Some clues to the history of the H-group chondrites

Earth and Planetary Science Letters, 54 (1981) 67-80 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

67

[51

Some clues to the history of the H-group chondrites Mireille Christophe Michel-Levy Laboratoire de Minbralogie-Cristallographie associ~ au CNRS, Universitb Pierre et Marie Curie, 75230 Paris (France) Received February 2, 1981 Revised version received March 25, 1981

SEM, optical and chemical observations have been performed on 12 H3-6 chondrites, 9 of them being also studied by other groups. Morphological features of chondrules and crystals (growth steps) are shown; the significance of the finely crystallised troilite in Menow and Ambapur Nagla is discussed in the light of the discovery that the NiFe blebs associated with it are Ni-rich (50-60% Ni). Sulphur should have been mobilized without shock evidence possibly as a result of solar heating. Pre-chondritic relict material is recognized by anomalous or variable mineral compositions, and in some cases, by the presence of overgrowths on relict cores. After short notes on individual chondrites, a tentative history of H chondrites is proposed. The chondrule-forming episode is considered as a remelting of pre-existing material. The accretion would immediately follow this event for type 6 (around 1000°C), and would occur at progressively lower temperature for types 5 and 4. Type 3 would represent material coming from an extended source region, an hypothesis consistent with the broader range composition of the particles and with their cooling before accretion to much lower temperatures (below 350°C).

1. Introduction A number of different techniques have been used recently to study meteorite samples, in order to improve our knowledge on their mineralogical, petrological and chemical composition and to try to understand their formation history. In particular, a group of ten H chondrites were chosen in 1975 under the initiative of R. Hutchison to be distributed by the British Museum to different laboratories for a consortium study. Some results

TABLE I Chondritesstudied H3

H4

H5

BremerviSrde Beaver Creek Allegan Tieschitz Guenie Ambapur Nagla Menow Sena Quenggouk Ste. Marguerite en Comines

have already been published [1-4] or await publication. We present here a few observations on the following chondrites from the British Museum and the Museum National d'Histoire Naturelle, listed in Table 1 by rank order according to the chemical -petrologic classification of Van Schmus and Wood [5]. Some results concerning Tieschitz and Sena have already been published [6,7]; Guenie has been studied as a 3rd cycle thesis subject by M. Bourot-Denise [8]. The H-group was chosen because of the occurrence of many slightly shocked stones which retain therefore most of their originaP characteristics. We shall compare our observations with those of other researchers on the same chondrites and other H chondrites as well.

H6 Butsura Kernouv6

2. SEM and optical observations 2.1. The chondrites

Chondrules are understood as all more or less rounded grains; the droplet chondrules have obvi-

0012-821X/81/0000-0000/$02.50 © 1981 Elsevier Scientific Publishing Company

68

1

2

3

4

5

6

7

8

9

Fig. 1. A 80-~m-diameter spherical chondrule found in Menow. Scratches may be noticed around the ball. Fig. 2. A 75-#m chromite-rich chondrule perched on an olivine crystal in Sena. Figs. 3, 4. A 500-~m-diameter cupulated droplet chondrule consisting

69 ously been molten; others are called lithic chondrules. They are better observed in the least shocked specimens; they mostly preserve their primary appearance in types 3 - 5 chondrites; in type 3 they may be surrounded by a fine-grained rim which prevents the direct observation of the chondrule surface [6,9,10]. The size of most chondrules ranges between 200 and 8 0 0 / t m , but we m a y find larger ones and a few smaller droplet ehondrules: contrary to a general opinion [11-13] that there is no correlation between composition and size, these usually have a chemical composition distinct from the average, being richer in silica, alumina and alkalis; calcium, magnesium, iron, titanium, chromium are also present. Note that Hughes [14] observes a gradual decrease-presumably compositionally r e l a t e d - - i n chondrule density with chondrule diameter. Three examples of peculiar chondrules are shown: (1) A 80 # m diameter round ball taken out of Menow, o n which scratches from low-velocity impact particles or from friction with sharp debris m a y be seen (Fig. 1). (2) A chromite-rich 75/~m diameter chondrule, perched on an olivine crystal, found in Sena (Fig. 2). (3) An oval-shaped, cupulated drop on the surface of which are ribbons of molten silicates, taken out of Ste. Marguerite (Figs. 3, 4). A polished section through this chondrule shows tiny hercynite spinels in glass. The size of these chondrules may be correlated with a low viscosity, a property which is generally not taken into account (see, however, Hughes [14]) when explaining the narrow size range of the majority of droplet chondrules which have also a limited range of chemical composition according to Lux et al. [15], at least in type 4 - 6 chondrites. The occurrence of bowl-shaped depressions on the surface of some chondrules (Fig. 3) m a y be

