Elemental abundances in chondrules from unequilibrated chondrites: Evidence for chondrule origin by melting of pre-existing materials

Earth and Planetary Science Letters, 50 (1980) 171-180 Elsevier Scientific Publishing Company, Amsterdam Printed in The Netherlands

171

[61

ELEMENTAl. ABUNDANCES IN CHONDRULES FROM UNEQUILIBRATED CHONDRITES: EVIDENCE FOR CHONDRULE ORIGIN BY MELTING OF PRE-EXISTING MATERIALS JAMES L. GOODING 1 and KLAUS KEIL Department of Geoh)gy and Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131 (U.S.A.)

TAKAAKI FUKUOKA 2 and ROMAN A. SCHMITT Department of Chemisto, and Radiation Center, Oregon State UniversiO', Corvallis, OR 97331 (U.S.A.)

Received April 21, 1980 Revised version received June 2, 1980

Bulk abundances of Na, Mg, AI, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, La, Sm, Eu, Yb, Lu, Ir, and Au were determined by neutron activation analysis of chondrulcs separated from unequilibrated t1-, L-, and LL-chondrites (Tieschitz, Hallingeberg, Chainpur, Semarkona) and correlated with chondrule petrographic properties. Despite wellknown compositional differences among the whole-rock chondrites, the geometric mean compositions of their respective chondrule suites are nearly indistinguishable from each other for many elements. Relative to the condensible bulk solar system (approximated by the C1 chondrite Orgueil), chondrules are enriched in lithophile and depleted in siderophile elements in a pattern consistent with chondrule formation by melting of pre-existing materials, preceded or attended by silicate/metal fractionation. Relative to nonporphyritic chondrules, porphyritic chondrules are enriched in refractory and siderophile elements, suggesting that these two chondrule groups may have formed from different precursor materials.

1. Introduction Determination of elemental abundances in meteoritic chondrules is an important part of efforts to distinguish between nebular condensation (e.g. [ i ] ) or other mechanisms (e.g. [2,3]) for chondrule formation. Whole-chondrule bulk compositions have been previously determined by a variety of techniques including instrumental and radiochemical neutron activation analysis (INAA, RNAA) [ 4 - 9 ] and electron microprobe analysis of fused chondrule pellets [ 1 0 - 1 4 ] . Compositions of chondrules in whole-chondrite thin-sections have been estimated by electron

1 Current address: Planetology Section 183-501, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91103, U.S.A. 2 Current address: Department of Chemistry, Gakushuin University, 1-5-1 Mejiro Toshima-Ku, Tokyo 171, Japan.

microprobe expanded-beam analysis [3,15,16] and by combined electron microprobe and modal phase analyses [17]. In support of earlier work [18,19] we report here extensive new data on bulk abundances of 19 major, minor, and trace elements in whole chondrules separated from four different unequilibrated chondrites (17 Tieschitz, H3; 24 Hallingeberg, L3; 14 Chainpur and 15 Semarkona, LL3; classified by Van Schmus and Wood [20]). We discuss previously unidentified correlations between bulk compositions and textures of chondrules and argue that chondrules formed by melting of pre-existing solid materials rather than by direct nebular condensation. Recognizing that solution of the problem of chondrule origins has been complicated by the absence of a consensus definition for "chondrule", it should be noted that the work reported here is limited to generally spheroidal or ellipsoidal objects of ~ 0 . 3 - 4 mm

0012-821X/80/0000-0000/$02.25 © 1980 Elsevier Scientific Publishing Company

172 size which possess internal textures implying rapid and/or incomplete crystallization of molten or partially molten droplets. Such objects display radial, finegrained granular, cryptocrystalline, porphyritic, or barred crystal arrays with some amount of glassy interstitial mesostasis of feldspathic composition which can be interpreted as solidified residual igneous liquid. A detailed account of chondrule textural types and their relative abundances is given elsewhere (Gooding and Keil, submitted for publication).

