Compositional continuity of enstatite chondrites and implications for heterogeneous accretion of the enstatite chondrite parent body

Geoehimicaet CosmoehimicaActa, Vol. 61, No. 22, pp. 4895-4914, 1997 Copyright© 1997Elsevier ScienceLtd Printed in the USA. All rights reserved 0016-7037/97 $17.00 + .00

Pergamon

PH S0016-7037(97 ) 00278-0

Compositional continuity of enstatite chondrites and implications for heterogeneous accretion of the enstatite chondrite parent body PING KONG, TADASHIMORI, and MITSURUEBIHARA Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan (Received October 31, 1996; accepted in revised form July 21, 1997) Abstract--Chemical compositions of eleven enstatite chondrites ( 1 EH5, 4 EH3, 3 EL3, 1 EL5, and 2 EL6) and their metal and nonmagnetic fractions determined by instrumental neutron activation analysis are reported. The abundances of nonvolatile lithophile elements, such as AI, Sc, and Mg, increase from EH to EL, while those of siderophile and moderately volatile elements decrease in the sequence of EH5, to EH3, EL3, and EL5,6. The continuity in compositions of enstatite chondrites and, in particular, the inverse variations of moderately volatile element abundances with petrographic type between EH and EL groups demonstrate that enstatite chondrites have been derived from a common parent body. The enstatite chondrite parent body formed by heterogeneous accretion of materials available in the accretional region; metal was effectively accreted into the core where EH5 chondrites were derived, and the abundances of metal decreased as accretion proceeded. In complement, silicate abundances gradually increased in the layers that accreted later. The lack of correlation between volatile element abundances and metamorphic degree demonstrates that losses of moderately volatile elements from enstatite chondrites cannot have resulted from parent body processes. Furthermore, it is expected that during accretion various components would mix and thus erase any early fractionation of moderately volatile elements of a single parent body. Variations of ambient gas temperatures during accretion are also impossible to produce the volatile element pattern of enstatite chondrites. It is suggested that moderately volatile elements in the enstatite chondrites were lost during local heating, the chondrule formation process. Formation of chondrules appears to have proceeded during the accretion of the enstatite chondrite parent body and lasted for a certain period. The mineralogical and textural features of enstatite chondrites can be explained in terms of two stages of metamorphism. The metamorphic trend from EH3 to EH5 resulted from internal heating whereas the trend from EL3 to EL6 was due to an external heat source. It seems very likely that the EH chondrites were metamorphosed during accretion, thus cooled rather rapidly. Activities of the early sun could serve as an energy source for the external heating. Copyright © 1997 Elsevier Science Ltd

1. INTRODUCTION The enstatite chonddtes are the most reduced group of meteorites discovered so far. Their characteristic minerals are believed to have formed in a region of the nebula where the C/O ratio was higher than the solar value (Latimer and Bartholomay, 1979). Kallemeyn and Wasson (1986) suggested that the highly reduced character of the enstatite chondrites is indicative of their formation in the innermost part of the solar nebula. Oxygen isotopic compositions of the enstatite chondrites, plotting along the terrestrial fractionation line (Clayton et al., 1984), may also imply formation of the enstatite chondrites in the inner region of the solar nebula. Hence, the study of the enstatite chondrites can provide not only clues to the formation of the terrestrial planets but may also provide a unique insight into conditions of the inner part of the early solar nebula. Enstatite chondrites comprise two groups: high-Fe EH and low-Fe EL (Sears et al., 1982). Before 1983 all EH members were assigned to the petrographic type 4 or 5, and all EL chondrites were of type 6. Prior to the discovery of EH3 and EL3 chondrites, the nonoverlapping petrographic types of EH and EL groups, the continuous variation of mineral compositions, and the gradual decrease of volatile element abundances from EH to EL and to aubrites (Wasson and Wai,

1970; Biswas et al., 1980) suggested a single evolutionary sequence for all enstatite meteorites. In contrast, the bulk compositional hiatus in nonvolatile major elements between EH and EL groups (Baedecker and Wasson, 1975; Keil, 1968) would require different parent bodies for the EH and EL chondrites. The newly recognized petrographic types EH3, EL3, and EL5 (Rambaldi et al., 1983; Prinz et al., 1984; Weisberg et al., 1995; Zhang et al., 1995; Sears et al., 1984) indicate that each of the two groups covers the range of metamorphic grades from 3 to 5 or 6. Keil (1989) reviewed this dispute and favored a multiparent body model for the enstatite meteorites. Bulk compositions of EH4,5 and EL6 chondrites are well known but few chemical data exist on EH3 and especially on EL3 chondrites, which are rare as falls. The heterogeneous distribution of minerals in the enstatite chondrites, making representative samples difficult to obtain, further complicates our understanding of the formation and evolution of the enstatite chondrite parent body(ies). In particular, classification based on mineral compositions, which are routinely determined with the electron microprobe, is difficult for the enstatite chondrites because mineral chemistry and petrologic textures of enstatite chondrites reflect various thermal histories (Skinner and Luce, 1971; Latimer and Buseck, 1974; Weeks and Sears, 1985; Fogel et al., 1989; Zhang et

4896

P. Kong, T. Mori, and M. Ebihara

al., 1996), and it is often difficult to decide whether these features reflect processes in the nebula or within a parent body. Subclassification of EH4,5 and EL6 chondrites is supported by the differences in bulk compositions of the enstatite chondrites, in particular, in the Fe contents (Sears et al., 1982). However, the difference between EL3 and EH3 chondrites is mainly determined by mineral chemistry (Weisberg et al., 1995; Zhang et al., t995). For example, the Si content in the metal of EH3 chondrites is about 2 wt%, in contrast to 0.5 wt% in the metal of EL3 chondrites. The M g / M n ratios of the Mg-Mn-Fe sulfide of EH3 and EL3 chondrites are also significantly different (Zhang et al., 1995). Chemically, Weeks and Sears (1985) determined bulk compositions of two EH3 chondrites and found their volatile element abundances lying at the low end of the EH range. This was later confirmed by Kallemeyn and Wasson (1986), who tentatively attributed the depletion of volatile elements in the EH3 chondrites to weathering. Zhang et al. ( 1995 ) also analyzed some EH3 and EL3 chondrites for bulk compositions, but their discussion was focused on mineralogical and textural features of these meteorites. For better understanding of the compositional interrelationship between the EH and EL groups, we performed new bulk chemical analyses of enstatite chondrites of various metamorphic grades. We especially focused on study of the EH3 and EL3 chondrites. In this paper we report results obtained by instrumental neutron activation analysis ( I N A A ) of eleven enstatite chondrites and magnetic separates (metal and nonmagnetic fractions) prepared from them. The purposes of this study are: (1) to precisely determine elemental concentrations in the enstatite chondrites and to study the compositional relationship between the EH3, EL3, EH5, and EL6 chondrites, and (2) to better understand the metamorphic histories of the EH and EL groups, in relation to that of the ordinary chondrites. 2. ANALYTICAL PROCEDURES Eleven enstatite chondrites were analyzed in this study. All but Qingzhen are from Antarctica; basic information is summarized in Table 1. LEW88180 has been classified as EH6 (Mason, 1990;

Table 1. Descriptions of the analyzed enstatite chondrites*. Meteorites

Classification

Weathering

%Fa

%Fs

Qingzhen* ALH84170 PCA91238 EET87746 LEW88180 MAC88136 ALH85119 PCA91020 TIL91714 ALH81021 LEW88714

EH3 EH3 EH3 EH3 EH5 # EL3 EL3 EL3 EL5# EL6 EL6

fall B Be Ce B/Ce A Be Ce C A C

0.2-2.8 0.6-28

0.4-12.5 0.9-17 1. l 0.6-2 0 - 13 0-3 0.3-12 0.2-3 0.4 0-1 0.1-6

1-2

0.1

* From Grossman (1994) unless otherwise indicated. # Zhang et al. (1995). Grossman et al. (1985) except the highest Fs value from Weisberg et al. (1994).

