Impact melt- and projectile-bearing ejecta at Barringer Crater, Arizona

Earth and Planetary Science Letters 432 (2015) 283–292

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Impact melt- and projectile-bearing ejecta at Barringer Crater, Arizona Gordon R. Osinski a,b,∗ , Ted E. Bunch c , Roberta L. Flemming a , Eric Buitenhuis a , James H. Wittke c a b c

Centre for Planetary Science and Exploration/Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7, Canada Department of Physics and Astronomy, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7, Canada Geology Program, School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ, 86011, USA

a r t i c l e

i n f o

Article history: Received 16 April 2015 Received in revised form 9 October 2015 Accepted 12 October 2015 Available online 3 November 2015 Editor: T.A. Mather Keywords: impact cratering impact melting glass iron meteorite carbonate melts

a b s t r a c t Our understanding of the impact cratering process continues to evolve and, even at well-known and wellstudied structures, there is still much to be learned. Here, we present the results of a study on impactgenerated melt phases within ejecta at Barringer Crater, Arizona, one of the first impact craters on Earth to be recognized and arguably the most famous. We report on previously unknown impact melt-bearing breccias that contain dispersed fragments of the projectile as well as impact glasses that contain a high proportion of projectile material – higher than any other glasses previously reported from this site. These glasses are distinctly different from so-called “melt beads” that are found as a lag deposit on the presentday erosion surface and that we also study. It is proposed that the melts in these impact breccias were derived from a more constrained sub-region of the melt zone that was very shallow and that also had a larger projectile contribution. In addition to low- and high-Fe melt beads documented previously, we document Ca–Mg-rich glasses and calcite globules within silicate glass that provide definitive evidence that carbonates underwent melting during the formation of Barringer Crater. We propose that the melting of dolomite produces Ca–Mg-rich melts from which calcite is the dominant liquidus phase. This explains the perhaps surprising finding that despite dolomite being the dominant rock type at many impact sites, including Barringer Crater, calcite is the dominant melt product. When taken together with our estimate for the amount of impact melt products dispersed on, and just below, the present-day erosional surface, it is clear that the amount of melt produced at Barringer Crater is higher than previously estimated and is more consistent with recent numerical modeling studies. This work adds to the growing recognition that sedimentary rocks melt during hypervelocity impact and do not just decompose and/or devolatilize as was previously thought. This has implications for understanding the processes and products of impacts into sedimentary rocks and for estimating the amount of climatically active gases released by impact events. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The impact of an asteroid or comet deposits vast amounts of energy into a planetary surface that can have far reaching consequences, not just for the geological make up and structure of the crust, but also for the environment and potentially for life itself. It is widely believed that the behavior of volatile-rich sedimentary rocks during impact differs from dense non-porous crystalline rocks, where large volumes of impact melt are generated (Dence, 1971). Carbonates are present in the target rocks of approximately one third of the world’s known meteorite im-

*

Corresponding author. Tel.: +1 519 661 4208; fax: +1 519 488 4721. E-mail address: [email protected] (G.R. Osinski).

http://dx.doi.org/10.1016/j.epsl.2015.10.021 0012-821X/© 2015 Elsevier B.V. All rights reserved.

pact structures (Osinski et al., 2008). In contrast to hypervelocity impacts into crystalline rocks, it has commonly been accepted that carbonates decompose during impact, releasing CO2 and producing CaO and MgO (Kieffer and Simonds, 1980). This led many workers to suggest that impacts into sedimentary rocks produce more destructive environmental effects than impacts of the same energy into crystalline rocks, as exemplified by the research into the environmental effects of the 65 Ma ∼180 km diameter Chicxulub impact structure – which impacted a ∼3 kmthick sequence of carbonates and evaporites overlying crystalline basement rocks – and its link to the Cretaceous–Paleogene mass extinction event (O’Keefe and Ahrens, 1989; Pierazzo et al., 1998; Pope et al., 1994). More recently, evidence for the shock melting of carbonates has been documented (see Osinski et al., 2008, for a

