The composition and origin of large microporphyritic chondrules in the Manych (L-3) chondrite

52

Earth and Planetary Science Letters, 39 (1978) 52-66 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [61

THE COMPOSITION AND ORIGIN OF LARGE MICROPORPHYRITIC CHONDRULES IN THE MANYCH (L-3) CHONDRITE R.T. DODD Department o f Earth and Space Sciences, State University o f New York, Stony Brook, N Y 11794 (U.S.A.J

Received October 4, 1977 Revised version received November 29, 1977

The majority (26/37) of the largest chondrules (d/> 1400 tzm) exposed in a thin section of the Manych chondrite are more or less rounded fragments of microporphyry, most of which contain from 50 to 80 vol.% olivine. Modal and phase analyses were used to calculate the approximate bulk compositions of nine such chondrules. Six vary modestly around the mean composition of L-group chondrites less most of their metal and troilite and are thought to have formed by bulk melting of L-group material with loss of an immiscible Fe-Ni-S liquid. Two other chondrules, which are olivine-rich and Na- and Si-poor, formed in the same way but with some loss of volatile constituents to a vapor phase. The ninth chondrule, an olivine-poor microporphyry, may be a non-representative sample of a coarser mieroporphyritic rock. Comparison of these microporphyritic chondrules with the products of controlled cooling experiments and with chemically similar olivine microporphyry in the St. Mesmin chondrite (LL-breccia) suggests that the microporphyritic chondrules are fragments of magmatie rocks which crystallized from masses of liquid no less than 10 cm across.

1. Introduction Wide textural and mineralogical variations among chondrules in type 3 ordinary chondrites indicate that these chondrules had highly varied individual histories before they accumulated in the meteorite parent bodies [1,2]. Though some are quenched droplets, most [ 3 - 5 ] are more or less rounded fragments o f igneous rock, much o f it microporphyritic, and b o t h drop- and clast-formed chondrules commonly bear evidence o f more or less deformation and/or recrystallization prior to accretion. The bewildering petrographic diversity o f chondrules contrasts sharply with their chemical variation, which is wide but apparently systematic [6,7]. It appears that the major element trends imposed on chondrules when they formed largely survived their varied subsequent histories. Thus these trends constitute the strongest available evidence for the process or processes o f chondrule formation. Dodd and Walter [7] used bulk analyses o f 61

chondrules from the Chainpur (LL-3) chondrite to evaluate various chondrule models. They suggested, following Dodd [1], that much o f the observed chemical variation is due to non-representative sampiing o f microporphyritic precursor rocks, fragments o f which persist as microporphyritic chondrules. Walter et al. [8] performed simple computer tests which suggest that this interpretation is plausible. The research reported here and in a following paper on drop-formed chondrules in Manych carries Walter and Dodd's [6,7] analysis further with correlated petrologic and bulk chemical analyses o f a suite o f chondrules. Manych was chosen for this study because earlier work [9] showed it to be free o f a perceptible metamorphic overprint, a conclusion reinforced b y the data presented here. This paper presents data for microporphyritic chondrules in Manych and attempts to identify the processes which formed them and caused their chemical variation. A second paper will present data for drop-formed chondrules and will draw the two sets o f

53 data together in a model for the pre-accretion history of chondrules in ordinary chondrites.

ficiently precise for the purposes to which they are put in this paper.

2.1. Modal analyses 2. Methods of study An ideal technique for chemical analysis of individual chondrules is yet to be devised. The approach which we have used in the past - microprobe analysis of glasses formed by fusing the chondrules with Li-tetraborate [6] - yields data for all major elements but sulfur (expelled during fusion), but it gives no information on the oxidation states of iron, yields no mineralogical or petrographic data, and biases the analyses toward drop-formed chondrules, which are most easily identified in crushed material. Analysis of chondrules in thin section by defocussed beam microprobe analysis [10] gets around many of these limitations and is suitable for glassy and microcrystalline chondrules. However, serious analytical uncertainties result when this approach is used for coarse samples [14], and it is difficult to be certain that a thin section through a coarse chondrule accurately represents the composition of that chondrule. As the present study required both bulk and phase compositions for microporphyritic chondrules, a combination of modal and phase analyses was used to estimate bulk compositions. The results are semi. quantitative for all but the most abundant elements, and the technique is clearly useful only for large and/or t'me-grained chondrules, but the data are suf-

Two set of modes were obtained for the largest chondrules in Manych (d >t 1400/am): (a) 250-point modes whose sole purpose was to estimate the olivine contents of large chondrules of all types;and (b) 500-point modes which were used, with phase compositions, to calculate the bulk compositions of selected microporphyritic chondrules. Where olivine is the only phenocryst phase - the case in most of the chondrules studied - this phase, groundmass (microcrystalline pyroxene and glass), and opaques were tabulated. Pyroxene was counted and analyzed separately in three chondrules in which it is coarse enough for reliable microprobe analysis. Replicate analyses (Table 1) show that a 500-point mode gives highly reproducible results for the major constituents but is imprecise for the opaque minerals, metal, troilite, and chromite. As these are sparse (typically less than 1%) and irregularly distributed, more detailed modes would be futile. We therefore excluded the opaques when calculating bulk compositions. This procedure has a trivial effect on other elements but results in systematic underestimation of Cr. The precision of the modes is one question, their accuracy another. Though restriction of the study to the largest chondrules minimizes errors due to nonrepresentative sections through chondrules, it cannot

TABLE 1 Modal compositions (vol.%) of selected mieroporphyritic chondrules in Manyeh (L-3). ND marks chondrules in which pyroxene is microcrystalline and included in matrix Chondrule No. 1 Diameter (t~m) 3225 Points 487 Olivine 53.6 Pyroxene 2.3 Matrix 42.9 Opaques !.2

2

3

7

17

20

30

34

35

35a *

35b *

3225 503 77.9 ND 21.5 0.6

1550 538 66.2 ND 32.2 1.7

2150 532 64.6 ND 34.8 0.6

1430 491 60.1 15.1 24.6 0.2

1800 488 71.9 ND 27.7 0.4

1650 498 48.2 18.9 31.9 1.0

1450 470 66.0 ND 33.8 0.2

1850 590 8.3 64.1 25.9 1.7

1850 278 7.5 63.7 27.7 1.1

1850 312 9.0 64.4 24.4 2.2

* 35a and b are replicate modes for 35.

54 wholly eliminate such errors. As non-representative sampling is one of several mechanisms entertained for the chemical diversification of chondrules, detailed consideration of sampling errors is deferred to the discussion.

