1. PSSRI, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
2. Laboratoire MAGIE, Université Pierre et Marie Curie, CNRS UMR 7047 case 110, 4 place Jussieu, 75252 Paris Cedex 05, France
3. Department of Geology, Rhodes University, PO Box 94, Grahamstown 6140, South Africa

Correspondence to: Richard C. Greenwood¹ Correspondence and requests for materials should be addressed to R.C.G. (Email: r.c.greenwood@open.ac.uk).

Abstract

Immediately following the formation of the Solar System, small planetary bodies accreted¹, some of which melted to produce igneous rocks^{2, 3}. Over a longer timescale (15–33 Myr), the inner planets grew by incorporation of these smaller objects^{4, 5} through collisions. Processes operating on such asteroids strongly influenced the final composition of these planets⁴, including Earth⁵. Currently there is little agreement about the nature of asteroidal igneous activity: proposals range from small-scale melting, to near total fusion and the formation of deep magma oceans². Here we report a study of oxygen isotopes in two basaltic meteorite suites, the HEDs (howardites, eucrites and diogenites, which are thought to sample the asteroid 4 Vesta⁶) and the angrites (from an unidentified asteroidal source). Our results demonstrate that these meteorite suites formed during early, global-scale melting ( 50 per cent) events. We show that magma oceans were present on all the differentiated Solar System bodies so far sampled. Magma oceans produced compositionally layered planetesimals; the modification of such bodies before incorporation into larger objects can explain some anomalous planetary features, such as Earth's high Mg/Si ratio.

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