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review); however, the response of carbonates to hypervelocity impact remains poorly understood and remains a subject of debate. Barringer Crater – also known as Meteor Crater – is one of the world’s best known meteorite impact craters and has been the subject of numerous geological investigations since its discovery over a century ago (Barringer, 1905). It is well preserved, accessible, and well exposed and is widely acknowledged as being a prototypical simple impact crater (Grieve and Garvin, 1984; Shoemaker, 1963). Despite this, over the past decade, Barringer Crater has continued to yield important new information, and some surprises, both with respect to the specifics of formation of this site, and the impact cratering process in general. This includes the first constraints on the stratigraphic extent of the melt zone at Barringer (Hörz et al., 2002), new insights into rim uplift and the mechanics of simple crater formation (Kumar and Kring, 2008; Poelchau et al., 2009), and contradictory numerical modeling studies, which suggest that Barringer Crater formed either by a lowvelocity impact event (Melosh and Collins, 2005) or a high-velocity impact of a fragmented projectile (Artemieva and Pierazzo, 2011). Because of its small size, young age, and well-constrained target stratigraphy, Barringer Crater provides an excellent opportunity to investigate the effects of meteorite impact into sedimentary targets. Here, we report the first documentation of impact glass- and projectile-bearing impact breccias in the continuous ejecta blanket of Barringer Crater. When coupled with our documentation of Ca–Mg-rich glasses and calcite melt globules, these results have important implications not just for our understanding of the Barringer impact event, but for our understanding of the processes and products of impact crater formation and the effects of impacts into sedimentary target rocks in general. 2. Geological setting Barringer Crater is a well-preserved 1.2 km diameter simple crater situated near Flagstaff, northern Arizona (32◦ 02 N, 111◦ 01 W). It comprises a topographic rim that rises from 30 to 60 m above the surrounding plains and a ∼180 m deep classic bowl-shaped depression. It formed ∼50,000 ago (Nishiizumi et al., 1991) from the impact of an ∼10–50 m diameter ∼30 t iron meteorite, fragments of which survived and are now known as the Canyon Diablo meteorite. This is a Group IAB iron meteorite with a bulk chemical composition of 6.9 to 7.1 wt% Ni, dominated by kamacite but with a diverse mineralogy including taenite, schreibersite, troilite and graphite (Moore et al., 1967). Barringer Crater formed in a thick sequence of flat lying sedimentary rocks of the Colorado Plateau (Shoemaker, 1963). Quartz sandstones (>95% quartz) of the 210–240 m thick Coconino Formation are the deepest lithologies affected by the impact event. The overlying thin, ∼3 m thick Toroweap Formation comprises impure sandstones and minor dolomites and is overlain by interbedded dolomites and sandstones of the ∼80 m thick Kaibab Formation. The uppermost rocks are of the Moenkopi Formation, a calcite-bearing siltstone, often with an iron-rich matrix (Mittlefehldt et al., 2005). Several types of impactites have been documented at Barringer Crater. Impact breccias are, however, rare and are typically poorly consolidated and, apparently, lacking whole rock impact melt materials (Shoemaker and Kieffer, 1988). Within the interior of the crater, Shoemaker (1963) mapped 3 main types of impact breccias. “Authigenic breccias” are monomict and typically found along fault planes. “Allogenic breccias” are found along the crater walls and floor and can be monomict or, more commonly, polymict; they are allochthonous and contain material shocked to a wide range of pressures and temperatures, including shock-melted sandstones (i.e., lechatelierite). So-called “mixed debris” lies above the allogenic breccias on the crater walls and as a ∼10 m thick lens within

the centre of the crater; the latter is normally graded. As with the allogenic breccias, this “mixed debris” is polymict and comprises variably shocked and shock-melted target rocks and impactor material. In accordance with the IUGS Subcommission on the Systematics of Metamorphic Rocks, the authigenic and allogenic breccias at Barringer Crater should be referred to as autochthonous and allochthonous impact breccias, respectively (Stöffler and Grieve, 2007), and we use these terms herein. Exterior to the rim, there is a continuous ejecta blanket that extends from 1.3 to 1.9 km from the crater centre, with original distribution up to ∼3 km (Roddy et al., 1975). It comprises an overturned rim sequence of inverted target strata overlain by allochthonous debris. Impact melt is noticeably rare with the only documented whole-rock melt occurring in the form of millimetre- to centimetre-size discrete particles of impact glass, or melt “beads”, found as a lag deposit on the present-day erosion surface around the crater rim (Hörz et al., 2002; Nininger, 1956). In this study, we provide the first documentation of allochthonous impact breccias within the continuous ejecta blanket, located ∼100 m down from the southeast crater rim. We first reported their discovery and existence in preliminary fashion in Osinski et al. (2006). These breccias appear to be a more consolidated and whole rock melt-rich version of the “allogenic breccias” of Shoemaker (1963) found in the crater interior. 3. Methods Polished thin sections were prepared and investigated using optical microscopy techniques and a JEOL JXA-8900 L electron microprobe (beam operating conditions of 15 kV and 20 nA), equipped with five wavelength dispersive X-ray spectrometer (WDS). A total of 216 spot analyses of glasses from 26 clasts were collected from the breccias and 68 spot analyses from 8 melt bead samples. Electron microprobe data were reduced using ZAF procedures incorporated into the operating system. Back-scattered electron (BSE) imagery was used to investigate the micro-textures of the various impactites. Hand specimen slabs and thin sections were studied in situ using a Bruker D8 Discover micro X-Ray Diffractometer (μXRD) (Flemming, 2007) and a Bruker M4 Tornado micro X-Ray Fluorescence (μXRF) spectrometer. In order to investigate the amount of impact melt beads in the present-day soils, we sampled an area 150 × 150 × 10 cm deep. We first sieved out the coarse rock fraction then performed a magnetic separation followed by a density separation. This was aimed at separating out local, non-magnetic rock particles (mostly Coconino and Kaibab formations). Subsequently, hand picking was used to remove the remaining rock contaminants, which were mostly the slightly magnetic Moenkopi Formation. Each fraction was then sieved and weighed in 3 g aliquots and the number of glass particles in 3 size bins was measured. 4. Results 4.1. Impact breccias The impact breccias crop out over an area of ∼ 45 m2 ∼ 100 m down from the southeast crater rim (Fig. 1). In hand specimen and under the optical microscope, these breccias are seen to consist of a fine-grained clastic groundmass containing clasts up to ∼20 cm in size (Fig. 2). With backscattered electron imagery (BSE) it is clear that there is a complete continuum from obvious mm- to cmsized ‘clasts’ to the fine-grained interstitial material or groundmass that ranges from 10s to 100s μm in particle size. Thus, a distinction between ‘groundmass’ and ‘clasts’ is somewhat arbitrary and depends on the resolution with which one views these breccias. Optical and BSE imagery together with μXRD show that the

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Fig. 2. A. Outcrop of impact breccias ∼100 m down from the top of the southeast crater rim. Note the prominent brick-red clasts of Moenkopi Formation siltstone and the pale white clasts of Coconino sandstone. 10-cm-long penknife for scale. B. Slab of impact breccia ∼12 × 6 cm.

