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A Petrologic and Isotopic Study of Winonaites: Evidence for Early Partial Melting, Brecciation, and Metamorphism

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A Petrologic and Isotopic Study of Winonaites: Evidence for Early Partial Melting, Brecciation, and Metamorphism Geochimica et Cosmochimica Acta, Vol. 62, No. 14, pp. 2535–2553, 1998 Copyright © 1998 Elsevier Science Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/98 $19.00 1 .00 PII S0016-7037(98)00166-5 A petrologic and isotopic study of winonaites: Evidence for early partial melting, brecciation, and metamorphism 1, 2 1,† 3 3 G. K. BENEDIX, *T.J.MCCOY, K. KEIL, D. D. BOGARD, and D. H. GARRISON 1Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA 2Dept. of Mineral Sciences, MRC NHB-119, National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560, USA 3Code SN2, NASA/Johnson Space Center, Houston, Texas 77058, USA (Received April 22, 1997; accepted in revised form April 16, 1998) Abstract—We have conducted detailed petrologic, chemical, and isotopic studies of winonaites to ascertain the genesis of this group of meteorites. Winonaites have reduced mineral compositions and mineralogy, and oxygen isotopic compositions distinct from primitive achondrite groups other than silicate inclusions in IAB and IIICD irons. However, winonaites differ from IAB and IIICD irons in that they lack the metallic matrices of the latter and consist mostly of silicates. On the basis of these criteria, Winona, Mount Morris (Wisconsin), Tierra Blanca, Pontlyfni, Y-74025, Y-75300, Y-8005, and QUE 94535 are winonaites and Y-75305 and Y-75261 may be winonaites. Winonaites are fine- to medium-grained, mostly equigranular rocks. Pontlyfni and Mount Morris (Wisconsin) contain what appear to be relict chondrules. Several winonaites contain mm-sized areas that differ substantially in grain size and/or silicate mineralogy from the surrounding matrix. Fe,Ni-FeS veins are common in many winonaites. Mineral compositions are intermediate between E and H chondrites, and reduced sulfides are observed in low-FeO winonaites. Bulk major element compositions are roughly chondritic, although REE elements are fractionated. The 39Ar-40Ar ages of three winonaites range from $4.40 Ga (Mount Morris, Wisconsin) to 4.54 Ga (Pontlyfni). Cosmic-ray exposure ages are ;20–80 Ma. Trapped noble gases in these winonaites resemble those in enstatite chondrites. We suggest that the winonaites formed from a chondritic precursor material unlike that of known chondrites in mineral and oxygen isotopic compositions, and this material may have been heterogeneous in composition. Extensive heating caused metamorphism and partial melting of both Fe,Ni-FeS and silicate material. Impact brecciation during cooling mixed lithologies with different thermal histories, and subsequent metamorphism produced recrys- tallization, grain growth, and reduction of mafic silicates. The 39Ar-40Ar ages indicate that cooling may have been more rapid than observed in IAB irons, although later resetting may have occurred. Copyright © 1998 Elsevier Science Ltd 1. INTRODUCTION winonaites (e.g., Graham et al., 1977; Prinz et al., 1980; Kimura et al., 1992). Detailed studies of asteroidal meteorites suggest that some In this paper, we concentrate on the petrologic, chemical, and asteroids are primitive and never melted, whereas others melted isotopic features of the winonaites. Previous investigators have and differentiated into cores, mantles, and crusts (e.g., 4 Vesta; studied only individual winonaites (e.g., Tierra Blanca; King et Gaffey et al., 1993). However, some meteorites exhibit features al., 1981) or a subset of the winonaites (e.g., Graham et al., intermediate between primitive chondrites and the more 1977; Kimura et al., 1992). In contrast, we examined all mem- evolved achondrites and irons. These rocks experienced incom- bers of the winonaite group. This is also the first study that plete partial melting, and their properties offer insights into and concentrates on the textural features, whereas previous authors constraints on the geologic processes which occurred on small have focused primarily on mineral and bulk elemental compo- bodies in the early solar system. Several groups of meteorites sitions. It should be noted that silicate inclusions in IAB irons which experienced incomplete partial melting have been rec- resemble the winonaites compositionally, mineralogically, and ognized, and these are collectively labeled “primitive achon- texturally. We discuss these meteorites and the relationship drites” (Prinz et al., 1983). These rocks are characterized by between winonaites and IAB irons in another paper (Benedix et chondritic bulk elemental compositions and chondritic mineral al., 1998). assemblages and by textures that vary from metamorphic to Here, we discuss the criteria for