Periodic Reporting for period 2 - SEPtiM (Solidification of Earth's Primitive Mantle)
Reporting period: 2023-03-01 to 2024-08-31
Earth’s mantle was extensively molten in its first 100 million years. Its solidification left a strong compositional and structural imprint on the mantle, still observable today in the geochemical and geophysical record. Isotopic variations in basalts and ancient crust reveal the existence of one or more mantle reservoirs that formed early in Earth’s history, and never fully remixed. Seismic tomography reveals large, chemically distinct, thermochemical structures in the lowermost mantle, anchored to the source of these anomalous basalts. These observations leave no doubt that the mantle still bears differentiated regions that preserve signatures of its solidification. However, their formation mechanisms and compositional evolution are still unknown. Unravelling those opens a new window in understanding the early stages of Earth and planetary evolution.
The goal of project SEPtiM is to understand how the Earth’s primitive molten mantle solidified through time. This is done through a multidisciplinary effort where we (1) carry out experiments to measure and then model the phase diagrams to know how solids form as a function of depth, and what their composition is, along with their densities with respect to melts. Then we include this in a novel two-phase (melts and solids) flow geodynamical code that we develop in 2D and in 3D that integrates the thermodynamics, the phase diagrams, element partitioning and densities to obtain a holistic approach of the problem.
The goal of project SEPtiM is to understand how the Earth’s primitive molten mantle solidified through time. This is done through a multidisciplinary effort where we (1) carry out experiments to measure and then model the phase diagrams to know how solids form as a function of depth, and what their composition is, along with their densities with respect to melts. Then we include this in a novel two-phase (melts and solids) flow geodynamical code that we develop in 2D and in 3D that integrates the thermodynamics, the phase diagrams, element partitioning and densities to obtain a holistic approach of the problem.
1) High pressure and temperature laser heated diamond cell experiments
We have measured for the first time the melting phase diagram and melt relations of the Earth’s mantle at deep (1500 km depth to the core-mantle boundary) mantle conditions and this was published early on. We since have covered all the depths of the mantle (paper in preparation), and we now know the melting relations and phase diagram on the molten mantle from core to crust. We can use it to calculate the solidification sequence at all depths in the mantle, and obtain the composition of the solids, melts, and their evolution during the process. This was one of the overarching goals of the project.
We have shown that Mars just like Earth has known a basal magma ocean and the Martian mantle has solidified from the top towards the bottom. We have also shown that iron isotopes will be fractionated differently in the Earth (or any large planet) than in the Moon (and any smaller body), because of two competing processes taking place in the magma ocean: redox vs. pressure and temperature.
These experiments and models are major breakthroughs since no one was able to carry out such measurements in those conditions relevant to the deep Earth; Now we are able to study the entire molten silicate Earth down to the core.
2) Geodynamics and geodynamical modelling and simulations
Our major achievements are the undertaking of a code that for the first time uses 2 phase flow theory to investigate differentiation at the planetary scale in a large planet like the earth, while taking into account thermodynamics and experimental constraints. This is called a self-consistent approach. We have shown that the final stage of mantle solidification always results a basal magma ocean, with the last residues of silicate melts lying above the core mantle boundary, regardless of starting hypotheses, and regardless of where the liquidus and adiabat meet, which until now was the standing paradigm. We show that this is irrelevant, shifting that paradigm for the Earth and all other terrestrial planets. We applied it to Mars because we had an opportunity with the end of the NASA/ESA InSight mission, and had two papers out of it in which we show that Mars also like the Earth has this final stage, but that in Mars that BMO is still present to this day and is compatible with seismic InSight data, while a non-BMO homogeneous Martian mantle is not.
We have measured for the first time the melting phase diagram and melt relations of the Earth’s mantle at deep (1500 km depth to the core-mantle boundary) mantle conditions and this was published early on. We since have covered all the depths of the mantle (paper in preparation), and we now know the melting relations and phase diagram on the molten mantle from core to crust. We can use it to calculate the solidification sequence at all depths in the mantle, and obtain the composition of the solids, melts, and their evolution during the process. This was one of the overarching goals of the project.
