a new martian meteorite from morocco: the nakhlite north west africa 817

A new Martian meteorite from Morocco:the nakhlite North West Africa 817

V. Sautter a;*, J.A. Barrat b, A. Jambon c, J.P. Lorand a, Ph. Gillet d,M. Javoy e, J.L. Joron f , M. Lesourd g

a Laboratoire de Mineralogie, FR32-CNRS, Museum National d'Histoire Naturelle, 61 rue Bu¡on, 75005 Paris, Franceb CNRS-UMR 6112 (Geodynamique et Planetologie) and Universite d'Angers, 2 bd Lavoisier, 49045 Angers Cedex, France

c Laboratoire Magie, Universite Pierre et Marie Curie, UMR-CNRS 7047 case 110, 4 place Jussieu, 75252 Paris Cedex 05, Franced Laboratoire des Sciences de la Terre, CNRS-UMR 5570, Ecole Normale Superieure de Lyon, 46 Allee d'Italie, 69364 Lyon Cedex 7,

Francee Laboratoire de Geochimie des Isotopes Stables, Universite de Paris VII, Institut de Physique du Globe et UMR-CNRS 7047,

4 Place Jussieu, 75251 Paris Cedex 05, Francef Laboratoire Pierre Sue, CEA-CNRS, Saclay, 91191 Gif sur Yvette, France

g CNRS et SCIAM, 2 rue Haute de Reculee, F-49045 Angers, France

Received 13 July 2001; received in revised form 22 October 2001; accepted 27 October 2001

Abstract

North West Africa (NWA 817) is a single stone of 104 g found in the Sahara (Morocco) by meteorite hunters inNovember 2000. The meteorite is an unbrecciated, medium-grained olivine-bearing clinopyroxenite with a cumulatetexture. It consists of zoned euhedral subcalcic augite (Wo42En38ÿ22Fs20ÿ36), olivine spanning a wide range ofcompositions (from Fa56 in the core to Fa86) with rare magmatic inclusions and an intercumulus mesostasis made of Fe-bearing albitic plagioclase, Si-rich glass, Ti-magnetite with unusual skeletal growth morphologies containing ilmeniteexsolutions, acicular pyroxene, olivine and cristobalite. Trace minerals are sulfide droplets and Cl-apatite. Mineralmodes (in vol%) are augite 69%, olivine 10%, mesostasis 20% and Fe^Ti oxides 1%. Pervasive alteration produced areddish clay mineral (hydrous ferrous silicate) in both olivine crystals and the mesostasis. The major elementcomposition of NWA 817 is very similar to that of the other nakhlites: high FeO, MgO and CaO concentrations reflectthe abundance of cumulus augite and olivine. Key element ratios such as FeO/MnO ( = 37), Na/Al ( = 0.40), K/La( = 449), Ga/Al ( = 3.9U1034) and oxygen isotopic composition (v17O = +0.37x) are clear evidence for a Martianorigin. The incompatible trace element pattern as in Nakhla displays a strong light rare earth element enrichmentrelative to chondrite (Lan/Ybn = 4.89). However, when compared to the other nakhlites, NWA 817 has specific features:(1) a higher modal proportion of mesostasis; (2) quench textures of Ti-magnetite and Fe-rich clinopyroxene; (3) moreMg-rich olivine core compositions whereas the augite core composition is identical for all nakhlites ; (4) a stronger Feenrichment toward crystal rims of these cumulus minerals. The intercumulus minerals (Ti-magnetite with skeletalgrowth morphology, acicular chains of clinopyroxene and Fe3�-rich feldspar) indicate rapid crystallization in responseto a high degree of undercooling at the end of the sample story. ß 2002 Elsevier Science B.V. All rights reserved.

0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 9 1 - X

* Corresponding author. Tel. : +33-1-40793525; Fax: +33-1-40793524. E-mail address: [email protected] (V. Sautter).

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Earth and Planetary Science Letters 195 (2002) 223^238

www.elsevier.com/locate/epsl

Keywords: SNC Meteorites; nakhlite; skeletal titanomagnetite; alteration

1. Introduction

The Martian meteorites described so far rangefrom basaltic rocks (the shergottites) to a varietyof ultrama¢c pyroxenitic (the nakhlites and ALH84001, an orthopyroxenite) or dunitic (Chassigny)cumulates. They all plot on a speci¢c O-isotopicfractionation line [1^3] and share petrological andchemical features indicating that they originatefrom the same large planetary body, probablyMars (e.g. [4]). Among the 17 Martian meteoritesdescribed so far, the most common are the sher-gottites. Only three of them (Nakhla, GovernadorValadares and Lafayette) belong to the nakhlitesubgroup: unbrecciated, medium-grained olivine-bearing clinopyroxenites with a cumulate texture([5^11]).

In this paper, we describe a new meteorite,named North West Africa 817 (NWA 817), the¢rst nakhlite found in a hot desert, the MoroccanSahara. We present a petrological and chemicalstudy of this 104 g stone and provide a compar-ison with the other nakhlites.

2. Petrography

Petrographic observations and quantitativechemical analyses of the various phases weremade on two polished sections and a thin sectionof NWA 817. Backscattered electron (BSE) im-ages were taken with a JEOL JSM6301-F scan-ning electron microscope (SCIAM Angers).

NWA 817 is an unbrecciated, medium-grainedolivine-bearing clinopyroxenite with a cumulatetexture. Modal analyses made on a polished sur-face of approximately 1.7 cm2 indicate the follow-ing mineral proportions (vol%): pyroxene 69%,olivine 10%, mesostasis 20% and Fe^Ti oxides1%. Pervasive alteration produced a reddish smec-tite-like mineral found in olivine crystals as wellas in the mesostasis. Shock e¡ects, observed insome olivine and clinopyroxene crystals, are mod-erate.

