Olivine in an_unexpected_location_on_vesta_surface

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Olivine in an unexpected location on Vesta’s surfaceE. Ammannito1, M. C. De Sanctis1, E. Palomba1, A. Longobardo1, D. W. Mittlefehldt2, H. Y. McSween3, S. Marchi1,4, M. T. Capria1,F. Capaccioni1, A. Frigeri1, C. M. Pieters5, O. Ruesch6, F. Tosi1, F. Zambon1, F. Carraro1, S. Fonte1, H. Hiesinger6, G. Magni1,L. A. McFadden7, C. A. Raymond8, C. T. Russell9 & J. M. Sunshine10

Olivine is a major component of the mantle of differentiatedbodies, including Earth. Howardite, eucrite and diogenite (HED)meteorites represent regolith, basaltic-crust, lower-crust and pos-sibly ultramafic-mantle samples of asteroid Vesta, which is the lonesurviving, large, differentiated, basaltic rocky protoplanet in theSolar System1. Only a few of these meteorites, the orthopyroxene-rich diogenites, contain olivine, typically with a concentration ofless than 25 per cent by volume2. Olivine was tentatively identifiedon Vesta3,4, on the basis of spectral and colour data, but otherobservations did not confirm its presence5. Here we report thatolivine is indeed present locally on Vesta’s surface but that, unex-pectedly, it has not been found within the deep, south-pole basins,which are thought to be excavated mantle rocks6–8. Instead, it occursas near-surface materials in the northern hemisphere. Unlike themeteorites, the olivine-rich (more than 50 per cent by volume)material is not associated with diogenite but seems to be mixed withhowardite, the most common7,9 surface material. Olivine is exposedin crater walls and in ejecta scattered diffusely over a broad area. Thesize of the olivine exposures and the absence of associated diogenitefavour a mantle source, but the exposures are located far from thedeep impact basins. The amount and distribution of observed olivine-rich material suggest a complex evolutionary history for Vesta.

The Visible and Infrared Mapping Spectrometer (VIR) on boardNASA’s Dawn spacecraft10 has been used in a global search for olivineon the Vestan surface (Supplementary Information). VIR revealed aglobal-scale dichotomy7,8 (Fig. 1), with diogenite-rich material exposedpredominantly in the deeply excavated southern hemisphere. Magma-ocean models for Vesta’s differentiation yield eucritic crust overlying adiogenite layer, with olivine-rich mantle rocks and a metallic core inthe deep interior11,12. These models predict mineralogical variationson a large vertical scale, with olivine-rich cumulates occurring belowolivine-poor diogenite. Alternative models, more consistent with thediverse trace-element geochemistry of diogenites, posit that diogeniticplutons occur at the crust–mantle boundary or within the basalticcrust13,14, resulting in association of olivine-rich and orthopyroxene-rich diogenites mixed on smaller scales.

VIR spectra did not provide definitive evidence for olivine withinthe two large basins in the southern hemisphere6–8. However, typicalolivine-bearing diogenites cannot be easily distinguished spectrallyfrom olivine-free diogenites1 because of the difficulty of identifyingolivine at low concentrations in the presence of abundant orthopyr-oxene15,16; thus, olivine may be present within the southern basins butonly in modest amounts (=25 vol%, comparable to that reported formost olivine-bearing diogenites2).

Unexpectedly, olivine-rich areas have now been discovered in thenorthern hemisphere. The VIR spectra of ejecta surrounding Arruntiacrater and the nearby Bellicia crater (Fig. 2) reveal clear olivine signatures(Fig. 3a), with the 1-mm band (hereafter BI) centred at slightly longerwavelength than the average Vesta spectrum, and the centre of the2-mm band (hereafter BII) is unchanged. Laboratory data demonstrate

that pyroxene features dominate the spectra of olivine–pyroxenemixtures15,16. Only olivine contents of $50 vol% produce a shift inthe centre of BI15,16, and the centre of BII remains unchanged withadmixture of olivine (Fig. 3b). The three parameters we used to inter-pret olivine–pyroxene mixtures are the positions of the respectivecentres of BI and BII and variations in the band area ratio16–18 (BAR).In the BI–BII diagram (Fig. 3c), Bellicia and Arruntia data lie distinctlyoff the linear HED trend, with high values for BI centres that reveal thepresence of olivine. Because the BII-centre position reflects the com-position of pyroxene in olivine–pyroxene mixtures and, in the Bellicia–Arruntia area, lies between those of eucrites and diogenites, we havedetermined that the olivine in this area is associated with the mixedlithology, howardite.