more often attributed to bursting bubbles than to impact features, as can be verified when sectioning these chondrules inside of which spherical holes are preserved. When the droplet is quenched, the surface of the chondrule remains rather smooth, the olivine or pyroxene crystals being frozen in the glass. When it cools more slowly, crystals may project over the curved surface; in addition, mineral particles passing by may be glued to its surface. Rather than showing examples of these chondrules, we prefer to show artificial silicate beads prepared by C.H. Donaldson (personal communication), the cooling law of which is known. Unfortunately, their composition is not similar to meteorite chondrules but to Apollo XII basalt so that pyroxene is much more prominent than olivine; however, olivine is the first phase to crystallise as shown in Figs. 5, 6. By comparison, the cooling rates of chondrules may be estimated to range from a few tenths of a degree up to tens of degrees per hour starting from the liquidus, according to the type of chondrule (granulated, porphyritic, microporphyritic, radiating, fibrous ...). Lithie chondrules are usually considered to have been rounded by some kind of rubbing or abrasion process, when travelling in a dusty atmosphere; this is not at all obvious and is difficult to prove, especially since even a light shock may have erased the pristine shapes. In Menow, which shows nearly no shock-effects, evidence has been found on an olivine crystal of what could possibly be considered as an abraded crest. Another plausible explanation for the lithic chondrules aspect is that they come from chips of broken and partially melted rocks; an example has already been shown in Tieschitz [6], another one is afforded by a chondrule in the Kiffa H4 stone (Figs. 7, 8). '

of tiny hercynite spinels in a calc-alkaline glass, found in Ste. Marguerite; glassy, micron-wide ribbons are sticking on its surface (detail Fig. 4, × 1500). Fig. 5. Artificial bead prepared by Donaldson, cooled at 5.8°/hour from 1270 to l l31 °C, then quenched. Isometric olivine crystals appear on the smooth glassy surface, x 100. Fig. 6. Artificial bead prepared by Donaldson; same composition as 5; cooled at 75°/hour from 1270 to 957°C; the mark of the holder is visible on the right side; olivinecrystals protrude from the irregular surface. X 50. Fig. 7. A rounded lithic chondrule which may be a chip of a partly molten rock. Kiffa; X 50. Fig. 8. A detail of the former chondrule: some glass (g) is covering the upper part of olivine crystals. X 500. Fig. 9. A hopper olivine crystal in Ste. Marguerite; the gulfs found in the crystal are not the result of corrosion but of an irregular fast growth. × 350.

13

b

Fig. 10. Growth steps on a broken crystal of olivine in Beaver Creek; the white small isometric crystal is a chromite. X 2500. Fig. I 1. A change in the growth conditions of these small olivine crystals may be noticed: the crystals are bounded by a few growth steps

71

2.2. The matrix The matrix may be recognized as a distinct unit only in the type 3 chondrltes, where fine particle coatings are found around chondrules; in Tieschitz where it is especially well developed, it consists mainly of less than 0.1 /~m particles of iron-rich olivines (as confirmed by X-rays); its black colour is attributable to the presence of carbon. The matrix may concentrate sulfide particles and probably most of the volatile elements. In types 4 and 5, it is indistinguishable on petrographic grounds from debris of crystalline material which attains a maximum abundance when the stone has been slightly shocked; only in type 6 has a distinct recrystallization occurred, both erasing the limits between chondrules and matrix and coarsening the matrix grains. SEM observations of the matrix are complicated by terrestrial oxidation which settles preferentially between chondrules, in loosely retained fines. Our observations are in good agreement with those of Ashworth [16] who obtains with HVEM many more details.

2.3. The minerals Interesting features are found for all mineral phases and may be related to their formation conditions.

Olivines andpyroxenes. Olivine porphyritic crystals are frequent and show unfinished hopper morphology [17] corresponding to a crystal growth happening in a few hours (Fig. 9); at the surface of the chondrules, these crystals acquire growth steps as shown in Fig. 10. Some microporphyritic rock fragments show a change in the morphology of crystals as if they had undergone different growth conditions, perhaps before and after the fragmentation of the chip (Fig. 11). In Sena and in other type 5 and 6, the growth of a rim around the