Zhang et al., 1993), but later reclassified as EH5 (Zhang et al., 1995; Rubin et al., 1997). The latter classification is used in the present study. One or two lumps weighing about lg for each chondrite were crushed in a stainless steel crusher. The resulting mm-sized pieces were ground in an agate mortar. A portion of the powder was stored for other purposes, and a portion was used for preparing duplicate or triplicate aliquots for bulk analyses. The remaining part was separated into magnetic and nonmagnetic fractions by a hand magnet. Both portions, silicate and metal, were subsequently ground and isolated. About 50 mg of the least magnetic fraction was prepared for INAA. The magnetic fraction was leached by conc. HF in an 80°C water bath for 2 min and then ultrasonified in deionized H20 for 10 min. The leaching residue was again subjected to magnetic separation, and the metal fraction was finally purified under an optical microscope. A quantity of 10-20 mg of each metal specimen was used for INAA. The rest of the metal fractions, in case sufficient amounts (100 mg) had remained, were boiled in conc. HF for 20 min for isolating taenite. The taenite fractions obtained differ from those in ordinary chondrites in appearance; they are spherical and are dispersed in the final residues of enstatite chondrites, in contrast to ordinary chondrites where taenite grains are clustering when isolated from other phases. The difference in the accumulating manner of taenite between ordinary and enstatite chondrites may imply that the taenite in enstatite chondrites carries weaker natural remnant magnetization (NRM) than in ordinary chondrites. This is consistent with the study by Sugiura and Strangway (1981) who found that cohenite, with a Curie temperature of 215°C, carries the most stable NRM in the enstatite chondrites. All meteorite samples, together with reference materials, and chemical standards were irradiated twice at a neutron flux of 1.5x 10 ~2 cm 2s- ~in the TRIGA-II reactor at the Institute for Atomic Energy, St. Paul's University. The first irradiation for the determination of short-lived radionuclides lasted 100 s and the second irradiation (6 h) was done for the determination of long-lived radionuclides. Counting was performed immediately after the short irradiation and was repeated several times within 40 days after the long irradiation. Our results for the reference materials agree within 3% with those in the literature (for comparison, see Kong et al., 1995). A more detailed report on INAA procedures is given by Kong et al. (1996). 3. RESULTS Our I N A A results for the bulk and the nonmagnetic fractions of the eleven enstatite chondrites are presented in Table 2 and Table 3, respectively. The bulk compositions are the averages of replicate analyses; a complete set of the I N A A data for the bulk samples is given in the Appendix. As only one EH5 chondrite was analysed in this study, literature values for some other EH4,5 chondrites (Abee from Baedecker and Wasson, 1975; Indarch, St. Marks, and St. Sauveur from Kallemeyn and Wasson, 1986) are cited and listed in Table 2. Group means for EH and EL chondrites from Wasson and Kallemeyn (1988) are also shown in Table 2 for comparison. Since silicate minerals contain tiny metal grains, complete separation of metal from silicates is almost impossible. This explains the rather high abundances of Co and Ni in the nonmagnetic fractions of enstatite chondrites (Table 3 ). Table 4 summarizes the I N A A data for the metal fractions of the enstatite chondrites. Even though the bulk metal samples are nearly free of silicates under the binocular microscope, the presence of lithophile elements, e.g., Na, in the metal fractions indicates up to 9% silicate contamination. In grinding the metal fractions resulting from the HF treatment, it was observed that portions of silicates were released from the metal; those silicates must have been enclosed in the

Compositional continuity of enstatite chondrites

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Compositional continuity of enstatite ehondrites metal grains. Studies of the Qingzhen and Yamato-691 meteorites showed a common enclosure of nonmetallic minerals in the metal grains (Wang, 1993; Kimura, 1988). Sodium abundances in the bulk metal of EL5,6 chondrites are less than 1% those for the corresponding nonmagnetic fractions, suggesting a thorough recrystallization of metal in the EL5,6 chondrites. Taenite is rare in enstatite chondrites; the taenite/ bulk metal ratios obtained in this work are 1-2%, significantly lower than those for H chondrites (in a range of 5 13%, unpubl, data). The taenite fractions are identified by their high Ni contents; they were not confirmed structurally. Metal/bulk abundance ratios for some siderophile and chalcophile elements whose contents were obtained in both the b u l l metal, and the b u l l sample are shown in Fig. 1. It is shown that most of these elements are equally enriched in the metal fractions, with the exceptions of Fe and Cu. Both elements also reside in the sulfide phase of enstatite chondrites (Allen and Mason, 1973). Unlike ordinary chondrites (Chou et al., 1973; Kong et al., 1995), Ir and Os are enriched in the metal of enstatite chondrites to the same level as Ni and Co. Nickel and cobalt are equally enriched

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Fig. 1. Metal/bulk abundance ratios for some siderophile elements in the enstatite chondrites. The siderophile elements, except Fe and Cu, are enriched in the metal fractions to a similar degree; Fe and Cu may partly reside in the sulfide phase. Iridium and osmium are enriched in the metals to the same degree as Ni and Co, in contrast to ordinary ehondrites. This plot suggests that Ir, Os, Ni, Co, Au, As, Ga, and Sb are mainly siderophile in the enstatite chondrites.

4899

Table 5. Proportions of Co and Ni in the nonmagnetic fractions of the enstatite chondrites. Proportion (in %) Meteorite

Co~

Ni¢

Ni-Co*

Qingzhen (EH3) ALH84170 (EH3) PCA91238 (EH3) EET87746 (EH3) LEW88180 (EH5)

9.4 5.1 20 17 6.9

25 14 26 26 5.9

16 9 6 9 0

MAC88136 (EL3) ALI-I85119 (EL3) PCA91020 (EL3) TIL91714 (EL5) ALH81021 (EL6) LEW88714 (EL6)

