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Shock-induced transformation of olivine to a new metastable(Mg,Fe)2SiO4 polymorph in Martian meteorites

Bertrand Van de Moortèle a, Bruno Reynard a,⁎, Paul F. McMillan b,c,Mark Wilson b, Pierre Beck a, Philippe Gillet a, Sandro Jahn d

a Laboratoire de Science de la Terre, Ecole Normale Supérieure de Lyon, Université Claude Bernard Lyon 1,CNRS, 46 Allée d'Italie, 69007 Lyon, France

b Department of Chemistry and Materials Chemistry Centre, Christopher Ingold Laboratories, University College London,20 Gordon Street, London WC1H 0AJ, UK

c Davy-Faraday Laboratory, Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, UKd GeoForschungsZentrum Potsdam, Department 4, Telegrafenberg, 14473 Potsdam, Germany

Received 3 April 2007; received in revised form 29 June 2007; accepted 8 July 2007

Editor: G.D. PriceAvailable online 24 July 2007

Abstract

Transient high pressures and temperatures generated during meteor or asteroid impacts induce mineral phase transformationsthat can mimic those occurring at depth within the silicate mantle of terrestrial planets. Olivine (α-(Mg,Fe)2SiO4), the primaryconstituent of the Earth's upper mantle and of chondritic meteorites, transforms to the high-pressure polymorphs wadsleyite andringwoodite (β-, γ-(Mg,Fe)2SiO4) that are observed in shocked chondrites. The observed phase transitions place constraints on theshock P–T conditions attained and they lead to models that describe the impact event. We studied the olivines present within twonewly catalogued Martian meteorites NWA 2737 and NWA 1950 using micro-Raman spectroscopy and high-resolutiontransmission electron microscopy (HRTEM) techniques. The shock conditions were not sufficient to cause melting ortransformation of the olivines into wadsleyite or ringwoodite (Mg,Fe)2SiO4. The shocked olivines are dark-coloured in handspecimen and thin section due to the presence of FexNiy metallic nanoparticles formed during the shock. Molecular Dynamicssimulations (MD) are consistent with the observation that the shocked olivines give rise to a new orthosilicate polymorph (ζ-(Mg,Fe)2SiO4) that is formed metastably during the shock process and that is subsequently recovered to ambient conditions. Thepresence of the new ζ-(Mg,Fe)2SiO4 polymorph in shocked ultrabasic rocks including meteorites may have remained undetecteddue to its structural and spectroscopic similarities with olivine. The existence of the metastable α–ζ phase transition also allowsrationalising previously unexplained results of shock compression experiments on olivines. ζ-(Mg,Fe)2SiO4 is also formed atambient pressure as a metastable intermediate during back-transformation from wadsleyite.© 2007 Elsevier B.V. All rights reserved.

Keywords: transition phase; olivine; zeta phase; HRTEM; Molecular Dynamics simulation

Earth and Planetary Science Letters 261 (2007) 469–475www.elsevier.com/locate/epsl

⁎ Corresponding author. Tel.: +33 4 72 72 81 02; fax: +33 86 77.E-mail address: bruno.reynard@ens-lyon.fr (B. Reynard).

0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2007.07.030

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1. Introduction

Ferromagnesian olivines form the major silicate frac-tion of the mantle of terrestrial planets that also pre-dominates in chondritic meteorites. The (Mg,Fe)2SiO4

