Experimental Constraints on the Origin of Volatile Elements on Earth – VOLTERRE

The objectives of our research program will be divided into three main targets: (1) Determining the S, Se, and Te metal-silicate partitioning at the direct pressure and temperature conditions of a deep magma ocean. These results will be used to test whether the abundances of these elements can be predicted by current models of Earth differentiation involving metal- silicate equilibrium. We will consequently evaluate if the addition of a given type of meteorite component following initial core formation can raise mantle abundances of S, Se, and Te to their current level. (2) Determining the sulfur isotopic fractionation between metal and silicate at high pressure and high temperature. The results will prove whether core–mantle differentiation generated the recently observed non-chondritic 34S/32S ratio of the silicate Earth. This will provide new independent constraints for the budget of sulfur in the core and the volatile accretion history of the Earth. (3) Determining the speciation of sulfur in (Fe, S) alloys at HP-HT for varying sulfur contents. The results will provide a mechanism to drive sulfur isotopic fractionation to the predicted higher 34S/32S ratio in the core than that of chondrites. Experimental methods developed will place this project at the frontier in between experimental petrology, stable isotopes geochemistry and mineralogy

-The sulfur partition coefficients measured from these experiments are an order of magnitude less than those obtained from extrapolation of previous results to core formation conditions. The mantle’s sulfur content is matched if sulfur is delivered with large bodies (3 to 10% Earth mass) during the last 20% of Earth’s accretion. In each accretion scenario, the core sulfur content remains below ~2 wt.% in close agreement with cosmochemical estimates and is a further indication that sulfur is not a dominant light element in the core.
-A substantial amount of K, and U is likely to have been segregate in the core during differentiation, producing more than 3 TW for about one billion years. This is sufficient to produce and sustain a magnetic field very early on and explain paleomagnetic observation.
-The present experimental work shows that, at sufficiently high temperatures, magnesium oxide can dissolve in core-forming metallic melts. As the core subsequently cooled, the ensuing exsolution of buoyant magnesium oxide would have generated enough gravitational energy to power an early geodynamo and produce Earth's ancient magnetic field.
-we present new results from laser-heated diamond anvil cell experiments where we measured gallium metal-silicate partitioning at the accurate P-T condition of Earth’s differentiation. We propose that gallium is not less siderophile during Earth’s accretion than expected, but less volatile. This can be explained by a reset in the condensation temperature that is usually assumed, due to Si- and O-rich atmosphere that surrounded the Earth following its bombardment; far from the H2-rich one of the solar nebula.

Among the objectives of the present proposal are the development of techniques and methodologies for:
(1) The determination of trace elements partitioning on DAC samples using the nanoSIMS. This powerful combination of HP-HT device and analytical instrument will be suitable for the study of the behavior of other volatile and siderophile elements (VSE) that are poorly integrated into any accretion and core formation models, such as Ge, As, In, Sb or Pb for instance.
(2) The determination of sulfur isotopic fractionation between metal and silicate at HP-HT. The development at IPGP of the promising approach of combining isotopic measurements and experimental petrology at HP-HT represents a first step toward numerous future works. For instance, the study of isotopic fractionation of other VSE need to be considered to provide additional constraints on the timing of volatile delivery on Earth. Cosmochemists/Geochemists at IPGP are currently developing new set of analytical techniques for the MC-ICP-MS measurements of non-traditional stable isotope systems for volatile elements (including Zn, Cr, Cu and Ga).
Finally we believe that once these techniques and methodologies have been established, they will find large applications for the study of deep Earth interior processes (e.g. mantle silicate partial melting, isotopic fractionation related to phase transitions at high pressure, crystallization of a deep magma ocean).

