Multiple impact events recorded in the NWA 7298 H chondrite breccia and the dynamical evolution of an ordinary chondrite asteroid

Earth and Planetary Science Letters 394 (2014) 13–19

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Multiple impact events recorded in the NWA 7298 H chondrite breccia and the dynamical evolution of an ordinary chondrite asteroid Jon M. Friedrich a,b,∗ , Michael K. Weisberg b,c , Mark L. Rivers d a

Department of Chemistry, Fordham University, Bronx, NY 10458, USA Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA c Department of Physical Sciences, Kingsborough College and Graduate School of the City University of New York, Brooklyn, NY 11235, USA d Consortium for Advanced Radiation Sources, University of Chicago, Argonne, IL 60439, USA b

a r t i c l e

i n f o

Article history: Received 18 December 2013 Received in revised form 7 March 2014 Accepted 10 March 2014 Available online xxxx Editor: C. Sotin Keywords: ordinary chondrite impact shock foliation petrofabric regolith

a b s t r a c t The major geologic process that has shaped the asteroids and led to development of their regoliths is impact. Petrofabrics in ordinary chondrites are undoubtedly the result of impact events on their asteroidal parent bodies and the foliation present in a chondrite serves as an enduring record of the magnitude of the most intense compacting event experienced by the material. An overwhelming majority of chondrites have an internally consistent petrofabric contained within the spatial dimensions of the entire rock, including across clasts or different petrographic domains. This indicates that the magnitude of the most recent impact to have affected the assembled chondrite was significant enough to impart a foliation across all lithologies. Information of any previous impacts is largely lost because of the consistent, realigned foliations. We present X-ray microtomography derived 3D petrofabric intensity and orientation data for three lithologies in the NWA 7298 breccia. The internally inconsistent petrofabrics among differing lithologies indicate that the magnitude of the final impact event was smaller than previous ones. This latter case preserves fabric information recorded during previous impacts and allows a more complete interpretation of the impact history of a local region of the asteroidal parent. We used our data to infer the sequence and intensity of distinct impact events affecting the NWA 7298 parent asteroid. We suggest a near-surface impact debris zone on the H chondrite parent asteroid as an origin for NWA 7298. These observations yield new opportunities for investigating and interpreting the dynamic collisional evolution of asteroids. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Asteroids are vital for understanding the evolution of the Earth and the other inner planets. The major geologic process that has shaped the asteroids and led to development of their regoliths is impact. Thus, crucial to understanding the evolution of asteroids and their regoliths is unraveling the collisional histories recorded in the meteorites derived from these bodies (e.g., Scott et al., 1989). However, asteroids have had complex, multiple impact histories and this strongly hinders the interpretation of the shock effects present in their meteorite offspring. Impact-related shock deformation of meteorites have proven to be “notoriously difficult to interpret” (Scott, 2002) because of the heterogeneous nature of shock propagation and collective effects of multiple impacts

*

Corresponding author at: Department of Chemistry, Fordham University, Bronx, NY 10458, USA. Tel: +1 718 817 4446; fax: +1 718 817 4432. E-mail address: [email protected] (J.M. Friedrich). http://dx.doi.org/10.1016/j.epsl.2014.03.016 0012-821X/© 2014 Elsevier B.V. All rights reserved.

on the rocks. To study the inherently cumulative effects of shock in ordinary chondrites in particular, studies have used rock and mineral compositions and textures (Dodd and Jarosewich, 1979; Stöffler et al., 1991; Rubin, 1992, 2002, 2003, 2004; Friedrich et al., 2008a, 2008b; Sasso et al., 2009), extent of brecciation (Bischoff et al., 2006), 40 Ar–39 Ar radiometric ages (Bogard 1995, 2011; Swindle et al., 2013), and cosmogenic chronometers (e.g. Meier et al., 2012) to interpret the collisional history of meteorites and their parent asteroids. Each of these has strengths, weaknesses, and broad uncertainties associated with them when the effects of multiple impacts are concerned. For example, highly porous post accretionary rocks with little fracturing or deformation apparent in the minerals were nevertheless shocked (Friedrich et al., 2013) and were subsequently compacted and shocked to a greater degree to make the well compacted, lower porosity chondrites (Consolmagno et al., 2008) that largely make up the chondrite record in our possession. An impact may have been energetic enough to have imparted enough energy to bring previously shocked target material to the point of melting in the case of impact melt rocks. Petrographic

