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Geochimica et Cosmochimica Acta 140 (2014) 334–348

Heterogeneous mineral assemblages in martian meteoriteTissint as a result of a recent small impact event on Mars

E.L. Walton a,b,⇑, T.G. Sharp c, J. Hu c, J. Filiberto d

a MacEwan University, Department of Physical Sciences, 10700-104 Ave, City Centre Campus, Edmonton, AB T5J 4S2, Canadab University of Alberta, Department of Earth & Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3, Canada

c Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287-1404, USAd Southern Illinois University, Department of Geology, Carbondale, IL, USA

Received 15 April 2014; accepted in revised form 19 May 2014; Available online 4 June 2014

Abstract

The microtexture and mineralogy of shock melts in the Tissint martian meteorite were investigated using scanning electronmicroscopy, Raman spectroscopy, transmission electron microscopy and synchrotron micro X-ray diffraction to understandshock conditions and duration. Distinct mineral assemblages occur within and adjacent to the shock melts as a function of thethickness and hence cooling history. The matrix of thin veins and pockets of shock melt consists of clinopyroxene + ringwoo-dite ± stishovite embedded in glass with minor Fe-sulfide. The margins of host rock olivine in contact with the melt, as well asentrained olivine fragments, are now amorphosed silicate perovskite + magnesiowustite or clinopyroxene + magnesiowustite.The pressure stabilities of these mineral assemblages are �15 GPa and >19 GPa, respectively. The �200-lm-wide margin of athicker, mm-size (up to 1.4 mm) shock melt vein contains clinopyroxene + olivine, with central regions comprisingglass + vesicles + Fe-sulfide spheres. Fragments of host rock within the melt are polycrystalline olivine (after olivine) and tis-sintite + glass (after plagioclase). From these mineral assemblages the crystallization pressure at the vein edge was as high as14 GPa. The interior crystallized at ambient pressure. The shock melts in Tissint quench-crystallized during and after releasefrom the peak shock pressure; crystallization pressures and those determined from olivine dissociation therefore represent theminimum shock loading. Shock deformation in host rock minerals and complete transformation of plagioclase to maskelynitesuggest the peak shock pressure experienced by Tissint P 29–30 GPa. These pressure estimates support our assessment thatthe peak shock pressure in Tissint was significantly higher than the minimum 19 GPa required to transform olivine to silicateperovskite plus magnesiowustite.

Small volumes of shock melt (<100 lm) quench rapidly (0.01 s), whereas thermal equilibration will occur within 1.2 s inlarger volumes of melt (1 mm2). The apparent variation in shock pressure recorded by variable mineral assemblages withinand around shock melts in Tissint is consistent with a shock pulse on the order of 10–20 ms combined with a longer durationof post-shock cooling and complex thermal history. This implies that the impact on Mars that shocked and ejected Tissint at�1 Ma was not exceptionally large.� 2014 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2014.05.023

0016-7037/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: MacEwan University, Department ofPhysical Sciences, 10700-104 Ave, City Centre Campus, Edmon-ton, AB T5J 4S2, Canada. Tel.: +1 780 497 4059; fax: +1 780 4975655.

E-mail addresses: waltone5@macewan.ca, ewalton@ualberta.ca(E.L. Walton).

1. INTRODUCTION

Virtually all meteorites have been modified by shockwaves, generated by hypervelocity impact on their parentbodies. Metamorphic effects in the shocked meteorites canbe described as deformational or transformational.

E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348 335

Transformation of host rock minerals includes local shockmelting observed as veins and pockets, which may comprisea significant volume of the host rock (e.g., 13.7 vol% inALH77005; Treiman et al., 1994). These shock melts areinterpreted to have formed in local hot spots (>2000–2500 K) by shock impedance contrasts or frictional meltingalong shear bands as shock waves traveled through hetero-geneous, cracked or porous materials (Langenhorst andPoirier, 2000; Beck et al., 2004, 2005). These hotspots areheated much more than the bulk rock (Sharp andDeCarli, 2006). Minerals stable at high pressures and tem-peratures, including (but not limited to) – wadsleyite, ring-woodite, majorite, akimotoite, silicate perovskite (vitrifiedupon decompression), stishovite, lingunite and tuite – occuralmost exclusively in and around shock melts (Langenhorstand Poirier, 2000; Beck et al., 2005; Xie et al., 2006; Liu andEl Goresy, 2007; Xie and Sharp, 2007; El Goresy et al.,2008; Fritz and Greshake, 2009; Imae and Ikeda, 2010).This association demonstrates that the high temperaturesassociated with shock melts are necessary to drive kineti-cally sluggish phase transformations (Sharp and DeCarli,2006). The formation and preservation of high-pressurephases, by crystallization of silicate liquids or solid-statetransformation of constituent minerals, is a function ofthe shock-melt cooling time and the duration of the shockpulse, defined as the time lag between shock compressionand arrival of the release wave. The duration of the shockpressure pulse in meteorites is related to pre-impact burialdepth, and impactor-specific variables such as size, densityand velocity (Melosh, 1985; Head et al., 2002; Artemievaand Ivanov, 2004; Fritz and Greshake, 2009).

In this study we investigate shock metamorphic effectsin the Tissint meteorite, a fall witnessed on July 18th,2011 and collected shortly thereafter. Tissint is classifiedas a depleted picritic shergottite � an Al-poor ferroanbasaltic rock from Mars (Chennaoui Aoudjedane et al.,2012). The igneous texture comprises mm-size macro-crysts of strongly zoned olivine (Fa19–60) and smaller oliv-ine grains (up to 0.4 mm) with relatively flat zoningprofiles (Fa29–53), embedded in a groundmass of pyroxene(pigeonite and augite) and plagioclase (now maskelynite)Minor minerals include merrillite, magnetite, pyrrhotite,chromite and ilmenite. A cosmic ray exposure (CRE)age of 1.10 ± 0.15 Ma (10Be; Nishiizumi et al., 2012)and 1.0–1.1 Ma (3He, 21Ne, 38Ar; Huber et al., 2013)pairs Tissint with many other depleted shergottitesincluding the well-studied Yamato 980459 and Dar alGani (DaG) 476/498 samples. Baziotis et al. (2013)described high-pressure minerals in Tissint and used oliv-ine transformation kinetics to conclude that the shockpulse was one second in duration and therefore Tissintcame from a large impact on Mars. In this work, wedescribe shock metamorphism of Tissint as deformationin the bulk rock, and mineral transformations that occurin and around shock melts. The location and distributionof these high-temperature and high-pressure phases pro-vide insight into shock conditions and post-shock thermalhistory. These allow us to constrain the size and natureof the impact event that caused shock metamorphismof this meteorite.

