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Accretion and Differentiation of Terrestrial Planets

Final Report Summary - EARLYEARTH (Accretion and Differentiation of Terrestrial Planets)

This project constrains (i) the formation of the earliest silicate reservoirs in planets using Nb-Zr chronometry and (ii) nucleosynthetic isotope heterogeneities in the solar system and (iii) the thermal evolution of small bodies that formed during the earliest history of our solar system and are represented by asteroids from which meteorite samples are available. To this end, new analytical techniques for high precision analyses of Pd, Pt and Zr isotopes were developed.

(i) Global differentiation in terrestrial silicate reservoirs may have taken place within the first 100-150 million years of our solar system based on new Sm-Nd isotope data. This timing, however, has been debated. The 92Nb-92Zr decay system (half-life 37 million years) is a potentially powerful chronometer to further constrain this issue. Its usefulness, however, has been hindered by uncertainties of the initial 92Nb abundance in the solar system. This value is needed to date early silicate differentiation. We obtained unequivocal evidence from differentiated meteorites and the oldest solids in our solar system i.e. calcium aluminium rich inclusions (CAIs) to settle this debate. Our results demonstrate that the disputed initial 92Nb/93Nb ratio of our solar system is low i.e. 1.57 (±0.09) x 10-5. Moreover, high precision Zr isotope analyses on a wide range of solar system materials (meteorites such as chondrites and achondrites, Martian and lunar samples) demonstrate the homogeneous distribution of 92Nb and 92Zr in the solar system. This is a prerequisite for the Nb-Zr system to be used as a chronometer. We applied the system to the oldest terrestrial rocks that formed during the Archean. Our data reveal the absence of 92Zr variations (generated by the decay of 92Nb) in these rocks. This provides strong evidence that early silicate differentiation on Earth occurred later than 60 – 110 Myr after the formation of CAIs. In contrast, Martian samples preserved 92Zr variations and this indicates the formation of very early silicate reservoirs on Mars within the first ~50 Myr.

(ii) Nucleosynthetic heterogeneities are quickly gaining importance for planet formation models. Each meteorite parent body (asteroids), Mars and the Earth shows unique isotope compositions for specific elements (e.g. Zr, Mo and Ru). These compositions - called nucleosynthetic variations - stem from the heterogeneous distribution of presolar grains in our solar system. Presolar grains are found in primitive meteorites. They were formed in previous generations of stars (AGB stars, supernovae etc.) and possess extreme isotope compositions, which serve as unique fingerprints of their production sites. Our Pd data illustrates the same nucleosynthetic heterogeneity as Zr, Mo and Ru data in asteroids and planets. This heterogeneity is traced back to the s-process that takes place in AGB stars. Palladium isotope variations are smaller than those of Zr, Mo and Ru. This shows that slightly volatile elements such as Pd are produced in AGB stars, but do not fully condense into stardust around the star. The Pd fraction that remained in the gas phase experienced extensive mixing with other material in the interstellar medium (ISM) (space between stars) and could not preserve its original nucleosynthetic signature. We also propose that the nucleosynthetic heterogeneity of planetary bodies is due to selective processing of the dust that formed in the ISM relative to ‘stardust’, which mainly condensed around AGB stars. To enable the identification of nucleosynthetic Pd variations, corrections are required for variations induced by exposure to galactic cosmic rays (GCR) during the travel in space from the meteorite parent body to Earth. We show that Pt isotopes are a powerful neutron dosimeter that can be utilised to correct for GCR induced effects on Pd isotopes and also effects that obscure Hf-W ages.

(iii) The application of such a rigorous GCR correction scheme to Hf-W ages sheds light on the debated origin of IAB iron meteorites. Our results show that the IAB asteroid experienced protracted heating such that metal-silicate separation only occurred 6.0 ± 0.8 Ma after CAIs. Thermal models of the interior evolution suggest that the asteroid underwent metal–silicate separation because of internal heating by short-lived radionuclides and accreted at around 1.4 ± 0.1 Ma after CAIs with a radius of greater than 60 km.