Fig. 1. Oblique aerial image of Barringer Crater looking north taken in 1964. The bright area in the southeast is highly shocked Coconino Formation sandstone. The region where impact melt-bearing breccias were sampled is highlighted with a white ellipse.

following target components comprise the groundmass and clasts within the impact breccias (Figs. 2, 3): (1) lithic clasts from the Moenkopi and Coconino formations and, more rarely, the Kaibab Formation; (2) individual quartz grains with planar deformation

Fig. 3. Backscattered electron (BSE) images showing various examples of impact melt glasses with impact breccias at Barringer Crater. A, B: Isolated sub-rounded glass (Gl) clasts. In BSE, the glasses always appear brighter than the surrounding matrix because of their FeO-rich nature. Clasts of quartz (Qtz) and lechatelierite (SiO2 Gl) are common within the glasses. C: Vesiculated glass clast. D, E: Irregularly shaped glass clasts encasing large quartz clasts. F: Highly vesiculated glass clast where the glass comprises <10% of the clast and where the glass forms a network essentially holding the clast together. G: Glass mantling a large fractured quartz grain. Impact-glass clast wrapping around a shocked quartz grain. H: Holohyaline glass clast virtually devoid of clasts and with no crystallites. I, J: Rare glass clasts with micron-sized pyroxene crystallites. K: Glass clast containing globules of calcite (Cc). L: Caliche on the original outer surface (bottom, black) of a sample. Note the well-laminated nature of the caliche and the intercalation of clays and fine sediment that appear darker in BSE images.

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Fig. 4. Individual WDS analyses of glass clasts from impact breccias at Barringer Crater. A, B. Harker diagrams of FeO and CaO, respectively. C. Ternary diagram of glass analyses. Red points are data from this study; green points are data from Horz et al. (2002); blue points are analyses of the Coconino (square), Moenkopi (triangle) and Kaibab (circle) formations (Mittlefehldt et al., 2005). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

features (PDFs); (3) individual grains of diaplectic quartz glass; individual grains of calcite and dolomite. A number of impact melt products are also present. Vesiculated SiO2 glass, also known as lechatelierite, is common throughout these samples, and has been documented previously at Barringer Crater (Shoemaker, 1963). We have also documented silicate impact melt glass in breccias for the first time at Barringer Crater. These glasses possess widely variable morphologies (Fig. 3) from compact and sub-rounded grains with up to ∼50% glass (Figs. 3A, B), to highly vesiculated, clast-rich examples where the glass comprises <10% of the “grain” (Fig. 3C). Quite frequently, large lithic or mineral clasts are encased in glass such that the glass appears more as a mantling (Figs. 3D–G). In some extreme examples, glasses can be best described an aggregate of shocked target material bound together by very thin glass stingers (Fig. 3F). The majority of the glasses are fresh and holohyaline (Fig. 3H), with pyroxene crystallites occurring in <5% of the grains studied (Figs. 3I, J). Quartz grains and vesiculated SiO2 glass are also present as isolated clasts within the silicate impact glass clasts, as are calcite spherules (Fig. 3K). In only one example, was a glass clast fractured and broken. Care was taken to avoid samples with a large build up of caliche or duricrust; although, this was not always possible. However, using BSE imagery it is relatively straightforward to identify caliche, which is well laminated and with interlayers of clay and fine sediment (dark in BSE imagery; Fig. 3L).

WDS analyses of a large number of glasses show that there is wide range in composition between different clasts (Fig. 4; Table 1) (e.g., SiO2 : 43–50 wt%, with one outlier at 39 wt%; FeO: 23–36 wt%; CaO: 9–20 wt%). There is also significant variation, up to ∼4 wt%, in major oxides within some individual clasts (Fig. 4). Overall, these glasses are notably more FeO-rich than those previously reported from Barringer Crater (Hörz et al., 2002). It should be noted that the glasses studied here do not correspond to the hydrothermally altered, palagonitic glasses studied by Hörz et al. (2002), which are also Fe-rich. Such glasses appear very dark in BSE images – due to hydration – and we did not find any such glasses in the impact breccias. Final constituents of the impact breccias are isolated Fe–Ni-rich “grains” up to ∼2 mm in diameter and Ni–Fe-rich “veins” in lithic clasts (Fig. 5). μXRD of these particles indicates the presence of kamacite, troilite, and taenite (Figs. 6A, B). A μXRF map of a slab of the breccias shows that such grains are widely dispersed throughout the breccia matrix (Figs. 6C, D). It is important to note that these kamacite-rich grains occur isolated within the breccia matrix (e.g., Figs. 7A–D) and within glass clasts (e.g., Figs. 7E–H) that are themselves clasts within the breccia. 4.2. Impact melt beads Irregular-shaped, millimetre to centimetre-size vesicular impactites and melt beads can be found scattered on the present-day erosion surface around Barringer Crater (Nininger, 1956). We have

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Table 1 Representative individual WDS spot analyses of glass clasts from impact breccias. Sample # Clast # Analysis #