fragmented core of a crystal has been observed both on olivine and orthopyroxene crystals [7]. The pyroxenes show finer crystal growth steps than olivine, corresponding to their striated morphology (Fig. 12); they also form fibrous or platy radiating chondrules as shown in Fig. 13a, b, c. These silicates may sometimes be affected by corrosion as shown in the case of Ambapur Nagla (Fig. 14) and Menow (Fig. 15) but we shall see that for those two chondrites, special features affect the troilite; the corrosion may be a late process correlated with a mobilisation of the sulfide, or the circulation of H 2S. While we find rather few differences between type 3 to 6 in iron-magnesian silicate morphology, the other minerals vary widely. Plagioclase feldspars crystallise either by devitrification in situ of glass in the chondrules, or as tiny globules that are found, for example, perched on pyroxene fibers, in types 4 and 5 (Fig. 13a). Only in well-recrystallized type 6, such as Kemouvr, is the normative feldspar of the stone wholly represented by intergranular 20- to 50-ffm crystals. In Tieschitz, albite, associated with some nepheline, forms part of the white matrix which has grown between the blackrimmed chondrules. Phosphates also show some mobility: in type 3, they are mainly associated with the metallic phases as shown in Fig. 16 (that is also true in type 3 carbonaceous chondrites); in type 4, we have found small crystals attached to the surface of chondrules (Fig. 17); in type 5 and 6, they may form wide interstitial grains between chondrules. Chromite is rare in type 3: tiny crystals are found in a few silicate and metallic chondrules. Euhedral crystals inside chondrules or associated with metal sulfide and phosphate are most common in type 4 and 5 whereas anhedral grains are more frequent in recrystallised type 6. Other rare chromian spinels occur in the lower types.

superposed to plane faces. Ambapur Nagla; X 1000. Fig. 12. The growth steps in pyroxenes are very finely delineated. Bremerv6rde; x 500. Fig. 13. Three aspects of pyroxene development in radiating chondrules: (a) A fibrous chondrule in Menow where tiny feldspar beads stick on some pyroxene fibers. X 300. (b) A detail in a large radiating chondrule in Ste. Marguerite showing better formed prisms. X 700. (c) Pyroxene laths in a Beaver Creek chondrule. X 700. Fig. 14. Corrosion holes with geometric contours are found in a few olivine crystals of Ambapur Nagla (arrows). X 250. Inset: detail of holes. X 6000. Fig. 15. Another aspect of corrosion of the crest of large olivine crystal in Mewow. X 1500. Fig. 16. A metallic chondrule in Tieschitz observed on a polished section. Pale grey, a- and "f-NiFe; grey, FeS; dark grey, silicates; the tiny black patches in the chondrule are calcium phosphate (P); notice the dark matrix rim around the chondrule. X 50.

72

17

18

20

,19

21

23

22

24

Fig. 17. An apatite grain sticking on the wall of a chondrule in Quenggouk. × 1000. Fig. 18. A kamacite crys;al in Ste. Marguerite; the lower part was moulded on silicates, but the free upper surface shows geometrical planes and growth steps. Fig. 19. Growth steps on

73

Nickel-iron and troilite. If high temperature affected the chondrule silicates which melted, the same was true for metal and sulfide which also melted to produce metallic chondrules. These are still recognizable in type 3 because they were quenched (Fig. 16), they were studied recently in Tieschitz by Bevan and Axon [3]. Because of the immiscibility between silicate and metallic melts and owing to an incomplete separation of these melts, there are a few metallic globules in some silicate chondrules, and a few phosphate and silicate blebs in metallic chondrules. Kamacite, taenite (often zoned and polycrystalline) and troilite are closely associated in the type 3 metallic chondrules; in the other types, they are also found as isolated crystals. The irregular form of the kamacite may be due to plastic deformation before crystallisation or to crystal growth in the voids between silicates; the surface sticking to silicates is moulded on them (Fig. 18); the free surface always shows more or less beautifully developed growth steps (Figs. 18, 19). The late history of the metallic phases may be inferred from: (1) New nucleation features. Dendritic micronsized kamacite needles growing in the 35-40% Ni cloudy core of zoned taenite in Tieschitz (Fig. 20) should be regarded as evidence of a very slow cooling at low temperature ( < 1o/10 6 years) * as should, in the same stone, the occurrence of anisotropic 50% taenite (tetrataenite, Clarke and Scott [19]), this superstructure phase becoming ordered when cooling down very slowly under 320°C; this is in strong contrast with the quenching of the metallic phase from a higher temperature inferred from the chondrule-like appearance of most of the metal, corresponding to the preaccumulation history, followed by a very slow postaccumulation cooling from some temperature under 350°C.

* By extrapolation of the Goldstein and Short curves [18].

