LETTERS PUBLISHED ONLINE: 20 JANUARY 2013 | DOI: 10.1038/NGEO1710 Solid-state plastic deformation in the dynamic interior of a differentiated asteroid B. J. Tkalcec 1 * , G. J. Golabek 2,3 and F. E. Brenker 1 Diogenite meteorites are thought to represent mantle rocks that formed as cumulates in magma chambers on 4 Vesta or a similar differentiated asteroid 1,2 . Northwest Africa 5480 is a harzburgitic diogenite 3,4 , composed mainly of heterogeneously distributed olivine and orthopyroxene. Here we present a microstructural analysis of olivine grains from Northwest Africa 5480, using electron backscatter diffraction techniques to quantify any preferred orientation of crystallographic lattice. We find that the preferred orientation in the olivine-dominated zones can be explained neither by cumulate formation nor by impact reprocessing near the asteroid’s surface. Rather, they represent high-temperature solid-state plastic deformation by the pencil-glide 5 slip system. The detected type of preferred orientation is well known from dry ultramafic rocks on Earth, where it is typically formed by mantle shear 5–7 at temperatures between 1,273 and 1,523 K. Numerical modelling indicates that our observations can be explained by large-scale downwelling inside the asteroid’s mantle, within the first 50 million years after formation of calcium–aluminium-rich inclusions. The discovery of solid-state plastic deformation in an asteroidal ultramafic rock represents compelling evidence of dynamic planet-like processes in asteroids. We conclude that long- lasting enhanced mass exchange occurred in the dynamic interior of a differentiated asteroid such as Vesta, and enabled accelerated chemical, structural and thermal equilibration. Diogenites belong to the Howardite–Eucrite–Diogenite (HED) group of achondrites thought to have originated from the differenti- ated asteroid 4 Vesta, or a Vesta-like body 1 . This study concentrates on the achondrite Northwest Africa (NWA) 5480, which is dominated by olivine (57 vol%) and orthopyroxene (42 vol%; ref. 2) and is classified a harzburgitic diogenite 3,4 . Diogenites have so far been thought to represent ultramafic cumulate rocks formed at deep crustal or upper mantle levels of the parent body 1,2 . Most studies of HEDs have concentrated on the geochemistry and petrology of these achondrites 1–3 . In this investigation we focus on the structural and textural properties, performing quantitative structural analysis on the olivine grains of NWA 5480 using electron backscatter diffraction (EBSD) to measure the crystallographic orientation of all crystal axes and determine any lattice-preferred orientation 7 (LPO). Within the Earth’s upper mantle, depending on conditions of pressure, temperature, water content, strain geometry and strain-rate, LPOs of olivine are formed during plastic deformation, preferentially via dislocation glide or dislocation creep. Slip is accommodated by (010)[100] (refs 7–9) and (001)[100] (refs 7–9) systems, a combination of both (pencil glide {0kl }[100]; ref. 7) or by (010)[001] (refs 7,9). The respective main slip systems active can be identified by the resulting LPO of olivine. Alternatively, compaction 1 Geoscience Institute, Goethe University, Altenhöferallee 1, 60438 Frankfurt am Main, Germany, 2 ENS Lyon, Laboratoire de Géologie, 46 Allée d’Italie, 69364 Lyon Cedex 07, France, 3 ETH Zürich, Institute of Geophysics, Sonneggstrasse 5, 8092 Zürich, Switzerland. *e-mail: tkalcec@em.uni-frankfurt.de. 1 mm Zone B Zone A Figure 1 | Stitched back-scattered electron image of NWA 5480 showing two distinct zones. Zone A is dominated by coarse-grained olivine; Zone B is dominated by orthopyroxene schlieren. White line indicates the approximated northeast–southwest direction of the foliation (relative to the bottom rim of the polished sample), based on the schlieren structure and main vein orientation. processes such as cumulate formation form a distinct LPO dominated by a shape-preferred orientation (SPO) of olivine 7,10 . Thus, quantitative analysis of the LPO of olivine in NWA 5480 and comparison with that of terrestrial samples or experimental data should expand our knowledge of the origin and formation of harzburgitic diogenites. The results offer new insights into the com- plex, polyphase textural and microstructural evolution undergone during the thermal history of accretion, heating, differentiation, compaction, deformation and cooling of the HED parent body. The studied harzburgitic diogenite 3,4 enables a unique, relatively undisturbed view into primary processes as, in contrast to most other diogenites 1 , NWA 5480 shows no sign of brecciation. The distribution of olivine and orthopyroxene is very heterogeneous, with some areas exhibiting up to 90% of either of the two min- erals, whereby some dominantly orthopyroxene regions exhibit schlieren-like patterns 2 . The sample can be subdivided into two pet- rographically distinct regions (Fig. 1) for targeted analysis: Zone A, the olivine-dominated region, and Zone B, the orthopyroxene- dominated schlieren region. Minor chromite, troilite and occa- sional metal iron grains are present throughout both zones as well as a couple of larger (500–1,000 μm) patches of chromite within the schlieren region. Throughout the sample there is a general NATURE GEOSCIENCE | VOL 6 | FEBRUARY 2013 | www.nature.com/naturegeoscience 93 © 2013 Macmillan Publishers Limited. All rights reserved.