1. LETTERS
PUBLISHED ONLINE: 20 JANUARY 2013 | DOI: 10.1038/NGEO1710
Solid-state plastic deformation in the dynamic
interior of a differentiated asteroid
B. J. Tkalcec1 *, G. J. Golabek2,3 and F. E. Brenker1
Diogenite meteorites are thought to represent mantle rocks
that formed as cumulates in magma chambers on 4 Vesta or
a similar differentiated asteroid1,2 . Northwest Africa 5480 is a
harzburgitic diogenite3,4 , 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-glide5 slip system. The detected type of preferred
orientation is well known from dry ultramafic rocks on Earth,
where it is typically formed by mantle shear5–7 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 longlasting 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 differentiated asteroid 4 Vesta, or a Vesta-like body1 . 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 diogenite3,4 . Diogenites have so far
been thought to represent ultramafic cumulate rocks formed at deep
crustal or upper mantle levels of the parent body1,2 . Most studies
of HEDs have concentrated on the geochemistry and petrology of
these achondrites1–3 . 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 orientation7
(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
Zone B
1 mm
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 olivine7,10 .
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 complex, 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 diogenite3,4 enables a unique, relatively
undisturbed view into primary processes as, in contrast to most
other diogenites1 , 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 minerals, whereby some dominantly orthopyroxene regions exhibit
schlieren-like patterns2 . The sample can be subdivided into two petrographically distinct regions (Fig. 1) for targeted analysis: Zone A,
the olivine-dominated region, and Zone B, the orthopyroxenedominated schlieren region. Minor chromite, troilite and occasional 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
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.
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2. NATURE GEOSCIENCE DOI: 10.1038/NGEO1710
LETTERS
a
[100]
Y
[010]
[001]
Zone A
[010]
[001]
Zone B
XO
b
[100]
5
10
Equal area, upper hemisphere,
half-width 10°, cluster size 20°, multiples of uniform density (m.u.d.)
Figure 2 | Stereographic projections of EBSD data for NWA 5480. Contour plots (Supplementary Fig. S1) show the measured crystallographic orientations
of olivine crystals from Zone A (a) and Zone B (b) according to the [100], [010] and [001] axes. Solid black line denotes the presumed
northeast–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] as
circumferential girdles perpendicular to the [100] maxima.
northeast–southwest foliation (Fig. 1, white line) relative to the
lower base of the polished sample, present mainly as veins in Zone A
and as schlieren-features in Zone B. The olivine grains within
Zone A are coarser (400–1,200 µm) compared to olivine grains
(50–300 µm) within Zone B. The Mg/Fe ratio of olivine is similar in
both zones, with an approximated Mg# value of 70. The FeO/MnO
ratio of olivine with 40.8–44.5 confirms previous findings2 .
Although using polarized light microscopy the olivine crystals
show no SPO, the EBSD results reveal a well-defined preferred
orientation of the main crystallographic axes [100], [010] and [001]
in both zones (Fig. 2a,b and Supplementary Information S1 and
Fig. S1). Yet, the measured LPOs of the olivine crystals from the
olivine-dominated Zone A (Fig. 2a) and those of the olivine crystals
from the orthopyroxene-schlieren region of Zone B (Fig. 2b) differ
significantly in intensity and orientation. This indicates two distinct
texture-forming events.
In Zone A the [100] axes form point maxima within the
foliation. The other two main crystallographic axes of olivine,
[010] and [001], show pronounced broad girdles perpendicular
to the [100] point maxima (Fig. 2a). The LPOs of Zone B show
a similar pattern. Here the [100] axes again form point maxima
within the assumed foliation but rotated roughly 90◦ along the
foliation. As in Zone A, the two other crystallographic axes, [010]
and [001], plot perpendicular to the [100] point maxima in
broad circumferential girdles (Fig. 2b). These pronounced LPOs
observed in both zones reveal the unexpected occurrence of plastic
deformation on the HED parent body.