(2) Deformation features, which, in our slightly shocked specimens, are reduced to stretching (Bremervorde, Allegan, Fig. 21) and scratching (Ste. Marguerite, Fig. 22) caused by hard silicate shards on soft metal. On polished sections, stretching (in Guenie) or Neumann bands have been observed in some of the kamacite grains. (3) Late mobilisation features. These are observed in the cases of Ambapur Nagla and Menow; the same processes operated in both stones but were more intense in the first one and were accompanied by corrosion of some silicates, as already noticed. The troilite does not occur as blocky crystals as usual but shows by its bireflectance a fine mosaicism; it also occurs in loose or curved flakes (Fig. 23). When in contact with nickel-iron, mixed zones may be observed, made of tiny irregular grains, not of droplets. These zones are often interpreted as melted zones indicative of a reheating to nearly 1000°C. But their appearance is clearly distinct from that of the melted mixed nickel-iron-troilite grains often found in highly shocked stones or just beneath the fusion crust of chondrites. On the other hand, Ramdohr [20] thinks that this "younger troilite"--as he calls it - - w a s formed at least in part from the metal phase by replacement, resulting from migration of sulphur. An argument in favour of this last hypothesis is the fact that the tiny grains of nickeliron left in the troilite are invariably very nickelrich (50-60% Ni) as we measured by X-ray energy dispersive analysis both in Ambapur Nagla and in Menow, and also in other chondrites showing the same phenomenon. When troilite crystallises as a replacement of kamacite or taenite, it has no use for the nickel of the alloy which would then be left over as very nickel-rich alloys; this replacement did not occur at a very high temperature since Pellas et al. [21] still find a partial fission track record in the phosphates of Menow where the replacement is less pronounced than in Ambapur

kamacite in Allegan. X 2500. Fig. 20. Dendritic kamacite (arrow) growing in cloudy taenite ( T ) in Tieschitz; polished section etched with nital. × 400. Fig. 21. The slight shock experienced by the Allegan meteorite has stretched this metallic grain. × 1000. Fig. 22. In Ste. Marguerite, when a slight shock occurred, some sharp silicate debris have scratched the NiFe and the marks are still visible. × 1500. Fig. 23. In Menow and Ambapur, free coma-like troilite ribbons or whiskers are found between the silicates. Here, a rolled lath in Menow. X2250. Fig. 24. Ni-Fe-S and silicates make up immiscible liquids as illustrated on this Tieschitz polished section where globular NiFe (white) and FeS (light grey) separate from a silicate (presently devitrified) glass (dark grey). × 100.

74 TABLE 2 Analysis result of the olivines and pyroxenes for F e / ( F e + Mg) Kernouv6 (H6)

Guenie (H4)

Ste. Marguerite (H4)

N u m b e r of crystals Olivine Fa (%)

15 17.5-19.7 *-20.5

30 17.7-19.2 * - 2 0

17 19.2-19.8 *-21.6

N u m b e r of crystals Pyroxene Fs (%)

12 16.9-17.3 * - 18.5

27 15.4-16.7 * - 18

18 13.5-17.2 * - 19.4

* Mean value

Nagla, where only few tracks if any are still present in the phosphates and no tracks at all in the adjacent pyroxenes (J.C. Lorin and P. Pellas, personal communication). This thermal event could have occurred late in the history of the meteorites since Turner [1] noticed in Menow a loss of 4°At during the last 2.5 × 10 9 years. In Ambapur and in other stones (see compilation by Schultz and Kruse [22]) it is connected-after correction for shielding effects--with a low 3He/2~Ne ratio which has been interpreted as evidence of solar heating [23]. This raises the interesting possibility that such a simple mineralogical record would allow an independent detection of an heating episode occurring during the exposure age.

3. Chemical observations on s o m e mineral phases

Major and minor elements analyses were carfled out either with a CAMECA MS 46 microprobe or with a CAMEBAX automatic probe on Kernouvr, Guenie, Ste. Marguerite and Sena; they supplement analyses of olivines and pyroxenes performed by Hutchison et al. on the 10 specimens of H-group chondrites they studied. Olivines and pyroxenes (Table 2). There is practically no difference in the scatter of M g / F e compositions between the H 4 - 6 stones. In Kernouv6 H6, the scatter is larger in olivine than in pyroxene; in others, the contrary is true; the great majority of crystals is quite near the mean value; only one or two crystals out of over 12 attain extreme values and there are generally trivial reasons for these deviations from the mean; for example, the Fsl85 pyroxene in Kernouv6 is surrounded

