority of the population between −2 and +5, in contrast to purely thermal effects, which predicted a ratio in the range of 1.7–2.6. Finally, the distribution is skewed to low velocity, while the distribution is Gaussian. Collectively, these observations point to thermochemical LLSVPs.

Heat flux at the Earth’s core-mantle boundary (CMB) partially controls the outer core dynamics and its associated geodynamo. On the mantle side, lateral variations in temperature above the CMB trigger lateral variations in heat flux with low temperature (typically, in and around subducted slabs) and high temperatures (at plumes roots and beneath hot thermo-chemical piles) areas being associated with high and low heat flux regions, respectively. Spatial and temporal variations in temperature are, in turn, controlled by details of mantle convection and mantle material properties. Here, we investigate the influence on CMB heat flux of two key parameters: the excess internal heating within piles of hot, dense material (also referred to as primordial material) modelling the large low shear-wave velocity provinces (LLSVPs) observed by global seismic tomography maps; and the temperature-dependence of thermal conductivity. For this, we perform a series of high-resolution numerical simulations of thermo-chemical convection in spherical annulus geometry using the code StagYY. Importantly, the total heating rate within the mantle is fixed, meaning that an excess heating within piles is balanced by a reduced heat released elsewhere. The initial condition on composition consists in a thin basal layer of chemically denser material, which subsequently evolves into piles of hot, primordial material on the top of which plumes are being generated. Our simulations show that the CMB heat flux is lower than the core adiabatic heat flux throughout the base of primordial material piles, and that it can be locally negative, i.e., heat flows from the mantle to the core. We further investigated the conditions needed for such patches to appear. As one would expect, a larger internal heating excess and a stronger temperature dependence of thermal conductivity both favor the development of negative heat flux patches. However, patches disappear if the piles excess heating gets too large. In this case, heat released in the regular mantle is strongly reduced, allowing plumes generated at the top of piles to extract more heat from these piles. Finally, our simulations predict relatively large CMB heat flux spatial heterogeneity, together with substantial temporal variations in this heterogeneity. Our findings have strong implications for core dynamics. In particular, they support the hypothesis that partial stratification at the top of the core can occur beneath LLSVPs, reconciling geomagnetic and seismic observations. In addition, and based on core dynamics studies, the CMB heat flux heterogeneity and temporal variations predicted by our simulations may play a key role in the occurrence of geomagnetic superchrons.

During the past 2 decades, data coverage and methodological developments have considerably improved the resolution of seismic tomography maps, refining our mapping of the deep Earth’s mantle structure. Nevertheless, the uneven distributions in sources (the earthquakes) and receptors (the seismic stations) leads to non-uniqueness of the solution and requires the prescription of a priori information (mostly damping and smoothing), the effect of which is to smear out seismic images and degrade their effective resolution. Alternatively, statistical quantities have been used to investigate the nature, purely thermal or thermo-chemical, of the structures observed by seismic tomography. In particular, it has long been recognized that the statistical distribution of shear-velocity anomalies (dlnVS) in the lowermost mantle shows some degree of asymmetry in the form of a slow velocity tail, and that this slow tail is associated with the large low shear-wave velocity provinces (LLSVPs), the prominent feature on lowermost mantle tomographic maps. This bimodal distribution appears from around 2200 km and persists towards the deeper mantle. Yet, the phase transition to post-perovskite (PPv) at depth ~2700 km, if not happens globally, implies a trimodal distribution for dlnVS. Here, we bring new insights on these questions. First, we investigate the effect of the seismic tomography ‘operator’ on seismic velocity anomalies triggered by different possible lowermost mantle thermo-chemical structures. For this, we first run simulations of thermal and thermo-chemical convection including or not the post-perovskite phase, and we calculate synthetic velocity anomalies predicted by these simulations. We then apply to these synthetic velocity anomalies a tomographic filter built for the tomographic model HMSL-SP06. We show that seismic signatures corresponding to different materials (regular mantle, thermo-chemical piles and PPv) are clearly distinct on statistical distribution of unfiltered shear-and compressional velocity anomalies, dlnVS and dlnVP, but get mixed or partially mixed after applying the filter. Interestingly, for synthetic velocity anomalies built from thermo-chemical simulations, a low velocity tail clearly appears on dlnVS histograms, but not on dlnVP histograms, similar to what is observed in real seismic tomography maps. For synthetic velocity anomalies built from purely thermal simulations, dlnVS histograms do not feature any low velocity tail, and distribution histograms for both dlnVS and dlnVP are fairly Gaussian. Overall, our results therefore support the hypothesis that the LLSVPs observed at the bottom of the mantle are composed of hot, chemically differentiated material. They further show that the mixing of seismic signatures due to tomographic filter, implying the statistical distribution of dlnVS and dlnVP may be richer and more complex than it appears to be from seismic tomography models. Acknowledging the mixing of seismic signatures inherent to tomography models, we then apply cluster analysis with trimodal distribution to four recent tomographic models: GLAD-M35, REVEAL, SPiRaL-1.4, and TX2019slab. We identify three velocity clusters, slow, neutral, and fast, which we associate with thermo-chemical piles, regular mantle, and PPv. Based on this analysis, we provide a probability map of the three clusters, which may be used to better understand the lowermost mantle structure and facilitate future geodynamic studies.