15 5.9 4.8 36 3.2 56

16 8.4 7.4 38 5.1 56

1 3 3 2 2 0

# Calculated by mass balance (for details, see text). * Differences between Ni and Co.

in the metal of EH5, EL3, and EL5,6 chondrites. In contrast, Ni is less enriched in the metal of EH3 chondrites compared with Co. The lesser enrichment of Ni in the EH3 metal is compensated for the high Ni contents in the nonmagnetic fractions of EH3 chondrites as will be shown below. Proportions of Co and Ni in the nonmagnetic fractions of the enstatite chondrites are calculated and shown in Table 5. The fractions of metal (designated as a) and nonmetal phases are calculated by mass balance, according to: MED + Se (1-0) = BE. ME, SE and BE are the abundances of element E in the metal, the nonmagnetic fraction and the b u l l sample, respectively. The proportion of Co in the nonmagnetic fraction of Qingzhen, for example, can be derived from the Sco( 1- 0) to BeD ratio. Because Co is almost quantitatively contained in the metal fractions, the difference between Ni and Co proportions (Ni-Co) reflects the presence of Ni in the minerals other than metal (sulfide, perryite, or phosphide). It is seen that significant fractions of Ni are present in the nonmetal phases of EH3 chondrites while nonmetal Ni is rare in the other types of enstatite chondrites. If EH3 chondrites are more primitive than the other types, the differences in chemical species of Ni between EH3 and the other types may reflect transformation of Ni from sulfide, perryite, or phosphide to metal during metamorphism. CI-normalized abundances of some nonvolatile elements for b u l l enstatite chondrites, including Abee (EH4), Indarch (EH4), St. Marks (EH5), and St. Sauveur (EH5) are shown in Fig. 2. Abundances for moderately volatile elements are plotted in Fig. 3. Zinc contents for the b u l l samples of TIL91714 and ALH81021 are below the detection limits; the plotted values are inferred from the Zn/Se ratios for the corresponding nonmagnetic fractions, assuming that these ratios are the same as for the bull. This assumption is justified because the chemical behavior of Se and Zn in the enstatite chondrites are very similar (Kallemeyn and Wasson, 1986). Likewise, b u l l Ga, Sb, and Ge data, which were below detection limits in the bulk analyses, are inferred from Ga, Sb, and Ge to Co ratios of the bulk metal fractions. As will be discussed later, concentrations of Ga, Sb, Ge, and Co for the b u l l metal are essentially unaffected by weathering.

P. Kong, T. Mori, and M. Ebihara

4900

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Fig. 2. Comparison of nonvolatile lithophile and siderophile element abundances between EH4,5, EH3, EL3, and EL5,6 chondrites. It is shown that the abundances of lithophile elements (A1, Sc, and Mg) can well distinguish the EH from the EL group, whereas those of siderophile elements decrease from EH, to EL3 and EL5,6 chondrites.

Gallium and germanium were found to be almost quantitatively concentrated in the metal fractions of enstatite chondrites (Fouch6 and Smales, 1967a; Rambaldi and Cendales, 1980). Although some fraction of Sb was observed in the nonmagnetic fraction of enstatite chondrites by Fouch6 and Smales (1967b), our results show that the enrichment factors of Sb in the metal fractions of the enstatite chondrites whose bulk Sb data were obtained are similar to those of Au and

Co (Fig. 1), suggesting that Sb is primarily siderophile in the enstatite chondrites. The inferred Ge contents for the EL5,6 chondrites are obviously lower than the average given by Wasson and Kallemeyn (1988; 21 ppm vs. 28 ppm). Because of a good agreement between our Ge data and those obtained by Easton (1986) for the bulk metal of EL5,6 chondrites (115 ppm vs. 113 ppm) and the well-characterized siderophile feature of Ge for the enstatite chondrites (Fouch6

4901

Compositional continuityof enstatitechondrites

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Fig. 3. Comparison of moderately volatile chalcophile and siderophile element abundances in the EH4,5, EH3, EL3, and ELS,6 chondrites. Data for Mn and Se are in the left Y-axis and for Zn are in the right Y-axis. Zinc contents for the bulk samples of TIL91714 and ALHA81021 are inferred from Zn/Se ratios of the cor~sponding nonmagnetic fractions. Bulk abundances of Ga, Sb, and Ge are inferred from Ga/Co, Sb/Co, and G¢/Co ratios of the bulk metal fractions. Abundances of all these moderately volatile elements vary in the same sequence of decrease from EH4,5, to EH3, EL3, and EL5,6.

4. WEATItERING TO TIlE ENSTATITE CHONDR1TES

Therefore, the compositions of enstatitechondrites are only compared between chondrites of the same petrographic types to see the effects of weathering. As shown in Table 6, there are no systematic differences in bulk compositions (including lithophile,siderophile, and chalcophile elements) of en-

As will be discussed later, enstatite chondrites of the same group but different petrographic types are anisochemical.

statite chondrites of different weathering types, suggesting that these meteorites were either equally affected by weather-

and Smales, 1967a; Rambaldi and Cendales, 1980), we suspect that the EL Ge concentration given by Wasson and Kallemeyn (1988) is too high.

P. Kong, T. Moil, and M. Ebihara

4902

Table 6. The effects of weathering on the balk compositions of enstatite chondrites. Lithophile

Siderophile

Chalcophile

Meteorite

Weathering type#

Na mg/g

Ca mg/g

A1 mg/g

Fe mg/g

Au ng/g

As #g/g

Sb ng/g

V ~zg/g

Mn mg/g

Se #g/g

Qingzhen (EH3) ALH84170 (EH3) PCA91238 (EH3) EET87746 (EH3) MAC88136 (EL3) ALH85119 (EL3) PCA91020 (EL3) ALH81021 (EL6) LEW88714 (EL6)

fall B Be Ce A Be Ce A C

5.98 5.46 5.75 5.60 5.80 5.90 6.08 5.75 6.50

10.0 9.08 8.32 8.68 10.7 10.5 11.2 9.87 7.21

8.12 8.17 8.00 8.35 9.50 10.0 9.91 10.4 10.7

285 307 267 297 266 264 262 213 219

325 354 283 342 255 284 268 238 222

3.27 3.72 3.02 3.87 2.93 2.73 2.72 2.09 2.40

162 218 154 190 164 145 146

56.2 50.7 54.0 52.0 49.6 57.2 53.5 50.1 49.3

1.92 2.12 1.87 2.39 1.88 1.86 1.91 1.11 1.37

20.7 26.4 23.3 28.8 19.1 20.3 18.1 13.5 15.9

A = minor rustiness; B = moderate rustiness; C = severe rustiness, and e = evaporative minerals visible to the naked eye (Grossman, 1994).

ing or there were no weathering related losses of major and trace elements. Therefore, the bulk data obtained in this study are equally used for the discussion. The above inference does not exclude the possibility that weathering caused redistribution of elements within meteorites. In fact, weathering types of chondrites are mainly determined by the degree of oxidation of metal Fe (type A: minor rustiness, B: moderate rustiness, and C: severe rustiness; Grossman, 1994). There is a trend of increasing concentrations of Ni, Au and As in the metal of enstatite chondrites from weathering type A to type C (Table 4), suggesting increasing degrees of oxidation of Fe from type A to type C enstatite chondrites. Thus, the concentrations of Ni, Au, and As in the metal of enstatite chondrites may increase by as much as 10 wt% from type A to type C. Concentrations of Co, Ga, Sb, and Ge in the metal remain, however, almost identical for chondrites of same petrographic types but different weathering types. Therefore, they are representative of those for the metal before arriving on Earth.

