LETTERSPUBLISHED ONLINE: 20 JANUARY 2013 | DOI: 10.1038/NGEO1710

Solid-state plastic deformation in the dynamicinterior of a differentiated asteroidB. J. Tkalcec1*, G. J. Golabek2,3 and F. E. Brenker1

Diogenite meteorites are thought to represent mantle rocksthat formed as cumulates in magma chambers on 4 Vesta ora similar differentiated asteroid1,2. Northwest Africa 5480 is aharzburgitic diogenite3,4, composed mainly of heterogeneouslydistributed olivine and orthopyroxene. Here we present amicrostructural analysis of olivine grains from NorthwestAfrica 5480, using electron backscatter diffraction techniquesto quantify any preferred orientation of crystallographic lattice.We find that the preferred orientation in the olivine-dominatedzones can be explained neither by cumulate formation nor byimpact reprocessing near the asteroid’s surface. Rather, theyrepresent high-temperature solid-state plastic deformation bythe pencil-glide5 slip system. The detected type of preferredorientation is well known from dry ultramafic rocks on Earth,where it is typically formed by mantle shear5–7 at temperaturesbetween 1,273 and 1,523 K. Numerical modelling indicates thatour observations can be explained by large-scale downwellinginside the asteroid’s mantle, within the first 50 million yearsafter formation of calcium–aluminium-rich inclusions. Thediscovery of solid-state plastic deformation in an asteroidalultramafic rock represents compelling evidence of dynamicplanet-like processes in asteroids. We conclude that long-lasting enhanced mass exchange occurred in the dynamicinterior of a differentiated asteroid such as Vesta, and enabledaccelerated 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 body1. This study concentrateson the achondrite Northwest Africa (NWA) 5480, which isdominated by olivine (57 vol%) and orthopyroxene (42 vol%; ref. 2)and is classified a harzburgitic diogenite3,4. Diogenites have so farbeen thought to represent ultramafic cumulate rocks formed at deepcrustal or upper mantle levels of the parent body1,2. Most studiesof HEDs have concentrated on the geochemistry and petrology ofthese achondrites1–3. In this investigation we focus on the structuraland textural properties, performing quantitative structural analysison the olivine grains of NWA 5480 using electron backscatterdiffraction (EBSD) to measure the crystallographic orientation ofall crystal axes and determine any lattice-preferred orientation7

(LPO). Within the Earth’s upper mantle, depending on conditionsof pressure, temperature, water content, strain geometry andstrain-rate, LPOs of olivine are formed during plastic deformation,preferentially via dislocation glide or dislocation creep. Slip isaccommodated 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 beidentified by the resulting LPOof olivine. Alternatively, compaction

1Geoscience Institute, Goethe University, Altenhöferallee 1, 60438 Frankfurt am Main, Germany, 2ENS Lyon, Laboratoire de Géologie, 46 Allée d’Italie,69364 Lyon Cedex 07, France, 3ETH 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 showingtwo distinct zones. Zone A is dominated by coarse-grained olivine; Zone Bis dominated by orthopyroxene schlieren. White line indicates theapproximated northeast–southwest direction of the foliation (relative to thebottom rim of the polished sample), based on the schlieren structure andmain vein orientation.

processes such as cumulate formation form a distinct LPOdominated by a shape-preferred orientation (SPO) of olivine7,10.Thus, quantitative analysis of the LPO of olivine in NWA 5480and comparison with that of terrestrial samples or experimentaldata should expand our knowledge of the origin and formation ofharzburgitic diogenites. The results offer new insights into the com-plex, polyphase textural and microstructural evolution undergoneduring the thermal history of accretion, heating, differentiation,compaction, deformation and cooling of theHEDparent body.

The studied harzburgitic diogenite3,4 enables a unique, relativelyundisturbed view into primary processes as, in contrast to mostother diogenites1, NWA 5480 shows no sign of brecciation. Thedistribution 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 exhibitschlieren-like patterns2. 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 wellas a couple of larger (500–1,000 µm) patches of chromite withinthe schlieren region. Throughout the sample there is a general

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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1710

[100] [010] [001]

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b

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XO

Equal area, upper hemisphere,half-width 10°, cluster size 20°, multiples of uniform density (m.u.d.)

