Impact melting in the Cumberland Falls and Mayo Belwa aubrites

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1 Meteoritics & Planetary Science 45, Nr 2, (2010) doi: /j x Impact melting in the Cumberland Falls and Mayo Belwa aubrites Alan E. RUBIN * Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California , USA * aerubin@ucla.edu (Received 29 July 2009; revision accepted 18 October 2009) Abstract Six chondritic clasts in the Cumberland Falls polymict breccia were examined: four texturally resemble ordinary chondrites (OCs) and two are impact melt breccias containing shocked OC clasts adjacent to a melt matrix. The six chondritic clasts are probably remnants of a single OC projectile that was heterogeneously shocked when it collided with the Cumberland Falls host. Mayo Belwa is the first known aubrite impact melt breccia. It contains coarse enstatite grains exhibiting mosaic extinction; the enstatite grains are surrounded by a melt matrix composed of 3 16 lm-size euhedral and subhedral enstatite grains embedded in sodic plagioclase. Numerous vugs, ranging from a few micrometers to a few millimeters in size, constitute 5 vol% of the meteorite. They occur nearly exclusively within the Mayo Belwa matrix; literature data show that some vugs are lined with bundles of acicular grains of the amphibole fluor-richterite. This phase has been reported previously in only two other enstatite meteorites (Abee and St. Sauveur), both of which are EH-chondrite impact melt breccias. It seems likely that in Mayo Belwa, volatiles were vaporized during an impact event and formed bubbles in the melt. As the melt solidified, the bubbles became cavities; plagioclase and fluor-richterite crystallized at the margins of these cavities via reaction of the melt with the vapor. INTRODUCTION Aubrites (enstatite achondrites) are highly reduced differentiated asteroidal meteorites that have experienced a significant collisional history. Most aubrites are brecciated (e.g., Watters and Prinz 1979; Keil 1989; Keil et al. 1989): the majority (e.g., Aubres, Bishopville, Pen a Blanca Spring) are monomict fragmental breccias (Watters and Prinz 1979), a few (Bustee, Elephant Moraine (EET) 90033, Khor Temiki, Lewis Cliff (LEW) 87007, Pesyanoe, Y ) are regolith breccias containing solar-wind-implanted noble gases (e.g., Eberhardt et al. 1965; Lorenzetti et al. 2003), and at least two are polymict breccias (Cumberland Falls and Allan Hills (AHL) 78113; e.g., Neal and Lipschutz 1981; Verkouteren and Lipschutz 1983; Lipschutz et al. 1988). Despite aubrites having experienced numerous collisions, impact melted material is rare. Norton County has an impact-comminuted matrix and a few clasts that appear to be impact melt breccias (Okada et al. 1988), but additional examples of impact melted material among aubrites have been lacking. I report here such material in two aubrites Cumberland Falls and Mayo Belwa. Cumberland Falls is a polymict breccia with a suite of chondritic inclusions (e.g., Binns 1969; Neal and Lipschutz 1981; Lipschutz et al. 1988) that are probably of ordinary-chondrite (OC) parentage (Kallemeyn and Wasson 1985; Wasson et al. 1993). As shown below, some of these clasts are impact melt breccias; they were presumably shocked and melted upon accretion to the Cumberland Falls host. I also characterize Mayo Belwa as the first known aubrite impact melt breccia. ANALYTICAL PROCEDURES Thin sections (Table 1) of Cumberland Falls and Mayo Belwa were examined microscopically in transmitted and reflected light with the Olympus BX60 petrographic microscope. Mineral compositions were determined with the JEOL 733 Superprobe electron 265 Ó The Meteoritical Society, 2010.

