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Periodic Reporting for period 3 - PRISTINE (High precision isotopic measurements of heavy elements in extra-terrestrial materials: origin and age of the solar system volatile element depletion)


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...


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.

Work performed

One of the key aspects of our project is to develop new high precision isotopic systems to be used in precious extra-terrestrial samples. During the first 18 months of this project we have already implemented the high precision isotopic measurement of gallium and rubidium and obtained new results on zinc and copper isotopes in lunar and meteoritic samples.
We found 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 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 (e.g. Kato et al. 2015 Nature Communications) 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 (Day et al. PNAS 2017).

We have so far published 39 papers in high impact journals and several papers are presently in review, among them 1 in Geology, and 1 in EPSL.
In addition, we have disseminating the data in various conferences (Goldschmidt Conference 2016 in Yokohama for student Pringle; Goldschmidt 2017 in Paris for PI Moynier, post-doc Sossi, students Kato and Amsellem), workshops (Symposium on the Solar System in Hokkaido for PI Moynier, students Pringle and Kato, workshop on the asteroid/comet connection in UCLA, USA for student Pringle) and invited seminar at different institutions (e.g. post-doc Sossi at Univ. of Bern in 2016, PI Moynier at Univ. of Kyoto in 2015 , Univ. of Durham in 2016, Univ. of Aarhus, in 2017).

Final results

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.
This has been critical during the first 18 months of our project since new methods had to be developed to resolve the scientific questions of the proposal.
Thanks to our new analytical progress we improved the precision of the isotopic measurements of Ga and Rb by more than a factor 10 compared to previous work. While we take advantage of this precision to answer the questions about the origin of volatile elements that are related to our project-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 (which is extremely difficult to measure for analytical reasons). 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.
In addition, we have recently developed the first high precision Sn isotopic measurements (technical paper by Creech et al. Chem. Geol. 2017) and we are now exploring this system in meteorites and lunar samples. To complement these methods we have also developed the Cr stable isotopic measurements by double spike approach and we should soon submit a paper to Nature on these new results.
In order to evaluate the relative volatility of the elements during evaporation in planetary systems we have 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. This paper is presently written by post-doc Sossi and should become a legacy paper.

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