phase diagram at high-P,T conditions is well establishedfrom experiments (Katsura et al., 2004; Inoue et al.,2006). The Mg2SiO4 end-member (forsterite; Fo) trans-forms first into wadsleyite-Mg2SiO4 and then spinel-structured ringwoodite-Mg2SiO4 at P=12–22 GPadepending upon the T, before disproportionating intoMgSiO3 perovskite+MgO atP=25GPa. Fe2SiO4 olivine(fayalite, Fa) transforms directly into the γ-spinelpolymorph at P=6–7 GPa. High-P,T transformations ofFo, Fa olivine solid solutions lead to the high-P mineralswadsleyite or ringwoodite (i.e., β-, γ-(Mg,Fe)2SiO4).These phase transitions explain the major seismicdiscontinuities observed within the Earth's upper mantleand transition zone (Frost, 2003; Ringwood, 1991).Wadsleyite and ringwoodite are also documented withinchondritic meteorites, where they are formed by shockmetamorphism of (Mg,Fe)2SiO4 olivines. Melting isalso reported to occur in certain meteorite samples. Herewe studied olivine grains within the newly-describedMartian meteorites NWA2737 and NWA1950 (Becket al., 2006; Gillet et al., 2005). Meteorite NWA1950 isa lherzolitic shergottite with a gabbroic cumulate texture.It contains 55% olivine (Fo66 to Fo75) crystals identifiedby optical microscopy. NWA2737 is the second exampleof a chassignite meteorite, found in the Western Sahara.It is an olivine-cumulate (dunite) containing minorpyroxenes and feldspar glass. It differs slightly fromChassigny by the dark colour of its olivine crystals(Fig. 1). The olivines in NWA1950 also appear black inhand specimen.

2. Petrological observations

From optical and Back-Scattered Electron (BSE)microscopy, individual olivine grains in the two me-teorites are clearly outlined by secondary calcite thatfills grain boundaries as well as fractures within thegrains. The shocked olivine crystals also contain light-coloured stripes. Some grains exhibit sets of subper-pendicular stripes, crosscut by calcite-filled fractures(Fig. 1). In BSE images, these stripes appear dark gray,indicating a density and/or compositional contrast withthe surrounding “olivine” crystals. Micro-Raman spec-tra of the clear stripes exhibit sharp peaks characteristicof (Mg,Fe)2SiO4 olivine (Fig. 1). The Raman spectra ofthe dark zones are different. Although they exhibit mainfeatures of the normal olivine spectrum, the peaks are

broadened. A broad background extends from 100 to600 cm−1, and also under the high frequency region(800–1000 cm−1). Such broad features could beattributed to highly disordered olivine crystals formedduring the shock process. In a few crystals, a weak peakobserved at 760 cm−1 might be assigned to formation ofSiOSi linkages or highly coordinated (SiO5, SiO6) spe-cies as “defects” within the structure (Durben et al.,1993). However, most grains examined did not showsuch a feature. Instead, the spectra showed additionalbroad bands at 650 and 686 cm−1 that cannot form partof the olivine spectrum, because the 600–800 cm−1

region corresponds to a well-defined gap in the vibra-tional density-of-states (VDOS) function (Price et al.,1987; Rao et al., 1988). Weak peaks are observed at thesame position in experimentally shocked olivine at59 GPa (Farrell-Turner et al., 2005). Also, the charac-teristic Raman mode of olivine at 600 cm−1 is missing(Fig. 1). Micro-Raman mapping experiments showedthat the “altered” olivine spectrum was homogeneouswithin both meteorite samples, both for individualgrains and among different crystals. The results indicatethe presence of a new phase that is structurally relatedto but is different from olivine, produced metastablyduring the shock process. We term this new orthosilicatepolymorph ζ-(Mg,Fe)2SiO4 for convenience in the dis-cussion below.

3. HRTEM observations

We determined the chemical composition of the darkzones and clear stripes within the shocked olivines using acombination of electron microprobe and nanoscale X-rayEnergy-Dispersive analysis (EDX) in the TEM studies.No difference in composition could be established. ForHRTEM studies we selected a single dark grain fromNWA2737 and prepared 50–250 nm thick sections bydiamond microtome (Diatome, Switzerland). TheHRTEM study confirmed that the sample was entirelycrystalline. Within some regions, selected area electrondiffraction (SAED) patterns could be analysed withinspace group Pbnm with lattice parameters α=0.4777,b=1.026, c=0.601 nm consistent with olivine. However,other HRTEM images indicated considerable distortion ofthe lattice planes, and the Fourier transformed (FT)images could no longer be indexed as olivine, althoughthe O2− anion sublattice maintained hexagonal (hcp)symmetry (Fig. 2). Similar results were reported forolivine crystals shocked in the laboratory and also in laser-heated Diamond Anvil Cell (DAC) experiments (Jeanloz,1980; Guyot and Reynard, 1992). We observed 5–30 nmmetallic inclusions distributed among the shocked olivine

