The composition of metal phases in Bruderheim (L6) and implications for the thermal histories of ordinary chondrites —Erratum

Earth and Planetary Science Letters, 102 (1991) 79-93 Elsevier Science Publishers B.V., Amsterdam

79

[CH]

The composition of metal phases in Bruderheim (L6) and implications for the thermal histories of ordinary chondrites - - Erratum t D . G . W . S m i t h a n d S. L a u n s p a c h 1 Department of Geology, University of Alberta, Edmonton, Alta. T6G 2E3 (Canada)

Received August 10, 1989; revised version accepted April 5, 1990

ABSTRACT The compositions and textures of metal phases in Bruderheim have been examined in detail using Nomarski differential interference contrast techniques, backscattered electron imaging and quantitative electron microprobe analysis for Fe, Ni and Co. The results clearly demonstrate substantial inter- and intra-grain compositional variations, and that these persisted through the high-temperature thermal event which largely homogenised the silicate phases forming the bulk of the meteorite. These results are difficult to reconcile with prograde thermal metamorphism. Instead, accretion at high temperatures is envisaged with homogenisation of silicates occurring at these temperatures. Relatively sparse metal grains were largely unaffected at these temperatures because they did not form three-dimensional networks (and hence lattice diffusion of metallic Fe and Ni was ineffective),and because, at the temperature of accretion, most grains were present as homogeneous particles of the 3' phase. The complexities now apparent in metal grains were established first during the decline of temperature and retrograde metamorphism after accretion and then later during relatively low-temperature burial metamorphism in asteroidal bodies. The reliability and significance of cooling rate calculations based on measurements of Ni concentrations in metal grains of such meteorites are questioned.

1. Introduction I n recent years we have p u b l i s h e d several papers which primarily, or incidentally, have a t t e m p t e d to draw a t t e n t i o n to the surprisingly variable compositions of metal grains i n chondrites [1-4]. These variations occur even in those meteorites which, o n the basis of the h o m o g e n e i t y of their silicate phases a n d the textures which they exhibit, have b e e n assigned to petrologic type 6. I n this p a p e r we e x a m i n e i n detail textural, c o m p o s i t i o n a l a n d mineralogical features of the m e t a l phases i n one well-known L6 c h o n d r i t e - - B r u d e r h e i m [5]. W e shall suggest how a p p a r e n t l y a n o m a l o u s composi-

t This article was published earlier in volume 99 of this journal, unfortunately with interchanged Figs. 1 and 4. As a result the scientific value was affected such that the Publisher decided to republish it in complete and correct form. 1 Present address: Sherritt Gordon Ltd., Analytical Services Division, Fort Saskatchewan, Alta. T8L 2P2, Canada. 0012-821X/91/$03.50

© 1991 - Elsevier Science Publishers B.V.

tional variations c a n be reconciled with the widely accepted view that such meteorites have at one stage i n their e v o l u t i o n u n d e r g o n e a high-temperature " m e t a m o r p h i s m " .

2. Investigative techniques and analytical methods Initial m e a s u r e m e n t s of the c o m p o s i t i o n s of i n d i v i d u a l c h o n d r i t i c m e t a l particles [6] were m a d e using electron m i c r o p r o b e s that were i n the early stages of d e v e l o p m e n t , a n d followed observations b y the t r a d i t i o n a l m e t h o d s of reflected light microscopy, often after samples h a d b e e n etched with " n i t a l " to reveal textural details n o t a p p a r e n t otherwise. I n o u r work we have avoided etching because of possible u n d e s i r a b l e effects o n analytical accuracy. Instead, we have used N o m a r s k i phase interference c o n t r a s t techniques [7] to observe subtle textural features revealed b y the differential polishing h a r d n e s s of the metal phases.

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The imaging capabilities of the electron microprobe have improved dramatically since the early work of Wood [6]. Thus backscattered electron (BSE) images with a spatial resolution of very much less than 1 /~m, can now be obtained, and atomic number differences of a small fraction of one, resolved. In some circumstances it is even possible to use electron channelling contrast effects to reveal the poly-crystalline character of material being investigated. Such capabilities allow textures and compositional variations amongst F e - N i phases to be made apparent very much more clearly than was previously possible. Details of our procedures have been published elsewhere [2]. In summary, we employed a modern equivalent of the calibration curve approach used by Wood [6]. A computer was used to fit a polynomial expression to x-ray intensity data for a series of five well-characterised standards ranging in composition from pure Ni to pure Fe. The advantages of this calibration curve approach are several. First, five standards are involved in calculating an expression for the curve, thereby minimising effects of any errors in either intensity or concentration for the standards. Neither background nor matrix corrections need be made for Fe and Ni. Because Co concentrations are low (almost always < 2.5%), they have insignificant effects on matrix corrections, as do other trace elements such as C, Si, P, Cr, Ga and Ge which together total < 0.5%. Co background corrections for samples can be made very accurately using measurements at the Co peak position on the Co-free F e - N i standards. Again, the computer is used to fit a background curve for Co to the data. Thus, statistical errors in background determination are greatly reduced and the effect of interference from an Fe satellite line is eliminated. Finally, matrix corrections for Co K radiation are calculated for each standard composition and a polynomial fit to these values is used to determine the appropriate correction for any analysed composition. This procedure allows very accurate determinations of Co if sufficiently long counting times are used. To document the variations in Ni, Co and Fe concentrations in Bruderheim very precisely, we used 100 s counting periods and crystal spectrometers for all measurements reported here. Although some indications of the variability of

