Exsolution and Shock Microstructures of Igneous Pyroxene Clasts in the Northwest Africa 7533 Martian Meteorite

Meteoritics & Planetary Science 51, Nr 5, 932–945 (2016) doi: 10.1111/maps.12637

Exsolution and shock microstructures of igneous pyroxene clasts in the Northwest Africa 7533 Martian meteorite

Hugues LEROUX1*, Damien JACOB1, Maya MARINOVA2, Roger H. HEWINS3,4, Brigitte ZANDA3,4,5, Sylvain PONT3, Jean-Pierre LORAND6, and Munir HUMAYUN7

1Unite Materiaux et Transformations, University of Lille & CNRS, F-59655 Villeneuve d’Ascq, France 2Institut Chevreul, University of Lille & CNRS, F-59655 Villeneuve d’Ascq, France 3Institut de Mineralogie, de Physique des Materiaux, et de Cosmochimie (IMPMC), Sorbonne Universite, Museum National d’Histoire Naturelle, UPMC Universite Paris 06, IRD & CNRS, 75005 Paris, France 4Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA 5Institut de Mecanique Celeste et de Calcul des Ephemerides, Observatoire de Paris, 77 Av. Denfert Rochereau, F-75014 Paris Cedex, France 6Laboratoire de Planetologie et Geodynamique, Universite de Nantes, 44322 Nantes, France 7Department of Earth, Ocean & Atmospheric Science and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA *Corresponding author. E-mail: [email protected] (Received 02 November 2015; revision accepted 26 February 2016)

Abstract–Northwest Africa (NWA) 7533 is a Martian regolith breccia. This meteorite (and its pairings) offers a good opportunity to study (near-) surface processes that occurred on early Mars. Here, we have conducted a transmission electron microscope study of medium- and coarse-grained (a few tens to hundreds of micrometers) Ca-rich pyroxene clasts in order to deﬁne their thermal and shock histories. The pyroxene grains have a high-temperature (magmatic) origin as revealed by the well-developed pigeonite–augite exsolution microstructure. Exsolution lamella characteristics (composition, thickness, and spacing) indicate a moderately slow cooling. Some of the pyroxene clasts display evidence for local decomposition into magnetite and silica at the submicron scale. This phase decomposition may have occurred at high temperature and occurred at high oxygen fugacity at least 2–3 log units above the QFM buffer, after the formation of the exsolution lamellae. This corresponds to oxidizing conditions well above typical Martian magmatic conditions. These oxidizing conditions seem to have prevailed early and throughout most of the history of NWA 7533. The shock microstructure consists of (100) mechanical twins which have accommodated plastic deformation. Other pyroxene shock indicators are absent. Compared with SNC meteorites that all suffered signiﬁcant shock metamorphism, NWA 7533 appears only mildly shocked. The twin microstructure is similar from one clast to another, suggesting that the impact which generated the (100) twins involved the compacted breccia and that the pyroxene clasts were unshocked when they were incorporated into the NWA 7533 breccia.

INTRODUCTION 7034, 7475, 7906, 7907, 8114, and 8171) are the first regolith breccias from Mars discovered so far (Agee Northwest Africa 7533 is a Martian regolith et al. 2013a; Humayun et al. 2013). The mineralogy and breccia. It consists of a fine-grained crystalline matrix, geochemistry are found to be in good agreement with fine-grained clast-laden impact melt rocks, and clasts the surface characteristics of Mars as seen by landers including medium- to coarse-grained igneous pyroxenes and orbiters (e.g., McSween et al. 1999, 2003; Gellert and feldspars. The meteorite and its pairings (NWA et al. 2004; Mustard et al. 2005; Poulet et al. 2005;

