Uranium in the silicate inclusions of stony-iron and iron meteorites

Geochimica ~Perga~~

et Cosmochimicrr

Acta Vol. 46, pp. 749 to 754

Press Ltd.1982. Printed inU.S.A.

Uranium in the silicate inclusions of stony-iron and iron meteorites GHISLAINE

CROZAZ,SCOIT

F. SIBLEY* and DOUGLASR.TASKER

Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, Missouri 63130, U.S.A. (Received May 26, 198 1; accepted in revised form December 11, 198 1) Abstract-The microdistribution of U has been studied, using fission track techniques, in eleven mesosiderites, seven pallasites and four iron meteorites with silicate inclusions. When concentrated, U is usually found in phosphates: merrillite and/or chlorapatite. As in stony meteorites, the U concentrations in a given phosphate phase are highly variable from meteorite to meteorite and sometimes also exhibit variations in the same meteorite. Uranium is found to be concentrated in merrillite (0.25 to 1.43 ppm) in all the mesosiderites except Bondoc where none was observed. No U-rich phase was identified in six of the seven pallasites. In the seventh, Marjalahti, there are merrillite grains with concentrations ranging from 0.06 to 0.14 ppm. Where observed, the phosphates from silicate inclusions in the irons appear to have U concentrat&s similar to the mesosiderites. INTRODUCTION IN a previous paper (Crozaz, 1%‘9), a study of uranium and thorium microdistributions in seventeen stony meteorites, using fission track techniques, was presented. The present work is an extension of this

study to twenty-one stony-iron and iron (with silicate inclusions) meteorites. The original motivation of this work was to identify in a given meteorite as many U-rich phases as possible which could then be used to help to unravel the geochemical behavior of plutonium and/or determine the thermal history of the meteorite. The decay of 2”Pu (an extinct radionuclide with a half-life of 82 my) produces fission tracks and rare gases which can be used to determine the abundance of 244Pu when different meteorites formed. This, in turn, has important implications on both the processes of nucl~synthesis and the relative ages of meteorites. However, it is critical to determine whether Pu and U were geochemically segregated or not when the meteorites formed. A ratio of Th/U (two incompatible elements like Pu) significantly different from 3.8 + 0.5 (the accepted solar system value) can be taken as indirect evidence of a plutonium fractionation, although the converse is not true: a Th/U ratio of -3.8 would not insure that plutonium did not fractionate. As suggested by Lugmair and Marti (1977), Wasserburg et al. (1977) and Boynton (1978) and as shown experimentally by Benjamin et al. (1979; 1980), Pu behaves more as a lanthanide than an actinide due to stabilization (under low fOZ conditions appropriate for meteorites) of trivalent Pu in contrast to tetravalent Th and U. In stony meteorites (Crozaz, 1974; 1979) phosphates, usually merrillite (formerly called whitlockite) and/or chlorapatite, are enriched in uranium and thorium, and the Th/U ratios of these phases differ considerably from the value of 3.8 indicating that * Present address: U.S. Bureau of Mines, Washington, D.C. 20241, U.S.A. 749

these elements (and most probably plutonium) were fractionated during the meteorite formation. In the present study, we concentrated on the determination of uranium. Thorium measurements using the technique developed by Crozaz (1979) require long fast neutron irradiations (on the order of months) using special facilities (developed by T. H. Blewitt) at the Argonne National Laboratory. As a consequence, Th was studied only in the mesosiderite Estherville. Because different minerals have different track retention temperatures, fission tracks can also be used in geothermometry. Pellas and Storzer (1975) first suggested measuring meteorite cooling rates by taking advantage of the different track registration properties of uranium-rich grains and adjacent uranium poor minerals. Their work in stony meteorites using this method has been recently summarized (Pellas and St(irzer, 1981). Cooling rates of meteorites can also be determined by metallographic techniques based on the Ni distributions in taenite and kamacite. Wood (1979), comparing meteorite metallographic rates and their ratiometric ages, has, however, suggested that the metallographic cooling rates when applied to stony and stony-iron meteorites might be in error by a factor of 6. There is thus a need for an independent method to evaluate the thermal histories of meteorites. What little is known about the U content of stonyiron and iron meteorites has been summarized by Morgan in the handbook o~~le~~~tal Abundances in Meteorites (Mason, 1971). In the metal phase of iron meteorites, the U concentrations are very low, 0.1 ppb or less (as first measured by Reed and Turkevich in 1955). The troilite phase is enriched relative

to the metal (4 to 17 ppb) which may indicate that U has distinct chalcophilic tendencies under reducing conditions. Whether the U is actually in the troilite crystal structure or in other phases included within the troilite is not clear. In pallasites, the U concentrations are equally low: 0.005 to 0.12 ppb in the metal of Brcnham (Reed et al., 1958) and 0.7 and