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indicators of the more mild event(s) have been overwritten because of the significant degree of compaction, fracturing, and possibly thermal annealing imparted by the more recent shock event (e.g. Rubin, 2004; Kring et al., 1999). 40 Ar–39 Ar ages can be used to recognize the existence of multiple shock (heating) episodes in a chondrite, but inferences about the relative intensity of each event can be complicated by the varying thermal intensity and duration of each. Brecciation as an indicator is truly only able to indicate a minimum of two impact events on an asteroid: an event to initially break up a rock and a second to compact and reconsolidate. Little is known about the relative intensity of each of possible additional impacts (if any). With few exceptions (e.g. Huss, 1980; Kring et al., 1996), from a textural perspective, the ordinary chondrite record consists of consolidated, shocked, and compacted materials where shock indicators and petrofabrics are uniform across the entire domain of a chondrite. Because of this, it is rare for petrographic techniques alone to yield an unambiguous chronological ordering rich with the details of multiple impact or shock events within a single ordinary chondrite. Here, we examine a unique situation within the H chondrite breccia Northwest Africa (NWA) 7298, where information on sequential impacts is retained within rock and mineral textures alone. We use a combination of traditional petrographic methods and synchrotron X-ray microtomography (μCT) to examine the impact history of these chondrites. We present a near-surface parent body (asteroidal) scenario for the creation of NWA 7298. This investigation of NWA 7298 is the first solely petrographic description of both the sequence and magnitude of multiple shock events on small bodies in the solar system and it exemplifies the utilization of 3D petrography in deciphering asteroidal physical structures and the impact histories recorded in meteorites. 2. Samples and methods We obtained a 35.1 g stone which initial classification indicated to be an unequilibrated (petrographic type 3.8) H chondrite breccia (Meteoritical Bulletin, 2013). Two adjacent petrographic thin sections were prepared for 2D optical and electron beam instrumental petrographic investigations. Two ∼2 cm3 portions of the stone were cut for μCT investigation. 2.1. Traditional mineralogy and petrography NWA 7298 is a breccia containing at least three distinct lithologies (labeled A, B and C; Fig. 1). We evaluated the shock stage (Stöffler et al., 1991) of each lithology and performed electron microprobe x-ray mapping (Fig. 1) of our two adjacent sections. X-ray maps were generated using the Cameca SX100 at the American Museum of Natural History and are a combination of wavelength dispersive (WDS) and energy dispersive spectral (EDS) maps. These are “stage maps” (moving stage, stationary electron beam). Operating conditions were 15 kV and 40 nA, with a dwell time of 12 ms on one micrometer beam spots spaced 6 μm apart. Quantitative mineral compositions were determined on nominally 1 μm spots using the Cameca SX100 electron microprobe. Natural and synthetic standards were chosen based on the compositions of the minerals being analyzed and an accelerating potential of 15 keV and a sample current of 20 nA was used. Counting times were 20 s on peak, and 10 s on background (off-peak) spectrometer positions. Relative uncertainties (2σ ), based on counting statistics, for major elements (Si, Fe, Mg) are calculated to be < 2% and for Ti, Cr, Mn and Ca they are 10%, 10%, 9% and 5%, respectively. Data reductions were carried out using methods described by Pouchou and Pichoir (1991).

Fig. 1. Composite backscattered electron (BSE) image of NWA 7298 (H chondrite breccia) created from two adjacent petrographic sections. The missing (black) area in the middle is at the edge of the respective sections created for this study. The images are arranged to reflect their original spatial orientation in the meteorite. Three distinct clasts with differing composition and textures can be identified in the sections. Unequilibrated (∼3.8) lithologies A and C (outlined) are intermingled, but their respective shock stages (S2 and S1), compositions (Tables 1, 2), and porosities are clearly distinct. Equilibrated (4–5) lithology B (shock stage S3) is a loosely attached, but clearly connected, low porosity equilibrated H chondrite fragment. Lithology B’s composition is also distinct from the others (Tables 1, 2). Table 1 Average (and range) of compositions of olivine in NWA 7298 host (lithology A) and clasts (B and C). See Fig. 1. (wt.%)