by nickel-iron: a local equilibrium prevailed over the general one; but it may also result from real heterogeneities as documented by Wlotzka and Fredriksson [24] for example, in the Morro de Rocio H5 chondrite, in H 4 - 5 specimens, the more extreme values are rather found in isolated debris: that is the case for example, for the FSl3.5 pyroxene of Ste. Marguerite which is the core of a crystal overgrown by a rim of a normal F s I 7 A composition. Surprisingly enough, we see that the highly recrystallised Kernouv6 chondrite displays a rather large scatter in its olivine Fa content. Obviously in this case, the present chemical composition reflects the initial F e / M g content of the chondrules and debris. On the other hand, the CaO content seems to be more sensitive to the thermal history of the meteorites: in Kernouv6 and Sena olivines, C a O < 0 . 0 2 % whereas in Guenie and Ste. Marguerite olivines, CaO < 0.06%. These figures are below 0.1% CaO reflecting rather slow crystallisation or recrystallisation history [25]. The CaO content found in the low-calcium pyroxenes confirms the analyses of Hutchison et al. [4] who find some scatter in type 4 - 5 chondrites and a narrow peak for H6: 0.56-0.87% CaO are found in Kernouv6 orthopyroxene while 0.17-1.17% CaO and 0.30- 1.08% CaO are found in Ste. Marguerite and Guenie pyroxenes. Distinctions between the composition of striated* and orthopyroxenes in * We call striated pyroxenes the type of low-calcium pyroxene found by Ashworth [2] to be an association of ortho- and clinopyroxenes lamellae, the former being more numerous than the latter; they are also known as low-calcium clinopyroxene [26] owing to the oblique extinction of the polysynthetically twinned lamellae, though Binns [27] noticed that their obliquity is always less than 12° and their birefringence low.

75 TABLE 3 Minor element contents in striated pyroxenes and orthopyroxenes Guenie

CaO A1203 TiO2 Cr203

Kernouv4

Sena

str.px

opx

str.px

opx

opx

0.49-0.67 0.11-0.37 0.11-0.21 ~<0.15

0.30-1.08 0.17-0.73 0.12-0.28 0.07- 0.43

0.40-0.60 0.10-0.20 0.05-0.15 ~<0.13

0.30- 1.10 0.10-1.15 0.05-0.45 0.10- 0.70

0.56-0.87 0.10-0.45 ~<0.36 ~<0.26

H 4 - 5 have been looked for. In Guenie, where 6 crystals of orthopyroxene and 9 of striated pyroxenes have been specially located for that purpose, somewhat greater variations of minor elements (Ca, A1, Cr, Ti) in orthopyroxene than in striated pyroxene were noticed; the same is true in Sena (Table 3). On the basis of the minor element repartition, two kinds of orthopyroxenes are found: (1) In H 4 - 5 chondrites, they show some scatter in their composition which may reflect some variations in prechondritic material composition (but in Sena, the orthopyroxene cores are mantled by a rim of newly formed orthopyroxene, the composition of which is more limited and similar to that of striated pyroxene; the rims have probably grown after accretion in a uniform environment). (2) In Kernouv6 H6, their compositions have narrow limits intermediate between those of striated and orthopyroxenes of H 4 - 5 , a result of metamorphic re-equilibration. Other recently analysed H6 chondrites have similar orthopyroxene composition: Seoni [28], Patora [29], Naragh [30] for example. Striatedpyroxenes may represent crystals grown and more or less quickly cooled from the proto form as assumed by Ashworth [2], in a melt or a glass produced during the chondrule-forming

event. Ashworth [2] finds the same minor element contents in the striated pyroxenes of H4 Quenggouk and H5 Allegan. If some orthopyroxenes represent pre-chondritic material, we should find also pre-chondritic olivines. These have been identified in Sena on a morphological basis: olivine irregular cores are mantled by newly formed rims. The minor element chemistry i s - - a s for the mantled pyroxenes of the same s t o n e - - m o r e uniform in the rims than in the cores. An olivine crystal rich in chromium and full of tiny chromite inclusions oriented in the host has been found in Guenie; the analyses of this olivine and its chromite inclusions are as shown in Table 4. The Cr content of olivine is as high as that found by Dodd et al. [31] in a St. Mesmin clast. A similar olivine crystal has been observed in Quenggouk where Ashworth [32] finds mostly olivine with homogeneously exsolved chromites of 10-100 nm long. An olivine precursor which should accept such a high level of A1, Cr, Ti is unknown; the size of the chromite particles is expected to depend not only on the cooling conditions but also on the degree of supersaturation of the host medium.