2. SAMPLES AND ANALYTICAL METHODS

Two polished thin sections of Tissint were investigatedby transmitted and reflected light microscopy (Fig. 1).Detailed microtextures were characterized by backscatteredelectron (BSE) imaging at the University of Alberta (UAb)using a Zeiss EVO MA scanning electron microscope(SEM) with a LaB6 filament. BSE images were acquiredwith a Si diode detector using a 20 kV accelerating voltageand a 8.0 mm working distance. SEM BSE images,imported into the image analysis software ImageJ, wereused to measure grain size and the thickness of shock melts.These measurements were made in the plane of the thin sec-tions and therefore represent apparent thicknesses in thatplane. Mineral modes of the shock melts in each thinsection were estimated by manual point counts on BSEimages. Major and minor elemental abundances weremeasured on the same instrument using energy dispersivespectrometry (EDS). Raster scanning was used to deter-mine the approximate bulk composition of shock veins.Crystals + glass within the vein matrix were rastered inareas of 10 lm x 10 lm and the results averaged. EDSwas used instead of wave-length dispersive spectrometry(WDS) because it enables rapid, simultaneous major ele-ment analyses operating at lower beam currents thanWDS, which minimizes alkali metal migration. Ramanspectra were collected with a Bruker SENTERRA spec-trometer at MacEwan University, using the 50X or 100Xobjective to focus the excitation laser beam (532 nm lineof Ar + laser) to a 3–4 micron and 1 micron spot size,respectively. A sequence of three-10 s exposures wereacquired using a laser power of 10–20 mW. Due to thefine-grained nature of some minerals, the analyses wererepeated several times to check reliability. These multipleexposures were then summed to achieve the final spectrum.Backgrounds of the spectra were graphically reduced usingthe OPUS v. 6.5 spectroscopy software.

Four areas of interest were excavated with a focused ionbeam (FIB) system using a FEI Nova 200 NanoLab. Trans-mission electron microscopy (TEM) imaging, selected areaelectron diffraction (SAED) and EDS analysis wereperformed on FIB sections using FEI CM200 and JEOL2000FX analytical TEM instruments, both operating at200 kV. All TEM work was performed in the LeRoy-Eyring Center for Solid State Science at Arizona StateUniversity (ASU). The same Tissint thin section was alsoanalyzed using synchrotron micro X-ray diffraction atGSE-CARS at the Advanced Photon Source, ArgonneNational Lab., using the sector 13BMD bending magnet,which has an X-ray beam of 5 � 12 lm and X-ray energyof 30 keV (k = 0.4137 A).

3. RESULTS

3.1. Deformational shock features

Olivine in Tissint is pale to dark brown in transmittedlight. The intensity of coloration increases with proximityto shock melting. Strong mosaicism is observed undercrossed polars. Olivine macrocrysts are heavily fractured,

0.2 cm

fusion crust

SMP

SMV

SMPol ol

ol

ol

ol

ol

olol

SMV

ol

ol

mi

mi

mi

SMP

ves

ves

Fig. 1. Transmitted light image of the two thin sections investigated in this study. Shock melts form opaque veins (SMV) or pockets (SMP)that cut across, and are heterogeneously distributed throughout, the host rock. The boxes show the locations of the shock melts investigated inthis study. Several olivine macrocrysts are labeled, containing round to ellipsoidal igneous melt inclusions (mi). A portion of fusion crust ispresent on both thin sections. The apparent shock melt thicknesses appear greater under transmitted light, due to a darkening of igneousminerals surrounding these features. This is attributed to injection of sulfides into cracks and fractures giving rise to a local “shockblackening”, or the formation of high pressure phases along their margins such as ringwoodite which are an extremely dark purple color.Vesicles (ves) are present in the largest shock melts. Note that many more vesicles are observed within the shock melts, typically concentratedin their centers, but are too small to be visible in this small scale overview. (For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

336 E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348

containing both irregular fractures and several sets ofplanar fractures. The spacing between planar fractures is�60–150 lm. Near to larger volumes of shock melt thesefractures are typically coated with thin films of Fe-oxideor Fe-sulfide, giving olivine a locally blackened appearance.Pyroxene displays pervasive fracturing and strong mosai-cism, indicated by a pronounced irregular optical extinctionon a very small scale. The fractures are predominantlyirregular, some cut through the entire crystal while othersare restricted to small areas inside the grains. Some defor-mation bands are also present. Polysynthetic mechanicaltwin lamellae, subparallel to (001), are present in somepyroxene grains. Under the microscope these twins appearas sets of narrow, lens-shaped bands �1–3 lm thick. Pla-gioclase has been completely converted to optically isotro-pic maskelynite throughout the host rock, with no relictbirefringence. Signs of vesiculation and flowing of plagio-clase grains, indicative of a normal glass rather than a dia-plectic one, are restricted to those grains near to shockmelts in the thin sections.

3.2. Shock melts

Both thin sections contain distinct, dark brown to blackshock melt veins or shock melt pockets, heterogeneouslydistributed throughout the non-brecciated host rock(Fig. 1). The abundance of shock melts in both thin sectionsaverages �15 vol%. Vein thickness ranges from thin20–150 lm to thick 450–1400 lm; likewise small, roundedisolated pockets (�150–300 lm) grade into larger pocketsof irregularly shaped melt (>1 mm). The latter are typicallyconnected by shock veins and contain round vesicles thatare concentrated in the center of the pocket.

3.3. Transformational shock features in and around thin

shock melts

Two thin shock veins (120 and 20 lm thick) and anisolated pocket of shock melt (300–450 lm thick) werecharacterized in terms of the mineral assemblages that crys-tallized from the melt, entrained clasts (shock-melt pocket

a

b

350 µm

12 µm

sheared olivine macrocryst

basal�c groundmass

shock vein

shock vein

olivine macrocrystbright rim + lamellae

0.6 µm

cOl[011]||Rwd[110]

011Ol

002Rwd

111Rwd

100Ol||111Rwd

(e)

(d)

(c)

(a)

(b)

Fig. 2. BSE images of olivine in Tissint. (a) A mm-size zonedolivine macrocryst is cut across and sheared by a �100 lm thickshock vein. (b) Close up of the area shown by the white box in (a).Portions of olivine in direction contact with the vein matrix aretransformed to a brighter phase. This is observed as an 8–12 micronthick rim and thin lamellae that extend into the host macrocryst. Theletters next to circles indicate the respective Raman spot analyses,with representative spectra given in Fig. 3. (c) A bright-field TEMimage of olivine with ringwoodite lamellae (dark) imaged nearlyalong the olivine [011] zone axis. The thinnest ringwoodite lamellaeare parallel to the (100) planes of olivine whereas the thickerlamellae are subparallel. The SAED pattern of the ringwoodite [110]and olivine [011] zone axes shows the 100*

Ol parallel to �111*Rw.

E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348 337

only), and transformation along the walls of host rock igne-ous minerals immediately adjacent to the shock melt. Here,we focus on those areas where veins cut across and shearolivine macrocrysts. This shearing is observed as an appar-ent displacement of igneous minerals at the shock veininterface, with movement on the order of �60–200 lm(Fig. 2a). Open fractures in host rock olivine are truncatedat vein margins and the contact with the shock veins issharp.