Lithology A – host

Lithology B – clast

Lithology C – clast

SiO2 TiO2 Cr2 O3 FeO MnO MgO CaO Total Fa (mol.%) 2σ error for Fa # grains

38.0 (36.0–41.1) <0.03 (bd–0.07) 0.06 (bd–0.23) 15.8 (0.99–20.8) 0.41 (0.11–0.59) 45.7 (38.0–57.9) 0.09 (bd–0.32) 100.1 16.4 (0.9–23.4) 5.18 19

37.8 (36.4–40.2) 0.04 (bd–0.19) 0.14 (bd–0.65) 18.6 (17.6–21.7) 0.43 (0.31–0.49) 43.6 (42.4–45.0) 0.03 (bd–0.19) 100.6 19.3 (18.0–22.3) 1.02 16

37.5 (35.8–38.8) <0.03 (bd–0.05) 0.04 (bd–0.2) 18.0 (17.2–21.3) 0.47 (0.37–0.54) 44.2 (41.9–44.7) 0.04 (bd–0.9) 100.3 18.5 (17.7–22.2) 1.07 15

bd = below detection limit. Table 2 Average (and range) of low-Ca pyroxene compositions in NWA 7298 host (lithology A) and clasts (B and C). See Fig. 1. (wt.%)

Lithology A – host

Lithology B – clast

Lithology C – clast

SiO2 TiO2 Al2 O3 Cr2 O3 FeO MnO MgO CaO Na2 O Total Fs (mol.%) Wo (mol.%) 2σ error for Fs # grains

55.4 (53.3–56.8) 0.10 (bd–0.32) 0.37 (0.09–1.8) 0.26 (0.07–0.68) 10.4 (2.4–18.5) 0.40 (0.11–0.58) 32.8 (25.7–39.1) 0.49 (0.13–0.76) 0.05 (bd–0.58) 100.3 14.9 (3.2–28.3) 0.9 (0.2–1.5) 4.29 27

54.8 (53.5–56.5) 0.04 (bd–0.06) 0.30 (0.16–0.42) 0.52 (0.29–0.87) 8.8 (6.1–12.9) 0.37 (0.25–0.65) 34.4 (29.6–37.0) 0.48 (0.21–1.6) 0.10 (bd–0.30) 99.8 12.5 (8.5–19.0) 0.9 (0.38–2.95) 4.51 7

54.0 (53.3–54.8) 0.12 (bd–0.19) 0.26 (0.09–0.57) 0.24 (0.11–0.86) 11.8 (11.3–14.1) 0.53 (0.45–0.81) 31.9 (29.2–32.6) 0.56 (0.22–0.76) 0.06 (bd–0.22) 99.5 17.0 (16.2–21.1) 1.0 (0.4–1.4) 1.41 6

bd = below detection limit.

2.2. 3D petrographic investigations We examined NWA 7298 with the 3D imaging technique of μCT (Ebel and Rivers, 2007) at the GSECARS 13-BMD beamline at the Advanced Photon Source of the Argonne National Laboratory. To obtain data on each of three lithologies in NWA 7298, we imaged two separate chunks of NWA 7298. The first chunk

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Fig. 3. Degree of preferred orientation of metal grains (given by the strength factor C , see Section 2.2) versus shock stage in ordinary chondrites (solid symbols, data from Friedrich et al., 2008a and Friedrich et al., 2013). The intensity of foliation for each lithology in NWA 7298 is distinct. The most intensely foliated lithology is B (see Figs. 1 and 2), which has foliation near the higher end for its observed shock stage of S3. The more mildly shocked lithologies A and C have foliation intensities reasonable for their shock stages. Lithology C is the most porous and mildly-compacted material, yielding the least intense foliation (Figs. 1 and 2).

Fig. 2. The relative preferred orientation of metal grains (impact-induced petrofabric) for each lithology of the NWA 7298 H chondrite breccia. On the left, equal area, lower hemisphere stereoplots of best fit ellipsoid major axes constructed around metal grains. On the right are density stereoplots for each lithology. Orientation of the sample is identical between each lithology, so the orientations of foliation of lithologies A and C are identical, but with differing strengths of foliation (Fig. 3). Lithology B clearly possesses an orientation that is different than that of A and C and the intensity of foliation is more pronounced in this lithology (Fig. 3).