Clinopyroxene appears in thin section as very

Fig. 1. Transmitted light photomicrographs of NWA 817and Nakhla: low-magni¢cation view (scale bar 1 cm = 0.5mm). (a) NWA 817. Cumulus augite (light colored grains)and two olivine crystals on the right hand side of the picture.The abundant mesostasis shows a ¢brous texture and whitishareas of feldspar. Black spots are artifacts of preparation ofthe thin section (bubble in the epoxy). (b) Nakhla. Note thatcumulus augites are more fractured and the mesostasis is lessabundant.

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V. Sautter et al. / Earth and Planetary Science Letters 195 (2002) 223^238224

Fig. 2. Backscattered electron images of NWA 817. (a) Augites are dark gray and olivines light gray. Note the di¡erence in zon-ing pattern between augite and olivine: Fe-rich bright rims on darker augite grains are very narrow and follow the crystal faceswhich is typical of overgrowth zoning; in olivines, the Fe-rich rim is wider and has a smooth transition to the more magnesiancore. (b) At higher magni¢cation, the olivine morphology appears clearly euhedral when in contact with the mesostasis, anhedralwhen adjacent to augites. Note the olivine crystal in the center that includes two small augite crystals. Rounded inclusions areobserved within two olivines in the bottom right hand side of the picture. (c,d) Skeletal Ti-magnetite with ilmenite exsolutions(bright) in the dark mesostasis. At this larger magni¢cation, pyroxene overgrowths become conspicuous. Skeletal Ti-magnetite(white) with ilmenite exsolution, pyroxene (light gray) and olivine (white) blades are surrounded by feldspathic background(black). (e,f) Inclusions in olivine are of three types: (1) rounded augite inclusion; (2) alteration mineral forming disconnectedrhombic-shaped areas and ¢lling fractures; (3) rounded inclusions connected to fractures and ¢lled by the mesostasis.

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V. Sautter et al. / Earth and Planetary Science Letters 195 (2002) 223^238 225

pale green crystals. The grains range from subhe-dral to euhedral, with well developed twinningand cleavages. Pyroxene laths average 250 Wmacross and 800 Wm length (up to 1 mm). Theaverage grain size is slightly larger in NWA 817than in Nakhla (Fig. 1). Simple twins on 100 arevisible on some elongated crystals. Clinopyroxeneexhibits complex serrate margins at the contactwith the mesostasis. These overgrowths consistof subparallel growth of tiny clinopyroxene aggre-gates. The only shock e¡ect is mechanical twin-ning.

Olivine is the second most abundant mineral,approximately similar in size to pyroxene and dis-playing a range of crystal morphologies depend-ing on the surrounding material : when borderingthe mesostasis areas, it has well de¢ned euhedralfaces whereas it appears anhedral when abuttingclinopyroxene grains (Fig. 2a,b,e,f). When in con-tact with the mesostasis, olivine, like augite, ex-hibits serrate overgrowths (Fig. 2e,f). Olivine con-tains two kinds of inclusions: (i) rounded augiteinclusion in their core (Fig. 2e); (ii) voids con-nected to fractures ¢lled by the mesostasis (Fig.2f). Dark lamellar exsolutions (augite^magnetiteintergrowth) as encountered in Nakhla olivine[12] have not been observed. Fractures in olivineare often ¢lled with an alteration phase, whichwill be described below.

The mesostasis is more abundant in NWA 817than in other nakhlites: up to 20% versus 5^13%[13]. Its texture and mineralogy (Fig. 2c,d) di¡eras well. NWA 817 is dominated by feldspar asso-ciated with glass, acicular chains of clinopyroxeneand olivine, Ti-magnetite with spectacular skeletalgrowth morphology containing exsolved ilmenite.Accessory phases are Cl-apatite and sul¢de drop-lets.

NWA 817 contains opaque minerals (Fe^Ti ox-ides and Fe-sul¢des), localized in the mesostasis,in similar proportions and compositions as Nah-kla. Fe^Ti oxides cover about 1 vol% of the rock(image analysis estimate on re£ected light photo-micrographs). The sul¢de modal abundance ismuch lower (6 0.01 vol%), well below the detec-tion limit of image analysis. A partly oxidizedpyrrhotite is the only sul¢de identi¢ed in re£ectedlight microscopy and electron microprobe (EMP)

analyses; it is scattered within the mesostasis,often as droplets, 5 Wm across. Such droplets ob-viously crystallized from immiscible sul¢deliquids, which sometimes nucleated over Fe^Tioxides.

Ti-magnetite crystals occur as an intercumulusphase. It is characterized by skeletal growth mor-phologies (Fig. 2c,d). Such features commonly oc-cur in quenched terrestrial basalts [14]. They havenever been reported in Nakhla or GovernadorValadares and appear only in the outermostpart of the fusion crust in Lafayette [16]. Accord-ing to the nomenclature [14], NWA 817 Ti-mag-netite crystals combine octahedral (rare), abortedoctahedron (Fig. 2c), cruciform (Fig. 2d) andcomplex growth shapes (common). Some grainsshow spectacular stacks of smaller (6 50 Wm) cru-ciform crystals, up to 1 mm in length, (Fig. 2d).Observations at high magni¢cation (s 1000)clearly indicate the spongy nature of all the Fe^Ti oxides, even those occurring as octahedral crys-tals. Whether massive or skeletal, Ti-magnetite al-ways contain ¢ne spindles of ilmenite along oneset of (111) planes (Fig. 2d); crowded ilmenitelamellae (6 2^3 Wm thick) along all sets of octa-hedral planes are also common. Such ilmenite tex-

Fig. 3. Transmitted light view of reddish alteration both inolivine and in the mesostasis around an oxide crystal. Notethe skeletal Ti-magnetite in the bottom part of the picture.

