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Martian Meteorite Earth and Planetary Science Letters 203 (2002) 431^444 www.elsevier.com/locate/epsl Aqueous alteration in the Northwest Africa 817 (NWA 817) Martian meteorite Ph. Gillet a;Ã, J.A. Barrat b, E. Deloule c, M. Wadhwa d, A. Jambon e, V. Sautter f , B. Devouard g, D. Neuville h, K. Benzerara i, M. Lesourd j a Laboratoire des Sciences de la Terre (UMR CNRS 5570), Ecole Normale Supe¤rieure de Lyon, 46 alle¤e d’Italie, 69346 Lyon, France b Laboratoire de Ge¤odynamique et Plane¤tologie (UMR CNRS 6112) and Universite¤ d’Angers, 2 boulevard Lavoisier, 49045 Angers Cedex, France c Centre de Recherches Pe¤trologiques et Ge¤ochimiques (UPR CNRS 2300), P.O. Box 20, 54501 Vandoeuvre les NancyCedex, France d Department of Geology, The Field Museum, Chicago, IL 60605, USA e Laboratoire MAGIE, Universite¤ de Paris VI, Paris, France f Laboratoire de Mine¤ralogie, Muse¤um National d’Histoire Naturelle, Paris, France g Laboratoire Magmas et Volcans (UMR CNRS 6524), Universite¤ Blaise Pascal, Clermont Ferrand, France h De¤partement des Ge¤omate¤riaux, ESA CNRS 7046, IPG Paris, 4 place Jussieu, 75252 Paris, France i Laboratoire de Mine¤ralogie et Cristallographie, Universite¤ de Paris VI, Paris, France j CNRS et SCIAM, Laboratoire d’Histologie, UFR de Me¤decine, Universite¤ d’Angers, 1 rue Haute de Recule¤e, 49045 Angers Cedex, France Received 4 April 2002; accepted 11 July 2002 Abstract Samples of a new Martian meteorite of the nakhlite family (NWA 817) contain traces of an iron-rich alteration product. Textural arguments indicate that this alteration product has been formed on the parent body of the meteorite (Mars). The chemical composition and structural data (X-ray diffraction, transmission electron microscopy and vibrational spectroscopy) show that the alteration mineral is a hydrous phase from the smectite family. Major elements and rare earth elements suggest that the formation of the alteration phase is related to the circulation of an aqueous fluid which composition is controlled by the dissolution of feldspars to account for a positive Eu anomaly, olivine and possibly apatite. Hydrogen isotope data display negative ND values ranging from 360 to 3280x in olivine and pyroxenes and from 3140 to 3181x in the alteration phase. The values of ND for the alteration product show a small scatter with a mean value of 3170 þ 14x. These values are lower than those previously obtained on other Martian meteorites, which give mainly positive ND values. These positive values have been interpreted as resulting from the interaction of the Martian meteorites with water from the Martian atmosphere. Ruling out the effect of terrestrial alteration, it is suggested that alteration in the NWA 817 meteorite was likely produced on Mars by the circulation of an aqueous fluid originating from a chemical reservoir, such as the Martian mantle, which has not equilibrated with a fractionated Martian atmosphere. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Martian meteorites; nakhlite; alteration; D/H; geochemistry * Corresponding author. E-mail address: [email protected] (P. Gillet). 0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S0012-821X(02)00835-X EPSL 6354 24-9-02 432 P. Gillet et al. / Earth and PlanetaryScience Letters 203 (2002) 431^444 1. Introduction composed of ¢ne-grained feldspars, acicular pyroxenes, a silica phase (cristobalite), minute The presence in the past of large quantities of amounts of sul¢de droplets, and Ti-magnetite in water on the surface of Mars has been inferred a glassy matrix [7]. NWA 817, like the three other from the study of valley networks and other mor- known nakhlites, shows no obvious shock fea- phological features. The identi¢cation and charac- tures like melt pockets, high-pressure minerals terization of water reservoirs (internal and exter- or planar deformation features [8]. The NWA nal) on Mars can also be done indirectly through 817 meteorite is an unbrecciated, olivine-bearing petrographic and chemical studies of pre-terres- clinopyroxenite with a cumulate texture [7,8].Mo- trial alteration products in the SNC meteorites dal analyses indicate the following mineral pro- which are thought to be pieces of Mars delivered portions: 69 vol% pyroxene, 15% olivine, 15% to the Earth’s surface [1]. Several such studies mesostasis and 1% Fe^Ti oxides. Key element ra- have been performed so far on the three known tios such as FeO/MnO ( = 37), Na/Al ( = 0.40), K/ meteorites of the nakhlite subclass (Nakhla, La- La ( = 449), Ga/La ( = 3.9U1034) and oxygen iso- fayette and Governador Valadares). These mete- topic composition (v17O = +0.37x) are clear evi- orites represent shallow crustal rocks showing dence for including this meteorite in the SNC clear evidence of pre-terrestrial low-temperature group [7]. Rare gas studies also indicate a martian aqueous alteration [2^5]. The predominant alter- origin for this meteorite [9]. ation products of these meteorites are: a material loosely termed iddingsite that ¢lls fractures