contained lithology A and B and was imaged at a resolution of 10.7 μm/voxel (a voxel is the 3D volume element equivalent of a 2D pixel) with monochromatic 55 keV X-rays. The second chunk was composed of lithologies A and C and it was imaged at a resolution of 7.7 μm/voxel with a 48 keV monochromatic X-ray beam. We used these volumes for the quantitative 3D examination of impact-induced petrofabrics (including orientation and intensity of foliation) within the chondrites. Petrofabric examination was accomplished by methods found in Friedrich et al. (2008a and 2013). In short, the ductile metal grains within a volume of interest are digitally isolated and bestfit ellipsoids are drawn around each. Orientation of the foliation can then be displayed by drawing a line through the long axis of each ellipsoid and collectively projecting points of intersection on a hypothetical sphere surrounding a sample on a stereoplot (Fig. 2). To obtain a quantitative value for the intensity of foliation, we used a variation of the orientation tensor method: the natural logarithm of the ratio of major over minor eigenvalues of the ellipsoids is computed to yield a strength factor, C (Woodcock, 1977; Woodcock and Naylor, 1983). The higher the strength factor, the greater the common orientation of the metal grains in the sample and the more pronounced the foliation (Friedrich et al., 2013). We applied these techniques to three digitally cropped μCT subvolumes of NWA 7298: a combined volume of lithology A (one from each chunk – see previous paragraph) and one each of lithology B and C (each from separate chunks, but associated with lithology A). Since the B and C lithologies were not imaged in the same chunk, but lithology A was imaged in both, we digitally rotated the A + C volume so that the foliation orientation of lithology A was identical among the separately imaged chunks. This allows us to

fruitfully compare relative orientations among all three lithologies. The two volumes of A were combined and used in toto to compute foliation parameters. 3. Results The two polished thin sections of NWA 7298 (-1 and -2) that were studied contain what is interpreted to be the host chondrite and two texturally distinct clasts (Fig. 1). NWA 7298-1 consists of the host chondrite (labeled A) and two clasts; a large (∼ 1 cm) oval-shaped clast (labeled B) and a smaller clast (C) that terminates with the edge of the section (Fig. 1). NWA 7298-2 consists of the host chondrite and more of clast C in the central part of the section (Fig. 1). Chondrules and fragments in the host (A) have sharp boundaries with the surrounding matrix and, based on its texture and mineral compositions, the host was previously determined to be an H3.8 (Meteoritical Bulletin, 2013). Mineral compositions of the three NWA 7298 lithologies (host and two clasts) are shown in Tables 1 and 2 (also see Fig. 1). In each case, mean olivine Fa and low-Ca pyroxene Fs compositions clearly fall within the known range of H chondrites (Rubin, 1990), but the substantial ranges indicate the host chondrite is an unequilibrated chondrite. From the pyroxene compositions clast C seems to be an approximately similar in petrologic grade to the host chondrite (H3.8). However, it has a smaller range in olivine composition compared to the host. Based on the range of olivine and pyroxene compositions, clast B is more equilibrated than the host chondrite and is H4 material. The lithology B petrographic type 4 classification is based on the sharp chondrule boundaries, no larger (>5 μm) Ca pyroxene grains being observed, and the occurrence of twinning in low-Ca pyroxene. Each of the three clasts possesses a different shock stage. To determine the shock stage we observed 20 olivine grains in the range of 50–100 μm in size in each lithology. In lithology A, the majority of olivine has sharp optical extinction and irregular fracturing and 30% of the olivine shows undulatory extinction with irregular fractures. This indicates a classification of shock stage S2. Lithology B has a higher shock stage of S3 with 72% of the olivine having