2.2. Phase analyses All phase analyses were performed on an ARL EMX/SM microprobe at Stony Brook, using oxide and simple mineral standards. Operating conditions, standard for this laboratory, were 15 kV and 0.015/2A (on brass). Beam current integration was used to minimize drift, and the data were reduced according to the procedure of Bence and Albee [11 ] with an on-line PDP-11 computer. The overall analytical procedure and uncertainties are as described by Dodd and Jarosewich [12]. Olivine and pyroxene were analyzed at random by stepping along traverses. From 10 to 23 complete analyses were obtained per phase, the number adjusted according to the observed degree of inhomogeneity. Seven elements - Cr, Mn, Fe, Ca, Mg, A1 and Si - were determined in both olivine and pyroxene, three others - Ni, Ti, and Na - in pyroxene alone, for spot checks showed these elements to be absent from olivine. The groundmass, a mixture of microcrystalline pyroxene and glass, with minor metal, troilite, and chromite, was analyzed for all of the elements listed above and for K. Sulfur, minor in all cases, was monitored with a non-dispersive detector. An expanded electron beam ( 2 0 - 5 0 #m) was used for the matrix analyses. In most cases, the groundmass phases are fine-grained enough so that grain-size effects are not significant [14]. It is unlikely that such effects are serious even in coarser materials (ca. 5 ~tm), for analytical sums for groundmass analyses consistently fell between 99 and 101 wt.% despite considerable variation of A1, which is prone to such effects.

groundmass and nil in other phases. The precision of the bulk analyses was estimated from probable errors in modal and phase analyses for chondrule No. 3. As this chondrule couples unusually inhomogeneous olivine with rather few groundmass analyses, the following estimates can be taken as maximum uncertainties: SiO~ + 2% (relative); FeO and MgO -+ 5%; A1203 and Na20 + 12%;and CaO + 19%. The precision for minor elements ranges from -+30% for MnO to -+70% for TiO2 and +94% for K20, at levels of 0.4, 0.1, and 0.1 wt.% respectively. These precisions are clearly poorer than those associated with the fusion technique and they preclude use of Ti and K data for interelement correlations. Improvement could be achieved by performing more analyses per phase, but the time spent in doing so would be wasted: As we shall see, the problem of non-representative sections, which is unavoidable, introduces errors of similar or greater magnitudes.

3. Results 3.1. Chondrule types in Manych and modal compositions Bulk analyses of individual chondrules from the Chainpur chondrite show more or less smooth normative variation from olivine-feldspar to pyroxenefeldspar and even quartz-normative compositions 15 ~-

'po[~_~-"Poikllific"

== I0 ¸

~]-Exc~lrorodlOl, Barrea

0 cO 0

DROP

5 -Q

3ROP

POlK

z 0

Modal and phase compositions and phase densities were used to calculate bulk chondrule compositions. In addition to the constituents reported here, each chondrule contains S and metallic Fe and Ni, all in concentrations which are well below 1% in the

DROP

Mictoporphyriti¢

"5

E

2.3. Bulk compositions

(LS)

M0nych

U'J

DROPI

0

20

I P°lK

40

60

80

I00

Olivine (Vol.%) Fig. 1. Modal olivine contents (250 points) of the largest chondrules in a thin section of the Manych chondrite. Error bar is based on replicate modes.

Fig. 2. Photomicrographs (plane polarized light) of microporphyritic chondrules in the Manych chondrite. (a) Chondrule No. 1, 3225 am long, with euhedral to subhedral olivine phenocrysts in microcrystaUine groundmass which includes minor metal and troilite. Absence of coarse pyroxene is typical of large microporphyritic chondrules in Manych. (b) Chondrule No. 17, 1430 am diameter. Olivine phenocrysts are mantled by pigeonite which is contiguous with that in the gxoundmass. (c) Chondrule No. 30, 1650 nm long. Olivine and subordinate clinobronzite phenocrysts, the latter elongate and twinned, in microcrystaUine groundmass. Minor chromite is present in some olivine crystals. (d) Chondrule No. 35, 1850 am long. Minor olivine, some as rounded insets in pyroxene phenocrysts. A large, embayed olivine crystal lies near the center of the chondrule. Groundmass is turbid, redbrown, and not resolvable with the microprobe.

56 [6,7]. This is a puzzling feature o f chondrule chemistry, for if fine-grained nebular dust were the immediate precursor o f chondrules, most of them should approach its composition. The model advanced by Dodd and Walter [1,7,8] specifically predicts that large chondrules, in particular microporphyries, should approach a chondritic composition. Fig. 1 suggests that this is true in Manych. Most large chondrules in this meteorite are microporphyritic, and the majority of these contain between 50 and 80% modal olivine. Interestingly, the largest chondrule observed in Manych - a 5700-pro barred chondrule - falls in the same modal range (77% olivine). Compositional convergence at large sizes is an important property of chondrules in ordinary chondrites.

3.2. Texture and mineralogy o f microporphyritie chondrules Nine microporphyritic chondrules, which span the modal range in Fig. 1, were selected for mineral and

chemical analysis. Five hundred-point modes for these are given in Table 1, and examples of various textural and mineralogical types are shown in Fig. 2. Most o f the chondrules studied resemble the one shown in Fig. 2a. Olivine, the only phenocryst mineral, occurs as subequant to slightly elongate euhedral crystals which are set in microcrystalline groundmass. Three chondrules contain coarse pyroxene as well (Fig. 2b, c, d) and in one o f these (Fig. 2d), olivine occurs as rounded insets in low-Ca pyroxene.

Olivines. The mean compositions of olivines in the nine analyzed chondrules are given in Table 2. Olivines in six chondrules (Nos. 1,3, 7, 17, 30, and 34) are closely similar: They show monotonic rimward increases of Fe and Mn relative to Mg and have CaO contents similar to those reported in similar chondrules in the Sharps (H-3) and Hallingeberg (L-3) chondrites [1,2]. The small amounts o f Cr and A1 reported in Table 2 persist despite careful background determinations (see also [13]), but the high Cr con-

TABLE 2 Mean compositions and FeO ranges of olivine in selected Manych microporphyritic chondrules Chondrule No. 1

Olivine (vol.%) Analyses Oxides (wt.%) SiO2 AI2O3 FeO MnO MgO CaO Cr203 Sum Max. FeO Min. FeO