04-040 1 5

04-040 2 12

04-040 3 25

04-040 4 31

04-040 5 43

04-040 6 59

04-040 7 75

04-040 8 83

04-040 10 106

04-012 1 7

SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 SO3 NiO Cl Total

48.34 0.16 1.39 29.47 b.d. 9.50 10.07 0.01 0.02 0.15 b.d.2 n.a. b.d. 99.12

46.54 0.10 1.48 34.89 b.d. 6.45 9.56 0.06 0.01 0.07 0.07 n.a. 0.02 99.26

49.25 0.10 1.99 30.55 0.05 6.51 9.89 0.01 0.40 0.16 0.52 n.a. 0.02 99.44

48.92 0.08 2.18 31.54 0.03 6.30 9.66 0.02 0.34 0.17 0.40 n.a. b.d. 99.62

49.64 0.10 2.06 29.47 0.03 6.69 9.89 0.05 0.41 0.12 0.50 n.a. b.d. 98.95

46.44 0.16 2.13 27.44 0.03 9.53 14.18 0.01 0.02 0.28 0.13 0.14 b.d. 100.50

47.84 0.10 1.90 33.85 b.d. 6.45 9.22 b.d. 0.27 0.19 0.38 0.07 b.d. 100.28

45.79 0.14 2.14 33.33 0.01 6.89 10.51 0.04 0.43 0.31 0.66 0.02 0.02 100.29

47.62 0.13 1.98 34.24 b.d. 6.66 8.86 0.03 0.05 0.20 0.01 0.53 b.d. 100.30

48.24 0.09 2.06 30.87 0.02 6.45 10.54 b.d.9 0.44 0.27 0.63 0.03 b.d. 99.62

Sample # Clast # Analysis #

04-012 2 14

04-012 3 31

04-012 4 35

04-012 6 51

04-010 1 3

04-010 3 22

04-010 4 34

04-010 5 43

04-010 7 61

04-010 9 79

SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 SO3 NiO Cl Total

47.38 0.24 1.95 31.84 b.d. 7.08 10.90 0.02 0.13 0.22 0.28 0.03 b.d. 100.06

45.96 0.12 2.18 30.23 0.04 7.90 11.94 0.07 0.42 0.40 0.55 0.04 b.d. 99.84

47.25 0.08 1.36 30.44 0.02 7.87 10.99 0.01 0.02 0.27 0.06 0.06 b.d. 98.42

48.09 0.10 1.99 31.93 b.d.7 6.83 10.06 0.04 0.42 0.27 0.71 0.03 b.d. 100.46

48.87 0.10 2.04 3b.d.5 0.03 7.36 10.66 0.00 0.37 0.31 0.34 n.a. b.d. 70.08

47.11 0.11 2.09 34.91 0.01 5.63 8.77 0.05 0.27 0.23 0.24 n.a. b.d. 99.41

46.98 0.15 2.16 30.25 0.01 7.72 11.20 0.03 0.43 0.27 0.42 n.a. b.d. 99.61

45.41 0.15 2.07 34.68 0.04 5.58 8.81 0.01 0.26 0.23 0.33 0.56 b.d. 98.12

47.64 0.10 1.97 33.59 0.01 6.33 9.64 0.03 0.39 0.12 0.56 0.02 b.d. 100.38

47.42 0.11 1.95 28.36 0.01 7.57 13.03 0.03 0.16 0.39 0.03 0.29 b.d. 99.35

b.d. = below detection; n.a. = not analyzed.

documented melt beads up to ∼5 km to the north, ∼8 km to the southwest and ∼2 km to the south of the crater. Thousands of impactites and a few melt beads are concentrated in water laid deposits that occur in small, flat lying depressions (<2 m diameter and <3 cm thick beds) on the south slope of the crater. These secondarily derived concentrations do not give information as to the original ejecta and impact plume fall out distributions, but they do provide large representative sample for examining melt/quench petrographic and compositional characteristics on small to intermediate sized impactites (0.01 mm to 2.55 cm). Our separation study of 0.225 m3 of soil yielded ∼9000 melt beads in the 3–4 mm size range, ∼17,000 in the 1.4–3 mm size range, and ∼25,000 particles in the <1.4 mm size range. This equates to 0.001715 m3 of glass per 1 m3 of soil. Elsewhere, extensive plant cover, soils, and wind and water deposited debris preclude finding sufficient samples for meaningful statistics and observations on the small to intermediate size fractions, although we have found large, solitary impactites (3.8 to 63 mm) in these terrains. A detailed study by Hörz et al. (2002) distinguished 3 major compositional groups of melt beads at Barringer Crater. We have documented a series of glasses that are notably more Ca- (up to 22 wt% CaO) and Mg-rich (up to 15 wt% MgO) (Table 2) and that contain irregularly shaped globules of calcite (Fig. 7; Table 2). This compares with maximum contents of <17 wt% CaO and <9 wt% MgO in the glasses studied by Hörz et al. (2002). The globules occur individually or as groups within silicate glass and may be coalesced, or partially coalesced. There are sharp boundaries and curved menisci between the calcite and silicate glass. Adjacent to large vesicles, the calcite displays feathery textures and contains thin (<5 μm thick) ‘lamellae’ of glass (Fig. 7B).

5. Discussion 5.1. Shock melting of carbonates The response of carbonates to hypervelocity impact remains a contentious topic. Once thought to largely decompose and devolatilize, a 2008 review of the evidence suggested that the melting of carbonates is common during impact (Osinski et al., 2008), with evidence at that time known from the Chixulub (Jones et al., 2000), Haughton (Osinski and Spray, 2001), Ries (Graup, 1999) and Tenoumer (Pratesi et al., 2005) impact structures. Despite these observations and recent new evidence from Steinheim (Anders et al., 2011), a recent article on the Ries impact structure argued against the melting of carbonates during that event (Stöffler et al., 2013); although the authors presented no new data to support their views. In their initial study of Barringer Crater melt beads, Hörz et al. (2002) did not observe carbonate melts and instead suggested that their “investigations show that CO2 is lost from the system and that refractory residues combine with quartz into a melt.” Most recently, these authors carried out a subsequent analysis of melt beads in which they conclude that devolatilization of carbonates was the dominant process at Barringer Crater (Hörz et al., 2015). It is outside the scope of this current study to address this recent contribution in its entirety; however, a brief discussion is warranted of the key points raised by Hörz et al. (2015) and that are relevant to this study; namely that most calcite in the melt beads is caliche (i.e., is of a modern low-temperature origin) and that dolomite and not calcite is the dominant carbonate mineral in the pre-impact target rocks. The complex textural relationships shown in Figs. 5Q–T and 7 in both the impact breccias and the melt beads, including intermin-