To better understand the nature of the texture-forming processes
the measured axes orientations are compared with compilations
of observed LPOs from common terrestrial mantle rocks and
experimental data. For ease of comparison, a schematic summary
of the LPOs of NWA 5480 Zone A, using the assumed northeast–
southwest foliation as the necessary reference frame, was compiled
and rotated approximately 45◦ clockwise. In almost all cases of
published LPOs, the [010] axes of olivine in olivine-rich natural
rocks plot perpendicular to a given lineation6,7,9–11 . We use this
general observation to set the orientation of the lineation (L) within
the 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 the
axes-plots [100], [010] and [001] as spotted, striped and solid
grey, respectively, and the foliation as a solid black line. Figure 3a
summarizes the LPO from NWA 5480 Zone A. Figure 3b
summarizes the olivine LPO from a wehrlite intrusion within the
Oman ophiolite12 , showing a well-developed SPO fabric typical of
compaction processes7,12 . The general observation for terrestrial
cumulates and compacted olivine typically reveals strong point
maxima of [010] normal to the foliation12 with the [100] and
[001] axes as girdles along the foliation7,12 . Being the slowest
growth direction for olivine, [010] represents the shortest axis.
If close to euhedral in shape, any olivine crystals sinking to
the bottom of a magma chamber will settle in the most stable
position with the shortest axis [010] parallel to the direction
of cumulation and perpendicular to any foliation. The other
two axes, [100] and [001], will be randomly distributed within
the foliation, perpendicular to the direction of cumulation. The
obvious dissimilarity to the LPO of NWA 5480 Zone A (Fig. 3a)
reveals that the latter was not caused by cumulation. According to
studies of more than 100 naturally deformed peridotite samples7,10
over 95% show LPO patterns related to (010)[100] (Fig. 3c),
the most active glide system during solid-state deformation in
the Earth’s mantle7,10,13 . Figure 3d summarizes a common glide
system (100)[001] (ref. 13) at low-temperature solid-state plastic
deformation7 . Figure 3e5,7,10 summarizes the LPO generated by
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3. NATURE GEOSCIENCE DOI: 10.1038/NGEO1710
a
LETTERS
b
c
NWA 5480 zone A
d
LPO examples of terrestrial and experimental olivine
L
L
e
f
g
[100]
[010]
[001]
Foliation
Figure 3 | Sketches summarizing LPO patterns found in olivine crystals. Comparison with LPOs of typical terrestrial mantle rocks, schematically
summarized and adapted from published literature as referenced (Supplementary Table S1). a, NWA 5480 Zone A (Fig. 2). b, Cumulate olivine from the
Oman 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 slip
system 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 along
the [100] direction. f, Olivine deformed under wet conditions14 . g, Olivine deformed with 6% melt17 .
a
Temperature (K)
500
1,000
1,500
Temperature (K)
2,000
500
240
1,000
1,500
Temperature (K)
2,000
500
240
1,500
2,000
240
260
Time = 8.65 Myr
260
280
T = 1,273 ¬ 1,523 K
280
280
300
300
320
260
Time = 30.12 Myr
Z (km)
Z (km)
300
Z (km)
1,000
320
Time = 50.12 Myr
320
340
340
340
360
360
360
380
380
380
400
240 260 280 300 320 340 360 380 400
400
240 260 280 300 320 340 360 380 400
400
240 260 280 300 320 340 360 380 400
X (km)
b
0
X (km)
Silicate melt fraction (%)
10
20
30
40
0
240
X (km)
Silicate melt fraction (%)
10
20
30
40
0
240
240
Time = 8.65 Myr
260
280
d = 50 km
ξ = 0.1 ¬ 4%
280
280
300
300
320
340
260
Time = 30.12 Myr
Z (km)
300
Z (km)
260
Z (km)
Silicate melt fraction (%)
10
20
30
40
320
340
Time = 50.12 Myr
320
340
360
360
360
380
380
380
400
240 260 280 300 320 340 360 380 400
400
240 260 280 300 320 340 360 380 400
400
240 260 280 300 320 340 360 380 400
X (km)
X (km)
X (km)
Figure 4 | Comparison of experimental constraints and numerical results. Magnification of the temporal evolution of temperature (a) and silicate melt
fraction (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 the
body. 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% of
silicate melt, and the red line is the maximum excavation depth (d = 50 km) of vestan meteorites22 .
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4. NATURE GEOSCIENCE DOI: 10.1038/NGEO1710
LETTERS
dunites and peridotites plastically deformed in the solid state
by pencil-glide5,7 , whereby multiple {0kl}[100] glide systems are
activated. Whereas the LPOs of Fig. 3b–d bear no resemblance
to the LPO of NWA 5480 (Fig. 3a), the LPO of Fig. 3e matches
that of NWA 5480 Zone A, indicating the latter was deformed
by the pencil-glide system. Figure 3f depicts the influence of
water on olivine fabric14 , which can be neglected here as the
resulting LPO bears no similarity to the axes orientations measured
in NWA 5480 (Fig. 3a). Furthermore, low volatile abundances
found in diogenites15,16 imply anhydrous mantle conditions in the
HED parent body15,16 . Similarly, the influence of partial melt can
be neglected here, as experimental studies17 yield olivine LPOs
(Fig. 3g) distinct from the fabric from NWA 5480 Zone A.