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crystals (Fig. 2). EDX analysis within the TEM showedthat the inclusions consisted of FexNiy alloys with theirstructure and composition dependent upon particle size.Asymmetric Fe–Kβ and Ni–Kα peaks indicate thepresence of a small amount of Co (too low to be quantify)which is consistent with Co enrichment in metal withrespect to olivine (Hirschmann and Ghiorso, 1994). Thenanoparticles cause the black colour of the shocked

olivines observed in hand specimen and thin section. Theappearance of FexNiy in the olivine samples results frommetal ion reduction and migration during the shockprocess (Van de Moortele et al., in press). The HRTEMobservations show that the olivine structure immediatelysurrounding the metal nanoparticles is highly ordered(Fig. 2). The clear stripes observed within the blackolivines do not contain metallic particles.

Fig. 1. Optical and electron microscopy of shocked olivine samples and Raman spectroscopy results. (a) Optical micrograph of a polished sectionof NWA2737 illustrating the unusual dark colour of the olivines in this rock. Some clear bands are observed within the black olivine grains, withwidths up to about 50 μm, oriented along two subperpendicular directions corresponding to {021} crystallographic planes. (b) Back-scatteredelectron (BSE) images show contrast between the clear bands and the majority black grains that reveals an electron density difference between thetwo. (c) Raman spectra recorded within the clear stripes correspond to pristine olivine. The spectra obtained within the dark zones resembleolivine upon first examination but they contain unusual features that cannot be assigned to an olivine structure. In particular there are broad bandsin the 600–700 cm−1 range that corresponds to a forbidden region within the VDOS of olivine crystals; also the characteristic 600 cm−1 peak ofolivine is absent. The spectrum is reproduced throughout the majority dark crystalline material found in shocked meteorite samples NWA2737 andNWA1950, and appears constant from micro-Raman mapping studies. The spectrum likely indicates the presence of a new (Mg,Fe)2SiO4

polymorph. d) band contrast image from EBSD mapping on the same area as (b). The bright lamellae consist of well-oriented olivine(b2° difference from point to point) and the darker majority zones to olivine-like pattern with large misorientations (5–10°) consistent with hcpstructure domains.

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4. Molecular Dynamics simulations

We carried out Dynamics simulations (MD) to iden-tify the nature of the new metastable Mg2SiO4 phase andits formation from metastably compressed olivine byshock transformation. An Anisotropic-Ion Model (AIM)is employed (Aguado et al., 2003), in which induced ion

moments and short-range size and shape deformationsof O2− are included to quadrupolar level. The AIM isparameterised using high-level electronic structurecalculations (Jahn and Madden, 2007). We showcalculated V(P) relations for α-, β-, γ- and ζ-phases ofMg2SiO4 generated within a simulation box with 672ions under constant P (variable V and cell dimensions

Fig. 2. High-resolution TEM images of shocked olivines within the Martian meteorite sample NWA2737. (a) HRTEM images of the new orthosilicateζ-polymorph derived from (Mg,Fe)2SiO4 olivine by shock compression. The Fourier transform of the HRTEM image is readily indexed to an hcplattice of O2− ions and disordered cation positions with lattice parameters a∼0.3 nm and c∼2a. Such a lattice has been proposed for high-P low-Ttransformations of olivine following DAC experiments (Guyot and Reynard, 1992). (b) HRTEM image of a metallic FexNiy particle (15 nm diameter)contained within the shocked olivine. (c) Annular dark field TEM image of sub-spherical metal inclusions contained within the shocked olivinegrains. (d) EDX spectra (normalized to the Si–Kα peak) in the precipitate and olivine matrix analysis spots (probe size 5 nm) depicted in (b).Deconvolution of analytical data yields a composition ∼Fe0.93Ni0.07 for the inclusion.