2

3

17

7

20

30

34

35

53.6 18

77.9 19

66.2 21

64.6 15

60.1 16

71.9 20

48.2 25

66.0 17

8.3 13

38.39 0,05 22,53 0.43 39.19 0.11 0.05

39.69 0.11 17.62 0,25 42.68 0.13 0.16

37.98 0.07 24.28 0.41 37.55 0.09 0,07

37.77 0,08 25.91 0.45 36.28 0,08 0.06

38.28 0.04 22.21 0.36 39.62 0.07 0.06

39.17 0.13 17.41 0.26 42.95 0.15 0.18

39.19 0.04 17.77 0.40 42.93 0.12 0.02

38.10 0.07 23.75 0.42 37.78 0.07 0.04

36.37 0.02 30.55 0.52 32.98 0.07 0.09

100.75

100.64

100.45

100.63

100.64

100.25

100.47

100.23

100.60

28.7 15.1

24.3 15.3

29.8 18.0

29,7 15,8

29.7 18.3

23.7 12.4

27.1 11.2

29.3 12.8

32.0 29.1

0.24 163 52

0.19 267 61

0.26 157 56

0.28 151 60

0.24 198 62

0.18 269 61

0.19 179 41

0.26 158 55

0.34 118 61

A tom& F/FM * Mg/Mn Fe/Mn

* F/FM = Fe/(Fe + Mg + Mn).

57 tent and strong Fe-Cr-Mn correlation observed in olivine microporphyry in the St. Mesmin breccia [15] are absent. Olivines in chondrules 2 and 20 are slightly different. They have narrower ranges and smaller mean contents of FeO and contain somewhat more A1, Ca, and Cr. That these chondrules are the most olivinerich of those studied suggests, and bulk analyses confirm, that they are chemically distinct from the other microporphyritic chondrules in Manych. The olivine in chondrule No. 35 is unusual. It is almost homogeneous, remarkably Fe-rich, and Capoor. As these are properties found in recrystallized chondrules [1,16], No. 35 may have undergone metamorphism (amd metasomatism) before it entered Manych. This possibility is explored further below. No. 35 is the only chondrule studied which shows evidence of secondary alteration. Pyroxenes. Pyroxene is present in all of the chondrules, but it is coarse enough for reliable microprobe TABLE 3 Mean compositions of phenocryst pyroxenes in selected Manych microporphyritic chondrules Chondrule No. 17 Analyses 10 Oxides (wt.%) SiO2 52.19 A1203 0.76 TiO2 0.07 FeO 19.54 MnO 0.77 MgO 20.34 CaO 4.35 Na20 0.47 Cr203 1.27 Sum Atomic F/FM Mg/Mn Fe/Mn Mg/Cr Fe/Cr

30

30 *

35

15

12

15

56.65 0.61 0.06 10.30 0.61 29.56 1.86 0.26 0.87

56.83 0.59 0.05 10.23 0.56 30.79 0.73 0.20 0.74

54.59 0.37 0.01 17.63 0.49 26.63 0.49 0.07 0.57

99.76

100.78

100.72

100.84

0.34 46.3 24.6 30.5 16.3

0.16 82.1 15.9 64.8 12.5

0.15 96.3 17.8 79.4 14.6

0.27 95.1 35.0 88.8 32.7

* Average excludes analyses which report more than 1.5 wt.%

CaO.

analysis only in Nos. 17, 30, and 35. Chemical data for pyroxenes in these chondrules are summarized in Table 3. In No. 17 (Fig. 2b), pyroxene occurs both as overgrowths on olivine phenocrysts and as elongate laths, many of which are contiguous with the overgrowths. The mean composition reported in Table 3 hides considerable variation, but all analyses suggest that the dominant or only pyroxene is pigeonite (2.3-8.4% CaO) whose range of Fe/Mg variation is small. The coarse pyroxene in No. 30 (Fig. 2c) occurs as polysynthetically twinned laths, most of which are Ca-poor and show covariation of Fe and Ca over a short range of Fe/Mg variation. Though a very few analyses show more CaO (to 13%), the dominant pyroxene in this chondrule appears to be clinobronzite. By contrast with the olivine in No. 35, the pyroxene in this chondrule is quite inhomogeneous ( 1 4 21% FeO). It contains little enough CaO (commonly less than 0.4%) to suggest that it is clinobronzite, but its Fe/(Fe + Mg + Mn) (F/FM) ratio (0.27) is much higher than is appropriate for the protobronzite precursor to this phase [17]. This observation and the olivine data suggest that No. 35 was altered before it entered Manych. Its bulk composition, in particular its F/FM ratio, must be viewed with some skepticism. Groundmass. Mean compositions of the matrices of the nine analyzed chondrules are given in Table 4. Several regularities are evident. All analyses show abundant normative feldspar and pyroxene, the latter with high to very high F/FM ratios. Acmite in some norms indicates that atomic Na commonly exceeds AI in the groundmass. (It should be noted that the Fe a÷ implied by acmite is not otherwise indicated. There is no evidence that the chondrules contain trivalent iron.) Table 4 also shows some systematic variations which appear to be related to the nature of the coarse phases in chondrules. Those chondrules (Nos. 17, 30, and 35) which contain coarse pyroxene as well as olivine have matrices which contain normative quartz and have unusually high F/FM ratios. On the other hand, the most olivine-rich chondrules (Nos. 2 and 20) contain Na- and Si-poor matrices which contain abundant normative anorthite and (No. 20) olivine. In anticipation of the discussion, we can note that these

58 TABLE 4 Bulk and normative compositions of microcrystalline groundmass of selected microporphyritic chondrules in Manych (sulfur omitted) Chondrule No. 1 Analyses

2 15

3 9

7 6

17 15

20

10

10

30

34

35

8

5

7

Oxides (wt.%) SiO2 A1203 TiO: FeO MnO MgO CaO Na2 O K20 Cr203 NiO

58.72 7.73 0.36 9.99 0.31 11.41 6.53 4.75 0.14 0.54 n.d.

50.00 17.90 0.78 7.17 0.20 6.95 14.46 1.71 0.09 0.53 n.d.

60.04 10.94 0.39 8.65 0.26 6.62 6.08 6.83 0.27 0.38 0.03

56.91 7.22 0.35 12.70 0.31 11.39 6.78 4.81 0.13 0.38 n.d.

67.24 7.29 0.31 13.04 0.30 2.63 3.51 4.86 0.33 0.14 n.d.

48.91 19.97 0.31 6.87 0.17 10.48 11.43 1.88 0.10 0.78 0.0l

61.08 9.10 0.41 9.06 0.46 5.76 6.88 5.63 0.24 0.59 n.d.

61.45 8.49 0.43 7.63 0.28 7.01 8.79 5.60 0.11 0.50 n.d.

