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Édith Kubik receives the 2022 CNFGG PhD Award

Édith Kubik received the 2022 Geophysics Prize for her PhD "Elemental and stable isotopic fractionations of siderophile elements – Implications for the delivery of Earth’s volatile elements".

Édith Kubik receives the 2022 CNFGG PhD Award

Publication date: 06/12/2022

Awards and Distinctions, Press, Research

Since 1988, the French National Committee for Geodesy and Geophysics (CNFGG) has awarded an annual PhD prize in geophysics to one or more young researchers who have defended outstanding PhDs in the field of geophysics, either for their fundamental or observational aspects or for their potential applications in society.

Experimental study of the Earth’s volatile delivery

Volatile elements and in particular water are intrinsically linked to the development of life as well as the onset of plate tectonics on Earth. The mechanism and timing of the terrestrial volatile delivery and the nature of the materials that carried them to Earth are highly debated topics.

Current scenarios suggest that the volatile elements could have been delivered to Earth in a continuous manner during the entirety of accretion, or on the contrary that they were brought at the very end of Earth’s formation, after the core–mantle differentiation was complete. In this second scenario, volatile elements were mixed with the terrestrial mantle without sustaining equilibration with the core, which means that their chemical signature in the mantle does not reflect core–mantle differentiation.

In this context, the behaviour of elements that are both volatile and affected by differentiation—elements that have an affinity for metallic iron, also called siderophile elements—can be used to distinguish between the proposed scenarios for the volatile delivery.

In this PhD work, I used experiments at high pressure and high temperature in the laboratory to reproduce the conditions of core formation. I measured the behaviour of volatile and siderophile elements (Sn, Cd, Bi, Sb and Tl) between metal and silicate with state-of-the-art tools in order to quantify the affinity of these elements for the core relative to the mantle.

Experiment performed in a piston cylinder press at high pressure and high temperature. In the centre, a metallic sphere is the analogue of the core and is surrounded by a silicate glass simulating the mantle. These two phases are contained in an experimental capsule made of magnesium oxide. © IPGP

I used these results to determine whether the signatures measured in the Earth’s mantle reflect a chemical equilibrium between core and mantle or alternatively a late addition of these elements during the accretional history. I parameterised the behaviour of these elements as a function of physical and chemical conditions (pressure, temperature, redox conditions and chemical composition), offering the possibility to predict their distribution between core and mantle at the conditions of Earth’s differentiation. I developed and used numerical models to recalculate the conditions of the core–mantle equilibrium at each step of Earth’s formation and reconstructed the distribution of volatile elements by simulating all the scenarios previously proposed.

The results of this work show that the studied elements possess a very strong affinity for the metallic core. This suggests that a homogeneous accretion would cause a depletion of these elements in the mantle by segregation into the core. A scenario of volatile accretion during the later phases of the accretion is necessary to explain the relatively high abundances of volatile and siderophile elements present in the terrestrial mantle.

Terrestrial volatile delivery model including all the findings reported in this PhD work (in bold). In this scenario, the volatile elements are accreted during the last 10–20% of the Earth’s accretion, by a giant impact possibly corresponding to the Moon-forming impact and of chondritic composition (carbonaceous chondrites). The core–mantle differentiation occurs by partial equilibration of the two reservoirs. A late veneer of 0.5% of Earth’s mass is accreted post-differentiation. The sulphur budget of the core is estimated at ca. 3wt.%. © IPGP
Terrestrial volatile delivery model including all the findings reported in this PhD work (in bold). In this scenario, the volatile elements are accreted during the last 10–20% of the Earth’s accretion, by a giant impact possibly corresponding to the Moon-forming impact and of chondritic composition (carbonaceous chondrites). The core–mantle differentiation occurs by partial equilibration of the two reservoirs. A late veneer of 0.5% of Earth’s mass is accreted post-differentiation. The sulphur budget of the core is estimated at ca. 3wt.%. © IPGP
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