An analytical electron microscope investigation of some pallasites

PHYSICS O F T H E EARTH AND PLANETARY INTERIORS ELSEVIER

Physics of the Earth and PlanetaryInteriors 103 (1997) 101-115

An analytical electron microscope investigation of some pallasites A. Desrousseaux a,., J.C. Doukhan a, H. Leroux a, J.C. Van Duysen b a Laboratoire Structure et Propri£t£s de l'Etat Solide (URA CNRS 234), Unicersit~ Sciences et Technologies de Lille, 59655 Villeneuee d'Ascq-Cedex, France b D~partement des Mat~riaux, Centre de Recherches EDF, 77250 Moret sur Loing, France

Received 2 January 1997; accepted 21 April 1997

Abstract Four pallasites (Brenham, Brahin, Esquel, and Omolon) were investigated by analytical transmission electron microscopy. All show very similar compositions and defect microstructures. The olivine grains contain a low density of c dislocations organized in tilt subgrain boundaries, indicating that the meteorites were annealed for a long time at high temperature. They also contain straight fractures parallel to {lk0} planes (with k = 0, 2, and 3) filled with a mixture of metal and sulfide, and alignments of tiny inclusions of metal + troilite. Both types of defects are shock indicators, the former ones are common in shocked olivine and the later ones must result from earlier shock events and healed during a post-shock annealing episode. The Widmanst~itten patterns in the metal are similar to those observed in iron meteorites. By using the calibration charts published by the Goldstein group the cooling rates of the pallasites were deduced from the Ni concentration profiles across taenite bands. These cooling rates are quite low ( < 5 K/MY). Kamacite represents approximately 90% of the metal and is constituted of large grains with low densities of free dislocations ( < 1013 m -2) exhibiting a marked screw character. One also detects some twin lamellae and a number of phosphide precipitates with compositions M3P and M2P as well (M = Fe and Ni). The twins probably result from a shock with moderate intensity. The M2P phosphides have never been characterized before. They probably precipitated at low temperature when their Gibbs energy became lower than the one of schreibersite (Fe, Ni)3P. The shock indices in both olivine and kamacite correspond to a moderate shock intensity. As their parent body (or bodies) suffered at least one strong shock which fragmented it, the source region of pallasites must lie at great depth within their parent bodies. © 1997 Elsevier Science B.V. Keywords: Pallasites;Electron microscopy; Shock indices; Widmanst~inenpatterns

I. Introduction Pallasites are peculiar meteorites containing essentially two phases in approximately equal amounts. Centimeter-sized olivine grains with a remarkably homogeneous composition are embedded (sus-

* Corresponding author. Tel.: +33-320436966; fax: +33320436591; e-mail [email protected]

pended) in a metallic matrix with a large grain size. In the sixties and seventies, a number of investigations were performed with the aim of understanding the formation mechanism of these meteorites and determine their location within the parent body (Anders, 1964; Buseck and Goldstein, 1969; Buseck, 1977; Scott, 1977a,b). Such models tried to explain how a relatively low-density mineral like olivine can remain within a heavy molten metal without physical

0031-9201/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0031-9201(97)00025-3

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separation (paradox of Wahl, 1965). The most popular model suggests, in analogy with the Earth, that pallasites represent the core-mantle boundary (D" layer) of differentiated asteroids. Urey (1966) pointed out that this D" layer should be extremely thin (a few meters at most) and would thus represent a very small volume fraction of the asteroid while approximately 40 pallasites are known and most of them likely stem from the same parent body (Hey, 1966; Buchwald, 1975; Scott, 1977b). Urey suggested an alternative model which he called 'raisin bread' where pallasites would represent the contact zones of isolated metal pools formed within the silicate mantle. Buseck and Goldstein (1968, 1969) suggested that pallasites formed in the center of the parent body which is the only region were the density contrast between olivine and metal might not induce physical separation. None of these models, however, appears fully satisfactory at the moment and the origin of pallasites still is a debated problem. We investigated by analytical transmission electron microscopy (ATEM) the defect microstructures in both olivine and metal phases with the hope that new information might help solving this problem. It is well known that the shock intensity produced by an impact decreases with the distance to the impact site. Assuming that the response of pallasites to shock stresses depends on their intensity, i.e., on the position of pallasites within their parent bodies, the density of shock indices in both their main components, olivine and kamacite should favor one of the competing genetic models. We detect a moderate density of shock defects in olivine grains (straight fractures injected with a mixture of metal and sulfide) and no clear shock indices in kamacite with the possible exception of twin lamellae. The dislocation microstructures in kamacite rather seem related to a slow deformation process. Pallasites thus appear weakly shocked. Their source region must have been deeply buried in the asteroid structure but our observations do not allow the discrimination between core-mantle boundary and center of the parent bodies.