5. DISCUSSION 5.1. Characteristics of Compositions of Enstatite Chondrites As concentrations of some elements, especially moderately volatile elements, appear different for enstatite chondrites of the same chemical group but different petrographic types (Figs. 2,3), the average compositions of EH4,5, EH3, EL3, and EL6 are calculated separately and shown in Table 7 and Fig. 4. Bulk abundances of Ga, Sb, and Ge are inferred from Ga, Sb, and Ge to Co ratios of the metal fractions. Data for LEW88714 are not included in the EL5,6 means because this meteorite exhibits some unique properties. Its Zn content is not as low as those in the other EL5,6 chondrites, and its Mg content is not as high as expected. The CI- and Mg-normalized rare earth element (REE) pattern for this meteorite is characteristic of EH chondrites as shown in Fig. 5. Mason (1992) suggested that LEW88714 is paired with LEW87119. The latter meteorite has some similarities in mineralogy with enstatite meteorites that have experienced

partial melting (Zhang et al., 1995). The bulk AI content of one of the two analyzed LEW88714 samples is only 0.49% (see Appendix), perhaps reflecting partial melting and local segregation of plagioclase in this meteorite. Since LEW88714 seems to have experienced a more intense metamorphism than the other EL5,6 chondrites, one may expect the Zn content for this meteorite to be similar to or lower than that of the other EL5,6 chondrites. The results, however, show the opposite. It seems that this meteorite has either a primitive composition or a thermal history distinct from the other EL5,6 chondrites. In the following, some unique compositional characteristics of enstatite chondrites are outlined in comparison with those of ordinary chondrites.

Table 7. Elemental abundances for average EH4,5, EH3, EL3, and EL5,6 chondiltes. Element

EH4,5

EH3

EL3

EL5,6

AI (mg/g) Ca (mg/g) Sc (#g/g) V (#g/g) La (#gig) Sm (/zg/g) Yb (#g/g) Mg (mg/g) Cr (mg/g) Mn (mg/g) Na (mg/g) Br (~zg/g) Se (~zg/g) Zn (~g/g) Ir (ng/g) Os (ng/g) Ni (mg/g) Co (/~g/g) Fe (mg/g) Au (ng/g) As (#g/g) Cu (/ag/g) G-a (#g/g) Sb (ng/g) Ge (#g/g)

8.04 8.4 5.46 57.3 0.24 0.140 0.16 109 3.35 2.29 6.96 1.9 27.0 294 566 622 17.8 857 297 355 3.59 196 17.4 218 44

8.16 9.02 5.87 53.2 0.24 0.134 0.19 106 3.09 2.07 5.70 2.14 24.8 257 569 612 18.4 879 289 326 3.47 200 14.2 181 40

9.80 10.8 7.26 53.4 0.25 0.151 0.19 125 3.18 1.88 5.93 2.07 19.2 213 603 625 18.7 804 264 269 2.79 207 13.0 152 33

10.4 10.7 7.06 50.9 0.20 0.135 0.17 137 2.70 1.42 5.58 0.60 12.6 6 532 577 13.7 677 213 232 2.13 11.2 94.4 20

Compositional continuity of enstatite chondrites t

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Fig. 4. CI-normalized abundances for average EH4,5, EH3, EL3, and EL5,6 chondrites. The CI composition is from Anders and Grevesse (1989). For lithophile elements the data ate also normalized to abundances of Mg and, for siderophile elements, normalized to Co. Abundances of LEW88714 are not included into the ELS,6 means because this meteorite exhibits some distinct properties (see text). The differences in REE pattern between EH and EL groups, La, Sm, and Yb form a flat line in the EH4,5 chondrites but form a rising line in the EL5,6 chondrites may reflect variations in relative proportions of oldhamite and pyroxene caused by heterogeneous accretion of the 6nstatite chondrige parent body. Co-normalized abundances of lx and Os for the EL group are higher than those for'the EH group, suggesting that Ni, Co, and Fe fractionated partly with moderately volatile elements in the EL chondrites.

5.1.1. Refractory lithophile elements The results show that the refractory elements AI, Se, and Ca can be used to distinguish the EH from the EL group, with their abundances in EH chondrites being systematically lower than in the EL chondrites (Table 7). Basically there are no variationsin Al, Sc, and Ca concentrations with petrographic type. The patterns of REEs are, however, different between E H and E L and even between EL3 and EL5,6 chon-

drites. Among chondritic meteorites, only EL6 chondrites exhibit this unique REE pattern, i.e., depletion of light REEs. The nonchondritic REE-pattern for EL6 chondrites has been noticed earlier by Kallemeyn and Wasson (1986).

5.1.2. Refractory and common siderophile elements Concentrations of Os, Ix, Ni, Co, and Fe are identical in EH4,5 and EH3 chondrites. However, they are significantly

4904

P. Kong, T. Mori, and M. Ebihara

• x o

Mean EH LEW88714 Mean EL

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•

•

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~° x

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composition, concentrations of noble metals in all these five groups increase with increasing volatility. Rhenium is more enriched in the metal of enstatite and ordinary chondrites compared with Os and Ir. Compared with Pd, Au is more abundant in the metal of H and L chondrites, and the enrichment of Au is more pronounced in the metal of enstatite chondrites. The concentrations of moderately volatile elements (from Au to Ge) decrease with increasing volatility. However, the decreasing degree is different between ordinary and enstatite chondrites. The losses of moderately volatile siderophile elements occurred more gently in the enstatite chondrites.

Fe

Fig. 5. CI- and Mg-normalized abundances for nonvolatile elements in LEW88714. Mean EH and EL data are taken from Wasson and Kallemeyn (1988). While the siderophile abundances of LEW88714 resemblethose of EL chondrites,the absoluteREE abundances and the REE pattern of LEW88714 seem to resemble those of EH chondrites.

different between EL3 and EL5,6 chondrites, with the latter chondrites having lower concentrations. Although Co and Ni concentrations are overlapping between EH3 and EL3 chondrites, there are many mineralogical features showing differences between the EH3 and EL3 chondrites (Zhang et al., 1995; Weisberg et al., 1995). In addition, concentrations of refractory lithophiles of EL3 chondrites are different from those of EH3 but similar to those of EL6 chondrites. Thus, EH3 and EL3 chondrites must have formed at different conditions in the solar nebula. Osmium and iridium concentrations are less variable compared with those of Ni, Co, and Fe between EH and EL chondrites (Table 7 and Fig. 2b). This may imply that Ni, Co, and Fe were once volatile in the EL chondrites. In contrast, in ordinary chondrites Os and lr concentrations are more variable compared to Ni, Co, and Fe (Kallemeyn et al., 1989).

5.1.3. Moderately volatile elements Some moderately volatile elements, e.g., Se in EH chondrites and Au in all enstatite chondrites, are enriched above the CI-level relative to the common elements Mg and Co, respectively (Fig. 4). This kind of enrichment of moderately volatile elements is unique and has not been found in carbonaceous and ordinary chondrites (Palme et al., 1988). This may imply that in the region where enstatite chondrites formed chemical fractionation processes are much more complicated.