classification of winonaites those suggesting partial melting. These groups include the and list the members of this group. We discuss the properties of acapulcoites and lodranites (Nagahara, 1992; Takeda et al., the group, with special emphasis on textural features and 39Ar- 1994; Mittlefehldt et al., 1996; McCoy et al., 1996b, 1997a, b), 40Ar ages. We also discuss implications for the nature of the silicate inclusions in IAB irons (e.g., Bunch et al., 1970; Bild, precursor chondritic material and the heating, partial melting, 1977; Choi et al., 1995) and IIICD irons (McCoy et al., 1993), metamorphism, and brecciation experienced by these rocks. brachinites (Nehru et al., 1996; Swindle et al., 1998), and 2. CLASSIFICATION *Present address: NASA/Ames Research Center, MS 245-3, Mof- fett Field, California 94035-1000, USA ([email protected]). Three criteria are used to classify the winonaites: (1) The †Also associated with the Hawaii Center for Volcanology. highly reduced mineralogy and mineral compositions, (2) the 2535 2536 G. K. Benedix et al. Fig. 1. Oxygen isotopic compositions for a variety of primitive achondrites (Clayton and Mayeda, 1996). Winonaites can be clearly distinguished from acapulcoites, lodranites, brachinites, and aubrites, but overlap with silicate inclusions in IAB and IIICD irons. Shown are the winonaites Winona, Mount Morris (Wisconsin), Pontlyfni, Tierra Blanca, QUE 94535, Y-74025, Y-75300, and Y-75261. oxygen isotopic compositions, and (3) the abundance and dis- compositions of the winonaites are identical to those of silicate tribution of metal and troilite. Previous workers (e.g., Mason inclusions in IAB and IIICD irons (Fig. 1). and Jarosewich, 1967; Graham et al., 1977; Prinz et al., 1980; Members of the winonaite group which satisfy both the Bild, 1977; Kimura et al., 1992) have demonstrated that the oxygen isotopic composition and mineralogical criteria include olivines of the winonaites are Mg-rich (Fa,5). In addition, the Winona, Pontlyfni, Tierra Blanca, Mount Morris (Wisconsin), winonaites contain some characteristic accessory minerals in- Yamato 74025, Yamato 75300, Yamato 75305, Yamato 8005, cluding alabandite, daubree´lite, schreibersite, and chromite. and QUE 94535 (Graham et al., 1977; Prinz et al., 1980; King Most of the winonaites are fine-grained and have granulitic et al., 1981; Kimura et al., 1992; Clayton and Mayeda, 1996; textures. However, these criteria alone cannot distinguish Mason, 1996; Yugami et al., 1996; R. N. Clayton, pers. com- winonaites from all other primitive achondrites such as the mun., 1996). Yamato 75261 has tentatively been classified as a acapulcoites, some of which are also highly reduced (e.g., ALH winonaite based on oxygen isotopic composition (Clayton and A81187/84190, Fa4.2) and have fine-grained, granulitic textures Mayeda, 1996). We have included literature data in addition to (McCoy et al., 1996). our own results on these meteorites where available. A more useful criterion of classification is the oxygen iso- topic composition (Clayton and Mayeda, 1996). These data 3. SAMPLES AND TECHNIQUES establish the winonaites as a group and allow discrimination We have concentrated our study on those meteorites (Table 1) for from acapulcoites and lodranites as well as some other groups which previously published mineralogical descriptions and oxygen of meteorites (e.g., ureilites, brachinites). The oxygen isotopic isotopic compositions confirm classification as winonaites. Where pos- Geologic History of Winonaites 2537 sible, more than one section of a meteorite was examined to determine for descriptive purposes, allowing us to thoroughly describe the the extent of heterogeneity within and between the individual meteor- range of textural features found in the winonaites. ites. All polished thin sections were examined with an optical micro- We had the opportunity to briefly examine thin sections of scope in transmitted and reflected light. Grain sizes were determined by measuring an average of twenty-five randomly selected grains in the Y-75261 (,51-1 and ,51-2) and Y-75305 in the optical micro- more abundant fine-grained regions (matrix) of each section. While scope at the National Institute of Polar Research. We find that averages calculated from a small number of grains carry large standard Y-75305 is among the coarsest-grained winonaites, and we deviations, they sufficiently illustrate the gross differences in grain grouped it with Tierra Blanca and Y-74025. On the other hand, sizes between individual winonaites (Fig. 2). Modal analyses of all thin sections were made in reflected light using a 40x objective. Olivine, Y-75261 is an impact-melt breccia, with fine-grained clasts up orthopyroxene, and clinopyroxene were not distinguished but were to 3 mm in diameter consisting of silicates and finely distrib- collectively determined
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