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Composition of Chondrule Silicates in LL3-S Chondrites and Implications for Their Nebular History and Parent Body Metamorphism

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Composition of Chondrule Silicates in LL3-S Chondrites and Implications for Their Nebular History and Parent Body Metamorphism .-,-- "., ~ Geochimiea el CosmtXhimica Aela Vol. 55, pp. 601-619 00 16-7037/91/$3.00 +.00 Copyright ~ 1991 Pergamon PreM; pic. Printed in U.S.A. Composition of chondrule silicates in LL3-S chondrites and implications for their nebular history and parent body metamorphism TIMOTHY J, MCCOY, ,·2 EDWARD R, D, SCOTT,' RHIA N H. JONES,2 KLA US KEIL; and G. JEFFREY TAYLOR ' 'Planetary Geosciences Division, Department of Geology and Geophysics. School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA 21nstitute of Meteoritics, Department of Geology, University of New Mexico. Albuquerque, NM 87 131, USA (Received April 26, 1990; accepled in revised form November 20. 1990) Abstract-Our petrologic studies of75 type IA and type II porphyritic olivine chondrules in nine selected LL group chondrites of type 3.3 to type 5 and comparisons with published studies of chondrules in Semarkona (LL3.0) show that compositions of silicates and bulk chondrulcs, but not overall chondrule textures, vary systematically with the petrologic type of the chondrite. These compositional trends are due to diffusive exchange between chondrule sil icates and other phases (e.g., matrix), such as those now preserved in Semarkona, during which olivines in both chondrule types gained Fe2+ and Mn2+ and lost H Mg2+, Cr , and Ca2+. In a given LL4-5 chondrite, the olivines from the two chondrule types are identical in composition. Enrichments of Fe2+ in olivine arc panicularly noticeable in type IA chondrules from type 3.3-3,6 chondrites, especially near grain edges, chondrule rims, grain boundaries, and what appear to be annealed cracks, Compositional changes in low-ea pyroxene lag behind those in coexisting olivine, consistent with its lower diffusion rates. With increasing petrologic type, low-ea pyroxenes in type IA chondrules become enriched in Fe2+ and Mn2+ and depleted in Mg2+, Cr3+, and AI 3+. These compositional changes are entirely consistent with mineral equili bration in chondritic material during metamorphism. From these compositional data alone we cannot exclude the possibility that chondritic material was metamorphosed to some degree in the nebula, but we see no evidence favoring nebula ovcr asteroidal metamorphism, nor evidence that the chondrule reacted with nebular gases after crystallization. Modelling of the equilibration ofchondrule olivines suggests that heterogeneous FeD concentrations in olivine could be preserved after cooling from 600°C at rates of 1- 1O°C/Ma for at least tens to hundreds of millions of years. This is consistent with published estimates for the maximum metamorphic temperatures in type 3 chondrites, thermal histories derived from metallographic and fission-track cooling rates, and 4.4 Ga 39 40 Ar_ Ar ages for ordinary chondrites. Since the lifetime of the solar nebula was not more than 106 years and there is abundant evidence that meteorite parent bodies were heated, some even above their melting point, we confidently conclude that the formation of type 3.3-5 ordinary chondrites from type 3.0 material by metamorphism occurred in parental asteroids, not the solar nebula. INTRODUCTION Some authors disagree with the conventional view and ar­ gue that nebular processes were important in modifying the BECAUSE OF THE COMPLEXITY of chondrites and the lack of properties of chondrules and other chondri tic components progress in characterizing conditions during their formation after they fonned (see HEWINS, 1989), KURAT (1988) argues and evolution, there are considerable disagreements con­ that the composition of mineral grains in ordinary chondrites cerning the thermal histories of chondrites in the solar nebula results solely from exchange between grains and nebular gases and on parent bodies and the relative importance of various and not from any asteroidal processing. For CV3 carbona­ heat sources that may have operated in these environments. ceous chondrites, PECK and WOOD (1987) and HUA et al. The conventional view, at least for ordinary chondrites, is . (1988) argue similarly that fayalite-rich rims on forsterite- that chondrules formed as a result of melting of pre-existing rich chondrules and FeD enrichments in chondrule interiors solids by unidentified heat sources in the solar nebula (TAY ~ are the result of nebular condensation and exchange. LOR et aI., 1983; GROSSMAN, 1988) and that chondrites were An entirely different view is that of REID and FREDRIKSSON metamorphosed in planetesimals as a result of heating by (1967) and FREDRIKSSON (1983) who conclude that diverse electrical induction or 26 AI decay (MCSWEEN et aI., 1988). crystallization