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& Planetary Science 45, Nr 2, 265–275 (2010) doi: 10.1111/j.1945-5100.2010.01022.x

Impact melting in the Cumberland Falls and Mayo Belwa

Alan E. RUBIN*

Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095–1567, USA *E-mail: [email protected] (Received 29 July 2009; revision accepted 18 October 2009)

Abstract–Six chondritic clasts in the Cumberland Falls polymict were examined: four texturally resemble ordinary (OCs) and two are impact melt 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 impact melt breccia. It contains coarse 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 . 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 ( and St. Sauveur), both of which are EH- impact melt breccias. It seems likely that in Mayo Belwa, were vaporized during an 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 et al. 1988), but additional examples of impact melted material among aubrites have been lacking. I report Aubrites (enstatite ) are highly reduced here such material in two aubrites—Cumberland Falls differentiated asteroidal meteorites that have and Mayo Belwa. Cumberland Falls is a polymict experienced a significant collisional history. Most breccia with a suite of chondritic inclusions (e.g., Binns aubrites are brecciated (e.g., Watters and Prinz 1979; 1969; Neal and Lipschutz 1981; Lipschutz et al. 1988) Keil 1989; Keil et al. 1989): the majority (e.g., Aubres, that are probably of ordinary-chondrite (OC) parentage Bishopville, Pen˜a Blanca Spring) are monomict (Kallemeyn and Wasson 1985; Wasson et al. 1993). As fragmental breccias (Watters and Prinz 1979), a few shown below, some of these clasts are impact melt (Bustee, Elephant Moraine (EET) 90033, Khor Temiki, breccias; they were presumably shocked and melted Lewis Cliff (LEW) 87007, Pesyanoe, Y 793592) are upon accretion to the Cumberland Falls host. I also regolith breccias containing solar-wind-implanted noble characterize Mayo Belwa as the first known aubrite gases (e.g., Eberhardt et al. 1965; Lorenzetti et al. 2003), impact melt breccia. and at least two are polymict breccias (Cumberland Falls and Allan Hills (AHL) 78113; e.g., Neal and ANALYTICAL PROCEDURES Lipschutz 1981; Verkouteren and Lipschutz 1983; Lipschutz et al. 1988). Thin sections (Table 1) of Cumberland Falls and Despite aubrites having experienced numerous Mayo Belwa were examined microscopically in collisions, impact melted material is rare. Norton transmitted and reflected light with the Olympus BX60 County has an impact-comminuted matrix and a few petrographic microscope. Mineral compositions were clasts that appear to be impact melt breccias (Okada determined with the JEOL 733 Superprobe electron

265 The , 2010. 266 A. E. Rubin

Table 1. Sections and petrologic characteristics of Cumberland Falls and Mayo Belwa. Thin section Shock Meteorite numbers Recovery stage Characteristics Cumberland Falls Fall Polymict fragmental breccia Host AMNH 664-2 S5 Host UCLA 574 S5 Clast 1 UCLA 512 S3 –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 604-11 S3 Ordinary chondrite–like Mayo Belwa AMNH 4465-1 Fall Impact melt breccia USNM 5873-3 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 lmin 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 5873-3 were determined using an automated point counter.

