Chondrules in H3 chondrites: textures, compositions and origins

Gcwhimuu <‘I Cesmochtmtc ‘I Ac,ru Vol 45. pp 615 10 685. 1981 Prmted in Grcar Btitam. All rlghts reserved

00167037’81~050675-11$02.c!o/0 Copyright Q 1981. Perpamon Press Ltd

Chondrules in H3 chondrites: textures, compositions and origins GAYLELux*, KLAUSKEIL and G. JEFFREYTAYLOR Department of Geology and Institute of Mcteoritics. University of New Mexico, Albuquerque, NM 87131, U.S.A. (Received 25 June 1980; accepted in revisedfirm 23 December 1980) Ah&act-Major and minor element bulk compositions of 90 individual chondrules and 16 compound chondrule sets in unequilibrated (type 3) H-group chondrites were determined in polished thin sections by broad beam electron probe analysis and the chondruies were classified petrographically into six textural types (barred olivine, porphyritic olivine, porphyritic pyroxene, barred pyroxene, radiating pyroxene, fine-grained). Although analyses of individual chondrules scatter widely, the mean composition of each textural type (except barred pyroxene) is rather distinct, as verified by discriminant function analysis. Ai205, TiOr and NasO are correlated in chondrules, but A&O3 and CaO do not correlate. Compound chondrule sets were found to consist almost entirely of chondrules or partial chondrules of similar texture and composition. The data suggest that composition played a conspicuous role in producing the observed textures of chondrules, though other factors such as cooling rates and degrees of supercooling prior to nucleation were also important. If compound chondrules formed and joined when they were still molten or plastic then the data suggest that chondrules of each textural type could have formed together in space or time. The correlation of A1203 and TiOr with NarO and not with CaO appears to rule out formation of chondrules by direct equilibrium condensation from the nebula We conclude that the most reasonable model for formation of the majority of chondrules is that they originated from mixtures of differing fractions of high-, intermediate- and low-temperature nebular condensates that underwent melting in space. A small percentage of chondrules might have formed by impacts in meteorite parent-body regoliths.

INTRODUCTION CHONDRITESare

stony meteorites containing millimeter-sixed spherules called chondrules, most of which are crystallized droplets of melts. Chondrules are typically composed of varying proportions of silicate minerals and glass arranged together in distinctive textural patterns. First recognixed in meteorites by HOWARD (1802) and named by Ros (18633, chondrules have long been a source of interest and pualement to scientists in many fields. This interest arises because chondrules are among the most primitive objects available for study in the laboratory, both in the sense that they are among the most unfractionated material in the solar system, and that they are older than any terrestrial and most lunar rocks studied to date. Because most geochemical processing of chondrules ceased early in the history of the solar system, they are of interest as indicators of physical and chemical conditions in the solar nebula as well as the record keepers of early events in the evolving solar system. The complex textures of chondrules that clearly crystallized from a melt have been studied extensively (e.g. TSCHERMAK,1883) and experiments with silicate melts have been carried out under controlled con* Present address: Department of Geological Sciences, Harvard University, and Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, U.S.A.

ditions in attempts to simulate chondrule textures (e.g. BLANDERet al., 1976; PLANNW and Knn, 1979; PLANNER,1980). Results of the experiments suggest that cooling rate and the degree of supercooling are important parameters in producing the observed textures of meteoritic chondrules. However, other factors must also control the textures of chondrules. Among these is composition, which affects the order in which minerals crystallize from the melt, diffusion rates in the melt (hence crystal growth rate), and the degree of supercooling produced by a given cooling rate. To unravel relationships between the compositions and textures of chondrules, we made a detailed study of compositional and textural variations among chondrules in unquilibrated (type 3) H-group chondrites. SAMPLES AND ANALYTICAL PROCEDURES Samples and microprobe analysis

Polished thin sections of the Sharps, Tieschitz and Bremerviirde chondrites, all H3 falls, were studied. Bulk compositions of 90 individual chondrules (30each in Sharps, Tieschitx and Bremervorde) were obtained in thin section by 1OOpm broad-beam electron probe analysis. The analyses were made with an automated ARL EMX-SM instrument, using procedures described previously (Lux et al., 1980). Concentrations of 13 components were determined (SiOl,

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Ti02, A1203, Crz03, FeO, MnO. MgO, CaO, Na20, K20, P205, FeS, Ni-Fe). Although most analyses totaled 98-lOOwt‘Y& those with sums in the range 96102 wt’?/,were accepted. We have reported good agreement between analyses of chondrules in thin section by the broadbeam method and analyses of those same chondrules by INAA (LLJXer al., 1980). The primary goal was to determine relative compositional differences between one textural type and another, rather than their absolute compositions. In this regard, any inherent errors that may have existed with broad-beam analysis are consistent insofar as the mineralogical differences in the different textural types allow. and should not affect comparative results. Petrographic analysis

This paper defines chondrules as bodies that are essentially circular in section, although they may be considerably elongate. They show textural evidence of an origin as quenched or crystallized molten droplets, such as a melt tension surface on which crystal nucleation may occur, or complete enclosure of euhedral crystals that appear to have crystallized from the melt of the chondrule. This definition of chondrules is most consistent with that of ‘fluid drop chondrule’ given by KING and KING (1978) but probably includes some of what DODD (1978a) called ‘microporphyritic chondrules.’ The chondrules included in this work consist of all rounded or sub-rounded bodies in the sections studied, and therefore represent all material in H3 chondrites that is not fine-grained matrix, metal and sulfide aggregates, or fragmental clasts (which are rare). Quantitatively, the chondrules analyzed comprise 5 SO-70% of the rock. Each chondrule was assigned to one of the following textural types: Barred oliuine chondrules. This type consists of bars of forsteritic olivine with interstitial, often glassy, material of high normative feldspar content. In many barred olivine chondrules the skeletal structure is optically continuous. in others several crystals in different orientations may be present. If the chondrule has an outer rim, it consists of ohvine that is crystallographically continuous with interior olivine. Rims commonly contain sparse to abundant inclusions of metal-sulfide droplets. Porphyritic chondrules. The most common type of chondrule consists of subhedral to euhedral crystals of olivine and pyroxene in glassy to microcrystalline interstitial material. The proportions of olivine and pyroxene vary considerably; thus porphyritic chondrules were designated as either porphyritic olivine (modal olivine B modal pyroxene), or porphyritic pyroxene (modal pyroxene > modal olivine) based on estimates made from petrographic examination. Radiating pyroxene chondrules. These chondrules consist of radiating laths of low-Ca pyroxene with or without minor glassy or microcrystalline material.