BSE images show that the matrix of the 120 lm shockvein is fine-grained (61–2 lm), devoid of vesicles, withsmall spheres of immiscible sulfides dispersed throughout.The walls of olivine along the veins are transformed to abrighter phase in BSE images, indicating a compositionaland/or density contrast between this phase and host rockolivine (Fig. 2b). This brighter material forms a homoge-neous rim (�5–8 lm thick), which grades into thin(61 lm) lamellae extending �4–12 lm into olivine(Fig. 2b). Lamellae occur in multiple orientations. Ramanspectra collected on the bright rim and thin lamellae, showa strong doublet at 790 and 842 cm�1 (Fig. 3a and b). Thesecorrespond to the expected vibrational modes for ringwoo-dite (McMillan and Akaogi, 1987). Additional peaksconsistently appear in ringwoodite spectra near 200, 290and 660–670 cm�1 suggesting the presence of an additionaloxide phase, most likely magnetite or magnesiowustite (deFaria et al., 1997). Sharp peaks near 817 and 847 cm�1

from spectra obtained within the olivine macrocryst(Fig. 3c), �20–30 lm distant from the shock vein, are char-acteristic of olivine. TEM imaging and diffraction confirmthat olivine has been transformed along the margins topolycrystalline aggregates, of sub-micrometer ringwooditecrystals, in contact with the melt. This texture grades intocoherent ringwoodite lamellae intergrown with olivine(Fig. 2c). The lamellae occur sub-parallel to olivine (100)with (100)Ol and [011]Ol parallel to {111}Ring andh110iRing, respectively. Raman analyses collected on thematrix of this shock vein yield three distinct spectral signa-tures: (1) flat spectra indicative of Raman-inactive phases,(2) a doublet at 668 and 1000 cm�1 characteristic of crystal-line clinopyroxene (Fig. 3d), and (3) clinopyroxene + pyr-rhotite identified by peaks near 668 and 1000 cm�1

(clinopyroxene) and 300–375 cm�1 (pyrrhotite) (Fig. 3e).TEM imaging and diffraction confirm the presence of clino-pyroxene throughout the quenched shock melt in this sam-ple. Several phases that do not appear in Raman spectra,including ringwoodite and stishovite, were also identified.The vein consists of sparse anhedral crystals of clinopyrox-enes, ringwoodite and stishovite needles in a predominantlyamorphous matrix.

The olivine megacryst along the margins of the thin-nest shock vein (20 lm), similar to that observed forthe 120 lm shock vein, exhibits a bright rim in BSEimages, grading to thin lamellae that extend away fromthe shock vein into olivine (Fig. 4a). Raman spectra con-firm that the bright rim and lamellae are ringwoodite,with sharp peaks near 790 and 840 cm�1. In direct con-tact with the shock vein, olivine has dissociated to form�2 lm rims, which parallel the shock vein (Fig. 4a andb). TEM imaging on this area reveals a mixture of

�50 nm crystals and glass (Fig. 4c). Electron diffractionpatterns collected on this dissociated olivine rim show amagnesiowustite ring pattern with a diffuse ring from

500 1000 1500

Raman shi� (cm-1)

ringwoodite rim

Rela

�ve

Inte

nsity

ringwoodite lamellae

200

290

670

795840

914948

690538

588olivine macrocryst

670

1003

670

1003

360333

314

pyroxene

pyroxene + pyrrho�te

(e)

(d)

(c)

(b)(a)

Fig. 3. Representative Raman spectra of mineral phases withinand around the shock veins. See text (Section 3.3) for details. Theapproximate location of these spectra, labeled (a), (b), (c) etc., areshown in Fig. 2.

338 E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348

an amorphous material (Fig. 4d). Single crystals of elon-gated ringwoodite (up to 1 lm in the longest dimension)also occur along the boundary between the dissociatedolivine and the shock vein (Fig. 4c). Within the shockvein, crystals of ringwoodite, sparse equant crystals ofclinopyroxene, and tiny needles of stishovite are embed-ded in a glassy matrix (Fig. 4c).

Clasts of olivine entrained within a smaller (�300–450 lm thick) shock melt pocket show transformation totwo distinct phases. This is similar to the two-phase assem-blage observed in a thin rim on olivine in contact with the20 lm-thick shock vein described previously. The dissoci-ated olivine clast has a symplectitic texture with many tinyblebs of high-contrast (BSE) material in a matrix of low-contrast material (Fig. 5a). This texture is coarsest incontact with shock melt and becomes progressively finerinto the transformed olivine clasts. Synchrotron X-raydiffraction patterns from this transformed olivine matchclinopyroxene (close to the clinoenstatite end member,space group P21/c). TEM images show 100–200 nm anhe-dral crystals (Fig. 4b). Selected area electron diffractionpatterns from a FIB section display strong diffraction frommagnesiowustite (111) and (220) as well as a ring patternfrom polycrystalline clinopyroxene (Fig. 4c). The quenchedmelt around the dissociated olivine clasts consist of fine-grained ringwoodite and clinopyroxene.

3.4. Transformational shock features in and around thick

shock melt

A thicker, irregularly-shaped shock vein within Tissint(up to 1.4 mm) transects the thin section (Fig. 1). Severalfeatures set this vein apart from those the thinner shock

melts described in Section 3.3. These include the nature ofthe host-rock shock-vein contact, variation in grain sizeand shape with proximity to the host-rock margin, andthe abundance of entrained mineral fragments. The contactbetween the shock melt and host rock is gradational(Fig. 6a), compared to the sharp contact between thinnershock veins and host rock. At its thickest (1.4 mm) theshock vein matrix consists of glass + vesicles + Fe-sulfidespheres. The glass ranges from schlieren-rich, observed asflow lines with contrasting greyscale in BSE images, tohomogeneous (uniform greyscale). The ratio of glass tocrystals decreases toward the host rock margin. A�200 lm zone at the shock vein margin is largely crystal-line, as are thinner areas of the vein (300–450 lm;Fig. 6b). Broad textural zones exist within the shock vein,with glass in the vein center grading into skeletal and den-dritic grain shapes toward the vein margin, followed byequant, euhedral grain shapes at the host rock margin (sul-fide spheres are finely dispersed throughout). The shock veinmatrix is opaque in transmitted light with several transpar-ent clasts of host rock minerals. These entrained clasts areoptically anisotropic and are polycrystalline based on thedistinct extinction position between grains within a givenfragment. BSE imaging reveals the presence of many smallersubrounded clasts not visible with the optical microscopethat are concentrated near the shock vein margin. EDS anal-yses confirm that the clasts are (compositionally): oliv-ine + pyroxene + chromite + plagioclase. Raman analysisof this thicker shock vein in Tissint focused on grains thatcrystallized from the melt, as well as those entrained clastsof host rock minerals. Representative Raman spectra forthis mm-size shock vein are shown in Fig. 7.