59.49 7.39 0.34 15.32 0.40 5.74 6.20 4.86 0.23 0.40 0.02

Sum

100.48

99.03

100.49

100.91

99.65

100.9l

99.21

100.29

100.39

0.5 0.6 0.8 40.3 1.1 25.3 28.3 3.0 -

1.1 0.6 0.6 15.3 39.0 27.0 16.3 0.1

0.5 0.4 1.6 56.5 2.4 23.6 10.0 7.5 -

0.4 0.4 0.8 38.0 3.5 26.4 16.2 14.2 -

0.5 0.2 2.0 38.9 4.8 14.4 18.5 20.7

0.4 0.8 0.5 16.5 44.5 8.6 13.6 15.0 -

0.6 0.7 1.4 48.4 1.8 27.4 15.1 4.7

0.5 0.6 0.5 44.9 3.7 34.4 11.2 4.1

0.5 0.4 1.4 39.2 3.8 24.8 25.7 4.2

0.32 71.0 34.5 58.8 167

0.37 58.3 34.3 73.0 294

0.41 41.5 30.0 139 , 140

0.38 71.3 44.0 150 352

0.72 16.5 45.3 92.6 235

0.26 131 47.5 93.3 253

0.46 24.2 21.2 47.0 182

0.37 43.8 26.5 87.3 232

0.59 23.8 35.3 36.0 77.1

Norm il cr or ab an ac di hy ol q

Atomic F/FM Mg/Mn Fe/Mn Fe/Cr Mg/Cr

n.d. = not detected.

variations preclude the possibility that the chondrules s t u d i e d are all f r a g m e n t s o f t h e same i g n e o u s rock. Clearly t h e y d i f f e r in c r y s t a l l i z a t i o n h i s t o r y .

3.3. Bulk compositions Bulk compositions calculated from the modal and p h a s e analyses in Tables 1 - 4 are given in Table 5. T h e m e a n c o m p o s i t i o n o f m e t a l - a n d troilite-free L-group m a t e r i a l [18] is i n c l u d e d for c o m p a r i s o n . T h e s e d a t a are t h e basis o f m u c h o f t h e f o l l o w i n g discussion.

4. Discussion The data presented here indicate that the Manych c h o n d r i t e c o n t a i n s a p o p u l a t i o n o f large m i c r o p o r p h y r i t i c c h o n d r u l e s , m a n y o f w h i c h have similar m o d a l a n d b u l k c o m p o s i t i o n s . T h e s e c h o n d r u l e s raise several q u e s t i o n s . First, w h a t process g e n e r a t e d t h e liquids f r o m w h i c h t h e y crystallized? S e c o n d , t o t h e e x t e n t t h a t c h e m i c a l v a r i a t i o n s a m o n g t h e m are real a n d n o t d u e t o a n a l y t i c a l e r r o r , w h a t processes caused

59 TABLE 5 Calculated bulk compositions and norms of Manych microporphyritic chondrules. Analyses listed in order of increasing SiO2 Chondrule No. 20

2

7

3

34

17

1

30

35

L*

Oxides (wt.%) SiO2 AI203 TiO2 FeO MnO MgO CaO Na20 K20 Cr203

40.3 5.1 0.1 20.2 0.4 30.4 3.0 0.5 0.1 0.3

41.4 3.6 0.2 15.7 0.3 35.3 3.0 0.3 0.1 0.3

43.4 2.5 0.1 21.6 0.4 28.1 2.2 1.5 0.1 0.2

44.2 3.3 0.1 19.9 0.4 28.0 1.9 2.1 0.1 0.2

45.3 2.8 0.1 18.8 0.4 27.8 2.9 1.8 0.1 0.2

46.5 1.7 0.1 20.0 0.5 28.2 1.4 1.1 0.1 0.3

47.1 3.5 0.2 16.9 0.4 26.7 2.9 2.1 0.1 0.3

47.8 2.7 0.1 13.5 0.5 29.1 2.3 1.6 0.1 0.3

53.9 2.0 0.1 18.2 0.5 21.6 2.1 1.3 0.1 0.5

46.4 2.8 0.1 17.1 0.4 29.3 2.2 1.1 0.1 0.5

0.1 0.2 0.3 4.0 11.1 2.3 0.4 81.6

0.2 0,2 0.0 2.8 7.6 5.1 2.7 81.4

0.1 0.2 0.3 12.5 . 0.6 8.5 2.3 75.4

0.2 0.2 0.5 13.3

0.1 0.2 0.3 7.1

0.2 0.3 0.3 17.8

0.6 7.2 12.7 65.2

0.2 0.2 0.3 14.3 . 0.8 10.8 6.0 67.3

0.6 5.6 36.4 49.7

11.0 10.6 59.8

0.2 0.3 0.3 13.6 0.2 9.0 25.1 51.2

0.1 0.6 0.3 10.7 0.2 8.1 71.0 9.0

0.2 0.6 0.8 9.3 2.2 6.5 23.6 56.9

0.27 152 56 93 253

0.20 221 55 73 294

0.30 126 53.5 150 352

0.28 140 55 139 350

0.27 139 52 87 232

0.28 117 46 93 235

0.26 134 47 59 167

0.20 121 31 47 182

0.32 77 36 36 77

0.24 122 40 34 122

Norm il cr or ab an ac di hy ol

.

.

.

A to mic F/FM Mg/Mn Fe/Mn Fe/Cr Mg/Cr

* L is the mean of L-group chondrites, less metal and troilite [18].

their variation? Finally, under what conditions did the chondrules crystallize? We address these questions in turn, though the first and second are obviously closely related.

4.1. Origin o f liquids Table 5 shows that most o f the analyzed chondrules, only the most olivine-rich (Nos. 2, 20) and -poor (No. 35) excepted, vary narrowly around the mean composition o f metal- and troilite-poor L-group chondrites. FeO is somewhat more and MgO somewhat less abundant in most o f them than in L-group chondrites, but the differences are slight. More serious discrepancies appear for Cr and Na.