Lux rr ul. The laths vary from less than a micron to 10 microns wide. Barred pyroxene chondrules. These chondrules consist predominantly of coarse parallel or sub-parallel pyroxene bars or laths with a glassy or microcrystalline interstitia. They do not, however, resemble barred olivine chondrules with regard to any other textural feature. The crystals of barred pyroxene chondrules are not crystallographically continuous, and in no case was a rim like that seen in barred ohvine chondruies observed. Fine-grained chondrules. Chondrules in this category are more or less devoid of recognizable crystals. They range from glassy to microcrystallinefaphanitic). They do not contain any recognizable textural characteristics attributed to the types described above. Compound chondrules

Sixteen compound chondrule sets, defined as two or more chondrules that are partially or completely embedded in one another, were noted and each chondrule of a set was analyzed and classified in one of the above textural categories. Because compound chondrules are not abundant in ordinary chondrites, we studied compound chondrules in some L- and LLgroup chondrites in addition to those in our H-group samples. All compound chondrules studied are from petrologic type 3 chondrites. with the exception of one from the Avanhandava (H4) chondrite. RESULTS Bulk composition

The average and ranges in bulk compositions of the six different textural types delineated are presented in Table 1. along with the FeO/MgO ratios, the average molS/, fayalite content of the normative olivine, the average apparent diameter as measured in thin section. and the number of chondrules studied. The following discussion refers to values presented in Table 1. FeS and Ni-Fe abundances were measured but are not reported because they are highly variable from chondrule to chondrule, causing the average values to have little meaning. This variability, to a large degree, is probably due to sampling error, since metal and sulfide tend to occur as isolated blebs in chondrules rather than as more disseminated grains. Not all differences in mean abundances of oxides shown in Table 1 are necessarily significant. The precise statistical confidence levels for differences in means cannot be determined because of differences in distributions about the mean and in sample sizes of the different chondrule populations. Although the ranges overlap for most chondrule types, there appear to be some systematic differences in the means that are significant. In a later section we present results of discriminant function analysis. which do show differences in the mean bulk compositions of the different textural types.

Chondrules in H3 chondrites

671

Table 1.Average compositions(wtY/,)anddiameters(mm)ofchondrule types

Barred olivine

SiO2

46.4

Porphyritic olivine

(2.6)4

45.8

(3.5)

Ti02 Cr203

K20 p205

FeO/MgOl norm. Fa2 Diam. No.~

Barred pyroxene

0.48(0.63) 0.08(0.14) .28 13.7 0.42 7

0.15(.18) 0.10(.14) .30 14.2 0.45 25

Radiating pyroxene

Finegrained

55.9 (2.3)

5;.;2[20; 2:7 (i.0 0.59t.11

Al203

Fe0 MnO MgO CaO Nan0

Porphyritic pyroxene

6.7 (4.0) 0.31(.13)

0.09(.11 O-04(.06 .29 13.5 0.40 30

.25 12.8 0.49 7

.23 13.1 0.51 8

.46 20.7 0.31 10

i Weight ratio. 2 MolOAfayalite in normative olivine. 3 Number of chondrules analyzed. 4 Numbers in parentheses represent one standard deviation of the mean.

Burred and porphyritic oliuine chondrules. Olivinerich chondrules are characterized by low Si02 and high Fe0 and MgO contents, relative to pyroxenerich chondrules. Barred olivine chondrules contain the greatest abundances of Ti02, A120J, Na20 and K20, and potphyritic olivine chondrules have the second-largest abundances of these oxides, with the exception of K20. (Fine-grained chondrules have the second-greatest abundances of K20. This is discussed in the section on fme-grained chondrules.) Barred olivine chondrules contain the lowest abundances of Cr203 and MnO. MnO contents of porphyritic olivine chondrules are higher than those for barred olivine types but are still significantly lower than the MnO content for any of the pyroxene-rich chondrules, including porphyritic pyroxene types. The Cr203 content of porphyritic olivine chondrules is approximately the same as that in the pyroxene-rich chondrules, but more than the Cr203 content of the barred olivine chondrules. CaO content in the olivine chondrules is slightly higher than in the pyroxene chondrules and the olivine chondrules contain the greatest amount of P205. The FeO/MgO ratios for barred and porphyritic olivine chondrules are similar, as are the fayalite contents of thier olivines. These values are similar to porphyritic pyroxene chondrules, but slightly higher than barred and radiating pyroxene chondrules. The diameters of the olivine-rich chondrules (measured in thin section) are very similar to one another as well as to the porphyritic pyroxene chondrules. Porphyritic, barred, and radiating pyroxene chon-

drules. Most of the differences between olivine- and

pyroxene-rich chondrules were pointed out in the previous section. Here we show the distinctions among the various types of pyroxene-rich chondrules. There is a slight increase in Si02 content from porphyritic, to barred, to radiating pyroxene chondrules, with a concomitant decrease in Fe0 (7. 14 and 63% of the porphyritic, barred, and radiating pyroxene chondrule types, respectively, are quartz normative). MgO content of the pyroxene-rich chondrules is lower than that of olivine-rich chondrules. Porphyritic and barred pyroxene chondrules contain similar amounts of Ti02, A1203, Na20 and CrzOJ, with radiating pyr oxene chondrules containing somewhat less of these oxides. K20 abundances increase slightly from porphyritic, to barred, to radiating pyroxene chondrules. CaO content of barred pyroxene chondrules is higher than that of porphyritic and radiating pyroxene types. P205 content is low overall and does not vary among pyroxene-rich chondrule types. The diameters of barred and radiating pyroxene chondrules appear to be slightly larger on average than those of the porphyritic pyroxene and olivine-rich chondrules. Fine-grained chondrules. The fine-gained chondrules generally have bulk compositions that coincide more closely with those of pyroxene-rich chondrules than with the olivine-rich chondrules, but they also have many unique features. First, the range of values for each oxide is generally much higher in the finegrained chondrules than in the other textural types. Second, the Fe0 content is much greater in finegrained types than any other chondrule type, hence the FeO/MgO ratio and mol% normative fayalite are higher overall for this group. Third, the MnO content

678

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Lvx et al.

of fine-grained chondrules is greater than that of other textural types, and K20 and P205 contents are high, similar to that of barred olivine chondrules. Finally, the fine-grain& chondrules appear to be significantly smaller than any other chondrule type. Discriminant function analysis