This situation is distinct from the olivine occurrence in HEDmeteorites, where only very small amounts of olivine (#3 vol%) occurin howardites19 (with the exception of the paired PCA 02 howardites,which nevertheless contain at most ,7% olivine20). Olivine in HEDmeteorites occurs mainly in diogenites, which range from orthopyr-oxenite to harzburgite to dunite21 (Extended Data Fig. 1 and ExtendedData Table 1). This observation is consistent with the interpretationthat HED meteorites sample lithologies from Vesta’s southern hemi-sphere that are associated with material ejected from the two largebasins22.

The Vestan olivine-rich spectra and derived parameters are consist-ent with a mixture of 50–80-vol% olivine with pyroxene occurring overa broad area of hundred-kilometre size, encompassing both Belliciacrater and Arruntia crater. Olivine-rich material occurs as severalhigh-albedo patches hundreds of metres across located high on thewalls of Bellicia crater (Fig. 2b–d). Some of these patches have positiverelief compared with the adjacent wall, suggesting more competentmaterial (Fig. 2f). Several fresh small craters (with diameters of order100 m) superposed on Bellicia ejecta also have high-albedo annuli witholivine spectral signatures (Fig. 2e). Olivine-rich material at Arruntiacrater is most common in the ejecta blanket (Fig. 2g, h). The geologicalsetting suggests that olivine-rich lithologies occur as a bright layerpartly obscured by slump deposits and regolith mixing of the surface.Unlike its occurrence in HED meteorites, mainly as a small volumefraction in diogenites, here a lithology rich in olivine ($50 vol%) inpatches hundreds of metres in size is mixed with howarditic regolith.

The detected olivine-rich materials have characteristics at odds withpre-Dawn ideas about Vestan olivine: they are not associated withdiogenites, they are located far from deeply excavated terrains in thesouthern hemisphere and they occur in large patches extending hun-dreds of metres.

Both exogenic and endogenic origins are possible. An exogenicorigin seems unlikely, considering how uncommon xenocrystic (chon-dritic) olivine is in howardites20 and the rarity of olivine-rich asteroidsin the main belt23 (Supplementary Information). Also, the large patchesseem inconsistent with the fact that impactors are normally disaggre-gated. On the other hand, endogenic olivine is a component of Vesta, as

1Istituto di Astrofisica e Planetologia Spaziali, INAF, 00133 Rome, Italy. 2NASA Johnson Space Center, Houston, Texas 77058, USA. 3Department of Earth and Planetary Sciences, University of Tennessee,Knoxville, Tennessee 37996, USA. 4NASA Lunar Science Institute, Boulder, Colorado 80302, USA. 5Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. 6Institut furPlanetologie, Westfalische Wilhelms-Universitat Munster, 48149 Munster, Germany. 7NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA. 8Jet Propulsion Laboratory, California Instituteof Technology, Pasadena, California 91109, USA. 9University of California, Los Angeles, California 90095, USA. 10Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA.

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demonstrated by its occurrence in diogenites and even in the PCA 02howardites, where the target rock for olivine-bearing impact melts inthese breccias was olivine-rich diogenite20.

Two main models for the origin of endogenic olivine are serial-magmatism models that consider fractional crystallization in diogeniteplutons emplaced at the base of, or within, the Vestan crust13,14, and

magma-ocean models that predict an olivine-dominated mantle atdepths of .20–40 km underlying an orthopyroxene-dominated (dio-genitic) lower crust11,12.