D.G.W. SM I T H A N D S. L A U N SPA CH

N i / F e are seen in the early plots of Wood [6; figs. 10 and 11], we believe that the intra- and intergrain variability of metals in chondrites has not been fully appreciated previously because early methods of quantitative analysis were more timeconsuming and hence deterred extensive investigations. The short time taken to acquire data by such a modern calibration curve approach means that hundreds of points may be investigated with an accuracy at least as good as, and probably better than, that in early work, particularly in the absence of surface etching. 3. Results

To investigate intra-grain compositional variations in Bruderheim, several metal particles of considerable compositional and textural complexity were selected. Some of these grains are shown in reflected light by Nomarski differential interference contrast in Figs. I and 2, which also show other grains typical of the Bruderheim section used. Differences within the various regions are apparent from the topography, but the full extent of heterogeneity is revealed even more clearly by BSE images (Figs. 3, 4). These composite photographs show distinctive areas which, for reference purposes, have been numbered. The areas were investigated by the microprobe techniques just outlined and the results shown in Table 1 and plotted in Fig. 5. To improve clarity, envelopes have been hand-drawn around points from individual regions for the grains represented in Fig. 5. For brevity, only Ni and Co values are given in Table 1, although Fe values were also determined. Grain 1, Fig. 3a, demonstrates particularly large compositional complexity within a single grain. Grain 11, Fig. 3b, is made up largely of plessite with some marginal areas of kamacite, taenite and probably tetrataenite. It is not possible to determine with the electron micoprobe whether the Ni-rich component of plessitic intergrowths is taenite or tetrataenite, and no attempt has been made at this time to establish the matter on the basis of optical anisotropy using oil immersion lenses [8]. However, evidence is now accumulating to indicate that in many instances tetrataenite is common in ordinary chondrites [8,9]. Grain 13 (Fig. 4a) is of a quite different type. At one end of the grain is a region of coarse

METAL PHASES IN BRUDERHE1M (L6) AND THERMAL HISTORIES OF ORDINARY CHONDRITES

plessite while at the other is an area of much finer plessite. Sandwiched between is homogeneous taenite, and embedded on one side of the grain, in contact with both plessite and taenite, is a substantial region of kamacite with a composition distinct from that of the kamacite component in either plessite. High magnification BSE images

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showed scattered areas < 1 g m in diameter of a relatively high atomic number phase, embedded in the kamacite. It is Ni-rich and apparently taenite or tetrataenite. Grains were too small for reliable analysis. Note that the bright edges which are seen particularly on one side of this grain, are artifacts of the imaging process.

Fig. 1. Nomarski differential interference contrast photographs of typical metal grains in Bruderheim. (A) is grain 11; (B) grain 13; (C) grain 14; (D) grain 15. None of the grains have been etched and the contrast seen within grains is entirely due to the minimal relief produced during polishing-a relief that is otherwise barely detectable even with a very carefully oriented sample and a finely tuned SEM image. Magnifications are indicated by the scale bars.

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Fe, N i a n d C o m u l t i - p e n c h a r t r e c o r d e r p r o f i l e s w e r e o b t a i n e d f r o m g r a i n 13 a c r o s s t h e b o u n d a r i e s b e t w e e n k a m a c i t e a n d t a e n i t e a n d b e t w e e n silicate a n d e a c h of the m e t a l phases. I n b o t h t h e

D . G . W . S M I T H A N D S. L A U N S P A C H

silicate-taenite and the kamacite-taenite interfaces, t h e r e was t h e u s u a l i n c r e a s e in N i e x t e n d i n g a few/~m into the taenite from the boundary. The r i m a d j a c e n t to t h e k a m a c i t e is o n l y a b o u t 3 /~m

Fig. 2. Nomarski differential phase contrast photographs of: (A) grain 16, a highly shocked composite metal grain showing the formation of fusion droplets; and (B) a grain made up largely of kamacite but with rather ill-defined lamellae and particles of taenite (or perhaps tetrataenite); (C) shows further detail of part of the grain seen in (B). Note that although the lamellae show some preferred orientation, they are not regularly arranged; (D) is a BSE image of part of a kamacite grain, showing minute bright speckled areas with high Ni contents. These are believed to be tetrataenite which separated from kamacite during low-temperature metamorphism. In the original photograph, different parts of the kamacite grain are represented by slightly different grey levels, probably reflecting electron channelling contrast. Magnifications are as indicated by the scale bars.

METAL PHASES IN BRUDER.HEIM

(L6) A N D T H E R M A L

HISTORIES OF ORDINARY

wide b u t the N i c o n t e n t increases f r o m - 38% up to n e a r l y 50%. A d j a c e n t to the silicate, N i enrichm e n t is slightly less, reaching a b o u t 45% a n d e x t e n d i n g o n l y a b o u t 1 to 2 /Lm into the taenite. However, e x a m i n a t i o n of the N i profile in adj a c e n t silicate m a t e r i a l shows t h a t at a b o u t 1 6 / ~ m a w a y from the b o u n d a r y in a grain of o r t h o p y r o x ene ( F s - 21.5), the N i increases progressively f r o m 0.12% until at the taenite b o u n d a r y it has ap-