Bibring et al. 2006; Murchie et al. 2009; Schmidt et al. 2013, 2014; Stolper et al. 2013; Sautter et al. 2014; Cannon et al. 2015). The study in the laboratory of this new type of brecciated Martian meteorites could thus provide important insights about the early evolution of the near surface of Mars, including aqueous alteration, since these meteorites contain more water than other Martian meteorites (Agee et al. 2013a; Muttik et al. 2014). The meteorite contains zircons with a crystallization age of 4.4 Ga later disturbed 1.4– 1.7 Ga ago (Humayun et al. 2013; Yin et al. 2014) and chlorapatite reset at 1.3–1.4 Ga (Bellucci et al. 2015). A Sm-Nd age on whole rock and pyroxene separates from NWA 7034 yielded 4.39 8Ga (Nyquist et al. 2016) consistent with the ancient U-Pb zircon ages. Plagioclases yielded K-Ar ages of ~1.6 Ga (Cartwright et al. 2014), and a more varied set <2Ga (Lindsay et al. 2014), which together with a Rb-Sr age of ~2.1–2.7 Ga (Agee et al. 2013a; Nyquist et al. 2016) reﬂect incomplete resetting by the later 1.4 Ga event. These ages reveal an ancient and complex origin, which likely included a succession of events before the formation of the breccia (e.g., Wittmann et al. 2015). Oxygen isotopic compositions of zircons reveal a complex series of interactions between the Martian lithosphere and its atmosphere and hydrosphere that interacted with the clasts in the breccia (Nemchin et al. 2014). The medium- and coarse-grained lithic clasts in NWA 7533 are dominated by pyroxenes (orthopyroxene, pigeonite, and augite) and feldspars (plagioclase and K feldspar) formed by a magmatic process (Humayun et al. 2013; Santos et al. 2015; Wittmann et al. 2015; Hewins et al., unpublished data). These two classes of minerals have wide ranges of solid solution and miscibility gaps (e.g., Lindsley [1983] for pyroxenes and Fuhrman and Lindsley [1988] for feldspars). They are, therefore, valuable tools for deciphering the thermal histories of rocks. In the case where the pyroxenes are of igneous origin, subsolidus exsolution microstructure can also be used as a cooling rate speedometer (e.g., Grove [1982] for lunar samples; Weinbruch and Muller€ [1995] for chondrules; Muller€ [1993] for Martian meteorites). Pyroxenes are also good indicators of shock deformation with a wide variety of Fig. 1. SEM-BSE images of the lithic monomineralic pyroxene shock features, including dislocations, twinning, planar clasts from which the FIB sections have been extracted. For microcracks, and planar deformation features (e.g., all images, arrows and white lines indicate the location of the Leroux 2001). FIB sections, all perpendicular to the exsolution lamellae. a) Here, we present a study of the microstructure of Coarse-grained augite grain showing pigeonite exsolution some Ca-rich pyroxene clasts by transmission electron lamellae. b) Augite grains with exsolution lamellae. The smaller grain, on the right side, shows evidence for pyroxene microscopy (TEM). The objective of the study is to decomposition into Fe-oxide and silica. c) Pigeonite grain in detail the exsolution and shock microstructures in contact with feldspar. The grain contains some white spots, order to better constrain the cooling history of the suggesting the presence of small iron oxides in its interior. 934 H. Leroux et al. clasts and their degree of shock metamorphism. We also describe the decomposition of pyroxene into silica and magnetite discovered in some of the clast samples.

SAMPLES AND METHODS

A polished section of NWA 7533 (named 7533 LMLM) was first studied by scanning electron microscopy (SEM) at Museum National d’Histoire Naturelle (MNHN) using a Tescan VEGA II LSU SEM (mainly at 15 kV and a probe current <20 nA). Backscattered electron (BSE) maps were recorded to locate Ca-rich pyroxene showing exsolution lamellae. Their compositions were measured with an SD3 (Bruker) energy-dispersive spectrometer detector. Four pyroxene grains were selected for the TEM study (Fig. 1). They consist of three augites (bulk Fig. 2. Low-magnification bright-field TEM image of pigeonite exsolution (marked by arrows) within the augite compositions En41Wo39Fs20,En47Wo37Fs15, En Wo Fs , numbered Aug#1, Aug#2, and Aug#3, host. The exsolution lamellae lie on the (001) planes. They are 44 31 25 crossed by thin twins on the (100) planes. respectively) and one pigeonite (Pig#1; bulk composition En54Wo10Fs36). One electron transparent TEM sample was extracted from each of the four RESULTS pyroxene clasts by the focused ion beam (FIB) technique. The planes of the FIB sections were Exsolution lamellae occur on the (001) lattice plane, oriented perpendicular to the exsolution lamellae. The both for augite and pigeonite hosts (Fig. 2). Their FIB work was performed using an FEI Strata DB 235 widths and spacings vary from one sample to another, at IEMN, University of Lille, France. The FIB sections ranging from averages of 0.25 to 1.5 lm for the width were studied by TEM with an FEI Tecnai G2-20 Twin and from 1.8 to 6.1 lm for the spacing. The width and (LaB6, 200 kV) and a Philips CM30 (LaB6, 300 kV) at the spacing of the lamellae are closely related, with the the electron microscopy center of the University of lower spacing associated with thinner width. Table 1 Lille, France. Both instruments are equipped with an summarizes the characteristics of the lamellae for each EDS detector for elemental microanalysis. The sample (average compositions, widths, and spacings). microstructure was characterized by conventional SAED patterns were recorded on the lamellae and the bright and dark-field imaging. Crystallographic host (Fig. 3). Augite and pigeonite share common orientations and characteristics were obtained by crystallographic orientations (both have a monoclinic microdiffraction and selected area electron diffraction structure with close cell parameters). In augite, the (SAED). The chemical analyses by EDS (energy- absence of h + k odd reflections confirms the C-centered dispersive X-ray spectroscopy) were done in scanning lattice, C2/c space group. In pigeonite, the h + k odd TEM (STEM) mode, where the electron probe size reflections are present, showing that it belongs to the ranges from 5 to 10 nm. The spectra were acquired as P21/c space group. A difference of 3° is measured for points or window areas. The EDS spectra were the b angle, as well as a small difference for the a and c quantified by applying the k-factor correction with k- lattice parameters, with apig < aaug and cpig < caug factors experimentally determined for the major (Fig. 3). The lamellae occasionally contain stacking elements (O, Mg, Fe, Ca, and Al) with standard faults on (100) planes in the pigeonite lamellae (host specimens using the parameterless correction method of augite), in particular when the exsolution lamella is Van Cappellen (1990). For minor elements (Ti, Mn, thick (Fig. 4). An offset of the pigeonite/augite Cr), k-factors were calculated from the theoretical interfaces is frequently observed at the intersection of curve of the detector response to different X-ray the planar defects with the interface. Occasionally, the energies, adjusted through the calibrated values of the stacking faults in pigeonite extend into the augite host major elements. The absorption correction was applied for a few tens of nanometers. These (100) stacking assuming electroneutrality of the sample, using the faults formed to relieve the strain associated with the method proposed by Van Cappellen and Doukhan C2/c ? P21/c phase transition (see Livi and Veblen (1994) for ionic compounds. [1989] and Putnis [1992] for details). For the augite Pyroxene microstructure in NWA 7533 935

