Planetesimal and Asteroidal earLy evoLution in the solAr System – PALLAS

* We determine the age of partially differentiated meteorites using the 182Hf-182W, 60Fe-60Ni and 26Al-26Mg chronometers; at the same time, the mass dependent isotope fractionations of Si, Ni and W permit identifying and quantifying the mechanism involved in the metal-silicate separation processes.
* The physical models of differentiation explore time- and length-scales inaccessible in the laboratory. We mainly focus on a 3-phases compaction model.
* In experimental petrology, we consider a 2-phase initial system made of metal and silicate - one phase being doped with the element of interest ; experiments are carried out under controlled atmosphere in a CO/CO2 mixing vertical furnace.
* Last but not least, mass dependent isotope fractionations measured in natural samples are compared with fractionations observed in the experiments and with the results of the ab initio calculations in order to determine the degree of equilibrium reached.

* Using the 182Hf-182W and 60Fe-60Ni chronometers, we showed that some small parent bodies experienced incipient metal segregation at a time when much larger bodies (such as iron meteorite parent bodies) were fully differentiated.
At equilibrium, no significant isotope fractionation of Ni is observed in the experimental charges, suggesting that the variations of isotope compositions detected in natural samples do not reflect metal-silicate differentiation at equilibrium. The absence of metal-silicate fractionation at high temperature is confirmed by ab initio calculations. Variations of the isotope composition are most likely controlled by evaporation/recondensation processes in the solar nebula.
Besides, the behaviour of W and its isotopes during alteration processes has been experimentally investigated. A chemical and isotopic fractionation is observed.

* On parallel, the thermal evolution of accreting planetesimals has been studied (see paper by Ricard et al. 2016). Modeling of the dynamical mixing of a metal diapir in a molten body focused on the effects related to the body rotation. It appears that these effects should considerably affect the chemical and thermal exchanges between metallic and silicated phases. The non-newtonian rheology also seems to play a major role, favoring the fragmentation when viscous effects dominate.

We first concentrated our efforts on the behaviour of nickel, the second most abundant element in the metallic phase. Now, we will focus on tungsten. Following the study of metal-rich chondrites, we will go forward with primitive achondrites. Noteworthy, the disturbances on the isotopic signature due to fluid-rock interactions will be taken into account, more particularly for tungsten.
Once the 26Al and 60Fe abundances have been established for the different parent bodies, these data will of course become input parameters for the compaction models simulating the differentiation processes and determining the thermal evolution of the bodies.

Y. Ricard, D. Bercovici and F. Albarède (2016) Thermal evolution of planetesimals during accretion, Icarus, submitted.
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Presentations at international conferences :

1. G. Quitté, F. Poitrasson and T. Zambardi (2016) Nickel stable isotopes in planetary reservoirs. Annual Meeting of the Meteoritical Society, Berlin, Germany. Poster.
2. J. Guignard, G. Quitté, M.J. Toplis, F. Poitrasson and M. Roskosz (2016) Core formation conditions in planetesimals: constraints from isotope fractionation experiments (Ni, Fe). AGU Fall Meeting, San Francisco, USA. Poster.
3. N.H.Yin, J. Guignard, S. Fabre and G. Quitté (2016) Tungsten Mobility during Alteration Processes: an Experimental Approach. AGU Fall Meeting, San Francisco, USA. Oral presentation.

This multi-disciplinary project is focused on metal-silicate differentiation processes in planetesimal size objects (on the order of tens to hundreds of kilometers) at the very beginning of the solar system history. When did differentiation take place? What triggered it? What was the role of the different heat sources? Under which conditions did metal segregation occur? What were the exact mechanisms of differentiation?
To address these questions, we will combine geochemical and isotope data obtained on partially differentiated meteorites with results of physical modeling, experimental petrology, mineralogy, and ab-initio calculations.
Metal-rich primitive meteorites are good candidates to provide precise information on the earliest stages of metal segregation in their parent bodies as they show evidence of incipient metal-silicate separation. We will determine their ages and study mass dependent isotope fractionation of Si, Ni, Cr and W, since stable isotopes permit the identification and quantification of the mechanisms involved in the processes. The short-lived 182Hf-182W radiochronometer is one of the best suited to study metal segregation. It will be associated with two others providing complementary information: the 60Fe-60Ni chronometer dates the cooling of the metal, while the 26Al-26Mg gives insight into the silicate phase. By combining the timescales deduced from the different chronometers having different closure temperatures, the thermal history of the body will be inferred. Besides, it has been recently argued that the distribution of the 26Al and 60Fe short-lived radionuclides was probably heterogeneous in the early solar system. Magnesium and Ni isotope measurements will allow assessing the initial abundance of these potential heat sources in each parent body. In parallel, we plan to explore systematically and in a statistical way the growth history of planetesimals, i.e. the accretion rates and thermal states of the bodies. These improvements will help to make comparisons with petrological and cosmochemical observations more meaningful. The results will also further help constraining the planetesimal initial state, which is of prime importance to model metal-silicate segregation in such bodies.
Physical differentiation models will involve two particularly innovative aspects which are: (1) to take into account the chemical and mineralogical evolution during melting, (2) to dynamically investigate metal-silicate isotope fractionation for a number of relevant chemical elements. At the same time, the compositional and isotopic consequences of segregation under a given set of conditions will be tested by experimental petrology. We will consider either a two-phase metal-silicate initial system, or the production of metal by reduction during the run. The metal-silicate system will be studied both at 1 bar in CO/CO2 mixing vertical furnace and at 1-3 GPa with a piston cylinder set up. Due to the volatility of sulfur, the piston cylinder will be preferred for sulfide-metal systems. A mineralogical and chemical characterization of the samples will precede determination of the isotope composition by mass spectrometry. Last but not least, we will compare the mass dependent isotope fractionation measured in natural samples, in experimental products, and obtained by ab-initio calculations to determine the degree of equilibration reached, as kinetic and thermodynamic effects can be identified by stable isotope fractionations.

The PALLAS project is a cutting-edge cooperation between geo-/cosmochemists, geodynamists, mineralogists, modelers, experimentalists from four French laboratories (IRAP in Toulouse, GET in Toulouse, ENS de Lyon, and UMET in Lille). The funding of the ANR to such an initiative is essential to provide the necessary support to the laboratories so that they can develop long term scientific projects at the highest international level.