2. Analytical methods and results

Detailed descriptions of all analytical procedures and data reduction methods may be found elsewhere [21] or obtained directly from the senior author (J.L.G.). Essential features are described below. Whole chondrules were mechanically separated from their host chondrites and physically cleaned of adhering matrix material (without chemical treatment). Chondrules spanning the entire observable size range were sampled (i.e., no effort was made to preferentially select large chondrules for ease of analysis) although bias against highly friable chondrules probably existed to a minor extent as previously mentioned by Osborn et al. [7]. Each chondrule was subjected to nondestructive INAA involving two separate high-flux neutron irradiations, utilizing both elemental and rock standards, and 7-spectrometry with a liquid nitrogen-cooled Ge(Li) detector interfaced with a multichannel analyzer. Final results were verified by comparison with results for 7 different aliquots of each of the standard rocks BCR-1, PCC-1, and JB-I [22] and the extensively analyzed carbonaceous chondrites Orgueil, Mighei, and Allende, all of which we analyzed along with the chondrules as chondrule-sized powdered aliquots. After INAA, each chondrule was sliced into multiple sections (using a precision wire saw) which were mounted, thinned and diamondpolished for subsequent petrographic study and electron microprobe analysis. Detailed results and discussion of correlations between the INAA and microprobe data will be presented elsewhere (see also (gooding [211). Our geometric mean results for Tieschitz and Chainpur chondrules (Table 1), respectively, are corn-

parable to those reported by Osborn et al. [7] except for higher lithophile and lower siderophile element abundances in our data, a difference which can be attributed to inclusion of several metal-rich chondrules (up to 65 wt.% bulk Fe) in the Osborn et al. [7] data set. For the elements determined, our Tieschitz and Chainpur data are also in general agreement with those of McSween [ 16] and our Chainpur data are comparable to those of Grossman et al. [9]. No comparison data are available for Hallingeberg and Semarkona chondrules (Dodd [23] reported mineral compositions but not bulk compositions tk~r Hallingeberg chondrules).

3. Normalized elemental abundances Since bulk Si was not directly determined in the present study (see Schmitt et al. [5] for Si data and discussion of INAA special requirements), the conventional geochemical practice of normalizing elemental abundances to 106 atoms of Si (e.g. [24]) could not be applied. However, current estimates of condensible solar system elemental abundances are based largely on chemical analyses of Type 1 (CI or C 1) carbonaceous chondrites (e.g. [24,25 ]), so that the bulk composition of the C 1 chondrite Orgueil, as determined in our study, was selected as the reference composition for normalization (Table 2, Fig. 1). Normalization to Orgueil (Fig. 1) is not meant to imply derivation of chondrules from C 1 material but is used simply to summarize the compositions of chondrules relative to that of the condensible bulk solar system. For tile elements considered here, minor bulk compositional differences between C 1 and C2 chondrites are not important and our use of Orgueil as a normalization standard is consistent with the general similarity of C 1/solar elemental abundances [26]. The Orgueil composition in Table 2 includes a mean Eu abundance derived from data of Evensen et al. [27], and selected in preference to our value which, at the present stage of data reduction, may be systematically high (see Gooding [21]). All other Orgueil elemental abundances are, within the stated uncertainties, in good to excellent agreement with various partial bulk analyses of Orgueil by Wiik [28], Schmitt et al. [29], Kr~ihenbi.ihl et al. [30], and Nakamura [31]. Our value for Lu may be systemati-

Hf(ppm)

Co (ppm) Ni (ppm) Ir(ppb) Au (ppb)

0.06 (8)

0.10 0.06 0.08 (13) 0.08 0.014 (16)

3.8 17 16 (13)

0.6 0.14 5.9 0.16 5.2 0.7 0.43

± 162 ± 3270 (16) ± 208 (16) ± 44

±

0.16

36 3140 152 49

+ -+ -+ ± ±

± ± ±

± + ± ± ± ± ±

0.38 0.25 0.11 0.26 0.038

10.8 77 25

Sc (ppm) V (ppm) Zn (ppm)

La (ppm) Sm (ppm) Eu (ppm) Yb (ppm) Lu (ppm)