Chromite, ilmenite and spinels. Data on these oxides have been gathered from Guenie. Besides the usual

TABLE 4 Chemical compositionof an olivine crystal in Guenie and its chromite inclusions

Olivine Chromite

SiO2 38.77 -

FeO 18.12 31.29

MgO 42.13 2.88

MnO 0.42 1.08

CaO 0.01 0.04

A1203

Cr203

TiO 2

'E

0.03 7.30

0.65 55.52

0.10 2.14

100.23 100.25

Fa (%) 19.4

76 occurrence of chromite (the mean composition of which is typical of the H-group chondrites), ilmenite is found associated with Ni-Fe and FeS or chromite in some debris, and with aluminous spinels in two chondrules. These aluminous spinels may be understood as relicts from chemical heterogeneities (see Kracher and Kurat [33]) to which are also presumably attributable the large MnO variations found in ilmenites: 1.5% MnO in spinel chondrules, 3.4-7.6% MnO in isolated crystals. In Guenie, only these accessory minerals show clear deviation from equilibrium and the same is true in other H4 chondrites such as Gobabeb [34] and Ste. Marguerite (occurrence of aluminous spinel) for instance.

4. Summary of the observations on individual chondrites H3 Tieschitz. Tieschitz stone has been the focus of much interest for the last few years. Whether Tieschitz is representative of typical type 3 chondrites is a matter of debate: indeed, the black rim of the chondrules is especially well preserved, the scatter of the composition of the chondrules is marked (e.g. [15]), the globular shape of most of the metal + sulfide associations is prominent. But the white matrix found in this chondrite [6] has not been foiand in other type 3's such as Chainpur, Bishunpur, Krymka, for instance; according to Rambaldi et al. [35], the siderophile element contents of this meteorite are more typical of an L chondrite than of an H chondrite. Besides, according to Turner [1], the 4°Ar-39Ar release pattern is markedly distinct from that of many other chondrites. Bevan and Axon [3] contend that chondrules and silicate/metal/troilite intergrowths were produced by the rapid solidification of immiscible silicate and Fe-Ni-S melt. Indeed, such an intergrowth is observed in Tieschitz (Fig. 24); phosphates separate as tiny blebs from the Fe-Ni-S melt and are associated with metallic chondrules rather than with silicates (Fig. 16). As Bevan and Axon underline, quenching from melting temperatures to subsolidus temperatures seems to be required to explain these features. A drastic change should, however, have occurred in order to

account for the very slow cooling rate inferred from the occurrence of tetrataenite and of micrometer-sized needles of kamacite in the core of cloudy taenites. This abrupt change in cooling rate must be related to the accretion of the chondrules, which happened after the coating of the chondrules with carbonaceous material which is now the black matrix. Heating experiments followed by the characterization of carbon in the black matrix show that it has never been heated over 350°C [36] which constitutes therefore an upper limit to the accretion temperature. H3 Bremervrrde. Dodd et al. [37] have found significant variations in the chemical composition of the ferromagnesian silicates justifying its classification as a H3 unequilibrated chondrite. We observe much more striated pyroxene than clinopyroxene in our section though Ashworth [2] found both varieties of calcium-poor pyroxenes; the metal and troilite are seldom found in globular association but more often in isolated irregular grains; clastic debris is preponderant in the matrix (see also Ashworth [2]) and many chondrules were broken and disaggregated by shock compaction of this veined stone which has thus many characteristics of type 4. Deformation of crystals occurs both in and outside the chondrules. H4 Beaver Creek shows a rather high porosity with few shock features; accordingly there is little clastic matrix; some fractured crystals are found crushed against compact chondrules, but growth steps are well preserved on the minerals, kamacite shows very few Neumann bands and troilite is monocrystalline; the occurrence of martensite in the core of zoned taenite points to a rather fast cooling as does the lack of cloudy taenite, an observation consistent with the cooling rate derived from fission tracks by Pellas and StOrzer [38]. However, this cooling rate should in no way be taken as representative of H4 chondrites as a whole, other H4 chondrites showing definite evidences in their nickel-iron phases of slower cooling processes. H4 Guenie is a compact stone, the compaction resulting from a slight shock: besides fracturing, deformation is found in some olivines outside chondrules; kamacite with some Neumann bands is stretched between the silicates; troilite may be