RESULTS

Cumberland Falls 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 The Cumberland Falls host contains enstatite grains twins. Some grains are faulted with displaced lamellae. and grain fragments ranging in size from 20 lmto Crossed nicols. 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 3, and 6) and those (clasts 4 and 5) that are impact melt displacement of the lamellae. About two-thirds of the breccias that appear to have been produced from OC- pyroxene grains exhibit strong mosaic extinction, like materials (see below). textural types in indicative of shock-stage S5 (using the criteria of the chondritic clasts include barred olivine (BO), Sto¨ ffler et al. 1991 and Rubin et al. 1997). Section porphyritic olivine (PO), porphyritic pyroxene (PP), AMNH 664-2 includes a single 100 · 180 lm size grain porphyritic olivine-pyroxene (POP), radial pyroxene of maskelynitized plagioclase. A few grains in (RP), and cryptocrystalline (C). The boundaries between the Cumberland Falls host are polycrystalline. In the chondritic clasts and the Cumberland Falls host are addition to coarse enstatite grains, there are clasts sharp (Figs. 2a and 2b). of comminuted material containing 10–250 lm size Clasts 1, 2, 3, and 6 (Table 1) contain enstatite fragments and multi-millimeter-size breccia 500–2000 lm in apparent diameter. The modal clasts that include 10–15 lm size grains of troilite. abundance of chondrules and chondrule fragments is Among the six chondritic clasts that I studied in 70–75 vol%. The clasts exhibit extensive silicate Cumberland Falls (Table 1), there are two types: those darkening, i.e., they have a dark appearance when that texturally resemble ordinary chondrites (clasts 1, 2, viewed microscopically in transmitted light due to the 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 Fig. 4. Adjacent olivine grains in clast 1 (dark gray) with clast (left); far fewer opaque phases occur in the coarse crystallographically oriented low-Ni metallic Fe blebs (white) enstatite grains in the aubrite host (right). Backscattered formed by reduction of FeO to Fe0. BSE image. electron (BSE) image. dispersion in the olivine and pyroxene grains of Many of the olivine grains in the clasts contain numerous tiny blebs of metallic Fe-Ni and sulfide. Also numerous small grains of low-Ni metallic Fe, in some present are small veins of troilite and metallic Fe-Ni cases apparently arrayed in crystallographically oriented (Fig. 3). Some metal grains include fragments of olivine directions (Fig. 4). Olivine grains are very magnesian and pyroxene grains. Fine-grained intergrowths (Fa0.4–0.7; Table 2). consisting of 2 lm size adjacent patches of metal and Low-Ca pyroxene grains in the clasts range from troilite occur at the boundaries between coarse (100– Fs2-21 in composition (Table 2). A compositional 400 lm size) grains of metal and troilite that also profile made across a typical low-Ca pyroxene grain in contain a few 30–100 lm size grains of daubre´elite. The clast 1 indicates that the grain is zoned—edges are more fine-grained metal-sulfide intergrowths appear to have magnesian than grain cores (Fig. 5). One grain of augite been quenched (cf. Scott 1982). Also present in clast 6 in clast 2 was analyzed (Fs2.5Wo38.4; Table 2). In clast are fine-grained intergrowths of troilite, metal, and 6, pyroxene grains in the PP chondrules have . clinoenstatite lamellae with polysynthetic twins. 6 .E Rubin E. A. 268

Table 2. Selected compositions (wt%) in Cumberland Falls clasts. Clast 1 1 2 2 2 2 2 3 3 4 4 5 5 5 6 6 Low-Ca Low-Ca Low-Ca Low-Ca Low-Ca Low-Ca Low-Ca Low-Ca Low-Ca Low-Ca Low-Ca Low-Ca Mineral pyx pyx Olivine pyx pyx pyx Augite pyx pyx pyx pyx pyx pyx Olivine pyx Olivine analysis # 12 14 25 23 26 27 28 31 32 41 39 44 45 46 19 20