No evidence for the high-pressure compositionalequivalents of olivine and pyroxene were identified basedon peak positions within Raman spectra, such as a strongpeak at 799 cm�1 (akimotoite; Ross and McMillan, 1984;Reynard and Rubie, 1996) or a doublet at 790 and847 cm�1 (ringwoodite; McMillan and Akaogi, 1987). Anal-ysis of host rock olivine in direct contact with the vein, aswell as those grains that have crystallized from the shockmelt exhibiting equant, euhedral and dendritic crystalshapes, reveal spectra with strong doublet at 818 and847 cm�1, with additional peaks at 670, 914 and 940 cm�1

that are well known from olivine (Fig. 7a–c). Likewise, thosecrystals (equant and dendritic) with pyroxene compositionfrom EDS analysis yield spectra with sharp peaks near330, 400, 668–670 and 1006 cm�1 (Fig. 7d), identical to hostrock pyroxene bordering the shock vein (Fig. 7e). Clasts ofFe,Cr-oxides within the melt yield a strong peak at 686 cm�1

with a broad hump centered over 550 cm�1 (Fig. 7g) consis-tent with chromite, as opposed to CaTiO4-structured chro-mite, which is the structure stable at high pressures (Chenet al., 2003). The glass-rich vein center yields weak Ramansignals (characteristic of glasses) with broad humps centeredover 660 and 1000 cm�1, indicating they are largely pyrox-ene glasses. A new shock-induced (Ca, Na,h)AlSi2O6

pyroxene, officially named “tissintite” (Ma et al., 2013),has also been identified in our thin sections by its occurrenceand texture, which is similar to descriptions by Ma et al.(2014). Here, tissintite occurs as fine-grained clusters of

thin shock melt

olivinedissocia�onrim

Mw + glass

2 µm

0.5µm

c

shock melt

a b

12 µm

olivine macrocryst

olivinemacrocryst

thin shock melt

ringwoodite rim + lamellae

olivine dissocia�on

FIB Sec�on

111

200110

d

100

210

S�

S�

120 nm

S� [001]

110

Fig. 4. (a) BSE image of the thin shock vein cutting across a large (mm-size) strongly zoned olivine macrocryst. Within the olivine macrocryst,approaching the vein margin, a thin rim of ringwoodite grades into a dissociated texture, shown at higher magnification in (b). (b) BSE imageof the dissociated olivine on the border of a thin vein, as well as the fine-grained shock vein matrix. (c) TEM image of the shock-vein matrixand the dissociation rim. The rim consists of magnesiowustite (inset SAED pattern) and glass with a relatively large (1 lm size) ringwooditecrystal at the boundary with the melt vein. The inset SAED pattern is a ring pattern from randomly oriented magnesiowustite crystals with therings labeled by hkl. (d) A higher magnification image of the shock vein showing acicular stishovite crystals in a predominantly glassy matrix.The inset SAED pattern is of a stishovite crystal viewed along the [001] zone axis. Mw = magnesiowustite; Rwd = ringwoodite;Sti = stishovite.

E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348 339

dendritic grains within the host rock bordering the shockvein, and as thin 1–3 lm size equant grains within plagio-clase clasts entrained in the shock vein matrix (Fig. 8a andb). The fine-grained dendrites appear to have nucleatedfrom the grain boundaries of the former plagioclase andradiate away from the shock vein (Fig. 8b). In both cases,tissintite is embedded in plagioclase glass. Raman spectracollected on tissintite contain peaks near 375, 520, 698 and1100 cm�1 (Fig. 7f). These peak positions are consistentwith those characteristic bands reported for jadeite (Yanget al., 2009), with which tissintite is closely related (tissintiteis the Ca-analogue of jadeite with 1=4 of the M2 Ca/Na sitesvacant; Ma et al., 2014). Tissintite is surrounded by glass,which gives a relatively weak Raman signal with a broadpeak centered over 450 cm�1.

3.5. Composition of shock melts

EDS raster scans were averaged and used to estimate thebulk composition of the shock veins in Tissint. The threeveins (20 lm, 120 lm and mm-thickness) show considerablecompositional variation between the average compositionof the shock melt, and even between individual raster scanswithin a single shock vein (Table 1). Amounts of sulfur(0.6–3.14 wt% SO3) are attributed to the iron sulfide spheresthat are dispersed throughout the vein matrix.

4. DISCUSSION

The shock P–T–t conditions experienced by a meteoritemay be estimated using high-pressure solid-state transfor-mations, the crystallization products of shock melts, andshock deformation of host rock minerals. The followingdiscussion focuses on constraining the shock conditionsexperienced by Tissint using the mineral, textural and com-positional observations reported in the results section.

4.1. Constraints on pressure from the crystallization products

of shock melts and olivine dissociation

The crystallization products of shock melts combinedwith experimental high-pressure melting relations providesa means of constraining the crystallization pressure inshocked chondrites (Chen et al., 1996). Depending on thecooling time of the shock melt and the shock duration,the crystallization pressure may be related to the shockpressure experienced by the meteorite (Xie et al., 2006;Sharp and DeCarli, 2006). The cooling history of Tissintshock melts is discussed in the following Section 4.2. Asummary of the mineral assemblages associated with shockmelt in Tissint is summarized in Table 2.

The quenched melt in the matrix of thin veins (20–120 lm), and that adjacent to clasts of dissociated olivine

Magnesiowüs�te

(111)(220)

12 �m

Transforma�onalborder

Olivine clast

Shock melt

10 µm

a b

0.5 µm

c

Fig. 5. (a) BSE image of a dissociated olivine clast within a 300–450 lm thick shock melt pocket. The texture of the transformational borderin the small white box is shown at lower-left. (b) TEM image of the border in the FIB section including magnesiowustite and glass. (c) SAEDpattern with strong spotty diffraction of (110) and (220) suggesting a topotaxial crystallographic relationship between grains.

340 E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348

within the shock melt pocket (�300–450 lm), consist ofclinopyroxene + ringwoodite ± stishovite embedded inglass (Table 2). An average of EDS raster scans providesan estimate of the bulk composition of major elements ineach vein (Table 1), showing that they are rich in a pyrox-ene component with lesser contributions from olivine, pla-gioclase and Fe-sulfide. This composition is similar tothat of, but richer in silica and alumina than, Allende(Agee et al., 1995) and KLB-1 peridotite (Herzberg andZhang, 1996). Based on Allende and KLB-1 phase dia-grams, the pyroxene-bearing assemblage could have crystal-lized stably up to about 16–18 GPa. However, the highersilica and alumina contents in Tissint make application ofthese phase diagrams problematic. In addition, the slowercrystallization of the basaltic shock melt, as indicated byabundant residual glass, may result in a higher potentialfor crystallization of metastable phases (see Table 2).