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tures correspond to the C2 stage of the oxy-exso-lution processes a¡ecting Ti-magnetite in ma¢crocks during subsolidus cooling (e.g. [14]). UnlikeNahkla, NWA 817 does not contain discrete il-menite grains (i.e. remote from the Ti-magnetite).

The predominant alteration product appears inthin section in plane polarized light as a red-or-ange mineral (Fig. 3), well crystallized undercrossed nichols but including an amorphousarea. Its occurrence in olivine is widespread andvery easy to recognize on BSE images (Fig. 2e,f).This material develops along cracks and formsdisconnected rhombic-shaped areas (Fig. 2e).Platelets in olivine cracks may extend over a fewhundred Wm (Figs. 2e,f and 3). A similar materialis present within cracks and grain boundariesthroughout the meteorite. In the mesostasis, itforms veinlets and patches (Fig. 3). A similar red-dish alteration phase has been previously de-scribed in other nakhlites and referred to as `id-dingsite' by [5,15^18], or clay [19].

3. Mineral chemistry

Microprobe analyses were performed using theCAMECA SX-50 facility at the University of

Paris VI (CAMPARIS) using an accelerating volt-age of 15 kV and a beam current of 20 nA. In-ternational mineral standards were used for cali-bration. Raman spectra were recorded with aDilor0 XY spectrometer equipped with confocaloptics and a nitrogen-cooled CCD detector. Amicroscope was used to focus the excitation laserbeam (488 nm and 514 nm lines of a SpectraPhysics0 Ar� laser) to a 2 Wm spot and to collectthe Raman signal in the backscattered direction.Accumulations lasting 120^300 s have been made.The laser power was 2^50 mW to avoid deterio-ration of the sample.

Clinopyroxene (Table 1 and Fig. 4): crystalcores display essentially homogeneous composi-tions (XMg = Mg/(Fe2�+Mg) = 0.61), with very lit-tle variation within and between grains of subcal-cic augite compositions near Wo40En38Fs24 (Table1). Moreover, this compositional cluster is almostidentical for all the nakhlites. Microprobe traver-ses together with BSE imaging (Figs. 2a,b and5) reveal a sharp zoning de¢ned by a strongFe enrichment toward the crystal rims up toWo40En15Fs45. These iron-rich rims are about 10Wm across (up to 50 Wm when the crystal faces arenot exactly perpendicular to the section plane).They mimic the euhedral faces of augite crystals

Table 1EMP analyses of pyroxene in nakhlites

Sample: NWA 817 Nakhlaa Gov. Val.a Lafayettea

Min. Cumulus Mesostasis Cumulus Cumulus Cumulus

(Wt%) Core Rim Core Rim Core Rim Core Rim Core Rim

SiO2 51.46 48.95 50.97 49.44 43.91 47.80 52.13 49.94 51.95 49.65 53.25 50.20TiO2 0.24 0.64 0.32 0.69 1.87 5.12 0.20 0.41 0.24 0.46 0.20 0.40Al2O3 0.83 1.99 0.90 1.89 3.51 8.60 0.75 1.60 0.79 1.82 0.76 1.47Cr2O3 0.49 0.00 0.32 0.00 0.21 ^ 0.38 0.17 0.39 0.13 0.43 0.17NiO 0.00 0.08 0.00 0.05 0.0 ^ ^ ^ ^ ^ ^ ^FeOT 14.02 22.24 14.14 20.12 35.57 26.85 14.09 20.13 14.03 19.74 13.29 18.08MnO 0.40 0.63 0.45 0.20 0.21 ^ 0.43 0.56 0.43 0.55 0.43 0.52MgO 12.90 6.95 12.79 8.84 2.69 3.31 13.41 9.39 13.33 9.46 13.62 10.82CaO 19.44 18.43 19.76 18.15 10.73 7.38 18.05 17.24 18.43 17.50 18.05 17.33Na2O 0.37 0.42 0.31 0.47 0.04 0.40 0.19 0.24 0.19 0.25 0.19 0.26K2O 0.00 0.00 0.00 0.02 0.01 0.3 ^ ^ ^ ^ ^ ^P2O5 0.00 0.03 0.00 0.00 0.1 0.05 ^ ^ ^ ^ ^ ^Total 100.41 100.57 100.28 100.07 98.9 101.13 99.64 99.65 99.77 99.56 100.18 99.24Wo 42 42 42 41 25 22 38 38 38 38 38 37En 38 22 38 27 9 14 39 28 39 29 40 33Fs 20 36 20 32 66 64 23 34 23 33 22 30a Lentz et al. [13].

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(see Fig. 2c) and develop exclusively where augiteis in contact with the mesostasis. The Fe-rich rimsare absent when in contact with olivine or anotheraugite crystal suggesting that they are benchmarksof growth zoning. The edges of augite crystals areextended by serrate overgrowths (Fig. 2c). Acicu-lar clinopyroxene crystals in the mesostasis haveFe-rich compositions and they are enriched inAl2O3. Compared to the other nakhlites, NWA817 shows compositional trends extending towardmore iron-rich compositions: XMg can be as lowas 0.17 versus 0.28 in Nakhla. Augite cores inNWA 817 and other nakhlites have Al, Ti andNa concentrations (Al2O3 = 0.9 þ 0.1; TiO2 =0.3 þ 0.05; Na2O = 0.3 þ 0.05) that remain rela-tively low with respect to terrestrial clinopyrox-enes, consistent with crystallization at low pres-sure. The rim is signi¢cantly enriched, up toAl2O3 = 3%, TiO2 = 1.2% and Na2O = 0.8%.