Thus, in comparison with the most common LPOs formed in
natural olivine-dominated terrestrial rocks the LPOs measured in
Zones A and B of NWA 5480 imply that the olivine crystals of
NWA 5480 were plastically deformed at least twice under solid-state
conditions in a temperature–pressure-strain rate regime where
{0kl}[100] pencil-glide is the most active glide system. This may
have occurred at depth within the mantle of the meteorite parent
body. Experimental work suggests that deformation by pencil-glide
is active at moderate strain-rates5 and dry14 , virtually melt-free17
conditions at temperatures between 1,273 and 1,523 K (refs 5–7),
delivering a temperature constraint for the plastic deformation
in NWA 5480. Therefore we draw our attention to the scenario
of a solid-state deformation mechanism within the early stages
after asteroid formation.
To identify a feasible mechanism for this dynamic process we
performed 2D numerical models18 (Supplementary Methods S3)
of Vesta and its early evolution. The modelled conditions at 8.6,
30 and 50 Myr (Fig. 4) reveal that after core formation a thin
thermal lid forms on top of the partially molten mantle. As the
partially 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 is
within the temperature range relevant for pencil-glide (Fig. 4a
and Supplementary Movie). These downwellings exhibit no or
only very small silicate melt fractions (Fig. 4b), in agreement with
observations for the solid-state deformation of NWA 5480 and
experimental studies17 revealing that the absence of melt bands in
NWA 5480 constrains the silicate melt fraction to <4% (Fig. 4b,
yellow contours). As the interior cools, the thickness of the rigid part
of the thermal lid grows over time20 . Thus, part of the lid material
experiences solid-state deformation without finally sinking towards
the core–mantle boundary, thereby fossilizing the deformation pattern in these regions. In our model the downwellings occur within
the first 50 Myr within a depth range (Fig. 4b, red line) from which
later impact excavation via the Rheasilvia crater at the south pole of
Vesta21 , the most likely source of HED meteorites1 , is feasible22 .
Whereas the LPO of NWA 5480 documents solid-state plastic
deformation in virtually melt-free conditions, deeper layers must
have been partially molten to allow viscosities low enough to induce
buoyancy-driven downwellings to develop within geologically relevant timescales. This has implications for the timing of solidification
of the HED parent body, which is still a matter of keen debate. In
line with our model, most geochemical data23–25 and numerical
1D models26,27 indicate that a considerable portion of Vesta’s
interior must have been partially molten during the first 100 Myr
after accretion. Recent isotopic data indicate solidification occurred
after 3–4 Myr (refs 16,28). Other isotopic studies of zircons found in
eucrites conclude magmatism occurred 7–24 Myr (ref. 29) after the
formation of calcium–aluminium-rich inclusions (CAI). Although
consolidation of these various data is necessary, our model
remains valid for virtually all temporal scenarios (Supplementary
Information S2 and Fig. S2), with downwellings beginning as early
as 3–4 Myr after CAI formation (Supplementary Movie).
1. McSween, H. Y., Mittlefehldt, D. W., Beck, A. W., Mayne, R. G. & McCoy, T. J.
HED Meteorites and their relationship to the Geology of Vesta and the Dawn
Mission. Space Sci. Rev. 163, 141–174 (2011).
2. Irving, A. J., Bunch, T. E., Kuehner, S. M., Wittke, J. H. & Rumble, D.
Peridotites related to 4 Vesta: Deep crustal igneous cumulates and mantle
samples. Lunar Planet. Sci. 40, 2466 (2009).
3. Beck, A. & McSween, H. Y. Diogenites as polymict breccias composed of
orthopyroxenite and harzburgite. Meteorit. Planet. Sci. 45, 850–872 (2010).
4. Wittke, J. H., Irving, A. J., Bunch, T. E. & Kuehner, S. M. A nomenclature
system for diogenites consistent with the IUGS system for naming terrestrial
ultramafic rocks. Meteoritics Planet. Sci. 46 (Meteoritical Society LXXIV),
Abstr. 5223 (2011).