2. Experimental techniques Relatively large pieces of Brahin, Brenham and Esquel ( = 30 × 30 × 5 mm 3) were obtained from

mineral dealers. In the case of Omolon (Wlotzka, 1992) only small pieces ( < 2 mm size) of olivine extracted from their surrounding metal were provided by the Russian Committee for meteorites. It was thus not possible to investigate the metal of this pallasite. A Philips 525 M scanning electron microscope (SEM) was used to image the metal phases previously etched in nital (3% HNO 3 in ethanol). Chemical analyses on the SEM were carried out with an EDAX PV 9900 energy dispersive spectrometer (EDS) operating at 20 kV. Thin foils of olivine for ATEM investigations were prepared by the usual technique of ion milling. Areas of interest were optically selected on 30 p,m thick petrographic sections. They were glued on 3 mm diameter copper rings and ion thinned down to electron transparency at a high tension _< 5 kV and an incident angle _< 15°. They were finally coated with a thin film of amorphous carbon ( ~ 300 A) to avoid charge problems in the electron microscope. Slices of metal ~ 500 /xm thick were cut with a low speed diamond saw. They were first mechanically polished and etched for examination by optical and scanning electron microscopy. Regions of interest were then thinned by jet electro-polishing at low temperature (starting temperature - 4 0 ° C ) in a solution of 2% HC103 in ethanol and a DC voltage of 40 V. Characterization of the defect microstructures was performed on a Philips CM 30 microscope operating at 300 kV. X-ray microanalyses were performed with a Noran X-ray EDS with a Ge detector and an ultra thin window. This equipment allows low energy characteristic X-ray lines like the O K line of oxygen (523 eV) or the Fe u of iron (705 eV) to be detected with a good sensitivity. In order to obtain precise analyses, the peak profiles of the elements of interest (K and L lines) were recorded on electron transparent samples with count rates and dead times similar to those used for microanalyses. Microanalysis in ATEM yields intensities or net counts ( I ) for the various elements detected. The corresponding concentrations (C) are related to the intensities by the Cliff and Lorimer kx/v factors C x / / C y = k X / v (/xflly)kabsorp. The absorption correction kabsorp depends on the thickness of the analyzed zone. We precisely determined the kx/v fac-

A. Desrousseaux et al. / Physics of the Earth and Planetao' Interiors 103 (1997) 101-115

tors by the parameterless method of Van Cappellen (1990). This method consists of performing a series of microanalyses at constant beam current on electron transparent specimens with known and homogeneous composition (synthetic silicate and silicide single crystals in our case). Spectra with a relatively large number of net counts (typically 100,000 in the peaks of the relevant elements) are recorded on areas with unknown but relatively small thickness. As thickness and count rate are more or less proportional the kx/v factors are obtained by extrapolating to zero thickness the ratio of the intensity (i.e., net count) of a given element X to that of a reference element Y (Si in our case). For instance we used synthetic quartz SiO2, forsterite (Mg 25iO4), fayalite (Fe2SiO4), Ni-olivine (Ni2SiO4), and Cr-disilicide (CrSi 2) for determining K o / s i , KMg/Si, KFe/Si, KNi/Si, and kcr/s i. Fig. la illustrates the method with the case of quartz (determination of ko/s~). Uncertainties on kx/si values depend on the number of recorded spectra and on the stability of the electron beam. In the most favorable cases this uncertainty is < 0.5% and it never exceeds 1%. If the analyzed zone is not extremely thin, quantitative microanalysis also requires an absorption correction but the fluorescence correction is always neglected because (i) a realistic calculation would require a precise knowledge of the shape of the thin foil which is never known, and (ii) this correction is generally extremely small. The absorption correction requires the knowledge of the thickness of the analyzed zone. Van Cappellen and Doukhan (1994) have shown that for stoichiometric oxides and silicates this data can be indirectly, but precisely, deduced from the intensity of the O K line. Indeed, due to its low characteristic energy, i.e., to its large absorption, the intensity of the O K line is very sensitive to sample thickness. The method consists in adjusting the thickness in the correction program until the atomic oxygen concentration reaches the expected value. For instance in an olivine sample with composition (Mg, Fe) 2SiO 4 the oxygen concentration is 4 / 7 = 57.14 at.%. This procedure avoids the direct measurement of the thickness (by a convergent beam technique or via the contamination stains). We extended this method to Fe-rich metal phases by using the intensity of the Fe L line, the absorption

103

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Fig. 1. Calibration of the microanalysis system. (a) Determination of k o / s i with a thin foil of synthetic quartz SiO 2. Their intensity ratio Io / l s i is plotted on a graph versus the total net count per second (1o + I s i ) / t (t = live time). A least square fit yields the ratio 1o / I s i at zero thickness (or zero c o u n t ) = 1.76 from which one deduces k o / s i = 1.14). (b) Experimental curves Fe L / F e K versus thickness in pure Fe metal.

