Meteoritics & Planetary Science 36, 869-88 1 (200 1 ) Available online at http://www.uark.edu/meteor Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid EDWARDR. D. SCOTT'*, HENNINGHAACKlt AND STANLEYG. LOVE2 IHawai'i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, Honolulu, Hawai'i 96822, USA *Mail Code CB, NASA Lyndon B. Johnson Space Center, Houston, Texas 77058, USA ?Present address: Geological Museum, University of Copenhagen, Oster Voldgade 5-7, DK-I 350 Ksbenhavn K., Denmark *Correspondence author's e-mail address: escottahigp.hawaii.edu (Received 2000 October 26; accepted in revisedform 2001 March 5) Abstract-We propose that mesosiderites formed when a 200-400 km diameter asteroid with a molten core was disrupted by a 50-1 50 km diameter projectile. To test whether impacts can excavate core iron and mix it with crustal material, we used a low-resolution, smoothed-particle hydrodynamics computer simulation. For 50-300 km diameter differentiated targets, we found that significant proportions of scrambled core material (and hence potential mesosiderite metal material) could be generated. For near-catastrophic impacts that reduce the target to 80% of its original diameter and about half of its original mass, the proportion of scrambled core material would be about 5 vol%, equivalent to -1 0 vol% of mesosiderite-like material. The paucity of olivine in mesosiderites and the lack of metal-poor or troilite-rich meteorites from the mesosiderite body probably reflect biased sampling. Mesosiderites may be olivine-poor because mantle material was preferentially excluded from the metal-rich regions of the reaccreted body. Molten metal globules probably crystallized around small, cool fragments of crust hindering migration of metal to the core. If mantle fragments were much hotter and larger than crustal fragments, little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally. The rapid cooling rates of mesosiderites above 850 "C can be attributed to local thermal equilibration between hot and cold ejecta. Very slow cooling below 400 "C probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris. We infer that our model is more plausible than an earlier model that invoked an impact at -1 km/s to mix projectile metal with target silicates. If large impacts cannot effectively strip mantles from asteroidal cores, as we infer, we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material. This may help to explain why relatively few olivine-rich (A-type) and metal-rich asteroids (M-type) are known. Some S-type asteroids may be scrambled differentiated bodies. INTRODUCTION dunites and rarer anorthosites. The largest clasts are fragments of coarse-grained orthopyroxene, olivine, gabbro and basalt Mesosiderites, which are breccias composed of roughly and are -10 cm in length (e.g., McCall, 1966; Ikeda et al., equal proportions of silicate and metallic Fe,Ni plus troilite, 1990; Mittlefehldt, 1980). Chemical, mineralogical and are among the most perplexing differentiated meteorites known. isotopic studies suggest that all these clasts were derived from About 45 mesosiderites are known including samples from the diverse levels of an igneously stratified asteroid that was similar 1-5 km diameter Eltanin asteroid, which was one of the larger but not identical to the parent body of the howardites, eucrites objects to hit the Earth in the last two million years (Gersonde and diogenites (the HED body), probably Vesta (Mittlefehldt et al., 1997). Eltanin samples were identified in impact ejecta ef al., 1998). Metallic Fe,Ni in mesosiderites is mostly in the found in sediment cores from the southeast Pacific Ocean (Kyte form ofmillimeter or submillimeter grains, which are intimately and Brownlee, 1985). mixed with similar-sized silicate grains (Fig. 1). Metal-troilite- The silicate fraction of mesosiderites consists of mineral silicate textures indicate that except for rare metallic nodules, and lithic clasts in a fine-grained fragmental or igneous matrix the metal crystallized after being mixed with silicate (Floran (see Mittlefehldt et al., 1998). The lithic clasts are largely 1978; Hewins, 1983). Metal in mesosiderites is remarkably basalts, gabbros, and pyroxenites with minor amounts of homogeneous and contains roughly chondritic proportions of 869 PYelLtde preprint MS#4453 0 Meteoritical Society, 2001. Printed in USA. 870 Scott et al. FIG. 1. Photograph of a 16 x 10 cm slice of the Emery mesosiderite showing an intimately mixed matrix of grains of metallic Fe,Ni (white) and silicates (gray), which are millimeter or submillimeter in size. Embedded in this matrix are a few volume percent of subcentimeter-sized silicate clasts, one rounded, centimeter-sized, metal nodule and a very large pyroxenite clast. (American Museum of Natural History sample number 