. a

The question arises as to the authenticity of the compositional differences between the chondrule textural types. Although most chondrule textural types we discuss are recognized by petrologists as separate types, some might be artifacts of the sectioning process. For example, barred pyroxene chondrules, which appear unique in thin section, might simply be different orientations of porphyritic or radiating pyroxene chondrules (BLANDERet al., 1976). or fine-grained chondrules could result from sectioning radiating pyroxene chondrules orthogonal to their pyroxene laths. In addition, differences in the thermal histories of chondrules might cause chondrules of the same composition to crystallize with different textures (BLANDERand ABEL-GAWAD,1969; BLANDERet al., 1976). To test whether or not the textural types we have described are compositionally distinct, we employed discriminant function analysis, a multivariate tool for studying the degree of divergence between specified groups of data. The mathematical objective of discriminant analysis is to weight and linearly combine the discriminating variables to maximize the differences between groups; that is, force them to be statistically distinct (for a detailed description of discriminant analysis see SNED~COR and COCHRAN,1967; LE MAITRE, 1968; COOLEYand Lo1971). With discriminant analysis, it was possible to test what percentage of the cases in each petrographically determined group were correctly classified by the compositional variables used. In this study, the ‘groups’ are the six textural types of chondrules, the ‘cases’ within each group are the individual chondrules, and the ‘discriminating variables’ are the 13 components determined for the bulk composition of each chondrule. Discriminant analysis was performed using the SPSS integrated system (NIE et al., 1975). Table 2 shows the numbers and percentages of individual chondrules that were predicted to be class&d

0

ENWtED

OLIVINE

FONPNYRITICWVINE

PoNPNYRITIC PYNOXENE

: RlQQAgamP4dE

1 -4

x FINE-6RAINCD 4

-6

9 -6 -4 CANONICAL

-2 0 2 DISCRIWNANT

4 6 FUNCTION

I

Fig. 1. Discriminant function 1 vs discriminant function 2 for individual chondrules. The tendency is for chondrules with the same textures to cluster.

in each chondrule textural type, based on chondrule bulk compositions. The vertical column on the left is the actual, petrographically-established textural type of the chondrules, the horizontal column across the top is their textural type predicted by discriminant analysis on the basis of their bulk compositions. All textural types of chondrules, with the exception of barred pyroxene chondrules, are predicted to be classified in their proper group with greater than 75% accuracy. Because there is little misclassification among groups, these textural types appear to be more-or-less distinct with respect to bulk composition. Barred pyroxene chondrules, a possible excep tion to this conclusion, are discussed in a later section. Figure 1 shows the individual chondrules of each type plotted in the plane of the two principal canonical functions derived from the discriminant analysis. Function 1 contributes 72% of the total variance and Function 2 contributes 18% (functions 3, 4 and 5. progressively more minnor, contribute the remaining 10% of the variance). Examination of this figure further suggests that we are dealing with rather distinct compositional types with regard to the barred olivine, porphyritic olivine, porphyritic pyroxene, radiating pyroxene and fine-grained textural types. Table 3 gives the standardized discriminant function coefficients (basically, the weighting factors) used by the two principal canonical functions. Ignoring the

Table 2. Percent of chondrules in each textural type classified correctly by discriminant function analysis Predicted Actual

Textural Type

Barred Olivine Porphyritic Olivine Porphyritic Pyroxene Barred Pyroxene Radiating Pyroxene Fine-Grained

No.of Cases 7

Barred Olivine

:z :

85.7 8.0 0.0 E

10

0:o

Porphyritic Olivine 14.3 88.0 3.3 0.0 0.0 0.0

Textural

Porphyritic Pyroxene 0.0 SE 28:6 0.0 20.0

Type Barred Pyroxene

Radiating Pyroxene

0.0 0.0 3.3 57.1 25.1 0.0

0.0 6"? 14:3 75.0 0.0

FineGrained 0.0 4.0 3.3 0.0 0.0 80.0

Chondrules in H3 chondrites Table 3. Standardized discriminant function co-efficients for the two major functions*

Func. SiO2 TiO, A1203 Cr203 Fe0 MnO M90 CaO Na20

-0.506 1.191 0.057 0.945 0.083 1.233 0.356 0.657

pK2: F;S5 FeNi

0.179 0.175 0.059 -0.202

1*

-0.501

L

Func. 2* 0.132 -0.802 -0.316 0.745 0.342 -0.720 0.755 0.552 1.155 -0.586 -0.352 0.523 0.178

*Ignoring the sign, each coefficient represents the relative contribution of its associated oxide to that function.

signs, each coefficient represents the relative contribution of the associated variable (oxide) to that function. Note that Function 1 is dominated by MgO, A1203 and FeO; Function 2, though more uniformly weighted, is controlled by NarO, TiOr, MgO, Cr20J and MnO. Correlation

of volatile and refractory element oxides

Figure 2 shows the correlation of bulk A&O, with TiOr, Na20 and CaO in the different chondrule textural types studied. Interestingly, A1203, a relatively refractory oxide, correlates well with TiOz, also refractory, but also with one of the more volatile oxides, NarO. (Note the correlation coefficients (r) of the regression lines.) Furthermore, A&O3 and CaO do not show a particularly good correlation. Textures and chondrules

bulk

compositions

of

tures of chondrules. Our data suggest that it had a conspicuous role: although analyses of individual chondrules scatter (both within and among textural groups), the mean composition of each textural group is rather distinct (Table 1). Discriminant function analysis confirms the distinct nature of the textural groups (Table 2, Fig. 1) and analysis of the source of the variance among the groups (Table 3) shows that major elements, which dominate the physical chemistry of the chondrule melts, tend to contribute most of the variance (exceptions are TiOr and Cr203 in Function 2). Nevertheless, it seems clear that other factors, such as cooling rates and degrees of supercooling prior to nucleation (BLANDER and ABDELGAWAD, 1969; BLANDER et al., 1976; PLANNER and KEIL, 1979; PLANNER, 1980X must also have influenced the textures observed in chondrules. Indeed, to some extent the compositional differences among textural groups might have given rise to diverse textures simply because composition affects melt structures, diffusion rates in melts, and the degree of undercooling caused by a given cooling :ate. It seems reasonable to conclude that chondrule textures are governed mainly by compositon and cooling rate, both of which affect the extent of super-cooling prior to nucleation. Discriminant function analysis indicates that the least distinct textural type is barred pyroxene chondrules. Only 57% of these were classified correctly, versus the average of 82% classified correctly