In the serial-magmatism hypothesis, a mixed region of eucrite, dio-genite and olivine-rich material could have been sampled by impactsthat did not excavate to great depth. The magma-ocean hypothesis

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Figure 2 | Olivine-rich region in the visible andnear-infrared wavelengths. a, Infrared mosaics ofVIR data (longitudes 30u–77uW, latitudes25u–60uN). Bellicia crater (38 km 3 43 km,longitude 48uW, latitude 38uN) and Arruntiacrater (,11-km diameter, longitude 72uW,latitude 40uN) are enclosed in black squares.Coordinates in Claudia system. The mosaicshave been made using VIR data from differentobservation cycles. The false colours (red, 1.25 mm;green, 1.93mm; blue, 1.64mm), emphasize in greenthe olivine-rich region. b, Stretched view of Belliciacrater in false colours (same as in a) showing ingreen the purest olivine exposures. c–f, FramingCamera images of Bellicia crater. c, Image (,65 mper pixel) highlighting example locations of brightmaterials carrying the olivine-rich spectralsignature (arrows). Numbered arrows show(1) olivine-rich material associated with a slumpdeposit downslope from a small crater;(2) comingled bright and dark materials on thecrater wall; (3) relatively dark material adjacent toolivine-rich material; and (4) small craters onBellicia ejecta that expose olivine-rich material.d, Image (,22 m per pixel) showing details of aportion of the crater wall. e, Image (,65 m perpixel) highlighting the small craters with olivineannuli. f, Image (,22 m per pixel) showing detailsof (1) portion of the crater wall. g, h, FramingCamera images of Arruntia crater. g, Image(,22 m per pixel) showing locations ofconcentrations of olivine-rich material (arrows) inArruntia ejecta. h, Image (,65 m per pixel)showing details of crater wall geology. Lenses ofbright material are present (5), and dark materialsare comingled with bright materials (6).

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Figure 1 | HED meteorite distribution map. Lithological map of Vesta’ssurface derived from VIR spectra8,10 using all the data acquired during theDawn orbital phases: red for diogenite, green for howardite, blue for eucrite,with overlapping fields of yellow for diogenitic howardite and cyan for eucritichowardite. The regions with magnesium-rich pyroxenes (red and yellow)correspond to a diogenite-dominated lithology. The distribution shows that thesouthern hemisphere is more rich in magnesiac pyroxene with areas of nearly

pure diogenite, whereas the equatorial region and the northern hemisphereare more basalt-rich (eucritic). Howardites—brecciated mixtures of theselithologies—are the most abundant rocks observed on Vesta’s surface. Arruntiaand Bellicia craters are indicated, as well as the rim of Rheasilvia and Veneneiabasins (dashed line). Howardites enriched in diogenites are visible in theruined northern basins and in Rheasilvia (see Supplementary Information forfurther details and Extended Data Fig. 2).

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implies that mantle olivine would be excavated only by large, basin-forming impacts.

In the Bellicia–Arruntia region, we see patches of nearly pure olivine,hundreds of metres in size, in a background of howarditic material thatsuggest a large olivine-dominated source, with coherent sub-kilometre-size ejecta. The serial-magmatism model envisions smaller scales ofpetrologic variation14,24, suggesting a mixed lithology of olivine andorthopyroxene that is not observed. The occurrence of several olivinespots a few hundreds of metres across, as seen in the walls of Bellicia,seems hard to reconcile with the plutonic origin.

In the magma-ocean model, the Rheasilvia basin, superimposed onthe older Veneneia25,26 basin, could have excavated and redistributedmantle material across Vesta27. The mineralogical diversity of theequatorial regions versus southern regions7,8 indicates that the lowercrust and upper mantle, which are dominated by diogenitic material,were exposed by these impacts and were deposited as an extensive areaof Rheasilvia ejecta in the northwest direction (Fig. 1), but most prob-ably not extending to the Bellicia–Arruntia region.

The presence of the olivine in the hemisphere opposite the largesouthern basins raises the question of antipodal focusing of energyleading to excavation of olivine-rich materials from depth. However,a large, high-velocity metallic core, such as in Vesta1, should defocusand deflect the energy away from the collision28. Thus, the olivine isprobably not due to antipodal excavation.