2 266 A. E. Rubin Table 1. Sections and petrologic characteristics of Cumberland Falls and Mayo Belwa. Meteorite Thin section numbers Recovery Shock stage Characteristics Cumberland Falls Fall Polymict fragmental breccia Host AMNH S5 Host UCLA 574 S5 Clast 1 UCLA 512 S3 Ordinary chondrite like Clast 2 UCLA 567 S2 Ordinary chondrite like Clast 3 UCLA 575 S2 Ordinary chondrite like Clast 4 UCLA 589 OC-like impact melt breccia Clast 5 UCLA 602 OC-like impact melt breccia Clast 6 USNM S3 Ordinary chondrite like Mayo Belwa AMNH Fall Impact melt breccia USNM AMNH sections from the American Museum of Natural History; USNM sections from the Smithsonian Institution s National Museum of Natural History; remaining sections from UCLA. microprobe at UCLA using wavelength-dispersive methods, natural and synthetic standards, a sample current of 15 na, an accelerating voltage of 15 kev, 20 s counting times per element, ZAF corrections, a focused beam (1 lm diameter) for the analyses of olivine, pyroxene and sulfide, and a beam 2 lm in diameter for plagioclase. Backscattered electron (BSE) images were made with the electron microprobe. Grain sizes were measured microscopically using a calibrated reticle and, for BSE images, with the automated scale bar. The modal abundances of matrix and pyroxene grains in Mayo Belwa thin section USNM were determined using an automated point counter. RESULTS Cumberland Falls The Cumberland Falls host contains enstatite grains and grain fragments ranging in size from 20 lm to 9 mm. Many of the grains have lamellae of clinoenstatite with polysynthetic twins (Fig. 1). Some of the enstatite grains have been faulted with concomitant displacement of the lamellae. About two-thirds of the pyroxene grains exhibit strong mosaic extinction, indicative of shock-stage S5 (using the criteria of Sto ffler et al and Rubin et al. 1997). Section AMNH includes a single lm size grain of maskelynitized plagioclase. A few troilite grains in the Cumberland Falls host are polycrystalline. In addition to coarse enstatite grains, there are clasts of comminuted material containing lm size enstatite fragments and multi-millimeter-size breccia clasts that include lm size grains of troilite. Among the six chondritic clasts that I studied in Cumberland Falls (Table 1), there are two types: those that texturally resemble ordinary chondrites (clasts 1, 2, Fig. 1. Aubrite host of the Cumberland Falls polymict breccia containing shocked enstatite grains of different sizes. Many of the grains have lamellae of clinoenstatite with polysynthetic twins. Some grains are faulted with displaced lamellae. Crossed nicols. 3, and 6) and those (clasts 4 and 5) that are impact melt breccias that appear to have been produced from OClike materials (see below). Chondrule textural types in the chondritic clasts include barred olivine (BO), porphyritic olivine (PO), porphyritic pyroxene (PP), porphyritic olivine-pyroxene (POP), radial pyroxene (RP), and cryptocrystalline (C). The boundaries between the chondritic clasts and the Cumberland Falls host are sharp (Figs. 2a and 2b). Clasts 1, 2, 3, and 6 (Table 1) contain chondrules lm in apparent diameter. The modal abundance of chondrules and chondrule fragments is vol%. The clasts exhibit extensive silicate darkening, i.e., they have a dark appearance when viewed microscopically in transmitted light due to the

3 Impact melting in Cumberland Falls and Mayo Belwa 267 Fig. 3. Thin veins of metallic Fe-Ni (white) and troilite (light gray) transect the silicate grains (dark gray) in clast 6. Coarse metal and sulfide grains occur at bottom left. BSE image. Fig. 2. Chondritic clast 6 in Cumberland Falls. a) Clast 6 (right) and coarse enstatite grains from aubrite host (white, left). The clast contains numerous PP, RP, BO, PO, and POP chondrules ranging from 600 to 2000 lm in apparent diameter. The clast exhibits extensive silicate darkening due to shock dispersion of melted opaque phases. Transmitted light. b) Shock veins and dispersed metal and sulfide occur in the clast (left); far fewer opaque phases occur in the coarse enstatite grains in the aubrite host (right). Backscattered electron (BSE) image. dispersion in the olivine and pyroxene grains of numerous tiny blebs of metallic Fe-Ni and sulfide. Also present are small veins of troilite and metallic Fe-Ni (Fig. 3). Some metal grains include fragments of olivine and pyroxene grains. Fine-grained intergrowths consisting of 2 lm size adjacent patches of metal and troilite occur at the boundaries between coarse ( lm size) grains of metal and troilite that also contain a few lm size grains of daubre elite. The fine-grained metal-sulfide intergrowths appear to have been quenched (cf. Scott 1982). Also present in clast 6 are fine-grained intergrowths of troilite, metal, and schreibersite. Fig. 4. Adjacent olivine grains in clast 1 (dark gray) with crystallographically oriented low-ni metallic Fe blebs (white) formed by reduction of FeO to Fe 0. BSE image. Many of the olivine grains in the clasts contain numerous small grains of low-ni metallic Fe, in some cases apparently arrayed in crystallographically oriented directions (Fig. 4). Olivine grains are very magnesian (Fa ; Table 2). pyroxene grains in the clasts range from Fs2-21 in composition (Table 2). A compositional profile made across a typical low-ca pyroxene grain in clast 1 indicates that the grain is zoned edges are more magnesian than grain cores (Fig. 5). One grain of augite in clast 2 was analyzed (Fs2.5Wo38.4; Table 2). In clast 6, pyroxene grains in the PP chondrules have clinoenstatite lamellae with polysynthetic twins.