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Figure 2 | Stereographic projections of EBSD data for NWA 5480. Contour plots (Supplementary Fig. S1) show the measured crystallographic orientationsof olivine crystals from Zone A (a) and Zone B (b) according to the [100], [010] and [001] axes. Solid black line denotes the presumednortheast–southwest foliation (Fig. 1). Zone A reveals a well-developed LPO with [100] point maxima within the foliation and broad girdles of [010] and[001] perpendicular to the [100] direction. Zone B shows a weaker LPO with [100] point maxima around the centre and [010] and [001] ascircumferential girdles perpendicular to the [100] maxima.

northeast–southwest foliation (Fig. 1, white line) relative to thelower base of the polished sample, presentmainly as veins in Zone Aand as schlieren-features in Zone B. The olivine grains withinZone A are coarser (400–1,200 µm) compared to olivine grains(50–300 µm) within Zone B. TheMg/Fe ratio of olivine is similar inboth zones, with an approximated Mg# value of 70. The FeO/MnOratio of olivine with 40.8–44.5 confirms previous findings2.

Although using polarized light microscopy the olivine crystalsshow no SPO, the EBSD results reveal a well-defined preferredorientation of themain crystallographic axes [100], [010] and [001]in both zones (Fig. 2a,b and Supplementary Information S1 andFig. S1). Yet, the measured LPOs of the olivine crystals from theolivine-dominated Zone A (Fig. 2a) and those of the olivine crystalsfrom the orthopyroxene-schlieren region of Zone B (Fig. 2b) differsignificantly in intensity and orientation. This indicates two distincttexture-forming events.

In Zone A the [100] axes form point maxima within thefoliation. The other two main crystallographic axes of olivine,[010] and [001], show pronounced broad girdles perpendicularto the [100] point maxima (Fig. 2a). The LPOs of Zone B showa similar pattern. Here the [100] axes again form point maximawithin the assumed foliation but rotated roughly 90◦ along thefoliation. As in Zone A, the two other crystallographic axes, [010]and [001], plot perpendicular to the [100] point maxima inbroad circumferential girdles (Fig. 2b). These pronounced LPOsobserved in both zones reveal the unexpected occurrence of plasticdeformation on the HED parent body.

To better understand the nature of the texture-forming processesthe measured axes orientations are compared with compilationsof observed LPOs from common terrestrial mantle rocks andexperimental data. For ease of comparison, a schematic summaryof the LPOs of NWA 5480 Zone A, using the assumed northeast–

southwest foliation as the necessary reference frame, was compiledand rotated approximately 45◦ clockwise. In almost all cases ofpublished LPOs, the [010] axes of olivine in olivine-rich naturalrocks plot perpendicular to a given lineation6,7,9–11. We use thisgeneral observation to set the orientation of the lineation (L) withinthe sample, plotting it horizontally within the foliation (Fig. 3).

We have summarized the data schematically in Fig. 3a–g(Supplementary Information S1.1 and Table S1) depicting theaxes-plots [100], [010] and [001] as spotted, striped and solidgrey, respectively, and the foliation as a solid black line. Figure 3asummarizes the LPO from NWA 5480 Zone A. Figure 3bsummarizes the olivine LPO from a wehrlite intrusion within theOman ophiolite12, showing a well-developed SPO fabric typical ofcompaction processes7,12. The general observation for terrestrialcumulates and compacted olivine typically reveals strong pointmaxima of [010] normal to the foliation12 with the [100] and[001] axes as girdles along the foliation7,12. Being the slowestgrowth direction for olivine, [010] represents the shortest axis.If close to euhedral in shape, any olivine crystals sinking tothe bottom of a magma chamber will settle in the most stableposition with the shortest axis [010] parallel to the directionof cumulation and perpendicular to any foliation. The othertwo axes, [100] and [001], will be randomly distributed withinthe foliation, perpendicular to the direction of cumulation. Theobvious dissimilarity to the LPO of NWA 5480 Zone A (Fig. 3a)reveals that the latter was not caused by cumulation. According tostudies of more than 100 naturally deformed peridotite samples7,10over 95% show LPO patterns related to (010)[100] (Fig. 3c),the most active glide system during solid-state deformation inthe Earth’s mantle7,10,13. Figure 3d summarizes a common glidesystem (100)[001] (ref. 13) at low-temperature solid-state plasticdeformation7. Figure 3e5,7,10 summarizes the LPO generated by