2.6 0.60 12.9 0.43 29.5 2.1 1.08

A120 3 (%) C r 2 0 3 (%) Fe (total) (%) MnO (%) MgO (%) CaO (%) N a 2 0 (%)

mean c

Tieschitz (H3) N=17 b

80 2030 74 32

0.15

0.37 0.25 0.084 0.25 0.036

10.3 75 20

2.5 0.58 11.7 0.39 29.1 2.0 1.01

geometric mean

0.29 142 4060 432 49

0.05 51 1230 52 20

0.07 140 3110 178 53

0.62 0.42 0.12 0.34 0.086

13.1 88 30

3.2 0.64 11.0 0.38 27.5 2.6 1.18

mean c

0.08 0.05 0.042 0.07 0.011

2.8 16 10

0.5 0.14 4.4 0.14 4.9 0.6 0.31

-SDGM e

0.39(11)

0.77 0.47 0.11 0.15 0.164

17.8 17 24

1.8 0.24 4.9 0.14 3.4 1.4 0.35

± 199 ± 8430 (23) -+ 1390 (23) -+ 57 (23)

±

~ + ± ± ±

± ± ±

-+ ± * ± ± ± ±

Hallingeberg (L3) N=24 b

0.11 0.06 0.084 0.10 0.015

3.8 2O 21

0.6 0.19 7.0 0.21 5.9 0.8 0.46

+SDGM e

Summary of chondrule bulk compositions determined by INAA a

TABLE 1

88 2020 71 32

0.19

0.49 0.34 0.094 0.32 0.057

10.0 86 23

3.0 0.60 10.0 0.34 27.3 2.4 1.11

geometric mean

129 3350 242 48

0.23

0.35 0.23 0.091 0.14 0.053

7.4 16 23

1.4 0.28 5.6 0.23 3.6 1.2 0.52

+SDGM e

52 1260 55 19

0.11

0.21 0.14 0.046 0.10 0.028

4.3 14 12

0.9 0.19 3.6 0.14 3.2 0.8 0.35

-SDGM e

0.14

lff(ppm)

0.03 (9)

0.05 0.04 0.12(11) 0.05 0.008

1.3 12 18 (10)

+ 244 ± 3720 ~ 196 • 44

+

± + ± -+ ±

± • ±

± + ± ± ± + ±

141 3320 81 35

0.14

0.36 0.24 0.079 0.27 0.027 (12)

9.1 77 24

2.3 0.63 12.2 0.38 26.8 1.7 0.97

241 4420 138 56

0.03

0.05 0.04 0.146 0.05 0.009

1.4 12 23

0.3 0.07 8.3 0.28 4.1 0.4 0.38

89 1900 51 22

0.02

0.05 0.03 0.051 0.04 0.007

1.2 10 12

0.2 0.06 4.9 0.16 3.5 0.3 0.27

0.14 (8)

0.10 0.15 0.03 0.08 0.008

2.3 12 6

0.7 0.37 4.1 0.10 4.0 0.5 0.34

~ 101 * 2089 ± 82 (14) -' 39 114)

•

0.33 139 2650 82 48

+ * ± + +

* * ±

+ ± * ± * ~t

0.38 0.32 0.11 0.27 0.044

8.4 80 8

2.5 0.68 12.3 0.50 26.2 1.9 0.90

geometric

105 1740 56 37

0.31

0.37 0.29 0.10 0.25 0.043

8.0 79 6

2.4 0.62 11.5 0.49 25.9 1.8 0.83

mean

139 3830 78 42

0.15

0.13 0.16 0.04 0.10 0.008

3.0 14 7

0.9 0.29 5.9 0.12 4.6 0.6 0.46

+SD(;M e

60 1200 33 20

0.10

0.10 0.11 0.03 0.07 0.007

2.2 12 3

0.6 0.20 3.8 0.10 3.9 0.5 0.30

SI)(;M e

a Weight percent (%), parts per million (ppm), and parts per billion (ppb); for a chondrule of - 1 . 5 mg (a value near ttle peak in the chondrule mass distribution; [21 ~), typical percentage m a x i m u m relative errors in final elemental abundance values (± one standard deviation from counting statistics) are as follows: AI(2), Cr(1), Fe(0.5), Mn(3), Mg(5), Ca(10), Na(3), S c (l ), V(5), Zn(10), La(l 5), Sm(10), Eu(10), Y b(l 5), Lu(20), C o ( l ) , Ni(5), lr(10), and Au(10). Uncertainties are similar for chondrules < 1.5 mg but are substantially smaller for chondrules > 1.5 mg. b For elements where fewer chondrules were successfully analyzed, numbe r of chondrules given in parentheses. c !1 standard deviation of the mean. d Excluding metallic chondrules SE-18, 21. e Standard deviation of geometric mean.