1. J. Siebert and A. Shahar (2015). An experimental geochemistry perspective on Earth's core formation. Geophysical Monograph Series, The Early Earth: Accretion and Differentiation, 103-121.
2. I. Blanchard, J. Badro, J. Siebert, F. J. Ryerson (2015). Composition of the Core from Gallium Metal-Silicate Partitioning Experiments. Earth and Planetary Science Letters, 427, 191-201.
3. G. K. Pradhan, G. Fiquet, J. Siebert, A-L. Auzende, G. Morard, D. Antonangeli (2015). Melting of MORB at core-mantle boundary. Earth and Planetary Science Letters, 431, 247-255.
J. Badro, J. Siebert, F. Nimmo (2016). An early geodynamo driven by lithophile element exsolutions from Earth’s core. Nature, 536, 326-328.
4. B. M. Mahan, J. Siebert, E. Pringle, F. Moynier. Elemental partitioning and isotopic fractionation of Zn between metal and silicate . Geochimica et Cosmochimica Acta, 196, 252-270.
5. T. A. Suer*, J. Siebert, L. Rémusat, N. Menguy, G (2017). Fiquet. A sulfur-poor terrestrial core inferred from metal-silicate partitioning experiments. Earth and Planetary Science Letters, 469, 84-97.
6. I. Blanchard, J. Badro, J. Siebert. Primordial volatilization on Earth inferred from the high-pressure metal-silicate partitioning behavior of gallium. Geochimica et Cosmochimica Acta (Submitted).
7. J. Siebert, P. Sossi, I. Blanchard J. Badro, F. Moynier. Chondritic Mn/Na ratio and limited post-nebular volatile loss of the Earth. Geochimica et Cosmochimica Acta (Submitted).
8. B. Mahan*, F. Moynier, Pierre Beck, Emily Pringle, Julien Siebert. Thermal history and volatile loss in carbonaceous chondrites:. Submitted to Geochimica et Cosmochimica Acta (in revision).
9. I. Blanchard*, J. Siebert, J. Badro. A 4.5 billion-year-old geodynamo. Submitted to Nature.
10. B. Mahan*, J. Siebert, I. Blanchard, J. Badro, F. Moynier. Partial equilibration of large impactor(s) during Earth’s accretion from Zn metal-silicate partitioning experiments. Submitted to Geochimica et Cosmochimica Acta.

Volatile elements (e.g. H, C, S) have a fundamental role in planetary evolution. But how and when budgets of volatiles were set in planets and the mechanism of volatile depletion in planetary bodies remains poorly understood and represents a fundamental obstacle in understanding the chemical processing of terrestrial planets. Two main theories exist. Either Earth accreted ‘dry’, with Earth’s building blocks completely devoid of volatile elements. Then, the Earth’s complement of volatile elements was only established later, once the Earth was differentiated into a core and mantle, by the addition of a late veneer. Or, the Earth accreted ‘wet’ where volatiles where present during the main stages of accretion and differentiation of the Earth. The imprint of core formation on the geochemistry of siderophile and volatile elements of the present mantle can discriminate between these two competing scenarios. We will use core formation experiments and the geochemical signatures from metal-silicate equilibration of three siderophile and volatile elements sulfur, selenium, and tellurium. An original and complementary multi-techniques approach combining experiments at high pressure and high temperature, and high-resolution analyses on quenched samples will be developed to obtain new constraints on the origin of volatiles elements on Earth. The objectives of our research program will be divided into three main targets: (1) Determining the S, Se, and Te metal-silicate partitioning at the direct pressure and temperature conditions of core formation in a deep magma ocean. These results will be used to test whether the abundances of these elements can be predicted by current models of Earth differentiation involving metal-silicate equilibrium. We will consequently evaluate if the addition of a given type of meteorite component following initial core formation can raise mantle abundances of S, Se, and Te to their current level. (2) Determining the sulfur isotopic fractionation between metal and silicate at high pressure and high temperature. The results will prove whether core–mantle differentiation generated the recently observed non-chondritic 34S/32S ratio of the silicate Earth. This will provide new independent constraints for the budget of sulfur in the core and the volatile accretion history of the Earth. (3) Determining the speciation of sulfur in (Fe, S) alloys at HP-HT for varying sulfur contents. The results will provide a mechanism to drive sulfur isotopic fractionation to the predicted higher 34S/32S ratio in the core than that of chondrites. Experimental methods developed will place this project at the frontier in between experimental petrology, stable isotopes geochemistry and mineralogy, as within the new scientific objectives of the Institut de Physique du Globe de Paris (IPGP).