Table 1. Summary of the characteristics of the pyroxene clasts studied, including the bulk composition of the grains, the average composition of the exsolution lamellae and their hosts, the average thickness of the lamellae and the average spacing between the lamellae. The secondary lamellae present in samples Aug#1 and Pig#1 are not taken into account here in the statistics. Average Average composition Average composition Average lamella Average lamella Sample composition of the host of the lamellae thickness (lm) spacing (lm)

Aug#1 En41Wo39Fs20 En39Wo45Fs16 En52Wo3Fs45 0.31 2.1 Aug#2 En47Wo37Fs15 En45Wo43Fs12 En61Wo4Fs35 0.38 2.5 Aug#3 En44Wo31Fs25 En40Wo40Fs20 En54Wo4Fs42 1.5 6.1 Pig#1 En54Wo10Fs36 En55Wo5Fs40 En45Wo40Fs15 0.25 1.8

Fig. 3. Selected area electron diffraction (SAED) pattern including a pigeonite lamella and its surrounding augite host with the beam parallel to the b axis. The pattern shows the superimposition of the a* and c* directions of augite and pigeonite. Note the splitting of the a* direction by a Db angle of about 3°, the absence of h + k odd reﬂections in the a* direction of augite, and the difference of the lattice parameter between augite (A) and pigeonite (P). sample containing the coarser exsolution lamellae, a second generation of exsolution is observed, both in the primary pigeonite lamellae and in the augite host (Fig. 5a). For the augite host, the secondary lamellae Fig. 4. Bright-ﬁeld TEM micrographs taken with the electron are thin (50–100 nm thick) and appear as extended beam approximately parallel to the b axis. a) Stacking fault on needles along (001) of augite. The secondary lamellae in (100) planes in a thick pigeonite lamella. Note the presence of pigeonite lamellae are smaller, typically 50–100 nm in a precipitate at the pigeonite/augite interface (arrows). Point length and 10 nm in width (Fig. 5b). Their orientation analysis showed the precipitate is rich in Ti, Cr, and Fe. b) Stepped interface (arrows) between a pigeonite lamella and its is close to (001) but systematically deviate by an angle augite host, related to the (100) stacking faults. of about 3° from the primary coarse exsolution lamellae. They are also frequently curved at their termination. The pigeonite sample also displays a primary exsolution lamellae–host composition pairs are secondary augite lamella set (average spacing 0.15 lm) plotted in the pyroxene quadrilateral on Fig. 6. parallel to the primary lamellae. Logically, there should be a relationship between the The primary lamellae are large enough to measure thickness of the lamellae and the closure temperature, their compositions. Representative compositions are with the coarser exsolution lamellae corresponding to given in Table 2 for each of the pyroxene grains. The lower closure temperatures. However, this tendency 936 H. Leroux et al.