225 45 80 141 52

0.36 0.24 0.13 0.27 0.028

9.2 78 29

Sc (ppm) V (ppm) Zn (ppm)

La (ppm) Sm (ppm) Eu (ppm) Yb (ppm) Lu (ppm)

2.3 0.64 13.7 0.42 27.1 1.7 1.01

AI203 (%) Cr203 (~/~) t:e (total) (%) MnO (%) MgO (%) CaO (%) NazO (%)

Co(ppm) Ni (ppm) lr(ppb) Au (ppb)

0.3 0.07 6.5 0.16 3.6 0.4 0.25

SDGM e

mean c

+SDGM e

mean c

geometric mean

Semarkona (LL3) N = 15b, d

Chainpur (LL3) N=14 b

TABLE 1 (continued)

-Z

175 TABLE 2 Bulk composition of the Orgueil (C1) chondrite a Mean -~ 1 SDM b AI203 ('/,) Cr203 (~}) l.'e ltotal) (5;) MnO C/) MgO (%) CaO (54) Na20 (91) Sc (ppm) V (ppm) Zn(ppm) La (ppm) Sm (ppm) Eu (ppm) Yb (ppm) Lu (ppm) Co (ppm) Ni (ppm) Ir (ppb) Au (ppb)

1.7 0.40 19.6 0.25 15.6 1.0 0.59 6.4 53 346 0.26 0.16 0.062 0.17 0.034 563 11,600 506 197

+ ± + + ± -+ *

Geometric mean 0.1 0.03 1.6 0.02 0.7 0.2 0.03

1.7 0.40 19.6 0.25 15.6 1.0 0.59

± 1.4 ± 5 ± 106 + + cl + +-

6.2 53 335

0.03 0.04

0.26 0.16 0.062 d 0.17 0.033

0.04 (6) 0.005 (4)

+ 44 -+ 1320 + 46 -* 20 (6)

562 11,600 505 196

+SDGM c 0.1 0.03 1.6 0.02 0.7 0.2 0.03 1.4 5 97

SDGM c 0.1 0.03 1.4 0.02 0.6 0.2 0.02 1.1 4 75

0.03 0.04

0.03 0.03

0.04 0.006

0.03 0.005

44 1310 45 21

41 1180 41 19

a Concentrations in weight percent (%), parts per million (ppm), and parts per billion (ppb) as determined by INAA in the present investigation, based on analyses of seven separate aliquots, except as noted in parentheses for Yb, Lu, and Au. b Standard deviation of the mean. c Standard deviation of geometric mean. d Average of two analyses by Evensen et al. [27].

cally high c o m p a r e d with the two-analysis m e a n o f 0.0253 ppm derived from N a k a m u r a [31]. In any case, uncertainties in the data o f Table 2 are comparatively small and do not affect the general conclusions discussed in the following sections.

4. Petrogenetic significance of elemental abundance patterns An i m p o r t a n t feature of the data in Table 1 is that, despite substantial c o m p o s i t i o n a l dispersions observed within each chondrule suite, the geometric mean (central t e n d e n c y o f log-normal distribution) compositions of the respective chondrule suites are remarkably similar to each other. In fact, the elemental abundance patterns o f the four different chondrule suites (Fig. 1) are virtually indistinguishable e x c e p t for a few m i n o r differences (e.g., higher Co and Ni and lower Lu abundances in Chainpur chondrules; greater depletion o f Zn in S e m a r k o n a chondrules). Thus, although bulk c o m p o s i t i o n a l dif-