SiO2 56.0 57.0 42.5 56.2 55.3 58.1 53.5 56.6 57.0 59.7 56.9 59.0 57.9 42.8 56.0 42.2 TiO2 <0.04 0.29 <0.04 0.06 <0.04 0.11 1.2 0.05 <0.04 <0.04 0.09 <0.04 <0.04 <0.04 <0.04 <0.04 Al2O3 <0.04 0.60 <0.04 0.23 0.46 0.95 3.0 0.11 0.09 0.05 0.38 0.12 0.18 <0.04 0.08 <0.04 Cr2O3 <0.04 0.42 0.05 <0.04 0.36 1.2 1.3 <0.04 0.05 0.14 0.44 0.15 0.18 <0.04 0.09 <0.04 FeO 13.1 2.5 0.76 10.3 14.3 1.3 1.6 13.2 10.9 1.5 10.3 4.2 7.2 0.31 12.9 0.54 MnO 0.50 0.47 0.11 0.37 0.50 0.43 0.40 0.40 0.29 0.42 0.13 0.33 0.37 0.17 0.40 0.14 MgO 29.9 35.6 57.7 32.5 30.0 36.1 20.8 30.9 32.6 39.0 32.5 37.5 35.4 58.5 31.5 57.6 CaO 0.21 1.2 <0.04 0.23 0.72 2.5 18.8 0.14 0.17 0.16 0.47 0.12 0.12 <0.04 0.18 <0.04 Na2O 0.06 <0.04 <0.04 0.15 <0.04 0.30 0.93 <0.04 <0.04 <0.04 <0.04 0.07 <0.04 <0.04 <0.04 <0.04 K2O <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 <0.04 <0.04 Total 99.8 98.1 101.1 100.0 101.6 101.0 101.6 101.4 101.1 101.0 101.2 101.5 101.4 101.8 101.2 100.5 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 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, Fig. 5. Compositional zoning profile across a typical low-Ca 0.1% Si, 0.2% Mn, 0.5% Ti and 0.6% Cr pyroxene grain in clast 1 showing relatively ferroan cores and (Table 3). magnesian edges indicative of reduction of FeO. Dotted lines Clast 4 is an impact melt breccia. It contains connect points where data have been excluded because of electron-beam overlap on small metallic Fe grains. 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, Table 3. Mean troilite compositions (wt%). millimeter-size POP chondrule. The largest olivine Ordinary Cumberland Cumberland phenocryst in this chondrule displays strong mosaic chondritea Falls clast 1 Falls clast 6 Cumberland extinction. The clast exhibits extensive silicate darkening No. grains 72 5 Falls hosta and has polycrystalline troilite grains and fine-grained Ca n.d. <0.04 <0.04 0.14 metal-sulfide intergrowths. The modally abundant Fe 63.1 62.4 63.3 61.3 matrix (85 vol%) has been impact melted; it consists Ti n.d. <0.04 <0.04 0.55 of 1–8 lm size angular olivine and pyroxene grains with Mg n.d. 0.06 <0.04 0.13 some interstitial microcrystalline feldspathic material, Mn n.d. 0.04 <0.04 0.18 possibly devitrified melt. This interstitial material Cr <0.04 0.17 0.21 0.58 includes 5–10 vol% tiny (0.2 lm size) rounded blebs S 36.8 36.7 37.3 37.4 of metal and sulfide; also present are coarser (up to Si n.d. 0.07 <0.04 0.09 15 lm), more angular grains of these phases. Relict Total 99.9 99.4 100.8 100.4 olivine grains embedded in the matrix have undulose to n.d., not determined. aOrdinary chondrite data from LL6 MIL 99301 (Rubin 2002). mosaic extinction and planar fractures. Low-Ca Cumberland Falls host data from Watters and Prinz (1979). pyroxene grains in the clast range from Fs1–15 (Table 2). Clast 5 is another impact melt breccia very clast portion have undulose extinction and planar similar to clast 4. It has a modally abundant matrix fractures; a few grains display weak mosaic extinction. (90 vol%) consisting of small olivine and pyroxene Low-Ca pyroxene grains in the unmelted portion of grains with minor interstitial microcrystalline the clast range from Fs6-10; olivine is very magnesian feldspathic material (Fig. 6). Metal- and sulfide-rich (Fa0.3–0.4; Table 2). veins and metal-sulfide spherules (some with fine- grained quench textures) also occur. In addition, 10– Mayo Belwa 15 vol% chondrule fragments and individual olivine and pyroxene grains are present in the matrix. At the Mayo Belwa is a brecciated aubrite containing edge of the thin section, the matrix abuts a large 50 lm to 9 mm size shock-darkened enstatite grains unmelted chondritic clast that closely resembles clasts (50 vol%; 843 ⁄ 1682 points) surrounded by a dark- 1, 2, 3, and 6 and contains 500–1100 lm diameter colored (in transmitted light), clast-laden, plagioclase- POP chondrules (Fig. 6) that constitute 85 vol% of bearing matrix (50 vol%; 839 ⁄ 1682 points) (Fig. 7a). this portion of the clast. This unmelted chondritic Some of the large enstatite grains are polycrystalline. clast portion exhibits extensive silicate darkening and More than 80% of the enstatite grains exhibit strong contains fine-grained metal-sulfide intergrowths, metal- mosaic extinction; a minority have weak mosaic and sulfide-rich veins, and grains of polycrystalline extinction and contain planar fractures. A few grains troilite. Many of the olivine grains in this unmelted appear to be clinoenstatite with polysynthetic twins. 270 A. E. Rubin

constitute 5 vol% of Mayo Belwa (A. W. R. Bevan, personal communication). In section USNM 5873-3, one 4 mm size vug is lined with (40–100) · (150–500) 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.