The apparent deficiency o f Cr in chondrules is almost certainly due to omission o f the opaque suite from the calculated bulk composition, but no such explanation can be raised for excess sodium, This difference between microporphyritic chondrules and their host chondrites is probably real, for Manych also contains many chondrules (Nos. 2 and 20 o f those considered here and almost half of the droplet chondrules to be discussed in a later paper) which are markedly deficient in Na relative to bulk L-group material. In their chemical similarity to metal- and troilitepoor chondritic material, the microporphyritic chondrules resemble the centimeter and larger clasts o f olivine microporphyry which occur in the Parnallee

60 chondrite (LL-3) [19] and in many chondritic breccias (e.g. [12,20,21 ]). Dodd and Jarosewich considered various means of accounting for liquids of such compositions and concluded that the most plausible is complete melting of chondritic material, accompanied by removal of an Fe-Ni-S immiscible liquid [12]. Fodor and Keil reached the same conclusion for similar material in the Plainview chondrite [20]. That separation of Fe-Ni-S and silicate liquids is rapid and efficient is evident from SneUenburg's observations on veins in chondrites [22]. How bulk melting was accomplished is less clear. Impact melting seems most plausible to the writer, but the data do not preclude solar heating. They do appear to exclude generation of the liquids by fractional melting in the parent body, as Dodd and Jarosewich noted [ 12].

trol on the olivine/pyroxene ratios of chondrules. The process by which this might operate is called here, for brevity, "redox/metal-loss". A final possibility is vapor-liquid fractionation. If melting entailed superheating, volatile constituents chiefly Na and Si among the major elements - could be lost to a vapor phase. This process would drive the remaining liquid toward high normative olivine contents. It is not clear how it could generate silica-rich material, but recondensation of silica-enriched vapor is one possibility [1 ]. Each of these differentiation mechanisms -crystalliquid separation, sampling, redox/metal-loss, and vapor-liquid fractionation - should yield a distinctive pattern of chemical variation. Interelement variations among the Manych microporphyritic chondrules are examined below to evaluate the role played by each mechanism.

4.2. Differentiation Normative ofivine-pyroxene-feMspar. As most of the Bulk melting and immiscible segregation account for compositions close to the L-group mean, but they do not account for the variations observed between the olivine-rich and -poor chondrules included in this study. We must consider several possible causes for this variation. One is crystal-liquid differentiation. As olivine is on the liquidus phase for L-group material and for microporphyritic chondrules, the olivine-rich and olivine-poor chondrules may be, respectively, cumulates and residua. Non-representative sampling is a second possibility. As all of the chondrules studied are rounded clasts, their chemical differences may reflect the vagaries of sampling rather coarse material on a f i n e scale [7]. Some of their variation may in fact be spurious and due to modal analysis of non-representative sections through chondrules. Sampling, in both senses, would produce varied compositions. A third source of variation is inherent in the process of melting and immiscible segregation. Clearly the distribution of Fe between Fe-Ni.S and silicate liquids depends on oxygen fugacity. At a very low fo2, Fe would be driven into the metal-sulfide liquid to leave an Fe-poor and Si-rich silicate liquid. Under oxidizing conditions, Fe would concentrate in the silicate liquid, leaving it relatively Si-poor. As Snellenburg [23] notes, redox may exert an important con-

chondrules in this study consist almost wholly of normative olivine, pyroxene, and feldspar, a plot of these constituents is useful for examining major element variations (Fig. 3a). Because of the nature of the norm calculation - some A1 is combined with Na and K in albite and orthoclase; the remainder goes

OL

Ca

\

FS"

"-

(o)

-

Na

(b)

Fig. 3. (a) Normative olivine (OL), pyroxene (PX), and feldspar (FS) and (b) atomic Ca, Na, and K in microporphyritic chondrules in Manych. The L-group mean [18] are also indicated, and exceptionally Na-poor chondrules are indicated with filled triangles. Dashed line in Fig. 3a shows the liquid trend for olivine subtraction. Solid line shows the trend and range of variation of residual silicate liquids after extraction of an Fe-Ni-S liquid from a L-group melt at various oxygen fugacities ("redox/metal-loss").

61 o

oL

o

.nnychl d - 5601Jm. 0 • 700 " 0

•

900

"

1250

"

•

FS

(Mol. % ) Fig. 4. Variation of normative olivine (OL), pyroxene (PX), and feldspar (FS) in samples of Manych chondrule No. 1. Ranges are similar for all sample sizes, but the large samples show a stronger central tendency. into anorthite - such a diagram shows, in effect, relationships among the ferromagnesian constituents (FM), SiO~, and A1203. Fig. 3a shows the same pattern as that found for all chondrules in the Chainpur meteorite [6,7] : Wide variation of the olivine/pyroxene (FM/SiO2) ratio which is independent of the abundance of normative feldspar (A1203). This trend is inconsistent with crystal-liquid fractionation, for both removal of olivine from a L-group melt and partial melting of L-group material would produce covariation of normative pyroxene and feldspar. The possibility that fragmentation of a microporphyritic parent rock would lead to the observed pattern of variation is explored in Fig. 4. Data for this diagram were obtained by preparing a modal grid for one of the largest microporphyritic chondrules in Manych, sampling areas of this grid equivalent to chondrules of various diameters, and converting the modes to norms using observed phase compositions. As the largest samples in Fig. 4 are only slightly smaller than many of the chondrules listed in Table 5, Fig. 4 also provides information on the variations to be expected to result from non-representative modes - sampling, in an analytical sense. It is evident from Fig. 4 that the trend of variation for Manych microporphyritic chondrules and for chondrules of all types in Chainpur is consistent with sampling and that some of this variation, e.g. among chondrules Nos. 3, 17, and 34 (the smallest in the pre-

sent study), is due to the vagaries of sectioning rather coarse material on a rather small scale. On the other hand, the range of variation observed in Manych is difficult to explain by sampling without postulating a parent rock which was coarser than the microporphyritic chondrules. Fig. 4 shows that large olivine-rich (Nos. 2 and 20, Table 5) and olivine-poor (No. 35) chondrules are unlikely to result from fragmentation of typical microporphyries. Fig. 3a shows the locus of compositions which can result from extraction of an Fe-Ni-S liquid from molten L-group chondritic material at various oxygen fugacities. The trend is indistinguishable from that produced by sampling, but the range is limited by two extreme cases in which all Fe is removed (reduction) or all is retained (oxidation). Unless different starting compositions are invoked, redox/metal-loss appears unable to yield extremely olivine-rich or -poor chondrules. Finally, vapor-liquid fractionation, involving loss of silica, can produce a trend similar to that in Fig. 3a. Loss of silica would produce weak covariation of normative olivine and feldspar, but this would be lost in the scatter in Fig. 3a. Accounting for pyroxenerich chondrules is a more serious problem, for though recondensation of silica-enriched vapor is a possibility, it seems unreasonable that materials formed in this way would fall so nicely on the trend defined by the more olivine.rich chondrules. Relationships among the alkalies and Ca bear on the role of vapor-liquid fractionation in the formation of chondrules. These relationships were poorly deffmed in our Chainpur study because of analytical problems [7], but are clear-cut in the present case. Fig. 3b shows that the microporphyritic chondrules in Manych fall into two distinct groups. Seven, including the most pyroxene-rich chondrule studied, cluster near the L-group mean. Two, the most olivine-rich chondrules, are much poorer in Na relative to Ca. These Na-poor chondrules are labelled separately in Fig. 3 and in the following diagrams. Covariation of Na and Ca in most microporphyritic chondrules indicates that little or no vaporliquid fractionation accompanied their formation. On the other hand, such fractionation may account for the low Na and Si contents of the olivine-rich chon-

Potassium-sodium-calcium.