CHONDRITES

83

p a r e n t l y r e a c h e d 0.41%. H o w e v e r , at the p r e s e n t time, the p o s s i b i l i t y t h a t this is an artifact p r o d u c e d b y c o n t i n u u m fluorescence c a n n o t be c o m pletely dismissed. F i n a l l y , as might b e a n t i c i p a t e d , there is no i n d i c a t i o n of e n r i c h m e n t in N i in the taenite r i m a d j a c e n t to the plessite. T h e final grain e x a m i n e d in detail (16) has suffered shock effects. T h e s e are e v i d e n c e d b y the p r e s e n c e o f a host of i m m i s c i b l e sub-circular grains

Fig. 3. BSE images of: (A) grain 1, the sharply defined criss-cross lines are scratches in the surface; and (B) grain 11 which shows a tendency towards lamellar exsolution of taenite from kamacite in the plessitic regions. Note deformation of these lameUae at the left side of the grain, an effect that must have occurred after low-temperature metamorphism and perhaps during shock events that affected Bruderheim. Magnifications are indicated by the scale bars. Numbers refer to areas described in the text.

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o f m e t a l in a silicate glass m a t r i x . O n e e x t r e m i t y o f this g r a i n a p p e a r s to h a v e r e s o l i d i f i e d a l m o s t at the m o m e n t it was s p a l l i n g off to f o r m a d i s c r e t e globule. T h e p r e s e n c e o f h i g h P in s o m e g l o b u l e s suggests t h a t earlier p h o s p h a t e g r a i n s ( m e r r i l l i t e o r

D . G . W . S M I T H A N D S. L A U N S P A C H

apatite) may have been partially incorporated into the metal. Note that although the kamacite-plessite b o u n d a r y r e m a i n s s h a r p , p l e s s i t e in p a r t o f t h e g r a i n h a s b e e n r e h o m o g e n i s e d b y t h e s h o c k effects. T h e s e m u s t , t h e r e f o r e , h a v e p o s t - d a t e d the

Fig. 4. BSE images of: (A) grain 13 clearly showing a central region of taenite, an "embedded" grain of kamacite at one side, and two distinct regions of plessite. This is a grain that was examined in detail by chart recorder traces to investigate the Fe, Ni and Co concentrations across boundaries between metal phases and between metal and silicate phases; (B) and (C) show the highly shocked grain 16. Regions of kamacite and plessite are preserved while at the other side of the grain is homogeneous taenite. Fusion has occurred to produce numerous droplets that have been dispersed into the adjacent fused silicate material. Preservation of such textures within a single grain is extraordinary but is attributed to both the very localised node of interfering shockwaves and the very brief period over which high temperatures were maintained. Magnifications are indicated by the scale bars. Numbers refer to areas described in the text.

M E T A L PHASES IN B R U D E R H E I M (L6) A N D T H E R M A L HISTORIES OF O R D I N A R Y C H O N D R I T E S

reheating that converted pre-existing martensite to plessite. We shall now turn to inter-grain compositional v a r i a t i o n s . I n t h i s case, a t o t a l o f s e v e n t e e n d i f f e r e n t g r a i n s all w i t h i n a n a r e a o f < 3 c m 2 w e r e examined. These grains are amongst the larger ones present. In this study the many very small grains which are also present, were not analysed. The seventeen grains show a variety of textures and compositions. Some appear to be homoge-

neous kamacite or taenite, others simple intergrowths of homogeneous kamacite and taenite. Plessitic intergrowths of varying coarseness also o c c u r as w e l l as c o m p l e x h e t e r o g e n e o u s i n t e r g r o w t h s s u c h as t h o s e d i s c u s s e d e a r l i e r . S o m e o f t h e s e g r a i n s a r e s e e n i n F i g s . 1 a n d 2. A n a l y t i c a l r e s u l t s o b t a i n e d f r o m t h e g r a i n s a r e s h o w n i n Fig. 6. T h e y m a y b e c o m p a r e d t o r e s u l t s o b t a i n e d i n early work on a completely different polished s u r f a c e o f B r u d e r h e i m [1; fig. 4]. T h e c r o w d i n g o f

TABLE 1 Metal compositions: grains 1, 11, 13 and 14 in wt.% Grain 1

11

Area

Ave. Ni

Ave. Co

Description

1 2 3 4 5 6 7 3, 4 5, 6 8 9 10 11 1 2 3 4 5 6

5.5 35 5.0 56 3.2 56 22 20 20.5 50 5.4 48 54 4.8 56 58 5.0 45.5 42 53 5.4 5.6 54 5.9 48 54 5.6 30 29.6 56 3.7 38.5 29.5 31.5 5.3 34 29 4.8 33

0.64 0.25 0.59 0.11 0.73 0.14 0.44 0.48 0.46 0.10 0.49 0.06 0.07 0.46 < 0.03 < 0.02 0.49 0.04 0.12 0.00 ) 0.56 0.60 0.01 0.54 0.05 0.00 ) 0.45 0.25 0.24 0.00 0.51 0.04 0.22 0.22 0.30 0.16 0.23 0.40 0.14

homogeneous kamacite homogeneous taenite kamacite in coarse plessite tetrataenite(?) in coarse plessite kamacite in fine plessite tetrataenite(?) in coarse plessite "homogeneous" taenite within plessite average plessite composition by rastering average plessite composition by rastering taenite rim to area 9 isolated kamacite "bleb" taenite rim to area I at grain margin taenite rim to area I at interior margin homogeneous kamacite with plessite tetrataenite(?) surrounding area 1 strained tetrataenite within plessite strained kamacite with plessite homogeneous(?) taenite (Ni ranges 44-47) taenite in very fine plessite showing a range of Ni and Co values marginal kamacite adjacent to area 6 marginal kamacite adjacent to area 6 tetrataenite(?) at margin of grain kamacite adjacent to area 9 tetrataenite(?) at grain margin showing range of values kamacite adjacent to 11 "homogeneous" taenite ave. plessite adjacent to area 13 tetrataenite(?) in coarse plessite kamacite in coarse plessite homogeneous taenite average of very fine plessite average of somewhat coarser plessite kamacite taenite in shocked grain plessite in shocked grain kamacite in shocked grain average of five "globules"