dislocations, clearly in glide configuration, attest to the mechanical origin of the twins. They are visible owing to a strain field contrast surrounding them when the twin boundaries are oriented edge on with respect to the electron beam (Figs. 8a and 8b). When inclined with respect to the electron beam, dislocations appear as straight lines lying in the twin walls, which display a fringe contrast (Fig. 8c). The width of the twins is clearly correlated to the number of partial dislocations within the walls. At the twins’ tips (Fig. 8b), the density of dislocations is higher. From these twin endings, we measured the increasing thickness of the twin as a function of the number of dislocations. We then calculated an increase of the twin width of 4.5 A per unit dislocation. This corresponds to half of the interplanar spacing of the (100) twin planes and confirms the partial nature of the dislocations, as also suggested by the contrast fringes within the walls (Fig. 8c). Image contrast analysis using several diffraction vectors suggests that the dislocations have a Burgers vector parallel to c. Figure 8d shows an SAED pattern taken on an area including a wide twin and its surrounding host. The (100) twin character of the lamella is confirmed by the presence of extra spots located at mirror positions along (100) with respect to the host spot positions. Most of the twins are continuous across the exsolution lamellae. When crossing a lamella, a shearing displacement is clearly visible for the exsolution domain with an amplitude Fig. 5. Bright-field TEM image: (a) Thick pigeonite primary proportional to the width of the twin lamella (Fig. 9). lamella and thin secondary lamellae (arrows) on (001) planes in augite. Note the presence of (100) twins crossing the We measured an associated shift amplitude as half of exsolution lamellae and an association of Fe-oxide and silica the twin thickness (Fig. 9b). This is fully compatible connected to the primary lamella. b) Magnified view of the with the (100) twin law and the mechanical character of primary pigeonite lamella showing a second generation of these twins. Finally, free dislocations are rare. Only a augite lamellae which consist of short needles frequently few of them have been detected, all having a c Burgers slightly curved at their edges. vector, in glide configuration within the (100) plane. During the SEM survey, we detected a few does not seem respected and may reflect different pyroxene clasts, both Ca-poor and Ca-rich, containing a thermal histories of the pyroxene clasts. The exsolution micron-sized association of Fe-oxide and silica internal lamellae in augite have a low Wo content. SAED patches (Fig. 10). There are two distinct occurrences of patterns showed that they consist of pigeonite (Fig. 3), this association. In some exsolved grains, Fe-oxide and despite their low Wo content. The close contact with silica inclusions decorate the exsolution lamellae of the C2/c augite host could have retained P21/c pigeonite pigeonite—such inclusions may protrude into the Ca- and prevented the conversion to orthopyroxene. It rich pyroxene host too (Fig. 10b). In other grains, they cannot be excluded that some of them have converted are randomly distributed along curve lines within the into orthopyroxene. Several compositional profiles were grain (Figs. 10c and 10d). One FIB section of each recorded across the lamellae and the surrounding host. occurrence has been extracted for TEM investigation Figure 7 shows one of these profiles taken in the (Aug#3 and Pig#1). Figure 11a shows a bright-field coarser lamellae. No composition gradient was detected. STEM image of an irregular array of Fe-oxide and The other dominant characteristic of the sample is silica inclusions within the Ca-poor pyroxene grain the presence of abundant twins on the (100) planes. (Pig#1). Figure 11b shows a dark-field STEM image of Their average spacing is close to 500 nm (Fig. 2). Their inclusions connected to pigeonite exsolution lamellae in widths are variable, ranging from a few nm to 120 nm. the augite grain (Aug#3). Figure 12 shows high- Partial dislocations are present on the twin walls. These magnification TEM images of the Fe-oxide + silica Pyroxene microstructure in NWA 7533 937

Table 2. Representative energy-dispersive spectroscopy (EDS) compositions extracted from the four studied pyroxene clasts, for the exsolution lamellae and their hosts. Data are given in oxide weight %, here normalized to 100%. The cation distribution is also given based on six oxygens. Absorption and k-factor corrections have been applied. Uncertainties on the values are typically 3% for the major elements (Mg, Si, and Ca) and 10% on the minor elements (Al, Ca, Ti, Cr, Mn, and Fe). Aug#1 Aug#1 Aug#2 Aug#2 Aug#3 Aug#3 Pig#1 Pig#1 Sample Host Lamella Host Lamella Host Lamella Host Lamella