of which is very sensitive to thickness. We calibrated the intensity ratio FeL/Fe K versus direct measurement of the thickness in a thin sample of pure iron (Fig. l b). We also verified that this curve remains practically unchanged for a Fe-Ni alloy with = 50% Ni and assumed that the this calibration holds for alloys with any composition comprised between Ni = 5 to 50%. We then used it to provide the correction program with the relevant data. We also verified that this calibration is not markedly affected for schreibersite (Fe, Ni)3P, i.e., the absorption of Fe e by the P atoms is small as compared to the one due

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to Fe and Ni atoms, the atomic concentration of which is three times larger. Uncertainties on the microanalyses can be estimated as follows. In F e - N i metal, the composition ratio is CNi/CF~ ( l N i / l F e ) " kFe/Ni " kabsorp. Assuming a negligible uncertainty on the absorption correction kabsorp, the relative uncertainty on CNi/CFe is the sum of the one on kvc/N i ( = kNi/s~/kFe/S i) plus the statistical error on intensities INi and IFc ( A l x / l x = l / f N at 1 o- with N = net counts in X peak). In Ni poor kamacite with net counts of the order of 10 5 for Fe K and 10 4 for Ni K, the relative u n c e r t a i n t y on CN~/CF~ at 3 cr is thus A k F e / N i / k F e / Y i + AIFe/IFe + A I N i / I N i = 1 + 1 + 3 = 5%. In Ni-rich metal with a Ni content of the order of 50% this relative uncertainty at 3 ar falls to a value _< 4%. The relative uncertainties on the atomic concentrations of Fe and Mg in olivine are similar but, due to the presence of O and Si atoms, one has [Fe] + [Mg] = 28.56 at.%. As shown below typical concentrations are F e = 3 and M g = 2 5 at.% ( O = 5 7 . 1 4 and Si = 14.3 at.%) yielding to a mean Fa content F e / ( F e + Mg) = 12 + 0.6 at.% (5% relative uncertainty at 3 ~r). Finally, a few extractive carbon replicas were prepared for investigating the small precipitates in the metallic phases. After polishing, the metal surfaces were slightly etched in nital and a carbon layer was sputtered on them. The metal was then dissolved in a solution of 10 vol.% bromine in ethanol. The precipitates (carbides, phosphides, sulfides) are not dissolved and lie in the thin and transparent carbon

foil; the size, spatial distribution, crystallography, and composition of the various precipitates are determined by standard A T E M methods.

=

3. Observations 3.1. OliL, ine 3.1.1. Composition As a general trend the compositions of olivine grains are remarkably homogeneous from grain to grain within a given pallasite. Their mean fayalite content is F e / ( F e + Mg) ranges between 11 and 13 at.% (see Table 1) in good agreement with previous measurements (Buseck and Goldstein, 1969; Klosteman and Buseck, 1973). In Brahin, Brenham, and Esquel, most of the olivine grains contain long cracks (up to 10 m m long), some being clearly filled with a mixture of FeS and metal. Some Ca or K rich tiny precipitates ( < 1 /xm) are observed in Brenham (Fig. 2); they most probably precipitated during cooling. We also observe, especially in Omolon, larger inclusions (1 to 5 /~m) of a mixture of FeS and F e - N i metal (Fig. 3). Concentration profiles show a marked enrichment in Ni (up to 24 wt.%) in the metal of the inclusions while the surrounding olivine is enriched in Fe. These inclusions are often aligned along some straight directions. 3.1.2. Defect microstructure All the investigated olivine grains of the four pallasites show an extremely low density of free

Table 1 Compositions of metal phases (Ni in wt.%) and olivine Pallasite

Brahin

Brenham

Esquel

Omolon

Mean Fa in olivine Fe/(Fe + Mg) at.% Minor elements in olivine

11 Na (0.5% cat. sites) Caa, K a, Mna, A1a, Ni~ 9 55 56 17/48 53-47

13 tiny Ca or K rich precipitates 10.5 53 53 17/48 53-47

12 _<0.1% Mn

13 _<_0.1% Mn

8.5 51 NA 17/48 53-47

NA NA NA NA NA

Mean Ni in metal Ni in duplex zone taenite Ni in needle-shaped taenite in black plessite Ni in martensite/tetra taenite in cloudy zone Ni range in clear taenite

aElement with a concentration too low to be quantitatively determined.

A. Desrousseaux et al./ Physics of the Earth and Planetary Interiors 103 (1997) 101-115

(a)

105

dislocations at their tips. We assume that these small cracks formed at low temperature when olivine was

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dislocations ( < 10 9 m-2). Almost all dislocations are organized in subgrain boundaries (SGB) which extend over large distances and are generally widely spaced ( > 50 /zm). Fig. 4a shows a rare occurrence of two neighboring SGBs separated by less than 10 /xm. SGBs are generally constituted of one family of straight, parallel, and very regularly spaced c dislocations. We also observe, especially in Brahin and Esquel, numerous cleavages or straight cracks parallel to a few planes with low crystallographic indices (100), (120) and (130). The larger ones are clearly filled with a mixture of FeS and oxidized F e - N i (Fig. 4b). This oxide which has a spinal structure must result from alteration. The smaller cracks along the injected fractures are empty and there are no

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Fig. 3. Inclusions of FeS and Fe-Ni in Omolon olivine (bright field). (a) Alignment of small inclusions. (b) Troilite and metal in a large inclusion. (c) Fa content profile around an inclusion.