Olivine (Table 2 and Fig. 2b^f) crystals aresmoothly zoned: core compositions are richer inforsterite compared to Nakhla, Governador Vala-dares and Lafayette (Fo41 vs. Fo35ÿ32 ; Fig. 4).The largest olivine crystal analyzed (nearly 500Wm in diameter; Fig. 5) exhibits a gradual Feenrichment over more than 100 Wm toward therims and a £at core zone 200 Wm wide (Fig. 5).As described above for cumulus augite, the rimsextend toward more Fe-rich composition com-pared to other nakhlites (Fig. 4: Fa86 vs.Fa71ÿ73 in Nakhla, Governador Valadares, andLafayette). Serrate overgrowths on the rim of cu-mulus olivine are similarly strongly enriched in Fe(Fa86). Such zoning pro¢les, distinct from sharpgrowth zoning observed in neighboring pyroxene,are typical of di¡usive zoning. These two kinds ofzoning have been already described in Nakhla[8,11]. The CaO content is high (0.62%) in olivinecore and decreases to 0.10% in crystal margin.These values are consistent with crystallizationat low pressure from a basaltic composition[20,21].

Despite its highly variable morphologies, theNWA 817 Ti-magnetite (Table 3: Fe 2�/Fe 3�

determined from stoichiometry) spans a narrowcompositional range (Usp31^Mt69 to Usp43:6^Mt56:3), akin to Ti-magnetite in Nahkla (Usp39^Mt61 to Usp45^Mt55 [22]). However, it is nearly

Fig. 4. Pyroxene and olivine compositions in nakhlites. Allthe known nakhlites show a cluster of augite compositionsaround Wo40En37Fs23, corresponding to crystal cores. ForNWA 817 the area labeled core represents 55 analyses andthat the corresponding composition Wo40En37Fs23 represents90% of each analyzed crystal. Augite rim has a higher Fecontent in NWA 817 individual grains than in all othernakhlites. Intermediate augite compositions (gray squares)are mixed composition obtained close to the crystal rim. Or-thopyroxene is lacking in NWA 817. Olivine shows a widerrange of composition compared to other nakhlites.

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Cr-free (6 0.1 wt%; Table 3). The very small sizeof ilmenite lamellae precluded detailed EMP anal-yses. The only analysis available corresponds to anear Mg-free, ilmenite containing about 11 mol%hematite ; however, the hematite content was cer-

tainly overestimated due to beam overlap ontothe host Ti-magnetite. The ilmenite composition,combined with the average Ulvo«spinel content ofthe Ti-magnetite (39 þ 5%), indicates that NWA817 Fe^Ti oxides crystallized at T6 750³C and

Table 2EMP analyses of olivine in nakhlites

Sample NWA 817 Nakhlaa Gov. Val.a Lafayettea

(Wt%) Coreb Rimc 1 Rim 2 Rim 3 Rim 4 Core Rim Core Rim Core Rim

SiO2 33.93 33.74 32.81 31.87 30.41 32.77 32.10 32.87 32.38 32.15 32.12TiO2 0.03 ^ 0.02 0.05 0.04 ^ ^ ^ ^ ^ ^Al2O3 0.01 0.03 ^ 0.02 0.00 ^ ^ ^ ^ ^ ^Cr2O3 0.04 ^ ^ ^ 0.00 ^ ^ ^ ^ ^ ^NiO 0.07 0.03 ^ 0.02 0.08 ^ ^ ^ ^ ^ ^FeO 46.36 47.83 52.55 56.75 62.07 49.94 53.16 50.00 52.25 52.59 52.57MnO 0.85 0.76 1.22 1.15 1.40 0.98 1.03 0.99 1.02 1.01 1.01MgO 19.75 18.50 14.32 11.28 5.67 15.46 12.62 15.01 13.08 14.04 13.87CaO 0.56 0.49 0.51 0.53 0.13 0.40 0.29 0.39 0.31 0.18 0.18Na2O 0.00 ^ ^ ^ 0.05 ^ ^ ^ ^ ^ ^K2O 0.02 ^ 0.01 ^ 0.03 ^ ^ ^ ^ ^ ^Total 101.63 101.37 101.44 101.69 99.88 99.54 99.19 99.26 99.04 99. 97 99.74Fo 41.2 40.8 32.7 26.2 14.0 35.5 29.7 34.8 32.6 32.2 32.0Fa 56.8 59.2 67.3 73.8 86.0 64.5 70.3 65.2 67.4 67.8 68.0a Lentz et al. [13].b Average 30 analyses.c Rim: 1 to 4 from core to rim.

Table 3EMP analyses of Fe^Ti oxides in nakhlites

Sample NWA817 Nahklaa

Min. TM TM TM TM TM TM Il TM TMComp. Oct Cruc. Oct. Comp. Oct Comp. Oct. Oct.

Wt%TiO2 11.60 12.52 10.60 10.91 14.57 10.77 46.60 12.96 14.92Al2O3 1.58 2.06 0.69 2.23 1.13 1.98 ^ 2.10 1.96Cr2O3 0.06 0.02 0.02 0.08 0.03 0.02 ^ 0.98 1.19Fe2O3 43.25 40.92 46.5 45.55 37.7 45.32 11.5 41.94 37.12FeO 41.30 42.60 40.76 40.26 43.13 39.75 41.10 41.15 42.65MnO 0.40 0.37 0.37 0.35 0.46 0.35 0.78 0.32 0.41MgO 0.08 0.14 0.05 0.12 0.20 0.07 0.18 0.52 0.56ZnO 0.13 0.09 0.02 0.06 0.10 0.05 ^ n.a. n.a.Total 98.38 98.77 98.50 99.94 97.33 98.50 100.16 99.98 99.00Mol%Usp 34.7 37.6 31.1 32.0 43.6 31.8 ^ 39.0 45.0Mt 65.3 62.4 68.9 68.0 56.3 68.2 ^ 61.0 55.0Ilm ^ ^ ^ ^ ^ ^ 89 ^ ^Hem ^ ^ ^ ^ ^ ^ 11 ^ ^

TM = Titanomagnetite; Il = 111 ilmenite exsolution in TM. Comp. = complex skeletal crystals; Oct = octahedral; Cruc. = cruciform(after Haggerty [14]); n.a. = not analyzed.a MNHN: J.P. Lorand (unpublished).

