Metallic minerals, thermal histories and parent bodies of some xenolithic, ordinary chondrite meteorites

Genchlmica

et Cosmnchlmico

.e Pergamon

Press Ltd

Acto Vol.

1981.

0016.7037/81!0101-0053Mt.00/0

45. pp. 53 to 67

Prmtcd inGreatBritain

Metallic minerals, thermal histories and parent bodies of some xenolithic, ordinary chondrite meteorites EDWARD R. D. Scorr*

and R. S. RAIAN

Department of Terrestrial Magnetism. Carnegie Institution of Washington. 5241 Broad Branch Road NW. Washington, DC 20015, U.S.A. (Receioed IO March

1980; accepted in revised form 28 August 19801

Abstract-We

have studied metal grains in the hosts and lithic fragments of widely differing petrologic types in four xenolithic chondrltes, using reflected-light microscopy and electron-probe analysis. In Weston and Fayetteville, which both contain solar-flare tracks and solar-wind gases, kamacite, taenite and tetrataenite (ordered FeNi) and troilite show a variety of textures. On a Wood plot of central Ni content vs dimension. taenite analyses scatter as if metal grains cooled at rates of 10-1000 and l-100 K/Myr respectively through 700 K, although metal in an H6 clast in Fayetteville plots coherently with a cooling rate of 50 K/Myr. We propose that metal grains oooled at these rates in chondritic clasts at different locations before host and clasts were compacted, and were not subsequently heated above 650 K. We predict a similar history for all gas-rich ordinary chondrites. By contrast. metallic minerals throughout Bhola and Memo-Madaras show more uniform textures and plot coherently giving cooling rates in the range 750 to -600 K of 0.1 and 1 K/Myr, respectively. We conclude that host and xenoliths in both chondrites were slowly cooled afrer compaction. Thus clasts in these chondrites experienced peak metamorphic temperatures and slow cooling through 700 K in different environments. According to the conventional onion-shell model for H, L or LL chondrite parent bodies, material of petrologic types 3-5 was arranged in successive shells around a type 6 core prior to catastrophic collisions which mixed all types intimately. But if peak metamorphic temperatures were reached during, not after accretion, as seems plausible, maximum metamorphism may have occurred in planetesimals c 10 km in radius. Cooling through 700 K may then have occurred in larger bodies that accreted from these planetesimals. Iron meteorites. mesosiderites and some achondrites may also have experienced melting in planetesimals and slow cooling in larger bodies.

INTRODUCIION

BINNS (1967) discovered that about 20% of the 383 chondrites which he examined contained chondritic clasts that belonged to the same chemical class as the host but had a different petrologic type (or metamorphic grade). These meteorites, which he called xenolithic chondrites, comprised 25% of the H group chondrites, 10% of L and 62% of LL. Among the various petrologic types, the unequilibrated, type 3 chondrites contain the highest proportion of xenolithic chondrites. Clasts generally vary in size from < 1 mm to at least several cm. Subsequent workers have confirmed that the great majority of chondritic lithic fragments in ordinary chondrites have a chemical composition which is close to that of the host (e.g. FODORet al., 1976; No@ NAN and NELEN, 1976). Several xenolithic chondrites also contain achondritic clasts but it is generally believed that these have been derived from host material by impact melting (e.g. FREDRIK~WN et al.. I975a: FOWR and KEIL, 1976). In I4 xenolithic ordinary chondrites, carbonaceous xenoliths have been

discovered (see WILKENING and CLAYTON, 1974; WILKENING1977; WASSON and WE’IHERILL, 1979). * Present address: Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131. U.S.A.

Several xenolithic chondrites are rich in solar-wind gases (see WASSON.1974, p. 104) solar-flare tracks and microcraters, and it appears that the gas-rich chondrites are a subset of the xenolithic chondrites. Because of the similarity of their properties to those of lunar breccias, it is widely accepted that gas-rich meteorites formed in a planetary regolith as a result

of collisional mixing of meteoritic material (e.g. PELLAS,1972; RAIAN, 1974). Since both gas-rich and gas-poor xenolithic ordinary chondrites contain similar carbonaceous and impact-melted lithic fragments, it seems very probable that gas-poor xenolithic ordinary chondrites also formed on planetary surfaces. Some xenolithic ordinary chondrites are amongst the most well-studied meteorite breccias. Very detailed studies of the silicate petrology in host and lithic fragments are available for the following meteorites: MezaMadaras (VAN SCHMUS, 1967; BINNS, 1968) Ghubara (BINNS, 1968), Sharps (FREDRIKSSON et al., 1969). Oro Grande (FOLKIRet al., 1972). Kelly (BUNCH and SNIFFLER,1974), St Mesmin (DODD, 1974). Pulsora (FREDRIKSSONet al., 1975a). Plainview (FODOR and KEIL, 1976), Abbott (FODORet al., 1976). Weston (NOONAN and NELEN, 1976), Allan Hills A76004 (OLSEN et al., l978), and Tysnes Island (KEIL and FODOR,1980). FODORand KEIL (1978) have also given detailed descriptions of clasts in 20 LL chondrites. 53