DISCUSSION

Cumberland Falls Clasts

All of the chondritic clasts in Cumberland Falls appear to be related. It is statistically unlikely that Fig. 6. Impact melted chondritic material. Clast 5 in several different chondritic projectiles unrelated to Cumberland Falls is itself an impact melt breccia. It contains a aubrites would be incorporated into Cumberland Falls. large, unmelted chondritic clast (right) with 500–1100 lm diameter POP chondrules. The black streaks in the clast at right Therefore, it seems reasonable to infer that the are metal- and troilite-rich veins. Abutting the unmelted chondritic clasts are all remnants of the same projectile chondritic clast is the impact melted portion of clast 5 (left) that impacted the Cumberland Falls region of the which consists of olivine and pyroxene grain fragments aubrite parent . If this is correct, then data embedded in microcrystalline material that probably represents gathered on one of the clasts can be inferred to be devitrified melt. The black circular object at left center is a metal-sulfide spherule with regions of a fine-grained intergrowth representative of the entire set of chondritic clasts. of these phases. Transmitted light. The clasts appear to have been derived from an OC rather than from an or carbonaceous Also present are grains of diopside and forsterite; chondrite. The inferred OC parentage of the chondritic Graham et al. (1977) reported millimeter-size rounded clasts is supported by several properties: (1) an LL- and sinuous aggregates of plagioclase. chondrite-like bulk chemical composition (Kallemeyn Many of the enstatite grains are transected by 10– and Wasson 1985), (2) O-isotopic compositions 30 lm thick veins of the plagioclase-bearing matrix (d18O = +5.53 to +5.8&; d17O = +3.5 to +4.00&; (Fig. 7b). Some of the veins include a few volume- D17O = +0.48 to +0.99&; data from R. N. Clayton percent of small (1–2 lm) angular grains of metal and reported in Verkouteren and Lipschutz 1983 and from troilite. Clayton and Mayeda 1978) that are in the OC range The matrix is composed of stoichiometric sodic and similar to those of some LL3 chondrites (d18O= plagioclase (Ab93Or4; Table 4) surrounding 3–16 lm +5.15 to +6.09&; d17O=+3.75to+4.24&; D17O= size euhedral and subhedral grains of enstatite +0.96 to +1.24&; Clayton et al. 1991), (3) chondrule (Fs0.05Wo0.8; Table 4) (Fig. 7c), olivine grains sizes that are in the range of LL chondrites (which have exhibiting silicate darkening, and small grains of a mean diameter of 570 lm; Nelson and Rubin 2002), and schreibersite (Fig. 7d). It appears that the (4) the occurrence of BO chondrules (which are matrix is a solidified melt; in some cases, small enstatite common in OC but rare in enstatite chondrites; e.g., grains in the matrix have nucleated on the surfaces of Rubin 2000), (5) the occurrence of RP chondrules coarse, partly resorbed enstatite grains (Figs. 7c and 7d). (which are appreciably more abundant in OC than in In some regions, small (2–5 lm size) patches of carbonaceous chondrites; e.g., Rubin 2000), (6) an OC- plagioclase are pincer-shaped, similar to those in the like chondrule modal abundance (70 vol%; e.g., Spade H-chondrite impact melt breccia (cf. Fig. 4 of Rubin 2000), and (7) core compositions of low-Ca Rubin and Jones 2003). Some of the pincer-shaped pyroxene (Fs20–21; Table 2; Fig. 5) that are within or plagioclase patches contain 2 lm size angular grains of close to the ranges of those in equilibrated L and LL metal and sulfide. chondrites (Fs19–22 and Fs22–26, respectively; fig. 181 Mayo Belwa is unique among aubrites in possessing of Brearley and Jones 1998) and are much more ferroan numerous vugs ranging in diameter from 2 lmto than typical grains in CM, CO, CR, CV, EH, and EL 4 mm (Figs. 7d and 8); Graham et al. (1977) chondrites. reported that some vugs reach 1 cm in size. The vugs If these Cumberland Falls clasts were derived from occur nearly exclusively in the matrix, and altogether OC then they must have suffered reduction. This 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 back- scattered electrons. process probably involved heating, which most likely Because diffusion is more sluggish in low-Ca occurred when the clasts’ precursor projectile accreted pyroxene than in olivine (e.g., Freer 1981; Chakraborty to the aubrite . During heating, FeO in 1997), the pyroxene grains did not completely the clasts’ olivine grains was reduced, forming low-Ni equilibrate and retained more ferroan compositions. blebs of metallic Fe (Fig. 4) surrounded by forsterite. Nevertheless, compositional zoning profiles across low- Through reduction, the olivine grains achieved Ca pyroxene grains show that they also suffered magnesian compositions (Fa0.3–0.7) that approached reduction—their margins are more magnesian than their equilibrium with the aubrite host (which has an olivine cores (Fig. 5). composition of Fa0.21; Table 6 of Watters and Prinz The occurrence of daubre´elite lamellae within some 1979). troilite grains in clasts 1 and 6 seems inconsistent with 272 A. E. Rubin