444 1 ; see Floran, 1978.) siderophiles, unlike iron meteorites, which show large chemical range of 39Ar-40Ar ages, 3.74.1 Ga (Bogard et al., 1990; variations due to fractional crystallization (Hassanzadeh et al., Bogard and Garrison, 1998). (These Ar-Ar ages and those in 1990; Shen et al., 1998). Since impacts do not efficiently melt Table 2 have been corrected for the -300 Ma error discovered asteroidal material and impact melt is largely lost (e.g.,Keil et by Bogard and Garrison (1998).) The 39Ar-40Ar ages of al., 1997), it is most likely that the metal was molten prior to mesosiderites have been interpreted in two different ways. A impact and that it crystallized when mixed with silicate. catastrophic impact may have disrupted the parent body at Various techniques have been used to constrain the cooling 4.2 Ga, heated the mesosiderites and left them deeply buried rates of mesosiderites over different temperature ranges (Bogard et al., 1990; Mittlefehldt et al., 1998). Alternatively, (Table 1) and these are now broadly consistent with one another. the 39Ar-40Ar ages may simply result from very slow cooling In the temperature range, 850-1 150 "C, mesosiderites cooled at depth in a large asteroid (Haack et al., 1996a). relatively rapidly at 104 to 105 "C/Ma, but below 500 OC they There are many puzzling features about mesosiderites. cooled at <0.5 "C/Ma, more slowly than any other meteorites. Why are crustal and core materials so abundant in mesosiderites Chronological constraints on mesosiderite history were yet fragments of olivine mantle are rare? Why are metal and reviewed by Mittlefehldt et al. (1998) and are summarized in silicate grains so intimately mixed? Why were mesosiderites Table 2. The mesosiderite parent body was globally cooled rapidly at high temperature yet very slowly at low differentiated at 4.56 Ga (Ireland and Wlotzka, 1992), -2 Ma temperatures? Although a wide variety of origins have been after the HED body (Wadhwa et al., 1999). From the Sm-Nd proposed (Hewins, 1983), most recent authors infer that molten ages of 4.42-4.52 Ga of four lithic clasts, Stewart et al. (1994) metal and silicate were first mixed by impact in the regolith of infer that clasts were mixed with molten metal -100-150 Ma a Vesta-like asteroid to form mesosiderite breccias that cooled after the solar system formed. Mesosiderites have a narrow rapidly. After many localized impact melting events, the Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid 87 1 TABLE1. Thermal history of mesosiderites. ____ Feature Temperature Cooling rate Reference ("C) (OC/Ma) Pyroxene exsolution > 1 OOO? 5.104 Miyamoto and Takeda (1 977) Pyroxene zoning 850-1 I50 2104 Ganguly et al. (1 994) I 05 Schwandt et al. (I 998) Plagioclase overgrowths 1100 2105 Ruzicka et al. (1 994) Taenite zoning 400 -0.03 Haack et al. (1 996a) Taenite zoning 400 0.2 Hopfe and Goldstein (2001) Kamacite zoning 400-300 -0.01 Haack et al. (1 996a) Fe-Mg ordering in pyroxene 250 51 Ganguly et al. (1994) 39Ar-4oAr ages <300 -0.2 Bogard and Garrison (1 998) TABLE2. Chronology of mesosiderites and comparison of two models for the evolution of the parent asteroid. Age Source Comparison of models: (G4 (A) This work; (B) Rubin and Mittlefehldt (1993) and Mittlefehldt et al. (1998) 4.56 Pb/Pb age of clast (I) (A) and (B): Global differentiation and formation of original crust. 53Mn-53Cr systematics (2) 4.42-4.52 Sm-Nd ages of four clasts (3) (A) After crustal remelting, an impact at -5 km/s mixed molten core with cooler crust and mantle. Metal-silicate mixtures were buried deeply in the reaccreted body, cooling to -800 "C within lo4 years. Slow cooling below 400 "C started after local thermal equilibration. (B) Projectile with molten core impacted at 1 kmls causing crustal remelting. Metal-silicate mixtures resided near surface of body. 4.42- 4.2 (A) Slow cooling at depth. (B) Many impacts caused localized melting and heating of near-surface metal- silicate mixtures followed by rapid cooling. 3.7-4.1 39Ar-4oAr ages of mesosiderites (4, 5) (A) Ar closure during slow cooling at depth. (B) An impact at 4.2 Ga caused disruption and reaccretion of body, Ar degassing, deep burial of mesosiderites, and the start of slow cooling. ~ ~ References: (1) Ireland and Wlotzka (1992); (2) Wadhwa et al. (1999); (3) Stewart et al. (1 994); (4) Bogard et al. (1990); (5) Bogard and Garrison (1 998). mesosiderites were deeply buried by an impact that disrupted Here we explore whether mesosiderites could have formed and reassembled the parent body, causing the Ar-Ar ages to be by impact-induced break-up and reassembly of a Vesta-like reset and the mesosiderites to cool slowly at low temperatures asteroid with a molten core (Haack et af., 1996a; Scott et af., (Bogard et al., 1990; Rubin and Mittlefehldt, 1993; Mittlefehldt 1996).