4 268 A. E. Rubin Table 2. Selected silicate mineral compositions (wt%) in Cumberland Falls clasts. Clast Mineral analysis # Olivine Augite Olivine Olivine 20 SiO TiO 2 < < < <0.04 < <0.04 <0.04 <0.04 <0.04 <0.04 Al 2 O 3 < < < <0.04 Cr2O3 < < < < <0.04 FeO MnO MgO CaO < < <0.04 Na 2 O 0.06 <0.04 < < <0.04 <0.04 <0.04 < <0.04 <0.04 <0.04 <0.04 K 2 O <0.04 <0.04 <0.04 <0.04 <0.04 < <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 Total Endmember Fs19.6 Fs3.8 Fa0.74 Fs15.0 Fs20.9 Fs2.0 Fs2.5 Fs19.4 Fs15.7 Fs2.0 Fs14.9 Fs5.8 Fs10.2 Fa0.30 Fs18.7 Fa0.52 Wo0.41 Wo2.3 Wo0.43 Wo1.3 Wo4.6 Wo38.4 Wo0.26 Wo0.31 Wo0.28 Wo0.88 Wo0.23 Wo0.21 Wo0.32

5 Impact melting in Cumberland Falls and Mayo Belwa 269 More than 25% of the olivine grains in clasts 1 and 6 have undulose extinction and planar fractures, characteristic of shock-stage S3 (Sto ffler et al. 1991); a few grains display weak mosaic extinction. Most of the olivine grains in clast 2 and 3 have undulose extinction, rare grains have weak mosaic extinction, and a few grains have planar fractures. Both clasts 2 and 3 are shock-stage S2. Troilite grains in the clasts are close to pure FeS in composition; the only other element appreciably above the detection limit is Cr (at 0.2 wt%) (Table 3). These grains are similar to those in ordinary chondrites and differ from those in the Cumberland Falls aubritic host which average (in wt%): 0.1% Mg, 0.1% Ca, 0.1% Si, 0.2% Mn, 0.5% Ti and 0.6% Cr (Table 3). Clast 4 is an impact melt breccia. It contains 15 vol% chondrule fragments and olivine and pyroxene grains ranging in size from 10 to 100 lm. The thin section also contains a single, mostly intact, millimeter-size POP chondrule. The largest olivine phenocryst in this chondrule displays strong mosaic extinction. The clast exhibits extensive silicate darkening and has polycrystalline troilite grains and fine-grained metal-sulfide intergrowths. The modally abundant matrix (85 vol%) has been impact melted; it consists of 1 8 lm size angular olivine and pyroxene grains with some interstitial microcrystalline feldspathic material, possibly devitrified melt. This interstitial material includes 5 10 vol% tiny (0.2 lm size) rounded blebs of metal and sulfide; also present are coarser (up to 15 lm), more angular grains of these phases. Relict olivine grains embedded in the matrix have undulose to mosaic extinction and planar fractures. pyroxene grains in the clast range from Fs1 15 (Table 2). Clast 5 is another impact melt breccia very similar to clast 4. It has a modally abundant matrix (90 vol%) consisting of small olivine and pyroxene grains with minor interstitial microcrystalline feldspathic material (Fig. 6). Metal- and sulfide-rich veins and metal-sulfide spherules (some with finegrained quench textures) also occur. In addition, vol% chondrule fragments and individual olivine and pyroxene grains are present in the matrix. At the edge of the thin section, the matrix abuts a large unmelted chondritic clast that closely resembles clasts 1, 2, 3, and 6 and contains lm diameter POP chondrules (Fig. 6) that constitute 85 vol% of this portion of the clast. This unmelted chondritic clast portion exhibits extensive silicate darkening and contains fine-grained metal-sulfide intergrowths, metaland sulfide-rich veins, and grains of polycrystalline troilite. Many of the olivine grains in this unmelted Fig. 5. Compositional zoning profile across a typical low-ca pyroxene grain in clast 1 showing relatively ferroan cores and magnesian edges indicative of reduction of FeO. Dotted lines connect points where data have been excluded because of electron-beam overlap on small metallic Fe grains. Table 3. Mean troilite compositions (wt%). Ordinary chondrite a Cumberland Falls clast 1 Cumberland Falls clast 6 No. grains clast portion have undulose extinction and planar fractures; a few grains display weak mosaic extinction. pyroxene grains in the unmelted portion of the clast range from Fs6-10; olivine is very magnesian (Fa ; Table 2). Mayo Belwa Cumberland Falls host a Ca n.d. <0.04 < Fe Ti n.d. <0.04 < Mg n.d < Mn n.d < Cr < S Si n.d < Total n.d., not determined. a Ordinary chondrite data from LL6 MIL (Rubin 2002). Cumberland Falls host data from Watters and Prinz (1979). Mayo Belwa is a brecciated aubrite containing 50 lm to 9 mm size shock-darkened enstatite grains (50 vol%; points) surrounded by a darkcolored (in transmitted light), clast-laden, plagioclasebearing matrix (50 vol%; points) (Fig. 7a). Some of the large enstatite grains are polycrystalline. More than 80% of the enstatite grains exhibit strong mosaic extinction; a minority have weak mosaic extinction and contain planar fractures. A few grains appear to be clinoenstatite with polysynthetic twins.

