23. A New Type of Antarctic Achondrites and their to S Asteroids and Chondrites

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1 No. 8] Proc. Japan Acad., 68, Ser. B (1992) A New Type of Antarctic Achondrites and their to S Asteroids and Chondrites Relationship By Hiroshi TAKEDA,*) Jun SAITO,*)'**) and Takahiro HIROI***) (Communicated by Ryoichi SADANAGA, M. J. A., Oct. 12, 1992) Abstract: Five meteorites recovered from Antarctica are unique achondrites with coarse-grained orthopyroxene and olivine crystals and variable amounts of Ni-Fe metal and FeS with additional augite or plagioclase. These mineral species can be found in chondrites, which are the most common types among the observed falls. Both Y74357 and MAC88177 contain considerable augite, but Y74357 is richer in olivine. Y contains minor plagioclase. Discovery of such meteorites with coarse-grained texture and similar major mineral chemistry with extensively modified chondritic bulk chemistries suggest that they are related meteorites with variable amounts of augite and plagioclase and variable degree of reduction. The variability of mineral abundance can be explained by different degree of removal or segregation of partial melts, by a radiogenic internal heating and collisional one. Reflectance spectra of some members of this group combined with those of iron meteorites resemble those of S asteroids common in the main belt. The trend of their variation in mineral assemblage is in line with those of the S asteroids. This model also explains the absence of chondritic asteroids in the main belt, because S asteroids may be modified products of larger chondritic bodies. Key words: Primitive achondrite; S asteroid; pyroxene mineralogy; chondrite. Introduction. One of the most common types of asteroids is S asteroid, which shows reflectance spectra having olivine and pyroxene absorption bands1 and the red continuum. There was a strong expectation that reflectance spectra of S asteroids would resemble ordinary chondrites which make up more than 75% of observed meteorite falls. However, it became apparent that spectra of ordinary chondrites actually had little similarity to S asteroids. To explain these facts it is natural to think that most S-type surface materials are differentiated and modified assemblages of minerals in ordinary chondrites.1),2) A group of unique meteorites recently recovered from Antarctica have shown such mineralogical features. Our first measurement of reflectance spectra of such meteorites demonstrated that a metal-troilite-rich meteorite with olivine and orthopyroxene would resemble some of S asteroids.2~ In this paper, we report mineralogy of new Antarctic meteorites related to this group (MAC88177, Y74357,, Y791491, Y and EET84302), and present a model mineral assemblage of S asteroid materials and propose processes to produce such materials from chondrite-like source materials. Pallasites arid mesosiderites are two major groups of stony-iron meteorites. Lodran was a one-of -a-kind stony-iron meteorite (lodranite) containing roughly 25% metal plus sulfide and 75% iron-magnesium silicates plus minor phases.3~ Y and Y found *) Mineralogical Institute, Faculty of Science, University of Tokyo, Hongo, Tokyo 113, Japan. **) Now at Control Systems Division, National Aerospace Laboratory, , Jindaiji Higashi-machi, Chofu-shi, Tokyo 182, Japan. ***) SN3, NASA/Johnson Space Center, Houston, Texas 77058, U.S.A.