SiO2 50.3 51.3 53.5 51.1 53.4 52.9 52.5 53.8 MgO 14.0 17.5 15.5 23.2 13.5 18.7 19.8 16.7 CaO 21.0 1.19 20.5 1.94 18.6 1.58 3.00 18.1 Al2O3 2.60 0.51 1.40 0.50 0.95 0.52 0.34 2.83 TiO2 0.23 bd 0.22 bd 0.28 bd bd bd Cr2O3 0.60 bd 0.31 bd 0.33 bd bd bd MnO 0.19 0.89 0.38 0.44 0.62 0.53 0.37 bd FeO 11.0 28.6 8.17 22.9 12.3 25.7 23.9 8.58 Formulae on the basis of six oxygens Si 1.88 1.97 1.98 1.89 2.01 2.01 1.98 1.97 Mg 0.79 1.01 0.86 1.28 0.76 1.07 1.12 0.92 Ca 0.84 0.05 0.81 0.08 0.75 0.06 0.12 0.71 Al 0.11 0.02 0.06 0.02 0.04 0.02 0.01 0.12 Ti 0.01 bd 0.01 bd 0.01 bd bd bd Cr 0.02 bd 0.01 bd 0.01 bd bd bd Mn 0.01 0.03 0.01 0.01 0.02 0.02 0.01 bd Fe 0.34 0.91 0.25 0.71 0.39 0.81 0.75 0.26 Wo 43 2 42 4 39 3 6 38 bd = below detection.

Fig. 6. Pyroxene quadrilateral diagram showing the augite– pigeonite compositional pairs in the four studied samples, as measured by analytical TEM. The diagram also shows the projection of the solvus isotherms of the two-phase field (after Lindsley 1983). inclusions. The silica is found to be amorphous in most of the occurrences and contains only a small amount of Fig. 7. Composition profile across the exsolution lamellae in other elements with concentrations well below 1 at%, the sample showing the coarser pigeonite lamellae in the augite host (see Fig. 5a). The secondary exsolution lamellae probably coming from the surrounding matrix by are clearly identifiable, but because of their small thicknesses, fluorescence or slight electron beam overlap. It is not their composition cannot be measured easily due to beam excluded that the amorphous state of the silica could be overlap with the augite host. due to electron beam amorphization during observation. In some occurrences, silica has been found to be crystalline, containing many planar defects (Fig. 12c). The Fe-oxide grains appear as single phases embedded Corresponding SAED patterns are indexed as a- in silica. They frequently show facets (Figs. 12a and cristobalite (Fig. 12d). The planar defects in cristobalite 12b). Microdiffraction patterns have been acquired on are found along (101) planes and correspond to twins. Fe-oxide grains. The patterns exhibit characteristic 938 H. Leroux et al.

Fig. 8. Bright-field TEM images (a) twins on (100) planes (dark lamellae) with the twin boundaries parallel to the electron beam. Note the large difference in thickness of the twins and the presence of strain contrast at the twin walls which propagates into the surrounding host. b) Thin twin which terminates as a needle (marked by an arrow). Its thickness is closely related to the number of partial dislocations present in the twin boundaries (here visible owing to the strain contrast they generate into the surrounding host). c) Partial dislocations in the twin walls, here inclined with Fig. 9. Plastic deformation associated with the (100) twins (a) respect to the electron beam. Note their regular spacing and TEM bright-field image showing the offsetting of a pigeonite the fringes which illustrate the stacking lattice defects at the exsolution lamella sheared by the (100) twins. b) Shear twin boundaries. d) Selected area electron diffraction pattern amplitude of exsolution lamellae as a function of the twin of a thick twin lamella and its surrounding host (zone axis thicknesses. The strong correlation attests to the mechanical ½011 ). origin of the twins, with shift amplitude of the exsolution approximately equal to the half of the twin thickness. distances and reflection extinctions, which are compatible with magnetite and exclude maghemite and 7533 pyroxene clasts shows that these clasts were hematite (Fig. 13). Composition profiles have been originally magmatic material and were later embedded recorded in pyroxene around the magnetite + silica in the brecciated rock. In magmatic rocks, exsolution association. No composition gradient has been found. lamellae are formed by subsolidus transformation occurring during cooling from high temperature when DISCUSSION the pyroxene enters the miscibility gap between augite and pigeonite. Coarsening of exsolution lamellae is The TEM investigation of pyroxene clasts from controlled by diffusion and, therefore, by time and NWA 7533 shows that the four grains studied have temperature. The width and spacing of exsolution similar characteristics, regarding both the exsolution lamellae are consequently related to the cooling rate. and shock microstructures. We have also highlighted The approach employed for estimating cooling rates is evidence for pyroxene decomposition, which is much based on a comparison with laboratory experiments less common in extraterrestrial samples and implies performed on compositional analogs. However, oxidation of the pyroxene. estimating the cooling rate from the observations reported here is difficult. Experimentally, the exsolution Thermal History—Exsolution Microstructure growth was mainly studied using pyroxene phenocrysts and continuous cooling rates (Grove 1982) or iron-free The formation of exsolution lamellae in calcic pyroxene samples under isothermal conditions, for pyroxenes is a well-known process, which has already periods of time from hours to months (McCallister been intensively described in the literature (e.g., Buseck 1978; Weinbruch et al. 2003, 2006). In NWA 7533, the et al. 1980; Putnis 1992). Their presence in the NWA width and spacing of the exsolution lamellae are Pyroxene microstructure in NWA 7533 939

