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ISOCORE Report Summary

Project ID: 616564
Funded under: FP7-IDEAS-ERC
Country: Germany

Mid-Term Report Summary - ISOCORE (New isotope tracers for core formation in terrestrial planets)

This project uses variations in the isotopic composition of meteorites and terrestrial samples to investigate the formation and chemical differentiation of 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 genetic heritage of different planetary bodies. For instance, Earth grew through the collision with several Moon- to Mars-sized planetary embryos and it is commonly thought that this building material of Earth is represented by meteorites. Our data show, however, that Earth and meteorites are isotopically distinct, reflecting a higher proportion of presolar dust derived from red giant stars in Earth compared to meteorites. Thus, contrary to common assumptions, the Earth cannot consist of material represented by meteorites. The isotope anomalies in meteorites not only help to understand the origin of Earth, but they also provide novel insights into the formation of Jupiter. Our isotopic data for meteorites reveal the existence of two contemporaneous but spatially distinct reservoirs that were separated by Jupiter: meteorites either derive from the inner solar system located inside the orbit of Jupiter (the non-carbonaceous meteorites), or they derive from the outer solar system beyond Jupiter (the carbonaceous meteorites). Thus, the present-day asteroid belt between Mars and Jupiter contains material that originally formed at much greater heliocentric distance in the outer solar system. Moreover, the presence of inner and outer solar system material within the meteorite suite makes it possible, for the first time, to date the 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's masses, must have formed within less than one million years of the solar system. This makes Jupiter the oldest planet of our solar system.

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 provide a new tracer for crust formation through Earth's geologic history. Ultimately we may be able to determine when crust formation by subduction zone processes started, marking the onset of modern plate tectonics on Earth.


Katharina Steinberg, (Head of Research Department)
Tel.: +49 2518322151
Record Number: 191613 / Last updated on: 2016-11-21
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