5.1.4. Metal composition CI-normalized abundances for metals from EH and EL chondrites are shown in Fig. 6 and compared with metals from ordinary chondrites. The data for ordinary chondrites are from Kong and Ebihara (1997). While concentrations of W, Mo, and Ga are low in ordinary chondrite metal, W and Ga are quantitatively contained in the metal of enstatite chondrites. The low Mo may indicate another Mo-host phase, sulfide. Interestingly when compared with the CI

5.2. Continuity of Concentrations of Moderately Volatile Elements in the Enstatite Chondrites From Fig. 3, it is seen that concentrations of moderately volatile elements are different between chondrites of different petrographic types. Although moderately volatile elements display different chemical affinities in the enstatite chondrites, Mn, Se, and Zn behave as chalcophiles (Kallemeyn and Wasson, 1986) and Au, As, Ga, Sb, and Ge are mainly siderophile (Fouch6 and Smales, 1967a,b; this can also be inferred from Fig. 1 ), their average abundances for the bulk samples all exhibit a decreasing trend from EH4,5, to EH3, EL3, and EL5,6. A decrease of moderately volatile element abundances from EH4,5 to EH3 has been reported by a number of authors (Weeks and Sears, 1985; Kallemeyn and Wasson, 1986; Kaczaral et al., 1988). Zhang et al. (1995) also reported INAA data for several enstatite chondrites of the two metamorphic sequences. The data for Mn and Se reported by these authors display a similar abundance trend among the various types of enstatite chondrites as found in this work (no data for Zn were reported in their INAA results). Zhang et al. (1995) gave, however, much lower abundances of common siderophiles (Ni, Co, and Fe) and moderately volatile siderophiles (Au, As, and Ga) for EL3 chondrites than those obtained in this study. We have analyzed duplicate samples of one EL3 chondrite and triplicate samples of the other two EL3 chondrites but never found siderophile abundances as low as those reported in the study of Zhang et al. (1995). There is, however, a possibility that all the EL3 chips we obtained are relatively enriched in metal, leading to the higher siderophile abundances in our samples compared to those of Zhang et al. (1995). Biases in sampling are always a problem in the study of metal-bearing chondrites (Jarosewich, 1990), and this problem is more pronounced when INAA is applied since the sample size with this technique is limited. In order to obtain a true chemical trend, possible variations by sampling need to be considered. Among the nonvolatile elements, V displays highly chalcophile and Co highly siderophile characteristics in the enstatite meteorites (Lodders et al., 1993). The chalcophile tendency of V in the enstatite chondrites can also be inferred from its depletion in the Qingzhen chondrules compared with other lithophiles of similar volatility (Grossman et al., 1985). Thus, comparison between V-normalized abundances of volatile chalcophiles and Co-normalized volatile sidero-

Compositional continuity of enstatite chondrites

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Fig. 6. Comparison of CI-normalized elemental abundances for the metal from enstatite chondrites with those for ordinary chondrites. Data for ordinary chondrites are from Kong and Ebihara (1997). In all these five groups, the concentrations of noble metals increase with increasing volatility, while those of moderately volatile elements (from Au to Ge) decrease with increasing volatility.

philes enable a better visualization of volatile element abundance trends, eliminating sample heterogeneity. Variations of V-normalized abundances of Mn, Se, and Zn and Conormalized Au, As, Ga, Sb, and Ge among the enstatite chondrites are shown in Fig. 7. For Ga, Sb, and Ge, abundance ratios of the bulk metal fractions were used for comparison. It is apparent that variations of Mn/V, Se/V, and Zn/V ratios among the enstatite chondrites resemble the changes of the absolute abundances of Mn, Se, and Zn. This is mainly due to the small variation of V contents among the enstatite chondrites. While average Au/Co and As/Co ratios are higher for EH than for EL chondrites, differences in Au/Co and As/Co ratios between EL3 and EL5,6 are indistinguishable. The subtle variations of Au/Co and As/ Co ratios among enstatite chondrites can be ascribed to similar volatilities of Co, Au, and As. Ga/Co ratios for the bulk metal of EH3 and EL3 are lower than those for the EL5,6 chondrites, contrary to the general trend of volatile elements. This can be explained in the context that Ga partly exists as nonmetal form in type 3 enstatite chondrites, since thermodynamically Ga is more easily oxidized than Fe, and some Fe has been observed in the silicates of type 3 enstatite chondrites (Weisberg et al., 1994). On the other hand, the fraction of Ga in the silicates, like Fe, was reduced and trapped into the metal fractions of metamorphosed enstatite chondrites during metamorphism. The Sb/Co and Ge/Co ratios clearly display a decrease from EH4,5, to EH3, EL3, and EL5,6. Antimony and germanium are the most volatile sider-

ophiles measured in this study and thus should be most sensitive to thermal processes. To summarize, it is clear that the general trend of decreasing abundances of moderately volatile elements within the sequence of EH4,5-EH3-EL3EL5,6 is independent of variations of proportions of various phases. Thus, the continuity in moderately volatile element abundances among the various types of enstatite chondrites must be indigenous and is not the result of sample heterogeneity. Kallemeyn and Wasson (1986) attributed the depletion of moderately volatile elements in EH3 chondrites compared to EH4,5 to weathering related losses of specific minerals. With this explanation it is expected that Se/V, As/Co, Ga/ Co, and Sb/Co ratios remain constant for EH3 and EH4,5 chondrites; data for these ratios from Kallemeyn and Wasson (1986) are shown in Fig. 8. Data for RPK A80259 are excluded from the plot because RPK A80259 shows mineral properties and bulk chemistry more like those of EL chondrites (Week and Sears, 1985; Zhang et al., 1995). It is seen that the Se/V, As/Co, Ga/Co, and Sb/Co ratios clearly resolve EH4,5 from EH3, with EH4,5 chondrites having higher ratios. Thus, the depletion of Se, As, Ga, and Sb in EH3 chondrites compared to EH4,5 cannot be attributed to weathering; it is not expected that only volatile elements would have been selectively leached from the sulfide and metal phases during weathering in Antarctica. Therefore, the difference in moderately volatile element abundances between EH3 and EH4,5 was established very early.

4906

P. Kong, T. Moil, and M. Ebihara

EH3 EH4,5 EH mean

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Fig. 7. To eliminate a possible bias in sampling, the abundances of volatile chalcophiles and siderophiles are normalized to those of the refractory ehalcophile element V and the less volatile siderophile element Co, respectively. M n / V and Se/V ratios are in the left Y-axis and Zn/V ratios are in the fight Y-axis. After these corrections, the continuous trends of Mn, Se, Zn, Sb, and Ge abundances still exist, although trends from EL3 to EL5,6 are weak for Au and As. Gallium shows a distinct trend, with relative abundances for EH3 and EL3 being lower than those for EL6 chondrites. This can be explained by assuming that Ga is partly present in the silicates of EH4 and EL3, while in EH4,5 and EL5,6 chondfites Ga entered quantitatively into the metal fraction during metamorphism.

For better evaluation of variations of the moderately volatile element abundances within the enstatite chondrites, data for Mn, Se, As, and Sb from both this study and from Kallemeyn and Wasson (1986) are plotted in Fig. 9. The figure shows that concentrations of these four elements vary in

the sequence of EH4,5-EH3-EL3-EL5,6, in contrast to the identical concentrations of moderately volatile elements in ordinary chondrite groups (Palme et al., 1988). Because the continuous decrease of moderately volatile element abundances from EH4,5 to EH3, EL3, and EL5,6

Compositional continuity of enstatite chondrites 21

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must be indigenous, we will first discuss the process that has been responsible for the fractionation of moderately volatile elements in the enstatite chondrites. 5.3. Fractionation of Moderately Volatile Elements in the Enstatite Chondrites There is some discussion in the literature regarding the origin of low contents of moderately volatile elements in the EL group relative to the EH group. The low volatile element content was either established in the nebula (Wasson and Wai, 1970; Lanl et al., 1973; Baedecker and Wasson, 1975; Takahashi et al., 1978; Hertogen et al., 1983; Kallemeyn and Wasson, 1986; Palme et al., 1988) or occurred within a parent body (Biswas et al., 1980; Dodd, 1981; Kaczaral et ai., 1988; Fogel et al., 1989). It is found in this work that the abundances of moderately volatile elements vary even among chondrites of the same chemical group but different petrographic types, e.g., EH4,5 and EH3. If the variations of moderately volatile element abundances with petrographic type were due to metamorphism in the parent body, i.e., losses of volatile elements by metamorphic heating, it would be hard to explain why the metamorphosed EH cbondrites (EH5) retain a higher fraction of volatile elements than the