5. Carter, N. L. & Avé Lallement, H. G. High temperature flow of dunite and
peridotite. Geol. Soc. Am. Bull. 81, 2181–2202 (1970).
6. Guegen, Y. & Nicolas, A. Deformation of mantle rocks. Annu. Rev. Earth
Planet. Sci. 8, 119–144 (1980).
7. Tommasi, A., Mainprice, D., Canova, G. & Chasatel, Y. Viscoplastic
self-consistent and equilibrium-based modeling of olivine lattice preferred
orientations: Implications for the upper mantle seismic anisotropy. J. Geophys.
Res. 105, 7893–7908 (2000).
8. Prior, D. J. et al. The application of electron backscatter diffraction and
orientation contrast imaging in the SEM to textural problems in rocks.
Am. Mineral. 84, 1741–1759 (1999).
9. Warren, J. M., Hirth, G. & Kelemen, P. B. Evolution of olivine lattice preferred
orientation during simple shear in the mantle. Earth Planet. Sci. Lett. 272,
501–512 (2008).
10. Ismaïl, W. B. & Mainprice, D. An olivine fabric database: An overview of
upper mantle fabrics and seismic anisotropy. Tectonophysics 296,
145–157 (1998).
11. Bystricky, M., Kunze, K., Burlini, L. & Burg, J-P. High shear strain of
olivine aggregates: Rheological and seismic consequences. Science 290,
1564–1566 (2000).
96
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To conclude, the LPOs measured from both zones of NWA 5480
are most likely formed by deformation via the pencil-glide system
{0kl}[100], indicating that NWA 5480 underwent at least two distinct solid-state plastic deformation events at temperatures between
1,273 and 1,523 K. Assuming a dry environment with low melt
fractions, this result contradicts previous generalized ideas30 about
texture-forming processes for diogenites, ruling out deformation as
a cumulate rock in a magma chamber. Our numerical models reveal
early downwellings of lid material on the HED parent body as a
feasible explanation for the documented solid-state plastic deformation of NWA 5480, suggesting a dynamic asteroid mantle and a
geologically active phase in the early evolution of planet-buildingblocks such as Vesta. This unexpected dynamic process in the solidified lid over a partially molten asteroid mantle will enable prolonged
fast mass exchange, therefore providing an effective mechanism for
late-stage chemical redistribution and homogenization.
Methods
A thick section (3 × 23 × 35 mm) of NWA 5480 was embedded in epoxy resin and
polished with Syton polish. It was thinly coated with carbon and surrounded on
all sides by a thin copper band to minimize ionic charging on the sample surface.
Analysis was performed in the Nanoscience Laboratory of the Geoscience Institute
at Goethe University Frankfurt using a Jeol Scanning Electron Microscope JSM
6490 equipped with an EBSD detector. The software used for imaging and mineral
composition analysis included an Inca Energy Dispersive X-ray Spectroscopy
system and HKL Channel 5 of Oxford Instruments and HKL Technology,
respectively. The first 84 back-scattered electron-images were collected at low
magnification and stitched together as an overview map. EBSD was performed with
an acceleration voltage of 15 kV, a beam current of 35–40 µA, a working distance
of 20 mm, a detector insertion of 176 mm and a Si-wafer as calibrant. Within
Zone A, the olivine-dominated zone, a total of 1361 crystallographic orientation
measurements of olivine were recorded manually over a total of 58 sample
locations, each about 800 × 500 µm2 in size. Indexing was monitored manually
and 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 148
crystallographic orientation measurements of olivine were recorded over a total of
20 sample locations (Supplementary Analytical Methods S1.1).
Received 21 June 2012; accepted 17 December 2012;
published online 20 January 2013
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© 2013 Macmillan Publishers Limited. All rights reserved.

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Acknowledgements
We thank T. V. Gerya for providing the code 12MART. Funding to G.J.G. was provided
by SNF grant PBEZP2-134461.
Author contributions
B.J.T. and F.E.B. conceived this project. B.J.T. carried out the EBSD measurements on the
sample. 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. analysed
and discussed the results. B.J.T. prepared the manuscript, which was then jointly edited
by B.J.T., G.J.G. and F.E.B.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints. Correspondence
and requests for materials should be addressed to B.J.T.
Competing financial interests
The authors declare no competing financial interests.
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