750

G. CROZAZ,

S. F. SIBLEY, AND D. R. TASKER Thermal neutron doses of 3 to 5 x lO”/cm* were used and uranium-rich phases were usually identified only when their sizes exceeded -10 pm and their uranium concentrations - $0 ppb. From one to three polished sections were analyzed for each meteorite studied (representing a total area of up to -5 cm*). In the case of the mesosiderite Estherville, thorium was also determined using a method developed by Crozaz (1979) which is based on the fission of thorium by fast neutrons.

4 ppb respectively in the olivine of Huchitta and Brenham (Mason, 197 1). The U content of the mesosiderites has not been the object of much investigation either. Clark et al. (1967) studied three mesosiderites (Estherville, Mount Padbury and Bondoc). It is not clear which phases were analysed in this work, which was subsequently criticized by one of the authors (Kuroda, 1969) who nevertheless reported a U concentration of 32 ppb for Esthervilie and 145 ppb for Mount Padbury. Fleischer et al. (1965) studied the microdistribution of U in the mesosiderite Vaca Muerta and concluded that U is concentrated in very rare zircon grains (1 to 4,000 ppm) as well as in minor merrillite (190 ppm). Given the rather meager data available, we undertook our study of the silicate fraction of stonyiron meteorites and also investigated some of the iron meteorites which contain silicate inclusions. As we shall see below, U is enriched in the merrillite of mesosiderites. We have already taken advantage (Crozaz and Tasker, 198 1) of this fact to conclude that the fission track record of these merrillite grains is inconsistent with the unusually low (lowest ever recorded for any meteoritic group) cooling rate of -0.1 OK/ lo6 yrs. suggested by Powell (1969) on the basis of his metallographic investigations of this group of meteorites.

RESULTS Eleven mesosiderites, seven pallasites and four iron meteorites with silicate inclusions were studied. The results are summarized in Tables 1 and 2. Crystal sizes indicated are the maximum observed. Only crystals 2 10 pm are included. Finding and identifying smaller grains is difficult and was not attempted as they would be too small for fossil track and rare gas studies. In view of the heterogeneous nature of many meteorites, it is possible that other uraniumrich phases were present but not observed; however, such phases could only be present in low abundances. In our search for uranium rich phases, no zircon was found.

ia] Mesosiderites

EXPERIMENTAL Meteorite sections were polished, covered with muscovite and irradiated with thermal neutrons which fissioned 235U. The fission fragments give etchable tracks in the mica, indicating the uranium microdistribution in the samples. The uranium-rich phases were identified with the use of a microprobe. The experimental procedures were the same as described by Crozaz (1979).

TAB&E 1 - URANIUM Group(l)

Meteorite

U-rich

phase

A summary of observations in mesosiderites is presented in Table 1 and below. These include eleven of the twenty-one known mesosiderites which have been grouped according to the classification scheme of Powell (1971) as modified by Floran ( 1978). Groups 1 through 3 correspond to mesosiderites with increasing degrees of recrystallization (or metamorphism) of the silicate phase; group 4 includes mesosiderites interpreted to be impact melts. A common

IN MESOSIDERITES

U concentration

Th/U

(ppm)

Crystal Size (microns)

Phosphate Content (2) (O/o)

1

Crab Orchard Mount Padbury Patwar Vaca Muerta

M 11 M M

0.52 0.94 0.70 0.87

2 0.05 + 0.08 + 0.04 +_ 0.05

2

Veram~n

M

0.5

- 2.5

3

Bondoc

none

Emery Estherville Morristown 4

Hainholz Pinnaroo

i < < <

170 200 400 150

z M !

(1978).

0.52 i 0.02 1.43 _+ 0.16 0.25 * 0.01 0.56 k 0.02 0.5 - 3.2

1 9.7 + 1.3 -

traces 3.3 2.2 3.0

< 100

1.0

< 500 < 300 < 250

0.2 3.7 1.0 1.7

< 100 < 400

:::

identified

M = merrillite (1) Powell (1971); Floran (2) Prinz et al. (1980).