Fig. 10. SEM-BSE images: (a) Low Ca-pyroxene containing augite exsolution lamellae decorated with submicron-sized Fe-oxide (bright dots) and silica grains (dark dots). b) Magnified view of the decorated lamellae. c) Low Ca-pyroxene with arrays of Fe- oxide and silica along some random directions. d) Magnification of the grain shown in (c). The association Fe-oxide + silica is not correlated with the augite exsolution (vertical darker gray bands on the image). typically one order higher than those synthesized in the different parent magmatic rocks and did not all cool at laboratory experiments, corresponding to cooling rates the same depth. lower by several orders of magnitude. Moreover, unlike most samples used for laboratory experiments, the Mechanical History—Shock Microstructure NWA 7533 pyroxenes are iron-rich. All of this makes it difficult to estimate the cooling rates of the pyroxenes Pyroxenes are good markers of shock from the clasts. Compared with the pyroxenes from the metamorphism. Shock defects include dislocations, SNC meteorites, pyroxenes of NWA 7533 show an mechanical twinning, and planar deformation features exsolution microstructure somewhat more developed. (e.g., Hornemann and Muller€ 1971; Leroux et al. 1994; We found an average distance between the exsolution Langenhorst et al. 1995). In pyroxenes from NWA lamellae in the range 2–6 lm in our selected clasts, 7533, the dominant defects are mechanical twins lying whereas in pyroxenes from the SNC, the average in the (100) planes. The mechanical nature of these distance is often positioned between 0.5 and 1 lm twins is clearly shown by the shear of the exsolution (Brearley 1991; Muller€ 1993; Mikouchi and Miyamoto lamellae at the intersection with the twins. The shear is 1998; Mikouchi et al. 1999; Leroux et al. 2004; parallel to the composition plane of the twins, and its Monkawa et al. 2004). It could suggest that the NWA amplitude is closely related to the thickness of these 7533 pyroxenes cooled slower thus were excavated from twins. From the shear effect of the exsolution lamellae, deeper crustal levels, likely by Noachian large impacts. the strain can be estimated. Its magnitude varies from However, it is important to note here that this one sample to another, with a maximum value of 8%. difference may be due to a sampling bias. Indeed, for The observations also revealed the presence of NWA 7533, we have intentionally selected clasts into numerous dislocations regularly spaced in the twin which the exsolution microstructure was well developed, boundaries. Counting the dislocations in the twin walls visible at the SEM scale. It is quite possible that other together with the associated width variation of the twins pyroxene clasts can have a finer microstructure shows that each dislocation corresponds to a variation exsolution or none. Anyway, as in the case of the SNC, in thickness of 4.5 A. This is compatible with the the NWA 7533 exsolution lamellae in pyroxenes display shearing of the (100) octahedral layers by partial a large range of characteristics (width and spacing of dislocations with Burgers vectors 1/2 c, as described lamellae) showing that the clasts may originate from earlier in detail by Kirby and Christie (1977). The twin 940 H. Leroux et al.

shocked (e.g., Ashworth 1980, 1985). They are also present in clinopyroxene in the laboratory deformed statically (Kirby and Christie 1977) or tectonically (Skrotzki 1995). The low shock intensity of NWA 7533 is also demonstrated by the absence of transformation of feldspars into maskelynite and any other shock feature except planar features in pyrite, a very brittle mineral (Lorand et al. 2015). It is compatible with the low shock intensity deduced from the paired NWA 7475 (Wittmann et al. 2015). The relatively low shock intensity of NWA 7533 strongly contrasts with the SNC meteorites which all display stronger shock levels (e.g., El Goresy et al. 2013). It is remarkable to note that the deformation microstructure is relatively comparable from one pyroxene clast to another. It is reasonable to conclude that the (100) twins formed during the same event, i.e., a mild shock deformation involving the brecciated rock. It also means that the clasts were unshocked prior to being incorporated in the breccia.