4907

primitive EH chondrites (EH3). Furthermore, heating experiments showed that at low fo2 (between 10 -16"5 and 10 -1z3 arm at 1050°C), Au, As, and Mn were not lost from the Allende meteorite during 4 day heating ( Wulf et al., 1994). Thus, if losses of moderately volatile elements in the EH3 chondrites compared with EH4,5 are explained in terms of shock-related reheating of the EH3 chondrites, the temperatures associated with shock on the EH3 region are required to be over 1050°C in order to reduce Au, As, and Mn from at least the EH4,5 level to the present amounts of EH3. Under these conditions, however, it seems impossible that the well delineated chondrules in the EH3 chondrites can still be preserved. Apparently, the moderately volatile element pattern of the enstatite chondrites cannot be attributed to any parent body process; the pattern must have been established before or during accretion in the solar nebula. Although various volatile element patterns in solids may have been established at a variety of heliocentric distances by different degrees of gas-loss during condensation (Wasson and Chou, 1974), one may expect that during accretion the various components will mix and thus erase differences in volatile element contents of a single parent body. If EH3, EH4,5, EL3, and EL6 were derived from different parent bodies with different volatile element patterns, it is strange that some parent bodies have been metamorphosed whereas others remain unmetamorphosed. The closely related redox states and oxygen isotopic compositions of enstatite chow drites (Baedecker and Wasson, 1975; Clayton et al., 1984; Clayton and Mayeda, 1985) imply that the timing of formation of the various classes of enstatite cbondrites was similar. It is, thus, unlikely that the volatile element patterns of enstarite chondrites were established prior to accretion; rather, it should have been established during agglomeration and accretion. The different patterns of moderately volatile elements for the various classes of enstatite chondrites can be related to either continuous variations of the ambient gas temperatures during accretion (Blander, 1971; Baede~ker and Wasson, 1975) or variable degrees of volatilization resulting from local beating during cbondrule formation (Anders, 1964; Larimer and Anders, 1967). As discussed above, variations of Au, As, and Mn from the EH4,5 level to those of EH3, EL3, and EL6 will require the ambient gas temperatures as high as 1000*C. In such a high temperature accretion process more volatile elements such as Ga, Se, and Zn will be completely lost as demonstrated by heating experiments (Biswas et al., 1980). Our results show, however, retention of some fractions of these elements in the enstatite chondrites (Table 7), suggesting that the accretion temperature was not very high. In addition, if accretion occurred in a gas at such a high temperature, it is not expected that EH chondrites could have cooled at rates >6*C/h as estimated by Skinner and Luce( 1971 ). We, therefore, prefer the hypothesis that moderately volatile elements in the enstatite chondrites were lost during the chondrule f o ~ o r L p r o c e s s . One may argue that efiondrnles are.m~nly composed of silicates whereas most/moderately volatile elements in the enstatite chondrites are chalcophile and si/~ophile. Kong and Ebihara (1996;/1997) proposed that metal and chon-

4908

P. Kong, T. Mori, and M. Ebihara

35

I

[

Se ( p p m ) 30

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25

EH4,5 EH3 EL3 EL5,6

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20

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O

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Fig. 9. Comparison of concentrations of Mn, Se, As, and Sb in various petrographic types of enstatite chondrites. Data are from this study (points which are underlined) and from Kallemeyn and Wasson (1986; the other points). It is shown that the concentrations of these four moderately volatile elements smoothly decrease from EH4,5 to EH3, EL3, and EL5,6 chondrites.

drules in ordinary chondrites are complementary components resulting from a common melting event. Although the arguments we have developed are not applicable to enstatite chondrites because of their highly reduced nature, there is textural evidence showing that metal and sulfide in the enstatite chondrites have also experienced melting. In Qingzhen oldhamite and troilite grains occur in a single metal grain (Fig. ld in Crozaz and Lundberg, 1995). This texture is not easily explained by condensation, rather, it may indicate association of these minerals by melting. Furthermore, it is not expected that metal and sulfide in the enstatite chondrites could escape, without modification, the heating event that

produced the chondrules. Thus, volatile siderophiles and chalcophiles could be vaporized during the thermal event that was responsible for chondrule formation. Although details about this process are not well understood, it is clear that the temperatures associated with the chondrule formation are high enough to enable vaporization of Au, As, and Mn, and the subsequent cooling is rapid to allow retention of some fractions of the more volatile elements Ge, Se, and Zn (Alexander, 1994; Connolly and Hewins, 1995). If the process of chondrule formation was responsible for the removal of moderately volatile elements from the enstatite chondrites, formation of chondrules seems very likely to have proceeded

Compositional continuity of enstatite chonddtes during the accretional stage and lasted for a certain period. As a result, moderately volatile elements in the enstatite chondrites were gradually lost as accretion proceeded. This may imply that collision between grains has played an important role in the formation of chondrules in the enstatite chondrites. The collision process appears to have been accompanied by agglomeration of dusts as suggested by the textures of chondrules which are fine-grained in EH chondrites, containing abundant volatile elements, but coarsegrained and volatile poor in EL chondrites (Grossman et al., 1995; Taunton et al., 1996).

5.4. Formation of the Enstatite Chondrite Parent Body As discussed above the abundances of moderately volatile elements constrain the formation sequence of enstatite chondrites, i.e., EH4,5 chondrites, having the highest content of moderately volatile elements, should have formed first, then the EH3, EL3, and EL5,6 chondrites formed. This sequence is compatible with enstatite chondrites either coming from a single parent body or from two parent bodies. If enstatite chondrites were derived from two parent bodies, as illustrated in Fig. 10, the later formed EL6 chondrites must have been metamorphosed by external heating. The metamorphism of EL6 chondrites could not have resulted from shock related heating, as all known EL6 chondrites were classified with shock stage 2 (Rubin et al., 1995). Recently, Rubin et al. (1997) suggested that the EL6 chondrites were previously shocked to stages 3 - 5 whose textural features were annealed during thermal metamorphism, and the meteorites were later reshocked to stage 2. Even if this was the case, a heat source, not from shock, is still required to induce the thermal metamorphism of EL6 chondrites. If the energy for the crystallization of EL6 chondrites was derived from T-Tauri activities of the sun, it is hard to envision that while leading crystallization of minerals in the later formed EL parent body, the external heating did not affect the early formed EH parent body. It can be noticed that the EH3 chondrites, which are less metamorphosed, were located in the outer part of the EH parent body (Fig. 10). Having similar oxygen isotopic compositions and redox properties, the EH and EL chon-

F o r m a t i o n sequence: E H 5 - 4 ~ E H 3 - - ~ E L 3 - - ~ - E L 6

Fig, 10. Two parent bodies assumed for the enstatite chondrites. The formation sequence is inferred from moderately volatile element abundances in the enstatite chondrites. It is apparent that the crystallization of silicates in the EL6 chondrites need an external heating. With this model, it is hard to envision that the external heating did not affect the less metamorphosed EH3 chondrites, which were located at the surface of the early formed EH parent body.