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including angles) at T=300 K (Fig. 3). The new ζ-Mg2SiO4 polymorph has a density intermediate betweenα- and β-Mg2SiO4. It is produced in the simulation byapplying a rapid P increment to metastably compressedα-Mg2SiO4 at P∼50 GPa, mimicking the actual shockconditions. The new structure maintains an hcp O2−

sublattice and isolated SiO44− units: it can be correlated

with the olivine structure by a model proposed by Hydeet al. (1982). These authors noted that the metal cationsublattice in olivine corresponds to an alloy structureNi2In, with Si(Mg,Fe)6 trigonal prisms linked by face-sharing to form a corrugated-prismatic structure. The γ-(Mg,Fe)2SiO4 spinel structure instead gives rise totruncated-tetrahedral Si(Mg,Fe)12 units as found in thealloy MgCu2. Both structures maintain isolated SiO4

4−

units and hcp O2− packing. The ζ-Mg2SiO4 structureproduced by MD shock compression is related to olivine

by reorganisation of the metal sublattice resulting in anincrease in the next-nearest-neighbour cation coordina-tion (Fig. 4). During natural and experimental shockconditions, substantial disorder in the cation packing canresult from the solid-state transformation, consistent withthe HRTEM and Raman results. The AIM-MD simula-tions show that ζ-Mg2SiO4 persists metastably upondecompression to P=1 atm. For comparison withexperiment, we calculated the Vibrational Densities OfStates (VDOS) and the SiO4

4− stretching and bendingmode contributions that dominate the Raman spectrum(Fig. 3). The VDOS are calculated both from thedynamical matrices (an instantaneous normal modeanalysis) and by constructing time correlation functionswhich directly probe the underlying silicon-centred localmodes of vibration (Pavlatou et al., 1997). The VDOSfor the simulated ζ-Mg2SiO4 is similar to that of olivine

Fig. 3. MD results. (a) V(P) relations showing the evolution of the high pressure ζ-phase. (b) Static (0 K) energy/volume relations for the α-, β-, γ-and ζ-phases. The figure also shows the dynamic (300 K) evolution of the system energy with the arrow highlighting the location of the pressure-driven phase transition. (c) Vibrational densities of states calculated from time correlation functions constructed to reflect the vibrational modes of theSiO4 tetrahedra. From bottom to top, the first three curves show the effect of pressure on the α-phase, whilst the upper two curves show the high andlow pressure states for the ζ-phase. (d) X-ray diffraction patterns calculated from the simulated structure factors for the α-phase starting material(lower curve) and the high pressure ζ-phase (upper curve).

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but is broadened so that features appear in the range600–800 cm−1, as observed experimentally (Figs. 1, 3).

5. Discussion

The conditions attained during shock are constrained bythe absence of high-P wadsleyite or ringwoodite phases.The peak shock pressure in the analogous Chassignymeteorite was estimated as P∼35±5 GPa from planarfractures within olivine grains, and is consistent with theobservation of small (250–500 nm in size) crystals ofwadsleyite and ringwoodite, indicating likely similar shockconditions for NWA2737 and NWA1950. The absence ofmelting indicates that T did not exceed 2500 K. Usingknown Fe–Ni–Mg interdiffusion coefficients for olivinewe can constrain the time–temperature (t–T) conditionsassociated with the nanoparticle formation. The averagespacing observed between nanoparticles determines acharacteristic diffusion length x=50±10 nm. Using x=(Dt)− 1/2 and diffusion data for Ni (Petry et al., 2004), wecalculate diffusion timescales on the order of t∼10 ms forT=2100±100 K and ∼ 1 s for T=1600±100 K. These

values correspond well with the expected duration oftypical thermal events following an initial shock of thismagnitude (Beck et al., 2005).