Table 4. Mean mineral compositions (wt%) in Mayo Belwa. Plagioclase Plagioclase in blades near Enstatite melt matrix vug No. grains 28 4

SiO2 60.6 67.4 ± 0.6 67.8 TiO2 <0.04 <0.04 <0.04 Al2O3 0.13 20.4 ± 0.3 21.1 Cr2O3 <0.04 <0.04 <0.04 FeO 0.04 <0.04 <0.04 MnO <0.04 <0.04 <0.04 MgO 39.9 0.17 ± 0.21 <0.04 CaO 0.44 0.73 ± 0.39 0.87 Na2O <0.04 10.7 ± 0.2 10.8 K2O <0.04 0.68 ± 0.19 0.76 Total 101.1 100.1 101.3 Fig. 9. Bundles of laths of plagioclase (plag laths) lining the Endmember Fs 0.05 Ab 92.7 Ab 91.7 wall of a vug (bottom) in Mayo Belwa. Wo 0.79 Or 3.8 Or 4.3

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 Fig. 8. Mayo Belwa whole rock showing numerous millimeter- chondritic fragments found on the : Bench Crater and sub millimeter-size vugs (arrows). Reflected light. Image is (a shocked CM1 chondrite recovered from the coarse from G. Benedix of the Natural History Museum, London fines fraction of an soil sample; McSween and is used with permission. 1976; Zolensky et al. 1996) and Hadley Rille (a partly impact melted EH chondrite with an agglutinate-like an OC parentage for these clasts because daubre´elite is rim recovered from the 1–2 mm size fraction of an a very rare phase in OC. Other chondritic clasts in soil sample; Haggerty 1972; Rubin 1997b). Cumberland Falls have additional phases (i.e., The overall shock stage of the Cumberland Falls and ferroan alabandite) (Kallemeyn and whole rock can be no higher than the lowest shock Wasson 1985) that are otherwise largely (or entirely) stages of its components. Hence, the whole-rock is restricted to enstatite meteorites (e.g., Rubin 1997a). shock stage S2. The fact that the OC-related clasts Because the Cumberland Falls clasts are not enstatite in Cumberland Falls have shock stages that are as low chondrites and were very likely derived from an OC as S2 and that the aubrite host is S5 indicates that the projectile, the occurrence of daubre´elite, oldhamite, and OC projectile accreted to the aubrite parent body after ferroan alabandite could be due either to (1) diffusive the host had experienced its maximum shock level. exchange between troilite in the clasts and sulfides in the aubrite host during annealing (e.g., Wasson et al. Mayo Belwa as an Impact Melt Breccia 1993), or (2) to brecciation involving incorporation of sulfide-bearing rock fragments from the aubrite host. In view of the significant shock experienced by However, because the daubre´elite-bearing sulfide grains Mayo Belwa and the presence of an intergranular melt Impact melting in Cumberland Falls and Mayo Belwa 273 matrix (containing euhedral silicate grains, small opaque The exclusive occurrence of fluor-richterite grains in grains and numerous vugs), it seems likely that the Mayo Belwa at the sides of vugs strongly suggests that matrix is an impact melt and that Mayo Belwa is an a vapor phase was necessary for their formation. I thus impact melt breccia. This is the first reported example infer that in Mayo Belwa, volatiles were vaporized of an impact melt breccia among aubrite whole rocks. during the impact event and formed bubbles in the melt (Fig. 8). As the melt solidified, the bubbles became Fluor-Richterite in Mayo Belwa cavities; relatively K-rich plagioclase and fluor-richterite crystallized at the margins of these cavities, presumably Bevan et al. (1977) and Graham et al. (1977) via reaction of the melt with the vapor. (The fine- reported that some vugs in Mayo Belwa are lined grained plagioclase in Mayo Belwa contains higher with bundles of acicular grains of the amphibole concentrations of K2O than plagioclase in other fluor-richterite [Na2Ca(Mg,Fe)5Si8O22F2]; some bundles aubrites [0.84 wt% versus 0.23–0.77 wt%; Watters and are up to 3 mm long and individual fluor-richterite Prinz 1979].) The bundles of plagioclase laths lining one blades are up to 1 mm long. I did not encounter this of the cavities (see above) supports this scenario. The phase in the two thin sections of Mayo Belwa that I flour-richterite may have reacted with the Na-rich examined. Other vugs in Mayo Belwa are lined with plagioclase melt that lined the vugs (Bevan et al. 1977). granular enstatite, acicular diopside, minor cristobalite, This is consistent with the high Na2O content (7.2 wt%) and bundles of laths of sodic plagioclase (Bevan et al. of fluor-richterite in Mayo Belwa (Bevan et al. 1977). 