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Fig. 5. BSE images (A, E, I, M, Q) and element maps of Fe–Ni-rich regions in the impact breccias. A–D: Rounded particle of kamacite rimmed with troilite within the breccia matrix. Note the SiO2 -rich glass surrounding this grain. E–H: Composite kamacite–troilite grain within SiO2 -rich glass, itself embedded in the breccia matrix. I–L: Unknown grain, likely troilite, within the breccia matrix. M–P: Vein of Ni–Fe-rich material within a sandstone clast in impact breccia. It is not known if the Ni–Fe-rich material is crystalline or glassy. Q–T: Calcite globules embedded in silicate impact glass.

gling and budding, of the silicate glass and carbonate globules indicates that both were in the liquid state at the same time (Osinski and Spray, 2001). These are diagnostic criteria for the recognition of carbonate impact melts (Osinski et al., 2008). Feathery carbonates are also indicative of rapid quenching of a melt (Jones et al., 2000). Interestingly, while they did not document any carbonate melts, Hörz et al. (2015) concur with this view, referring to previous work by Osinski et al. (2008), stating that “spherical blebs of pure calcite exist in the impact melts of [Barringer Crater] and that textural evidence suggests immiscible melt relationships between carbonate and silicate melts.” These authors go on to say that this carbonate that this is a “minor population”, with a lowtemperature caliche origin being the dominant mechanism for the formation of calcite in their samples. Because these two studies focused on different sample sets, we cannot question the conclusions of Hörz et al. (2015); however, in our samples, care was taken during sample preparation to avoid areas of heavy caliche development, but in instances when this was not possible, it is possible to easily texturally differentiate between caliche (Fig. 3L) and the globular and feathery calcite that we interpret as igneous in origin

(Figs. 5Q–T, 7). Thus, while we do not disagree that devolatilization may have occurred or that some calcite is of a caliche origin, this study clearly shows that carbonates also underwent melting during the formation of Barringer Crater. It is well known that the dominant carbonate at Barringer Crater is dolomite (Shoemaker and Kieffer, 1988). Because of this, Hörz et al. (2015) posed the question “how is it possible to derive an essentially calcitic melt from a mixture of dolomite and quartz?”. They use this as an argument against primary, shock-produced calcite melts at Barringer Crater. Unfortunately, our knowledge of the behavior of dolomite during impact events is even less than calcite, where a high P –T phase diagram exists (Ivanov and Deutsch, 2002). In addition, very few experiments have been conducted on dolomite. The most extensive to date are by Skála et al. (2002) who suggested that dolomite remains stable up to 60 GPa. This is remarkable given most silicates show evidence for substantial phase changes and/or melting and pressures at or below this pressure. At pressures of exceeding 68 to 70 GPa the containers underwent catastrophic failure with no recoverable physical fragments leading to the suggestion that dolomite under-

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Fig. 6. A: μXRD pattern showing the presence of kamacite and troilite at spot 8 in the polihedthin section shown in B: The quartz is showing due to beam overlap with the surrounding lithic clast and matrix. C, D: Image of polished slab and corresponding μXRF Fe–Ni–Si element.

goes melting or devolatilization above these pressures (Skála et al., 2002). Experiments and phase relations are, thus, inconclusive about the fate of dolomite during meteorite impact events. Based on the observations presented here from Barringer Crater and consideration of the literature, we propose the following two mechanisms for the production of calcite melt during impact events. The first is the direct melting of limestone or other calcitebearing sedimentary lithologies to produce a calcite melt. This is likely unimportant at Barringer Crater given how rare calcite is in the pre-impact target rocks, but is likely the dominant mechanism for the formation of calcite melt at other craters such as the Ries impact structure, Germany (Graup, 1999; Osinski, 2003), where limestone is the dominant target rock. Where dolomite is the dominant target rock and mineral, we propose a second mechanism whereby calcite precipitates from a primary dolomitic melt. This has previously been suggested based on data from the Haughton impact structure (Osinski and Spray, 2001; Osinski et al., 2005) where the target rocks consist of ∼1.9 km of sedimentary rocks with dolomite the dominant rock type. This perhaps surprising mechanism to form igneous calcite from a dolomite target is routed in the literature dedicated to understanding carbonatites. Experiments have shown that for the systems CaO–MgO–CO2 –H2 O (Lee et al., 2000) and CaO–MgO–SiO2 –CO2 –H2 O (Otto and Wyllie, 1993), calcite is the liquidus phase for a wide range of compositions and

pressure–temperature conditions. Thus, the prediction is that when dolomite-rich target rocks are shock melted, calcite will typically be the first phase to crystallize out of the melt, with dolomite only forming at lower temperatures upon slow cooling. Given the obvious rapid quenching of the Barringer Crater melts – and indeed many impact melts – igneous dolomite will be a rare occurrence. As a result of the crystallization of calcite, MgO will be enriched in the residual melt along with constituents from the melting of other target rocks. At Barringer Crater, this includes contributions from the projectile and other sandstone-rich units (see subsequent discussion sections), hence providing an explanation for the production of MgO-rich glasses (Tables 1, 2). Our proposal that carbonates underwent melting at Meteor Crater is further supported by preliminary results published in abstract form by Hagerty and Gaither (2014, 2013), who note the presence of “carbonate lithic inclusions and carbonate melt globules within several impact melt fragments” in drill cores through the ejecta blanket. Whether these globules are indeed of impact melt origin remains to be proven; however, we hypothesize that the drill cores being studied by these authors represent a subsurface continuation of the impact melt-bearing breccias studied here. 5.2. Impact melt production at Barringer Crater Historically, it has been widely discussed that the amount of documented impact melt at Barringer is much lower than