ferences a m o n g whole-rock H-, L-, and LL-chondrites are well k n o w n (e.g. [29,32]), differences a m o n g the mean bulk compositions o f their respective chondrule suites are apparently m u c h less p r o n o u n c e d and possibly nonexistent or, at least, nonsystematic for some elements. Thus, chondrule suites in unequilibrated H-, L-, and LL-chondrites, on the average, appear to have formed from starting materials o f the same or similar bulk compositions. A second major feature o f Fig. 1 is that, in general, chondrules appear to be enriched in lithophile elements and depleted in siderophile elements relative to the condensible bulk solar system (CBSS). The strong depletion o f Zn, relative to CBSS, suggests that chondrules are also depleted in chalcophile elements or other Zn-rich carrier (such as c h r o m i t e ; [33]). A l t h o u g h the apparent depletion o f Lu relative to the other rare earth elements in chondrules m a y be an artifact o f normalization to a systematically high Orgueil Lu abundance (see previous section), the

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Fig. 1. E l e m e n t a l a b u n d a n c e p a t t e r n s in c h o n d r u l e suites, relative to the c o n d e n s i b l e b u l k solar s y s t e m , d e r i v e d b y n o r m a l i z a t i o n o f g e o m e t r i c m e a n c h o n d r u l e d a t a (Table 1) to g e o m e t r i c m e a n Orgueil (C1) c h o n d r i t e d a t a (Table 2). I n s e t s h o w s n u m b e r s o f chondrules analyzed.

fundamentally important siderophile/lithophile fractionation in chondrules seems unmistakable. Furthermore, Fe in chondrules is depleted less than other siderophiles while additional inter-siderophile fractionation is suggested by the chondrule geometric mean weight ratios of Ir/Au = 1.5-2.3 (Table 1) which are lower than that of bulk Orgueil (Ir/Au = 2.58, Table 2; 3.43 + 0.19, computed from table 2 of Krghenbiihl et al. [30]). The existence of inter-lithophile fractionations is less certain although Mg/AI and Ca/A1 are noteworthy candidates. Our chondrule suite geometric mean ratios ofMg/A1 = 1 0 - 1 4 (Table 1) tend to be higher than that ofCBSS (Mg/AI = 10, Table 2; 11, computed from Orgueil data of Von Michaelis et al. [34] and Ahrens et al. [35]). Geometric mean suite ratios of Ca/A1 = 1.0-1.2 (Table l) in chondrules are nearly the same as the bulk chondrite grand mean of 1.10 [34,35] although fractionation relative to CBSS cannot be convincingly demonstrated due to uncertainty

in the Ca/AI ratio of bulk Orgueil (e.g., 0.79 + 0.16, recalculated arithmetic mean data, Table 2; 1.6 rejected in favor of 1.0, Ahrens and Von Michaelis [36];1.1, [35]). Regardless of the degree to which inter-lithophile and inter-siderophile elemental fractionations may exist in chondrules, the gross chondrule/CBSS fractionation patterns (Fig. 1) remain highly significant. These patterns imply fractionation based on geochemical affinity rather than volatility of elements since both refractory and volatile siderophiles (e.g, Ir and Au, respectively) are depleted by similar factors while both refractory and volatile lithophile (e.g., A1 and Na, respectively, or Sc and Mn, respectively) are likewise enriched by similar factors. Thus, formation of chondrules by direct condensation from a cooling solar nebula, a process expected to introduce refractory/volatile fractionations, seems very doubtful. Chondrules more likely formed by melting of preexisting solid materials although clearly not by iso-

177 chemical, bulk melting of CBSS (C 1 chondrite) material. For C 1 chondrite material (or its CBSS isochemical equivalent) to have served as the chondrule precursor, partial or fractional melting (with net separation of silicates from metal and sulfides) must be invoked. Alternatively, chondrules could have formed by bulk melting of solids which had already experienced silicate/metal fractionation, or by limited re-mixing of previously separated silicate and metal components. In any case, the chondrule/CBSS fractionations imply significant geochemical processing of chondrule precursor materials before or during chondrule formation.