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fO2 near the QFM bu¡er assemblage using thecalibration of the Fe^Ti oxide oxygen thermoba-rometer [23]. Although based on only one ilmeniteanalysis, our estimate agrees well with the T^fO2

estimate deduced from the Fe^Ti oxide assem-blages in Nakhla [15].

The mesostasis is dominated by feldspar (Table4) containing variable amounts of Fe (up to 10%Fe2O3). Crystal compositions vary betweenAb74An13Or14 and Ab69. An17Or15. Zoning hasbeen detected for one crystal from Ab70 in thecore to Ab55 in the rim. Raman spectrum of thisalbitic feldspar is given in Fig. 6. Iron-richK-feldspars up to 13% Fe2O3 in sanidine havebeen reported in terrestrial lava, mainly lamp-roites [24]. Such a Fe-rich alkali feldspar has notbeen described in the other nakhlites. Extensivesolid solutions between KAlSi3O8 and KFeSi3O8

were established experimentally [25]. Further crys-tallographic study should be performed to betterconstrain the crystal structure of feldspars. Cris-

tobalite occurs as tiny crystals (10 Wm in size) inthe mesostasis in association with albitic feldspar(10^20 Wm in size). Its crystalline nature has beenidenti¢ed by Raman spectroscopy (Fig. 6).

The alteration phase (Table 5) is an iron-richhydroxysilicate with 13% H2O (as OH groups).Oxide components except SiO2, FeO, and MgOare below 1%. It is close to the compositionfound in Lafayette except for lower Al andCa abundances. Structural formulae calculatedon a nine oxygen basis, by reference to serpen-tine or septochlorite, indicate non-stoichiomet-ric compositions with excess Si (Sis 2). Prelimi-nary X-ray di¡raction indicates that it is apoorly crystallized phase. A more accurate de-termination is in progress (Gillet et al., in prep-aration). As in Lafayette, the SiO2/FeO ratiodepends on the adjacent phases: the alterationmineral is more Fe-rich in olivine (38% FeO;40% SiO2) and more silica-rich in the mesosta-sis (47% SiO2 ; 26% FeO).

Table 4EMP analyses of feldspar in NWA 817 mesostasis

Wt%SiO2 60.38 62.69 59.17 64.57 63.07 61.65TiO2 0.72 0.43 0.41 0.06 0.18 0.71Al2O3 17.97 18.82 16.87 20.30 18.97 18.43Cr2O3 0.00 0.00 0.00 0.01 0.00 0.00Fe2O3 7.20 5.35 9.89 2.76 4.29 6.44MnO 0.05 0.09 0.12 0.02 0.02 0.05MgO 0.60 0.25 0.78 0.27 0.48 0.45CaO 2.27 2.82 2.82 2.57 1.92 2.32Na2O 7.15 7.84 6.42 8.10 6.23 7.51K2O 2.44 2.11 2.08 2.26 4.80 2.26P2O5 0.31 0.30 0.32 0.01 0.09 0.27NiO 0.00 0.01 0.02 0.00 0.01 0.00Total 99.08 100.70 98.90 100.92 100.05 100.08Si 2.772 2.811 2.745 2.858 2.853 2.791Ti 0.025 0.015 0.014 0.002 0.006 0.024Al 0.972 0.995 0.922 1.059 1.011 0.983Fe3 0.25 0.181 0.35 0.103 0.146 0.219Mn 0.002 0.003 0.005 0.001 0.001 0.002Mg 0.041 0.017 0.054 0.018 0.032 0.031Ca 0.113 0.136 0.140 0.122 0.093 0.113Na 0.636 0.682 0.577 0.695 0.55 0.659K 0.143 0.121 0.123 0.128 0.277 0.132P 0.012 0.011 0.013 0.000 0.003 0.010Total 4.96 4.97 4.94 4.98 4.97 4.96XAn 0.13 0.14 0.17 0.13 0.10 0.12XAb 0.71 0.73 0.69 0.74 0.60 0.73XOr 0.16 0.13 0.15 0.14 0.30 0.14

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4. Bulk rock composition

A 500 mg fragment was ¢nely ground using asapphire mortar and pestle. An aliquot was usedto determine major and trace element abundancesby inductively coupled plasma atomic emissionspectrometry (ICP-AES) and inductively coupledplasma mass spectrometry (ICP-MS), respectively.Another aliquot of the same powder was used forthe determination of additional trace elementabundances (As, Mo, Br, Sb, Au and Ag) by in-strumental neutron activation analysis (INAA).Our procedures are the same as those describedby [26,27]. Analytical results are reported in Table6. The accuracy of major and trace element anal-yses is better than 5% in most cases except for As(15%), Mo (20%), and Au (30%). Oxygen isotopeswere determined by laser £uorination: V1 mgaliquots were oxidized in a 100 mbar BrFs atmo-sphere in a CO2 Melles Griot laser beam. Theresulting oxygen was puri¢ed over a KBr furnaceand analyzed as O2 in a VG Optima dual inletmass spectrometer. Quartz and tholeiitic glass sec-ondary standards were routinely run in the BrF5

chamber together with the meteoritic samples.Because hot desert alteration is liable to signi¢-

Fig. 5. Zoning in cumulus olivine and augite. Zoning in oli-vine: olivine exhibits an homogeneous core zone of 160 Wmand a characteristic di¡usion length of 100 Wm. Zoning inaugite: augite shows strongly zoned rims with a steep transi-tion between core and rim. Note that the pro¢le is not sym-metrical, the left hand side corresponding to the crystal facewhich is oblique relative to the section plane. Zoning at theolivine^augite contact: pro¢les 1, 2 and 3.