histories, not metamorphism, are responsible Most authors believe that the mineralogical and textural dif­ for the homogeneity of chondrule silicates in types 4-6 and ferences between type 3-6 ordinary chondrites result largely their heterogeneity in type 3 chondrites. In support of their from asteroidal metamorphism of material that resembled views, these authors argue that grain growth occurs more type 3 chondrites (V AN SCHMUS and WOOD, 1967; DODD, rapidly than silicate homogenization and that friable chon­ 1969; Huss et aI., 198 1; Lux et aI. , 1980; SEARS and HASAN, drites with homogeneous sili cates such as Bjurb61e could not 1987). In panicular, these authors conclude that the com­ have formed from a heterogeneous type 3 precursor by meta­ position of homogeneous silicates in chondrules of type 4-6 morphism. FREDRIKSSON (1983) argues funher that silicates chondrites was established by equilibration of heterogeneous with unequilibrated compositions are common in ordinary silicates in chondrulcs and matrix of type 3 chondrites. type 4-6 chondrites and that these meteorites are not meta- 601 602 T. J. McCoy et al. morphic rocks. SCOTT et al. (1985) agree that these chondrites markona chondrules are largely consistent with closed-system arc breccias, but they argue that the components experienced fractional crystallization. Because H3.O and L3.0 chondrites diverse parent body metamorphic histories. Detailed petro­ are not known (SEARS and HASAN, 1987), we have only stud­ logic studies of ordinary chondrites of type 3- 6 are needed ied LL chondrites in this work. However, analogous studies to resolve these issues. of types IA and II chondrules in C03 chondrites (SCOTT and Although much information about the thermal history of JONES, 1990) suggest that our conclusions may be applicable chondrites has been deduced from studies of compositional to other chondrite groups. We conclude in this paper that zoning in grains of metallic Fe,Ni (WOOD, 1967; WILLIS and porphyritic olivine chondrules in LL3.3-5 chondrites were GOLDSTEIN, 1981a, 1983), less progress has been made from derived from chondrules like those in Semarkona by meta­ analogous studies of silicate zoning in chondrules. This is morphic equilibration of chondritic material. The time scales partly because diffusion rates in silicates at low temperatures for metamorphism arc too long for nebular processing, but are known to lower accuracy than those for Fe-Ni diffusion are consistent with heating in asteroidal bodies. in metal. A more fundamental difference, however, is that zoning in chondrule silicates probably results from both ig­ SAMPLES AND TECHNIQUES neous and metamorphic processes (DODD, 1969), whereas Table I lists the section numbers and sources of the nine LL chon­ drites we studied. Our data for chondrule silicates in two LL4 chon­ zoning in metal grains, at least in unshocked chondrites, is drites, Kelly and Soko-Banja, and two LL5 chondrites, Tuxtuac and entirely metamorphic in origin. This is because any igneous Krahenberg, are not presented here, as the compositions of their zoning in metal grains in unshocked chondrites was erased chondrules are almost identical to those of Sevilla (LL4) and Olivenza during slow cooling; Ni heterogeneities in taenite developed (LL5), except for very minor differences in FeO. during kamacite growth below 600·C (WILLIS and GOLD­ Parnallee (LL3.6) was selected because a preliminary survey iden­ tified it as one of relatively few type 3 ordinary chondrites in which STEIN, 1981 b). Characteristics of chondrule silicates that result the chondrules appear to have had similar metamorphic histories from igneous and metamorphic processing must be distin­ (SCOTT et al., 1983; ScOTT, 1984). In addition, chondrule silicates guished carefully before precise models can be developed to in Parnallee are roughly midway in composition between those in explain silicate zoning. Semarkona (LL3.0) and the equilibrated LL chondrites. Lastly, Par­ nallee does not appear to have experienced sufficient shock reheating Previous studies of the compositions of chondrule silicates to alter mineral compositions. Allan Hills A81251 (hereafter ALH have mostly been comparative studies of the bulk properties A81251) was selected because its subtype of 3.3 is intermediate be­ of chondrules in chondrites of diverse petrologic type (DODD tween those ofSemarkona and Parnallee. A survey of five LL4 chon­ et aI., 1967; Lux et aI., 1980). Until recently, the only detailed drites found only one, Bo Xian, with homogeneous olivines and het­ study of zoning in chondrules was by MIYAMOTO et a1. (1986). erogeneous pyroxenes. No LL6 chondrites were studied, because of the difficulty in idcntifying types IA and II chondrulcs. These authors analyzed chondrule silicates in two type 3.6- Scanning electron microscopy and wavelength dispersive analysis 3.7 ordinary chondrites and modelled their
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