4909

dritcs should have been closely related in space. It appears, therefore,thatcnstatitechondrites may not have been derived from two parent bodies. The continuity in compositions of enstatite chonddtes and, in particular,the inverse variations of moderately volatileelement abundances with petrographic type between E H and EL groups may indicate that all enstatitechondrites form a single evolutionary line and have been derived from a single parent body. The following pieces of evidence are consistent with the formation line of enstatite chondrites: (I) As seen from Table 7 the fractionationof nonvolatile elements among the enstatite chondritcs are strongly dependent on the chemical affinity of the elements. Abundances of Al, Ca, and Sc, which are contained in the silicates,increase from the E H to the EL group. In contrast, the abundances of V and Cr, which are sited in the sulfide phase, decrease weakly, and those of Ix, Os, Ni, Co, and Fe, which are concentrated in the metal phase, decrease manifestly from EH4,5, to EH3, EL3, and EL5,6. It is apparent that the variation of nonvolatile element abundances depends on the density of the minerals where these elements are contained. This may imply some kind of differentiation during gravitational sedimentation on the enstatite chondrite parent body. Metal, having the highest density, would be more effectively accreted into the core then the sulfide and the silicates should be least affected. Such a differentiation has been preserved in the enstatite chondrites that were agglomerated at different times. EH5 chondrites, which are rich in metal, must have accreted first and EL6 chondrites, which are metal-poor, should have accreted later. Sulfide has a density intermediate between those of metal and silicates. Thus, the abundances of sulfide are less variable among enstatite chondrites compared with those of metal and silicates. (2) The cosmic-ray exposure ages of enstatite chondrites display an increase from EH5, to EH4 then to EL6 (Wasson and Wai, 1970; Crabb and Anders, 1981). The exposure age of Qingzhen falls between those of EH4 and EL6 (Crabb and Anders, 1982). Cosmic-ray exposure ages may provide information regarding clustering relationship of meteorites within a parent body. The longer exposure ages of EL6 chondrites may indicate an earlier shock on the EL6 chondrites and, hence, EL6 chondrites should be in the outer part of the parent body. It is remarkable that the sequence of exposure ages of enstatite chondrites is parallel to the sequence of their chemical compositions. This implies that the original structure of the enstatite chondrite parent body has not been completely disrupted at an early stage, as in the case of the ordinary chondrites (Crabb and Schultz, 1981 ). Rather, the enstatite chondrite parent body seems to have been gradually eroded inward by shock. (3) I-Xe data show that the EL group evolved 4.0 myr longer than the EH group, if both groups formed from a homogeneous iodine isotope reservoir (Kennedy et al., 1988). If the I-Xe age was fixed during condensation and agglomeration, the longer-evolved EL cbondrites must have accreted later than EH, i.e., Eli chondrites were located in the interior of the parent body. If the I-Xe age reflects metamorphism in the parent body, the I-Xe dating must have set 4.0 myr later in the EL cbondrites than in the EH chondrites. If EL6 chondrites were deeply buried, a longer and a more intense metamorphism of EL chondrites would leave its trace

4910

P. Kong, T. Mori, and M. Ebihara

in the EH chondrites, at least in their metal phases. While Ni contents in the taenite of EL chondrites are around 2 0 30% (Table 4), taenite is rare in EH chondrites ,and its Ni content is very low (about 10%, Keil, 1968). This implies that the metamorphism of EL chondrites did not overprint the EH chondrites and, hence, the EL chondrites cannot have been located in the deeper part of the parent body than the EH chondrites. Based on the above discussion, it is concluded that all enstatite chondrites were derived from a single parent body and were arranged in the sequence of EH5, EH4, EH3, EL3, and then EL6 from inner part to outer part of the parent body. The enstatite chondrite parent body formed by heterogeneous accretion of various materials in the solar nebula. The nonvolatile element abundances of ordinary chondrites of the same chemical group are identical. One would ask why heterogeneous accretion did not occur during the formation of the ordinary chondrite parent bodies. One possible explanation is the gravitational field was quite significant for the formation of enstatite chondrites but not so important for the formation of ordinary chondrites. To express it more explicitly, dusts may have first agglomerated into planetesireals in the location where ordinary chondrites formed, and then the ordinary chondrite parent bodies formed by accumulation of great numbers of planetesimals. In contrast, in the location where enstatite chondrites were born, formation of planetesimals might not be substantial and dusts fell directly to the enstatite chondrite parent body. The latter inference is also consistent with the inference that chondrules were simultaneously produced as the agglomeration of the enstatite chondrite parent body proceeded. A preferential aggregation of metals during the early stage of accretion of the enstatite chondrite parent body, as suggested by Wasson and Wai (1970), may also predict a variation of bulk compositions of ordinary chondrites of the same chemical group. Such a variation for ordinary chondrites, however, has not been observed. Hence, it seems very likely that the difference in gravitational fields is an important determinant for the different accretional histories of ordinary and enstatite chondrite parent bodies. The nonchondritic ratios of REEs for EL6 chondrites, as noted by Kallemeyn and Wasson (1986), have been confirmed in this study as seen in Fig. 4. Compared with CI, the abundances of REEs increase from La to Sm and Yb for EL5,6 chondrites. Leaching experiments performed by Ebihara (1988) on Yamato-691(EH3) showed a relative enrichment of light REEs (LREE) and a depletion of heavy REEs (HREE) in the acid-soluble fraction, with the reverse trend appearing in the residue, which is mainly composed of low-Ca pyroxene. Ebihara (1988) suggested accordingly that while oldhamite is the major carder for LREEs, some HREEs should reside in silicates. Larimer and Ganapathy (1987) and Crozaz and Lundberg (1995) confirmed the enrichment of LREEs relative to HREEs in oldhamite. Crozaz and Lundberg (1995) further showed that enstatite in unequilibrated enstatite chondrites carries detectable amounts of REEs by using an ion-probe, indicating that sulfide is not the sole carrier-phase for REEs in primitive enstatite chondrites. Our analyses of bulk samples of enstatite chondrites imply a subtle decrease of sulfide abundances but an

increase of silicate amounts as accretion proceeded. Thus, the bulk REE pattern of the enstatite chondrites will depend on the relative proportion of the carrier phases of REEs. The EH chondrites appear to contain sulfides and silicates in a proportion making the REEs in the bulk samples in chondritic ratios. However, oldhamite is depleted and silicates are relatively enriched in EL6 chondrites. Hence, a bulk REE pattern for EL6 chondrites reflects largely the distribution of REEs in the silicates.