Selected area electron diffraction patterns collected onigneous olivine macrocrysts in and around small volumesof shock melt show a magnesiowustite ring pattern with(1) a diffuse ring from an amorphous material (Fig. 5c),or (2) a ring pattern from polycrystalline clinopyroxene(Fig. 4c). The breakdown of olivine to magnesiowustiteand a pyroxene composition phase is interpreted to be areaction to (Mg,Fe)SiO3-perovskite plus magnesiowustite.This dissociation has been observed in olivine (Fa34-41)adjacent to, or entrained within, shock melts in shergottiteDaG 735 (Miyahara et al., 2011). The amorphous materialand the polycrystalline pyroxene were silicate pervoskite athigh pressure that transformed to glass and pyroxene in a

hot post-shock environment. The pyroxene + magne-siowustite assemblage corresponds to the hotter post-shockenvironment associated with larger melt veins. In bothcases, the original crystal shapes of the transformed olivineare well preserved with sharp boundaries contacting theshock melt, which indicates the phase transformationoccurred in the solid state. Solid-state phase transitions thatoccur during shock generally represent metastable reactions(Sharp and DeCarli, 2006), but they still can provide usefulconstraints on shock pressure. We do not assume chemicalequilibrium, but the fact that olivine dissociated indicatesthat the perovskite plus magnesiowustite had a lower Gibbsfree energy than olivine at the P–T conditions of transfor-mation. The equilibrium boundary between ringwooditeand silicate perovskite + magnesiowustite, which occurs atabout 24 GPa (Ito and Takahashi, 1989), is not relevantbecause the transformation did not involve ringwoodite.Instead, the metastable olivine to perovskite + magne-siowustite phase boundary, which occurs at about 19 GPain the forsterite system (Fig. 9), represents a minimumtransformation pressure with the actual pressure signifi-cantly overstepping this metastable boundary. The break-down of olivine to form silicate perovskite andmagnesiowustite is a eutectoid-type reaction, similar toeutectoid reactions that occur in metals, where the productphases form lamellae or granular intergrowths (Poirieret al., 1986; Kubo et al., 2002). This reaction is favored athigh temperature because it requires long-range chemicaldiffusion at the olivine interface and Poirier et al. (1986)found that granular intergrowths were favored at high

a

110 µm

12 µm

basal�c groundmass

thin sec�on edge

shock vein

clinopyroxene

olivine

olivine

60 µm

c

basal�c groundmass olivine

olivine

equant, euhedral crystals (vein margin)

15 µm

dendri�c olivine

homogeneous glass

b

d

(g)

(f)

(e) (d)

(c)

(b)

(a)

Fig. 6. BSE images of the mm-thick shock melt in Tissint. (a) The contact between the host rock (basaltic groundmass = pyroxene + formerplagioclase) and the shock melt is gradational. The interior of the shock vein consists of relatively homogeneous glass + Fe-sulfide spheres. (b)Those grains that have crystallized from the melt show dendritic shapes closer to the vein center. (c) Approaching the shock vein margin, veinshapes progress to equant, euhedral. Olivine is present in direct contact with the now-crystallized shock melt margin. (d) Olivine andclinopyroxene within the vein matrix. The letters next to circles indicate the respective Raman spot analyses, with representative spectra givenin Fig. 7.

500 1000 1500Raman shi� (cm-1)

Rela

�ve

Inte

nsity

674

850

olivine (shock vein - equant)

pyroxene (shock vein)

olivine (shock vein - dendri�c)

818

818850

914 944

674

850818

914

olivine (host rock)

pyroxene (host rock)

�ssin�te330

406660 676

686

chromite

1100520

1006

(g)

(f)

(e)

(d)

(c)

(b)

(a)

698

550

Fig. 7. Representative Raman spectra from the mm-size shockvein. See text for details. The approximate location of these spectra,labeled (a), (b), (c) etc., are shown in Fig. 6.

E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348 341

temperature. The olivine breakdown reaction is likely to befavored at higher temperatures relative to the polymorphictransformation of olivine to ringwoodite.

The thick shock vein yields a crystallization assemblageof olivine plus clinopyroxene (Table 2). Although olivineand pyroxene are generally considered low-pressure phases,they occur together at elevated pressures up to approxi-mately 14 GPa (Gasparik, 1992; Agee et al., 1995). Thegranular assemblage of olivine and clinopyroxene thatoccurs near the margins of the large shock veins thereforecrystallized at a pressure at or below 14 GPa. The occur-rence of vesicles in the vein interior is consistent with liquidremaining after pressure release and solidifying under ambi-ent pressure conditions. The assemblage of dendritic olivinein a pyroxene-rich glass have been produced experimentallyusing synthetic glasses representative of the composition ofshock melt pockets in shergottites DaG 476 and SaU 150(Walton et al., 2006). In these experiments olivine + clino-pyroxene crystallize under dynamic cooling conditions at1 bar. Tissintite, (Ca, Na,h)AlSi2O6, the Ca-analogue ofjadeite (Ma et al., 2014), has been identified in clasts thatwere originally plagioclase, and in the host rock adjacentto the shock vein. The textures observed in our sampleare consistent with crystallization of tissintite from theplagioclase-normative melt, rather than a solid-state phasetransformation (Fig. 8). Due to the similarity betweentissintite and jadeite, synthetic and natural occurrences of

µm

shock vein

�ssin�te + glass

a

20

µm

shock vein

�ssin�te

b

15

plagioclase glass

c

�ssin�te

µm0.6

Px[100]

001

010

glass

000

Fig. 8. BSE images of the plagioclase breakdown texture oftissintite + glass, observed within former plagioclase grains thatwere entrained as fragments within the shock vein (a) and in thehost rock in direct contact with the now-crystallized melt (b). (c)Bright-field TEM image of tissintite crystals and glass in atransformed plagioclase grain. The inset SAED pattern is fromthe labeled tissintite crystal along the [100] zone axis. The presenceof h + k – 2n reflections indicates that this pyroxene has aprimitive lattice, consistent with a P21/c space group rather thanthe C2/c space group of jadeite.

342 E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348

jadeite can be used to shed light on possible modes oftissintite formation; jadeite is stable between 2.5 and22 GPa (Liu, 1978; Tutti et al., 2000). According to thehigh-pressure, high-temperatures experiments on albite(Yagi et al., 1994; Tutti, 2007) and TEM investigations

on natural and synthetic samples (Miyahara et al., 2013),this phase transforms with increasing pressure toalbite! jadeite + coesite or stishovite! lingunite. InTissint, the characteristic Raman peaks for either SiO2

polymorph (coesite or stishovite) are absent; likewisecoesite and stishovite were not encountered in our TEManalysis of transformed plagioclase. Jadeite, with a lackof stishovite, has also been documented in shock veins ofhighly shocked L6 chondrites (Kimura et al., 2000;Ohtani et al., 2004; Ozawa et al., 2009; Kubo et al., 2010;Miyahara et al., 2013). Jadeite forms from amorphous pla-gioclase, with the delay in nucleation of other minerals suchas stishovite attributed to sluggish nucleation kinetics(Kubo et al., 2010; Miyahara et al., 2013).