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irregular fractures and 28% showing planar fracturing. The third distinct lithology (C) has a shock stage of S1, based on sharp optical extinction in nearly all olivine. Lithologies A and C are tightly intermingled and the boundary between them is vague, such that identification of lithology C was only confirmed with close inspection of the textural and compositional differences. Lithology B, on the other hand, has a sharp boundary between it and lithology A, marked by a fracture. While lithology B is more loosely associated, it was sufficiently fused to the rest of the stone to survive atmospheric entry, landing, and a probable extended terrestrial residence with typical hot desert weathering. Orientations of the impact-induced foliation in NWA 7298 the intermingled lithologies A and C are identical (Fig. 2). In thin section and in μCT stacks, the orientation of foliation in lithology B is markedly stronger and different than that of lithologies A and C. Fig. 2 qualitatively shows that the intensity of the foliation varies between each lithology. Quantitatively, the intensity of foliation in an ordinary chondrite can be used as an indicator of the degree of shock-induced compaction experienced by the sample and this correlates well with shock stage (Friedrich et al., 2008a, 2013). The foliation intensity of NWA 7298 is no exception (Figs. 2 and 3). Foliation intensity within each lithology of NWA 7298 is consistent with the shock stage of each. Lithology B (Fig. 1) may have a hint of foliation evident in the BSE image, and our 3D petrographic investigations shows it is among the more pronounced foliations present for an S3 shock stage OC. Our BSE and μCT data show that the porosity-related textures of the three lithologies in NWA 7298 are distinct. Qualitatively, the porosity (Fig. 1) clearly increases lithology B < lithology A < lithology C. For quantitative μCT-based porosity values, however, resolutions of 1–3 μm/voxel on separate chips are necessary (Friedrich and Rivers, 2013) and we did not collect this data. There is, however, a clear relationship between higher porosity and lower compaction-induced petrofabric intensity among NWA 7298 lithologies (Fig. 2), as may be expected. 4. Discussion First, we present plausible scenarios for the formation of NWA 7298 on the H chondrite parent asteroid using our collective petrographic evidence. We contrast our evidence with other chondrites and impact generated rocks to place our findings in a greater context. 4.1. Impact history and asteroidal provenance for NWA 7298 Foliation found in ordinary chondrites is exclusively induced by uniaxial compaction (Gattacceca et al., 2005; Friedrich et al., 2008a) and, since the process is irreversible, it permanently records the most intense event experienced by a chondrite. Overwriting a foliation can only occur when a compacting event (impact) of a greater magnitude than others previously experienced by a chondrite takes place. In principle, shock stages of ordinary chondrites should record the magnitude of the most intense shock event experienced by the rock, but this may be overwritten by thermal annealing due to residual heat from the original or subsequent shock events (e.g. Rubin, 2004). Additionally, little is known about more mild shock events that post-date the most intense shock episode. By using textures and foliation in conjunction with shock stages, we can place stricter limits on the sequence and intensity of shock events experienced by NWA 7298 and the lithologies in it. Our BSE and μCT imaging of lithologies A and C show some porosity, consistent with a relatively mild shock stage and the lower degree of collective metal grain orientation relative to lithology B: some degree of ancient post-accretionary porosity remains

(Friedrich et al., 2008a; Sasso et al., 2009). We propose that the material that would eventually become NWA 7298 began as highly porous (>20% porosity) unconsolidated (and chemically unequilibrated) chondritic material. For ease of illustration, we place the unequilibrated host lithology (A) of NWA 7298 near the surface of the H chondrite parent. A workable model for chondritic parent bodies invokes an onion shell structure (e.g. Trieloff et al., 2003) where more deeply buried material experienced higher degrees of thermal metamorphism than the unequilibrated rocks near the surface because of the greater ability of the deeper materials to retain their heat. However, we point out that recent work (Ganguly et al., 2013) indicates that a simple onion shell is not the only plausible structure for the H chondrite parent. Although for the remainder of the discussion we place NWA 7298 near the surface of the asteroid, rather than deeper within it, our outlined scenario for its formation would also be applicable to more deeply buried material. NWA 7298 contains three distinct H chondrite lithologies, each with a different shock stage, porosity (Fig. 1), and petrofabric properties (Figs. 2 and 3). We construct an asteroidal scenario for the origin of NWA 7298 (Fig. 4) while utilizing our collective evidence. We first consider a simple double impact setting and later consider if additional impacts are needed or are possible. We begin with a single impact near the surface of the H chondrite parent asteroid (Fig. 4), which created a zone of variably shocked debris and fallback from an impact into our initially highly porous unequilibrated material. Lithologies A and C comprise different portions of the resulting brecciated near-surface material. Lithology A was somewhat more compacted and shocked by this event, but still retained appreciable porosity and a low (S2) shock stage. The lesser-effected lithology C retained more porosity and experienced less shock loading than lithology A. While lithologies A and C have different textures and porosities, their foliation orientation is identical in direction and foliation intensity is similar (Figs. 2, 3). They record at least one simultaneous compaction event; however, we suggest that this is the result of a later event after the impact that gave rise to their differing porosities and brecciation. We discount the idea of this initial impact heterogeneously shocking and compacting material within such close (mm) proximity, but rather suggest that transport of variably shocked and compacted material was necessary. Lithology B’s metamorphic grade, and foliation orientation and intensity is different than the others, and its low-porosity texture correlates well with its higher degree of compaction and S3 shock stage (Figs. 2, 3). From this evidence, it is clear that lithology B saw a stronger degree of shock loading than the other lithologies. Because of the different foliation orientation, this event was necessarily spatially separated from the other lithologies with a later transport of lithology B material into the region that will become NWA 7298. However, temporally separating the event is not necessary: it is plausible that the same impact event that only mildly affected lithologies A and C yielded the more significant shocked lithology B in a nearby region more central to the impact event (Fig. 4) and in the aftermath, the three lithologies were mixed in the fallback and debris region. Fig. 4 (top) shows the resulting assembly of materials after our proposed post first impact. Loosely consolidated and brecciated H chondrite material from a local region of the asteroid is now in place to become NWA 7298. We propose that a second consolidating impact mildly compressed the materials to form the resulting rock. This impact was strong enough to align the initially variably compacted lithologies A and C, but not strong enough to realign the fabric in the already shocked lithology B. A weak second impact is also supported by the severe boundary between lithologies A and B. It is possible, but not necessary that this consolidating impact also resulted in the ejection of NWA 7298 from the parent