62 drules. Covariation of Na and Ca is of course to be expected of a redox/metal-loss process. It also follows from sampling, for both elements are strongly concentrated in the chondrule groundmass (compare Tables 3 and 4).

Ca depends on groundmass content, pyroxene content, and the type of pyroxene in the parent rock. The Ca/A1 ratio should, of course, be constant in a redox/metal-loss process and in vapor-liquid fractionation.

Calcium-sodium-aluminum. Seven of nine analyzed chondrules contain Na and A1 in approximately 1:1 atomic proportions, though both elements vary quite widely (Fig. 5). This pattern is consistent with both sampling and redox/metal-loss fractionation mechanisms. Low Na/AI ratios in the two olivine-rich chondrules (Nos. 2 and 20) are consistent with a history for these chondrules which included vapor-liquid fractionation. By contrast with the situation for Na and Al, Ca and A1 covary in all of the chondrules studied (Fig. 5), but with scatter that exceeds that expected to result from the rather wide analytical uncertainties. Though covariation is consistent with both sampling and redox/metal-loss, the scatter is a more likely result of sampling. This follows from the fact that the Al contents of samples depend almost wholly on the amount of groundmass incorporated in them, while

Lithophile element ratios. In fact, strict constancy of all lithophile element ratios should follow from fractionation by a redox/metal-loss process and these ratios provide an important test of that model. Table 5 and Fig. 6 suggest that these ratios are by no means constant in the microporphyritic chondrules, even if we disregard those chondrules which may have lost silica to a vapor phase. All ratios show somewhat more scatter than can be explained by analytical uncertainties. Mg/Si and, with less certainty, A1/Si decrease as silica increases. These relationships are inconsistent with redox/ metal-loss, but they follow from sampling, in which elemental abundances and ratios depend on the proportions of several phases in the samples. Scatter in

0

1.0 t

(P 0.5 I

i

I

I

2

-

O

(NJ (3 Z I

i

,x/

t

(L)

L

A

•

,a/

Lx--

"~-

,

,

,

,

I

I

I

I

0.10~

O.IO 0.05

z~ (L) 2

A ~ ~; I

I

!

I

2

3

4

5

Wt. %

U1 - -

Z~

i

~

i

.

t

.

.

t

t

.

/x

ZX

i

i

i

•

[

:

~

~

:

:

:

0

J

:/'

'

A

0.05.

o

A

I

t

~

AI203

Fig. 5. Na-Ca-AIrelationships in microporphyritic chondrules. Symbols conform with Fig. 3, Solid line in the upper diagram marks a 1:1 atomic ratio of Na and A1.

1'I4

I

/X i

!

1

i

O

C~ (D

i

U~

.

0

t

I

O.15.&

4: 0

20tA

t

•

A

i

ZX

,

I

I

I

i

~

;

&

:

:

,

+1

zx-X-----~rn ~ zx

1.0

A-zx---~__~ t, A 0.6 ~ , 4O

i

p

45 J

.

:

;

:

:

50

:

:

~

A

55

Wt. % SiO 2

Fig. 6. Variation of lithophile element ratios vs. silica in Manych microporphyritic chondrules. Symbols conform with Fig. 3. Horizontal lines denote the constant ratios anticipated for redox/metal-loss.

63 those ratios which involve Al follows from the fact (Fig. 4) that considerable variation in groundmass abundance occurs among samples of microporphyry which are nearly as large as those studied here. On the other hand, the Mg-Si relationship, which depends almost wholly on the olivine/pyroxene ratios of sampies, should be more regular. A smooth decrease in Mg/Si with increasing Si is expected and observed.

Ferromagnesian element ratios. It is unfortunate that Cr cannot be determined accurately with the approach used here, for patterns of variation of the Mg/Cr ratio against silica should differ sharply between sampling and redox/metal-loss processes. Mg/Cr should remain constant during redox/metalloss, or it may increase if the oxygen fugacity is low enough to drive Cr into the Fe-Ni-S liquid. On the other hand, the strong preference of Cr for pyroxene (Table 3) and its near absence from olivine (Table 2) require that sampling produce a strong inverse correlation between Mg/Cr and SiO2. Such a relationship is evident in Table 5, but again, we cannot exclude the possibility that it is an artifact. Fig. 7 summarizes data for the other ferromagnesian elements. If we disregard the most silica-rich chondrule (No. 35), whose history may include recrystallization and chemical alteration, the remaining data are broadly consistent with differentiation by a redox/metal-loss process. An apparent decrease of Mg/Mn is not consistent with such differentiation,

60

÷

,,~ 4 0

z~ i

+

o ~: 2 o o , [:: ....

,A

z~

z~

o

E u_

0,4} A

0.2 i

4O

45

50

55

Wt. % SiO2

Fig. 7. Variation of ferromagnesian elements against silica in rnicroporphyritie chondrule. Symbols conform with Fig. 3. Solid lines show trends predicted for redox/metal-loss fraetionation. F/FM -- Fe/(Fe + Mg + Mn).

but it is poorly defmed and the error bars are wide, The Fe/Mn and Mg/Mn data are also consistent with sampling. As both ratios are higher in olivine (Table 2) than in pyroxene (Table 3), they must decrease as the olivine/pyroxene ratio decreases. The case of F/FM is less clear. Whether the ratio of Fe to ferromagnesian elements should increase or decrease with increasing silica depends on whether this ratio was higher in olivine or pyroxene of the sampled rock. The pyroxene analyses in Table 3 admit both possibilities: Sampling of a rock similar to chondrule No. 17 (F/FM higher in pyroxene) would yield a positive correlation between F/FM and SiO2; sampling of rock similar to No. 30 or 35 (F/FM higher in olivine) would yield an inverse correlation. The pyroxene data presented here do not indicate which F/FM-silica trend should be typical of a sampiing process, but more extensive data for microporphyritic chondrules in Sharps [1 ] and Hallingeberg [2] suggest that F/FM is typically lower in pyroxene (clinobronzite) than in olivine. Thus sampling should in general produce an inverse correlation between F/FM and silica, but the slope of this relationship will vary according to the specific mineralogy of the sampled rock and exceptions to it may be common. Thus variation of the F/FM is a weak test of the sampiing model, though it is a strong criterion for redox/ metal-loss.