5.5. Metamorphic History of the Enstatite Chondrite Parent Body Chondrules are well delineated in EH3 and EL3 chondrites, suggesting that these meteorites are less metamorphosed. In contrast, EH5 and EL6 have been metamorphosed to some degree. The arrangement of enstatite chondrites in the sequence of EH5, EH4, EH3, EL3, and EL6 from inner part to outer part of the parent body thus need two stages of metamorphism in the enstatite chondrite parent body, Metamorphism on the EH chondrites should be invoked by internal heating whereas a metamorphic trend from EL6 to EL3 must have been imprinted by external heating. The two stage metamorphism model has been proposed to account for the difference in taenite components between equilibrated and unequilibrated ordinary chondrites of L group (Kong and Ebihara, 1996). It has been suggested that EH5 chondrites were equilibrated at temperatures between 700 and 800°C during metamorphism, in contrast to about 200°C for the equilibration of EL6 chondrites (Skinner and Luce, 1971 ). The difference in equilibrium temperatures implies different cooling rates for EH5 and EL6 chondrites during metamorphism. Zhang et al. (1996) also suggested that the metamorphosed EH chondrites underwent a more rapid cooling than the metamorphosed EL chondrites, which was inferred from the structure of enstatite in the EH4,5 and EL6 chondrites. The difference in cooling rates during metamorphism between EH and EL groups indicates that metamorphosed EH and EL chondrites cannot have resulted from a single metamorphic event. The above conclusion is further supported by the difference in taenite composition between EH and EL chondrites; taenite is very rare and has a very low Ni content (10%, Keil, 1968; Rubin and Keil, 1983) in the EH chondrites, whereas the Ni content in the taenite of EL chondrites is relatively high (20-30%, Table 4). The low taenite Ni content implies that EH chondrites must have cooled rather rapidly. By comparing the microstructure of carbon in the Abee metal with those in the laboratory-prepared alloys, Herndon and Rudee (1978) gave a very rapid cooling rate for Abee (EH4), less than 10 h from above 700°C to room temperature. Based on studies of quenching rates of coexisting sulfide phases, Skinner and Luce ( 1971 ) pointed out that EH4,5 chondrites must have cooled below 800°C at rates greater than 6°C/h. With such rapid cooling, the energy leading to the internal metamorphism cannot have been derived from any radioactive decay. Some authors suggested that the fast cooling rates of EH chondrites were related with heating imposed by a late shock event (Rubin and Keil, 1983; Torigoye and Shima, 1993). The 4.5 Gyr K-Ar and

Compositional continuity of enstatite chondrites U, Th-He ages for Abee, whose cooling rate has been well defined, clearly show no severe reheating after accretion (Crabb and Anders, 1981; Wacker and Marti, 1983; Bogard et al., 1983). Breaking up the enstatite chondrite parent body as early as 4.5 Gyr ago would lead to the cosmic-ray exposure ages of enstatite chondrites overlapping between various classes, as the case of ordinary chondrites (Crabb and Schultz, 1981 ). The actual observations show, however, no overlap for enstatite chondrites. The rapid cooling of EH chonddtes, therefore, must reflect characteristics about accretion rather than a late reheating event. It seems that the fast cooling of EH chondrites can only be ascribed to hot accretion in a cold nebula, and EH chondrites have been metamorphosed during accretion. If this is the case, accretion of the enstatite chondrite parent body seems to have occurred drastically, especially during the early stage when EH4,5 chondrites formed. The early formed meteorites (EH4,5) may have been heated by energy converted from gravatitional potential energy to temperatures of 700-800°C for mineral equilibration (Skinner and Luce, 1971). Some meteorites may have been partly melted, repidly cooled, and then reheated during accretion, as suggested for the case of Abee by Rubin and Scott (1997). Koug and Ebihara (1996) proposed that the internal metamorphism must be responsible for the transformation of taenite into tetrataenite in both equilibrated and unequilibrated ordinary chondrites. Hence, the internal heating for ordinary chonOrites must have lasted rather long (around 100 myr; Wood, 1967), ended after the external heating. The difference in internal metamorphic histories may imply that enstatire chondrites accreted later than ordinary chondrites, if radioactive decay was responsible for the internal metamorphism of ordinary chondrites. Enstatite chondrites should have formed after the decay of major radionuclides and, as a result, no energy could be accumulated to heat their parent body, If this is the case, the energy that resulted in the internal metamorphism of ordinary chondrites should have come from the decay of short-lived radionuclides, such as 26A1. The energy which provide the external heat to the enstatite and ordinary chondrite parent bodies could have been derived from the same source. Kong and Ebihara (1996) suggestod that either activities of the early sun or Jupiter have supplied energy for the external heating of the L chondrite parent body. Because of the broad influence to the asteroid bodies (ordinary and enstatite chondfite parent bodies), it appears more likely that the external heat source was derived from the early activities of the sun. 6. CONCLUSION Our replicate analyses of various types of enstatite chondrites show a compositional continuity for lithophile, chalcophile, and siderophile elements. Abundances of nonvolatile lithophiles increase from EH4,5, to EH3, EL3, and ELS,6. In contrast, those of nonvolatile and less volatile chalcophile elements, such as V and Cr, decrease weakly, and those of nonvolatile siderophiles and moderately volatile elements decrease apparently within this sequence. The decrease of moderately volatile element abundances from EH4,5, to

4911

EH3, EL3, and ELS,6 is independent of the mineralogy of the enstatite chondrites. The smooth variation in composition of enstatite chondrites and, in particular, the inverse variations of moderately volatile element abundances with petrographic type between EH and EL groups suggest that enstatite chondrites are derived from a single parent body. The enstatite chondrite parent body may have formed by heterogeneous accretion of materials in the nebula. Additional fractionation may have occurred by small gravitational separation of iron-metal. Metal, with the highest density, was concentrated in the central region, where EH chondrites were derived, and silicates, with lower densities, were gradually enriched in the layers that accreted later. The continuous decrease of moderately volatile element abundances from EH4,5, to EH3, EL3, and EL6 could not have resulted from parent body processes. The temperatures required for vaporizing Au, As, and Mn are so high (1000°C) that well defined chondrules could not be preserved in the EH3 and EL3 chondrites. Furthermore, one may expect that any fractionation of moderately volatile elements prior to accretion would be erased during assembly of a single parent body. Hence, the different patterns of moderately volatile elements for the various classes of enstatite chondrites should have been established during accretion. If the fractionation of moderately volatile elements was related to variations of ambient gas temperatures, again for effective removal of Au, As, and Mn, the temperatures required for the ambient gas would have been too high to allow the more volatile elements such as Ge, Se, and Zn to be preserved in the enstatite chondrites. It is, therefore, suggested that moderately volatile elements in the enstatite chondrites were lost during local heating, during the chondrule formation process. The metamorphic features of EH and EL chondrites are explained in terms of two stages of metamorphism: internal metamorphism, which was responsible for the crystallization of EH chondrites, and external metamorphism, which resulted in the crystallization of EL chondrites. The internal metamorphism lasted short, suggesting absence of internal heat sources; EH chondrites seem more likely to be metamorphosed during accretion. Crystallization of EL chondrites was produced by external heating. The broad influences on the asteroid bodies (enstatite and ordinary chondrite parent bodies) that formed at different heliocentric distances may suggest that the activity of the early sun was the energy source for the external heating.

Acknowledgments We thank the Meteorite Working Group of NASA/NAF for supplying the Antarctic meteorite samples used in this study and Prof. D. Wang for donating the Qingzhen meteorite. We are grateful to Prof. H. Palme for helpful suggestions and discussions and a formal review of the manuscript. Useful reviews are also provided by M. K. Weisberg and an anonymous referee to whom we are acknowledged. We thank the reactor committee of the University of Tokyo for the cooperative use of the facilities at the Institute for Atomic Energy, St. Paul's University. P. Kong is indebted to the JSPS fellowship. This work was partly supported by a Grant-in-Aid for Scientific Research of the Ministry of Education, Science and Culture, Japan (No. 05453012). REFERENCES

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