1 SEARCHING FOR CHIPS OF KUIPER BELT OBJECTS IN METEORITES. M. E. Zolensky , G. Briani2, 1 4&5 6 3 1 M. Gounelle2, T. Mikouchi3, K. Ohsumi , M. K. Weisberg , L. Le , W. Satake3 & T. Kurihara , ARES, NASA 2 Johnson Space Center, Houston, TX 77058, USA ([email protected] ); Muséum National d’Histoire Naturelle, 57, rue Cuvier, 75005, Paris, France; 3University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; 4Kingsborough Community College, Brooklyn, NY 11235, USA; 5Department of Earth and Planetary Sciences, American Museum of Natural History, NY, NY 10024, USA; 6ESCG Jacobs, Johnson Space Center, Houston, TX 77058, USA. Introduction: The Nice model [1&2] describes a LEW 87015, Y-793497, Elemeet, Y-791834; Ordi- scenario whereby the Jovian planets experienced a nary Chondrites: Wells, Abbott, Willard (b), Pa- violent reshuffling event ~3:9 Ga – the giant planets rambu, Y-790048, Zag, DAG 577, Plainview, DAG moved, existing small body reservoirs were depleted 581, NWA 1848, Sahara 98328, NWA 4846, DAG or eliminated, and new reservoirs were created in par- 369, Oubari, Mezzo Madaras, NWA 4686, NWA ticular locations. The Nice model quantitatively ex- 5386, Parnalee, Siena, St. Mesmin, Leighton, Dah- plains the orbits of the Jovian planets and Neptune [1], mani, Y-82055, Cynthianna, Sharps, Broken Hill, the orbits of bodies in several different small body Tsukuba; Carbonaceous Chondrites: Cold Bok- reservoirs in the outer solar system (e.g., Trojans of keveld, El Djouf 001, Al Rais, Renazzo, ALH 85085, Jupiter [2], the Kuiper belt and scattered disk [3], the PAT 91546, PCA 91467, QUE 94411, HaH 237, Ish- irregular satellites of the giant planets [4], and the late eyevo, Tagish Lake, NWA 2086, NWA 760, NWA heavy bombardment on the terrestrial planets ~3:9 Ga 2364, EET 96026, Ningqiang, NWA 801, NWA 2140, [5]. This model is unique in plausibly explaining all of Vigarano, Allende, LON 94101, Bencubbin; Aubrite: these phenomena. One issue with the Nice model is Cumberland Falls; Ungrouped: Kaidun. that it predicts that transported Kuiper Belt Objects We are performing a complex suite of bulk compo- (KBOs) (things looking like D class asteroids) should sitional and mineralogical analyses to test the hypothe- predominate in the outer asteroid belt, but we know sis that KBO pieces are present in meteorites, espe- only about 10% of the objects in the outer main aster- cially regolith-rich meteorites, and that these pieces oid belt appear to be D-class objects [6]. However (xenoliths) can be recognized and exploited for cos- based upon collisional modeling, Bottke et al. [6] ar- mochemical information on the earliest history of the gue that more than 90% of the objects captured in the outer solar system, and by extension, other solar sys- outer main belt could have been eliminated by impacts tems. if they had been weakly-indurated objects. These dis- A few of these xenoliths have previously under- rupted objects should have left behind pieces in the gone thorough mineralogical work - we don’t list the ancient regoliths of other, presumably stronger aster- references for much of this work because of length oids. Thus, a derived prediction of the Nice model is limitations, but important work has been performed by that ancient regolith samples (regolith-bearing meteor- Keil, Krot, Weisberg, Prinz, their coworkers and many ites) should contain fragments of collisionally- others. None of these materials have had major-, mi- destroyed Kuiper belt objects. In fact KBO pieces nor- or trace-element compositions analyzed and few might be expected to be present in most ancient re- have experienced any kind of isotopic investigation. golith-bearing meteorites [7&8]. We recognize several distinct classes of xenoliths (in addition to many unique xenoliths), which we briefly Clasts: We have previously searched through re- describe here. All types are shown in Figure 1. These golith-bearing meteorites, to locate and characterize types are preliminary groupings only, and will cer- the most common types of meteorite xenoliths (Figure tainly change as we gather more detailed information 1). At that time these materials were generally called on all clasts. “C1-,” “C2-clasts”, or “dark inclusions” in the litera- Type FGA (fine-grained anhydrous) xenoliths are ture, and were reported in all types of chondrites in widespread. These are fragmental breccias with a bi- addition to HEDs and other achondrites. We con- modal size distribution. Coarse (1-100 µm), generally cluded that these xenoliths were most commonly simi- fragmented grains of olivine, low-Ca pyroxene, Fe-Ni lar to CM2 and CR2 chondrites [9-12], but that signifi- sulfides are set within a fine-grained, anhydrous cant differences exist, and in fact similarities to un- (which we need to verify) groundmass principally of melted Antarctic micrometeorites were more apparent ferromagnesian silicates. Some classes also have mi- [13&14]. Following a long search, the meteorites