701

. 6ARRED OLIVINE PORPHWTIC OLIWNE D PDRPHYRITICP%DicNE . RADIATING PYROXENE . BARRED PYROXENE . FINE-GRAINED

l

I

%

compound

Table 4 gives the bulk composition and textural type of each chondrule (or partial chondrule) in a given compound chondrule set. Clearly, most compound chondrule sets consist of chondrules that have the same texture. Of the 16 chondrule sets studied, only three consist of two chondrules of different types. All three are porphyritic olivine chondrules with barred olivine chondrules embedded in them, two textural types that are close in composition and mineralogy. Furthermore, except for chondrule sets consisting of different textural types, each chondrule set consists of two or more chondrules that have very similar compositions. In addition, the majority of compound chondrule sets observed consist of radiating pyroxene

679

r = .4l

r.v

R g

1

-

5.0

.

chondrules. 20 DlSCUSSlON

Chondrule textures

One goal of this study was to elucidate the role that composition played in producing the observed tex-

61) 0.00 WEIGHT X Al203

Fig. 2. AlaO vs Ti02, NaaO and CaO for individual chondrules. Clustering by textural type is more obscure than in Fig. 1. A&OS, TiOt and NaaO are correlated (note r1. but Al,O, and CaO are not.

680

GAYLE

Lux er al. Table 4. Compositions (wt3,)

Tieschitz (1) a(radiating pyroxene) b(radiatin pyroxene) Tieschitz (23 albarred olivine) btbarred olivine) Bremervorde atradiating pyroxene) blradiating pyroxene) Hallingeberg (1) airadiating pyroxene) blradiating p roxene) Hallingeberg (27 atradiating pyroxene) b(radiating pyroxene) c(radiating pyroxene) Hallingeberg (3) a(radiating pyroxene) b(radiating pyroxene) c(radiating pyroxene) Bishunpur a(radiating pyroxene) b(radiating pyroxene) Krymka a(radiating pyroxene) b(radiating pyroxene) Inman (1) a(porphyritic olivine) b(barred olivine) Inman (2) a(barred olivine) b(barred olivine) Inman (3) a(porphyritic olivine) b(porphyritic olivine) Semarkona a(radiating pyroxene) b(radiating pyroxene) Chainpur (1) atradiatinq wroxene) biradiating ijroxene) Chainpur (2) a(porphyritic olivine) b(barred olivine) Dha'ala af radiating pyroxene) b(radiating pyroxene) Avanhandava a(porphyritic olivine) b(barred olivine)

Si02

Ti02

Al 203

55.6 57.9

0.24 0.15

6.4 3.6

0.20 0.23

0.98 1.4

52.6 50.2

0.10 0.09

2.9 1.8

0.63 0.70

15.5 16.3

56.3 56.8

0.07 0.12

2.2 2.0

0.58 0.53

9.3 9.2

54.7 56.7

0.10 0.10

2.4 2.4

0.75 0.61

13.1 12.2

52.7 54.0 55.0

0.09 0.07 0.08

2.2 1.9 1.8

0.62 0.54 0.57

15.1 16.7 16.4

55.5 57.2 56.7

0.12 0.10 0.07

3.0 2.4 2.1

0.70 0.74 0.80

10.6 10.7 11.5

52.9 50.0

0.09 0.07

2.2 1.4

0.46 0.55

15.8 16.4

52.3 51.1

0.11 0.09

2.6 2.2

0.58 0.63

16.0 16.6

48.3 52.8

0.09 0.12

2.2 3.0

0.52 0.58

15.1 12.5

39.7 38.7

0.32 0.24

7.9 6.5

0.25 0.28

18.7 17.3

43.4 48.5

0.11 0.11

3.1 3.2

0.35 0.52

8.5 10.6

61.1 57.9

0.10 0.11

2.1 2.2

0.57 0.56

8.4 7.6

57.2 57.2

0.14 0.12

3.3 2.0

0.74 0.82

7.5 8.4

43.6 47.4

0.13 0.30

2.9 10.0

0.44 0.17

18.6 5.4

54.5 57.7

0.11 0.11

2.6 2.9

0.65 0.72

6.9 6.1

47.0 47.0

0.10 0.06

4.9 7.0

0.17 0.25

12.2 11.4

Cr203

Fe0

* The letters a, b and c refer to individual chondrules in each compound set: refer to specific compound sets in those chondrites where more than one

(Table 2). We infer that barred pyroxene chondrules are probably artifacts of slice geometry that actually belong to either radiating- or porphyritic-pyroxene types. BLANDER et al.(1976) illustrated how sectioning a radiating pyroxene chondrule at different orientations can yield a barred chondrule. It seems likely that certain orientations of porphyritic pyroxene chondrules might exhibit pyroxene laths that are

parallel or subparallel, and hence, resemble barred pyroxene chondrules. Although chondrule textural types appear to be distinct when their mean overall bulk compositions are considered, their compositions with respect to certain oxides do not define random or discrete groups, but seem instead to delineate a range or sequence (Fig. 2). This observation suggests that the process that

Chondrules in H3 chondrites

681

of compound chondrules* MnO

M90

CaO

Na,O

K20

p205

FeS

FeNi

0.06 0.06

27.5 30.2

9::

2.0 1.3

0.15 0.17

0103

0.03 0.29

2.1

97.68 100.11

0.53 0.52

24.9 25.8

2.1 0.63

0.04 0.03

0.03 0.03

0.33 0.55

0.11 0.11

101.87 98.80

0.39 0.39

23.8 25.6

5.2 3.5

1.2 1.0

0.19 0.11

0.02 0.01

0.36 0.14

0.68 0.23

100.29 99.63

0.71 0.62

23.5 23.0

E

1.2 1.3

0.04 0.05

0.06 0.07

0.69 0.55

0.23 0.08

98.98 99.29

0.56 0.58 0.63

21.4 20.2 22.1

1.6

1.1

;:;

1:;

0.04 0.18 0.05

0.05 0.05 0.08

2.8 0.44 0.38

1.1 0.08

99.36 97.31 97.97

0.64 0.69 0.86

25.0 24.8 24.1

1.9 :::

