Mantle convection | EPFL Graph Search

Mantle convection is the very slow creeping motion of Earth's solid silicate mantle as convection currents carry heat from the interior to the planet's surface. The Earth's surface lithosphere rides atop the asthenosphere and the two form the components of the upper mantle. The lithosphere is divided into a number of tectonic plates that are continuously being created or consumed at plate boundaries. Accretion occurs as mantle is added to the growing edges of a plate, associated with seafloor spreading. Upwelling beneath the spreading centers is a shallow, rising component of mantle convection and in most cases not directly linked to the global mantle upwelling. The hot material added at spreading centers cools down by conduction and convection of heat as it moves away from the spreading centers. At the consumption edges of the plate, the material has thermally contracted to become dense, and it sinks under its own weight in the process of subduction usually at an ocean trench. Subd

Chemical Characterisation of Deep Earth Reservoirs

Hélène Marie Piet

Seismic imaging of the internal Earth has emphasized the inhomogeneity of its mantle. This interface layer of nearly 3000 km surrounding the central metallic core conducts heat released from the latter, which played a significant role in shaping the actual mantle as it currently is. The present state of the deep Earth (mainly the lower mantle and the core) is also a consequence of the physical properties of the material composing them. Large-low shear velocity provinces atop the core-mantle boundary, for example, are composed of a denser material than their surrounding environment but their origin, formation and survival over billions of years to mantle convection, is still largely debated. Here we use chemical partitioning to characterise the current composition of those deep Earth reservoirs and better understand the processes leading to their formation. In the first part, experiments were conducted in the laser-heated diamond anvil cell and the latest advances in microanalysis were used to refine iron partitioning between minerals of the lower mantle. We reconcile long-standing discrepancies in previous partitioning data and show that significant variations of silicate iron content are associated to changing ferric iron incorporation. We also note that such chemical variations might significantly affect the physical properties of the silicate, which might in turn provoke mantle rheology variations. Our results and observations thus provide a mineral physics based argument for the stabilization of deep Earth reservoirs and especially large structures like large low-shear-velocity provinces (LLSVPs). In the second and numerical part of the thesis, previous results on siderophile (iron-loving) element partitioning between metal and silicate were used to constrain some aspects of core formation. We constrain the chemical and physical state of the magma ocean during core formation explaining both the siderophile composition of the mantle and the light element composition of the core. We simulate the Moon-Forming Giant Impact in the last steps of Earth accretion, where we constrain the size of the impactor to be smaller than 15% of Earth's total mass, a roughly Mars-sized object, and further show that melting of the large fractions of the mantle upon impact by the giant impactor is consistent with siderophile composition of the bulk silicate Earth, which can have great implications for the geodynamical evolution of the mantle. In conclusions, given the fact that the mantle may have been almost fully molten, further crystallization may have isolated a layer of liquid primitive material at the base of the mantle possibly explaining the seismic signature of the LLSVPs associated with a strong thermochemical contrast with the surrounding mantle. The layered composition in iron of the actual mantle also provides a chemical argument for a viscously layered mantle, which could have helped sustain LLSVPs for billions of years from efficient convection.

EPFL2016

Density currents driven by differential cooling in lakes: occurrence, dynamics and implications for littoral-pelagic exchange

Tomy Doda

Cross-shore flows exchange water laterally in lakes, with ecological implications for the ecosystem. One example is the convective circulation induced by differential cooling, also known as the thermal siphon. This lateral flow forms when the sloping sides of lakes experience surface cooling. Shallow areas cool faster, which generates lateral density gradients and drives a cold downslope gravity current and an onshore surface flow. The role of this two-layer-circulation in lateral exchange in lakes is poorly understood, due to the lack of long-term and high-resolution measurements of thermal siphons. This thesis aims at filling this gap by investigating the occurrence and dynamics of thermal siphons from extensive in situ observations in a wind-sheltered Swiss lake (Rotsee).We first quantified the seasonality of thermal siphons from one-year-long observations in Rotsee. Thermal siphons occurred on a daily basis from late summer to winter and flushed the littoral region in ~ 10 h. Their duration increased but their intensity decreased over the same period. We linked this seasonality to the change in forcing conditions by testing scaling relationships from theoretical and laboratory-based studies.We then focused on the short-term temporal variability of the transport by quantifying the dynamics of thermal siphons over a diurnal cycle and their interaction with convective plumes. Our results reveal that convective plumes penetrated into the gravity current at night and eroded its upper interface. This vertical mixing generated vertical interface fluctuations and reduced the lateral transport at night. The maximal transport was delayed to daylight conditions when radiative heating weakened penetrative convection.After having quantified the physical water transport, we assessed the role of thermal siphons in the lateral exchange of dissolved gases. We found that both branches of the circulation were capable of transporting gases laterally. The downslope current brought littoral gases to the base of the mixed layer in the stratified region whereas the surface flow transported gases towards the shore. We quantified this exchange for oxygen and methane.Finally, we generalized our observations to other lakes with different bathymetry and forcing conditions. The frequent occurrence of thermal siphons observed in six lakes confirmed that this lateral transport process is ubiquitous in lakes with shallow littoral regions. We showed that our results from Rotsee were applicable to other systems, where the same scaling relationships predicted the formation and intensity of thermal siphons. Based on these results, we proposed a procedure to predict the contribution of thermal siphons for lateral transport in any lake.This thesis provides a comprehensive understanding of the formation and dynamics of thermal siphons and paves the route for integrating this lateral transport process in lake ecosystem research.

EPFL2022

Spin state transition and partitioning of iron: Effects on mantle dynamics

James Badro

Experimental studies at pressure and temperature conditions of the Earth's lower mantle have shown that iron in ferropericlase (Fp) and in Mg-silicate perovskite (Pv) undergoes a spin state transition. This electronic transition changes elastic and transport properties of lower mantle minerals and can play an important role in mantle convection. Here we focus on the geodynamic effect of the spin-induced density modifications caused by the volume collapse of Fp and by the variation of Fe partitioning (KPv-FP) between Fp and Pv. Since KPV-FP behavior strongly depends on alumina content, we explore two end-member compositions, one Al-bearing (with 4.7 wt% Al2O3 in Pv) and the other Al-free. We use the theoretical model by Sturhahn et al. (2005) to calculate the spin configuration of Fp over a range of pressure-temperature conditions, and use experimental results to model Fe partitioning. We then apply the Mie-Gruneisen-Debye equation of state to obtain the density of the mineral assemblages. The calculated amplitude of the density change across the spin state transition is less than 1%, consistent with experiments by Mao et al. (2011); our density profiles differ from PREM by less than 1.5%. The spin-induced density variations are included in a three dimensional convection code (Stag3D) for a compressible mantle. We find small temperature differences between models with and without spin state transitions, since over billions of years the relative temperature difference is less than 50 K. However the relative RMS vertical velocity difference is up to 15% for an Al-free system, but only less than 6% for an Al-bearing system. (C) 2015 Elsevier B.V. All rights reserved.