6 270 A. E. Rubin constitute 5 vol% of Mayo Belwa (A. W. R. Bevan, personal communication). In section USNM , one 4 mm size vug is lined with (40 100) ( ) lm size bundles of laths of sodic plagioclase (Ab92Or4; Table 4). Individual laths are 4 10 lm thick. Some of the plagioclase bundles are straight; others are bent (Fig. 9). A few bundles are aligned perpendicularly to the vug wall; others are oriented at more oblique angles. Cumberland Falls Clasts DISCUSSION Fig. 6. Impact melted chondritic material. Clast 5 in Cumberland Falls is itself an impact melt breccia. It contains a large, unmelted chondritic clast (right) with lm diameter POP chondrules. The black streaks in the clast at right are metal- and troilite-rich veins. Abutting the unmelted chondritic clast is the impact melted portion of clast 5 (left) which consists of olivine and pyroxene grain fragments embedded in microcrystalline material that probably represents devitrified melt. The black circular object at left center is a metal-sulfide spherule with regions of a fine-grained intergrowth of these phases. Transmitted light. Also present are grains of diopside and forsterite; Graham et al. (1977) reported millimeter-size rounded and sinuous aggregates of plagioclase. Many of the enstatite grains are transected by lm thick veins of the plagioclase-bearing matrix (Fig. 7b). Some of the veins include a few volumepercent of small (1 2 lm) angular grains of metal and troilite. The matrix is composed of stoichiometric sodic plagioclase (Ab93Or4; Table 4) surrounding 3 16 lm size euhedral and subhedral grains of enstatite (Fs0.05Wo0.8; Table 4) (Fig. 7c), olivine grains exhibiting silicate darkening, and small grains of kamacite and schreibersite (Fig. 7d). It appears that the matrix is a solidified melt; in some cases, small enstatite grains in the matrix have nucleated on the surfaces of coarse, partly resorbed enstatite grains (Figs. 7c and 7d). In some regions, small (2 5 lm size) patches of plagioclase are pincer-shaped, similar to those in the Spade H-chondrite impact melt breccia (cf. Fig. 4 of Rubin and Jones 2003). Some of the pincer-shaped plagioclase patches contain 2 lm size angular grains of metal and sulfide. Mayo Belwa is unique among aubrites in possessing numerous vugs ranging in diameter from 2 lm to 4 mm (Figs. 7d and 8); Graham et al. (1977) reported that some vugs reach 1 cm in size. The vugs occur nearly exclusively in the matrix, and altogether All of the chondritic clasts in Cumberland Falls appear to be related. It is statistically unlikely that several different chondritic projectiles unrelated to aubrites would be incorporated into Cumberland Falls. Therefore, it seems reasonable to infer that the chondritic clasts are all remnants of the same projectile that impacted the Cumberland Falls region of the aubrite parent asteroid. If this is correct, then data gathered on one of the clasts can be inferred to be representative of the entire set of chondritic clasts. The clasts appear to have been derived from an OC rather than from an enstatite chondrite or carbonaceous chondrite. The inferred OC parentage of the chondritic clasts is supported by several properties: (1) an LLchondrite-like bulk chemical composition (Kallemeyn and Wasson 1985), (2) O-isotopic compositions (d 18 O = to +5.8&; d 17 O = +3.5 to +4.00&; D 17 O = to +0.99&; data from R. N. Clayton reported in Verkouteren and Lipschutz 1983 and from Clayton and Mayeda 1978) that are in the OC range and similar to those of some LL3 chondrites (d 18 O= to +6.09&; d 17 O=+3.75to+4.24&; D 17 O= to +1.24&; Clayton et al. 1991), (3) chondrule