-

U IN METEORITES

feature of mesosiderites of all groups is that U is systematically found enriched in the merrillite phase. After neutron irradiation, this phase is easily identified following a 30 second etch in HN03 0.25% (which partially reveais the fossil tracks in merrillite and leaves it with a characteristic dark and eroded appearance, when viewed under reflected light). In mesosiderites, all U-rich regions correspond to merrillite grains (and vice-versa). A general observation which applies to all the mesosiderites studied here, is that the merrillite is generally associated with the metal (Fuchs, 1968). Merrillite grains not found in intimate-intact with the metal are relatively scarce and invariably small. Group 1 Crab ~~c~u~~-Merrillite grains in Crab Orchard are rare and rather small, in agreement with the observations of Prinz et al. (1980) who could only detect traces of phosphate in this meteorite. The U concentration in the merrillite is -0.5 ppm (average of 14 crystals) and is constant from grain to grain within the limits of unce~ainty (- 10% standard deviation). Mount Padbury-With 3.3 volume % phosphate (all the phosphate modal values referred to here and below are from Prinz et al. (1980)), Mount Padbury is one of the mesosiderites which contain the most merrillite. However, most grains are very small (~20 ym) with occasional crystals up to 200 Frn in size. There is no obvious U variation from grain to grain. The average U concentration (measured on 20 of the largest merrillite grains) is 0.94 ppm. Putwar-Patwar is also relatively rich in phosphate (2.2%) and merrillite grains, up to 400 am size, are abundant. However, small and irregular merrillite crystals, whose areas are difficult to measure, are by far the most common. Within lS%, the U concentration seems constant (average of 0.7 ppm measured on 16 of the largest crystals). Vuca Mtrertu-This meteorite has a phosphate content of 3% and contains abundant uranium rich merrillite grains. However, the crystals are generally small or plucked and measuring their U concentration is difficult. The average U concentration (measured on the 16 largest crystals) is -0.9 ppm with no apparent grain to grain variation. It is possible that the uranium concentration of the smaller crystals which we did not attempt to measure is somewhat different. Whether this is the case or not, the conclusions of our study of fossil tracks in the merrillite of Vaca Muerta (Crozaz and Tasker, 1981) remain unchanged as we limited this study to the largest phosphate grains available. No zircon (reported in this meteorite by Marvin and Klein, 1964) was present in our samples. Group 2 Veramin-Only two mesosiderites are recognized as members of group 2: Veramin and Clover Springs. Veramin, with a phosphate content of only l%, contains only few and very small merrillite grains which makes the U determination particularly difficult. As usual, merrillite is associated with the metal phase which in this meteorite is very finely disseminated. The average U concentration (measured on 11 crystals) is 0.9 ppm but with extreme values ranging from 0.5 to 2.5 ppm. Group 3 Bon&c-Although Bull and Durrani ( 1980) reported merrillite in this meteorite which contains 0.2% of phos-

751

phate, we could not find any merrillite after the analysis of about 4 cm’ of polished sections. The inhomogeneous distribution of merrillite, which is clearly apparent in a number of mesosiderites, is thus particularly striking in Bondoc. Bull and Durrani (1980) obtained an average U concentration of 1.4 ppm in the merrillite (with extreme values ranging from 0.6 to 2 ppm). Emery-The largest and most abundant merrillite grains (more than 1.5%) are found in this meteorite whose phosphate content is also the highest (3.7%). Many of the merrillite grains (and all the largest) are in close contact with the metal, Their U concentration is constant (0.52 ppm) even though it was measured in over 30 crystals with sizes ranging from 20 firn to 500 pm. Esfhervifle-Merrillite is very heterogeneously distributed in this meteorite but its U concentration (measured on 12 crystals) seems homogeneous f 1.4 ppm). This is the only mesosiderite in which Th was determined as well. The Th/U ratio is (9.7 f 1.3) quite different from the accepted solar system ratio of 3.8 but similar to the Th/U ratio of (11.8 f 1.4) measured in the merrillite of the LL6 chondrite St. Severin (Crozaz, 1974). There was thus fractionation of Th and U when the meteorite formed. Although Dowty (1977) showed that terrestrial and extraterrestrial Ca-phosphates are structurally distinct and argued that using partition coefficients to describe the behavior of minor and trace elements in these minerals in extraterrestrial samples may be invalid, it is interesting to compare the Th/U ratio of -9.7 measured in Estherville merrillite with the partition coefficient ratio determined by Benjamin et al. (1980). These authors measured partition coefficients for the actinide elements U, Th, and Pu between diopsidic clinopyroxene, merrillite and silicate liquid at 20 kbar and reported a Th/U fractionation factor for merrillite of 2.4. Assuming a solar system ThjU ratio of 3.8, Benjamin et al. (1980) would thus predict a Th/U ratio of 9.1 in the merrillite, a value in very good agreement with the one measured here. In addition, their measurements also show that Pu would be fractionated with respect to both U and Pu. At an oxygen fugacity of -10m9 atm, they report that Pu is much more readily incorporated into both whitlockite and clinopyroxene and interpret this behavior as resulting from the different valence state of Pu (3+) on one hand and U and Th (+4) on the other. All the evidence available thus points towards a Pu fractionation for phosphates. Morristown-This meteorite contains some large merrillite grains with rather constant but relatively low U concentration (-0.25 ppm measured on 16 crystals).