5. Evidence for different chondrule source materials The close similarities among population characteristics of different chondrule suites, as discussed above, should not be allowed to overshadow the importance of the chemical and petrological variations among individual chondrules which occur within any single suite [ 19,21 ]. Specifically, we suggest that some petrogenetically significant correlations may exist between the textures and bulk compositions of individual chondrules. Although the major types ofchondrule textures are well known (e.g. [37]) and probably attributable to formational control by both bulk composition and cooling/crystallization history [38], we further suggest that at least some of the different textural types may also represent different precursor materials or formational sources. No conventional taxonomic system has yet been formally adopted for chondrules although at least eight different chondrule primary textural types can be defined and reproducibly identified in unequilibrated ordinary chondrites ([21]; Gooding and Keil, submitted for publication). Following the phyric/aphyric division applied to volcanic rocks, chondrule textures can be roughly divided into porphyritic (olivine -+ pyroxene phenocrysts in glassy or cryptocrystalline inesostasis; barred olivine) and nonporphyritic (radial pyroxene; granular olivinepyroxene; cryptocrystalline) categories. The corresponding division of chondrule bulk compositions (Table 3, Fig. 2) suggests that porphyritic chondrules are enriched in refractory and siderophile elements relative to nonporphyritic chondrules.

We emphasize that the porphyritic/nonporphyritic compositional dichotomy illustrated in Fig. 2 may not be statistically significant for all elements. Although most statistical tests for differences between sample mean values assume Gaussian distributions, while many elements in chondrules appear to follow log-normal distributions [7,8,21 ], application of the t-test (e.g. [39]) to our data is informative. The t-test, for example, shows that the arithmetic mean (+ one standard deviation of the mean) of 11.9 +5.6 wt.% Fe for the porphyritic chondrules, at the 95% level of confidence (c~ = 0.05, f = 22), is not significantly different from the value of 13.1 -+ 4.8% Fe for the nonporphyritic chondrules. However, the same test shows that tire mean A1203 abundance of the porphyritic set (3.0 + 1.3%) is probably significantly different from that of the nonporphyritic set (2.2+0.5%). Computation of t-statistics for the respective Fe and A1203 geometric means (Table 3) yields the same qualitative indications. From these results and the general pattern of Fig. 2 we suggest that the subtle but systematic compositional contrast between porphyritic and nonporphyritic chondrules is significant for most elements (though probably not for Na, Cr, or Fe). The required porphyritic/nonporphyritic chemical mass balance appears permissible since the enrichments shown in Fig, 2 (ratio >1) can be accommodated by the depletion (ratio <1) of Si which, although not shown in Fig. 2, is known from microprobe analyses of the chondrules [21 ], to be generally depleted (ratio 40.9) in porphyritic chondrules relative to nonporphyritic chondrules. The nature of the compositional contrast between porphyritic and nonporphyritic chondrules seems to preclude simple derivation of one type from the other. The relative enrichment of refractory lithophile elements in porphyritic chondmles exists despite dilution effects which might be expected from the generally greater metal abundances in porphyritic chondrules [21 ]. Furthermore, porphyritic and nonporphyritic chondrules appear to have compositionally different siderophile components (i.e., nonidentical Ir/Au, Co/Fe, Ni/Fe ratios) such that exact derivation of one type from the other by closedsystem oxidation/reduction is generally not possible. Likewise, the geometric mean compositions of porphyritic chondrules cannot be explained by simple mixing of metal which material having the geometric

178 TABLI" 3 Geometric mean bulk compositions of porphyritic and nonporphyritic chondrules calculated from INAA results a Porphyritic a

Nonporphyritic b

N = 47 c,d

+SI)(;M f

AI203 ('f) C r 2 0 3 ('~) Fe (total) CX) MnO (<;'~) MgO (9;) CaO ('A) N a 2 0 (5~)

2.8 0.60 10.7 0.35 28.7 2.2 1.04

1.0 0.27 6.4 0.22 4.1 '0.9 0.45

Sc (ppm) V (ppm) Zn (ppm)

10.5 84 18

5.4 17 29

(45)