Summary. The variations observed among microporphyritic chondrules in Manych appear to reflect histories which included: (1) bulk melting of L-group material; (2) extraction of immiscible Fe-Ni-S liquids; (3) crystallization of the remaining silicate liquids to produce microporphyry; and (4) fragmentation and rounding to produce the millimeter-scale chondrules which we observe. Coherence of Na and Al in seven of nine chondrules studied suggests that bulk melting typically occurred at temperatures low enough to preclude loss of volatile major elements to a vapor. Two olivinerich and Na- and Si-poor chondrules may reflect volatile loss caused by local superheating of the bulk melts. Although variations o f f o 2 during separation of immiscible Fe.Ni-S liquids might yield silicate liquids with different silica contents, it is unlikely that this process is responsible for the observed chemical dif-

64 ferences among microporphyritic chondrules. Several trends reported here, in particular inverse correlations of A1/Si and Mg/Si with silica, are inconsistent with a major role for redox/metal-loss in the differentiation of chondrules. It appears that much of the observed variation is due to non-representative sampling of the microporphyritic precursor rocks. Some is no doubt due to analysis of non-representative sections through chondrules, whose effects mimic those of sampling per se. The narrow compositional range observed for most large microporphyritic chondrules in Manych is all the more striking in view of the certainty that some of this variation is analytical. Whether sampling can account for pyroxene-rich microporphyritic chondrules is less clear. Fig. 4 suggests that suitable precursors for such chondrules would be substantially coarser grained than most microporphyritic chondrules. As pyroxene-rich droplet chondrules are far more common than pyroxenerich microporphyries, further discussion of this problem is deferred to a following paper on drop-formed chondrules in Manych.

4.3. Dimensions of parent rocks All of the chondrules included in this study are more or less rounded fragments of larger objects. It remains to estimate how large these parent or precursor rocks were. This question can be approached in two ways: by comparing the texture and mineralogy of microporphyritic chondrules with those produced in controlled cooling experiments, and by comparing them with other natural materials whose cooling histories are known. The experimental approach is preferable and has been applied by several workers to a wide variety of lunar and terrestrial magmatic rocks [2426]. However, most experiments thus far reported used materials which contain much more Ca and A1 and less Fe and Mg than microporphyritic chondrules. The principal exception [27] used compositions in the system Mg2SiO4-SiO2. As melt composition is one of several variables which control olivine morphology [26], these experiments can give only qualitative insights when extrapolated to chondrule compositions. Data from several laboratories suggest that sub-

equant, non-skeletal olivine crystals like those typical of microporphyritic chondrules form during relatively slow cooling and with little or no supercooling. Representative cooling rates are 0.5°C/hr [26], <20°C/hr [24], and <50°C/hr [25]. The subequant skeletal crystals present in some microporphyritic chondrules imply more rapid cooling and/or supercooling, but the cooling rates are still, in general, less than lO0°C/hr. By contrast, the platy, skeletal, and dendritic olivine crystals typical of droplet chondrules appear to require much more rapid cooling and/or deeper supercooling [27]. These data, though still qualitative, confirm earlier textural and mineralogical arguments that microporphyritic chondrules are more or less rounded fragments of igneous rocks which were much larger than themselves [3,19,1]. The minimum dimensions of these parent rocks can be estimated by comparing microporphyritic chondrules with chemically similar olivine microporphyries which occur as clasts in the St. Mesmin chondrite (LL-group breccia) [12]. Two parameters which are useful for this comparison are the maximum size of olivine crystals and nucleation density, the latter estimated from the number of olivine crystals encountered per centimeter of traverse across each sample. Such data for large microporphyritic chon-

E E

1.0

Manych (L-5):

/k /k

/x

(D N

CO

/k

z~

Microporphyritic Chondrules ( 5 0 - 8 0 vol.% 01)

-I-,-

>',- 0.5

(..)

A

Zt

~

c~

A

20

4'0

/,,

A

o

S.la. 0 5 S.M. D3

/X 0~'~

6'0 8'0 100 I;~0 Crystals/cm.

Fig. 8. Maximum olivine size compared with the number of olivines encountered per centimeter of traverse across microporphyritic chondrules. Data points marked "S.M." refer to coarse, homogeneous olivine microporphyry in the St. Mesmin chondrite (D5) and a fine-grained microporphyry (D3) in the same meteorite [121.

65 drules in Manych and for the St. Mesmin microporphyries are summarized in Fig. 8. The chondrule data scatter badly in Fig. 8, for the largest olivine seen in section is not necessarily the largest present in a chondrule. Nonetheless, maximum crystal size and nucleation density follow a crude inverse relationship which suggests that the chondrules cooled at various rates. The coarser of the St. Mesmin microporphyries (D5) falls near the "fastcooling" end of the chondrule trend; the finer (D3), a 3-cm clast with a chilled contact, plots far off scale. Fig. 8 suggests that most of the microporphyritic chondrules in Manych cooled more slowly than the coarser St. Mesmin microporphyry. Further evidence to this effect is the fact that olivine crystals in the latter, though subequant, commonly consist of parallel plates separated by glass, which suggest that they first crystallized as bars, each of which preserves a pattern of igneous zoning [ 12]. By contrast, zoning is monotonic in most olivines in microporphyritic chondrules. Finally, the St. Mesmin olivines are much more calcic (0.1-0.4% CaO) than those in the Manych chondrules and in similar chondrules in other type 3 ordinary chondrites [1,2]. In this respect, they more closely resemble olivines in droplet chondrules in these meteorites. As both of the olivine microporphyries in St. Mes. rain are clasts, only their minimum dimensions are known. They were certainly no smaller than 6 cm across before breakup, suggesting that the parent rocks from which microporphyritic chondrules formed were at least this large. 10 cm appears to be a safe lower limit for the sizes of these precursor rocks and the melts from which they crystallized. This conclusion rests on the assumption that the parent rocks crystallized in environments like those which produced the St. Mesmin microporphyries. Clearly, bodies of liquid as small as the chondrules could cool slowly if, for example, they were immersed in a hot medium. However, the need to explain both coarse microporphyritic chondrules and quenched droplets makes this alternative unattractive. It appears that melting of chondritic material on a scale of or larger than 10 cm produced the rocks which were parental to microporphyritic chondrules. Experimental determination of the cooling rates of such rocks is needed to establish an upper limit for the scale of melting of chondritic material.