We have shown that Mars just like Earth has known a basal magma ocean and the Martian mantle has solidified from the top towards the bottom. We have also shown that iron isotopes will be fractionated differently in the Earth (or any large planet) than in the Moon (and any smaller body), because of two competing processes taking place in the magma ocean: redox vs. pressure and temperature.
These experiments and models are major breakthroughs since no one was able to carry out such measurements in those conditions relevant to the deep Earth; Now we are able to study the entire molten silicate Earth down to the core.
2) Geodynamics and geodynamical modelling and simulations
Our major achievements are the undertaking of a code that for the first time uses 2 phase flow theory to investigate differentiation at the planetary scale in a large planet like the earth, while taking into account thermodynamics and experimental constraints. This is called a self-consistent approach. We have shown that the final stage of mantle solidification always results a basal magma ocean, with the last residues of silicate melts lying above the core mantle boundary, regardless of starting hypotheses, and regardless of where the liquidus and adiabat meet, which until now was the standing paradigm. We show that this is irrelevant, shifting that paradigm for the Earth and all other terrestrial planets. We applied it to Mars because we had an opportunity with the end of the NASA/ESA InSight mission, and had two papers out of it in which we show that Mars also like the Earth has this final stage, but that in Mars that BMO is still present to this day and is compatible with seismic InSight data, while a non-BMO homogeneous Martian mantle is not.
1) High pressure and temperature laser heated diamond cell experiments
The present limitation on experimental determination of melt relations and melt phase diagrams corresponds to the pressure limit of the multi-anvil press using tungsten carbide anvils. This is a routine experimental protocol in large-volume presses, but a similar protocol has remained technically unmanageable in the laser-heated diamond anvil cell (LHDAC), the only tool reaching higher pressures and temperatures to cover all plausible conditions of magma ocean crystallisation on Earth, for two reasons. The first stems from the inability to accurately control experimental conditions in the LHDAC at extreme conditions, specifically the precise control of temperature required to slowly crystallise the melt and attain controlled degrees of crystallisation (e.g. fractions of residual melt). Secondly, the size of the phases produced in the LHDAC is nanometric, so that the analysis of the run products requires the use of high-resolution analytical techniques that enable at least a 100-fold gain in spatial resolution.
Now we have undertaken a novel approach to dridge that gap, and establish phase diagrams and melting relations in the LHDAC. Our protocol consists in carrying out a series of experiments where samples are molten at a fixed pressure and then slowly cooled down and fractionally crystallised to various degrees in a narrow temperature range, a feat deemed impossible just two years ago in the LHDAC. Using FIB and TEM, we track the crystallisation sequence of silicate melts, and are able to unambiguously determine the mineralogy and composition of liquidus phase, e.g. the first mineral phase to crystallise in a cooling magma ocean, as well at the composition of the residual liquid. As crystallisation proceeds, the liquid line of descent follows a cotectic melt composition while crystallising equilibrium phases, whose mineralogy and composition are also measured, thus delineating the melting phase diagram. With this phase diagram in hand, we can calculate (at any given pressure, or depth, inside the Earth's mantle) the composition, modal abundance, and evolution of solids and liquids as temperature drops and they start solidifying. We have also provided a new method based on non-negative matrix factorization-aided phase unmixing to quantify trace element concentrations down to 100 ppm level in our LHDAC samples, paving the way to full geochemical characterisation using our expeirmental advances.