Group 4 Huinholz-The merrillite grains in this meteorite are small and irregular. and it is difficult to estimate their sizes. Their U concemration, measured on I 1crystals, seems constant (0.56 ppm). Pinnaroo--In contrast with Hainholz, Pinnaroo (also believed to be formed by an impact melt) contains many big merrillite grains but their U concentrations vary by a factor of -6 (from 0.5 to 3.2 ppm) with an average U ~ncentration (measured on 25 crystals) of 0.9 ppm.

(b) Pallasites

Seven pallasites were studied (Table 2). All showed evidence of corrosion, Admire, Brenham and Mount Vernon being the most altered. Although the pallasites are mainly composed of Fe-Ni metal and olivine, with minor troilite, they also contain a variety of

152

G. CROZAZ, TABLE 2 - URANIUM Type

S. F. SIBLEY, AND D. R. TASKER

IN PALLASITES

Meteorite

AND IRON METEORITES

U-rich phases

WITH SILICATE

U concentration (ppm)

Pallasite

Iron

Admire Brenham Mount Vernon Thiel Mountains Salta Springwater El Taco Toluca Four Corners Landes Persimmon Creek Woodbine

INCLUSIONS

Crystal Size (microns)

no U-rich phase identified

(Diopside) (Diopside) Chlorapatite Phosphate Chlorapatite Chlorapatite

0.01; 02pi.20(1) 1:3 0.5

: ;3; < < < <

60 50 60 20

(1) Shirck et al. (1969). (2) Fleischer et al. (1968).

minerals present in very minor amounts (Buseck, 1977; Buseck and Holdsworth, 1977). In their careful study of 3 1 pallasites, Buseck and Holdsworth ( 1977) noted that phosphate minerals are widespread minor constituents of this type of meteorite. Merrillite, the most abundant of these phosphates, was reported in Brenham, Marjalahti, Mount Vernon, Salta and Springwater. Although it was not observed in Thiel Mountains (and the meteorite Admire was not included in the study), the authors believed that it was probable that all pallasites contained this phosphate phase. In certain cases, it could not be detected because only small (1 cm*) polished sections from a given meteorite were available. Buseck and Holdsworth (1977) observed that phosphates are hard to recognize because of their small size and their strong resemblance to olivine which they typically border. These authors recognized them primarily on the basis of the relief developed at their contacts with olivine (the phosphates being softer than the olivine). If the merrillite grains in pallasites contain U as in other types of meteorites, they could be used to date these objects for which no radiometric age is available. With this goal in mind, we first studied six pallasites with special emphasis on Springwater which Buseck (private communication) found comparatively rich in merrillite and only slightly corroded. Because Buseck’s samples were too thick and would have acquired a prohibitive radioactivity after neutron irradiation, we prepared new polished sections with a total area of - 13 cm* (5 cm* for Springwater alone). Unfortunately, we were unable to locate any U-rich region corresponding to merrillite. For Springwater, Admire, Brenham and Mount Vernon, the U map consists only of rare clusters of tracks (“point stars”); for Salta and Thiel Mountains, tracks are found aligned at the border between metal and silicate and even between metal and epoxy strongly suggesting a terrestrial contamination. The absence of any track (and thereby U) concentration (except for obvious contamination) suggested that

either merrillite is so rare in these meteorites that our sections were devoid of it or that U was not enriched in pallasitic merrillite. However, our recent study of the uranium concentration of 30 merrillite grains extracted from a silicate sawing residue from the Marjalahti pallasite (Pellas et al., 198 1) suggests that the former explanation is correct. Indeed, merrillite in Marjalahti contains uranium; the median U concentration of the 30 grains is 85 ppb with values ranging from 60 to 140 ppb. Although Pellas et al. (1981) reported on a fission track investigation of the merrillite in Marjalahti, fission track and rare gas studies in the merrillite-poor pallasites will be challenging. (c) Irons