Diogenite-rich materials in the northern regions are concentrated inan area broadly corresponding to a 180-km ruined crater25 near Belliciaand in other large craters farther north (Fig. 1). Thus, the northerndiogenitic material might have been ejected by these other ancient largeimpacts. However, the depths of the old basins near Bellicia andArruntia are 10 and 15 km (ref. 25), respectively, possibly makingthe basins too shallow to reach the mantle.

A generalized geologic history for these olivine-rich materials couldbe as follows: ancient large impacts excavated and incorporated largeblocks of diogenite-rich and olivine-rich material into the eucriticcrust, and subsequent impacts exposed this olivine-rich material inArruntia and Bellicia. This produced olivine-rich terrains in a howar-ditic background, with diogenite-rich howardites filling nearby, eroded,older basins.

The large exposures of olivine-rich material and their associationwith howardite may favour a magma-ocean model for the origin of theolivine. However, the apparent absence of olivine concentrations inRheasilvia, where the excavation depth is greater, may suggest that theinternal distribution of lithologies was heterogeneous, perhaps sup-porting the serial-magmatism model, or that the crust–mantle boundarywas deeper in the region excavated by Rheasilvia than in the Bellicia–Arruntia region. In any case, the lack of pure olivine in the southerndeeply excavated basins and its unexpected discovery in the northernhemisphere of Vesta indicate a more complex evolutionary historythan inferred from pre-Dawn models.

Online Content AnyadditionalMethods, ExtendedData display items and SourceData are available in the online version of the paper; references unique to thesesections appear only in the online paper.

Received 10 April; accepted 13 September 2013.

Published online 6 November 2013.

1. Russell, C. T. et al. Dawn at Vesta: testing the protoplanetary paradigm. Science336, 684–686 (2012).

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Figure 3 | Spectral characteristics of the olivine-rich areas. a, Continuum-removed average Vestan spectrum and continuum-removed spectrum of theolivine-rich area in Bellicia. Olivine-rich spectra show a large asymmetric BI,typical of olivine-rich mixtures, whereas BII indicates that pyroxene is alsopresent. The BI centre is at a slightly longer wavelength with respect theaverage spectrum, but the BII centre does not shift, as would be the case foriron-rich pyroxenes typical of eucrites or for high-calcium clinopyroxenes.b, Coloured lines show spectra of mixtures of olivine (Ol) and orthopyroxene(Ortho-Pyx) (data from the RELAB database) and the green points show thespectrum of the olivine-rich area. Laboratory olivine spectra exhibit only abroad, asymmetric 1-mm feature due to the overlapping of three individualabsorptions29, whereas orthopyroxene exhibits two well-defined, symmetricabsorptions near 1mm and, respectively, 2mm (refs 17, 30). Spectra of mixturesof olivine–orthopyroxene show that large olivine contents (.50%) producedistortion of the band shape near 1mm from that of pure pyroxene. Moresensitive indications of olivine in a mixture are a shallow depression near1.3mm and a reduction in depth of BII pyroxene absorption. c, Scatter plot ofband centres. HED meteorite data are represented as coloured circles and lie ona linear correlation trend: eucrites and diogenites data are well separated, withthe howardite data between them. For olivine–orthopyroxene mixtures (30–70%, 50–50%, 70–30%; orange squares), the BI centre shifts towards longerwavelengths for increasing olivine content as illustrated by the arrow, but littleor no shift is registered in the BII centre. The olivine-rich area inside Bellicia(green squares), Bellicia walls (brown cross) and a control area nearby Bellicia(cyan cross) are also represented. The olivine-rich points scatter above the HEDmeteorite trend and separate from the control area, which lies in the HEDmeteorite field. The Bellicia walls data lie between the olivine-rich area and thecontrol area, suggesting a mixing of both.

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2. Beck, A. W. & McSween, H. Y. Diogenites as polymict breccias composed oforthopyroxenite and harzburgite. Meteorit. Planet. Sci. 45, 850–872 (2010).

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4. Binzel, R. P. et al. Geologic mapping of Vesta from 1994 Hubble Space TelescopeImages. Icarus 128, 95–103 (1997).

5. Li, J. Y. et al. Photometric mapping of asteroid (4) Vesta’s southern hemispherewith Hubble Space Telescope. Icarus 208, 238–251 (2010).