sizes that are in the range of LL chondrites (which have a mean diameter of 570 lm; Nelson and Rubin 2002), (4) the occurrence of BO chondrules (which are common in OC but rare in enstatite chondrites; e.g., Rubin 2000), (5) the occurrence of RP chondrules (which are appreciably more abundant in OC than in carbonaceous chondrites; e.g., Rubin 2000), (6) an OClike chondrule modal abundance (70 vol%; e.g., Rubin 2000), and (7) core compositions of low-ca pyroxene (Fs20 21; Table 2; Fig. 5) that are within or close to the ranges of those in equilibrated L and LL chondrites (Fs19 22 and Fs22 26, respectively; fig. 181 of Brearley and Jones 1998) and are much more ferroan than typical grains in CM, CO, CR, CV, EH, and EL chondrites. If these Cumberland Falls clasts were derived from OC then they must have suffered reduction. This

7 Impact melting in Cumberland Falls and Mayo Belwa 271 Fig. 7. Mayo Belwa enstatite and melt matrix. a) Large enstatite grain fragments surrounded by a dark-colored, impact melt matrix. b) The coarse enstatite grains are typically fragmental; many are transected by the melt (dark veins in the grains). c) Outer portion of a large enstatite grain at right. The grain has a fractured interior and a smooth edge. Portions of the edge of the grain appear to have been partly resorbed by the melt matrix. Thin veins of the melt penetrate the large orthoenstatite grain (center and bottom center). The matrix consists mainly of 3 16-lm-size euhedral and subhedral enstatite grains surrounded by sodic plagioclase. Also present are small grains of kamacite and schreibersite. Black areas are plucked regions. opx = orthopyroxene. d) Portion of melt matrix consisting of small, quasi-equant enstatite grains (light gray) and metallic Fe-Ni and schreibersite (white) surrounded by patches of plagioclase (very light gray). Also present are numerous vugs that are a few micrometers in size. At bottom center is a coarse, partly resorbed orthopyroxene grain. a, b in transmitted light; c, d in backscattered electrons. process probably involved heating, which most likely occurred when the clasts precursor projectile accreted to the aubrite parent body. During heating, FeO in the clasts olivine grains was reduced, forming low-ni blebs of metallic Fe (Fig. 4) surrounded by forsterite. Through reduction, the olivine grains achieved magnesian compositions (Fa ) that approached equilibrium with the aubrite host (which has an olivine composition of Fa0.21; Table 6 of Watters and Prinz 1979). Because diffusion is more sluggish in low-ca pyroxene than in olivine (e.g., Freer 1981; Chakraborty 1997), the pyroxene grains did not completely equilibrate and retained more ferroan compositions. Nevertheless, compositional zoning profiles across low- Ca pyroxene grains show that they also suffered reduction their margins are more magnesian than their cores (Fig. 5). The occurrence of daubréelite lamellae within some troilite grains in clasts 1 and 6 seems inconsistent with

8 272 A. E. Rubin Table 4. Mean mineral compositions (wt%) in Mayo Belwa. Plagioclase Enstatite Plagioclase in melt matrix blades near vug No. grains SiO ± TiO 2 <0.04 <0.04 <0.04 Al 2 O ± Cr 2 O 3 <0.04 <0.04 <0.04 FeO 0.04 <0.04 <0.04 MnO <0.04 <0.04 <0.04 MgO ± 0.21 <0.04 CaO ± Na 2 O < ± K 2 O < ± Total Endmember Fs 0.05 Ab 92.7 Ab 91.7 Wo 0.79 Or 3.8 Or 4.3 Fig. 8. Mayo Belwa whole rock showing numerous millimeterand sub millimeter-size vugs (arrows). Reflected light. Image is from G. Benedix of the Natural History Museum, London and is used with permission. an OC parentage for these clasts because daubre elite is a very rare phase in OC. Other chondritic clasts in Cumberland Falls have additional phases (i.e., oldhamite and ferroan alabandite) (Kallemeyn and Wasson 1985) that are otherwise largely (or entirely) restricted to enstatite meteorites (e.g., Rubin 1997a). Because the Cumberland Falls clasts are not enstatite chondrites and were very likely derived from an OC projectile, the occurrence of daubre elite, oldhamite, and ferroan alabandite could be due either to (1) diffusive exchange between troilite in the clasts and sulfides in the aubrite host during annealing (e.g., Wasson et al. 1993), or (2) to brecciation involving incorporation of sulfide-bearing rock fragments from the aubrite host. However, because the daubréelite-bearing sulfide grains Fig. 9. Bundles of laths of plagioclase (plag laths) lining the wall of a vug (bottom) in Mayo Belwa. are not surrounded by coarse enstatite grains, the second possibility seems unlikely. The OC projectile was shocked and some fragments of it were shock melted upon collision with the aubrite parent body. Heterogeneous shock effects are common among OC impact melt breccias, e.g., Rose City (Mason and Wiik 1966), Shaw (Taylor et al. 1979) and Cat Mountain (Kring et al. 1996). Some portions of these rocks contain significant amounts of silicate melt; other portions are shocked, but unmelted. Additional examples of projectiles that were shocked when they collided with a foreign parent asteroid include CM clasts in HED breccias (Zolensky et al. 1996) and in OC regolith breccias (e.g., Rubin and Bottke 2009). There are also reports of two devolatilized chondritic fragments found on the Moon: Bench Crater (a shocked CM1 chondrite recovered from the coarse fines fraction of an Apollo 12 soil sample; McSween 1976; Zolensky et al. 1996) and Hadley Rille (a partly impact melted EH chondrite with an agglutinate-like rim recovered from the 1 2 mm size fraction of an Apollo 15 soil sample; Haggerty 1972; Rubin 1997b). The overall shock stage of the Cumberland Falls whole rock can be no higher than the lowest shock stages of its components. Hence, the whole-rock is shock stage S2. The fact that the OC-related clasts in Cumberland Falls have shock stages that are as low as S2 and that the aubrite host is S5 indicates that the OC projectile accreted to the aubrite parent body after the host had experienced its maximum shock level. Mayo Belwa as an Impact Melt Breccia In view of the significant shock experienced by Mayo Belwa and the presence of an intergranular melt

9 Impact melting in Cumberland Falls and Mayo Belwa 273 matrix (containing euhedral silicate grains, small opaque grains and numerous vugs), it seems likely that the matrix is an impact melt and that Mayo Belwa is an impact melt breccia. This is the first reported example of an impact melt breccia among aubrite whole rocks. Fluor-Richterite in Mayo Belwa Bevan et al. (1977) and Graham et al. (1977) reported that some vugs in Mayo Belwa are lined with bundles of acicular grains of the amphibole fluor-richterite [Na 2 Ca(Mg,Fe) 5 Si 8 O 22 F 2 ]; some bundles are up to 3 mm long and individual fluor-richterite blades are up to 1 mm long. I did not encounter this phase in the two thin sections of Mayo Belwa that I examined. Other vugs in Mayo Belwa are lined with granular enstatite, acicular diopside, minor cristobalite, and bundles of laths of sodic plagioclase (Bevan et al. 1977; Graham et al. 1977; this study). Lin and Kimura (1998), citing their own unpublished data, mentioned that fluor-richterite occurred in some aubrites, presumably in addition to Mayo Belwa. Fluor-richterite has been reported in only two other enstatite meteorites; both are EH-chondrite impact melt breccias: Abee (Rubin and Keil 1983; Rubin and Scott 1997; Rubin 2008) and St. Sauveur (Keil 2007; Rubin 2008). In Abee, fluor-richterite occurs as rare 3.5-mmlong radiating acicular grains bundled in clusters in association with enstatite, troilite and keilite (Douglas and Plant 1969; Olsen et al. 1973). In St. Sauveur fluor-richterite occurs as lm size subhedral grains (Rubin 1983). The morphologies of these grains and their apparently exclusive occurrence among enstatite chondrites within EH-chondrite impact melt breccias imply that fluor-richterite crystallized in these rocks from the impact melt (Rubin 2008). Another F-rich phase, fluorphlogopite [KMg 3 (Si 3 Al) O 10 F 2 ], occurs in the EH impact melt rock Y as rare subhedral, lm size grains in association with enstatite, silica, and albite (Lin and Kimura 1998). This phase also crystallized from the impact melt. It seems likely that after impact melting, most of the F in Abee, St. Sauveur, and Y that was not driven off was scavenged by the crystallizing grains of