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Fig. 7. A: BSE image of a vesiculated melt glass bead containing irregularly-shaped globules of calcite. The calcite in the right hand side of the image displays feathery textures under plane polarized light. B: Higher resolution zoom in of another portion of the melt bead shown in A. Note the delicate textures developed by the calcite globules and the intermingling with silicate glass. C: BSE image of calcite globules embedded in silicate glass. The bright region to the left of the image is predominantly clinopyroxene. D–F: Element maps of Si, Ca, and Mg, respectively, of the area shown in C. Table 2 Representative WDS spot analyses of calcite-bearing melt beads. Sample # Analysis #

137 34

137 36

148 12

148 14

04-001b 4

04-001b 8

04-017a 20

04-017a 23

140 21

140 25

SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 SO3 NiO Cl Total

51.51 0.33 3.74 3.72 0.02 16.09 24.37 0.01 0.10 0.47 b.d. 0.18 0.02 100.56

52.40 0.46 4.14 2.96 0.03 16.26 22.92 0.02 0.23 0.45 0.01 0.22 b.d. 100.09

53.57 0.04 0.65 24.80 0.02 12.50 7.42 b.d. 0.04 0.12 0.01 0.29 b.d. 99.47

47.22 0.12 1.42 31.43 0.06 4.96 13.81 b.d. 0.04 0.26 0.01 0.27 0.01 99.62

54.30 0.26 4.70 0.74 0.03 16.43 22.10 0.04 0.73 0.37 0.02 n.a. 0.01 99.72

54.31 0.24 4.72 0.64 0.07 14.71 23.87 0.06 0.75 0.33 0.01 n.a. 0.03 99.73

50.59 0.12 2.05 29.67 n.a. 6.52 10.05 n.a. 0.44 0.28 0.28 0.05 n.a. 100.04

50.32 0.15 2.05 29.94 n.a. 6.60 10.08 n.a. 0.41 0.31 0.26 b.d. n.a. 100.11

45.16 0.21 3.70 19.79 b.d. 9.17 21.66 b.d. 0.10 0.38 b.d. b.d. b.d. 100.16

49.45 0.00 2.00 18.15 b.d. 10.10 20.51 b.d. b.d. b.d. b.d. b.d. b.d. 100.21

b.d. = below detection; n.a. = not analyzed.

theory and models would predict (Kieffer and Simonds, 1980; Shoemaker, 1963). This led Melosh and Collins (2005) to suggest the possibility of a low-velocity (∼12 km s−1 ) impact – resulting in a much smaller volume of melt. Subsequent numerical models suggested that this couldn’t be the case (Artemieva and Pierazzo, 2009) and that a high velocity scenario must have occurred. Thus, the reason(s) for the dearth of impact melt at Barringer Crater remains elusive. This study represents the first detailed analysis of coherent impact breccias from the ballistic ejecta blanket at Meteor Crater, which we discovered and reported on preliminary fashion in Osinski et al. (2006). Previously, the only reported breccias were from within the crater, where they were interpreted as being “fallout” in origin (Shoemaker, 1963). An important observation is the presence of projectile fragments and silicate impact glasses in these breccias, in addition to shock-melted sandstones and carbonates (see above). When taken together with ongoing investigation of drill cores (Hagerty and Gaither, 2014, 2013), this suggests that the abundance of silicate and carbonate melt-bearing breccias within the ejecta blanket at Barringer is much greater than

previously thought. If our estimates for the amount of finely dispersed melt beads present in surficial soils can be extended around the impact site, the melt estimates increase further. A simple explanation is, thus, that the estimated amount of melt present at Barringer Crater is artificially low because much remains hidden under the present-day erosional surface within the ejecta blanket. This would be most consistent with numerical models of the Barringer impact event (Artemieva and Pierazzo, 2011, 2009). The glasses within the impact breccias studied here are interesting as they differ considerably from the melt beads studied here and by Hörz et al. (2002). Melt beads are typically crystallized, texturally homogeneous and fall into two classes based on FeO content: 5–10 wt% (“low” FeO glasses) or 20–30 wt% FeO (“high” FeO glasses) (Hörz et al., 2002), which is consistent with our analyses of melt beads (Table 2). The melt beads are notably heterogeneous in terms of SiO2 content (43–65 wt%). In contrast, the glass clasts within the impact breccias from this study are typically pristine, glassy and uncrystallized, and texturally heterogeneous. Furthermore, they are different in composition, being more SiO2 poor (43–50 wt%) overall and FeO-rich (up to 36 wt% FeO) (Fig. 4). They