7 8 9 10 11

13

16

12 13 14 1 2 3 4 5 6 1 2 3 4

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D.G.W. SMITH A N D S. LAUNSPACH

1.0

0.8

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Bruderheim Grains 1, 11, 13, 16

1.5

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1.3

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Ni W e i g h t

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Fig. 5. C o / N i plots of (A) data from g a i n s 1, 11, 13 and 16. For a description and discussion of the data, see text.

1.0 Bruderheim Grains 1 - 17 08

E • 0.6 13.. -t-* tO3 O 0.4

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Fig. 6. C o / N i plots of data from all seventeen grains investigated in this study. For a description and discussion of the data, see text.

M E T A L PHASES IN B R U D E R H E I M (Lr) A N D T H E R M A L HISTORIES OF O R D I N A R Y C H O N D R 1 T E S

data points does not allow envelopes to be drawn around results for individual grains, but in most cases similar multiple groups for an individual grain were observed. Two particularly important features are apparent in Figs. 5 and 6. Firstly, different grains show kamacite, plessite, taenite a n d / o r tetrataenite with different Ni and Co contents. Secondly, Co may vary substantially from grain to grain even when the Ni contents of those grains or areas are very similar. Throughout Bruderheim there are many kamacite grains which show no substantial areas of associated taenite or plessite. However, within them there are many minute areas of very distinctly different Ni content. These occur either as ill-formed lamellae, generally of the order of 1-2 /xm in diameter (Fig. 2b, c), or else as speckles (similar to those described for grain 13) which are generally < 1 # m in diameter and which occur throughout the body of the kamacite grain (Fig. 2d). These speckles may be the material that Wood [6] took to be schreibersite during his examination of Bruderheim. In general, both lamellae and speckles are too small for successful quantitative electron microprobe analysis. However, semiquantitative measurements indicate Ni levels > 40 wt.% and hence we are possibly observing tetrataenite compositions. The lamellae might be due to exsolution of excess Ni as temperatures fell from that of accretion, whereas the speckles may have formed as a result of low-temperature exsolution contemporaneous with the formation of piessite from martensite elsewhere. If this is the case, it suggests that the polycrystalline kamacite, which is evidenced by electron channelling effects in BSE images (Fig. 2d), formed from pre-existing monocrystalline kamacite during reheating that accompanied burial metamorphism. 4. Discussion of results

4.1. Origin of metal grains Controversy over the origin of metal particles in chondritic meteorites can be followed at least as far back as 1921 when Merrill [10] suggested they were of secondary, not primary origin. Later, this view was challenged by Urey and Mayeda [11] who pointed to the enormous textural complexity and to differences between grains in individual

87

chondrites as clear evidence that they must be of primary origin and have acquired their characteristics prior to accretion. However, Wood [6] was able to produce compositional evidence not previously available and, following a study of metal grains in 35 different chondrites, concluded that these grains formed in situ and not in some system pre-dating the chondrites. In reaching this conclusion, Wood presented several arguments which it is appropriate to review at this time. First he pointed out that textures observed and reported by Urey and Mayeda [11], if they had been formed before accretion could not have survived the thermal event that, since the time that paper was written, had been shown to have affected the silicates in many chondrites. He also felt that the shapes of metal grains are suggestive of growth or recrystallisation in situ rather than of fragmentation of pre-existing larger grains. He further claimed that there is an identity between the compositions of minute metal grains within chondrules and larger metal grains embedded in the matrix. He then pointed to the similarity in the total Fe + Ni contents of all chondrites and the variations in the F e / S i and F e / N i ratios to suggest " a n intimate relationship between the metal and silicate minerals in chondrites". Finally, he placed considerable weight on the results of his heating experiments (see below). To the first of these arguments we would respond that the chemical and textural inhomogeneity amongst kamacite, taenite and martensite (now plessite) reflects primary features inherited during high-temperature aggregation perhaps subsequently modified somewhat by retrograde effects during cooling. The arguments concerning the shapes of grains are subjective. Metal particles certainly do stand in strong textural contrast to the silicate phases amongst which they lie--they tend to be relatively large, often elongate and frequently show cuspate margins. However, these characteristics might easily be attributed to the much more malleable character of the metal than the silicate. In many metal particles we see deep embayments filled with silicate grains that have the appearance of having been embedded by impact (Figs. 1 and 2). Prior's law, concerning the total Fe + Ni content and the variation of Fe with Si and Ni, can equally well be satisfied if the "intimate relation-