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NATURE GEOSCIENCE DOI: 10.1038/NGEO1710 LETTERS

[100]

[010]

[001]

Foliation

LPO examples of terrestrial and experimental olivineNWA 5480 zone A

L L

a b c d

e f g

Figure 3 | Sketches summarizing LPO patterns found in olivine crystals. Comparison with LPOs of typical terrestrial mantle rocks, schematicallysummarized and adapted from published literature as referenced (Supplementary Table S1). a, NWA 5480 Zone A (Fig. 2). b, Cumulate olivine from theOman ophiolite12 showing the typical SPO pattern. c, One of the most common glide systems of olivine, (010)[100] (refs 7,10). d, A further common slipsystem in terrestrial olivines, the (100)[001] (ref. 13). e, The pencil glide system {0kl}[100] (refs 7,10), whereby multiple planes are activated to slip alongthe [100] direction. f, Olivine deformed under wet conditions14. g, Olivine deformed with 6% melt17.

Temperature (K)

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d = 50 km = 0.1 ¬ 4%ξ

Figure 4 | Comparison of experimental constraints and numerical results. Magnification of the temporal evolution of temperature (a) and silicate meltfraction (b) from a global numerical model of a Vesta-sized body with r= 265 km and tstart=0.5 Myr. The white line in a and b depicts the surface of thebody. Black lines in a show the temperature range where pencil glide is the dominant deformation mechanism5–7. Yellow lines in b mark 0.1% and 4% ofsilicate melt, and the red line is the maximum excavation depth (d= 50 km) of vestan meteorites22.

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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1710

dunites and peridotites plastically deformed in the solid stateby pencil-glide5,7, whereby multiple {0kl}[100] glide systems areactivated. Whereas the LPOs of Fig. 3b–d bear no resemblanceto the LPO of NWA 5480 (Fig. 3a), the LPO of Fig. 3e matchesthat of NWA 5480 Zone A, indicating the latter was deformedby the pencil-glide system. Figure 3f depicts the influence ofwater on olivine fabric14, which can be neglected here as theresulting LPO bears no similarity to the axes orientations measuredin NWA 5480 (Fig. 3a). Furthermore, low volatile abundancesfound in diogenites15,16 imply anhydrous mantle conditions in theHED parent body15,16. Similarly, the influence of partial melt canbe neglected here, as experimental studies17 yield olivine LPOs(Fig. 3g) distinct from the fabric fromNWA5480 Zone A.

Thus, in comparison with the most common LPOs formed innatural olivine-dominated terrestrial rocks the LPOs measured inZones A and B of NWA 5480 imply that the olivine crystals ofNWA5480were plastically deformed at least twice under solid-stateconditions in a temperature–pressure-strain rate regime where{0kl}[100] pencil-glide is the most active glide system. This mayhave occurred at depth within the mantle of the meteorite parentbody. Experimental work suggests that deformation by pencil-glideis active at moderate strain-rates5 and dry14, virtually melt-free17conditions at temperatures between 1,273 and 1,523K (refs 5–7),delivering a temperature constraint for the plastic deformationin NWA 5480. Therefore we draw our attention to the scenarioof a solid-state deformation mechanism within the early stagesafter asteroid formation.

To identify a feasible mechanism for this dynamic process weperformed 2D numerical models18 (Supplementary Methods S3)of Vesta and its early evolution. The modelled conditions at 8.6,30 and 50Myr (Fig. 4) reveal that after core formation a thinthermal lid forms on top of the partially molten mantle. As thepartially molten silicate layer beneath exhibits a low viscosity,this favours large growth rates of Rayleigh–Taylor instabilities19,20.Assuming dry conditions, the resulting downwelling material iswithin the temperature range relevant for pencil-glide (Fig. 4aand Supplementary Movie). These downwellings exhibit no oronly very small silicate melt fractions (Fig. 4b), in agreement withobservations for the solid-state deformation of NWA 5480 andexperimental studies17 revealing that the absence of melt bands inNWA 5480 constrains the silicate melt fraction to <4% (Fig. 4b,yellow contours). As the interior cools, the thickness of the rigid partof the thermal lid grows over time20. Thus, part of the lid materialexperiences solid-state deformation without finally sinking towardsthe core–mantle boundary, thereby fossilizing the deformation pat-tern in these regions. In our model the downwellings occur withinthe first 50Myr within a depth range (Fig. 4b, red line) from whichlater impact excavation via the Rheasilvia crater at the south pole ofVesta21, themost likely source ofHEDmeteorites1, is feasible22.