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are, however, compositionally more uniform than the melt beads. A further notable observation is the complete apparent lack of the “low” FeO glasses in the breccias in contrast to the melt beads. Synthesizing these observations, the simplest explanation is that the impact melts incorporated into the breccias were derived from a more constrained sub-region of the melt zone that also had a larger projectile contribution. When taken together with the fact that the impact breccias contain abundant projectile fragments and clasts of Moenkopi – the uppermost target rocks – this suggests that the source for the components within the impact breccias was very shallow and proximal to the projectile–target interface. This also has implications for reconstructing the melt zone at Barringer Crater. Hörz et al. (2002) faced a dilemma in trying to link melt beads to target formations. These authors suggested that the Fe-poor group of glasses were derived from the Moenkopi and upper Kaibab formations, with the Fe-rich glasses comprising 15–20% meteorite plus 50–70% Kaibab. They noted, however, that the melt beads are more SiO2 -rich than average Kaibab so that “the additional quartz may have been derived from Coconino or the upper Kaibab, implying melt depths >90 m or <30 m, respectively”. They also write “on balance, we favor a shallow melt zone at Meteor Crater, yet we cannot positively exclude that it extended into the Coconino Formation”. Our discussion above of a shallow origin of melts in the impact breccias, together with documentation of overall slightly more SiO2 -poor glasses together with evidence for melting of carbonates, likely from the Kaibab Formation, suggest that the shallow melt zone of <30 m depth at Barringer Crater is indeed more likely to be the case. This is consistent with the most recent numerical model of Barringer Crater (Artemieva and Pierazzo, 2011) who suggested that “only melts from an initial depth of less than 30–50 m are ejected and dispersed beyond the crater rim”. 5.3. Fate of the projectile The presence of meteorite fragments in the vicinity of what is now known as Barringer Crater has been known for well over a century (Foote, 1892) and was one of the major pieces of evidence that Barringer (1905) used to propose an impact origin for this site. In addition to fragments of what was later named the Canyon Diablo meteorite, various particles formed either via direct melting and/or condensation from vapor, have been documented (Kelly et al., 1974; Nininger, 1956). Historically, several workers have documented “impactite metallic particles” and “metallic spheroids”. The latter are found individually within soils up to several kilometres away from the crater. The particles we have studied here appear to correspond to descriptions of impactite metallic particles typically found within what we and Hörz et al. (2002) refer to as melt beads. One important observation from this study is the evidence for forcible injection of projectile-derived melt into target rocks prior to their excavation as clasts and subsequent deposition in ejecta (Figs. 5M–P). Recent numerical models have substantially increased our understanding of the fate of the projectile at Barringer Crater. These simulations indicate that for the all likely scenarios, a large proportion of the projectile (∼26% on average) is ejected in the solid state, with ∼45% partially to completely melted, and the remaining ∼29% partially vaporized (Artemieva and Pierazzo, 2011). These authors noted, however, that “the deficiency of solid projectile material is still an enigma”. It is currently not possible to estimate the current or original volume of the impact breccias we document here; however, if these breccias are abundant, as preliminary data from drill cores would suggest (Hagerty and Gaither, 2013), then we propose that they may represent an important reservoir for projectile material at Barringer Crater. Importantly, the Fe-Ni-rich

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projectile is preserved as solid fragments and as a major melt contributor to the impact glasses present in these impact breccias. One further outstanding question at Barringer Crater is why the impact melts are so rich in projectile material, both solid and incorporated into the melt zone. Fragments of projectile are common in craters <1.5 km in diameter and exceptionally rare for those larger (Goderis et al., 2012). In addition, the majority of impact melts possess << 1 wt% projectile material (Goderis et al., 2012), with very rare exceptions (e.g., Palme et al., 1978). A notable exception to both cases is the ∼70 km diameter Morokweng structure in South Africa where the projectile contribution to the impact melts is estimated at 5.7 wt% and a projectile fragment has been documented (Hart et al., 2002). This begs the question of why some craters possess much more projectile material than others. Given what we know of the impact cratering processes there appears to be no strong argument as to why we wouldn’t expect projectile material to be present in all impact craters (Melosh, 1989). To our knowledge, systematic modeling of the fate of projectile with increasing diameter have not been conducted; however, it is well known that the volume of impact melt relative to the size of the transient cavity increases with the size of the event – the so-called differential melt scaling effect (Grieve and Cintala, 1992). In terms of clast content, Grieve and Cintala (1992) postulated that the relative amount of clastic material derived from the transient cavity, and available for incorporation in the final impact melt deposits, decreases with increasing diameter such that impact melts become more clast-poor with increasing diameter. It is suggested that similarly, that the relative amount of projectile material decreases with increasing diameter such that impact melts become systematically poorer in projectile material as crater diameter increases. Thus, the lack of projectile material – both in solid and originally liquid form – in large craters is likely due to a combination of the dilution of any projectile signature due to differential melt scaling, the difficulty of identifying some projectile types and/or their ease of weathering (e.g., achrondrites), and/or the lack of preservation of suitable impact melt material to search for a projectile signature in many craters on Earth. 6. Summary and conclusions Studies going back to the 1960s have shown that when large asteroids or comets impact into crystalline targets, such as the Canadian Shield, large volumes of impact melt are generated that subsequently cool and crystallize to produce classic igneous structures (e.g., columnar jointing) and textures. For a long time, the widely held view was that impact melt was either lacking or was volumetrically very minor at craters formed in sedimentary rocks (Kieffer and Simonds, 1980). Beginning in 1999 with the discovery of limestone-derived melts at the Ries impact structure in Germany (Graup, 1999), a series of studies provided evidence that limestone and dolomite undergo melting during hypervelocity impact (reviewed in Osinski et al., 2008). The melting of carbonates has recently, however, been called into question again (Stöffler et al., 2013; Hörz et al., 2002). In this study, we investigated the well-known Barringer Crater in Arizona and show that despite over a century of study, there are still hidden secrets to be revealed. We have provided the first description of coherent impact breccias in the ejecta blanket of this structure. These unusual breccias contain dispersed fragments of the projectile as well as impact glasses that contain a very high proportion of projectile material, providing important new insights into the fate of the projectile during impact events. We also provide the first definitive evidence that carbonates underwent melting during the Barringer impact. The products are complex, however, and we propose that the melting of dolomite produces Ca–Mg-rich melts from which calcite is the dominant