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Fig. 4. A model for a sequence of collision events occurring on the H chondrite parent body producing NWA 7298. After one or more impacts (a single initial one is shown to simplify the diagram) variably shocked material that will become NWA 7298 is locally emplaced in a near-surface debris zone on the H chondrite parent asteroid. Initial impact energy on a highly porous body is focused on compressing the rock rather than ejecting material from the crater. Adjacent to the crater, significant porosity can be retained, whereas under the crater, materials become well compacted and heated. Unequilibrated material in the fallback and debris zone would be a mixture of variably, but mildly, shock-effected material including highly porous precursors to material seen in lithologies A and C of NWA 7298. Lithology B of NWA 7298 would have necessarily originated from an area of more significantly compressed, shocked, and heated material, but mixed with more mildly shocked material in the debris zone in the chaotic aftermath of the impact event. A subsequent impact (bottom) into target material that would become NWA 7298 compacted and foliated lithologies A and C and consolidated all three lithologies. The second impact imparted less energy into uniaxial compaction to incipient NWA 7298 material than was originally seen by lithology B, otherwise, foliations across all three lithologies would be identical, like that seen most chondrites. A more complex series of events is possible, but each ends with a mild consolidating impact that retains the relative foliations in the lithologies (see Section 4.1).

asteroid. Our data cannot distinguish between these two possibilities. The genesis of NWA 7298 can be fully explained by the simple case of two impacts on the H chondrite parent asteroid. However, although we have information on the sequence and intensity of at least two impacts creating NWA 7298, we cannot rule out additional impacts playing a role in its formation, only that fewer impacts cannot completely explain NWA 7298’s properties. The current variable shock stages and degrees of compaction of lithologies A and C may have been the result of more than one impact with subsequent mixing of the two variably shocked materials. It is also completely plausible that a previous impact on the same asteroidal parent created and deposited lithology B into area that later became the target rocks creating lithology A and C. We have no cosmogenic radionuclide evidence regarding the transported configuration of NWA 7298 from its parent to Earth. It is possible that an additional event beyond that which consolidated the material was needed to eject it from the asteroidal surface. Numerous combinations are possible. Throughout all of these scenarios, one thing is clear: we can discount the ambiguous interpretation of heterogeneous shock propagation creating the variable shock stages seen in NWA 7298 because of the two distinct foliations seen across lithology B and lithologies A and C combined. We have evidence of a minimum of at least two distinct impact events and their relative intensities. Although some uncertainties remain about the shock history of NWA 7298, we can contrast our information with that contained in other chondrites to see that we have gained additional information with the combination of our textural and shock stage information.