Europa is characterized by a thin ice I _h shell overlying a subsurface ocean and a large solid core. Estimates of the outer ice shell's thickness range from a few kilometers to several tens of kilometers, with strong implications for Europa's thermal and geological history. Here, we model the thermal evolution of Europa's ice shell using a parameterized convection approach that explicitly accounts for internal heat release. We explore changes in the thickness of this ice shell as a function of ice reference viscosity, tidal heating, and ocean composition. We further consider the effect of cyclical variations in tidal heating in response to changes in the eccentricity of Europa's orbit. Our calculations show that the ice shell thickness is mostly influenced by both the ice viscosity and tidal heating. While significant in the absence of tidal heating, the ocean composition has no or little influence when such heating is accounted for. For dissipated tidal power and viscosity around 1 TW and 10 ¹⁴ Pa·s, respectively, which are within the expected range of values for these parameters, our calculations predict an ice shell thickness in the range of 20–45 km including, at the top part of this shell, a stagnant lid ∼10 km in thickness. These values are in agreement with recent estimates of impact basin morphology. Our calculations further indicate that a 10% change in orbital eccentricity may trigger variations in the ice shell thickness of approximately 15 km, which further helps to reconcile estimates based on geological features and modeled thermal history.

The Earth’s core-mantle boundary (CMB) hosts structures and processes that strongly affect our planet’s thermal and chemical evolution. Spatial and temporal changes in CMB heat flux, Φ _CMB , influence core dynamics and its associated geodynamo. These variations, in turn, are controlled by details of mantle dynamics and may be affected by lowermost mantle structures, in particular the large low shear-wave velocity provinces (LLSVPs), which commonly interpreted as reservoirs of hot, chemically differentiated material. Here, we perform simulations of thermo-chemical convection that account for changes in thermal conductivity with temperature and for excess heating in piles of dense material, modelling LLSVPs, and show that beneath these piles, Φ _CMB can be locally negative, i.e., heat flows to the core. These patches do not appear if excess heating is too large. We further show that throughout the piles lateral extension, Φ _CMB remains lower than the adiabatic core heat flux, while slab arrival at the CMB triggers temporary high Φ _CMB spikes, increasing lateral heterogeneity in Φ _CMB . Our findings support the hypothesis that partial stratification occurs at the top of the core, reconciling present-day geomagnetic and seismic observations in this region. At timescales of 100s Myrs they may explain the onset and termination of superchrons.

Above Earth's core-mantle boundary (CMB), seismic studies revealed numerous enigmatic, small-sized patches of ultralow velocity zones (ULVZs) with anomalously lower velocities and higher density than ambient mantle. These regions may be enriched in Fe-rich oxides, and their thermal conductivity Ʌ would critically influence regional heat-flux and thermochemical evolution around CMB, but remains poorly constrained. Here we experimentally show that Ʌ of (Mg_0.75,Fe_0.25)O, Ʌ_Fp25, and (Mg_0.25,Fe_0.75)O, Ʌ_Fp75, both decrease across an iron spin-transition at 573 K, while such reduction is ~30-40% smaller than at room temperature, suggesting their Ʌ are less-affected across the spin-transition under deep-mantle's high temperatures. The temperature dependences of low-spin Ʌ_Fp25 and Ʌ_Fp75 (T ^-0.39 and T ^-0.23, respectively) are weaker than the conventional T ^-0.5 for high-spin state. If made of Fe-rich oxides (e.g., Fp75), ULVZs should have an ultralow thermal conductivity ( ~ 3.4 W m^-1 K^-1). Such strong thermal insulation enhances local temperature, vitalizing regional mantle dynamics and thermochemical evolution, and growth of thermal plumes. The significant Ʌ discontinuity across CMB would induce heterogeneous amplitude and pattern of CMB heat-flux, potentially impacting geodynamo and geomagnetic stability.