88

ship" is established at some stage prior to accretion. Wood's statement concerning the similarity in composition of metal grains from place to place in a chondrite is at variance with our own observations and with those of other later workers (e.g., Hutchison and Bevan [12]). In particular, the assertion that the composition of minute metal particles found in chondrules is identical to that of metal grains in the matrix, is demonstrably untrue. It has been shown that the latter varies all the way from low-Ni kamacite to high-Ni taenite. Wood's heating experiments [6] were carried out with an artificial mixture of components similar in bulk composition (but apparently not in terms of the compounds used) to the Renazzo carbonaceous chondrite. He found that Ni, introduced as NiO powder, migrated to pure Fe particles to form a fairly homogeneous metal (Fe93_89NiT_ll) under the influence of a temperature of 950°C for 2 h and 850°C for a further 200 h. No information on the grain size of the powder was given and the lengths of diffusion paths between NiO and metal grains cannot be estimated. Furthermore, the experiments might also be discounted on the grounds of the dissimilarity of the starting Ni compound to the Ni-bearing spinels which are more likely in Wood's postulated carbonaceous chondrite precursor of ordinary chondrites. Wood laid great store by the presence of Ni-rich rims on taenite grains in chondrites, interpreting them as evidence that Ni could diffuse readily through the silicate phases from the hypothetical Ni-oxide particles to a precursor Fe-rich metal phase. The additions to or losses from metal grains, we suggest, were very minor in their volume, the rims being typically only a few/~m wide. Wood also recorded Ni-rich rims around plessite with enrichment up to Ni55. These compositions are very similar to those we find intergrown with kamacite in the plessite and we suggest that some of these rims may have developed, or at least been accentuated, during the much later formation of plessite, either by grain boundary diffusion or possibly by lattice diffusion within the metal.

4.2. Interpretation of metal textures and inhomogeneities It appears to us inconceivable that textures such as those seen in grains 1, 11 and 13 could

D.G.W. SM I T H A N D S. LAUNSPACH

have originated as suggested by Wood [6]. It seems inevitable that within the area covered by a microprobe mount, all the resulting grains of kamacite would have the same composition if the mechanism suggested by Wood were correct. Similarly, taenite grains would have largely achieved the composition that coexisted in equilibrium with the kamacite at whatever temperature obtained. Instead we observe a wide range of taenite compositions and a small but significant range of kamacite compositions. In particular, it would be difficult to account for the very variable Co concentrations in metal grains of the same F e / N i ratio. There can be no doubt that the Ni-enriched rims, regarded by Wood as crucial evidence of the growth of the metal grains in situ, are commonly (but certainly not always) present. However, the absence of such rims on even a few of the taenite grains would be difficult to explain by Wood's hypothesis. The origin of the Ni-rich rims proposed by Wood [6] is crucial to his use of taenite compositions and the distances to grain boundaries to compute cooling rates using theoretical diffusion profiles. Wood limited his cooling rate estimates to the range 550-450°C and assumed a linear cooling history. Although there was much scatter in his data, he felt able to estimate cooling rates for four ordinary chondrites (two L6, one L5 and one L4) lying between about 1 and 100°C/Myr. For several unequilibrated chondrites he obtained estimates between about 0.01 and l ° C / M y r . Although the sizes and compositions of rims found around taenite or plessite grains may reflect in some way the duration of the process that caused them, if they are not formed as envisaged by Wood, it is doubtful whether such cooling rate estimates are valid. Furthermore, the assumption of a linear cooling history may not be well based. Even if cooling involved only black body radiation, the rate of change of temperature would not be linear. In reality, various other factors may have been involved. For example, the rate of waning of the thermal event that produced the "metamorphism" could have been important at any stage in the process, as could also the size of the accreted body that was cooling. Furthermore, after the agglomerated material had cooled to the ambient temperature in solar system space at that time, it was probably buried in an asteroidal body,

METAL PHASES IN BRUDERHEIM

(L6) A N D T H E R M A L

HISTORIES OF ORDINARY

after which reheating may have become an important factor. This is suggested by the common presence of plessite which is taken to be a decomposition product of martensite. Such reheating might well have produced temperatures up to 300-350°C (in view of the presence of tetrataenite) and these temperatures could have persisted over hundreds of millions of years until the parent bodies were eventually broken up and the material once again returned to the ambient temperature of space. The formation of plessite with the individual phases 1-5 /~m in size (e.g., grain 13, Fig. 5) indicates that diffusion of Ni over at least these distances was possible at the temperature existing, and in the times available. Such movement may also have contributed to the highNi rims around some taenite and plessite grains. The original material now represented by Bruderheim, by analogy with type 3 members of this group, accreted with phases showing significant ranges in composition. Such variations may have been particularly large amongst metal grains because of the possibility that random fragmentation of previously-formed composite k a m a c i t e / taenite grains during collisions and impacts, could produce wide variations in the bulk compositions of the fragments. It might also be noted that homogenisation of silicates in higher petrologic types has, in general, been demonstrated only on very local scales. Compositional investigations are normally made by microprobes with one polished thin section or slab being cut more or less randomly from one individual of a given meteorite. To the best of our knowledge, seldom, if ever, has homogeneity been demonstrated between many individuals of a shower, or between several distinct regions in a single large individual. As a result, we really know little about the homogeneity of the different types on a scale much larger than a few cm. Furthermore, it has been demonstrated that ultrafine, unequilibrated silicate particles occur amongst the larger particles in certain equilibrated chondrites, indicating that equilibration preceded the final aggregation of components [13,141. A model must be sought which will allow the heterogeneities found amongst the silicate phases of L3 chondrites to be virtually eliminated by equilibration in types 6 and 7, while leaving metal grains largely unaffected. The model we envisage