E. R. D. SCOTTand R. S. RAJAN

54

However comparative studies of the metal have not been made in these or other xenolithic chondrites. The most important information provided by metallographic studies is the cooling rate in the approximate temperature range 800-6OOK. In his brilliant and pioneering study of metal in 36 chondrites, Wool (1967) analyzed kamacite and taenite grains with the electron probe. Using a method he devised for iron meteorites in which central compositions of taenite grains are compared with those calculated using the Fe-Ni phase diagram and experimental diffusion data, Wood derived cooling rates of 0.1 to 100 K/Myr for the chondrites. Microscopic studies of the metal provide additional qualitative information on relative cooling rates and identify any meteorites which have been briefly reheated above 650 K. ’ There is general agreement between the metallographic cooling rates and those derived from 244Pu fission track data (PELLASand STORZER,1977, 1980). The latter gives rates of 0.7-1.6 K/Myr for ordinary chondrites (recently revised to I->20 K/Myr), which are fairly close to the geometric mean of metallographic cooling rates, 4 K/?vlyr (WOOD, 1967; TAYLOR and HEYMANN,1971). However, two chondrites Tillaberi and Shaw give metallographic cooling rates (CHRISTOPHEMICHEL-LEVYand MALEZIEUX, 1976; TAYLORet al., 1979) at least 100 times faster than track cooling rates. These discrepancies may arise because the two techniques measure cooling rates over different temperature ranges. Insufficient meteorites have been analyzed by both techniques to test whether there is any correlation between the two sets of data. Upper limits of 1.4 to 4.2 Gyr to the time of brecciation of some gas-rich meteorites including St Mesmin, Plainview and Kapoeta have been deduced by radiometric dating of individual clasts (e.g. SCHULTZand SIGNER,1977; BCGARD,1979; RAJANet Table 1. Sources of samples and host classification for the xenolithic chondrites studied

Meteorite

Host*

Source+

Sanp,le

No.

LL

a?&!

1806

Fayetteville

H (%4)

us?IE.I

1734- 2

E&zzii-\Qdaras

L3

Bhola

Weston

H(%4)

2915 !?$ Yale USNM

(=: #4) 3210

USW Yale*

1180 _-

* For references see text. + Source abbreviations: USNM-Smithsonian Institution, National Museum of Natural History; VS-W. R. Van Schmus, see VAN SCHMUS(1967); Yale-Yale Univer-

sity. $ Polished thick sections; the remainder were polished thin sections.

al., 1979). This suggests that brecciation continued for many Gyr on the meteorite parent bodies. In this work we have studied metal in the host and clasts of four xenolithic chondrites, Weston and Fayetteville, which are rich in solar-flare tracks (LAL and RAJAN, 1969; PELLASet al., 1969 ; RAJAN, 1974) and solar-wind gases (SCHULTZand KRUSE, 1978) and Bhola and MezGMadaras. Bhola lacks solar-flare tracks (B. K. KOTHARI,private communication), while MezG-Madaras is known to lack solar-wind gases (SCHULTZ and KRUSE, 1978); presumably both are deficient in solar tracks and gases. Our aim was to relate information from metallography concerning the thermal history of host and clasts to that obtained from track studies, and hence to learn something of the early history of the meteorite parent bodies. Initial results from work on metal (Scar and RAJAN,1979b) and tracks (KOTHARI and RAIAN, 1980) have been published elsewhere. SAMPLES AND METHODS Table 1 shows the sources and host classification of the four meteorites (all falls) analyzed in this study. Additional thin sections from the Smitdsonian Institution collection were studied with reflected and transmitted light microscopy. Sections were lightly etched in 2% nital to reveal metallic microstructures. Electron-probe analyses were performed at 15kV with a specimen current of 0.04 PA, using pure Fe, Ni and Co standards and a ZAF correction program. Only grains with relatively uniform borders of tetrataenite (ordered FeNi) and cloudy taenite were selected. Apparent distances to the edge of the grains were subsequently measured by means of the microscope. If the probe contamination marks were not central, data were discarded. Cooling rates were derived by selecting cooling curves to fit the lower envelopes of data on Wood plots (WOOD, 1967). The error in Ni analyses is _ 0.5% Ni. Uncertainty in the apparent distance to the edge due to irregular shapes varies from _ 1.5 to 3 pm for grains 10-50 pm in diameter. WOOD (1967) estimates that cooling rates can be determined with a precision of a factor of 1.5, and an accuracy of a factor of 2.5. As discussed below, there is evidence that some cooling rates may be as much as 5 times too low. Normal taenite grains with M-shaped Ni profiles are not single crystals as Wool (1967) supposed. Instead, on cooling below 620 K they develop rims usually l-5 pm wide of tetrataenite. ordered FeNi (ALBERTSEN et al., 1978; SCOTT and CLARKE,1979; CLARKEand Scan, 1980). Next to the tetrataenite rim is the cloudy taenite border or core, which appears brown on etching (SCOTT,1973; BUCHWALD,1975) and is now known to be a two-phase intergrowth containing tetrataenite. The effect of the growth of tetrataenite on Wood’s method of estimating cooling rates is uncertain, though probably small. However, the extent of its growth at such low temperatures tends to indicate that calculated diffusion rates in taenite are too low (SCOTTand CLARKE. in preparation). The apparent dimensions of taenite grains were measured to the outer edges of the tetrataenite rims.

METALLOGRAPHY AND MICROANALYSIS a. Weston This meteorite was described by REICHENBACH in 1860 as “a conspicuous example of ‘meteorites in a meteorite’, i.e. one which contains so many inclusions of all sorts of crys-

Fig 1. Reflected-light photomicrographs of etched metal in the host of Weston gas-rich H chondritc (USNM 1180). (a) Normal slow-cooled taenite grains containing plessite (P), cloudy taenite (CT), kamacite (K) and tetrataenite (Tt) with troilite (Tr) adjacent in black silicate matrix. (b) Lamellae of kamacite and zoned taenite with cloudy rims resembling pearlitic plessite found in iron meteorites. Tips of kamacite lamellae are oxidized at upper left.

