Final Report Summary - ISOCORE (New isotope tracers for core formation in terrestrial planets) This project used variations in the isotopic composition of meteorites and terrestrial samples to investigate the early evolution of the solar system, from its formation to the accretion and chemical differentiation of the planets. Isotopic variations may arise through the heterogeneous distribution of stellar-derived dust in the solar protoplanetary disk, and in this case provide information on the dynamics of material transport in the disk, the formation location of distinct planetary objects, and the genetic heritage of planetary building blocks. Our results reveal that meteorites (fragments of asteroids) derive from two contemporaneous but spatially distinct reservoirs. While the non-carbonaceous meteorites represent material from the inner solar system, carbonaceous meteorites represent the outer solar system. Our results reveal that the distinct compositions of both reservoirs reflect heterogeneities in the composition of the solar system's parental molecular cloud, which have not been homogenized by processes within the accretion disk and were maintained through the early and rapid formation of Jupiter. By dating meteorites from the non-carbonaceous and carbonaceous groups we demonstrate that the solid core of Jupiter, consisting of about 20 Earth masses, must have formed within less than one million years of the solar system. This makes Jupiter the oldest planet of our solar system. The subsequent growth and migration of Jupiter led to scattering of bodies from beyond its orbit into the inner solar system, accounting for the co-occurrence of non-carbonaceous and carbonaceous objects in the present-day asteroid belt between Mars and Jupiter. Our results also show that Earth accreted carbonaceous bodies from the outer solar system during the final stages of its formation, most likely through the giant impact that also led to the formation of the Moon. This event, therefore, delivered most or all of Earth's volatiles and water. As such, our results suggest that Earth's habitability is strongly tied to the very late stages of its formation, and to the formation of the Moon.Isotope anomalies in meteorites and terrestrial samples may also reflect the uneven distribution of isotopes according to their mass. Such isotope fractionations may occur during a variety of chemical processes. In general, the magnitude of the isotope fractionation decreases dramatically with increasing temperature, and so only recently has it become possible to identify isotope fractionations associated with high-temperature geological processes. We have developed new analytical techniques to be able to identify such small isotope fractionations, and we show that the stable isotope compositions of molybdenum (Mo) and tungsten (W) provide constraints on the conditions of terrestrial core formation; in addition they also provide a novel tracer for the evolution of Earth's mantle and crust. We find that the Mo and W isotopic compositions of Earth's mantle is indistinguishable from that of bulk Earth, indicating that the temperatures during core formation on Earth were too high to result in resolvable isotope fractionations. Such high temperatures imply that core formation on Earth occurred in a deep magma ocean, and that the metal cores of Earth's building blocks at least partly equilibrated within Earth's mantle before entering Earth's core. We also find that the Mo and W isotopic compositions of different silicate reservoirs on Earth vary. While the Mo isotopic compositions of oceanic basalts are highly variable and bear testimony to the compositional heterogeneity of Earth's mantle and large-scale anterior partial melting of the mantle, variations in W isotopes predominantly seem to be related to subduction zone processes. As subduction zone magmatism is the dominant site of continental crust production, W isotopes may provide a novel tracer for crust formation through Earth's geologic history.