1977; Graham et al. 1977; this study). Lin and Kimura (1998), citing their own unpublished data, mentioned Commonality of Brecciation among Aubrites that fluor-richterite occurred in ‘‘some aubrites,’’ presumably in addition to Mayo Belwa. There are 64 aubrites and anomalous aubrites Fluor-richterite has been reported in only two other currently listed in the on-line Meteoritical Bulletin enstatite meteorites; both are EH-chondrite impact melt Database (MBD). One of these (NWA 2828) is certainly breccias: Abee (Rubin and Keil 1983; Rubin and Scott not an aubrite; it is a peculiar enstatite chondrite 1997; Rubin 2008) and St. Sauveur (Keil 2007; Rubin (unpublished data of A. Irving and T. Bunch; my 2008). In Abee, fluor-richterite occurs as rare 3.5-mm- petrographic observations) and will not be considered long radiating acicular grains bundled in clusters in here. After taking probable pairings of the remaining 63 association with enstatite, troilite and (Douglas aubrites and anomalous aubrites into account, based on and Plant 1969; Olsen et al. 1973). In St. Sauveur their on-line descriptions, I conclude that there are 27 fluor-richterite occurs as 40 · 100 lm size subhedral unique aubrites listed in the MBD, nine of which grains (Rubin 1983). The morphologies of these grains appear to be unbrecciated (including Shallowater and and their apparently exclusive occurrence among Mount Egerton; Keil et al. 1989; McCall 1965; Watters enstatite chondrites within EH-chondrite impact melt and Prinz 1980) and two of which are only poorly breccias imply that fluor-richterite crystallized in these characterized (NWA 5419 and Yamato 793592). rocks from the impact melt (Rubin 2008). Shallowater is probably from a different parent asteroid Another F-rich phase, fluorphlogopite [KMg3(Si3Al) than the majority of aubrites (Keil et al. 1989) and is O10F2], occurs in the EH impact melt rock Y 82189 as not included in the present statistical count. Thus, the rare subhedral, 10–30 lm size grains in association with proportion of brecciated aubrites is approximately 69% enstatite, silica, and albite (Lin and Kimura 1998). This (18 ⁄ 26). This number greatly exceeds the proportion of phase also crystallized from the impact melt. It seems breccias among H and L chondrites (25% and 10%, likely that after impact melting, most of the F in Abee, respectively) and is similar to that of LL chondrites St. Sauveur, and Y-82189 that was not driven off was (62%) (Binns 1967). scavenged by the crystallizing grains of fluor-richterite At small heliocentric distances (where the enstatite and fluorphlogopite. meteorites probably formed; e.g., Wasson 1988), orbital The retention of F in impact melted enstatite periods are shorter and orbital velocities are higher. This meteorites is consistent with a high original abundance results in higher collision probabilities and higher average of F. Enstatite chondrites are particularly rich in F: e.g., collision velocities (e.g., O¨ pik 1951; Bottke et al. 2006), F in EH chondrites has an average Mg-normalized accounting for the pervasive brecciation among aubrites abundance ratio of 3.40 · CI (Rubin and Choi 2009). and the occurrence in them of impact melted materials. The F in Abee, St. Sauveur and Y 82189 may have initially condensed as simple metal halides that were Acknowledgments—I thank the curators at the American incorporated into enstatite-chondrite precursor materials Museum of Natural History, the Smithsonian (Rubin and Choi 2009). Institution, and the NASA Johnson Space Center for 274 A. E. Rubin the loan of thin sections. Reviews by A. El Goresy, Keil K. 1989. Enstatite meteorites and their parent bodies. T. J. McCoy, Y. Lin, a couple of anonymous referees, Meteoritics 24:195–208. and associate editor C. Floss on different versions of Keil K. 2007. Occurrence and origin of keilite, (Fe>0.5, Mg<0.5)S, in enstatite chondrite impact-melt rocks and this manuscript were helpful in making revisions. I am impact-melt breccias. Chemie der Erde 67:37–54. grateful to G. K. Benedix of the Natural History Keil K., Ntaflos T., Taylor G. J., Brearley A. J., Newsom H. Museum, London for an image of Mayo Belwa, to E., and Romig A. D. 1989. 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