1.6 1.6 1.3

0.05 0.03 0.07

0.01 0.01 0.03

0.19 0.08 0.14

-

99.31 99.95 99.47

0.56 0.50

23.2 23.9

1.9 1.9

1.2 0.67

0.18 0.12

0.15 0.14

0.33 0.71

0.15 -

99.12 96.36

0.63 0.68

24.4 24.6

5::

1.7 1.4

0.31 0.23

0.13 0.11

0.44 0.33

0.08 -

101.38 100.07

0.53 0.52

29.5 26.8

1.7 1.9

1.1 1.6

0.48 0.64

0.04 0.04

0.14 0.27

0.38 0.15

100.08 100.92

0.29 0.21

26.0 29.4

4.1 4.3

0.82 0.33

0.04 0.05

0.03 0.07

0.03 0.05

0.68 0.83

98.86 98.26

0.13 0.22

38.3 31.1

2.0 1.6

0.39 1.0

0.02 0.07

0.06 0.03

0.08 0.08

1.1 1.1

97.54 98.13

0.88 0.97

23.2 23.4

0.30 0.25

-

100.19 96.25

0.57 0.83

24.9 27.4

1.8 1.9

1.8 1.1

0.29 0.15

0.30 0.09

0.04

98.84 100.05

0.28 0.14

28.5 26.8

1.5 6.0

1.9 2.5

0.37 0.05

0.40 0.03

0.33 0.05

-

98.68 98.84

0.43 0.59

25.1 25.9

2.0 1.9

1.4 1.6

0.08 0.23

0.01

2.6 0.08

1.6 0.11

97.98 97.94

0.32 0.32

30.4 27.9

2 2 3:1

0.28 0.39

0.09

;:"o

1.2 0.05

1.4 -

0.06

Total

101.66 98.53

their textural type is designated in parentheses. The numbers in parentheses

compound chondrule set was observed.

formed most chondrules might be systematic or sequential with regard to certain aspects of that process; i.e. most chondrule types are related by a common process in their formation. This observation does not, however, apply to fine-grain4 chondrules. Their range of compositions (Table 1, Fig. 2) tends to set them apart from the other textural types. Compound chondrules The close

similarity in composition

and texture of

compound chondrule sets (Table 4) is consistent with that reported by BLANDERand ABDEL-GAWAD(1%9) and MCS~EEN (1977).The presence of joined chondrules in chondrites suggests that prior to their incorporation into the rock, two or more chondrules impacted one another while at least one of them was molten or partly molten (plastic). It is possible that the join served as a point of crystal nucleation for the molten droplet(s) (BLANDER and ABDEL-GAWAD, 1969);however, it is also reasonable to assume that,

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GAYLE

because the droplets were of similar composition, they nucleated to form the same textures. We can also infer that because most compound chondrules consist of two or more chondrules of similar composition and texture, chondrules of one textural type formed together in time or space; i.e. at the critical period when chondrules were distinct entities, yet still not completely crystallized, they shared an environment only with chondrules of very similar composition. In three cases, the compound chondrules we studied consisted of two chondrules of different textures. In each case, a barred olivine chondrule is embedded in a porphyritic olivine chondrule. Although the bulk compositions of the chondrules in these sets diverge more than chondrule sets consisting of chondrules of the same texture, we still observe two chondrule types that are similar in composition; i.e. we do not, for example, observe a chondrule set consisting of a barred olivine chondrule and a radiating pyroxene chondrule. Furthermore, the fact that in all three cases, a barred olivine chondrule is embedded in a porphyritic olivine chondrule suggests that the porphyritic olivine chondrule was molten at the time of contact and the barred olivine chondrule was crystalline. The high percentage of radiating pyroxene chondrules in compound chondrule sets suggests that during the formation of this chondrule type there was a greater probability of collision with nearby droplets. Either the mechanism that formed chondrules was more energetic here or there existed a greater density of these chondrule types. It may also be possible that they were more viscous, and tended to maintain their identities instead of flowing together into one droplet.

Lvx rc al.

have relatively constant CaO + Alz03 contents relative to those in carbonaceous chondrites and chondrules in ordinary chondrites have higher Na,O and KZO and lower CaO contents than do those in carbonaceous chondrites. Furthermore, although chondrules in unequilibrated H, L and LL ordinary chondrites have recently been found to be compositionally indistinguishable (GOODINGet al.. 1978a), they may display very different forms. For example, in Land LL-group chondrites chondrules with circular sections (e.g. spheres) are the exception rather than the rule, whereas in H-group chondrites, spherical or ‘fluid drop chondrules’ are common (GCXJDING er al.. 1978b. G~ODING, 1979; M-TIN and MILLS, 1976: 1978). Many of the distinctions between chondrules in different types of chondrites may simply be due to different preaccumulation histories after their formation by a common process, but this cannot be assumed automatically. Therefore, the following discussion applies specifically to chondrules in the H-group ordinary chondrites (Sharps, Tieschitz, and BremervGrde) and attempts to explain their genesis alone. Whether or not any of the interpretations apply to chondrules in other types of chondrites awaits further study. Origin as direct condensation products. The data we report here appear to preclude direct equilibrium condensation from the nebula to form chondrules. This agrees with conclusions reached by DODD (1978b). GRO~.VMAN et al. (1979a, b) and GILDING er al. (1980). The condensation process should have led to compositional differences between early formed condensates and later formed condensates. This is observed on a broad scale with respect to Ca-Al-rich inclusions, chondrules (in a broad sense) and matrix; i.e. Implications for chondrule origin the Ca-Al-rich inclusions appear to represent an early-formed condensate of high-temperature conModels for the origin of chondrules play many stituents, the chondrules an intermediate-temperature variations on two broad themes: (1) chondrules as primary objects-models that pic- condensate. and the matrix a later-formed condensate containing low-temperature constituents (LARIMER ture chondrules as products of direct condensation from the solar nebula (SUES$ 1949; WOOD, 1963; and ANDERS,1967). By the same reasoning we expect BLANDERand KATZ, 1967; BLANDERand ABDEL- to see a sequence within the chondrule population. GAWAD, 1969; Wool and MCSWEEN, 1977: That is. we should observe a range of chondrule compositions, from those that contain more abundant MCSWEEN, 1977) or from proto-planetary atmoshigh-temperature components to those that contain pheres (POWLAK and CAMERON,1974); low-temperature components. In (2) chondrules as secondary objects--models that more abundant view chondrules as remelted solids (TSCHERMAK. 1883: other words, abundances of volatile and refractory RINGWOOD, 1959; FREDRIKXIN and RINGWOOD, elements ought to vary inversely. Our results, how1963; FREDRIKSSON,1963; WHIPPLE, 1966, 1972; ever, indicate a positive correlation between volatile KURAT, 1967; W-N, 1972. KING ef nl., 1972; and refractory elements (Fig. 2) and, therefore, are CAMERON, 1973; KIEFFER,1975; DODD, 1978b; GROSS- contrary to the trend expected for condensation, In MANet al., 1979&b; GCMDDING, 1979; G~ODING et al.. addition, the refractory element oxides that condense at similar temperatures should show a strong positive 1980). Attempts to explain the origin of all types of chon- correlation. We do not observe this for the oxides of drules in all types of chondrites by a single model are Al and Ca. Although our results do not include trace element probably futile. As DODD (1978b) has pointed out. chondrules in carbonaceous chondrites differ from data, GROSSMANet (I/., 1979a. b) have reported that the volatile elements Ge. Ga Zn and Cd are not those in ordinary chondrites in form, texture, mineralogy and chemical composition. MCSWEEN (1977) strongly depleted in Chainpur chondrules relative to average LL-group composition or to the whole rock. has shown that chondrules in ordinary chondrites