we crochondrules of all types, but principally barred, have since found clasts in are: HEDs: MAC 02666, microcrystalline or glassy. These xenoliths have re- EET 87513, Bholghati, Jodzie, Kapoeta, LEW 85441, ceived little detailed characterization. The fact that LEW 85300, Malvern, Lew 87295, Mundrabilla 020, they are anhydrous leads us to believe they may have the greatest potential to be KBO pieces, since the Wild be reliable indicators. For example, it will be interest- 2 samples are to date anhydrous. ing to measure the D/H ratios and O isotopic composi- Type FGH (fine-grained hydrous) xenoliths are per- tions of the fluid inclusions in halite which accompany haps the most widespread type and are often called the Zag clast, which is work in progress. “C1”, “CR” or “CI” in past studies. These xenoliths tend to be rather small, probably reflecting low- F'W_III • k' strength. The ones that have been analyzed typically consist of 0.5-10 I.Lm sized Fe-Ni sulfides and magnet- ite set within serpentine and saponite. Occasional fragmented grains of olivine are found in the larger / xenoliths, which indicate that these are not truly petro- 1 ¶•• logic type 1. Gounelle et al. [13&14] have pointed out a e •' that these xenoliths are mineralogically most similar to hydrous micrometeorites though some differences are apparent. One clast from Leighton which we studied contains abundant Ca-carbonate grains and Fe sulfides which synchrotron X-ray diffraction (SXRD) showed to be very poorly crystalline or amorphous – the prob- able result of shock. Type CGH (coarse-grained hydrous) xenoliths are Figure 1. Several xenoliths we have identified in chondrites almost as widespread as FGH and have frequently – all except (e) are BSE images, showing a fraction of the been called “C2”or “CM”. These xenoliths tend to be diversity of possible KBO chunks. (a) A 1 cm-long FGH xenolith from the Zag (H5). A large carbonate suitable for significantly larger than the FGH. The ones that have Mn-Cr dating is arrowed. (b) A FGH cast from the Tsukuba been analyzed typically consist of 0.5-10 I.Lm sized Fe- chondrite (H5-6), which measures 0.9 cm across. (c) A large Ni sulfides and partially-aqueously-altered chondrules, CGH xenolith from Plainview (H5), measuring 2 cm across. fragmented olivine and low-Ca pyroxene set within (d) A fine-grained xenolith in NWA 2364 (CV3), measuring serpentine and (lesser) saponite. These xenoliths have 3 cm across. (e) Reflected light image of a generally fine- long been recognized as being very similar to CM2 in grained xenolith in Y-791834 (eucrite) (5 mm across). (f) terms of mineralogy [9-12], but a definite relationship Strange xenolith of uncertain mineralogy from the Cumber- has never been established (but could be by bulk com- land Falls aubrite (1 cm across). positional measurements, including O isotopes). References: [1] Tsiganis et al. (2005) Nature 435, Could these clasts derive from KBOs? Can we 459-461. [2] Morbidelli et al. (2005) Nature 435, 462- reliably identify certain xenoliths with a KBO origin? 465. [3] Levison et al. (2009) Icarus, in press. [4] How would this be done? Campins and Swindle [15] Nesvorný et al. (2007) Ap. J. 133, 1962-1976. [5] recommend looking for dark, weak, porous lithologies Strom et al. (2005) Science 309 1847-1950. [6] Bottke which have nearly solar abundances of most elements, et al. (2008) LPSC XXXIX (1447). [7] Gounelle et al. and have elevated C, N, and H contents. We are also (2006) MAPS 41, 135-150. [8] M. Gounelle, et al. comparing the mineralogical components and petro- (2008) in: The Solar System beyond Neptune, pp. 525. graphies of these clasts with Wild 2 samples. One [9] Zolensky et al. (1993) Meteoritics 27, 596-604. potential strategy is to look at compositional variation [10] Zolensky et al. (1996) LPSC XXVII, pp. 1507- of major and minor elements in olivine and low-Ca 1508. [11] Zolensky et al. (1996) MAPS 31, 518-537. pyroxene, which we are systematically doing. As we [12] Buchanan et al. (1993) Meteoritics 28, 659-682. have recently shown [16&17], Wild 2 olivine and low- [13] Gounelle et al. (2005) Geochimica et Cosmo- Ca pyroxene have the widest possible compositional chimica Acta. 69, 3431-3443. [14] Gounelle et al. ranges for Mg/Fe, and elevated minor element compo- (2003) Geochimica et Cosmochimica Acta 67, 507- sitions, as compared to all known chondritic materials, 527. [15] Campins and Swindle (1998) LPSC XXIX except anhydrous chondritic IDPs. In addition, a sig- (1004). [16] Zolensky et al. (2006) Science 314, 1735- nificant degree of shock in clasts (as we have seen for 1740. [17] Zolensky et al. (2008) MAPS 43, 161-172. the hydrous Leighton clast), since impacts between KBOs and main belt asteroids would be at a generally higher velocity than those purely between main belt asteroids. We are assessing the shock state of these clasts using SXRD and Electron Backscattered Elec- tron Diffraction. Finally, isotopic compositions may