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High precision isotopic measurements of heavy elements in extra-terrestrial materials: origin and age of the solar system volatile element depletion

Periodic Reporting for period 4 - PRISTINE (High precision isotopic measurements of heavy elements in extra-terrestrial materials: origin and age of the solar system volatile element depletion)

Reporting period: 2019-10-01 to 2021-09-30

The objective of this project is to understand the origin of volatile elements in the terrestrial planets. Volatile elements controls many fundamental characteristics of the terrestrial planets, including their ability to develop and sustain life as well as the geochemical properties that make each planet unique. For example, the abundance of volatiles in the Earth’s mantle controls the rheological properties of rock central to mantle convection and is possibly the driving force behind plate tectonics. In addition, the presence of volatiles in the Earth’s outer core creates a chemical convection, which is in part responsible for the Earth’s magnetic field.
Despite the key role volatiles play in an understanding of the terrestrial planets, the origin of volatile elements has historically been a central problem in Earth sciences. The major difficulty is that the terrestrial planets formed in the inner solar system close to the young Sun where temperatures were too high (>1000K) for volatiles to condense. How to explain the presence of volatiles in the inner solar system has been and is still a subject of intense debate, but two main theories exist. It has historically been proposed that the Earth accreted devoid of volatiles (the “dry” Earth hypothesis), and that all volatiles were brought to Earth after the main stage of planetary accretion and differentiation (i.e. 100-200 Myrs after the formation of the solar system). An alternative explanation is that the volatiles were added to the Earth early, during the main stage of planetary formation, and that the Earth accreted “wet”.
Volatilization is known to fractionate isotopes while condensation only fractionates isotopes in some very specific cases; therefore, comparing the isotopic compositions of volatile elements is a very powerful tool to understand the origin of volatile element abundance variations and therefore test the different hypothesis (evaporation or partial condensation). In other words, we are using the isotopic composition of some specific elements as a signature of the origin of their depletion.
In this project we are developing novel analytical techniques for measuring stable isotope ratios at high precision in terrestrial, meteoritical and lunar samples of well selected elements (e.g. rubidium, gallium) which have different geochemical properties (siderophile, chalcophile, lithophile). In order to interpret the isotopic data that we will measure, we also have to calibrate the possible source of isotopic variations by performing laboratory experiments (e.g. metal/silicate fractionation to simulate core formation of heating experiments to simulate volatilization). In addition, we will date the timing of volatile depletion by using a radioactive element, 87Rb that decay to 87Sr and apply that to lunar samples in order to determine the timing of volatile depletion in the early solar system.
In this project we have developed new high precision isotopic systems that we applied to terrestrial and extra-terrestrial samples. Isotopic ratios record the signature of specific physical processes and via the measurements in natural samples they can be used to decipher the history of the natural samples and the relation to the history of their parent bodies (e.g. Mars, the Moon).

Along the course of this project we have developed the first high precision isotopic measurements of gallium, rubidium, zirconium , tin , platinum and palladium as well as improved and obtained new results on zinc, copper , silicon, calcium, chromium, iron, potassium and titanium isotopes in lunar and a multitude of meteoritic samples.

We discovered that Ga and Rb are isotopically different between bulk meteorites and bulk silicate Earth, suggesting that the terrestrial Ga and Rb have not been inherited from the late arrival of meteoritic material after its formation but reflect an isotopic fractionation processes such as core formation or volatilisation-this is a major result in order to tackle the issue of the origin of volatile elements. By using Zn, and Cr isotopes we have found that volatile elements are re-distributed during the magma ocean phase of the Moon and that the Moon may actually be even more volatile depleted than previously thought suggesting the Moon is water poor. We have also worked on the most volatile rich Moon rock, the so-called rusty rock, and found that this sample is the isotopically lightest sample ever measured in the solar system and suggest that the origin of volatiles in this rock (including water) must be due to secondary condensation and it is not primordial. We also show that the isotopes of elements with affinity with iron (siderophile elements) that partitioned into the Earth's core, can be used to trace the origin of subduction on Earth, which is linked to the origin of plate tectonics.

In summary, through the combination of various isotopic measurements we demonstrated that the chemical composition of terrestrial planets is the consequence of volatility processes during and after the accretion of planets and planet embryos. This is a change in our understanding of planet formation, as it is opposite to previous dominating idea that the composition was inherited from more ancien nebular processes.

We have published over 60 peer-reviewed papers in high-impact journals (e.g. PNAS, Science Advances, Nature Astronomy, Earth and Planetary Science Letters...) as well as provided oral presentation in multiple international conferences, such as the Goldschmidt conferences or the American Geophysical Union.
Since a large part of our research in cosmochemistry is spent to the development of novel analytical techniques to measure the isotope ratios to be measured to unprecedented precision and accuracy, which includes both new chemical purification schemes and mass spectrometry techniques.

Thanks to our various analytical development on unprecedentedly precise isotopic tools of various elements we solved many cosmochemistry questions related to the goals of the proposal, but these new methods will certainly find many more applications in the future. For example there is an expending field of using these new isotopic tracers developped in cosmochemistry to medical sciences in order to trace the transport of trace metals in the human body and possibly track diseases that are related to a change in the metabolism of these metals. For example rubidium is often substituted to potassium and therefore by using Rb isotopes as a proxy for change in the K stable isotopes composition. In the same idea, Ga is often substituated to aluminium, which is a major element in the Earth's crust-however, Al has only one stable isotopes and it is therefore not possible to track Al behavior with isotope geochemistry and gallium isotopes may also be used as a proxy for Al.
Also, our isotopic data has motivated the development of experimental tools to calibrate the isotopic fractionation during evaporation. In order to evaluate the relative volatility of the elements during evaporation in planetary systems we have also performed series of experiments at various oxygen fugacities and developped a new scale of volatility applicable to the formation of the Earth and the Moon as well calibrated the isotopic fractionation of Cu and Zn under various conditions.
Artist view of a giant impact (credit IPGP).