Chondrules in H3 chondrites They interpret these data to preclude any direct-condensation model for the origin of chondrules, since condensation would have occurred at too high a temperature to condense out significant fractions of these elements. This conclusion, of course, rests on the assumptions that chondrules in Chainpur were not affected by thermal metamorphism after accretion and that the chondrules analyzed were not contaminated by adhering, volatile-rich matrix materials. Both assumptions seem reasonable: based on data on the abundances and compositions of fine-grained, opaque matrix and recrystallized matrix in unequilibrated (type 3) ordinary chondrites, Huss et al. (1978, 1980) argue that Chainpur experienced little thermal metamorphism. The problem of contamination by volatile-rich matrix that rims many chondrules was evaluated by GROSSMAN et al. (1979b) who found that etching chondrules before neutron-activation analysis did not decrease the abundances of volatile trace elements. We conclude, therefore, that unless an unlikely sequence of events took place during condensation or the condensation process did not approach equilibrium chondrules in H-group chondrites did not form by direct condensation from gas to liquid. Consequently, they must be secondary objects, formed by the melting of pre-existing solids. We consider these models next. Origin as melted or remelted solids. At least five varieties of models view chondrules as products of melted or remelted solids: (1) Volcanism (TXHERMAK, 1883; RINGWOOD, 1959; FREDRIK~~~Nand RING WOOD,1963); (2) melting by impact into a planetary regolith (FREDRIKSSON, 1963; KURAT, 1967; Kmc et ai., 1972); (3) fusion of condensed dust grains or aggregated dustballs by lightning discharge in space (WHIPPLE,1966; CAMERON,1966; MARTINand MILLS, 1976); (4) high velocity collisions between dust grains or silicate aggregates in space (WHIPPLE, 1972; WA+ SON,1972; CAMERON,1973; K~EFFER,1975; GROSSMAN et al., 1979a b), and (5) recondensation of a gasdust mixture following a transient energetic event (WOOD and MCSWEEN, 1977). (The last model is really a direct-condensation model because it involves equilibrium condensation combined with a fractionation of dust from gas, however, it does involve remelting of solids.) DODD (1978b) suggested a combination of some of the above models whereby melting of nebular dust produced a microporphyritic rock. Later fragmentation and remelting (by impact) resulted in porphyritic and fluid-drop chondrules, respectively. Although volcanism represents one of the oldest ideas suggested to explain how chondrules formed (TSCHERMAK, 1883). it suffers from three main faults: first, it is hard to visualize how volcanism could produce and concentrate so many chondrules (well over 50:< of the volume of an H-group chondrite consists of chondrules and chondrule fragments). Although terrestrial volcanoes certainly produce droplets of silicate melt, most of the volume of erupted material

683

(especially during eruptions of basic magmas) is in the form of flows, not millimeter-sized spherules. Second, if widespread volcanism on chondrite parent bodies produced chondrules, it must also have made intrusive rocks and lava flows with compositions like those of chondrules. Such meteorites, however, have not been reported. Third, volcanism capable of dispersing the erupted melt into droplets requires the presence of a significant amount of a gas phase as a driving mechanism. Although much of the gas could have escaped into space, surely some evidence for its presence would be recorded in ordinary chondrites. Such evidence is lacking. Formation of most chondrules in the regolith of a chondrite parent body seems unlikely, although we cannot rule it out completely. The main problem with this model is that impact melting into a planetary surface is an inefficient process (e.g. G’KEEFE and AHRENS,1975) yet the great abundance of chondrules in chondrites (> 50%) suggests that a pervasive process formed them. Furthermore, the only sample of an impacted planetary regolith available to us-the lunar soils-is certainly not littered with chondrule-like spherules. In fact, most of the lunar surface consists of glass-bonded aggregates of angular fragments of rock and minerals; i.e. although glass (melt) is produced, most of the impact debris is fragmental (KERRIDGE and KEFFER, 1977). Although we recognize that the surfaces of the moon has a large gravitational attraction (which affects impact velocities and the flight times of drops of melted regolith), we doubt that impact onto a planetary surface could possibly produce the majority of the chondrules that populate ordinary chondrites. Gn the other hand, impact into a regolith clearly does produce impact melts, so the idea is not without merit. Turning again to the lunar analogue, we know that impacts into the regolith of the moon can produce spherules that resemble meteoritic chondrules in texture (FREDRIKSSONet al., 1970; 1971; KEIL et al., 1972; KING et al., 1972; KURAT et al., 1972; NELENer al., 1972). The fine-grained chondrules described in this study could be candidates for an origin by impact melting in the meteorite parent body regolith. We recognize that this is purely speculative because no property of fine-grained chondrules directly implies a regolith origin, however they do possess some unique properties relative to the other textural types: (i) compositionally, they show a large range in values for most oxides of their bulk composition, yet they are chemically distinct from any of the other textural types; (ii) they lack exotic textures, (iii) their fine-grained nature suggests that they cooled more rapidly than the other textural types, and (iv) they appear to be significantly smaller, on average. We can say with some certainty, however, that finegrained chondrules are not simply quenched forms of the other textural/compositional types. If fine-grained chondrules were merely disguised versions of the other textural types, they would have been classified with these types in the multivariate analysis (Table 2).

684

GAYLELLJXet al.