Fig. 3. Thin section of Fayetteville H chondrite showing contact between dark host containing chondrule and rock fragments, and a lighter colored, H6 lithic fragment above. Irregular black grains are metal and troilite. 55

Fig. 5. Reflected-light photomicrographs of etched metal grams in Me&Madaras. (a) and (b) Grains in host and L4 clast respectively contain kamacite (K), cloudy taenite (CT) with well developed rims of tetrataenite (Tt). (c) Large metal-sulfide nodule in partly melted clast contains tiny silicate grains I-10 pm in size. It solidified quickly but developed normal cloudy taenite and tetrataenite after compaction of host and clasts during slow cooling through 700 K. (d) Unusual grain from host in which kamacite matrix contains highly irregular tetrataenite (white) and troilite (Tr). It may have formed by slow cooling of a fine intergrowth of silicate. metal and troilite grains. 56

Fig. 7. Thin sections of Bhola LL chondrite. (a) Host is largely composed of rock fragments ranging from ~0.1-20 mm in size, chondrules are scarce. The two inclusions above the center are both type S-6 clasts, the black one is heavily shocked. The clast on the right side has an igneous texture with olivine crystals set in a black, partlydevitrified, K-rich glass. (b) Part of large LL4 clast with abundant well defined chondrules.

57

Fig. 8. Bhola LL chondrite-reflected-light micrographs of etched metal grains in various clasts: (a) tiny coarse-grained, type 6 fragment (400 pm in size) in host, (b) heavily shocked, black LL5-6 clast at center of Fig. 7a [note finely dispersed metal and troilite (Tr) in silicate matrix], (c) lightly shocked clast on left of Fig 7, (d) LL4 clast shown in Fig 7b. All metal shows well-developed kamacite (K) and cloudy taenite (CT) with tetrataenite rims (Tt) indicative of normal slow cooling Large grams, e.g. in (d), formed from polycrystalline taenite; strips of tetrataenite 5-15 pm wide between areas of cloudy taenite mark previous locations of taenite grain boundaries.

58

Metallic minerals. thermal histories and parent bodies of xenolithic meteorites

59

On a Wood plot (Fig. 2) data for Weston metal do not tals and other bodies as to resemble a breccia or sandstone define a uniform cooling rate. The wide scatter of data conglomerate” (see FARRINGTON,1915). Weston contains cannot be attributed to reheating of the whole meteorite as H3-7 clasts set in a host made of tiny rock and chondrule fragments and some chondrules (NOONAN and NELEN. normal tetrataenite and cloudy taenite are found in the host. However, the scatter is consistent with the wide var1976). The host was classed as H4 (see WASSON,1974), but Noonan and Nelen suggest that it is actually a mixture of iety of metal textures observed, and with the presence in Weston of solar-flare tracks and solar-wind gases (see unequilibrated (0.1-36 mol% fayalite) and equilibrated (Fa later). We propose that the metal grains cooled at rates 18-19x) material. 10-1000 K!Myr through the temperature range 8-00 K Host metal is largely made of kamacite and zoned taein separate environments before they were incorporated nite with cloudy rims or cores and sometimes contains into the meteorite. plessite (Fig. la). Grains of tetrataenite, which TAYLORand HEYMANN(1971) called clear taenite, are present. One unb. Fuyerrrcillv usual metal grain in the host contained subparallel lamelThe dark host of Fayetteville contains chondrules and lae of kamacite and taenite resembling pearlitic plessite rock fragments like Weston and was classified as H4 by observed in iron meteorites (Fig. I b). MASON (1975). Metal grains in an H6 fragment (Fig. 3) By contrast, metal in a black H3 clast in Weston does conform to the coarse grain structure of the surrounding not show normal slow-cooled textures such as cloudy taesilicates. while in the less equilibrated host. metal is surnite and tetrataenite rims. Instead kamacite is polycrystalrounded by fine-grained matrix. This is consistent with line and taenite grains are usually featureless suggesting what is observed in normal type 4 and 6 chondrites. some reheating has occurred. Tiny metal and sulfide grains Host metal has normal slow-cooled textures with zoned are distributed locally in silicate and a few troilite and taenite grains and kamacite, some of which contains tetrametal areas are filled with pm-sized silicate grains. The taenite grains up to 60 pm in size. No evidence for shock or presence of sulfide veins in silicate suggests that intimate reheating was observed. Two grains of pearlitic plessite, local mixing of sulfide, metal and silicate and the reheating similar to Fig. lb, and two others with unusual finewere due to shock. This shock must have predated the grained plessite lacking normal high-Ni rims were found in compaction of the breccia as shock effects are not observed in the host. One taenite grain from this xenolith with 22% the host. In the type 6 clast, taenite uniformly shows cloudy borders and there is also no evidence for reheating. Ni is plotted in Fig. 2. However, reheating probably altered Figure 4 shows the compositions of 29 taenite grains in taenite compositions, preventing cooling rates from being the Fayetteville host and 14 in the H6 clast; tetrataenite deduced in the normal way. compositions are not plotted. Clast data are consistent One large metal-troilite grain (3 x 2 mm in size) and with a cooling rate of SOK/Myr, but host data scatter several smaller grains in the host have structures and minbetween the 0.5 and lOOK/Myr cooling curves; a much eral compositions (SCOTT,in preparation), which resemble wider spread than Wool (1967) observed in other ordinary those of rapidly solidified and rapidly cooled Fe-FeS melts (BEGEMANNand WLOTZKA, 1969; BLAU and GOLDSTEIN, chondrites. Wood also found that data from both light and dark components of Fayetteville plotted incoherently. 1975). We suspect that the 5Og metal nodules which Shepard found in Weston in 1848 (see FARRINGTON,1915) Excluding tetrataenite analyses. his data scatter between the 1 and 100 K/Myr curves, in agreement with ours. As in have a similar structure. Weston. we believe the metal grains experienced .slow cooling (at l-100 K/Myr) in separate environments before the meteorite was compacted.

d

6

5

v ._

z

Apparent

distance

from edge (pm)