2 116 H. TAKEDA, J. SAIT0, and T. HIROI [Vol. 68(B), in Antarctica are similar to Lodran, but they contain plagioclase.2),4) Their similarity to other ungrouped meteorites with chondritic major bulk chemistry and with achondritic textures has been suggested and the term "primitive achondrite" has been proposed.5) The members of this primitive achondrite group include also winonaites, Acapulco-type chondrites, Brachina and silicate inclusions in TAB and IIICD irons.5) Acapulco-type chondrites and lodranites have now been shown to be a separate group called acapulcoites.6) However, lodranites have much coarser-grained textures and higher degree of deviation from chondritic chemistry. MAC88177 was initially described as a carbon-free ureilite.7) Bild and Wassong) pointed out a relationship of a lodranite with ureilites. Although this proposal is not well supported, there is reported evidence of remnants of primitive materials in this meteorite and ureilites,9) both of which preserve high abundances of planetary-type noble gases. Oxygen isotopic compositions similar to that found in the carbonaceous chondrites have also been detected in ureilites. 10) Because of this similarity between the two groups, we proposed to app]ly a planetesimal-scale collision model," proposed to ureilites, to the production of primitive achondrites with an internal heat source. 12) Materials and[ methods. We studied polished thin sections (PTS) of MAC88177,55,7) Y74357,62-113) and Y791491,61-2,2) Y791058,51-2,13) and EET84302,19 by electron probe microanalyzer (EPMA) and scanning electron microscope (SEM). Color processed back-scattered electron images (BET) and chemical map analysis (CMA) utilities on a JEOL 840A SEM system equipped with Energy Dispersive Spectrometer and Kevex Super 8000 unit, have been employed to obtain mineral distribution maps of the PTS's. Results. MAC88177, Y74357, Y791491, EET84302, and Y are coarse-grained ultramafic rocks with subrounded olivine and orthopyroxene (Opx) crystals up to 2 mm in diameter and variable amounts of Fe-Ni metal and troilite. In MAC88177, Y74357 and Y791491, elongated metal-troilite veins appear to fill the interstices or grain boundaries of rounded mafic silicates, and look like those of Lodran and other primitive achondrites (Fig. 1). These meteorites have affinity to Lodran in that their major phases are olivine and Opx, and are called tentatively as lodranites. Y contains more metal than silicates and island-like aggregates of silicate crystals distribute in metal. EET84302 is an intermediate between lodranites and Y Their volume percentages of metal-troilite (+chromite) decrease from Y (82%), EET84302 (36.5%), Y (29.5%), Y74357 (8%) to MAC88177 (5%). Along one edge of EET84302,1/3 of the PTS is richer in metal (32%) than other portion (19%). Relative abundances of olivine, Opx + Cpx (clinopyroxene, mainly augite) and plagioclase + phosphates within the silicate portions of these meteorites are plotted in Fig. 1. Photomicrograph of MAC Width is 10 mm.

3 No. 8] Antarctic Achondrites and S Asteroids 117 Fig. 2. The mineral assemblages of silicates of MAC88177 (M88) and Y74357 (Y74) plotted in pyroxene (Opx+Cpx)-olivine (Ol)-plagioclase- Ca-phosphate (flag+phos) diagram for primitive achondrites.5~ Cpx is mainly Aug. Lines: Acapulcoites and winonaites (W) including A78 (ALH78230) and Y80 (Y8002). Dotted line: JAB silicates5~ except for two (open squares). IIICD silicates5~: solid squares. Open circles (present study): lodranites including, Y79 (Y791491); solid circles (Print et al., 1983)5: lodranites including Lodran (Lo), Y79 (Y791493) and Y75 (Y75274). Others: magnesian ureilite LEW85440 (U)," (H6) and Brachina (Br). Y82111 chondrite Fig. 3. Chemical compositions of pyroxenes in primitive achondrites (PA) and ureilites (Ure) plotted in an enlarged pyroxene quadrilateral (shaded area on top). Dotted area are for Ure, Mg-Ure (Mg-ureilites) and PA. Opx in MAC88177: solidd circles; Y74357: open circles; Y791491: solid squares; Lodran: open squares; Y791058: open triangles; EET84302: solid triangles. Fe x 100/(Mg+Fe) mol % of Opx in ordinary chondrites (type 6) plot between 17 and 27. Fig. 2. The amounts of olivine and Opx in MAC88177 and Y are nearly equal but Y74357 is rich in olivine (83%). MAC88177 and Y74357 contain appreciable amounts of augite (Aug. 6-3%) in comparison with Y791491(0.07%) and Lodran (0.05%). The presence of plagioclase in Y is outstanding because of its absence in all other lodranites. In contrast, Y contains more plagioclase than any other primitive achondrites. EET84302 is rich in orthopyroxene and chromite. Although bulk chemistries of these meteorites are extensively modified chondritic, the preservation of Fe-Ni-S in the metals

118 H. TAKEDA, J. SAITO, and T. HIROI [Vol. 68(B), and troilites and Ca and Al in Aug and plagioclase imply that these meteorites have more affinity to chondritic materials than Lodran.