Instead, we observed that they were indeed a distinct group. Fusion of condensed dust grains and/or aggregates by lightning discharge, and high velocity collisions between dust grains or silicate particles in space circumvent some of the problems posed by direct condensation from the nebula or impact melting in a regolith. Some advantages of forming chondrules by melting solids in space are: (1 f large numbers of chondrules can be produced in this fashion. (2) The compositional variability observed in chondrules can be explained if we postulate that, prior to melting, the chondrule parent material consisted of a number of different ‘mixtures’ (GROSSMANet at., 1979a,bj, more or less homogeneous within, but each containing different fractions of high-, intermediateand low-temperature nebular condensate. When melting occurred, by whatever method, these different mixtures of dust or silicate aggregates were converted to melt dropiets that later crystallized into the different chondrule textural/compositional types. Each group, conceivably, could have had its own unique thermal history (i.e. some chondrule types might have experienced supercooling to a greater or lesser degree than others, or some could have crystallized under differ~g P, ‘I; X conditions). (3) Since we envision melting of condensed nebular dust or silicate aggregates, this model explains certain systematic properties among chondrule types that parallel the condensation sequence (e.g. the A1203 and TiOt correlationj. (4) The observations regarding compound chondrules are also consistent with this model; that is, when chondrules were still molten or plastic enough to embed and stick together. they shared an environment with other chondrules of similar composition. (5) The fact that different chondrule types are not present in chondrites in similar proportions (porphyritic chondrules are by far the most abundant in H-group chondritesj can be explained readily because certain mixtures occurred more frequently, thus favoring the production of a particular chondrule type (e.g. porphyritic chondrulesj, for a given cooling rate. (6) The I-Xe data reported by PO~SEK (i969) suggesting that chondrules postdate the matrix is consistent with this model. Unfortunately, there are problems in conceiving chondrules as formed in space by melting solids. First, complete melting of the material is required. It seems likely that some material would have been only partially fused, yet there is no evidence of chondrules with unmelted cores or partially melted dustballs. Second, some aspects of the model are contrived and leave unanswered questions: how and why did these specific mixtures form 7 How did all the textural types of chondrules later become mixed together to form the proportions we observe in chondrites now? Third. both mechanisms for heating in space have been criticized on astrophysicat grounds. SONETT (1979j, for example, acknowledge that lightning discharges in the soiar nebula might have melted solids to form

chondrules but noted that is not known whether processes taking place in the nebula actually produce lightning. WOODand MCSWJZEN (1977) argue that formation of chondrules by collisions between centimeter-sized meteoroids is unlikely to have occurred in the solar nebula because of the difficulty of obtaining the required > 3 km/set relative velocities needed to form melts (KIEFFER,19751 and also unlikely to result in accumulation of chondrules if the collisions took place after the nebula was dissipated. (We note that these arguments are only as good as the general level of understanding of the physical processes that operated in the solar nebula. Althou~ the arguments are sound enough to make one skeptical of both the lightning-discharge and small-body collision hypotheses for the origin of chondrules, they are not on such solid foundations that these models ought to be discarded.) It seems we are still left with the problem of how chondrules originated. It would be appealing as well as satisfying to account for the genesis of chondrules by a single, simple theory; however, the great diversity of chondrules and the incongruous nature of their bulk compositions and textures makes it difficult to explain their origin by any simple, str~~tforw~d model. It seems necessary to adopt the complex notion that most chondrules originated from mixtures of differing fractions of high-, intermediateand lowtemperature nebular condensates that underwent melting in space. A small percentage might have formed by impacts on the regoliths of the meteorite parent bodies. Acknowlodgemenrs-Useful reviews were provided by R. T. DODD,J. F. KERRIDGE, and H. Y. MCSWEEN.JR.This work was supported by NASA Grant NGL 32404-064 (K. KEIL, Principal Investigator).

REFERENCES BLANDERM. and KATZ J. (1967) Condensation of primordia1 dust. Gewhim. Cosmoehim. Acta 31, 1025-1034. BLANDERM. and AB~EL-GAWADM. (1969) The origin of meteorites and the constrained equilibrium condensation theory. Geochim. Cosmochim. Acra 33, 701-716. BLANDERM., PLANNERH. N.. KEIL K., NELSONL. S. and RICHARD~~FI N. L. (1976) The origin of chondrules: experimental investigation of met&able liquids in the system Mg,Si04-SiO*. Geoehim. Cosmochim. Acta 40, 889-896. CAMERON A.

G. W. (lY66) The accumulation of chondritic material. Earth Planet. Sci. Lett. 1, 93-96. CAMERONA. G. W. (1973) Accumulation processes in the primitive solar nebula. Icarus IS, 407-450. COOLEYW. W. and LOHNESP. R. (1971) ~uiti~ariafe Data Anafysis. Wiley. DODD R. T. (1976) Accretion of the ordinary chondrites. Earrh Planet. Sri. Lett. 30, 281-291. DODD R. T. (1978a) The composition and origin of large microporphyritic chondrula in the Manych (L-3) chondrite. Earth Planet. Sci. Lett. 39, 52-66. DODD R. T. (1978b) Compositions of droplet chondruks in the Manych (L-3) chondrite and the origin of chondrules. Earth P&net. Sci. Letr. 40. 71-82.

Chondrules in H3 chondrites FREDRIKSS~N K. (1963) Chondrules and the meteorite parent bodies. Trans. N.Y. Acad. Sci. 25, 156769. FREDRIKS~ON K. and RINGW~ODA. E. (1963) Origin of meteoritic chondrules. Geochim. Cosmochim. Acra 27, 639-641. FREDRIK~ON K., NELEN J. and MELON W. G. (1970) Petrography and origin of lunar breccias and glasses. Proc. ADO//O I I Lunar Sci. Conf., Vol. 1. DD. . . 419-432. Pergambn Press. FREDRIKS~N K., NELENJ. and NOONANA. (1971) Lunar chondrules (abstract). Meteorirics 6, 270-271. GOODING J. L. (1979) Petrogenetic properties of chondrules. Ph.D. Dissertation, University of New Mexico. GOODING J. L., KEIL K., FUKUOKAT. and SCHMITT R. A. (1978a) Chemical-petrological comparison of individual chondrules from the Chainpur (LL3) and Tieschitz (H3) chondrites (abstract). Mereoritics 13, 475-476. GCQMNGJ. L.. KEIL K. and HEALEYJ. T. (1978b) Physical properties of individual chondrules from ordinary chondrites (abstract). Meteoritics 13, 476477. GOODING J. L., KEELK., FUKUOKAT. and SCHMITT R. A. (1980) The origin of chondrules as secondary objects: evidence from chemical-petrological heterogeneities (abstract). In Lunar and Planetary Science XI, pp. 345-347. Lunar and Planetary Institute. GROSSMAN 1. N., KRACHERA. and W,Q~~N J. Y. (1970a) Compositional and petrographic constraints on the origin of Chainpur chondrules (abstract). In Lunar and Planetary Sciences X, pp. 464-466. Lunar and Planetary Institute. GROSSMAN1. N., KRACHERA. and WASSONJ. T. (1979b) Volatiles in Chainpur chondrules. Geophgs. Res. Lerr. 6, 597-600.