To summarize our results, we see a range of pressuresrecorded by the mineral assemblages preserved in andaround shock melts in Tissint. The transformation of oliv-ine to ringwoodite and to silicate perovskite plus magne-siowustite indicates that the peak shock pressure reachedin this sample was greater than about 19 GPa. The meltin the thinnest vein (20 lm) and adjacent to thetransformed olivine in a thicker shock melt pocket(�300–450 lm) consist of clinopyroxene, ringwoodite andstishovite, suggesting crystallization at moderate pressures(�15 GPa). Larger mm-size volumes of shock melt havecrystallized during pressure release through to ambientpressure conditions. The inconsistency between transforma-tion and crystallization pressure, and the range of observedcrystallization pressures, supports our interpretation thatcrystallization of the melt occurred during pressure releasewhich continued after decompression in the largest volumesof melt. The thinnest shock veins cool fastest and record thehighest-pressure crystallization, while the largest shockveins and pockets solidified on a time scale longer thanthe shock pulse duration.

4.2. Cooling times of shock melts

The distribution of mineral assemblages preserved inand around shock melts in Tissint is variable, and is a func-tion of vein thickness (Section 4.1). In Tissint, severalobservations point to the formation of the shock veins byshear-induced frictional melting. These include the offsetand displacement of igneous minerals along the vein mar-gin, strong variation in vein thickness, sudden changes invein orientation and sharp vein contacts. All of these fea-tures are akin to experimentally-produced shear melts(Kenkmann et al., 2000) and point to a similar mode of for-mation. Shock melts, therefore, represent local mineralmelts and their bulk compositions are variable betweenindividual veins and pockets in a single meteorite (Waltonet al., 2010). The composition of the shock melt will alsoaffect the mineral assemblages that crystallize; however,detailed discussion of this variability is beyond of the scopeof this paper. This shock melt, once formed, will cool byconduction of heat to the host rock. Crystallization, there-fore, begins at the margins and progresses inwards to thecenter. As discussed by Xie et al. (2006), the mineral assem-blage that crystallizes within the shock vein is dependent onthe duration of the shock pressure, defined as the time that

Table 1EDS analysis of shock melts in Tissint.

Thin Shock Veins Thick Shock Vein

wt% (20 mm) (120 mm) mm-size

Oxides avg min max avg min max avg min max

SiO2 39.39 33.44 43.84 41.30 38.03 45.77 38.88 33.93 42.24TiO2 1.08 0.39 1.43 0.64 0.49 0.80 0.46 0.15 1.05Al2O3 8.23 5.98 10.90 5.91 2.43 7.76 4.17 1.74 12.17Cr2O3 0.52 0.33 0.72 0.77 0.50 1.05 0.64 0.33 1.10FeO 21.99 16.70 25.20 22.68 21.55 24.04 20.49 15.71 26.29MnO 0.71 0.34 1.16 0.48 0.39 0.68 0.74 0.57 0.97MgO 15.19 12.57 24.08 16.55 14.98 18.80 17.04 11.41 20.66CaO 9.41 5.52 13.98 8.68 7.66 9.66 7.34 5.07 9.62Na2O 1.33 0.93 1.73 1.31 0.51 1.68 0.01 b.d. 0.06K2O 0.03 b.d. 0.08 0.01 b.d. 0.03 0.22 b.d. 0.55P2O5 1.05 0.11 6.22 0.24 0.05 0.56 1.04 0.32 1.84SO3 1.20 0.85 1.48 1.88 0.61 2.92 1.21 0.18 3.14n 40 22 40

n = number of raster scans represented by the data range.b.d = below detection limits.

Table 2Minerals associated with shock melts in Tissint.

Shock melt

Thickness* 20 lm 120 lm 300–450 lm 1.4 mm

Crystallization products rgt + cpx + stish rgt + cpx + stish cpx + rgt cpx + ol (rim)glass + ves (center)

Transformation in ol!rgt! ol!rgt n.o. plag!tissintiteHost rock mw + am sil perov ol!recryst olTransformation in no fragments no fragments ol!cpx + mw ol!recryst olEntrained clasts px!recryst px

plag!tissintiteEstimated cooling time <10 ms 10 ms 20–50 ms 1 s

n.o. = not observed; rgt = ringwoodite; stish = stishovite; cpx = clinopyroxene; ol = olivine; ves = vesicles; plag = plagioclase;mw = magnesiowustite, am sil perov = amorphous silicate perovskite; recryst = recrystallized.* Thicknesses of shock melts were measured in the two dimensions of the thin section and so represent apparent thicknesses.

E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348 343

elapses between arrival of the shock front and decompres-sion, and the vein quench time. Here the post-shock ther-mal history of Tissint is assessed, with consideration of itsrelationship to the shock duration in Section 4.3.

The time required for cooling and partial crystallizationof shock melts in natural martian meteorite samples,including DaG 476 � an olivine-pyric shergottite proposedto be paired with Tissint based on similar CRE age(Chennaoui Aoudjehane et al., 2012; Eugster et al., 2006)� has been modeled by Shaw and Walton (2013). In theirmodels, the size, geometry and spatial distribution of shockmelts were taken into consideration. This is important inconsidering the post-shock cooling history of Tissint, asthe shock melt distribution and thickness is extremely var-iable within the studied thin sections (Fig. 1). The conduc-tive cooling time from the initial temperature of the shockmelt (2500 �C) to the background temperature of the hostrock (500 �C) was calculated to 900 �C, since at this temper-ature diffusion of even the fastest moving components (e.g.,Na) would be negligible. The minimum cooling time for a1 mm2 shock melt pocket is 1.2 s; this is comparable tothe thickest shock melts in investigated in this study.

Scaling their models to consider 100 lm thick shock meltpockets decreased cooling times by a factor of 102, andcooling over the interval 2500 �C to 900 �C was achievedin 10 ms. This is within the range of those calculated cool-ing times of 2 ms (Beck et al., 2005), 0.6 ms (Langenhorstand Poirier, 2000) and 40 ms (Xie et al., 2006) for the sameshock melt thickness. Therefore, the thin shock veins inves-tigated in this study (20 lm and 120 lm) are likely to havecooled rapidly by conduction to the colder host rock(�10 ms). The presence of ringwoodite, stishovite, clinopy-roxene and glass in these shock veins is consistent withrapid quench and crystallization of a metastable assemblagein a supercooled melt. Individual EDS raster scans withinthe shock vein matrix show considerable heterogeneity,which supports their melting and partial crystallization atan extremely fast rate, limiting mechanical and chemicalmixing of the melt. The larger volumes of shock melt (upto 1.4 mm) would have remained hot over a longer time-scale, cooling within �1 s.