4 118 H. TAKEDA, J. SAITO, and T. HIROI [Vol. 68(B), and troilites and Ca and Al in Aug and plagioclase imply that these meteorites have more affinity to chondritic materials than Lodran. The chemical compositions of these silicate minerals are uniform within and between the crystals. The Opx compositions of MAC88177 and Y74357 distribute close to those of Lodran (Fig. 3). Olivine Fa13.3 and Opx Ca3Mg84Fe13 and Aug (bulk) Ca42Mg52Fe6 in MAC88177 are in equilibrium and the Opx-Aug pair gives equilibrium temperatures of 1100 to 900 C.14)'hhe CaO contents of Opx decrease towards the rims (1.8 to 0.9 wt. %). An Aug crystal up to 1.5 mm along the c axis shows exsolution lamellae of low-ca pyroxene up to 0.6 um wide with 4 um intervals. The CaO contents of olivine, 0.01 to 0.03 wt. %, are far less than those of ureilites ( ). The texture of Y74357 differs from MAC88177 in that its euhedral olivine crystals contain dislocations decorated mainly by troilites. The chemical compositions of olivine and Opx in Y74357 are not an equilibrated pair. Olivine Fa7.9 is richer in Mg than Opx Ca2.3 Mg84Fe13.7. In Y74357 Opx crystals, chemical zoning of Mg/(Mg+Fe), CaO and MnO has been detected within 20 um from the rims. The outer most rim composition is Ca13Mg90Fe8 7. In a core of some Opx crystals in MAC88177 and EET84302, there are dusty opaque inclusions (mostly troilite). The plagioclase compositions of Y and Y are close to chondritic values. The Ab values of Y distribute from 12 to 20, and that of Y is 18. Discussion. Relationship to other meteorite groups. There is an apparent textural similarity of Y74357 and MAC88177 to some ureilites11 but the presence of carbon veins in ureilites is major difference. Both meteorite groups are rich in planetary-type noble gases9~ and show differences in oxygen isotope composition.6~'10~ The oxygen isotope composition of MAC88177 does not fall within the range of ureilites,10~ but lies in the region of acapulcoites.6~ Mineralogical differences of MAC88177 and Y74357 from the ureilites are: (1) presence of chromite, which is typically absent in ureilites; (2) near chondritic modal abundances except for the lack of plagioclase; and (3) the low abundances of CaO in olivine. The MnO/FeO ratios of pyroxenes of MAC88177 and some ureilites are similar. The olivine and pyroxene compositions of MAC88177, Y and Y74357 are close to those of Lodran (Fig. 3). The MAC88177 pyroxene composition is outside the compositional ranges known for the ureilitic pigeonites and is at the Fe-rich end of the ureilite Opx trend of the magnesian ureilites" (Fig. 3). The lodranite pyroxenes are located at the Fe-rich end of the primitive achondrites in the pyroxene quadrilateral.5~ The reduced rims of Y74357 have a pyroxene composition close to Y This fact is in agreement with the formation of some primitive achondrites from Fe-richer and carbon-poorer, chondritic source materials by reduction. On the basis of Fig. 2, we can classify primitive achondrites with near chondritic compositions as acapulcoites and we can preserve a name lodranite for much coarser-grained variety with less than 5% of plagioclase + phosphates. As discussed above, lodranites in broad sense including MAC88177, Y74357 and Y seem to be partially differentiated meteorites, containing much of their original complement of planetary-type noble gases, but have lost more low melting silicates than acapulcoites among the primitive achondrites. A model of their formation. In order to preserve primitive signatures of ureilites during differentiation, normal magmatic models encounter many difficulties. A planetesimal-scale collision (PSC) model has been proposed for ureilites" to produce an olivinepyroxene-rich assemblage without total melting. Because the chondritic chemistries are better preserved in the primitive achondrites than in ureilites, models of their formation process do not encounter many of the difficulties that occur in the ureilite formation models especially when additional heat sources are combined. Ca and Al in these meteorites, especially acapulcoites are contained in Aug or plagioclase, typically absent in ureilites and