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A. Desrousseaux et al./ Physics q[the l~zrth and Planetary Interiors 103 (1997) 101 115

1977c). Large grains ( = 1 mm) of troilite (FeS) and schreibersite ((Fe, Ni)3P) with a Ni content = 40 wt.% are also clearly visible in the vicinity of the olivine grains. 3.2.1. Kamacite

Fig. 4. Defect microstructure in olivine. (a) Two closely spaced subgrain boundaries in an olivine grain of Brahin (bright field; the bar stands for 5 /xm). (b) Straight crack in Esquel filled with a mixture of FeS and oxidized Fe-Ni surrounded by smaller empty cracks (arrows).

well below its ductile-brittle transition. They probably formed to relax the stresses induced by the differential thermal expansion of the injected metal and the surrounding olivine. 3.2. Metal

A survey by optical microscopy of polished and etched sections of the metal parts of the three pallasites shows the typical Widmanstiitten patterns with a proportion of kamacite = 90%. X-ray microprobe analyses performed in SEM mode on areas as large as possible including both kamacite and taenite yield mean Ni contents of 9 - 1 0 wt.% (Table 1) in good agreement with previous determinations (Scott,

X-ray microprobe analyses reveal homogeneous compositions of the kamacite grains in the three pallasites with a mean Ni content of = 7 wt.% (however, [Ni] decreases down to = 3.3% in the immediate vicinity of the kamacite-taenite interfaces). As a general trend the kamacite grains show a low to moderate dislocation density and these dislocations present a marked screw character parallel to (111) directions. In Esquel, the dislocation density is very low ( --- 10 t~ m 2): they are concentrated in a few narrow bands with practically no free dislocations between them (Fig. 5a). In Brahin, the dislocation density is slightly larger ( = 10 ~ m 2) and more homogeneous (Fig. 5b). In Brenham, this density is homogeneous and reaches = 1013 m 2. We also detect in Esquel and Brahin a few twin lamellae with quite a large dislocation density in their boundaries (Fig. 5c). As already suggested by Jago (1974), such twin lamellae are probably responsible for the optical contrast called Neuman bands. We also observe in the three pallasites a number of tiny phosphide precipitates. Some of them are surrounded by a dislocation density markedly larger than in the bulk crystal (Fig. 5d). There is, however, no evidence of severe pinning of the dislocations by the precipitates. 3.2.2. Taenite

Goldstein and his coworkers (e.g., Reuter et al., 1988; Zhang et al., 1993) have studied in detail the microstructures of taenite bands in metallic meteorites. They have shown that [Ni] presents a typical 'M-shaped' profile with maximum Ni at the taenite-kamacite interfaces. They distinguish various zones corresponding to different microstructures and Ni contents (see for instance Figs. 1 and 2 in Reuter et al., 1989). Clear taenite is a single phase region corresponding to the highest Ni zone, in the immediate vicinity of the interface with kamacite. By moving toward the center of the band the following zones are successively observed.

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107

Fig. 5. Typical dislocation microstructures in kamacite. (a) Dislocation band in Esquel. (b) Low and homogeneous dislocation density in Brahin. (c) Twin lamellae in Brahin. (d) Phosphide precipitates in Esquel surrounded by an increased density of dislocations.

The cloudy zone results from spinodal decomposition. Its microstructure is a mixture of rounded taenite grains embedded in a martensitic matrix (martensite is a distorted BCC metastable phase which nucleates and grows rapidly by a diffusionless process; its formation has been extensively studied by metallurgists, e.g., Hornbogen, 1983). The black plessite zone is a mixture of small grains of Ni rich taenite and martensite. Taenite and martensite are generally epitaxially related by the following relationship {111}v//{ 110}M. Finally, the central part of the band (duplex plessite) consists of fine grained kamacite with intergranular elongated precipitates of taenite. Our ATEM investigations reveal the same complex microstructure of the taenite bands in the three pallasites, with the same succession of zones. The

observations and characterizations presented here below hold for any of them. Fig. 6a illustrates an M shaped [Ni] profile. The central part (duplex plessite) consists of = 1 /zm large martensite grains with grain boundaries decorated by elongated taenite precipitates (typical size = 0.2 X 1 /zm 2, Fig. 6b). In the black plessite zone the martensite grains reach --~ 3 /zm. They contain a large and pervasive density of small intracrystalline needle-shaped taenite precipitates (typical size ---0.1 × 0.5 /xm) in epitaxial relationships with the host crystal (Fig. 6c). The size of the taenite globules in the cloudy zone reaches 200 nm at the boundary (Fig. 6d).