4.2. Variable shock-induced textures in chondrites: the rare case of NWA 7298 Because of the cumulative nature of compaction and shock events, within most OCs, petrofabrics are identical among all lithologies in a rock, so unlike the situation in NWA 7298, information on previous shock events is lost. In previous reported (Friedrich et al., 2008a, 2013) and unreported efforts we have examined the petrofabrics of ∼40 ordinary chondrites. In all of these cases, including samples from multiple portions of the same fall, same stone, and those breccias that contain obvious multiple lithologies {e.g. Zag [H3-6], Tysnes Island [H4], Felt (b) [L3.5-5]} have coherent fabrics that are congruent and have a uniform intensity across all portions of the material. The existence of variably shocked material in the same ordinary chondrite is rare, but previously observed. Huss (1980) reported that one petrographic section the Sharps H (3.4) chondrite contained evidence for moderate shock (facies d in the Dodd and Jarosewich, 1979 shock taxonomy), while another section contained characteristics indicative of only mild shock (facies b). However, we hypothesize that Sharps would nevertheless have identical petrofabrics stretching across the entire stone, implying a situation like found in NWA 7298, where the last shock event was less intense than previous ones. Ohnishi and Tomeoka (2002) found evidence of strongly shocked (S3–S4) dark inclusions contained in the S1 shock stage host material in the CV3 chondrite Mokoia. They concluded that Mokoia is a mixture of materials resulting from differing degrees of shock and thermal metamorphism from the near surface of the parent body. The dark inclusions were more strongly shocked and were mixed into a region on the asteroid that escaped most impacts.

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This is akin to our own ideas for the origins of NWA 7298. Obtaining 3D petrographic evidence for the foliation undoubtedly present in Mokoia would shed light on the collective impact history of the clast and host material. Impact melt rocks and impact melt breccias often contain some evidence of variably shocked materials. Kring et al. (1996) found moderately to highly shocked (S3–S4) and even shock-melted clasts that were crosscut by veins and dikes contributed by the shock-melted host material within the Cat Mountain L chondrite. However, other than the fact that they were melted, we have little information on the cumulative processes that have shaped the shocked and melted vein material. Similarly, Kojima et al. (2000) found shock-melted clastic material in Manych, an LL 3.4 chondrite, in the same petrographic section that contained both unmelted (but substantially shocked) precursor material and similarly shocked matrix. However, an obvious foliation is evident across the entire section of Manych (see images in Kojima et al., 2000), indicating that the most recent impact has largely obliterated detailed information on previous impact processing of the clasts in Manych. Based on compositions, Rubin and Read (1984) argued for pairing a group of three L6 chondrites [now: Ness County (1894)] with different Dodd and Jarosewich shock facies, which likely comprised a historical strewn field. They discussed the presence of glassy shock veins in some specimens, while others seem very little effected by shock (facies a–b). One hypothesis to explain this was that Ness County (1894) is a fragmental impact breccia, containing mixed material from different regions that experienced different shock pressures and peak temperatures, including impact melting, which is reasonable for an energetic impact into an already shocked and fragmented body. Impacts into non-asteroidal bodies have also probably experienced multiple impacts, but the interpretation is more difficult because of the complexities of material mixing during an impact into a low-porosity body. Although Floran et al. (1972) and Sclar (1972) interpreted complex brecciated textures in Apollo 12 samples as indicators of multiple impacts, later works on Apollo 12 (e.g. Quick et al., 1981), Apollo 14 (Wilshire and Jackson, 1972), and Apollo 17 samples (e.g. Wolfe et al., 1981) recognized the complex nature of impacts into low porosity bodies and the complex “breccia-within-breccia” textures that single impacts can produce (e.g. French, 1998). What these complex rocks are informing us about is not the heterogeneous nature of shock events affecting the same material, but the complexity of material mixing during a single impact into low-porosity target rocks. The difference between these complex rocks and the case of NWA 7298 is that in addition to shock stage, compaction-related effects are also apparent from the variably foliated textures contained across the lithologies and the fact that a more intense impact has not erased the effects of previous impacts. NWA 7298 was created by at least two impacts into a porous body rather than a solid, differentiated body with low porosity. This yields additional information on which we can further interpret the evolutionary impact history of NWA 7298 and demonstrates the utility of petrofabric analysis to the interpretation of impact histories. 5. Conclusions NWA 7298 is the result of multiple impacts near the surface of the H chondrite parent body, probably in an impact debris zone. Based on the petrofabrics and shock stages apparent in three different lithologies of NWA 7298, we outlined a plausible impact sequence on an initially porous H chondrite parent asteroid to explain the textures contained in NWA 7298. The simplest sequence of events is a two-impact scenario where variably-affected shocked material is first mixed and later shocked again. Our use of both petrofabrics and shock stages in inferring the impact history of