Fig. 2. Central Ni concentrations of taenite grains in Weston host plotted against apparent distance from edge of grain. Data do not lie parallel to WOOD’S(1967) cooling curves. We propose that metal grains experienced cooling rates of lO-lOOOK/Myr in different locations before the meteorite was assembled.

c. Me:&Madaras BINNS(1968) and VAN SCHMUS(1967) have described in detail the silicate petrology of the L3 host. which contains abundant well-defined chondrules, few rock fragments and is typical type 3 material (DODD et al., 1967). and the L4 lithic fragments which can exceed 8 cm in length. Van Schmus also found a carbonaceous clast and one with L group composition that had been partly melted (his inclusions A and C). We have studied metal in the host and partly melted, carbonaceous and L4 clasts. (Samples are listed in Table 1 in this order.) Metal in the host and L4 lithic fragment seems fairly uniform in texture (Fig. 5a,b). Metal particles commonly contain 3-6 taenite grains, which contain cloudy cores and well developed tetrataenite rims. In localized areas throughout the meteorite, shock has caused kamacitr and cloudy taenite to recrystallize (see Fig. 2D of BINNS, 1967). Such areas were avoided when choosing taenite grains for cooling rate measurements. With these exceptions, all the metal grains show features characteristic of very slow cooling. The host also contains a few unusual metal grains with very irregular. small tetrataenite grains in kamacite but no cloudy taenite (Fig. 5d). Compositions of kamacite (4.3-5.3’,; Ni) and tetrataenite (48-53x Ni) were identical to normal occurrences. Throughout the metal and troilite are dispersed many pm-sized silicates. which are not observed in normal metal. We suggest that these grains may have formed by annealing of metal-troilite-silicate mixtures with crystal sizes of 2 10 pm. Metal in the carbonaceous and partly melted clasts

E. R. D. SCOITand R. S. RAJAN

60

2

4

10

20

40.

100

4

10

20

40

loo

Apparent distance to edge (pm)

Fig. 4. Wood plot for Fayettcvillechondrite showing compositions and dimensions of taenite in (a) host, (b) type 6 clast. Clast data are coherent and detine a cooling rate of about 50 K/Myr, but host data show wide scatter between 1 and 100 K/Myr curves. also contains normal kamacite, cloudy taenite cores, and tetrataenite rims and crystals, although the size and shape of the metal grains are different from host grains, as VAN SCTHMUS (1967) notes. In the partly melted clasf metalsulfide grams are large and rounded, and in places show a very coarse dendritic structure (Fig. 5~). Unlike the rapidly solidified metal in Weston,metal in this clast contains taenite with normal cloudy cores and tetrataenite rims. Metalsulfide grains in the carbonaceous xenolith are much smaller (CZOOpm in size) and less abundant than in the rest of the meteorite. Figure 6 is a Wood plot showing the composition-

2

4

10

20

40

dimension relationships for taenite grains in host and partly melted, carbonaceous and L4 clasts of Me& Madaras. (Analyses were made on cloudy taenite, as all taenite contains cloudy cores.) All sets of data plot coherently. and their lower envelopes define a cooling rate of I K/Myr for host and xenoliths in the range 760-6GOK, consistent with WOOD’S(1967) result for an unidentified sample. [These are the temperatures at which taenite with the maximum and minimum Ni concentrations (_ 50 and 32%) are in equilibrium with kamacite.] The chance that four meteorites with the diverse compositions and textures of host and clasts in Mezo-Madaras would have indis-

100

4

10

20

40

100

Apparent distance to edge (p m) Fig. 6. Wood diagram for Mezo-Madaras showing central Ni concentrations and dimensions of taenite: (a) L3 host, (b) partly melted clasf (c) carbonaceous inclusion (Fig. 5a), and (d) L4 clast. On all plots, the lower envelopes of the data are close to the 1 K/Myr cooling curve. This suggests that metal in host and clasts cooled at this rate in the range 760600 K afterthe meteorite was compacted.