6. McSween, H. J. et al. Composition of the Rheasilvia basin, a window into Vesta’sinterior. J. Geophys. Res. 118, 335–346 (2013).

7. De Sanctis, M. C. et al. Spectroscopic characterization of mineralogy and itsdiversity across Vesta. Science 336, 697–700 (2012).

8. Ammannito, E. et al. Vestan lithologies mapped by the visual and infraredspectrometer on Dawn. Meteorit. Planet. Sci. http://dx.doi.org/10.1111/maps.12192 (13 September 2013).

9. De. Sanctis, M. C. et al. Vesta’s mineralogical composition as revealed by the visibleand infrared spectrometer on Dawn. Meteorit. Planet. Sci. http://dx.doi.org/10.1111/maps.12138 (8 July 2013).

10. De Sanctis, M. C. et al. The VIR spectrometer. Space Sci. Rev. 163, 329–369 (2011).11. Righter, K. & Drake, M. J. A magma ocean on Vesta: core formation

and petrogenesis of eucrites and diogenites. Meteorit. Planet. Sci. 32, 929–944(1997).

12. Ruzicka, A., Snyder, G. A. & Taylor, L. A. Vesta as the howardite, eucrite anddiogenite parent body: implications for the size of a core and for large-scaledifferentiation. Meteorit. Planet. Sci. 32, 825–840 (1997).

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Supplementary Information is available in the online version of the paper.

Acknowledgements We gratefully acknowledge the support of the Dawn Instrument,OperationsandScience teams, and, inparticular, theDawnFramingCamera team.Thiswork was supported by Italian Space Agency grant I/004/12/0 and by NASA throughthe Dawn mission and the Dawn at Vesta Participating Scientists Program.

Author Contributions M.C.D.S., E.A., E.P. and A.L. contributed to the data analysis.M.C.D.S., E.A., S.M., D.W.M., H.Y.M. and C.M.P. contributed to the data interpretation andto writing and improving the manuscript. E.A. and M.C.D.S. provided calibrated VIRdata. F.T. provided geometric data. F.Z. and A.F. provided the projected and mosaickedVIR data. All authors contributed to discussion of the results.

Author Information All Dawn data are available at PDS: Small Bodies Node(http://pdssbn.astro.umd.edu/data_sb/missions/dawn/index.shtml), and VIR dataare also available at the ASI Data Center (http://www.asdc.asi.it/). Reprints andpermissions information is available at www.nature.com/reprints. The authors declareno competing financial interests. Readers are welcome to comment on the onlineversion of the paper. Correspondence and requests for materials should beaddressed to M.C.D.S. ([email protected]) or E.A.([email protected]).

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Extended Data Figure 1 | Ternary diagram of orthopyroxene, olivine andclinopyroxene in diogenites. Proportions of orthopyroxene, olivine andclinopyroxene in diogenites normalized to 100%, with fields fororthopyroxenitic (red), harzburgitic (green) and dunitic diogenites (yellow).Data taken from Extended Data Table 1.

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Extended Data Figure 2 | Distribution of the band centres for the HEDmeteorites. The difference in spectral properties of diogenites, howardites andeucrites can be quantified using a scatter plot of the BI-centre position versusthe BII-centre position. We used spectra in the RELAB database to definethe different HED meteorite spectral areas9. The HED meteorite distributionmap has been derived as explained in refs 6, 8, 9. In this diagram, diogenites andeucrites populate distinct areas because both the BI-centre position and theBII-centre position are sensitive to the pyroxene compositions. Howardites,which are physical mixtures of diogenite and eucrite, plot between, and partlyoverlap, these fields. By associating a colour indication of composition withevery region in the scatter plot (red for diogenite, green for howardite andpurple for eucrite, with overlapping fields of yellow for diogenite–howarditeand cyan for eucrite–howardite), we constructed the correspondence map inFig. 1 using the same colour scheme.

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Extended Data Table 1 | Average modal mineralogy of diogenites (vol%)

Average modal mineralogy of diogenites compiled from different literature sources (refs 2, 19, 21 and refs 33–35 in Supplementary Information).

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