Redox state of deep Earth through its history – OxyDeep

The project requires advanced experimental techniques and, in particular, performing high P and T. We will use both resistive heating in the large volume press (LVP) and the laser-heating in the diamond anvil cell (LH-DAC) for P-T conditions up to (25 GPa-2500 K) and (135 GPa-5000 K), respectively. Such conditions cover P-T ranges of core-mantle interactions in early Earth and at present time. The most extreme pressures are necessary to reproduce the conditions in the D«-layer in the lowermost mantle. Variable redox conditions will be established on the basis of the composition of the starting materials. Sample properties will be investigated using a series of characterization techniques including: X-ray diffraction (XRD) in the LVP and LH-DAC; X-ray absorption fine spectroscopy (XAFS); X-ray fluorescence technique (XRF).

Concerning Task 1: Core-mantle interactions in the primitive Earth and at the present-day core-mantle boundary, two experimental studies were already performed: (i) Partitioning of sulphur between liquid Fe-rich metals and silicates was investigated at temperatures from 1800°C to 2400°C, pressures from 2 to 23 GPa and oxygen fugacities from 3.5 to 1.5 log units below the iron-wüstite buffer. (ii) Partition coefficients of Sm and Nd between molten chondritic-type mantle and various Fe-alloys. (iii) In a third study, we now investigate the role of volatile elements, in particular water, on the core-mantle partitioning of various geochemical tracers.
Concerning Task 2: Role of magma ocean crystallization on the inherited mantle heterogeneities, we performed two complementary studies: (i) assessing how much ferric iron can be incorporated in bridgmanite during magma ocean crystallization. (ii) we detected the onset of melting of samples of carbonaceous chondrite composition, at pressure and temperature conditions relevant to mantle depths up to 1000 km. (iii) In a third study, we initiated a geodynamical modelling of the cooling and crystallization of the magma ocean.
Concerning Task 3: Role of the D”-layer in global mantle dynamics, we performed an original study to determine the chemical reactions taking place today at the core-mantle boundary (CMB). Using the laser-heated diamond anvil cell, we used µ-XANES at the ID24 beamline of ESRF to determine the Fe behavior in lowermost mantle minerals.

We pursue the program outlined in the results section to confirm our original findings and get prepared for writing publications.

1. Boujibar et al. (2014) Metal-silicate partitioning of sulphur and thermodynamical constraints on planetary accretion, Earth and Planetary Science Letters, 391, 42-54
2. Bouhifd et al. (2015) Superchondritic Sm/Nd ratio of the Earth: Impact of Earth’s core formation, Earth and Planetary Science Letters, 413, 158-166

Oxygen is the key element that regulates the Earth (and other planets) dynamics because it allows for abundant elements (Fe, Si, S, H) to change valence and therefore their distribution between the Earth’s major reservoirs (core, mantle, crust, hydrosphere). The role of iron is major since it can change its oxidation state from Fe0 to Fe2+ and Fe3+. Early in history, a vertical layering in oxygen concentration is inherited from core-mantle differentiation, magma ocean crystallization, and formation of the crust. Consequently, the different Earth's reservoirs have distinct mineralogical contents as well as different redox states. The Earth is a tectonically active planet and its materials are recycled over geological times: oxidized material from the surface returns to the mantle in subduction zones; deep material raises to the surface following convection movements and form numerous hot spots; the oceanic crust deepens in subduction processes, reaches the core mantle boundary and eventually interacts with the outer core; even the inner-core crystallization is an issue for the Earth's redox state since its progressive growth induces rejection of light elements, including O, to the outer core. This induces mixing of material originating from different parts of the Earth. Interactions between reservoirs are expected to be firstly dominated by redox reactions, potentially associated with O-exchanges at the interfaces. These redox reactions are not well established and their consequences remain largely unknown. A better understanding of these reactions is critical for understanding the generation of the different Earth’s reservoirs as well as the history of this planet as a whole.

The OxyDeep project is planned over 36 months. We aim at reproducing and characterizing the phase transformations such that take place in the early Earth as well as at present time in the deep Earth. We will track changes in redox state, chemical reactions and partial melting. The project requires advanced experimental techniques and, in particular, performing high P and T. We will use both resistive heating in the large volume press (LVP) and the laser-heating in the diamond anvil cell (LH-DAC) for P-T conditions up to (25 GPa-2500 K) and (135 GPa-5000 K), respectively. Such conditions cover P-T ranges of core-mantle interactions in early Earth and at present time. The most extreme pressures are necessary to reproduce the conditions in the D"-layer in the lowermost mantle. Variable redox conditions will be established on the basis of the composition of the starting materials. Sample properties will be investigated using a series of characterization techniques including: X-ray diffraction (XRD) in the LVP and LH-DAC; X-ray absorption fine spectroscopy (XAFS); X-ray fluorescence technique (XRF).

The team is composed of members of the LMV plus external researchers with whom the LMV-members established long-term scientific collaborations for several years. We share complementary expertise and common interests. The expertise encompasses both experimental techniques and scientific areas. To realize this project, we request ½ PhD position (co-financed by Region Auvergne) and 1 post-doc position. The PhD research work will be dedicated to the "Redox properties in the early Earth". It will yield new results on the Early core-mantle interactions as well as on the magma ocean crystallization. The Post-doc subject will be dedicated to the "redox properties of silicates in the deep mantle". He will address the redox state in the D"-layer and the exchange of material occurring at the core-mantle boundary.