Metallic minerals, thermal histories and parent bodies of xenolithic meteorites tinguishable cooling rates must be very small, judging from the total range of cooling rates observed: 0.1 to 100 K/Myr in normal meteorites and IO-* to 10” K/yr in reheated chondrites (SMITH and GOLDSTEIN,1977). We therefore conclude that the metal grains cooled from 760 K to -600 K at the rate of I K/Myr u&r host and lithic fragments were compacted. VAN SCHMUS(1967) found that metal in the host and two inclusions had simiiar mineral compositions, and also eoneluded that there had heen mild metamorphism after compaction. Since this argument would imply that Weston and Fayetteville had heen metamorphosed after compaction, we suggest that identical Wood-type cooling rates must he obtained on at least three and preferably more, types of clasts before such a conclusion is reached. d. Blrola As in many other xenolithic chondrites, though not Mez&Madaras, the host material in Bhola (Fig 7a) contains few chondrules but many sub-mm rock fragments of various types so that the petrologic type of the host cannot he specified uniquely (FREDRIKSSON et al.. 197530;NOONAN et al., 1978). Figure 7 shows an LL4 clast and recrystallized LLS-6 inclusions, some of which are shock-blackened. There are also dark fragments with an igneous texture consisting of euhedral and skeletal olivines in a partly devitrilied, K-rich glass. FODORand KEIL (1978) and WLOTZKAet ai. (1979) also described Bhola clasts. Metal in host and clasts is composed of kamacite, cloudy taenite and tetrataenite with textures indicative of very slow cooling around ?oOK (Fig. Sad). Although some metal particles contain a single taenite grain (Fig. 8c), most contain several (Fig. 8a,d), as in Mez&Madaras. But in Bhola, neighboring taenite grains are usually not separated by kamacite, as they are in Me&Madaras, presumably as a result of higher Ni concentrations in metal from LL chondrites. Grain boundaries within what was once polycrystalline taenite are marked by strips of tetrataenite, which are generally double the width of tetrataenite strips that separate cloudy taenite from kamacite or silicate (Fig. 8d). Grain boundaries within tetrataenite do not etch with nital, but with crossed polars they can he observed in favorable locations approximately midway between the edges of cloudy taenite grains. Tbe uniformity of the tetrataenite rim on what was once a single crystal in polycrystalline taenite suggests that grain boundary di~sion of Ni in polycrystalline taenite or tetrataenite is as rapid as diffusion along silicate-metal boundaries. Thus zoning in a taenite grain is not dependent on whether the grain is enclosed by kamacite, silicate or by other taenite grains. Woods technique can therefore he applied equally to monocrystalline and ~ly~ystalline taenite. In estimating the apparent distance to the edge of a grain in polycrystalline taenite for Woods technique, it was sufficiently accurate to assume that grain boundaries were equidistant from adjacent cloudy taenite grains. In the shocked, black lithic fragment at the center of Fig. 7a, metal and troilite are distributed as ~-sized drop lets in the silicate. Larger grains up to 200 pm in size (Fig. 8b) contain cloudy taenite and tetrataenite with unshocked textures like those shown in the matrix. Clearly normal slow cooling through 700K postdated the shock event which dispersed the metal and sulfide droplets. (In Fig. 8b tetrataenite adjacent cloudy taenite shows a cfoudy border as a result of heavy etching. Light etching turns only cloudy taenite brown; tetrataenite remains clear as in Fig. 8a and d.) Tbe clast at tbe left side of Fig 7a, wbicb is more lightly shocked, also contains normal slow-cooled cloudy taenite and tetrataenite (Fig. SC). Fragments composed of olivines in K-rich glass (Fig. 7a) contain a few tiny metal grains but only two, which were 20-25 w in size, could he analyzed. Both contain cloudy taenite with concentrations of 34 and 37% Ni, 1.6 and 2.9%

61

Co, at their centers. Kamacite contains 4.1% Ni, S.9:( Co and tetrataenite 55% Ni, 0.7% Co. Whether because of high Co con~trations of for other reasons, the two taenite grains are much poorer in Ni than other grains of comparable size. Figure 9 shows Wood plots for metal in the host of Bhola, and in the heavily shocked, lightly shocked and LL4 clasts shown in Fig. 7. Although Wood’s cooling curves were calculated for metal with IO’% Ni, which is much lower than the Ni concentration of LL metal viz, c 25-r0°% Ni (MA~c~Nand WIIK, 1964). the curves may also he valid for some LL chondrites (WOOD, 1967, 1979; TAYL~X,1976). Large grains with central Ni concentrations close to the bulk concentration of the metal may give underestimates of the cooling.rate as the cooling curves will converge towards tbe bulk Ni concentration. Although the mean Ni concentration of Bhola metal is not known. this effect does not seem to he apparent in Fig. 9. One host metal grain with a central Ni concentration of 31% (Fig. 9a) lies well helow the 10 K/Myr curve for reasons unknown. Its central Co concentration of 1.44%was much higher than in other taenite (0.5-0.9%). Apart from this grain and a few larger grains, the Bhola data are reasonably coherent and define a cooling rate of 0.1 K/Myr in the range of 750 to -600 K for host and clasts. As with Me&iMadaras, we propose that this slow cooling occurred after the litbic fragments and host were compacted. DISCUSSION Polycrystalline taenite WOOD (1967) recognized that some taenite in chondrites was polycrystallme. but nearly all the grains which he described were single grains surrounded by kamacite and silicate. We observed polycrystalline taenite in all four chond&es, most especially in Bhola and found that it was very similar in mineralogy and composition to monocrystalline taenite. All taenites have a cloudy core and a tetrataenite rim containing 48-57% Ni irrespective of whether they are surrounded by taenite, silicate or kamacite. In Bhola and MezGMadaras, central Ni contents of interior grains in ~ly~ystalline taenite and of mon~rystalline taenite are

2

4

IO 20 Apparent

40

80 distance

4

IO 20 40

80

to edge (rrn)

Fig. 9. Wood diagram for Bhola showing central Ni concentrations and dimensions of taenite grains in host (a). and shock-blackened (b), lightly shocked (c) and LL4 (d) ciasts which are shown in Fig. 7. With a few exceptions the data define a cooling rate of 0.l K,&fyr. The uniformity of the cooling rate in host and clasts suggests that this slow cooling occurred ufier their compaction.

62

E. R. D. Scorr and R. S. RAJAN

indistinguishable on Wood plots. Thus below 750 K, the host metal is commonly located in recognizable rate of Ni diffusion along taenite grain boundaries is so microscopic chondtitic clasts. When one or more recmuch higher than volume diffusion that the factor which ognizable clasts in a xenolithic chondrite plot cohercontrols zoning in interior grains is volume diffusion, just ently on a Wood diagram but the total scatter of as for single taenite grains. taenite compositions is well outside that normally exBEVANand AXON(1980) observed polycrystalline taenite in Tieschitz, and state, we believe incorrectly, that it cannot perienced (e.g. Fig. 4). it seems reasonable to conclude be accounted for by WOOD’S model (1967) of kamacite that the metal is recording true cooling rates from growth. They suggest instead that it is a relict solidification different locations. structure, which was prevented from annealing to monoIt is difficult to reconcile the very slow cooling rates crystalline taenite by rapid cooling from 1700 to 1000 K in of 0.1 to 1 K/Myr of mesosiderites and some chonless than a few months. They argue that the observed zoning in taenite results from the enhancement of solidification drites with other data on their thermal history. In zoning by diffusion during subsequent kamacite growth general, radiometric ages for irons and chondrites are below 8OOK, and that Wood underestimated the cooling rate during kamacite growth by a factor of 10’. HLJTCHI~~N 4.4-4.5 Gyr implying that these meteorites cooled to

et al. (1980)argue by analogy that other H chondrites with polycrystalline taenite cooled rapidly from c 1700K, and that their Wood-type cooling rates are also incorrect. We suggest that grain boundaries in polycrystalline taenite are not, in general, a result of solidification. If metal grains were small (C 20 pm in size) following accretion, it is likely, especially in unequilibrated chondrites like