Themechanisms of transformations betweenα-,β- andγ-polymorphs of (Mg,Fe)2SiO4 have been studiedextensively (Brearley et al., 1992; Brearley and Rubie,1994; Rubie and Brearley, 1990; Price et al., 1982). Aseries of "spinelloid" structures exists for AB2O4 com-pounds between γ-spinel and a hypothetical structuredesignated the ω- or ε⁎-phase, formed by shifting spinelblocks relative to each other along crystal planes to createSiOSi linkages (Hyde et al., 1982; Madon and Poirier,1983). β-Mg2SiO4 corresponds to one spinelloid poly-morph: others have been discovered in the Ni2SiO4–NiAl2O4 and NiGa2O4 system, among Mg-gallogerma-nates and in fayalite–magnetite solid solutions (BarbierandHyde, 1986;Davies andAkaogi, 1983;Hammond andBarbier, 1991; Ross et al., 1992). Formation of the ω- orε⁎-phase provides an intermediate step in α-(β,γ) phasetransformations and leads to other spinelloid structures(Hyde et al., 1982; Madon and Poirier, 1983). However,the appearance of the distorted olivine phase observedfollowing shock compression and described above cannotbe explained by the same transformation mechanism.The spinelloids are based on cubic packing of the O2−

sublattice, rather than the hcp symmetry observed here,and they contain SiOSi linkages between polymerisedSiO4 units that would give rise to strongRaman peaks bothat 600–700 cm−1 and 950–1100 cm−1 (McMillan andAkaogi, 1987; Choudhury et al., 1998). Instead, theRaman spectra of the shocked olivines remain character-istic of an orthosilicate structure containing isolated SiO4

4−

units (Fig. 1). It is thus likely that the new ζ-(Mg,Fe)2SiO4

polymorph might be present in many shocked ultrabasicrocks including meteorites, but its presence has remainedundetected due to its structural and spectroscopicsimilarities with olivine (Fig. 3).

We calculated stable and metastable G(P) relations forα-, β-, γ- and ζ-polymorphs of Mg2SiO4 (G=H−TS isGibbs' free energy; H and S are enthalpy and entropy)(Fig. 4). The AIM-MD model predicts α–β and β–γtransitions at P=12 and 13 GPa compared with P=9,13 GPa from experiment after linear extrapolation toT=0 K (Katsura et al., 2004; Inoue et al., 2006). The highkinetic barriers associated with these transitions allowolivine to be compressed metastably beyond these limits atlow T (Durben et al., 1993). The predicted ζ-polymorphbecomes stabilised relative to olivine at 38 GPa. Atambient P, the free energy of the ζ-phase lies betweenthose of α- and β-Mg2SiO4 so that metastable back-transformation of wadsleyite can result in formation ofthe ζ-polymorph. This analysis agrees with experimental

Fig. 4. (a) Simulated free energy curves calculated as a function ofpressure. The free energies are expressed relative to that of the α-phasein order to emphasize the thermodynamic phase relations. (b)MolecularDynamics snapshot of the high pressure ζ-phase shown in the ab plane.TheMg, Si and O atoms are shown as the red, blue and magenta circlesrespectively. The green box highlights the underlying unit cell. (Forinterpretation of the references to colour in this figure legend, the readeris referred to the web version of this article.)

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studies by Raman spectroscopy and X-ray diffraction,that show formation of a new metastable structure after"gentle" heating of metastably-decompressed β-Mg2SiO4

recovered from high-P,T synthesis experiments (McMillanet al., 1991; Reynard et al., 1996; Tsukimura et al., 1988).

We have thus discovered a new metastable polymorphof (Mg,Fe)2SiO4 orthosilicate within Martian meteoritesin which the olivine phase was subjected to PN35 GPashock pressure at low T. The conditions were notsufficient to cause melting or transformation to stablehigh-P polymorphs. Such shock conditions are commonamong natural impacts (Stoffler et al., 1991).

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