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melt phase rather than dolomite. This is in keeping with studies of other craters in dolomite-bearing targets where calcite appears to be the dominant melt product. Finally, this work adds to the growing recognition that carbonates and other sedimentary rocks melt during hypervelocity impact and do not just decompose and/or devolatilized as was previously thought (e.g., Kieffer and Simonds, 1980). Caution is, therefore, urged when estimating the amount of climatically active gases released by impact events into such target rocks. Acknowledgements The primary author (GRO) is extremely grateful to the Barringer Family Fund for Meteorite Impact Research and the Natural Sciences and Engineering Research Council of Canada (NSERC) for funding this research. We thank F. Hörz, D. Kring, and H.J. Melosh for their detailed and constructive reviews of this manuscript. References Anders, D., Kegler, P., Buchner, E., Schmieder, M., 2011. Carbonate melt lithologies from the Steinheim impact crater (SW Germany). In: 42nd Lunar Planet. Sci. Conf. Abstract# 1997. Artemieva, N., Pierazzo, E., 2009. The Canyon Diablo impact event: projectile motion through the atmosphere. Meteorit. Planet. Sci. 44, 25–42. Artemieva, N., Pierazzo, E., 2011. The Canyon Diablo impact event: 2. Projectile fate and target melting upon impact. Meteorit. Planet. Sci. 46, 805–829. Barringer, D.M., 1905. Coon Mountain and its crater. Proc. Acad. Nat. Sci. Philadelphia 57, 861–886. Dence, M.R., 1971. Impact melts. J. Geophys. Res. 76, 5552–5565. Flemming, R.L., 2007. Micro X-ray diffraction (μXRD): a versatile technique for characterization of Earth and planetary materials. Can. J. Earth Sci. 44, 1333–1346. Foote, A.E., 1892. A new locality for meteoric iron with a preliminary notice of discovery of diamonds in the iron. Proc. Am. Assoc. Adv. Sci. 40, 279–283. Goderis, S., Paquay, F., Claeys, P., 2012. Projectile identification in terrestrial impact structures and ejecta material. In: Osinski, G.R., Pierazzo, E. (Eds.), Impact Cratering: Processes and Products. Wiley-Blackwell, Chichester, pp. 223–239. Graup, G., 1999. Carbonate–silicate liquid immiscibility upon impact melting: Ries Crater, Germany. Meteorit. Planet. Sci. 34, 425–438. Grieve, R.A.F., Cintala, M.J., 1992. An analysis of differential impact melt-crater scaling and implications for the terrestrial impact record. Meteoritics 27, 526–538. Grieve, R.A.F., Garvin, J.B., 1984. A geometric model for excavation and modification at terrestrial simple impact craters. J. Geophys. Res. 89, 11561–11572. Hagerty, J.J., Gaither, T.A., 2013. The USGS Meteor Crater sample collection: results and insights. In: 44th Lunar Planet. Sci. Conf. Abstract #2128. Hagerty, J.J., Gaither, T.A., 2014. Compositional contradictions recorded within impact-generated materials from Meteor Crater, Arizona: implications for crater formation. In: 45th Lunar Planet. Sci. Conf. Abstract #2397. Hart, R.J., Cloete, M.C., McDonald, I., Andreoli, M.C., 2002. Siderophile-rich inclusions from the Morokweng impact melt sheet, South Africa: possible fragments of a chondritic meteorite. Earth Planet. Sci. Lett. 198, 49–62. Hörz, F., Archer, P.D., Niles, P.B., Zolensky, M.E., Evans, M., 2015. Devolatilization or melting of carbonates at Meteor Crater, AZ? Meteorit. Planet. Sci. 50, 1050–1070. Hörz, F., Mittlefehldt, D.W., See, T.H., Galindo, C., 2002. Petrographic studies of the impact melts from Meteor Crater, Arizona, USA. Meteorit. Planet. Sci. 37, 501–531. Ivanov, B.A., Deutsch, A., 2002. The phase diagram of CaCO3 in relation to shock compression and decompression. Phys. Earth Planet. Inter. 129, 131–143. Jones, A.P., Claeys, P., Heuschkel, S., 2000. Impact melting of carbonates from the Chicxulub Crater. In: Gilmour, I., Koeberl, C. (Eds.), Impacts and the Early Earth. In: Lecture Notes in Earth Sciences, vol. 91. Springer-Verlag, Berlin, pp. 343–361. Kelly, W.R., Holdsworth, E., Moore, C.B., 1974. The chemical composition of metallic spheroids and metallic particles within impactite from Barringer Meteorite Crater, Arizona. Geochim. Cosmochim. Acta 38, 533–543. Kieffer, S.W., Simonds, C.H., 1980. The role of volatiles and lithology in the impact cratering process. Rev. Geophys. Space Phys. 18, 143–181. Kumar, P.S., Kring, D.A., 2008. Impact fracturing and structural modification of sedimentary rocks at Meteor Crater, Arizona. J. Geophys. Res. 113. http:// dx.doi.org/10.1029/2008JE003115.

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