The 1-dimensional conductive cooling model of Baziotiset al. (2013) may be used to estimate the cooling conditionsfor the intermediate �300–450 lm thick shock melt pocket

Fig. 9. Phase diagram calculated for the Mg2SiO4 system to illustrate the role of metastable vs. stable phase boundaries. Stable phaseboundaries are illustrated with black lines and stable assemblages are forsterite (Fo), wadsleyite (Wad), ringwoodite (Rwd), akimotoite (Aki),periclase (Peri), MgSiO3-perovskite (Mg-pv), majorite (Maj) and stishovite (St). The blue (lower) dashed line is the metastable boundarybetween forsterite and ringwoodite and the red (upper) dashed line is the metastable boundary between forsterite and MgSiO3-perovskite pluspericlase. The metastable boundaries represent the minimum P–T conditions under which forsterite can transform to ringwoodite orMg-perovskite plus periclase. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version ofthis article.)

344 E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348

investigated in this study. In their model, a 350 lm shockmelt pocket in Tissint initially at 2230 �C, was cooled to730 �C in contact with a host rock at a uniform temperatureof 180 �C. This simulation suggested cooling within �22 msbetween 15 and 25 GPa, and �50 ms for the center of themelt pocket to solidify down to 730 �C. Thus cooling within20–50 ms is a reasonable estimate for the intermediate-sizedshock melt pocket (300–450 lm) considered in this study.

4.3. Constraints on shock duration

Three scenarios for shock melt crystallization as afunction of shock duration and quench time are considered.(1) The shock duration exceeds the quench time. In this casecrystallization occurs at the peak pressure. (2) The shockduration is shorter than the quench time, resulting incrystallization of the vein during pressure release. (3) Thevein remains molten after pressure release. In Tissint, wedocument evidence for crystallization of shock melts underall three conditions that is largely a function of theirthickness.

The thinnest shock veins (20 lm) quenched at the peakpressure to preserve the solid state disproportionation ofolivine to magnesiowustite + silicate perovskite which sub-sequently vitrified after pressure release. The high-pressureminerals clinopyroxene + ringwoodite + stishovite crystal-lized in the matrix of this thin vein. In the intermediateveins and pockets (�120–450 lm), which remained partiallymolten after decompression, the vitrified silicate perovskitewas hot enough to transform to clinopyroxene. The 1-mm

scale shock vein crystallized after pressure release to crystal-lize a low-pressure olivine + pyroxene assemblage at themargins with a glass-rich, highly vesiculated vein center.Combining this thermal history with the shock melt coolingtimes discussed in Section 4.2, allow us to place constraintson the shock duration experienced by Tissint. The thinnestshock veins quench at the peak shock pressure in 10 ms, theintermediate shock veins quench during pressure releasewithin 20–50 ms and the largest shock melts (�1 mm)quench after decompression within 1s. It thus follows that10–20 ms is a reasonable estimate for the shock durationexperienced by Tissint. This implies that there is a lot ofcomplex mineralogy occurring on a very short (millisecond)timescale, supported by study of shock veins in other mar-tian meteorites (Langenhorst and Poirier, 2000; Walton,2013; Greshake et al., 2013).

These results call the findings of Baziotis et al. (2013)into question. In their study on Tissint, Raman spectros-copy was used to document the presence of high-pressurephases maskelynite, ringwoodite, lingunite, majorite,akimotoite, tuite, stishovite and amorphosed silicate perov-skite. These phases, with the exception of maskelynite,occur exclusively in and adjacent to shock melts. Basedon an assumed ringwoodite crystal size of 1 lm and aringwoodite growth rate calculated from experimentally-determined ringwoodite growth kinetics for interface-controlled growth mechanism (Mosenfelder et al., 2001),Baziotis et al. (2013) infer that the shock metamorphismin Tissint is the result of a 1s shock pulse. Using thisestimate and the equation Dimpact = s � t, where s = the

E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348 345

estimated shock duration (1 s) and t is the impact velocity(10 km s�1), they calculate a 10 km impacting body and a90 km impact crater. This scenario is inconsistent with thecrystallization of shock melt during or after release froma short shock duration proposed from this study. If theshock duration was long, on the order of 1 s as proposedby Baziotis et al. (2013) then the shock melt would be morelikely to crystallize during shock compression, formingminerals that record a crystallization pressure directlyrelated to the shock P–T history of the host rock. Indeedthis is the case for strongly shocked L6 chondrites, wherethicker shock melts crystallize at a constant equilibriumshock pressure (Chen et al., 1996; Kimura et al., 2000;Xie et al., 2006; Ozawa et al., 2009). For example, in theL6 chondrite Yamato 791384, shock veins 2 mm thick(twice that of the thickest Tissint shock veins investigatedin this study) have crystallized a majorite-pyrope solidsolution at the peak shock pressure of 18–23 GPa (Ohtaniet al., 2004). A shorter shock duration is also consistentwith previous calculations for martian meteorites (10 ms),compared to ordinary chondrites (100 ms) (Beck et al.,2005; Fritz and Greshake, 2009). Two theories are consid-ered to explain the discrepancy between our work onTissint and that of Baziotis et al. (2013): (1) the ringwoo-dites in Tissint consist of polycrystalline aggregates ofsub-lm ringwoodite crystallites as described in all otherreports of natural ringwoodite, (2) the duration calculatedfrom growth kinetics of ringwoodite was long because thetransformation temperature was underestimated.

The difference in shock duration between martian mete-orites and ordinary chondrites is attributed to distinctimpact conditions on their respective parent bodies; unlikethe L-chondrite parent body, which was impacted �460 Maby a very large meteoroid (McConville et al., 1988), theimpact that caused the shock in Tissint was likely muchsmaller, resulting in a much shorter shock pulse. It is gener-ally accepted that martian impact ejection events were smallbecause of their young CRE ages, the decrease in size-fre-quency distribution of impact craters over the history ofour solar system, and that there are too few young largeimpact craters on Mars to account for all different ejectionevents observed (Head et al., 2002). Not only is excavationof martian meteorites in a recent small impact event consis-tent with impact crater size-frequency statistics, but it isalso consistent with the observation that shergottites areformed by crystallization of mafic magmas on or near thesurface of Mars (e.g., Greshake et al., 2004; Gross et al.,2011, 2013; Filiberto et al., 2014). The shock effects thatwe report here are consistent with the smaller impactorsthat have produced the youngest craters on Mars.

4.4. Constraints on shock pressure from deformation and

transformation in the bulk rock

In Tissint, the veins quenched or quench-crystallizedduring and after pressure release, therefore, the mineralassemblages in and around the shock melts in Tissint donot represent the peak shock pressures. The pressureestimates derived in this study by the crystallization pres-sure of the clinopyroxene + ringwoodite + stishovite

assemblage (�15 GPa) and the dissociation of olivine to sil-icate perovskite (>19 GPa), therefore, represent the mini-mum shock loading experienced by Tissint. Shockdeformation in host rock minerals, extensive shock meltingand the complete transformation of plagioclase tomaskelynite, indicate that Tissint was strongly shock meta-morphosed. If the composition of An58-66 reported byBaziotis et al. (2013) for Tissint host rock maskelynite iscompared with shock recovery experimental data (Stoffleret al., 1986; Fritz et al., 2005), the minimum shock loadingrecorded by Tissint maskelynite is �29–30 GPa. This pres-sure range is required for complete isotropization of thismineral in shock recovery experiments. Indeed, plagioclasewith the composition of Tissint maskelynite (An58–66), ifshocked to 19 GPa (our minimum shock loading estimate),would still be birefringent (Stoffler et al., 1986). The shockeffects in olivine and pyroxene may also be used to estimatethe shock pressure. Based on shock recovery experiments,the development of strong mosaicism, planar fractures andmechanical twinning (pyroxene only), are indicative of a�30 GPa shock pressure. These pressure estimates supportour view that the peak shock pressure in Tissint was signif-icantly higher than the minimum 19 GPa required to trans-form olivine to silicate perovskite plus magnesiowustite.

