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44th Lunar and Planetary Science Conference (2013) 1407.pdf

VESICULAR IMPACT-MELT CLASTS IN CARBONACEOUS : EVIDENCE FROM THE CV3 LAR 06317 AND RELEVANCE TO SURFACE PROCESSES ON THE 4 VESTA. N. G. Lunning1 H. Y. McSween1 and C. M. Corrigan2, 1Department of Earth and Planetary Sciences, Uni- versity of Tennessee, Knoxville, 1412 Circle Drive, Knoxville, TN 37996-1410, USA; [email protected], 2Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA

Introduction: Recent findings by the DAWN mission eral analyses were conducted at 15 keV, 30 nA with a 1- indicate that carbonaceous impactors contributed diameter micron beam. Glass analyses were conducted at 15 / to 4 Vesta’s regolith [1,2]. This is consistent keV 10 nA with a 10 micron diameter beam. 120-200 point with the occurrence of clasts in how- analyses set-up in grids were used to estimate bulk composi- ardites, which are the best meteorite analog for Vesta’s rego- tions. The modal was determined using the NIH lith [3]. The carbonaceous chondrite clasts identified in how- software program ImageJ64 to analyze backscatter electron ardites retain their water content [4,5]; therefore, the shock (BSE) images. Pixel values for each mineral were assigned the carbonaceous chondrite impactors experienced when they based on prior EMP point analyses. collided with Vesta was not great enough to metamorphically Results & Discussion: LAR 06317 is an oxidized CV3 dehydrate them [e.g. 6,7]. meteorite. It contains shock features, including planar frac- The contribution of water and other volatiles to the oth- tures in olivine grains, fizzy-textured sulfide grains and poly- erwise volatile-depleted asteroid may have changed surface crystalline sulfide grains. In ordinary chondrites, these fea- processes on Vesta. Pitted terrains [8] and gullies [9]) on tures correspond to a shock grade of at least S3 [20, 21]; this Vesta may have formed by release of volatiles. Mineralogi- scale can be extrapolated to carbonaceous chondrites [11]. cally bound water in carbonaceous chondrites might have Impact-melt clasts in LAR 06317. Four fragmental im- been liberated by impacts [8]. There may be direct evidence pact-melt clasts and one microbreccia clast ( frag- for this in . It is common for ments in a pseudotachylite-like groundmass) have been iden- to contain impact-melt glasses [e.g. 10], which in some cases tified in LAR 06317. Several of these clasts contain circular may have formed from carbonaceous-chondrite-bearing voids that are likely cross sections of vesicles formed from howardite material. vaporization of volatiles. These clasts have textures that To identity evidence for carbonaceous chondrite source match impact-melt clasts identified in ordinary chondrites: material in howardite impact-melt glasses, it is important to euhedral equant olivine grains with sizes on the order of tens understand impact-melting of carbonaceous chondrites. The of microns with circular blebs of sulfide and/or FeNi-metal effects of impacts on the volatile component of carbonaceous [12]. The impact-melt clasts in LAR 06317 may be geneti- chondrites are not well studied, partially because impact-melt cally related, as they have similar mineral textures and chem- clasts have been reported in carbonaceous chondrites [11]. In istries. Each clast, however, was quenched or ceased - this study, we describe such impact melts. lization at progressively different stages; this is reflected in Impact-melt in chondrites. Impact-melt clasts have been the variation from inclusion-free groundmass glass to vitro- reported in a number of ordinary chondrites [12-15]. Carbo- phyric glass, and finally to an ophitic-textured clast with naceous chondrites are expected to respond to shock com- pyroxene and plagioclase between olivine grains. pression from an impact similarly to ordinary chondrites The bulk compositions of these impact melts are evi- [11], but they contain water, whereas most ordinary chon- dence of their precursor meteorite [19]. Estimates of bulk drites do not [16]. Most of the water in carbonaceous chon- compositions are very similar to published CV3 bulk compo- drites is structurally bound in clay minerals within OH- sitions [16]. The Al/Si ratio of CV3 is fairly diagnostic as it groups. [17]. Carbonaceous chondrites contain between 0.6- is the highest of any chondrite group [16]. The notable ex- 18 wt. % water; CV3 chondrites contain 2.5 wt. % H2O [16]. ception is that the impact-melt clasts have lower Fe than In ordinary chondrites, impact-melt clasts have bulk published bulk values for CV3 chondrites, possibly explained compositions close to the meteorite/material from which they by immiscibility of sulfides in impact-melts and the scale of melted [15, 18]. Incipient partial melts, which are feldspathic the clasts (100 µm to 3.5 mm). in chondrites, have only been identified as small pockets in Mineralogy of impact-melt clasts. These four impact- chondrites [19]. Deviations in impact-melt clasts from the melt clasts contain unequilibrated and strongly zoned olivine bulk composition of their parent meteorites have been ex- grains (10-100 µm). The olivine grains have Mg-rich cores plained by processes that occurred after bulk melting, such as that are homogenous within each grain but vary between loss of volatiles or separation of immiscible sulfides/metals grains, Fo80-95. The rims (4-15 µm) have Mg-rich cores and from silicate melts [12, 15]. Fe-rich rims; with outermost rims as Fe-rich as Fo44, consis- Methods: Thin sections LAR 06317,2 and 06317,11 tent with rapid crystallization. Some olivine grains with the were examined with transmitted and reflected light using a most Mg-rich cores (>Fo90) exhibit distinct optical disconti- petrographic microscope. Two of the four impact-melt clasts nuities between the cores and rims. This may indicate that identified in these sections have been rigorously analyzed so these are relic grains that did not equilibrate with the initial far: Clast A from LAR 06317,11 and Clast B from LAR impact melt. 06317,2 (henceforth referred to as Clast A and Clast B, re- All four clasts contain accessory Al-rich chromite (2-10 spectively). µm), most commonly as intergrowths with the Fe-rich oli- Electron microprobe (EMP) analyses were performed vine rims, and occasionally as euhedral grains in the with a Cameca SX-100 EMP at the Univ. of Tennessee. Min- groundmass. The chromites from each impact-melt clast fall 44th Lunar and Planetary Science Conference (2013) 1407.pdf