5. Conclusions Most of the large chondrules exposed in a thin section of the Manych (L-3) chondrite are rounded fragments of olivine microporphyry, many of which are compositionally similar to the host meteorite less most of its metal and troilite. The rocks from which these chondrules were drawn formed from L-group liquids which were depleted in Fe, Ni, and S by separation of immiscible Fe-Ni-S and silicate liquids. In some cases, melting and immiscible segregation were accompanied by vapor-liquid fractionation to yield a small population of Na. and Si-poor chondrules. Much of the chemical variation of microporphyritic chondrules in Manych is due to non.representative sampling of the parent rocks, and some to non-representative modal analysis of the chondrules themselves. Little or none of this variation can be charged to redox variations during formation of the parent rocks. Sampling may also be responsible for occasional pyroxene-rich compositions, but only if some of the parent rocks were much coarser than the microporphyritic chondrules now present in Manych. Comparison of microporphyritic chondrules with centimeter-sized masses of olivine microporphyry in the St. Mesmin chondrite suggests that the latter cooled more rapidly than most such chondrules. It is likely that the rocks from which microporphyritic chondrules were derived, and the melts from which these precursor rocks crystallized, were no less than 10 cm across.

Acknowledgments L.G. Kvasha of the Academy of Sciences, U.S.S.R., kindly provided the sample of Manych on which this study was conducted. Elana Benamy, Cynthia Sue Cook, and Robin Spencer assisted with drafting and manuscript preparation. Discussions with John V. Heyse and Dr. Timothy Grove were valuable to the writer during the study. Support for this research is provided by the U.S. National Science Foundation under grant No. EAR 7421801A01.

References 1 R.T. Dodd, The petrology of chondrules in the Shaxps meteorite, Contrib. Mineral. Petrol. 31 (1971) 201.

66 2 R.T. Dodd, The petrology of chondrules in the Hallingeberg meteorite, Contrib. Mineral. Petrol. 47 (1974) 97. 3 G.P. Merrill, On chondrules and chondritic structure in meteorites, Proc. Natl. Acad. Sci. 6 (1920) 449. 4 R.T. Dodd and L.S. Teleky, Preferred orientation of olivine crystals in porphyritic chondrules, Icarus 6 (1967) 407. 5 A.F. Noonan and J.A. Nelen, A petrographic and mineral chemistry study of the Weston, Connecticut, chondrite, Meteoritics 11 (1976) 111. 6 L.S. Walter and R.T. Dodd, Evidence for vapor fractionation in the origin of chondrules, Meteoritics 7 (1972) 341. 7 R.T. Dodd and L.S. Walter, Chemical constraints on the origin of chondrules in ordinary chondrites, in: L'Origine du Syst~me solaire, H. Reeves, ed. (C.N.R.S., Paris, 1972) 293. 8 L.S. Walter, R.T. Dodd and P. Smidinger, Sampling model of chondrule compositional variations, Meteoritics 8 (1973) 449. 9 R°T. Dodd, W.R. van Schmus and D.M. Koffman, A survey of the unequilibrated ordinary chondrites, Geochim. Cosmochim. Acta 31 (1967) 921. 10 H.Y. McSween, Jr., Carbonaceous chondrites of the Ornans type: a metamorphic sequence, Geochim. Cosmochim. Acta 41 (1977) 477. 11 A.E. Bence and A.L. Albee, Empirical correction factors for the electron microanalysis of silicates and oxides, J. Geol. 76 (1968) 382. 12 R.T. Dodd and E. Jarosewich, Olivine microporphyry in the St. Mesmin chondrite, Meteoritics 11 (1976) 1. 13 R.T. Dodd, Minor element abundances in olivines of the Sharps (H-3) chondrite, Contrib. Mineral. Petrol. 42 (1973) 159. 14 A.L. Albee, J.E. Quick, and A.A. Chodos, Source and magnitude of errors in "broad-beam analysis" (DBA) with the electron probe, Proc. 8th Lunar Sci. Conf. (in press) 15 R.T. Dodd, D.J. Morrison-Smith, and J.V. Heyse, Chromium-bearing olivine in the St. Mesmin chondrite, Geochim. Cosmochim. Acta 39 (1975) 1621.

16 R.T. Dodd, Recrystallized chondrules in the Sharps (H-3) meteorite, Geochim. Cosmochim. Acta 32 (1968) 1111. 17 A.C. Turnock, personal communication (1972), cited in R.T. Dodd, J.E. Grover and G.E. Brown, Pyroxenes in the Shaw (L-7) chondrite, Geochim. Cosmochim. Acta 39 (1975) 1585. 18 B. Mason, The chemical composition of olivine-bronzite and olivine-hypersthene chondrites, Am. Mus. Novitates No. 2223 (1965). 19 R.A. Binns, An exceptionally large chondrule in the Parnallee meteorite, Mineral. Mag. 37 (1968) 319. 20 R.V. Fodor and K. Keil, Carbonaceous and non-carbonaceous lithic fragments in the Plainview, Texas, chondrite: origin and history, Geochim. Cosmochim. Acta 40 (1976) 177. 21 W. Kempe and O. M/iller, The stony meteorite Kratlenberg, its chemical composition and the Rb-Sr age of the light and dark portions, in: Meteorite Research, P. Millman, ed. (Dordrecht, D. Reidel, 1969) 721. 22 J.W. Snellenburg, The melt phase in veins and porphyritic chondrules in ordinary chondrites, EOS Trans. Am. Geophys. Union 56 (1975) 1016 (abstract). 23 J.W. Snellenburg, Mn/Fe as a petrogenetic indicator in porphyritic chondrules in the Semarkona (LL-3) meteorite, EOS Trans. Am. Geophys. Union 58 (1977) 429 (abstract). 24 G. Lofgren, C.H. Donaldson, R.J. Williams, O. MuUins, Jr. and T.M. Usselman, Experimentally reproduced textures and mineral chemistry of Apollo 15 quartz normative basalts, Proc. 5th Lunar Sci. Conf. 1 (1974) 549. 25 D. Walker, R.J. Kirkpatrick, J. Longhi and J.F. Hays, Crystallization history of lunar picritic basalt sample 12002: phase equilibria and cooling rate studies, Geol. Soc. Am. Bull. 87 (1976) 646. 26 C.H. Donaldson, An experimental investigation of olivine morphology, Contrib. Mineral. Petrol. 57 (1976) 187. 27 M. Blander, H.N. Planner, K. Keil, L.S. Nelson and N.L. Richardson, The origin of chondrules: experimental investigation of metastable liquids in the system Mg2SiO4-SiO 2, Geochim. Cosmochim. Acta 40 (1976) 889.