Tieschitz, that the metal was never annealed sufficiently to produce large single crystals of taenite. In some chondrites, taenite grain boundaries might have been introduced by deformation and subsequent annealing. Even supposing that rapid solidification had enriched the Ni concentration at the rims of taenite crystals, as BEVANand AXON (1980) propose, any slow cooling and consequent solid-state diffusion which allowed the growth of 100 pm kamacite crystals from taenite should have obliterated any solidification zoning in smaller taenite crystals. Other evidence against their theory is our finding that the central Ni contents of taenite grains are not dependent on whether taenite is enclosed by kamacite, troilite, silicate or by other taenite grains. In Bhola and Mezo-Madaras, taenite grains define the same cooling curve on Wood plots. irrespective of their location. As discussed below. there is evidence that Wood’s cooling rates are too low, but the arguments of BEVANand AXON(1980) that Wood’s technique gives cooling rates in error by a factor of IO’ are incorrect.

Metallographic cooling rafes Our taenite compositions in an H6 clast from Fayetteville plot coherently whereas data from the adjacent host scatter considerably. We conclude that host metal in Fayetteville is derived from chondritic material that cooled slowly in widely separate environments before compaction of host and clasts. Since the metal shows textures like those in normal slow-cooled chondrites and exhibits no signs of local shock heating which might perturb taenite compositions, we believe that Fayetteville metal grains have retained a true record of the cooling rates they experienced before compaction i.e. l-100 K/Myr. WOOD (1967) argued from petrological evidence that metal in chondrites had cooled in situ, and had not cooled in a separate location as UREY and MAYEDA(1959) claimed. Nevertheless, when he found that light and dark portions of Salles had different metallographic cooling rates, he concluded that the two components must have evolved in different places. In Fayetteville and Weston, it is possible that some individual metal grains were mixed with silicate after slow cooling as Urey and Mayeda argued, but

below the isotopic closure temperatures in less than about 15OMyr. If published Ar closure temperatures are correct, metallographic cooling rates are too low by factor of about 6 (WOOD, 1979). Wood has attempted to resolve these difficulties by devising an alternative thermal model for meteorite parent bodies which allows fast cooling at high temperatures and slow cooling below 8OOK, and by arguing that isotopic closure temperatures are much higher than 5OOK. Nevertheless he believes (private communication) that it is more likely that metallographic cooling rates are too low by a factor of -6. We discuss below additional evidence in support of this conclusion. KOTHARI and RAJAN(1980) have measured the density of fission tracks in phosphates from Bhola. From estimates of the U and Pu concentrations of phosphates in ordinary chondrites (KIRSTENet al., 1978) they estimate that Bhola cooled below 300-400 K, the track retention temperature of phosphate (PELLASand STORZER,1977), 4.1 Gyr ago. Thus they conclude that Bhola cooled from 740K (a lower limit to the maximum metamorphic temperature deduced from our metal data) to 400 K in less than 500 Myr, giving a lower limit to the cooling rate of 0.7 K/Myr. Uncertainty in the Pu concentration of a factor of 2 causes the cooling period to change by 80 Myr (the half-life of 244Pu). Giving an upper limit of a factor of 5 to the Pu uncertainty, means that the cooling rate could not have been less than 0.5 K/Myr. Thus our metallographic cooling rate of 0.1 K/Myr must be in error by at least a factor of 5. Most of our conclusions are not dependent on the size of this error, but we will identify those that are. Xenolithic chondrites On the basis of our detailed studies of metal in four chondrites, we propose that xenolithic chondrites can be divided into two subgroups according to whether they experienced slow cooling in the range 800-600 K before compaction of host and clasts, like Weston and Fayetteville, or after compaction, like MezG-Madams and Bhola. This conclusion is entirely consistent with the presence of solar-flare tracks in Weston and Fayetteville and their absence in the other two. Tracks in olivines and orthopyroxenes would be erased by heating to 700K in less than a year

Metallic minerals. thermal histories and parent bodies of xenolithic meteorites

compaction, some xenolithic chondrites were buried at great depths (10-100 km). The same sequence of metamo~hism or melting, breaking up, recompaction and subsequent deep burial at great depths is also required for the mesosiderites, group IC and IIE irons (see SCOTT, I979), and the Shaw chondrite (SCOTI-and RAJAN, 1979a; TAYLORet al., 1979). Such complexity requires that accretion continue during metamorphism and melting of meteorite parent bodies. For Bhola and MeziiMadaras, the thermal history below 750 K recorded in the metal and the maximum metamorphic temperatures (up to 800-1200 K) recorded by the sibcates refer to entirely different periods of metamorphism. The same may be true for all chondrites, contrary to the general assumption of slow monotonic cooling after peak temperatures were reached. If so, it would help to explain the lack of an inverse correlation between metallo~phic cooling rate and petrological type for members of the same group that would be expected if all metamorphism occurred in one internally-heated parent body which accreted before metamorphism began (Fig. 10). PELLASand STQRZER(1977, 1980) find that their fission-track cooling rates increase with decreasing petrologic type in each ordinary chondrite group, but more data are needed to establish significant correlations. If meteorites like Bhola and Mez&Madaras are relatively common, as we believe, no correlation should exist. At the time of BIN& (1967) study of xenolithic chondrites, it was assumed that if the host material was, for example, petrologic type 5, then the meteorite must have been heated to temperatures appropriate