within the same compositional range. The impact-melt-clast Scully J.E.C. et al. (2013) LPSC, this vol. [10] Beck, A.W. et chromites are distinctly more Al-rich than chromites com- al. (2012) MAPS 47, 947 [11] Scott et al. (1992) GCA 56, monly found in within CV3 meteorites [17, 22]. 4281. [12] Bogard, D.D. et al. (1995) GCA 59, 1383 [13] All of the clasts contain accessory intergrown sulfides (5-50 Bogard, D.D. (2011) Chemie de Erde 71, 207 [14] Corrigan, µm): troilite and << 1 µm immiscible pentlandite. C.M. & Lunning, N.L. (2013) LPSC, this vol. [15] Mittle- Neither the olivine grains nor the sulfides in the impact- fehldt, D.W. & Lindstrom, M.M. (2001) MAPS 36, 439 [16] melt clasts exhibit the shock features observed in the rest of Wasson, J.T. & Kallemeyn, G.W. (1988) Phil. Trans. R. Soc. this meteorite. Lond. 325, 535 [17] Brearley, A.J. & Jones, R.H. (1998) In Of the four fragmental impact-melt clasts found, Clast A Planetary Materials. Rev. in Min. Vol. 36 [18] Dodd, R.T. et appears to have been quenched earliest in its crystallization al. (1982) EPSL 59, 364 [19] Dodd, R.T. & Jarosewich, E. sequence, as its groundmass glass does not contain vitrophy- (1982) EPSL 59, 355 [20] Bennett, M.E. & McSween, H.Y. ric phases (Figure 1, Table 1); the calculated norm from (1996) MAPS 31, 255 [21] Bischoff, A. & Stoffler, D. (1992) composition of this glass would crystallize more olivine plus Eur. J. Mineral. 4, 707 [22] Johnson, C.A. & Prinz, M. diopside and plagioclase. In contrast, Clast B appears to have (1991) GCA 55, 893 [23] Benedix et al. (2008) GCA 72, progressed the farthest in its crystallization path (Figure 2, 2417 [24] McCoy et al. (2006) EPSL 246, 102. Table 1), because it has an ophitic texture and in place of the intersertal glass in Clast A, Clast B contains Al-rich Ti-rich clinopyroxene (En20-29 Wo52-56) and plagioclase (An36-59). Table 1: Modal The zoned oli- vine grains in Clasts Clast A Clast B A and B have simi- Olivine 67 82 lar zoning profiles, Glass 32 -- but the outermost rim of the olivine Pyroxene -- 5 grains extend to Plagioclase -- 9 more Fe-rich com- Chromite 0.7 1.7 positions in Clast B (Fo ) compared Sulfides 0.6 0.8 44-48 to Clast A (Fo53-62). This further supports the idea that Clast B progressed farther in its crystallization sequence, while Clast A was quenched earlier. Vesicles in LAR 06317 impact-melt clasts. Both Clasts A and B contain circular voids (Figures 1 & 2). The voids in Clast A are small (1-5 µm) and many are associated with sulfide blebs. Those in Clast B are larger (20-150 µm). These circular voids appear to be 2-dimensional sections through vesicles. Vesicles have only been observed in the PCA 91501 L-chondrite impact melt. These vesicles formed from vola- tilization of sulfur (from sulfides) that was unable to escape because of confining [23]. On the surface of a small airless body, are expected to escape and fragment an impact-melt or flow rather than form vesicles [24]. It is possible that these impact melts were fragmented but the escape of some volatiles, but cooled too quickly for all the volatiles to escape. Then the fragments re-aggregated with other components of their precursor CV3 meteorite that did not melt but exhibit this as shock features. Implications: Whether escaping gases fragment impact melts or are trapped in them as vesicles may influence how volatile-bearing regolith geomorphologically responds to impact events. The behavior of volatiles during impacts onto carbonaceous chondrite-bearing material may help us to better understand surface processes and the formation of the pitted terrains and gullies on Vesta. References: [1] Prettyman T.H. et al. (2012) Science 338, 242 [2] DeSanctis M.C. et al. (2012) Ap. J. Lett. 758, L36 [3] Beck, A.W. et al. (2012) 75th Ann. Met. Soc. Mtg #5328 [4] Zolensky, M.E. et al. (1996) MAPS 31, 518 [5] Rubin, A.E. & Bottke, W.F. (2009) MAPS 44, 701 [6] McSween, H.Y. (1976) EPSL 31, 193 [7] Zolensky, M.E (1997) MAPS 32, 15 [8] Denevi B.W. et al. (2012) Science 338, 246 [9]