CHONDRITES

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involves metal grains initially of widely divergent bulk composition. The thermal event that followed and largely homogenised individual silicate phases, was able to produce this effect because the proportions of the silicates present are such as to form a random, three-dimensional network of grains. Thus it seems likely that thermal equilibration of the silicate phases was achieved largely by lattice diffusion. The only other mechanism that may have been effective in space is grain boundary diffusion. Metal phases in all chondrites are much less abundant than silicates. They are well below the volume where a three-dimensional network of grains could be present. This was verified experimentally by measuring grain-to-grain electrical conductivity on polished surfaces. None of the samples tested from the LL, L and H groups showed any significant grain-to-grain conductivity, indicating that there is no continuous diffusion path between metal grains. Thus, during the thermal event which homogenised silicates, although lattice diffusion may have homogenised individual metal grains to some extent, grains in different parts of the meteorite will not have achieved equilibrium with one another. Diffusion of Ni in taenite is sluggish at temperatures below - 6 0 0 ° C , as evidenced by the development of " M " profiles in taenite lamellae of iron meteorites. The diffusion of metallic, not ionic, Ni and Co via a three-dimensional olivine and orthopyroxene network which would be necessary to homogenise metal compositions throughout the meteorite, is clearly likely to be even less effective. Similarly, compositional heterogeneities are found amongst chromites, ilmenites and phosphates in type 6 chondrites [15,16]. The absence of continuous three-dimensional networks of such minerals may have been important in preserving these inhomogeneities. The temperatures attained and duration of the event that converted chondrites to the different petrologic types, are still by no means agreed. For L6 chondrites, depending on the geothermometer used, estimates by various workers of maximum temperatures have ranged between - 7 5 0 and 1080°C [17]. Cooling rates have generally been estimated to lie between about 2 and 1 0 ° C / M y r . No matter what the actual temperatures of accretion, above - 7 5 0 ° C all metal particles with

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Ni >~ 15 wt.% would exist within their stability field. Above - 9 0 0 ° C (and below melting) this would be true of any N i - F e particle irrespective of its composition. Thus such particles would be accreted as individually homogeneous metal grains, with the 7 structure. Immediately after aggregation at these temperatures, the only compositional changes that might be anticipated are some elemental exchanges which would tend to equalise the Ni:Fe ratio amongst the grains. Since there was no continuous three-dimensional network of metal grains, this process might well have been ineffective. Only when metal particles cooled sufficiently for the solvus between kamacite and taenite to be intersected, would there be any tendency for originally homogeneous individual metal grains to separate into two phases. For those with low average Ni contents (up to - Ni20 ) there may have been a substantial period at temperatures above 600°C where the processes of decomposition into two phases, and the implied intra-grain diffusion, were effective and composite grains of kamacite and taenite could have been formed. But as the temperature fell through - 6 0 0 ° C and movement of Ni became sluggish, additional compositional changes would have been limited to those of a very local nature, such as the enhancement of Ni in taenite rims adjacent to kamacite. The extent to which even this process occurred would have been controlled by the cooling rate. Eventually, the path of crystallisation would leave the kamacite/taenite solvus and, as pointed out by Wood [6], a wide range of compositions could invert to the martensite (a2) structure. The temperature at which this transition occurs is dependent on taenite composition. Some particularly Ni-rich taenite will not undergo the transition at all. Once formed, the martensite structure may persist indefinitely at lower temperatures. Thus from such processes we might end up with discrete kamacite grains, kamacite-martensite intergrowths, kamacite-taenite intergrowths, discrete martensite and discrete taenite grains. However, grains such as 13 (Fig. 4), made up of taenite, kamacite and two somewhat different areas of martensite (now represented by plessite), would not be anticipated. Proponents of a prograde metamorphism might suggest that the plessitic areas were formed by reaction of kamacite and taenite regions of the grain in response to increas-

D . G . W . S M I T H A N D S. L A U N S P A C H

ing temperature. However, this seems highly unlikely given the absence of a zone of plessite between kamacite and taenite where they actually contact one another. It is far more probable that such composite particles represent agglomerations of grains that had separate origins. The persistence of such grains with their distinctive compositions argues for temperatures of aggregation towards the low end of the range proposed, so that they were not homogenised by Ni diffusion. It would appear that plessite now forming the bulk of many grains (e.g., 1, 11 and 13) was at one stage present as martensite. The different average plessite compositions from grain to grain are inconsistent with an origin by growth in situ by diffusion, because all grains would be expected to achieve similar equilibrium bulk compositions. They are also inconsistent with homogenisation of the metal phases during any subsequent prograde thermal event that " m e t a m o r p h o s e d " the chondrite. Grains within a few mm of one another must certainly have been subjected to identical thermal "metamorphic" effects and one cannot, therefore, attribute the different compositions of various regions of plessite to exsolution of different precursor taenite from kamacite. Rather they must reflect differences in taenite composition that existed prior to and survived thermal "metamorphism". The average plessite composition in grain 13, areas 3 and 4 is particularly significant because it is very close to 30% Ni. Martensite of this composition does not begin to form until temperatures below 0°C are reached. If it were assumed that areas 3 and 4 had accreted with the martensite structure, this could have persisted, only until temperatures of - 5 0 0 ° C were reached during any reheating, when it would have undergone a diffusionless reverse transformation to taenite [18]. Thus survival of martensite through a prograde thermal event seems unlikely. Instead, it must have formed (or reformed) during cooling after "metamorphism". This is an important conclusion for it means that the meteorite reached sub-zero temperatures after "metamorphism" and that subsequently it must have been reheated to produce the plessite. Grain 11 shows interesting textural features within the coarser plessitic areas. Exsolution of tetrataenite appears to have adopted the form of

ME T A L PHASES IN B R U D E R H E I M (L6) A N D T H E R M A L HISTORIES OF O R D I N A R Y C H O N D R I T E S

discontinuous lamellae with a suggestion of a preferred orientation. At the left edge of Fig. 3b the lamellae have been deformed. This may have occurred during a shock event evidenced by textures illustrated in Fig. 4 b, c. If so, it demonstrates that shock effects postdated plessite formation rather than causing it.