77 cracked or twinned; the occurrence of cloudy taenite, martensite and plessite points to a slower cooling rate than Beaver Creek. Normal contents of rare gases (L. Schultz, personal communication) confirm the rather quiet history of the stone after its accretion. Though, as already seen, silicates are essentially equilibrated, a proof of the diversity of the original material resides in the occurrence of a variety of oxides and of a dark clast in the three sections studied. The 5-mm-wide banded clast is devoid of chondrules and made of fragments of olivine and pyroxenes having nearly the same composition as the bulk, in a comminuted matrix; the occurrence of polycrystalline kamacite in the clast leads us to think that it has suffered a heating episode before its agglomeration in the chondrite. H4 Menow shows a high porosity, few broken crystals, little matrix, nice growth steps on the silicates, replacement features (as already discussed) in sulfide, nickel-iron oxide intergrowths and troilite curved laths. Menow is a fall and is not expected to have suffered oxidation due to weathering. Though oxidation may be due to drying after sawing the specimen under water, some of the oxide, finely intergrown with sulfide or replacing kamacite in plessite, may have been formed at the same time as the troilite replacement. The reheating event--underlined by a loss of radiogenic rare gases [1] and possibly spallogenic rare gases [22] and fading of the fission tracks in phosphates--has been mild and unconnected with any shock: no Neumann bands in the kamacite nor deformation of the silicates; it has not been accompanied by homogenisation of the nickel-iron alloys: zoned taenite, martensite, piessite and clear taenite are still visible. H4 Quenggouk has been compacted by a slight shock which had little effect inside the harder chondrules (see also Ashworth [16]) but which broke rock fragments and deformed crystals between them. Polycrystallinity on the border of the grains or Neumann bands across them are found in kamacite; troilite is polycrystalline without twinning; the occurrence of cloudy taenites points to a quiet cooling history. H4 Ste. Marguerite is a stone which fell in 1962 and was briefly described at that time [39]. It is

very porous and brittle, and chondrules separate easily from the mostly clastic matrix. The metallic grains are rounded as if quickly cooled and are scratched by silicate fine debris: this occurred probably when submitted to a slight shock which broke material but did not stick it together. The abundance of hopper olivine in the chondrules and the occurrence of a little residual glass point also to a fast cooling of the silicate phases. The cooling rate following accretion has been high as indicated by the lack of cloudy taenite, and the presence of globular martensite associated with kamacite.i The cooling rate of Ste. Marguerite has been higher than that of Beaver Creek, which itself was higher than that of Guenie and Quenggouk. H5 Allegan, like Ste. Marguerite, is very brittle, some chondrules bearing curved cracks and peeling as onions, following a slight shock which fragmented what was frail, deformed some silicates and kamacite outside the chondrules and broke the troilite. It did not disturb the slow cooling as proved by the occurrence of cloudy taenite. On the contrary, H5 Ambapur Nagla shows no shock effect on our sample and has very little clastic matrix. The undeformed silicates show beautiful growth steps; some of them have been corroded as shown by negative crystals on some faces (Fig. 14). A relation betweencorrosion of silicates and mobilisation of sulfide may exist if H2S or H2SO 4 has been produced during the late heating event which happened as proved by the loss of radiogenic and spallogenic rare gases [21], the healing of radioactive track damage in the silicates (J.C. Lorin and P. Pellas, personal communication) and the interaction sulfide-nickel iron. The already described [7] H5 Sena stone shows neither shock effects nor reheating event and would have been proposed as a model of an H chondrite with an undisturbed history had Turner [1] not found a unique argon release pattern in this object. H6 Butsura shows many chondrules but the presence of orthopyroxene as unique low-calcium pyroxene together with plagioclase feldspar leads to its classification in the H6 group. It has been shock compacted: some deformations are still visible in silicates between the chondrules. Ashworth [16] has seen irregular low deformations with some

78

recovery. Turner [1] found a 4.48-b.y. plateau age without visible loss of argon. The occurrence of tetrataenite points to a slow cooling at low temperature. In contrast Kernouvb shows few chondrules; the occurrence of fine grained zones along linear directions, together with a linear accumulation of metal visible in the main stone are interpreted as ancient veins; accordingly, the meteorite may have suffered an early shock event prior to its recrystallisation, at the beginning of its cooling history. As the silicates show no deformation and contain numerous bubble-like empty inclusions (except feldspars) and as some cloudy taenite is found together with kamacite and a little plessite, the stone would have quietly recrystallised since then. 4°Ar-39Ar measurements give an age of 4.45 ___0.03 b.y. so that the petrological data are the only indication of an early shock. The track data [38] are well preserved and define a slow cooling. The postulated early shock event has then most probably occurred when the stone was in a rather deepseated location, thermally insulated and shielded from later impact events.