3.2.3. Precipitates A relatively large density of precipitates is detected in kamacite and a few ones in plessitic zones.

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Fig. 6. Typical microstructure of a taenite band. (a) M-shaped Ni concentration profile (Brenham). (b) Duplex zone in Brahin (the 3' precipitates are indicated by arrows). (c) Needle-shaped tetrataenite precipitates in martensitic matrix in the black plessite zone (Brenham). (d) Ni-rich globules embedded in a martensitic matrix in the cloudy zone (Brcnham).

With the exception of a few large ( > 1 mm) FeS grains at olivine rims, all the foreign phases detected in metal are phosphides. The bigger grains (0.1 to 1 mm) are more easily detected by SEM. They are rounded and lie in the vicinity of olivine. They have homogeneous compositions corresponding to the usual schreibersite solid solution with an atomic ratio [Ni]/([Ni] + [Fe]) = 56 4- 2%. The technique of ATEM was used for characterizing the smaller phosphides. We observed electropolished thin foils and carbon replicas. Carbon replicas allow all the precipitates to be easily detected and characterized but their possible interaction with the surrounding matrix (possible variation of local chemical composition or dislocation density) cannot be investigated. These

pieces of information were obtained by investigating electropolished thin metal foils. We detected by these techniques numerous rounded-shaped precipitates (mean diameter of the order of 2 /zm), needle-shaped ones (typical cross section 0.5 × 0.5 /zm 2, length up to 5 /_tm) and small platelet-shaped ones (typical larger dimension of the order of 0.3 /zm but this dimension can reach, in a few cases, up to 2 /zm). A number of needle- and platelet-shaped precipitates are aligned along some directions (Fig. 7a). We find, however, no crystallographic relationships between these phosphide precipitates and the kamacite matrix. The compositions of the not too small precipitates and of their surrounding matrix can unambiguously be determined by tilting the electro-polished foil

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A. Desrousseaux et aL / Physics of the Earth and Planeta~ Interiors 103 (1997) 101-115

!IIIIIW

Fig. 7. Phosphide precipitates. (a) Elongated schreibersite in Esquel aligned along two directions. (b) Low magnification view of a carbon replica showing the spatial distribution of the extracted precipitates. (c) and (d) Zone axis diffraction patterns with smallest lattice repeats.

until the narrow electron beam intersects only the precipitate. We find for the rounded-shaped and the needle-shaped precipitates the usual schreibersite composition (Fe, Ni)3P with mean compositions corresponding to [Ni]/([Ni] + [Fe]) = 66 _+ 2 at.% for the rounded ones and [Ni]/([Ni] + [Fe]) = 74 _+ 2 at.% for the needle-shaped ones. In the immediate vicinity of these precipitates the matrix systematically shows a slight depletion in Ni (5.7 wt.% at the interface and = 7 wt.% at 0.1 to 0.5 /zm away from the interface). In a number of cases the platelet-shaped precipitates are embedded in the matrix and even if some can be oriented parallel to electron beam, their composition cannot be determined by the above technique because of beam broadening effects. We thus characterized them (composition and crystal structure) on extractive replicas (Fig. 7b). Taking into account the absorption correction we find composi-

tions close to (Fe, Ni)2P. Their [Ni]/([Ni] + [Fe]) ratio clearly shows a bimodal distribution with two maxima close to 68 and 98 at.% respectively. Their diffraction patterns (Fig. 7c,d) are not consistent with quadratic schreibersite. A similar composition M2P was already detected in the Ollague pallasite by Buseck (1969) who called it Barringerite. However, the crystal structure determined by Buseck is not consistent with our diffraction patterns. In addition our M2P precipitates represent a very small volume fraction of the metal phase which could not be detected by the X-ray technique used by Buseck. Therefore, these tiny platelet-shaped precipitates are a new phase never observed before in meteorites or in metallic alloys. A detailed investigation of a number of zone axis diffraction patternsoYields the fol-o lowing lattice parameters a = 6.29 A, b = 4.45 A, c = 3 . 9 3 A, a = 1 0 8 °, / 3 = 4 2 °, and y = 9 1 °. The corresponding unit cell is primitive (it would be

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A. Desrousseaux et al. / Physics qf the Earth and Planetary Interiors 103 (1997) 101-115

quadratic if y = 90°). We detect similar (Fe, Ni) 2P tiny precipitates in grain boundaries of the plessitic zone of Esquel.