fluor-richterite and fluorphlogopite. The retention of F in impact melted enstatite meteorites is consistent with a high original abundance of F. Enstatite chondrites are particularly rich in F: e.g., F in EH chondrites has an average Mg-normalized abundance ratio of 3.40 CI (Rubin and Choi 2009). The F in Abee, St. Sauveur and Y may have initially condensed as simple metal halides that were incorporated into enstatite-chondrite precursor materials (Rubin and Choi 2009). The exclusive occurrence of fluor-richterite grains in Mayo Belwa at the sides of vugs strongly suggests that a vapor phase was necessary for their formation. I thus infer that in Mayo Belwa, volatiles were vaporized during the impact event and formed bubbles in the melt (Fig. 8). As the melt solidified, the bubbles became cavities; relatively K-rich plagioclase and fluor-richterite crystallized at the margins of these cavities, presumably via reaction of the melt with the vapor. (The finegrained plagioclase in Mayo Belwa contains higher concentrations of K 2 O than plagioclase in other aubrites [0.84 wt% versus wt%; Watters and Prinz 1979].) The bundles of plagioclase laths lining one of the cavities (see above) supports this scenario. The flour-richterite may have reacted with the Na-rich plagioclase melt that lined the vugs (Bevan et al. 1977). This is consistent with the high Na 2 O content (7.2 wt%) of fluor-richterite in Mayo Belwa (Bevan et al. 1977). Commonality of Brecciation among Aubrites There are 64 aubrites and anomalous aubrites currently listed in the on-line Meteoritical Bulletin Database (MBD). One of these (NWA 2828) is certainly not an aubrite; it is a peculiar enstatite chondrite (unpublished data of A. Irving and T. Bunch; my petrographic observations) and will not be considered here. After taking probable pairings of the remaining 63 aubrites and anomalous aubrites into account, based on their on-line descriptions, I conclude that there are 27 unique aubrites listed in the MBD, nine of which appear to be unbrecciated (including Shallowater and Mount Egerton; Keil et al. 1989; McCall 1965; Watters and Prinz 1980) and two of which are only poorly characterized (NWA 5419 and Yamato ). Shallowater is probably from a different parent asteroid than the majority of aubrites (Keil et al. 1989) and is not included in the present statistical count. Thus, the proportion of brecciated aubrites is approximately 69% (18 26). This number greatly exceeds the proportion of breccias among H and L chondrites (25% and 10%, respectively) and is similar to that of LL chondrites (62%) (Binns 1967). At small heliocentric distances (where the enstatite meteorites probably formed; e.g., Wasson 1988), orbital periods are shorter and orbital velocities are higher. This results in higher collision probabilities and higher average collision velocities (e.g., O pik 1951; Bottke et al. 2006), accounting for the pervasive brecciation among aubrites and the occurrence in them of impact melted materials. Acknowledgments I thank the curators at the American Museum of Natural History, the Smithsonian Institution, and the NASA Johnson Space Center for

10 274 A. E. Rubin the loan of thin sections. Reviews by A. El Goresy, T. J. McCoy, Y. Lin, a couple of anonymous referees, and associate editor C. Floss on different versions of this manuscript were helpful in making revisions. I am grateful to G. K. Benedix of the Natural History Museum, London for an image of Mayo Belwa, to A. W. R. Bevan of the Western Australian Museum, Perth for his observations of Mayo Belwa, and to W. F. Bottke and T. D. Swindle for discussions. This work was supported by NASA Cosmochemistry Grant NNG06GF95G (A. E. Rubin). Editorial Handling Dr. Christine Floss REFERENCES Bevan A. W. R., Bevan J. C., and Francis J. G Amphibole in the Mayo Belwa meteorite: First occurrence in an enstatite achondrite. Mineralogical Magazine 41: Binns R. A Structure and evolution of noncarbonaceous chondrites. Earth and Planetary Science Letters 2: Binns R. A A chondritic inclusion of unique type in the Cumberland Falls meteorite. In Meteorite research, edited by Millman P. M. 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