Fig. 5 (continued).

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cantly a¡ect the composition of meteorites (e.g.[28^32]), the e¡ects of weathering are evaluated¢rst. No signi¢cant evidence of terrestrial altera-tion was detected except for two mobile elements:Pb and Ba concentrations are relatively high andwill not be further discussed. The lack of othernoticeable terrestrial e¡ects is illustrated by the

Br, U and Sr abundances which are sensitive in-dicators of surface processes. Previous determina-tions of halogens in nakhlites suggest that theseelements are indicative of terrestrial contamina-tion [33] ; Br concentrations range between 3.5and 8.4 ppm in Nakhla whereas NWA 817 con-tains only 1 ppm Br, an abundance relativelyclose to the level reported for Lafayette (0.6ppm). NWA 817 displays a normal Th/U ratioof 4.4 identical to the value obtained for Nakhla[34]. The Sr/Nd ratio of 16.1 is in the range of thevalues obtained for Nakhla and Governador Va-ladares [33,34]. Hence, our analysis can be used toaddress the nature of the parent body and thegenesis of NWA 817.

The major element concentrations of NWA 817are comparable to those of other nakhlites: highFe, Mg and Ca concentrations re£ect the abun-dance of cumulus augite and olivine. No notice-able di¡erence between NWA 817 and othernakhlites is observed for siderophile and chalco-phile element abundances such as Ni, Co, Cu andAu. Furthermore, key element ratios such asFeO*/MnO ( = 37), Na/Al ( = 0.40), K/La( = 449) or Ga/Al ( = 3.9U1034) con¢rm that itis a nakhlite with a¤nities to other Martian me-teorites. This conclusion is con¢rmed by the bulkrock analysis for oxygen isotopes, which yieldsN18O = +5.44x, N17O = +3.20x, and v17O =

Table 5EMP analyses of alteration minerals in nakhlites

Sample NWA 817 NWA 817 NWA 817 NWA 817 Lafayettea Nakhlab

(Wt%) in olivine in olivine in meso. in meso. clay `rust'

SiO2 41.26 43.06 45.05 46.73 40.2 40.2TiO2 0.01 0.03 0.05 0.00 ^ 0.02Al2O3 0.80 0.02 1.75 2.86 3.2 0.74Cr2O3 0.02 0.03 0.00 0.02 ^ 0.03FeO 38.36 34.19 31.32 26.14 32.1 34.10MnO 0.72 0.53 0.35 0.23 1.1 0.63MgO 5.96 5.99 6.32 8.22 10.8 6.82CaO 0.84 0.10 0.46 0.08 1.4 1.14Na2O 0.20 0.00 0.16 0.07 0.5 1.16K2O 0.61 0.32 0.67 0.29 0.4 0.60P2O5 0.00 0.00 0.03 0.00 ^ 0.06NiO 0.04 0.00 0.00 0.00 ^ 6 0.01Total 88.81 84.27 86.15 84.64 89.7 86.3a Bridges and Grady [18].b Average of 78 EDS analyses, Gooding et al. [17].

Fig. 6. Raman spectra of cristobalite and albitic feldspar ob-served in the mesostasis of NWA 817 and compared to refer-ence samples. Diamonds indicate traces of feldspar in thecristobalite spectrum. The reference spectrum has been re-corded on an ordered low-temperature albite; in NWA 817,feldspar is disordered leading to broader Raman bands.

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Table 6

NWA 817 (this work) Nakhla

ICP-AES/MS INAA Dreibus et al. [35] Nakamura et al. [33] Kong et al. [36]

Mass g 0.1069 0.052 0.158 0.048SiO2 wt% 49.33TiO2 wt% 0.61 0.35 0.37Al2O3 wt% 3.28 1.64 1.98FeO wt% 19.84 21.70 17.50MnO wt% 0.53 0.55 0.56MgO wt% 10.31 11.82 12.02CaO wt% 13.07 14.30 19.45Na2O wt% 0.94 0.57 0.55K2O wt% 0.32 0.17 0.078 0.16P2O5 wt% 0.10Li ppm 7.43 3.8Be ppm 0.44Sc ppm 47 55 64V ppm 181 192 251Cr ppm 1519 1710 2050Co ppm 49.0 54 43.9Ni ppm 71 90 77Cu ppm 12.7 6.7Zn ppm 71.52 220 92Ga ppm 6.77 8.82Rb ppm 6.06 3.15Sr ppm 145 51.4Y ppm 9.86Zr ppm 29.72Nb ppm 4.60Cs ppm 0.25 0.43Ba ppm 167 34 31.3La ppm 5.92 2.14 2.28 2.52Ce ppm 14.70 5.6 6.20Pr ppm 2.11Nd ppm 9.02 2.85 4.06Sm ppm 1.97 0.78 0.855 1.110Eu ppm 0.576 0.23 0.276 0.321Gd ppm 1.96 0.84 2.21Tb ppm 0.305 0.13 0.208Dy ppm 1.81 0.808Ho ppm 0.360 0.17Er ppm 0.953 0.441Yb ppm 0.817 0.4 0.397 0.51Lu ppm 0.121 0.062 0.0557 0.0848Hf ppm 0.78 0.29 0.34Ta ppm 0.248 0.09W ppm 0.45 0.176 0.15Pb ppm 1.9 0.509Th ppm 0.60 0.2357U ppm 0.136 0.0559As ppm 0.67 0.015Mo ppm 0.17 0.137Br ppm 0.97Sb ppm 0.025 0.006Au ppm 0.001 0.0029 0.00105Ag ppm 6 0.05

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+0.37x. These values are in agreement withthose measured on other nakhlites [1^3].