(FLEISCHERer al., 1967). Similarly, isotopic ratios of solar-wind He, which is abundant in Weston and Fayetteville and absent in Mez~Madar~ (data are not yet available for Bhola), would be easily modified by heating. The isotopic ratios of He and Ne also impose stringent constraints on the thermal history of gas-rich chondrites, and are consistent with those deduced from the presence of solar-flare tracks. Since all gas-rich chondrites are believed to contain solarflare tracks, we would expect metal grains in these meteorites to retain a record of their thermal history before compaction. TAYLOR and HEYMANN (1971) derived a cooling rate of 15 K/Myr for Gee Vee, which is rich in solar-wind gases, although their data show an appreciable spread on the Wood diagram. it is unlikely that all the metal in a xenolithic chondrite experienced identical thermal histories prior to compaction, and we suggest that additional sampling would increase the spread of cooling rates. Our Fayetteville data suggest that its metal grains cooled from at least 800 K to below 620K at rates varying from 1-1~K~Myr. Thus compaction of the meteorite could not have occurred until at least 130 Myr after the clasts had formed. Errors in the metallographic cooling rates might reduce this interval to 25 Myr. Thus at present metallographic data alone cannot give detailed information about the time of formation of meteorite breccias. However, taken with the fission track record, the data can give useful constraints on the time of brecciation. The uniform slow metallographic cooling rates in Bhoh and Mez&Madaras strongly suggest that after

I

”

I

I

I

I

3

4

5

63

I I

63

I

I

4

PetKdogic

I

63

I

I

I

I

4

5

I I

6

5+ype

Fig.

10. M~ailo~aphic cooling rates in the range 8004XKJK (from Woon, 1979) plotted against petroIogic type for H, L and LL chondrites. Assuming that petrologic type is coatroW by the extent of metamorphism during radioactive heating within parent bodies, as seems probable, negative correlations

between cooiing rate and petrologic type would be expected if each group of &on&&es was heated in a single parent body. Negative correlations are not observed, however; the limited data are more consistent with positive correlations. This implies that either chondrites were metamorphosed in a different way (e.g. Fig. ilb), or petrologic types were mixed by collisions before they cooled through 8W-600 K.

E. R. D. Scorr and R. S. RAJAN

64

to type 5 (say lOOOK) after it was compacted. But with the discovery of type 2 carbonaceous and unequilibrated clasts in equilibrated host material it became obvious from petrological evidence alone that this was not true. ASHWORTHand BNCBER(1976) have used electron microscopy to show that grains in gasrich meteorites, like those in lunar breccias, have been stuck together by an amorphous cementing material. Even mild shocks that leave tracks in large grains unaltered can cause local melting along grain boundaries and in pores. Me&Madams and Bhola appear to be the only two xenolithic chondrites that have definitely suffered prolonged annealing to 750K or above after compaction.

model (PELLASand STORZER,1980) for a body of the dimensions discussed above. The proportions of types 3-6 were chosen to provide the observed fall frequencies of H group petrologic types (WASSON, 1974). ANDERS(1978) shows the corresponding diagram for the L group body, which has a higher proportion of type 6 chondrites. The only evidence that fall frequencies are representative is that the relative proportions of types 3-6 are roughly similar in H, L, and LL groups. Internal radioactive heating is assumed in onion-shell models. There is no evidence that xenolithic chondrites, whether gas-rich or gas-poor, formed in different locations from more homogeneous chondrites. as their silicate mineralogy, cosmic-ray exposure ages and Parent bodies metallographic cooling rates do not appear to be atyThe virtual absence of intermixing of H, L and LL pical. Thus if onion-shell parent bodies with a radius material in xenolithic chondrites suggests that each of 50 km or more ever did exist, they must have been group comes from a separate parent body (BINNS, thoroughly mixed by collisions. If our samples are 1967; FODORand KEIL, 1973 ; KEIL and FOWR, 1973). representative, 60% of the LL parent body (BINNS. Wool (1967) established lower limits of 100-I 50 km 1967) was reduced to fragments < 20 cm in size. for the radii of these bodies. Even assuming that Peak metamorphic temperatures in an initially Wood’s cooling rates are too low by a factor of 5, the isothermal body heated by 26A1 would be fairly uniradii were probably 50 km or more. form (assuming no melting occurred), except in the Many workers envisage that three parent bodies for outer few km (e.g. HERNDONand HERNDCIN,1977; the H, L and LL chondrites had type 6 material in MINISTERand ALL~GRE,1979; MINEAR et al.. 1979). their cores surrounded by successive shells of type 5-3 This is because heat would be conducted only a few material, The chemical and petrographic characterkm through chondritic material before 26A1 had istics of the petrologic types may have been establargely decayed. Thus a body with a radius of lished by accretion and metamorphism respectively, 50-100 km would provide a very high proportion of with type 6 material accreting first and experiencing type 5 and 6 material, as is observed. However, for the highest metamorphic temperatures (e.g. LARIMER such a body to have existed, it must have accreted in 1973; TAKAHASHIet al., 1978). Alternatively, metaless than a few million years, otherwise maximum morphism alone may have been responsible for both metamorphic temperatures, and hence petrologic chemical and petrographic characteristics (e.g. DODD, types:would have been established in smaller bodies. 1969). Figure 1la shows a conventional onion-shell Time scales for accretion are not well-known, but Q.Onron-shell model

I

b. Metamorphosed-planetesirnat

Fig. 11. Schematic models for ordinary chondrite parent bodies. (a) Conventional onion-shell mode) in which radioactive heating after accretion produces a radial distribution of metamorphic types 3-6 (drawn to represent the observed H group fall frequencies). (b) Metamorphosed-planetesimal model in which maximum metamorphic temperatures are reached in small planetesimals (d 10 km in radius) before the chondrite parent body has accreted. Abundant mixing of types 36 in xenolithic chondrites (especially in LL chondrites) suggests that if (a) ever existed, it must have been thoroughly mixed by subsequent collisions; (b) seems to fit the data better.