5 No. 8] Antarctic Achondrites and S Asteroids 119 Ni-Fe metal and troilites are still retained at grain boundaries. In some groups of primitive achondrites including silicate inclusions in iron meteorites, Ni-Fe metal is enriched. Whatever the formation model for primitive achondrites is, removal of low temperature fraction (partial melts) with such elements as Ca, Al, Fe, Ni, and S need not be as perfect as in ureilite formation. We suggest that the presence of volatiles in the carbonaceous source materials of ureilites may make a large difference in the removal of partial melts and residues of crystal growth in the case of the ureilites. Because primitive achondrites still preserve parts of the partial melts at grain boundaries, it should be easy to trace their formational processes. The presence of dusty cores in the mafic silicates of MAC88177 and EET84302 is similar to that of partly shock-melted, recrystallized diogenites (e. g. Y74013).15) This fact suggests that shock partial melting is responsible for recrystallization and that the total melting did not occur. The distribution of Co/Ni in MAC88177 metals is not uniform, indicating that they may not be formed by a typical magmatic process. The non-uniform distribution of metal-troilite in EET84302 suggests also non-magmatic segregation processes. The preservation of Ca0 zoning in Opx suggests that the growth speed of the MAC88177 Opx is nott so slow in spite of the coarse-grained textures and uniform Mg/Fe compositions. The decrease of Ca0 towards the rims is in line with the removal of Ca-enriched partial melts during the crystal growth. The Mg/Fe homogenization can happen in a magmatic process but may have also taken place during the high temperature episode in the solid state. As pointed out by Kallemeyn and Wasson,16~ the primitive achondrites are rated according to their compositional `pristinity', i.e., their fidelity to chondritic abundance patterns. The acapulcoite group keeps more pristinity than the lodranite one. Since melting for an appreciable period of time invariably leads to major fractionations of the sort associated with igneous processes, a chondritic composition in an igneous-textured rock implies a very brief period of incomplete melting, such as that produced by impact-generated) shock. 16) Because such crater-forming impact produces large amounts of cold breccias, we prefer collision as in the PSC model to produce more heat without breaking up a planetesimal. Miyamoto's calculationl2) showed that internal temperature of a chondrite-like parent body with an acapulcoite composition and a diameter larger than 100 km will produce partial melt by decay of 26A1. The PSC of a body with partial melts produced by internal heating will help the silicate melts migrate towards the surface or other place and are subsequently lost by collisions or impacts. 17) The presence of more plagioclase in Y is in agreement with the concentration of partial melt in other places. The presence of clear rims around dusty cores in EET84302 and MAC88177 supports an idea of incomplete melting and of crystal growth after a collisional event. The discovery of Y791491, Y74357 and MAC88177 links lodranites and acapulcoites among the primitive achondrites. The differing degrees of partial melting and reduction and removal of its low-temperature melting silicates produced by internal heating and PSC can produce a variety of primitive achondrites. Origin of S asteroids and absence o f the chond rite parent bodies. The reflectance spectra of lodranites have been measured for the first time by Hiroi and Takeda.2~ On the basis of additional spectra, Hiroi et al.ls> directly compare them with the spectra of S asteroids. The fractures with reduced metal-troilites in Y74357 reduce the grain size of olivine and also reduce the absorption band strength of olivine. The reduction of Fe in olivine also reduces the absorption.. It is pointed out that reduction in Fe content and grain size are important processes to produce materials like S asteroids. This process as was observed in Y74357 may be similar to a process called "space weathering". Such processes