5. Conclusions

- - A s is often the case for classifications, the Van Schmus-Wood classification which proved to be such a useful tool is none the less artificial and more differences are found between two H3 chondrites like Bremervi3rde and Tieschitz than between H4 Quenggouk and H5 Allegan. Binns' [40] classification of chondrites into primitive, transitional and recrystallised types is perhaps closer to the observations in spite of the genetic connotation implied, and though the term "primitive" is perhaps somewhat inappropriate to characterize already very complex objects. - - N o n - c l a s t i c matrix is visually recognizable in some unequilibrated stones where it mantles the chondrules. For the others, it may only be inferred from the chemical abundance of volatile trace elements which may be related to the pre- and post-accumulation history of the material. The abundance of clastic matrix is in direct relationship with the occurrence of shock events, which break much of the more fragile material; it is thus

in inverse relationship with the chondrule abundance. A large abundance of chondrules coupled with a small abundance of matrix material is characteristic of unshocked stones, independently of their cooling history. - - T h e sequence of the different types of chondrites has been interpreted as a metamorphic series either as a progressive metamorphism, necessitating a reheating of cold accreted material with partial recrystallisation growing in intensity from type 3 to 6 (e.g. [25]) or as a retrograde metamorphism allowing, after a hot accretion, a slower cooling and thus a more complete recrystallisation from type 3 to 6 (e.g. Hutchison et al. [4] for a late version o f the model). Our observations, coupled with the data of the literature, suggest the possibility of a third model in which initial differences in accretion temperatures play a key role. Each chondrite has a double thermal history: (1) The chondrule formation: We cannot elude the fact that chondrules have been molten or partly molten silicate and metal objects; this necessitates temperatures around 1500-1300°C. Records of the cooling rates of the discrete grains may be found in the morphology of the crystals inside the chondrules and quantitatively interpreted by comparison with synthetic melts: they range from a few degrees to tens of degrees per hour for most of them. The principal chemical characteristics of the minerals, mainly olivine and pyroxenes were then acquired, depending upon the composition of the prechondritic material and the redox conditions. The existence of preexisting condensed silicates and metals is controversial but will be admitted here. It is in our opinion the only way to explain the occurrence of chondrules such as those shown in Figs. 1-3, for instance; the olivine and orthopyroxene cores of Sena isolated crystals are considered as relicts of broken pre-existing crystals; also present in other chondrites, they are generally less easy to identify. Like Hutchison et al. [4] we think that each chondrule mineral association is explained by its particular chemical composition and that an early Fe-Ni-S differentiation was acquired at the same time as the chondrule formation. (2) The accretion: Some grains stuck together when still viscous, indicating proximity of formation and reflecting the density of the dust cloud

79 they come from. In this case (all types above 3), the accretion quickly followed the formation of chondrules, after a drop of temperature that m a y have introduced differences between type 4 to 6. T y p e 6 is the only one where recrystallisation is an effective process: the accretion temperature m a y have been above 1000°C as inferred from the chemical composition of some mineral phases, and the high-temperature form of oligoclase feldspars, the solidus of which is near 1100°C. The clinop y r o x e n e / o r t h o p y r o x e n e equilibrium for F e / M g distribution as for Ca content stands around 900°C (Bunch and Olsen [41] and our own analyses). However, the high temperatures were not kept a sufficiently long time to allow grain growth above 1 m m and complete homogenisation of ironmagnesian silicates. As more and more chondrules accreted, the build-up of an insulating cover allowed a slower cooling to settle (calculation in W o o d [42]) which has been recorded in the 244pu fission tracks. This insulating cover accreted at a progressively lower temperature, but still above 700°C (transition o r t h o / c l i n o b r o n z i t e : the pyroxene rim in H5 Sena is orthorhombic) or at a faster cooling rate (much of the striated pyroxene does not convert to orthopyroxene in H 4 - 5). This would be consistent with the oxygen isotope equilibrium data of O n u m a et al. [43] for types 5 and 6 chondrites (950 -+- 100 °) and with the rare gas data of Marti [44]: a prolonged high-temperature heating in an o p e n system would result in diffusive losses and fractionation of the noble gases according to a pattern which is not observed in the types 4 and 5. Finally, type 3 would represent material coming f r o m a large cloud allowing more variations in the chemical and redox composition of the particles travelling therein, therefore, cooling before accretion at a m u c h lower temperature than the types 4 - 6 and having time to glue very fine and volatilerich dust. Since the study of the free c a r b o n fraction of this dust in Tieschitz shows it has never been heated to 350°C this temperature should be an upper accretion limit. But its slow post-accretion cooling rate remains a puzzle if we consider H3 chondrites as a shallow deposit: or we must admit at that time a hot atmosphere around the parent body, which was slowly dissipating, or the cover-

age by a blanket of a very efficiently insulating material, o r - - a s calculated for example by W o o d [ 4 2 ] - - a thermal quasi-steady state condition, the thermal source being possibly the radioactive decay of 26A1 [45,46].

Acknowledgements The author thanks C. Vilbert and P. Blanc for their help using the SEM, M. Bourot-Denise for microprobe analyses, J.C. Lorin for discussion leading to improvements of earlier drafts and the reviewers for their helpful remarks. This research was partly funded through D G R S T grant 76-70899.

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