Metallic minerals. thermal histories and parent bodies of xenohthic meteorites 10-100 yr for l-2 km planetesimals and 10’ yr for 50 km bodies appear to be plausible values (see WE’LL, 1978). Assuming 26Al was the major heat source, then it is possible that maximum metamorphic temperatures were reached in small planetesimals 5 10 km in radius, and that these accreted into larger bodies before their residual heat had been lost by conduction. In this model (Fig. 1lb), type 4-6 chondrites would have cooled relatively rapidly from their peak temperatures, and more slowly through 700K in larger bodies. If volatile abundances were established during nebular fractionation, an inverse correlation with petrologic type could be a natural result of early accretion, as bodies which accreted later contained less 26Al. This new model (Fig. I1 b) has a number of advantages over the conventional onion-shell model: (a) slow cooling at 700 K can be reconciled more easily with old radiometric ages, (b) the absence of an inverse correlation between metamorphic type and metallographic cooling rate (Fig. 10) is explained, and (c) the necessity for completely fragmenting and then reaccreting several 50 km bodies is removed. However, the slow cooling rates of some Weston and Fayetteville material still require major collisions to have excavated this materiakfrom great depths. WA=N (1972. 1974) also proposes that chondrite metamorphism occurs in planetesimals. but he envisages that bodies l-1OOm in radius were externally heated by the sun during a brief Hayashi phase. He suggests that gas-rich meteorites formed in regoliths during the accumulation of these small planetesimals into larger bodies. However the young ages of clasts in St Mesmin and Piainview (see BOCARD,1979) and the slow-cooling rates of some Weston and Fayetteville material suggest that these gas-rich chondrites formed after l-100 m planetesimals had accreted into larger bodies. Reheated meteorites like Bhola and Mezii-Madams may have been formed during early stages of accretion. Wool (1979) has investigated a model in which accretion occurs during 26A1 heating, but he envisages that it is thermally insulating particulate matter which accretes. not planetesimals. Like chondrites. iron meteorites and mesosiderites may have experienced maximum temperatures in small planetesimals and slow cooling through 700K in larger bodies that formed from these planetesimals (Scorr. 1979). Impact heating was probably involved in the production of droplet chondrules and shocked and melted xenoliths, and could also have been an important source of heat for metamorphism, as FREDRIKSSON ef al. (1975) propose. In this case, both parent bodies and planetesimals probably lacked concentric zoning of petrologic types. MITTLEFEHLDT(1979) favors impact heating for forming achondrites as he finds evidence for remelting. However, gentle collisions between molten and solidified planetesimals could also cause remelting. Electromagnetic induction heating. if it occurred, was brief (<106yr) and pro-

65

duced local metamorphism, not radial heating as in Fig 11 (HERBERTand SONN~, 1979). Another unresolved puzzle is why we have so little chondritic material that was heated to higher than type 6 levels @cot? and RAJAN, 1979a). WILKENING (1979) has reviewed other meteorite and astronomical evidence relevant to discussions of meteorite parent bodies. CONCLUSIONS 1. Bhola and Me&Madaras chondrites were annealed to 2750 K and then slowly cooled after compaction of host and lithic fragments. Cooling rates determined by Wood’s technique are 0.1 K/Myr and 1 KMyr respectively, but these may be too low by at least a factor of 5 judging from 244Pu fission track data on Bhola phosphates (KOTHARI and RAJAN,1980). 2. Weston and Fayetteville chondrites show a wide variety of lextures in their metal, and taenite compositions which correspond to cooling rates between lO-loo0 K/Myr and l-100 K,Myr respectively. Chondritic clasts cooled slowly at these rates in different environments before compaction of host and xenoliths and were not subsequently heated above 650K. We predict a similar history for other gas-rich chondrites. 3. Many chondrites, like chondritic clasts in Bhola and MezGMadaras, may have experienced maximum metamorphic temperatures and slow cooling through 700 K in different environments. 4. Zoning of Ni in normal polycrystalline taenite cannot be attributed in part to solidification as BEVAN and AXON(1980) propose. Their arguments for asserting that Wood’s cooling rates are too low by a factor of at least 10’ appear to be incorrect. 5. An onion-shell model for ordinary chondrite parent bodies in which type 6 material is located in the core and types 5-3 in successive shells is not incompatible with xenolithic chondrite data assuming that these bodies subsequently suffered catastrophic collisions. However, we prefer another model in which maximum metamorphic temperatures are reached in km-sized planetesimals and slow cooling through 700 K occurs in larger bodies that accreted from these planetesimals. Acknowledgements-We

thank W. R. VAN SCHMUS. K.

FREDRIKSSON, R. S. CLARKEJR. A. F. NOONANand K. K. TUREKIAN for their help in locating and loaning suitable

polished sections of xenolithic meteorites, W. R. VAN SCHMUSfor loaning his photographs of Mezii-Madams sections, and our colleague B. K. KOTHARI for many fruitful discussions. R. BRETT,J. I. GOLDSTEIN. J. A. WOODand A. W. R. BEGANgave helpful reviews and comments. and C. G. HADIDIACO~and D. J. GEORGEof the Geophysical Laboratory kindly provided electron probe facilities.- This work was partly supported by NASA grant NAGW-38.

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