It has been emphasized above that NWA 817displays a higher proportion of mesostasis com-pared to other nakhlites. As a consequence, its Ti,Al, Na and K contents are about two times higherthan those of Nakhla (e.g. [35,36]). Similarly,NWA 817 displays the highest Th, U and rareearth element (REE) concentrations ever reportedfor a nakhlite. Nevertheless, the shape of its in-compatible element pattern (normalized to chon-drite) is similar to Nakhla (Fig. 7) and character-ized by a strong light REE enrichment (Lan/Ybn = 4.89). The slight negative Eu anomaly(Eu/Eu* = 0.90) displayed by NWA 817 is theonly noticeable di¡erence between the two pat-terns. Chlorapatite is a major carrier of REEs innakhlites, having a pronounced negative Euanomaly (Eu/Eu* = 0.2^0.3) [37]. We thereforesuggest that the Eu anomaly in NWA 817 is the¢ngerprint of phosphate, which occurs in traceamounts in the mesostasis.

The strong compositional similarity betweenNWA 817 and the other nakhlites is impressiveand suggests they share a similar (if not the same)parental melts.

5. Discussion

Petrographic data, bulk chemistry, and O-iso-topic compositions indicate that NWA 817 isMartian and can be classi¢ed as a nakhlite. Ourdatabase raises the following points that may shedsome new light on the petrology of the nakhlites.

The augite core composition (XMg = 0.61) inNWA 817 is the same as in all the other nakhlites.This `core' composition represents at least 90% ofthe volume of each grain (e.g. £at core zone inFig. 5 and BSE images of Fig. 2). When twoaugite crystals are in contact, no compositionalgradient is observed. One may therefore concludethat augite crystals were in contact before lateovergrowth. In NWA 817, where the mesostasisis more abundant compared with other nakhlites,augite crystals form clusters. The clusters oftenexhibit broken crystals at their margin (e.g. Fig.2b). Augite crystals are clearly xenocrysts.

The olivine core composition (XMg = 0.43) is ho-mogeneous for all large crystals (diameter s 200Wm). It is less magnesian than augite cores(XMg = 0.61). Therefore olivine cores are not inequilibrium with augite cores as observed in theother nakhlites. NWA 817 olivine cores are, how-ever, more Mg-rich compared to the other nakh-lites (0.37^0.39 in Nakhla and Governador Vala-dares, 0.31 in Lafayette).

Cumulus phase^mesostasis relationship : cumuluscrystals in contact with the mesostasis are zoned;augites show sharp growth zoning (about 10 Wmacross) whereas it is a well developed zoning inolivine (100 Wm across) as already observed inNakhla and Governador Valadares. As shownfrom BSE imaging (Fig. 2a,b), olivines seem tobe homogeneous when abutting augite crystals.Therefore, it results exclusively from an interac-tion with the Fe-rich melt that produced the mar-ginal overgrowths. Di¡usive subsolidus re-equili-bration between Mg-rich core and Fe-rich rimacts di¡erently in augite and olivine: it smoothedslightly the step-like pro¢les in augite margins,whereas it was more penetrative in olivine. Thisis due to Fe^Mg interdi¡usion rate in olivinewhich is ¢ve to six times faster compared to cli-nopyroxene [11,38]. The Fe^Mg interdi¡usion was

Fig. 7. Trace element patterns of NWA 817 and Nakhla [33].The reference chondrite is from Evensen et al. [45].

EPSL 6087 11-2-02


e¤cient enough in olivine to relax inwardly themarginal growth zoning over 100 Wm during latemagmatic to subsolidus cooling. However, thischaracteristic di¡usion length of 100 Wm is smallerthan the crystal radius of 250 Wm of the crystalcut through its center (see Fig. 5 where olivinecore composition is preserved over 160 Wm).Therefore, zoning in cumulus crystals, producedby overgrowth and di¡usional exchange, does nota¡ect the homogeneous core composition. Corecompositions of olivine and augite do representtwo di¡erent primary magmatic compositions.NWA 817 appears to be the least re-equilibratedof the nakhlites extending the trend seen fromLafayette (highly equilibrated) to Governador Va-ladares (partially equilibrated) to Nakhla (lessequilibrated).

The contact between olivine and augite is partic-ularly interesting (see Fig. 5, pro¢les 1^3): Far-thest from the mesostasis (pro¢le 1), augite is notzoned and its composition at the boundary isnearly identical (XMg = 0.59) to the average augitecore composition (XMg = 0.61). Olivine is quitedi¡erent as it is already zoned with anXMg = 0.27 at the contact and a pro¢le extendingover about 25 Wm inward. Two hypotheses can bemade: (i) olivine was already zoned when it abut-ted onto augite which, at that moment, had notstarted to develop any overgrowth. The olivinerim may then be interpreted as an overgrowthwhich started before augite overgrowth; (ii) oli-vine zoning at this contact is an analytical arti-fact : thinning like a wedge of this particular oli-vine towards the pyroxene contact would allowinteraction with intercumulus magma from below.