HOWARDE. (1802) Experiments and observations on certain stony and metallic substances, which at different times are said to have fallen on the earth; also on various kinds of native iron. Phi/. Trans. R. Sot. London, 162-212. Huss G. R, KEIL K and TAYLORG. J. (1978) Composition and recrystallization of the matrix of unequilibrated (type 3) ordinary chondrites (abstract). Mereorirics 13, 495-497.

HUSSG. R., KEIL K. and TAYLOR G. J. (1980) The matrices of unequilibrated ordinary chondrites: implications for the origin and history of chondrites. Geochim. Cosmochim. Acra 45, 33-51. KEIL

K., KURAT G., PRINZ M. and GREEN J. A. (1972) Lithic fragments, glasses and chondrules from Lunar 16 fines. Earth Planer. Sci. Lea. 13, 243-256. KERRIDGE J. F. and K~EFFERS. W. (1977) A constraint on impact theories of chondrule formation. Earth Planer. Sci. Lerr. 35, 35-42. K~EFFERS. W. (1975) Droplet chondrules. Science 189, 333-340. KING T. V. V. and KING E. A. (1978) Grain size and petrography of C2 and C3 carbonaceous chondrites. Meteoritics 13, 47-72. KING E. A., CARMANM. F. and BUTLERJ. C. (1972) Chondrules in Apollo 14 samples: Implications for the origin of chondritic meteorites. Science 175, 59-60. KURAT G. (1967) Zur entstehung der chondren. Geochim. Cosmochim. Acra 31,491-502.

KURAT G., KEIL K., PRINZ M. and NEHRU C. E. (1972) Chondrules of lunar origin. Proc. 3rd Lunar Sci. ConJ, pp. 707-721. LARIMERJ. W. and ANDERSE. (1967) Chemical fractionations in meteorites. II. Abundance patterns and their interpretation. Geochim. Cosmochim. Acra 31, 1239-1270.

685

LEMAITRER. W. (1968) Chemical variation within and between volcanic rock series-a statistical approach. J. Petrol. 9, 220-252.

Lux G., KEIL K. and TAYLORG. J. (1980) Metamorphism of the H-group chondrites: Implications from compositional and textural trends in chondrules. Geochim. Cosmochim. Acra 44,841-855. MARTINP. M. and MILLS A. A. (1976) Size and shape of chondrules in the Bjurbole and Chainpur meteorites. Earth Planer. Sci. Lerr. 33, 239-248.

MARTINP. M. and MILLS A. A. (1978) Size and shape of near spherical Allegan chondrules. Earrh Planer. Sci. Lerr. 38, 385-390.

MCS~EEN H. Y., JR (1977) Chemical and petrographic constraints on the origin of chondrules and inclusions in carbonaceous chondrites. Geochim. Cosmochim. Acra 41, 1843-1860.

NELENJ., N~~NAN A. and FREDRIK~S~NK. (1972) Lunar glasses, breccias and chondrules. Proc. 3rd Lunar Sci. Conf, Vol. 1, pp. 723-737. NIE N: H., HAL~C. H., JENKINSJ. J., STEINBRENNER K. and BENT D. H. (19751 SPSS: Srarisrical Packaae ” *ior the Social Sciences. McGraw-Hill. G’KEFFEJ. D. and AHRENST. J. (1975) Shock effects from a large impact on the moon. Proc. 6th Lunar Sci. Cont. Vol. 3. pp. 2831-2844. PLANNERH. (1980) Chondrule thermal history implied from olivine compositional data. Ph.D. Dissertation. University of New Mexico. PLANNERH. and KEIL K. (1979) Evidence for an isothermal event in chondrule thermal history (abstract). Mereorirics 14, 517-518. POD~LAKM. and CAMERONA. G. W. (1974) Possible formation of meteoritic chondrules and inclusions in the precollapse of Jovian protoplanetary atmosphere. Icarus 23, 326-333. Ponoa~~ F. (1969) Dating of meteorites by the high temperature release of iodine-correlated Xe’*‘. Geochim. Cosmochim. Acra 34, 341-366. R~NGWOODA. E. (1959) On the chemical evolution and densities of the planets. Geochim. Cosmochim. Acra 15, 257-283. Rosa G. (1863) Beschreibung und Eintheilung der Meteoriten auf Grund der Sammlung-im mineralogischen Museum zer Berlin. Abh. Akad. Wiss. Berlin. 81-151. SONF~TC. P. (1979) On the origin of chondrules. Geophps. Res. Lerr. 6, 677680.

SNEDECORG. W. and COCHRANW. G. (1967) Srarisrical Merhods. Iowa State Univ. Press. Sums H. E. (1949) Zue Chemid der Planeten und Meteoritenbildung. Z. Elecrrochem. 53, 237-241. TSCHERMAKG. (1883) Beitrag zur Classification der Meteoriten. Sirzber. Akad. Wiss. Wien 88 (Part 1). 347-371. W-N J. T. (1972) Formation of ordinary chondrites. Rev. Geophys. Space Phys. 10, 71 l-759. WHIPPLEF. L. (1966) Chondrules: suggestion concerning the origin. Science 153, 5456. WHtPPLEF. L. (1972) On certain aerodynamic processes for asteroids and comets. In From Plasma to Plane: (ed. A. Elvius), pp. 21 l-232. Almquist & Wiksell. WOODJ. A. (1963) On the origin of chondrules and chondrites. lcarus 2, 152-180. Wool J. A. and MCSWEENH. Y., JR (1977) Chondrules as condensation products. In Comers, Asteroids. Mereorires: Inrerrelarions, Euolurion, and Origins (ed. A. H. Delsemme). Univ. of Toledo.