4. Discussion

Our investigations allow some parts of the thermal and mechanical history of the pallasites to be deciphered. Cooling rates can be reestimated by using the methods developed by the Goldstein group (critical review in Saikumar and Goldstein, 1988). Some pieces of information about their late thermal history (400 to 200°C) can be inferred from the tiny (Ni, Fe) phosphide precipitates. Defect microstructures in the major phases (dislocations and remnants of shock defects) cast some light on their mechanical history. These points are discussed below. 4.1. Thermal history 4.1.1. Cooling rates During the last twenty years our knowledge on the F e - N i - P equilibrium phase diagram has been considerably improved (Buchwald, 1966; Doan and Goldstein, 1970; Romig and Goldstein, 1980, 1981; Reuter et al., 1989; Zhang et al., 1994a,b; Yang et al., 1996). After the pioneering work of Wood (1964), Goldstein and coworkers have developed a diffusion-based model of the taenite--* kamacite transformation in slowly cooled F e - N i - P alloys representative of meteoritic metal phases (Narayan and Goldstein, 1985; Dean and Goldstein, 1986; Saikumar and Goldstein, 1988; Herfer et al., 1994). Three independent methods were developed for determining the cooling rates of iron meteorites. All are based on the diffusivity of Ni in taenite which is the metallic phase into which this diffusion is slower. The first method initiated by Wood (1964) is based on a relationship between cooling rate and ratio of [Ni] in the middle of a taenite band over its thickness. The corresponding measurements are easily performed with a SEM. The second method (profile matching, Goldstein, 1965) is based on the slope of the [Ni] gradient in taenite at the taenite-kamacite boundary. This method requires analyses at closely spaced points. ATEM is thus well adapted for this method. The third method (Yang et al., 1993) is based on an empirical relationship between cooling

rate and the largest size of tetrataenite globules in the cloudy zone. As these globules are quite small. ATEM or high resolution SEM are to be used for determining their size. Our investigations provide us with all the necessary information for any of the three methods. Comparison with the charts published by the Goldstein group for other meteorites yields the following results. Brahin 1st method < 25°C/My 2nd method _< 5°C/My 3rd method 5 + 2°C/My

Brenham < 25°C/My < 5°C/My 5 _+ 2°C/My

Esquel < 25°C/My < 5°C/My 3 + I°C/My

The first method does not yield precise values because of the lack of published data for very slow cooling rates. The second and third method yield results very similar for the three pallasites and in good agreement with previous determinations of cooling rates of pallasites (Buseck and Goldstein, 1969). It is also to be mentioned that, as previously noted by a number of authors (e.g., Scott, 1977c; 1976) the metal of the main group of pallasites is very similar to the one of IIIAB iron meteorites (similar composition and cooling rate). Both types might stem from the same parent body.

4.1.2. Precipitates A part of the metal cooling history can be traced with the composition of precipitates. The most frequent foreign phases in metal are phosphides and sulfides. Large (mm-sized) FeS grains close to the olivine rims were the first non metallic solids to crystallize with the metal (taenite) at, or close to, the eutectic temperature ( = 1000°C). The solubility of S in both the FCC and BCC phases being extremely low we can ignore this element in the following and only consider the solid phases of the F e - N i - P diagram. In contrast with S the solubility of P in both taenite and kamacite is quite large at high temperature (900-1000°C) and severely decreases as T decreases (Romig and Goldstein, 1980, 1981). As a consequence metal cooling is accompanied by the precipitation of Fe-Ni phosphides (Clarcke and Goldstein, 1978). Down to temperatures T-~ 600°C the diffusivity of Ni is large enough to allow

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schreibersite (Fe, Ni)3P precipitates to grow with a composition close to equilibrium, i.e., with a composition governed by the equilibrium phase diagram. This is typically the case of the large (up to 1 mm) schreibersite precipitates with an atomic ratio [Ni]/([Ni] + [Fe]) close to 56%. These precipitates must have nucleated and grown at high temperature in a taenite matrix which then transformed into kamacite while the composition of the precipitates did not change appreciably. Indeed, if they had directly nucleated and grown in kamacite their Ni content would have been appreciably lower in the nucleation stage and should have drastically increased as T decreased. Considering their large size, this reequilibration would have implied Ni diffusion over unrealistically large distances at moderate temperature. As T decreases below T = 600°C the diffusivity of Ni drastically decreases and kamacite becomes slightly supersaturated in Ni. The solubility of P decreases too but, in contrast with Ni, the mobility of this element still is appreciable. The growth rate of the large precipitates practically vanishes and new tiny schreibersite precipitates nucleate on preferential sites (presumably on dislocation cores, leading to the observed beads of precipitates) with a larger Ni content. Their growth rate is quite low and they are probably Ni deficient, i.e., their [Ni]/([Ni] + [Fe]) ratio does no more reach its equilibrium value. Below some temperature this schreibersite becomes highly unequilibrated. It can thus become less stable than the (Fe, Ni) 2P phase. Because of the sluggishness of precipitation reactions we do not know the stability fields at low T of (Fe, Ni)3P and (Fe, Ni)2P in kamacite. The low temperature part of the F e - N i P phase diagram of Romig and Goldstein (1980, 1981) corresponds to the system schreibersitemartensite. Therefore, the stability field of schreibersite in this system may appreciably differ from the one of the system kamacite-phosphide(s). We can thus interpret the precipitation of this new (Fe, Ni) 2P phase in assuming that its Gibbs energy (and/or its surface energy?) is lower than the one of Ni-deficient schreibersite in kamacite. The large Ni content of some of them does not imply Ni diffusion over large distances because the volume of these plateletshaped precipitates is very small. The specific shapes of the needle and platelet precipitates probably stems

