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Early formation of evolved asteroidal crust


Mechanisms for the formation of crust on planetary bodies remain poorly understood1. It is generally accepted that Earth’s andesitic continental crust is the product of plate tectonics1,2, whereas the Moon acquired its feldspar-rich crust by way of plagioclase flotation in a magma ocean3,4. Basaltic meteorites provide evidence that, like the terrestrial planets, some asteroids generated crust and underwent large-scale differentiation processes5. Until now, however, no evolved felsic asteroidal crust has been sampled or observed. Here we report age and compositional data for the newly discovered, paired and differentiated meteorites Graves Nunatak (GRA) 06128 and GRA 06129. These meteorites are feldspar-rich, with andesite bulk compositions. Their age of 4.52 ± 0.06 Gyr demonstrates formation early in Solar System history. The isotopic and elemental compositions, degree of metamorphic re-equilibration and sulphide-rich nature of the meteorites are most consistent with an origin as partial melts from a volatile-rich, oxidized asteroid. GRA 06128 and 06129 are the result of a newly recognized style of evolved crust formation, bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early-formed materials in the Solar System.

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Figure 1: Bulk composition of the GRA 06128/9 achondrite meteorites.
Figure 2: δ 18 O–Δ 17 O plot for GRA 06128/9 versus achondrite meteorites and lunar and terrestrial materials.
Figure 3: Highly siderophile element and Re–Os isotope systematics of GRA 06128/9 and Brachina.

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We thank the ANSMET 2006/2007 field team, the Meteorite Working Group and the Smithsonian Institution of Washington for collection and provision of the GRA 06128/9 and Brachina meteorites. D. Mittlefehldt and R. Greenwood provided reviews that improved the quality of this paper. A. Patchen and P. Piccoli provided assistance with electron microprobe analysis. Portions of this study were supported by the NASA Cosmochemistry Program: NNX07AM29G (R.J.W.), NNX08AH76G (W.F.M.), NNG05GG03G (L.A.T.).

Author Contributions All authors participated in data collection and interpretation and commented on the manuscript. J.M.D.D led the project and wrote the paper.

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Correspondence to James M. D. Day.

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Day, J., Ash, R., Liu, Y. et al. Early formation of evolved asteroidal crust. Nature 457, 179–182 (2009).

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