6 120 H. TAKEDA, J. SAITO, and T. HIROI [Vol. 68(B), are most commonly observed in primitive achondrites. The red continuum of S asteroids can be simulated by addition of metal. If the body is composed of metallic masses segregated from the partial melt in a differentiated silicate matrix, differential corrosion stemming from the greater strength of metal might have led to an increase in the amount of metal on the asteroid surface. 17) The presence of a metal (opaque mineral)-rich portion in EET84302 demonstrates the segregation process of metal in the primitive achondrite parent body. Large variations of olivine/pyroxene ratio (Fig. 2) and (metal + troilite)/silicate ratio have been observed both for primitive achondrites by us and S asteroids by Gaffey et al. 19) If these materials are modified products of chondritic materials related to S asteroids, the absence of chondrite parent bodies in the main asteroid belt may no longer be problems. Large S asteroids in the main belt might have had higher chances of thermal metamorphism and may have smaller chances for injecting their fragments into Earth-crossing orbits. On the other hand, small asteroids near unstable orbits such as those around the Kirkwood gaps may have less chances of being highly differentiated and have higher chances of ejecting their fragments, which will eventually be delivered to the Earth. We thank the Antarctic Meteorite Working Group and National Inst. of Polar Res. (NIPR) for the meteorite samples, and Dr. Martin Prinz for critical reading of the manuscript. We are indebted to Professor R. N. Clayton and Dr. T. Mayeda for oxygen isotope information on MAC88177, and Drs. H. Nagahara, P. Bell, C. Pieters, K. Keil and M. Miyamoto for discussion. Y74357 is a consortium sample of NIPR. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) Bell, J. F.: Lunar Planet. Sci., 22, (1991). Hiroi, T., and Takeda, H.: Proc. NIPR Symp. Antarct. Meteorites, Natnl Inst. Polar Res., Tokyo, 4, (1991). Prinz, M. et al.: Lunar Planet. Sci., 9, (1978). Nagahara, H., and Ozawa, K.: Mem. Natnl. Inst. Polar Res. Spec. Issue, 41, (1986). Prinz, M. et al.: Lunar Planet. Sci., 14, (1983). Clayton, R. N., Mayeda, T. K., and Nagahara, H.: ibid., 23, (1992). Mason, B.: Antarctic Meteorite Newslett., 13, no. 2, 23 (1990). Bud, R. W., and Wasson, J. T.: Miner. Mag., 40, (1976). Gobel, R., Ott, U., and Begemann, F.: J. Geophys. Res., 83, (1978). Clayton, R. N., and Mayeda, T.: Geochim. Cosmochim. Acta, 52, (1988). Takeda, H.: Earth Planet. Sci. Lett., 93, (1989). Miyamoto, M., and Takeda, H.: Abstr. 17th Symp. Antarct. Meteorites. NIPR, Tokyo, pp (1992). Yanai, K., and Kojima, H.: Photographic Catalog of the Antarctic Meteorites. NIPR, Tokyo, p. 216 (1987). Lindslay, D. H., and Anderson, D. J.: Proc. Lunar Planet. Sci. Conf., 13th, A (1983). Takeda, H., Mori, H., and Yanai, K.: Mem. Natnl. Inst. Polar Res. Spec. Issue, 20, (1981). Kallemeyn, G. W., and Wasson, J. T.: Geochim. Cosmochim. Acta, 49, (1985). Taylor, G. J.: Lunar Planet. Sci., 22, (1991). Hiroi, T. et al.: ibid., 23, (1992). Gaffey, M. J. et al.: ibid., 21, (1990).

Keiko YuGAMI 1 *, Hiroshi TAKEDA 2, Hideyasu KonMA 3 and Masamichi MIYAMOTo 1

Keiko YuGAMI 1 *, Hiroshi TAKEDA 2, Hideyasu KonMA 3 and Masamichi MIYAMOTo 1 Antarct. Meteorite Res., 12, 117-138, 1999 COMPARISONS OF TEXTURAL AND CHEMICAL VARIATIONS OF MINERALS IN SOME PRIMITIVE ACHONDRITES AND AN H7 CHONDRITE, WITH REFERENCE TO THEIR FORMATION AND COOLING HISTORIES