Pyroxene Decomposition into Magnetite and Silica

Some of the clasts display evidence of pyroxene decomposition into magnetite and silica. Two different configurations have been identified. In some cases, the pyroxene decomposition took place within the pigeonite lamellae exsolved in augite, showing that it occurred after their formation. In other cases, the magnetite + silica association was found inhomogeneously distributed along random directions within the grain, likely in healed fracture systems Fig. 11. STEM images of the Fe-oxide and silica association cutting across the exsolution lamellae, and thus clearly within pyroxene grains. a) Low Ca-pyroxene containing an postdating the exsolution event. The fact that the array of inclusions oriented along a random direction (bright field). The Fe-oxides are in dark gray and the silica in light decomposition is only observed in some specific gray. b) Inclusions of Fe-oxide + silica connected to the pyroxenes (most of them do not show any evidence for pigeonite lamellae in an augite grain (dark field). The Fe- this decomposition) argues in favor of an event that oxides are the bright regions and the silica is in the dark gray occurred before they were incorporated in the NWA regions. 7533 breccia as clasts. The presence of magnetite is the result of an characteristics deduced from the TEM investigations are oxidizing event, involving a change in valence state of in agreement with the twin law and the associated glide iron from Fe2+ (in pyroxene) to Fe3+ (in magnetite). elements described by Raleigh and Talbot (1967) and The phase assemblage pyroxene + magnetite + silica Kirby and Christie (1977). suggests that the pyroxene was brought out of its Mechanical twinning in the (100) planes in NWA stability field at high oxygen fugacity (fO2) according to 7533 pyroxene is likely due to a low-intensity shock the following equilibrium reaction: event as indicated by the moderate twin density and the absence of other shock defects, such as free dislocations, 3FeSiO3ðin pyroxeneÞþ1=2O2 ¼ Fe3O4 þ 3SiO2 (1) mechanical twinning on (001), mosaicism, planar deformation features, or microfractures. Twins in (100) Another possibility to explain the magnetite + silica are commonly found in clinopyroxene deformed by association would be the incorporation of hematite or shock as found in meteorites (Hornemann and Muller€ maghemite from the matrix along fractures in pyroxene 1971; Muller€ 1993; Langenhorst et al. 1995; Leroux followed by a heating event. In this case, the equation is et al. 2004), including those that have been mildly as follows: Pyroxene microstructure in NWA 7533 941

Fig. 12. Magnetite + silica association (a and b) TEM bright-ﬁeld images showing details of the association magnetite + silica. Note that some of the magnetite grains are facetted. c) Dark-ﬁeld TEM image showing a silica crystalline inclusion containing planar defects (twins). The shape of the inclusion is underlined by a dashed line and the magnetite grain by a continuous line. d) SAED pattern recorded on the SiO2 crystalline inclusion, indexed as a-cristobalite. The pattern reveals that cristobalite is highly twinned on ð101Þ (zone axis [010]).

€ FeSiO3ðin pyroxeneÞþFe2O3 ¼ Fe3O4 þ SiO2 (2) the Fe-Mg interdiffusion coefficient described by Muller et al. (2013) for clinopyroxenes. This curve shows that According to these reactions, the volume ratios for relatively short times (not related to a metamorphic silica/magnetite are 1.75 for (1) and 0.58 for (2). event), the transformation necessarily operated at high From the STEM images, we measured the amount of temperatures, probably above 1000 °C. A formation at SiO2 compared to the one of magnetite in samples high temperature is also confirmed by the presence of affected by this decomposition (Aug#3 and Pig#1). We cristobalite (high temperature polymorph of SiO2) found a volume ratio silica/magnetite equal to 1.80 for associated with magnetite. The equilibrium stability field sample Aug#3 and equal to 1.74 for sample Pig#1, of cristobalite at 1 bar extends from 1470 to 1730 °C. suggesting that reaction (1) is more likely. This reaction This temperature range is above the liquidus requires an interdiffusion of Fe and Mg(Ca), the two temperature of augite, which is close to 1350 °C for the latter diffusing back into the pyroxene host. However, composition of the studied pyroxene grains (e.g., no concentration profile has been found in the pyroxene Huebner and Turnock 1980). However, it appears in contact with silica and magnetite. This means that clearly that the pyroxene decomposition into magnetite the elements have diffused over relatively large and silica occurred at subsolidus conditions. Cristobalite distances, thus at high temperature, making an eventual likely formed at lower temperature, under metastable concentration gradient undetectable. Considering that conditions, as frequently observed in the case of the average distance between silica + magnetite presence of impurities (e.g., Leroux and Cordier [2006] precipitates is about 1 lm, we calculated the time- and references therein). temperature couples (T-t) needed to erase a possible A lower limit for the redox conditions is estimated gradient (Fig. 14). This corresponds to a distance from the ferrosilite–magnetite–quartz buffer, which approximately equal to half the diffusion distance corresponds to fO2 at least 2–3 log units above the between these precipitates (0.5 lm). The couplespffiffiffiffiffiffi T-t fayalite–magnetite–quartz buffer (FMQ) (Lindsley et al. have been calculated using the equation x2 ¼ 2 Dt and 1990; Frost and Lindsley 1991, 1992; Lindsley and 942 H. Leroux et al.