SDGM f

N = 23 c,e

0.7 0.18 4.0 0.14 3.6 0.6 0.31

2.2 0.62 12.1 0.48 24.7 1.6 0.90

3.6 14 I1

7.6 72 13

+SDGM f 0.6 0.11 6.4 0.19 3.9 0.5 0.50

0.5 0.09 4.2 0.13 3.4 0.4 0.32

2.2 11 13

(17)

f

SDGM

1.7 10 6

La (ppm) Sm (ppm) Eu (ppm) Yb (ppm) ku (ppm)

0.44 0.31 0.11 (43) 0.31 0.048 (44)

0.24 0.16 0.098 0.11 0.038

0.15 0.10 0.053 0.08 0.021

0.34 0.24 0.056 (20) 0.23 0.032

0.09 0.11 0,044 0.07 0.010

0.07 0.07 0.024 0.06 0,008

tlfippm)

0.21

0.17

0.09

0.16

0.12

0.07

Co (ppm) Ni (ppln) lr/ppb) Au (ppb) a b c d e f

(19)

111 2470 95 42

201 4970 259 60

(45) (44) (45)

72 1650 69 24

(17)

76 1670 39 22

71 1610 34 25

37 820 18 12

Porphyritic olivine-pyroxene, porphyritic olivine, porphyritic pyroxene, barred olivine. Radial pyroxene, granular olivine-pyroxene, granular pyroxene, cryptocrystallinc. l:or elements where fewer chondrules were successfully analyzed, n u m b e r of chondrules is given in parentheses. 11 Tieschitz (1t3), 20 Hallingeberg (L3), 7 Chainpur (LL3), 9 Semarkona (LL3). 6 Tieschitz, 4 tlallingeberg, 7 Chainpur, 6 Semarkona. Standard deviation of geometric mean. I

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'

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Fig. 2. Compositional contrast between porphyritic and nonporphyritic chondrules derived from data of Table 3. Bulk Si (not shown) plots at ratio <0.9 as indicated by electron microprobe analyses [21 ]. Inset shows n u m b e r s of chondrules analyzed.

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179 mean composition of nonporphyritic chondrules since differences in lithophile element ratios (e.g., V/Sc and Cr/Mn) between the two types would remain unexplained. Consequently, porphyritic and nonporphyritic chondrules may represent two compositionally overlapping but statistically different populations of precursor materials. Such compositional differences would not be inconsistent with apparent differences in the physical (Gooding and Keil, submitted for publication) and thermal [38] histories of the same two chondrule categories. The extent to which compositional/textural correlations and corresponding populations of precursor materials can be further resolved will be discussed in later papers.

Acknowledgements We thank R.S. Clarke, Jr. (Smithsonian Institution) for providing the Hallingeberg and Semarkona samples, M.-S. Ma for assistance with INAA, and V. Berry and S. Officer, and F. Van Ness for expertly typing the manuscript. H.Y. McSween, Jr. and two anonymous reviewers made useful comments on an earlier draft. Partial financial support was provided by NASA grants NGL 32-004-064 (K. Keil) and 32-002039 (R.A. Schmitt) while additional support for manuscript preparation was provided by the Jet Propulsion Laboratory under NASA prime contract NAS7-100.

References 6. Conclusions From studies of approximately equal numbers of chondrules ( 1 4 - 2 4 each) separated from the Tieschitz (H3), Hallingeberg (L3), Chainpur (LL3), and Semarkona (LL3) chondrites, we conclude the following: (1) Despite well-known bulk compositional differences among the whole-rock chondrites, the geometric mean compositions of their respective chondrule suites are remarkably similar for 19 different elements, suggesting that chondrules in unequilibrated H-, L-, and LL-chondrites formed from similar precursor materials. (2) Relative to the composition of the condensible bulk solar system (represented here by the C 1 chondrite Orgueil), chondrules are enriched in lithophile elements and depleted in siderophile (and possibly chalcophile) elements. This lithophile/siderophile fractionation, which is apparently based on geochemical affinity rather than elemental volatility, argues strongly against direct condensation of chondrules from the primitive solar nebula but favors chondrule formation by melting of pre-existing materials, preceded or attended by silicate/metal fractionation. (3) Relative to nonporphyritic chondrules, porphyritic chondrules are enriched in refractory and siderophile elements, suggesting that these two chondrule groups may have formed from different precursor materials.

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