NWA 7298 shows that the last impact to affect it was mild in nature, which preserved the previous impact intensities of each lithology contained in NWA 7298. This is unlike the case found in most chondrites, where consistent petrofabrics are the norm or in the case of impact melt rocks, where complex mixing of potentially previously shocked material is further thermally altered by an intense impact. To explain NWA 7298, we do not need to invoke an ad hoc situation of “inhomogeneous shock effects” to explain our observations of multiple shock stages in ordinary chondrites. Our methodology and observations yield new opportunities for investigating the dynamic collisional evolution of asteroids and shows that the H chondrite parent asteroid experienced several generations of impacts. Acknowledgements J.M.F. acknowledges a Theodore Dunham, Jr. Grant for Research in Astronomy (Fund for Astrophysical Research) for assistance in the acquisition of computer equipment used for portions of this study. M.K.W. is supported by NASA Cosmochemistry grant NNX12AI06G. Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences (EAR-1128799) and U.S. Department of Energy – GeoSciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The authors thank D. Kring (LPI) for helpful discussion. References Bischoff, A., Scott, E.R.D., Metzler, K., Goodrich, C.A., 2006. Nature and origins of meteoritic breccias. In: Lauretta, D.S., McSween, H.Y. (Eds.), Meteorites and the Early Solar System II. The University of Arizona Press, Tucson, pp. 679–712. Bogard, D., 1995. Impact ages of meteorites: A synthesis. Meteorit. Planet. Sci. 30, 244–268. Bogard, D.D., 2011. K–Ar ages of meteorites: clues to parent-body thermal histories. Chem. Erde 71, 207–226. Consolmagno, G.J., Britt, D.T., Macke, R.J., 2008. The significance of meteorite density and porosity. Chem. Erde 68, 1–29. Dodd, R.T., Jarosewich, E., 1979. Incipient melting in and shock classification of Lgroup chondrites. Earth Planet. Sci. Lett. 44, 335–340. Ebel, D.S., Rivers, M.L., 2007. Meteorite 3-dimensional synchrotron microtomography: methods and applications. Meteorit. Planet. Sci. 42, 1627–1646. Floran, R.J., Cameron, K.L., Bence, A.E., Papike, J.J., 1972. Apollo 14 breccia 14313: a mineralogic and petrologic report. In: Proceedings of the Third Lunar Science Conference Geochim. Cosmochim. Acta Supplement 3, pp. 661–671. French, B.M., 1998. Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. LPI Contrib., vol. 954. Lunar and Planetary Institute, Houston. 120 pp. Friedrich, J.M., Rivers, M.L., 2013. Three-dimensional imaging of ordinary chondrite microporosity at 2.6 micrometer resolution. Geochim. Cosmochim. Acta 116, 63–70. Friedrich, J.M., Wignarajah, D.P., Chaudhary, S., Rivers, M.L., Nehru, C.E., Ebel, D.S., 2008a. Three-dimensional petrography of metal phases in equilibrated L chondrites – effects of shock loading and dynamic compaction. Earth Planet. Sci. Lett. 275, 172–180. Friedrich, J.M., Macke, R.J., Wignarajah, D.P., Rivers, M.L., Britt, D.T., Ebel, D.S., 2008b. Pore size distribution in an uncompacted equilibrated ordinary chondrite. Planet. Space Sci. 56, 895–900. Friedrich, J.M., Ruzicka, A., Rivers, M.L., Ebel, D.S., Thostenson, J.O., Rudolph, R.A., 2013. Metal veins in the Kernouvé (H6 S1) chondrite: Evidence for pre- or synmetamorphic shear deformation. Geochim. Cosmochim. Acta 116, 71–83. Ganguly, J., Tirone, M., Chakraborty, S., Domanik, K., 2013. H-chondrite parent asteroid: a multistage cooling, fragmentation and re-accretion history constrained by thermometric studies, diffusion kinetic modeling and geochronological data. Geochim. Cosmochim. Acta 105, 206–220. Gattacceca, J., Rochette, P., Denise, M., Consolmagno, G., Folco, L., 2005. An impact origin for the foliation of chondrites. Earth Planet. Sci. Lett. 34, 351–368.

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