Other contacts in the more marginal zones aredi¡erent (Fig. 5, pro¢les 2 and 3). Olivine is stillzoned with progressively Fe-enriched composi-tions (XMg = 0.20 and 0.14 for pro¢les 2 and 3respectively). Augite is zoned and at the contactwith olivine exhibits the same XMg. This suggeststhat olivine and augite simultaneously developedtheir overgrowth rims.

From these preliminary observations an originfor the cumulus crystals may be discussed. Cumu-lus phases would have crystallized initially fromdi¡erent magmas. In a ¢rst stage augite crystalsgrew from a more ma¢c melt forming a cumulate

as indicated by their close association in clusters.In a second stage it was dismantled (broken crys-tals) by another melt which crystallized olivine(Fo43). After some time olivine (Fo43) and augite(XMg = 0.61) settled to form a new cumulate. Rap-id cooling started and rapid growth ensued. Oli-vine continued to grow in a closed system devel-oping a rim up to Fo27 ; pyroxene saturation wasreached and augite was subjected to rapid over-growth. During settling of the cumulus phase, in-tercumulus melt was trapped and reaction be-tween cumulus phases and intercumulus liquidensued. Whatever the origin of cumulus phases,rapid crystallization ended with a ¢nal stage ofserrate overgrowth armoring augite and olivine.

Late-magmatic solid-state di¡usion re-equili-brates substantially olivine margins. Although itmay be possible to use the olivine pro¢le to cal-culate cooling rates, the uncertainties attached todi¡usion data and initial conditions (T0 the tem-perature at which late magmatic cooling startedand initial compositional pro¢le) lead to great un-certainties [39]. For the sake of exercise, we usedthe geospeedometer of Jaoul and Sautter ([39],equation 19) and di¡usion data of Misener([40]: experiments performed between 900³ and1100³C and for a wide range of compositions(0.16XMg 6 0.8)). This geospeedometer, basedon `compensation law', greatly reduces uncer-tainty of the di¡usion parameter but still propa-gates error due to initial temperature. A di¡erenceof 100³C in T0 induces an uncertainty by a factor10 on cooling rate estimate: for a T0 of 1100³Cwe obtain a cooling of 0.11³C/day, for 1000³C weobtain 0.012³C/day. This would be a lower limitobtained while assuming an initial stepwise zoningin olivine.

Skeletal Ti-magnetite, the most striking featureof NWA 817 compared to other nakhlites, is com-mon in terrestrial hypabyssal basalt. Skeletal mor-phology is usually interpreted in terms of rapidcrystallization due to a high degree of undercool-ing [41]. It is important to note the late stage ofcrystallization of oxides and sul¢des never ob-served included in cumulus phases. Ti-magnetiterapid crystallization decreased locally Fe concen-tration of intercumulus liquid. As sulfur solubilityis tight to the melt FeO content [42], iron oxide

EPSL 6087 11-2-02


crystallization would trigger the ¢nal precipitationof immiscible sul¢de liquid.

Olivine and pyroxene overgrowths are systemati-cally more iron-rich than those seen in the othernakhlites (e.g. Fig. 4). This Fe enrichment, togeth-er with the high Fe3� content of feldspar from themesostasis, skeletal growth morphology and larg-er proportion of the mesostasis in NWA 817 in-dicate faster crystallization at the end of mag-matic cooling.

6. Conclusions

Like other nakhlites [8], NWA 817 is a cumu-late igneous rock, crystallized near the Martiansurface, and consisting of accumulated grains ofaugite and olivine coated with post-cumulus over-growths. The remaining liquid crystallized to a¢ne-grained mesostasis (Fe-rich feldspar, glass,clinopyroxene and olivine needles, skeletal Fe^Ti-oxides and sul¢de droplets) among the cumu-lus grains and post-cumulus overgrowth.

NWA 817 shows, however, somewhat distinctand unique features in its mesostasis. The cumu-lus crystals in NWA 817 are less compacted com-pared to Nakhla, Governador Valadares or La-fayette, and contain spectacular Ti-magnetitequench crystals. From analysis of late-magmaticzoning in cumulus minerals we conclude thatNWA 817 spent a shorter time near the solidustemperature compared to the other nakhlites. Thelarger proportion of mesostasis in NWA 817 com-pared to Nakhla, Governador Valadares and La-fayette may be related to faster quenching whichdid not allow maturation of the cumulate andmelt extraction by ripening of the textures.Quenching could also explain the greater Fe en-richment of cumulus crystal margins.

Noble gas measurements [43] give for NWA817 an average exposure age of 9.7 þ 1.1 Ma.This age agrees with the time of ejection fromMars of the other nakhlites [44], suggesting thatNWA 817 was ejected from the same collisionevent. Petrological and chemical similarities sug-gest possible slow crystallization in a commonmagma body before mesostasis formation. How-ever, di¡erences in mineral zoning, proportions of

mesostasis and quench oxide textures indicate thatNWA 817, once cumulus minerals settled, crystal-lized more rapidly than the other known nakh-lites. If rapid crystallization was due to rapidcooling, then NWA 817 could have been locatedcloser to the top of the cumulus pile. However,rapid crystallization may also be due to a highdegree of undercooling related for instance to asudden outgassing which signi¢cantly rose the sol-idus temperature.

Acknowledgements

Bruno Fectay and Carine Bidaut are thankedfor providing us with fragments of this meteorite.This research has made use of NASA's Astro-physics Data System Abstract Service and theMars Meteorite Compendium-1998 compiled byCharles Meyer. Thanks are due to CSEEM andCNRS FR 32 for ¢nancial support. Discussionwith G.G. Carlier improved our knowledge offeldspar composition. The present manuscriptbene¢tted from helpful comments of G. Crozaz,M. Macri and the two reviewers H.Y. McSweenJr. and R.P. Harvey.[AC]

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a new martian meteorite from morocco: the nakhlite north west africa 817

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