4. 3. Impfications for chondrite formation Thus we suggest that the complex compositional variations amongst metal particles documented here for Bruderheim and elsewhere for other ordinary chondrites [1-4], can only have been inherited from an earlier stage prior to aggregation into the present meteorite. Furthermore, it seems most unlikely that the textures observed within individual grains could have been either a product of, or have persisted through, a prograde thermal metamorphic event that has been estimated to have reached temperatures between - 7 5 0 and 1080°C [17]. We suggest, therefore, that the metamorphism that homogenised silicate compositions and which blurred or destroyed textural features that are so clear in chondrites of petrologic type 3, was retrograde in n a t u r e - - a conclusion recently reached by several other workers from quite different lines of evidence [19,20]. While members of petrologic type 6 were aggregating from individual particles, chondrules and clasts, they were held for some time at the temperatures inferred for their peak of metamorphism. The precursor particles were probably not in total chemical equilibrium with one another at the time of aggregation, perhaps due to the effects of mechanical mixing of nebular condensates a n d / o r fragments from some earlier stage of planetary formation. The effects that would then be observed following aggregation would not differ greatly from those that might have been associated with prograde metamorphism of similar particles that had accreted with variable compositions at low temperatures. However, it could be anticipated that a prograde metamorphism of substantial, tightly compacted bodies of particles, followed by their slow cooling, would have been much more effective in homogenising metal grains than a single cooling cycle of perhaps loosely compacted aggregates. It is necessary to postulate a source of heat to raise the temperature of particles before they ag-

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gregated. But this is no more difficult, and per: haps much easier, than finding a mechanism for heating meteorites to such temperatures after they have aggregated. The model we envisage differs little from that outlined by Wasson [21], except that agglomeration occurred while particles produced by earlier processes were being subjected to reheating. This may have occurred, as suggested by Wasson, during the Hayashi phase of solar evolution. In this scheme, members of/lower petrologic types within one group could have aggregated in regions which were shielded in some way from heat radiation, for example by being towards the centre or at the back of an annular dust cloud, in the shadow of particles that were to form higher petrologic types. Alternatively, they could represent particles that had suffered similar heating to those in higher petrologic types but which had already cooled again before they agglomerated. Relative formation ages calculated from measurements of 1291/1271 do not support the latter alternative, although it might be remarked that the spread of these ages is small and experimental errors and uncertainties might disguise any relationship between age and petrologic type. In our model, the differences in volatile element contents of the different petrologic types could be accounted for quite simply by differences in the diffusive loss of these elements at high temperatures from the small agglomerations of condensate particles. This model is similar to one proposed for H-group chondrites [22]. Thus we envisage that these losses occurred before aggregation into the somewhat larger planetesimals. Volatile elements trapped in lower petrologic types aggregated at lower temperatures, would be less inclined to be lost by diffusion and hence would be less depleted in the planetesimals. The overall chemical and mineralogical similarity of meteorites within each group is consistent with an origin in similar regions of the nebular dust cloud whilst minor differences which are found, for example between petrologic types 3 and 4, can be ascribed to inherent small differences in the composition of dust clouds in that region, differences associated either with time or with position. 5. Conclusions This study has demonstrated, unequivocally, we believe, the very variable compositions of metal

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grains in Bruderheim, a member of the L6 group of chondrites. The compositional variability is interpreted as a characteristic inherited at the time of accretion and not eliminated during thermal "metamorphism" because of the absence of a continuous diffusion path through a network of metal grains. This is similar to the situation that exists for oxide and phosphate phases but contrasts with that for silicates which do form three-dimensional networks and thus offer continuous paths for lattice diffusion. Any equilibration between isolated grains of metal must occur by grain boundary diffusion or by elemental (not ionic) diffusion through the silicate n e t w o r k - - a process which is likely to be very slow indeed. Thus, to a large extent, individual metal grains in Bruderheim have acted as isolated chemical systems during "metamorphism". Subsequent to accretion, all metal grains in a given meteorite must have had essentially identical thermal histories, except for very local shock heating effects which, anyway, are likely to have been of very short duration. However, the responses of the grains to the thermal history has varied according to their initial bulk composition as well as accidental features such as the proximity to other metal grains, and the presence of inclusions, grain boundaries and imperfections which formed sites for nucleation of lower temperature phases. Furthermore, we believe there is no possibility that the complexity of N i / F e and N i / C o patterns can be attributed to shock effects. Although Bruderheim shows obvious textural evidence of shock, other L group chondrites with equally complex N i / F e and N i / C o patterns, show no such evidence. Furthermore, the heavily shocked Peace River [14] shows unusually simple patterns [2] while Catherwood, also heavily shocked with fusion veins containing ringwoodite and majorite [23], shows complex N i / F e and N i / C o patterns [1]. This interpretation of the compositional characteristics of phases making up ordinary chondrites renders a prograde metamorphic event unnecessary or even unlikely. All the observed characteristics could be attributed to retrograde metamorphism during cooling of chondritic material accreted and held for a while at temperatures within the range proposed for peak "metamorphism". In such circumstances, attempts to de-

D.G.W. SMITHAND S. LAUNSPACH

termine cooling rates from taenite compositions and distances to grain boundaries, are fraught with uncertainty and may give results that are quite meaningless--not simply because of the variable and unknown three-dimensional geometry of the exsolved phases, but also because of the effects of a prolonged low temperature annealing which is evidenced by the decomposition of a martensite phase into plessite. The existence of martensite (now represented by plessite) which, from its composition, could only have formed at temperatures at or below 0°C, indicates that piessite formation must have occurred during later reheating events. This may have been associated with burial metamorphism in asteroidal bodies.