cross- cutting magmatic minerals and mesostasis, sider- 3. The alteration products: textural observations ite and evaporitic minerals including halite, gyp- sum and anhydrite. Hydrogen isotope systematics Alteration in NWA 817 is readily apparent in indicate that these rocks have been altered by a hand samples and in thin sections as reddish £uid that equilibrated with the fractionated Mar- patches or veins (Fig. 1A). The reddish alteration tian atmosphere [6]. A fourth nakhlite, Northwest product is clearly post-magmatic as it often cross- Africa 817 (NWA 817), has recently been discov- cut olivines, the mesostasis and, more rarely, py- ered in the Sahara [7]. roxenes. In the mesostasis it occurs as patches The present study provides the description and rather than as well-de¢ned veinlets (Fig. 1B). In interpretation of the alteration products observed olivines, it occurs mostly as veinlets with, in some in this meteorite. They have been characterized cases, irregular and saw-tooth margins very sim- with the following analytical techniques: optical ilar to those described in the Lafayette meteorite microscopy, scanning electron microscopy (SEM), [3,4] (Fig. 1C). The veinlets appear to be mostly electron microprobe, secondary ion mass spec- homogeneous, with the exception of some compo- trometry (SIMS), high-resolution transmission sitional zoning mostly near their margins (Fig. electron microscopy (TEM), X-ray di¡raction, 1D). These zoning features as well as the texture IR and Raman spectroscopy (see Appendix for suggest that the veins may be hydrothermal in the description of these techniques). origin, with multiple episodes of deposition of material. Unlike the ‘iddingsite-like’ alteration materials reported from Nakhla and Lafayette 2. NWA 817: a Martian meteorite [2], optical and SEM observations show no evi- dence for the presence of a complex polycrystal- NWA 817 is a 104 g stone found in the Moroc- line and polymineralic mixture in this alteration can desert. The mineralogy consists of euhedral product. No siderite or other carbonates were ob- subcalcic augite (Wo38^40, En38^27, Fs24^34) served in association with the orange material. olivine (extensively zoned from Fa-57 in the core The extraterrestrial origin of this alteration can to Fa-86 in the rim) with crystallized magmatic only be demonstrated indirectly since we were not inclusions and a multi-component mesostasis able to observe relationships between the fusion EPSL 6354 24-9-02 P. Gillet et al. / Earth and PlanetaryScience Letters 203 (2002) 431^444 433 crust and the alteration veinlets in the samples we with the X-ray data. Additional spectroscopic have used for making thin sections and polished data further con¢rm the mineralogical nature of sections. However, other arguments strongly sug- the alteration phase. Raman spectra exhibit broad gest that the orange material was formed during peaks at low wavenumbers (216 cm31, 278 cm31, pre-terrestrial alteration of the meteorite. A gen- 598 cm31 and 760 cm31), con¢rming its ill-orga- eral inspection of the main mass of the meteorite nized nature (Fig. 2C). The spectrum bears strong using a binocular microscope did not reveal or- similarities with those of smectites and kaolinites ange veinlets crosscutting the fusion crust or or- [13]. Infrared spectra show a very intense and ange weathering products at the surface of the large absorption band near 3600 cm31, suggestive fusion crust. The alteration product occurs in tex- of the presence of OH or H2O molecules (Fig. tural settings similar to those observed for the 2D). pre-terrestrial ‘iddingsite-like’ material in the three other nakhlites. Finally, textural evidence indi- cates that NWA 817 underwent only very mini- 5. The alteration product: chemical mal, if any, alteration in the terrestrial environ- characterization ment, thereby also lending support to a pre- terrestrial origin of the alteration of the magmatic The major element composition of the altera- minerals. Most meteorites recovered from the Sa- tion phase occurring in olivines and in the meso- haran desert have su¡ered di¡erent degrees of al- stasis is given in Table 1 and Fig. 3A. Fig. 3A teration. We have studied several of these mete- shows that the composition does not overlap orites, including three new shergottites [10^12]. with the compositional ¢elds de¢ned by the major Typically, terrestrial alteration is marked by the minerals. The low analytical totals (86^88 wt%) deposition of calcite from terrestrial £uids circu- and the absence of signi¢cant amounts of Cl, F lating in the cracks and fractures in these rocks. and C suggest the presence of water, which is Such calcite, however, is present in very minor consistent with the infrared data. The major ele- amounts in NWA 817. Additionally, the rare oc- ment composition, which is dominated by Si, Mg currences of sul¢des (which are known to be and Fe, resembles that of a ferromagnesian smec- highly susceptible to terrestrial alteration) in tite.
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