4.5. Shock metamorphism and ejection of Tissint in a recent

small impact event on Mars

The extensive melt in Tissint (�15 vol%), coupled withshock deformation and shock transformation effects, indi-cates that it was strongly shocked by a high velocity impac-tor. The heterogeneous distribution of shock-inducedphases in Tissint, mineral zoning within shock melts (fromrim! center), and the presence of vesicles, testifies to theshort shock duration experienced by this meteorite. Keywt% element ratios such FeO/MnO (39.7), Al/Ti (7.2),Na/Ti (1.41), Ga/Al (3.9 � 10�3), Na/Al (0.20) and D17O(+0.301&) confirm Mars as the Tissint parent body, andclassify this meteorite as a picritic shergottite (ChennaouiAoudjehane et al., 2012). The timing of Mars ejection canbe approximated by the meteorite’s cosmic ray exposure(CRE) age, or more precisely by combining the CRE agewith the terrestrial residence age. Due to the circumstancessurrounding Tissint’s recovery (i.e., within a few months ofthe observed fall), its CRE age (Nishiizumi et al., 2012;Huber et al., 2013) gives the time that this meteorite waslaunched from the surface of Mars, forming a tight clusterwith other depleted olivine-phyric shergottites at1.05 ± 10 Ma. We contend that this ejection event repre-sents the timing of shock metamorphism of this meteorite;this is based on the observation that no evidence for multi-ple impact processing is present in the studied thin sections.Obvious signs of a multi-generational impact history wouldbe breccia formation, as in the case of the much older(2.089 ± 0.081 Ga) martian meteorites Northwest Africa7034 (Agee et al., 2013) and the �4 billion year oldALH84001 (Treiman, 1998). Furthermore, all known mar-tian meteorites have experienced shock pressures of at least5–14 GPa (Fritz et al., 2005), supporting predictions fromnumerical modeling that material cannot be ejected from

346 E.L. Walton et al. / Geochimica et Cosmochimica Acta 140 (2014) 334–348

the surface of Mars without experiencing a shock pressureof at least 10 GPa (Artemieva and Ivanov, 2004). If theshock metamorphic effects in Tissint were from an earlierimpact event on Mars with ejection in a later, smallerimpact, we would expect to see fractures overprinting shockmelts in this sample, as is seen in ALH84001 (Treiman,1998). This is not the case; fractures in igneous mineralsare truncated at the shock melt margins and no such fea-tures cut across shock melts. Numerical simulationsdesigned to study the delivery dynamics of martian meteor-ites also support a multiple small impact scenario, withlaunch as small rocks from the near surface of Mars overthe past few million years (Gladman, 1997). El Goresyet al. (2013) used the various shock-induced mineral assem-blages in Tissint to argue for multiple shock events. How-ever, this apparent inconsistency in pressure estimates,derived from various minerals assemblages in an aroundshock melts in Tissint, does not require multiple shockevents. Variable high-pressure minerals in the bulk rockcan be explained by pressure heterogeneities during shockloading combined with thermal heterogeneities that controltransformation rates and mechanisms. Three-dimensionalcomputational models of shock wave propagation throughheterogeneous materials analogous to meteorites haveshown that pressure equilibrates within �1 ls (Baer, 2000;Baer and Trott, 2002). In contrast, thermal heterogeneitieswill dissipate by conduction of heat from hotter areas(shock melts) to the colder host rock over a much longertimescale (10 ms for the thinnest melts up to >1 s for mm-size shock melts; see discussions in Section 4.3). The appar-ent variation in shock pressure in Tissint is therefore theresult of a relatively short duration of the shock pulse(10–20 ms) combined with a complex thermal history anddoes not imply that the sample record multiple impactevents. Using these lines of evidence we deduce that Tissintwas ejected in a recent (1 Ma), single and relatively small(short shock duration), impact event on Mars.

5. SUMMARY AND CONCLUSIONS

Evidence of shock deformation including strong mosai-cism and fracturing, mechanical twinning, mineral transfor-mation and extensive shock melting, indicate that Tissintwas strongly shocked. However, the mineral assemblagespreserved in and around shock melts vary depending onvein thickness. The apparent inconsistency in pressure(1 bar versus 15 and >19 GPa) indicates that crystallizationof shock melt in Tissint occurred during and after pressurerelease, and so represent the minimum shock loading expe-rienced by this meteorite. The shock features in olivine(strong mosaicism, irregular and planar fractures), pyrox-ene (strong mosaicism, mechanical twinning, pervasivefracturing and formation of planar deformation bands)and plagioclase (complete transformation to maskelynite)throughout the bulk rock suggest a peak shock pressure�29–30 GPa. Based on our detailed SEM, Raman, TEMand synchrotron micro-XRD investigation of shock meta-morphism in Tissint, the following scenario is proposedfor the shock history of this meteorite: (1) shock loadingcausing plastic deformation and cracking of host rock

olivine and pyroxene and complete transformation of pla-gioclase to maskelynite, (2) frictional melting along shearbands during shock compression, (3) quenching of thinshock veins during pressure release to form clinopyrox-ene + stishovite + glass rather than silicate perovskite inthe transformed olivine, (4) quenching of thicker veins afterdecompression; in (3) earlier-formed silicate perovskiteback-transformed to clinopyroxene after pressure release.We see no evidence for multiple impact events as recordedin Tissint or for exceptionally long shock durations from alarge impacting body.

ACKNOWLEDGEMENTS

This work has been funded by the Natural Science and Engi-neering Research Council Discovery Grant RES0007057 awardedto E. Walton. T. Sharp and J. Hu acknowledge NASA Cosmo-chemistry NNH08ZDA001 N-COS Grant NNX09AG41G sup-porting TEM analyses. NASA Grant MFR #NNX13AG35G,awarded to J. Filiberto, facilitated purchase of the Tissint materialused in this study. We gratefully acknowledge the use of FIB/TEMfacilities within the LeRoy Eyring Center for Solid State Science atArizona State University. Thanks for De-Ann Rollings for SEMassistance at the University of Alberta. Constructive commentsby Axel Whittmann, Jorg Fritz and an anonymous reviewerimproved the overall quality of this manuscript.

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Associate editor: Marc Norman