Whereas the LPO of NWA 5480 documents solid-state plasticdeformation in virtually melt-free conditions, deeper layers musthave been partially molten to allow viscosities low enough to inducebuoyancy-driven downwellings to develop within geologically rele-vant timescales. This has implications for the timing of solidificationof the HED parent body, which is still a matter of keen debate. Inline with our model, most geochemical data23–25 and numerical1D models26,27 indicate that a considerable portion of Vesta’sinterior must have been partially molten during the first 100Myrafter accretion. Recent isotopic data indicate solidification occurredafter 3–4Myr (refs 16,28). Other isotopic studies of zircons found ineucrites conclude magmatism occurred 7–24Myr (ref. 29) after theformation of calcium–aluminium-rich inclusions (CAI). Althoughconsolidation of these various data is necessary, our modelremains valid for virtually all temporal scenarios (SupplementaryInformation S2 and Fig. S2), with downwellings beginning as earlyas 3–4Myr after CAI formation (SupplementaryMovie).

To conclude, the LPOsmeasured from both zones of NWA 5480are most likely formed by deformation via the pencil-glide system{0kl}[100], indicating that NWA 5480 underwent at least two dis-tinct solid-state plastic deformation events at temperatures between1,273 and 1,523K. Assuming a dry environment with low meltfractions, this result contradicts previous generalized ideas30 abouttexture-forming processes for diogenites, ruling out deformation asa cumulate rock in amagma chamber. Our numerical models revealearly downwellings of lid material on the HED parent body as afeasible explanation for the documented solid-state plastic defor-mation of NWA 5480, suggesting a dynamic asteroid mantle and ageologically active phase in the early evolution of planet-building-blocks such as Vesta. This unexpected dynamic process in the solidi-fied lid over a partiallymolten asteroidmantle will enable prolongedfast mass exchange, therefore providing an effective mechanism forlate-stage chemical redistribution and homogenization.

MethodsA thick section (3×23×35mm) of NWA 5480 was embedded in epoxy resin andpolished with Syton polish. It was thinly coated with carbon and surrounded onall sides by a thin copper band to minimize ionic charging on the sample surface.Analysis was performed in the Nanoscience Laboratory of the Geoscience Instituteat Goethe University Frankfurt using a Jeol Scanning Electron Microscope JSM6490 equipped with an EBSD detector. The software used for imaging and mineralcomposition analysis included an Inca Energy Dispersive X-ray Spectroscopysystem and HKL Channel 5 of Oxford Instruments and HKL Technology,respectively. The first 84 back-scattered electron-images were collected at lowmagnification and stitched together as an overviewmap. EBSD was performed withan acceleration voltage of 15 kV, a beam current of 35–40 µA, a working distanceof 20mm, a detector insertion of 176mm and a Si-wafer as calibrant. WithinZone A, the olivine-dominated zone, a total of 1361 crystallographic orientationmeasurements of olivine were recorded manually over a total of 58 samplelocations, each about 800×500 µm2 in size. Indexing was monitored manuallyand only those EBSD measurements with a mean angular deviation (MAD) of<1◦ were accepted and recorded. Within Zone B, the schlieren zone, a total of 148crystallographic orientation measurements of olivine were recorded over a total of20 sample locations (Supplementary AnalyticalMethods S1.1).

Received 21 June 2012; accepted 17 December 2012;published online 20 January 2013

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AcknowledgementsWe thank T. V. Gerya for providing the code 12MART. Funding to G.J.G. was providedby SNF grant PBEZP2-134461.

Author contributionsB.J.T. and F.E.B. conceived this project. B.J.T. carried out the EBSDmeasurements on thesample. B.J.T. and F.E.B. analysed and discussed the results of the EBSD measurements.G.J.G. designed and implemented the numerical model. B.J.T., G.J.G. and F.E.B. analysedand discussed the results. B.J.T. prepared the manuscript, which was then jointly editedby B.J.T., G.J.G. and F.E.B.

Additional informationSupplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints. Correspondenceand requests for materials should be addressed to B.J.T.

Competing financial interestsThe authors declare no competing financial interests.

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