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from anisotropy of their interface energies and it is surprising that no epitaxial relationships with the kamacite matrix were detected. The occurrence of a relatively large density of very small phosphide precipitates (volume as small as 5.10 -3 /zm 3 for the platelet-shaped ones) clearly means that precipitate nucleation continued down to quite a low temperature owing to a low nucleation energy and a very low cooling rate of the meteorites. Finally, it can be noted that the fact that this new phosphide phase was never detected in iron meteorites may simply stem from a lack of detailed investigations.

4.2. Mechanical history Like other large asteroids, the pallasite parent body (or bodies) must have suffered a series of shock events which may have occurred at any time of their history. No clear shock indices are detected in the metal phases, with the possible exception of the few twin lamellae in kamacite. The low to moderate densities of screw dislocations in kamacite are rather consistent with a low strain rate deformation regime (Peierls regime). Shocks have been shown to induce in iron-based metals huge dislocation densities of the order of 10 j5 m - : (e.g., Gordon, 1970) and twinning. Indeed mechanical twinning is generally considered as the response to a high differential stress inducing a high strain rate which can only be achieved by the cooperative motion of partial dislocations leading to the formation of twin lamellae. If the pallasitic parent bodies were heavily shocked, most of the shock defects in kamacite appear to have been erased by efficient post-shock annealing or recrystallization episode(s). Although the olivine grains must have been affected by this annealing process, they still contain a number of shock indices. The fractures injected with metal and the beads of rounded inclusions of FeSand Ni-rich metal must result from a shock. Fractures are the most common shock indices in olivine (Sttiffier et al., 1991). They are opened by the rare faction wave. Strong shocks induce local melting of the assemblage troilite + metal (TMelt = 1000°C for the eutectic composition) and this melt is injected in the opened fractures. The rounded inclusions of FeSand Ni-rich metal most probably result from subsequent healing of such injected fractures during the post-shock annealing stage. The high Ni content in

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the rounded inclusions and the associated Fe enrichment in the surrounding olivine comfort this hypothesis (the reaction between injected material and olivine leads to the oxidation of metallic Fe and the dissolution of FeO in olivine). The rounded inclusions must thus result from shock events which occurred in the early history of the parent body when its temperature still was rather high and allowed post-shock annealing. Alternately the annealing might result from the temperature increase induced by a very strong shock but this hypothesis is not supported by observations. A very strong shock would induce a very large density of fractures. It would also partially transform olivine into its high pressure polymorphs, ringwoodite and wadsleyite. We also detect a number of injected fractures which are not healed. They must result from shocks which occurred later on when the parent body was colder, preventing the healing process to occur. The history of the parent body (bodies) must have ended with a shock strong enough to induce its (their) fragmentation. The impact between two high velocity bodies generates in both of them a shock wave which propagates from the impact point with a decreasing amplitude (Melosh, 1989). Small bodies constituted of weakly stuck grains (chondrites with low metamorphic grade) have a low toughness and weak shocks can fragment them. As a result no shock defects are imprinted in their constitutive grains and the corresponding meteorites present a very low shock degree in the StiSffler shock scale. In contrast, large differentiated asteroids with radii > 100 km are constituted of materials with a much higher cohesion and an appreciably larger toughness. Stronger shocks (i.e., larger rarefaction waves) are necessary for fragmenting them. Such shocks are expected to generate a large density of shock defects and a large temperature increase, at least in regions close to the impact. At some distance from the impact, for instance at their core-mantle boundary or at their center, the shock intensity must be drastically reduced but fractures can still propagate in a very brittle material. Both the main minerals of pallasites, kamacite and olivine, become very brittle at low temperature (the mean temperature in the asteroid belt region is -- 80 K). The final shock which fragmented the pallasitic parent body (bodies) may thus have been a very strong shock which induced huge stresses and a large