Earth's presently‐active magnetic field is thought to be generated by geodynamo via convection of liquid outer core, primarily composed of Fe alloyed with Ni and light elements, for example, Si. Core thermal conductivity critically controls its thermal evolution, dynamics, and available energy (thermal vs. compositional) powering the geodynamo over Earth's history. Here we precisely measured thermal conductivity of solid Fe _0.85 Ni _0.06 Si _0.09 (Fe–7wt%Ni–5wt%Si) to 148 GPa/2950 K and Fe to 155 GPa/3246 K, respectively. We show that a liquid outer core with Fe _0.85 Ni _0.06 Si _0.09 has a moderate thermal conductivity of ∼73 W m ⁻¹ K ⁻¹ , in the moderate zone over the previously reported broad range (∼20–250 W m ⁻¹ K ⁻¹ ) that includes various core compositions. Such a modest isentropic, conductive heat flow at core‐mantle boundary (CMB) (∼11 TW) is smaller than present‐day CMB heat flow (∼15 TW), suggesting a presently‐active thermally‐driven geodynamo and a maximum inner core age of ∼0.8–1.3 billion‐years.

Europa’s radial structure consists of a thin ice Ih shell overlying a subsurface ocean and a large solid core. The thickness of the outer ice shell is strongly related to Europa’s thermal and geological histories. Estimates of this thickness range from a few kilometers to several tens of kilometers, with values deduced from the analysis of surface geological features being on the lower end (a few kilometers and less), and values predicted by modelling thermal evolution being on the upper end (up to a few tens of kilometers). Here, we model the thermal evolution of Europa's ice shell using a parameterized convection approach that explicitly accounts for the release of heat (assumed to have a tidal origin) within this shell. We explore changes in the thickness of this ice shell depending on several parameters, including the bulk ice viscosity, amount of tidal heating, and ocean composition. We further consider possible cyclical variations in the amount of tidal heating in response to changes in the eccentricity of Europa’s orbit. Our calculations show that ice shell thickness is mostly influenced by both the ice bulk viscosity and tidal heating. While significant in absence of tidal heating, the ocean composition has no or few influence when tidal heating is accounted for. Interestingly, for dissipated tidal heat and viscosity around 1 TW and 1014 Pa·s, respectively, which are within the expected range of values for these parameters, our calculations predict an ice shell thickness in the range 15-45 km and, at the top of this shell, and a stagnant lid around 10 km in thickness, in agreement with recent estimates from impact basin morphology. Our calculations further indicate that a 10% change in orbital eccentricity may trigger variations in the ice shell thickness of approximately 15 km, which further helps to reconcile estimates based on geological features and modelled thermal history.

Heat flux at the Earth’s core-mantle boundary (CMB) partially controls the outer core dynamics and its associated geodynamo. On the mantle side, spatial and temporal variations in this flux are, in turn, controlled by details of mantle convection. Previous simulations of mantle dynamics showed that CMB heat flux may be locally negative, i.e., in these regions heat flows from the mantle to the core. Here, we investigate the conditions needed to generate such patches of negative CMB heat flux. For this, we perform a series of high-resolution numerical simulations of thermo-chemical convection in spherical annulus geometry using the code StagYY. The compositional initial condition consists in a thin basal layer of chemically denser material (alos referred to as primordial material), which subsequently evolves into piles of hot, primordial material, modelling the large low shear-wave velocity provinces (LLSVPs) observed on global seismic tomography maps. We more specifically explore the influence of two key parameters that promote temperature increase within the piles of primordial material: the excess internal heating within these piles ; and the temperature-dependence of thermal conductivity. We quantify the size and amplitude of negative heat flux patches depending on these parameters. As one would expect, a larger internal heating excess and a stronger temperature dependence of thermal conductivity both favor the development of negative heat flux patches within piles of dense material. However, these parameters also alter the piles stability, such that there is no straightforward relationship between them and the size and amplitude of the negative heat flux patches. Finally, we discuss possible consequences of our findings for core dynamics and geodynamo.