Fig. 14. Time–temperature couple required to erase a concentration gradient in pyroxene around the silica + magnetite precipitates.

Another possibility is to consider a deuteric cooling < > Fig. 13. 110 zone axis microdiffraction pattern acquired on of oxidized magmatic rocks, thus an event predating the an Fe-oxide grain. The pattern exhibits clear extinction of the 110 type reflections. These extinctions are compatible with the incorporation of the pyroxene grains in a regolith. magnetite face-centered Bravais mode (h, k, l all odd or even) Indeed, the trails of magnetite + silica inclusions closely and not with the maghemite primitive Bravais mode. Other resemble fluid inclusion trails (see for instance Figs. 10c symmetry elements of the maghemite space group cannot and 10d). Deuteric cooling of magmatic rocks on Earth explain 110 extinctions (space group P4132; reflection = are well known to produce highly oxidizing fluids far conditions for general positions: for h00: h 4n, with h, k, l – permutable). beyond the magnetite hematite synthetic buffer assemblage at subsolidus temperature (e.g., Sato and Frost 1992). This fO2 value is well above the one Wright 1966; Haggerty 1976). If water is quickly deduced from studies for Martian basalts for which the degassed, hydrous silicates do not form. In NWA 7533 FMQ values range from 3to1 (e.g., Herd et al. and pairings, it is known that igneous-textured clasts 2002). In NWA 7533 and paired meteorites, there are represented in the different generations of lithic clasts several lines of evidence for oxidizing postmagmatic show evidence of formation under strongly oxidizing conditions. The oldest zircon dated at 4.4 Ga shows conditions (cf Ce anomaly in zircon, Nemchin et al. REE evidence of crystallization in a high fO2 setting 2014; magnetite–ilmenite assemblages, Agee et al. (FMQ + 1 to FMQ + 2 log units; Nemchin et al. 2014). [2013b] and Hewins et al. [unpublished data], for The same is also true for ilmenite-Ti magnetite instance). It, thus, appears that the oxidizing conditions composite clasts that give a similar range of fO2 (Agee seem to have prevailed very early and throughout most et al. 2013b; Santos et al. 2015; Hewins et al., of the history of NWA 7533. The data we report here unpublished data). The breccia also contains, mostly in indicate that some of the pyroxene clasts were exposed its fine-grained matrix, a high abundance of Fe-oxides to similarly oxidizing conditions. with substantial ferric iron (Agee et al. 2013a, 2013b; Gattacceca et al. 2014; Muttik et al. 2014). Alteration CONCLUSION products, such as ferric iron oxides, phyllosilicates, hydrogen peroxide, or chlorate-perchlorate, can be The TEM study of several Ca-rich pyroxene clasts candidates for explaining the source of oxygen allows us to elaborate some elements of the history of responsible for the pyroxene breakdown. It is to be noted the breccia and its components, a sequence of cooling that this breakdown likely occurred at high temperature from igneous temperatures, subsequent subsolidus (as discussed above), is only observed in some pyroxenes, oxidation for some of the pyroxenes, and finally a mild and is not limited to the margins of the pyroxene grains. shock event. First, the pyroxene crystal clasts likely These observations do not favor an oxidation event originated from igneous rocks for which the within the NWA 7533 regolith breccia (but could have characteristics (cooling rates) do not significantly differ occurred in a previous regolith generation). from the SNC meteorites. Second, some of the pyroxene Pyroxene microstructure in NWA 7533 943

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Martian low-albedo regions from the reﬂectance spectrum This late event could be responsible for the launch of of Northwest Africa 7034. Icarus 252:150–153. the rock from Mars. Cartwright J. A., Ott U., Herrmann S., and Agee C. B. 2014. Modern atmospheric signatures in 4.4 Ga Martian meteorite NWA 7034. Earth and Planetary Science Letters Acknowledgments—The authors thank T. Mikouchi and 400:77–87. F. McCubbin for detailed reviews and suggestions that El Goresy A., Gillet P., Miyahara M., Ohtani E., Ozawa S., helped improve the article. The authors thank J. F. Beck P., and Montagnac G. 2013. Shock-induced Dhenin and A. Addad for their assistance at the Lille deformation of shergottites: Shock-pressures and perturbations of magmatic ages on Mars. Geochimica et (France) electron microscopy facility. They thank D. Cosmochimica Acta 101:233–262. Troadec for the preparation of high-quality FIB Frost B. R. and Lindsley D. H. 1991. 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