temperature increase in the vicinity of the impact point. At great depth, stresses and temperature peaks were appreciably lower. Stresses must have been large enough for inducing the fragmentation of the inner part of the asteroid. The large cold kamacite grains probably broke by cleavage, a process known to easily occur in cold BCC metals. Areas close to the tips of the propagating fractures were most probably severely affected by the shock which generated in them a high density of shock defects (dislocations and twins). Farther from the cleavage surfaces, the fragmentation process may have induced no appreciable change of the defect microstructure. Our observations suggest that the metal grains investigated in this study come from regions far from the cleavage surfaces, i.e., from zones free of shock-defects. The density of shock defects should be markedly larger in pallasites coming from areas closer to the fracture surfaces. Along the same lines, it is to be remembered that the metal of pallasites of the main group show remarkable similarities of compositions and cooling rates with the IIIAB group of iron meteorites which seem to be heavily shocked (Buchwald, 1975). Finally, the somewhat complicated scenario proposed above appears to be the only coherent explanation which reconciles the standard model of impact fracturing with our observations. This scenario which holds for the four pallasites investigated here can be summarized in the paragraphs below. (1) The parent body (bodies) suffered a series of impacts on the surface of the asteroids. The genetic regions of pallasites being deeply buried suffered only moderate shock intensities which induced in olivine fracturing (but no phase transformation toward high pressure polymorphs) and a large density of e screw dislocations, and in metal dislocations and twins. Some of these shocks occurred when the temperature in the pallasite source region still was relatively high (T > 800°C?) and the cracks in olivine healed. As cracks injected with the eutectic mixture FeS + metal healed, they transformed into partially reequilibrated rounded inclusions of metal and troilite. The other typical shock defects of olivine, like the large density of c screw dislocations (Joreau et al., 1996) annealed and formed well organized tilt SGBs constituted of one family of edge e dislocations. In kamacite the dislocations must have rapidly

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annealed leading to well organized SGBs. Some recrystallization may have also occurred leading to a reduced grain size and a severe decrease of the density of twins. (2) Other shock events occurred later on, when the pallasitic material was colder and large amounts of kamacite had grown at the expense of taenite (T < 600°C). Like for previous shocks, olivine was fractured. Due to the heterogeneity of the shock wave, a limited amount of local melting of FeS + metal may still have occurred and this melt injected some of the opened fractures. However, no subsequent healing and no significant chemical reequilibration between injected metal and olivine rims occurred. In kamacite these shocks induced further twinning and an increase of the density of screw dislocations. These later ones were able to recover, presumably by cross-slip, and formed SGBs. Depending on temperature, recrystallization might or might not have occurred. (3) At still lower temperature a limited amount of ductile deformation at low strain rate occurred in kamacite. Depending of the amount of plasticity, only some deformation bands or a pervasive density of screw dislocations resulted. The microstructure of Esquel corresponds to a very small strain ( < 0.1%). The ones of Brahin and Brenham correspond to slightly larger strains (1 to 5%). Such densities, however, are markedly lower than the ones detected so far in iron meteorites ( = 1015 m -2, e.g., Gordon, 1970; Jago, 1974; Desrousseaux, unpublished results). The observed dislocations do not seem to be related to a shock event. They most probably result from the growth of kamacite and martensite at the expense of taenite and from the different thermal expansion coefficients of the various phases in contact. At this temperature no dislocations were activated in olivine which was no more ductile. (4) A final strong shock fragmented the cold parent body (bodies). Because the pallasite source region was cold and at great depth in the parent body, it suffered a moderate shock intensity which induced shock defects only in the vicinity of cleavage fractures. The material investigated here stems from regions far from these surfaces and is practically unaffected by this final shock. The low density of shock defects in the four pallasites investigated clearly shows that the source

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region of pallasites cannot be close to the surface of their parent bodies. Our observations do not support the 'raisin bread' model of Urey. Furthermore, the rare pieces of information on the densities of dislocations and twin lamellae in metallic meteorites already published (Gordon, 1970; Jago, 1974; Desrousseaux et al., 1996) and our rapid survey of Gibeon, Mundrabilla and Mondieu (Desrousseaux, unpublished results) reveal densities larger than the ones observed in the four pallasites investigated here. It is clear that the defect microstructures have been checked in a very small number of pallasites (four among a total of forty) and in a similar small number of metallic ones as well. Such an incomplete information does not allow the discrimination between the two other models (D" layer or center of the asteroid). However, if further observations on other pallasites would confirm this systematic low density of shock defects, the genetic model of Buseck and Goldstein (center of the asteroid) would be comforted.

5. Conclusion Our observations show remarkable similarities of composition and defect microstructures of the four investigated pallasites. These defect microstructures are typical of an efficient annealing at high temperature which erased or healed most of the shock indices. This is exemplified in olivine by the perfectly organized SGBs and, in kamacite, by the low densities of dislocations. This similarity comforts the hypothesis that these pallasites originate from the same parent body, an idea suggested years ago by Buseck and Goldstein (1968) and Buseck (1977). The large grain sizes in plessitic zones indicate quite low cooling rates and this in turn shows that pallasites stem from quite a large parent body. The very tiny precipitates of the new phase (Fe, Ni)2P in kamacite are also consistent with a low cooling rate. The low density of shock indices in both olivine and kamacite (as compared to the densities detected in kamacite areas of the few metallic meteorites investigated so far) implies that pallasite originate from a very deep region in their parent body, but these pieces of information do not allow a clear

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discrimination between the D" layer or the asteroid center as a source region.

Acknowledgements The authors are pleased to thank Dr. Goldstein and Dr. Buchwatd for detailed and constructive comments. They acknowledge the Programme National de Plan~tologie for financial support.

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