2) Geodynamics and geodynamical modelling and simulations
Recent mathematical and numerical advances now allow to extend the multiphase flow formalism to geodynamic regimes appropriate for magma ocean solidification dynamics with vigorous convection (expressed by the thermal Rayleigh number > 109) and high melt fraction (from 0 to 100\%). We used that to create a new code, in which we model the solid-liquid multiphase physics using a numerical implementation of a multiphase flow mathematical formalism based on the averaging method under the assumption of infinite Prandtl number and Boussinesq approximation. Our code accounts for thermochemical convection in both the liquid and solid states, solid-melt phase equilibrium, mineralogical phase change and solid-melt phase separation. Convective motion in a solidifying magma ocean is driven by three types of density differences. These originate from thermal expansivity (due to temperature differences), compositional differences (due to changes in iron content) and phase changes (due to the varying melt fraction). An additional mechanism that generates motion in a partially molten convective medium is the solid-liquid phase separation driven by shear deformation, in addition to the density contrasts between the melt and the solid. Phase separation is limited by matrix deformation, i.e. compaction, and viscous friction between the melt and the solid via Darcy’s law. We implemented a depth-dependent density contrast between liquid and solids, and used a thermodynamically self-consistent compositional evolution of melts and solids based on our (WP1) experimental melting phase diagrams.
The present limitation on experimental determination of melt relations and melt phase diagrams corresponds to the pressure limit of the multi-anvil press using tungsten carbide anvils. This is a routine experimental protocol in large-volume presses, but a similar protocol has remained technically unmanageable in the laser-heated diamond anvil cell (LHDAC), the only tool reaching higher pressures and temperatures to cover all plausible conditions of magma ocean crystallisation on Earth, for two reasons. The first stems from the inability to accurately control experimental conditions in the LHDAC at extreme conditions, specifically the precise control of temperature required to slowly crystallise the melt and attain controlled degrees of crystallisation (e.g. fractions of residual melt). Secondly, the size of the phases produced in the LHDAC is nanometric, so that the analysis of the run products requires the use of high-resolution analytical techniques that enable at least a 100-fold gain in spatial resolution.
Now we have undertaken a novel approach to dridge that gap, and establish phase diagrams and melting relations in the LHDAC. Our protocol consists in carrying out a series of experiments where samples are molten at a fixed pressure and then slowly cooled down and fractionally crystallised to various degrees in a narrow temperature range, a feat deemed impossible just two years ago in the LHDAC. Using FIB and TEM, we track the crystallisation sequence of silicate melts, and are able to unambiguously determine the mineralogy and composition of liquidus phase, e.g. the first mineral phase to crystallise in a cooling magma ocean, as well at the composition of the residual liquid. As crystallisation proceeds, the liquid line of descent follows a cotectic melt composition while crystallising equilibrium phases, whose mineralogy and composition are also measured, thus delineating the melting phase diagram. With this phase diagram in hand, we can calculate (at any given pressure, or depth, inside the Earth's mantle) the composition, modal abundance, and evolution of solids and liquids as temperature drops and they start solidifying. We have also provided a new method based on non-negative matrix factorization-aided phase unmixing to quantify trace element concentrations down to 100 ppm level in our LHDAC samples, paving the way to full geochemical characterisation using our expeirmental advances.
2) Geodynamics and geodynamical modelling and simulations
Recent mathematical and numerical advances now allow to extend the multiphase flow formalism to geodynamic regimes appropriate for magma ocean solidification dynamics with vigorous convection (expressed by the thermal Rayleigh number > 109) and high melt fraction (from 0 to 100\%). We used that to create a new code, in which we model the solid-liquid multiphase physics using a numerical implementation of a multiphase flow mathematical formalism based on the averaging method under the assumption of infinite Prandtl number and Boussinesq approximation. Our code accounts for thermochemical convection in both the liquid and solid states, solid-melt phase equilibrium, mineralogical phase change and solid-melt phase separation. Convective motion in a solidifying magma ocean is driven by three types of density differences. These originate from thermal expansivity (due to temperature differences), compositional differences (due to changes in iron content) and phase changes (due to the varying melt fraction). An additional mechanism that generates motion in a partially molten convective medium is the solid-liquid phase separation driven by shear deformation, in addition to the density contrasts between the melt and the solid. Phase separation is limited by matrix deformation, i.e. compaction, and viscous friction between the melt and the solid via Darcy’s law. We implemented a depth-dependent density contrast between liquid and solids, and used a thermodynamically self-consistent compositional evolution of melts and solids based on our (WP1) experimental melting phase diagrams.