Acknowledgements This work was supported financially by grant A4254 from NSERC (Canada) to the first author. We are grateful to Profs. Klaus Keil, Richard Lambert and Mike Wayman and to Drs. Edward Cloutis, Robert Hutchison, Stephen Reed, John Wood and Ian Wright for helpful discussions of various aspects of this work. We alone are responsible for the interpretation of the data presented herein.

References 1 D.G.W. Smith, The mineral chemistry of the Innisfree meteorite. Can. Mineral. 18, 433-442, 1980. 2 D.G.W. Smith and S. Launspach, Determination of the compositions of metal phases in chondritic meteorites, in: Microbeam Analysis 1983, Proc. 18th Annu. Conf. Microbeam Anal. Soc., San Francisco Press, San Francisco, Calif., pp. 47-50, 1983. 3 Y. Miflra, D.G.W. Smith and S. Launspach, The Ni, Fe and Co contents of metal phases in the Allende, Holbrook and Nuevo Mercurio chondrites. Proc. 8th Symp. Antarctic Meteorites, Natl. Inst. Polar Res., Tokyo, pp. 224-236. 1983. 4 D.G.W. Smith, Y. Miflra and S. Launspach, Ni, Fe and Co variations in some Antarctic chondrites, Abstr. 9th Symp. Antarctic Meteorites, Natl. Inst. Polar Res., Tokyo, pp. 56-58, 1984. 5 H. Baadsgaard, F.A. Campbell, R.E. Folinsbee and G.L. Cumming, The Bruderheim meteorite, J. Geophys. Res. 66, 3574-3577, 1961. 6 J.A. Wood, Chondrites: their metallic minerals, thermal histories and parent planets, Icarus 6, 1-49, 1967. 7 I.D. Muir, Microscopy: transmitted light, in: Physical Methods of Determinative Mineralogy, J. Zussman, ed., pp. 35-108, Academic Press, New York, N.Y., 1977.

METAL PHASESIN BR.UDERHEIM(L6) AND THERMALHISTORIES OF ORDINARY CHONDRITES 8 R.S. Clarke and E.R.D. Scott, Tetrataenite-ordered FeNi, a new mineral in meteorites, Am. Mineral. 65, 624-630, 1980. 9 T. Nagata and M. Funaki, Tetrataenite phase in Antarctic stony meteorites, in: Proc. l l t h Symp. Antarctic Meteorites, Mem. Natl. Inst. Polar Res., Spec. Issue 46, 245-262, 1986. 10 G.P. Merrill, On metamorphism in meteorites, Geol. Soc. Am. Bull. 32, 395-416, 1921. 11 H.C. Urey and T. Mayeda, The metallic particles of some chondrites, Geochim. Cosmochim. Acta 17, 113-124, 1959. 12 R. Hutchison and A.W.R. Bevan, Conditions and time of chondrule accretion, in: Chondrules and their Origins, E.A. King Jr., ed., Houston Lunar Planet. Inst., pp. 162-179, 1983. 13 K. Fredriksson and Wlotzka, Morro do Rocio: an unequilibrated H5 chondrite, Meteoritics 20, 467-478, 1987. 14 K. Fredriksson, Crystallinity, recrystallisation, equilibration, and metamorphism in chondrites, in: Chondrules and their Origins, E.A. King Jr., ed., Houston Lunar Planet. Inst., pp. 44-52, 1983. 15 S.J.B. Reed and D.G.W. Smith, Ion probe determination of rare earth elements in merrillite and apatite in cbondrites, Earth Planet. Sci. Lett. 72, 238-244, 1985.

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16 G.D. Price, A. Pumis, S.O. Agrell and D.G.W. Smith, Waddsleyite, natural (Mg,Fe)2SiO4 from the Peace River Meteorite, Can. Mineral. 21, 29-35, 1983. 17 R.T. Dodd, Meteorites: a petrologic-chemical synthesis, Cambridge University Press, Cambridge, 368 pp, 1985. 18 L. Kaufman and M. Cohen, The Martensitic transformation in the iron-nickel system, Trans. AIME 26, 1391-1401, 1956. 19 R. Hutchison, A.W.R. Bevan, S.O. Agrell and J.R. Ashworth, Thermal history of the H-group of chondrite meteorites, Nature 287, 787-790, 1980. 20 J.R. Ashworth, L.G. Mallinson, R. Hutchinson and G.M. Biggar, Chondrite thermal histories constrained by experimental annealing of Quenggouk orthopyroxene, Nature 308, 259-261, 1984. 21 J.T. Wasson, Formation of ordinary chondrites, Rev. Geophys. Space Phys. 10, 711-759, 1972. 22 M. Christophe Michel-Lrvy, Some clues to the history of the H-group chondrites, Earth Planet. Sci. Lett. 30, 143-150, 1981. 23 L.C. Coleman, Ringwoodite and majorite in the Catherwood meteorite, Can. Mineral. 15, 97-101, 1977.