with silicate

inclusions

In order to complete our survey of the U microdistribution in meteorites, we included in our study four iron meteorites with silicate inclusions: Four Corners, Landes, Persimmon Creek, and Woodbine (see Table 2). Shirck et al. (1969) previously investigated diopside and enstatite crystals from El Taco and found no enrichment in the enstatite whereas the diopside contained 13 to 20 ppb of U, a concentration that is too low by our criteria to qualify it as a U-rich phase. In the meteorite Toluca, Fleischer et al. (1968) also measured a U concentration of 20 ppb in the diopside. Essentially, the same following remarks apply to the four iron meteorites studied here: the U is found enriched in rare phosphate grains which are small and fragile (Sibley, 1974). Actually most of the phosphate grains appear to have been broken during the polishing step. The U concentrated in holes in the sections where it was associated with P (and usually Cl). In each of the Four Corners and Persimmon Creek sections, only one chlorapatite grain (<60 pm) could be located. In Woodbine, P and Cl were found associated with a U concentration in a hole. In Landes, which was studied by Bunch et al. (1972)

U IN METEORITES

no phosphate could be found, although these authors reported both chlorapatite and merrillite present in nearly equal abundances. Benkheiri et al. (1979) noted the large variability of the abundance of phosphates among the inclusions of both Landes and Copiapo, two IA irons with silicate inclusions. Extreme phosphate contents range from 0 to -3%. In contrast with the observation of Bunch et al. (1972), the phosphates they found in Landes are mostly apatites containing 0.19 ppm U (only two merrillite grains were detected in 5 inclusions), and exclusively apatites in all the 6 Copiapo inclusions they studied. The U content of the apatites from Copiapo is 0.96 ppm (Benkheiri et al., 1979). We conclude that the phosphates in iron meteorites are U-rich but present in low abundances and distributed heterogeneously. The measured U concentrations are consistent with 0.6 ppm U which Benjamin (private communication) estimates for phosphates coexisting with diopside containing 13 to 20 ppb of U (measured U concentrations in Et Taco and Toluca diopside). This estimate is based on the data on clinopyroxene partitioning in (low or) P-free systems given in Benjamin et al. (1980). CONCLUSIONS

As in the case of stony meteorites, our hope to identify a number of different U-rich phases in stony iron meteorites has been frustrated. Uranium is found to be concentrated in phosphates which are inhomogeneously distributed in the meteorites and

whose U concentration is variable from meteorite to meteorite. In a given meteorite, the uranium concentration tends to be uniform in the phosphate but in some cases variations of up to a factor of 6 are observed. Mesosiderites, in which the phosphate content ranges between 0 and 3.7 volume % contain up to 1.5 volume % of merrillite, a value much in excess of what is found in stony meteorites. The U contents of merrillite in mesosiderites range from 0.25 to 3 ppm (approximately the same range as measured in stony meteorites). As observed by Fuchs (1968), the merrillite is always irregularly shaped and occurs along the borders of and within the kamacite areas, with lesser amounts dispersed in the stony portion away from the metal. Fuchs (1968) suggested that the phosphate formed by oxidation of P dissolved in the metal but this explanation has recently been questioned by Harlow er al. (1980) who tried to explain the conspicuous abundances of tridymite and phosphate in mesosiderites and ended up concluding that they could not unless the metal was unusually rich in P or unless the P was extracted from a larger volume of metal than now present in mesosiderites. It is thus not clear at this time where the P came from. At any rate, the uranium in the merrilhte cannot have been extracted from the metal phase because it does not concentrate in this phase, and thus

753

requires another source. Chlorapatite, though reported by Marvin and Klein (1964) in Vaca Muerta, was not observed in our mesosiderites and thus must be present in very minor amounts. The ratio of Th/ U in Estherville merrillite is - 10, similar to the ratio measured in the same phase in the chondrite St. Severin; Th and U were thus fractionated, and presumably Pu and U as well, at the time of formation of the meteorite. In contrast with mesosiderites, pallasites and irons with silicate inclusions contain very little phosphate. Although a U-rich phase (usually chlorapatite) was identified in the silicate inclusions of some irons, its low abundance would make a track study very difficult and a rare gas investigation, unless huge amounts of material were available, essentially hopeless.

Acknowledgements-Louis M. Ross, Jr., who helped in the final stages of the experimental work, is gratefully acknowledged and the help of the personnel of the Research Reactor Facility, Columbia, MO, and in particular of Andy Meyer, is deeply appreciated. Roy S. Clarke, Jr., U.S. National Museum, Devendra Lal, Physical Research Laboratory in Ahmedabad, Carleton Moore, Center for Meteorite Studies of Arizona State, Edward Olsen, Field Museum of Natural History, Paul Pellas, Museum d’Histoire Naturelle in Paris, Robert Pepin, University of Minnesota, and Martin Prinz, American Museum of Natural History, very kindly supplied meteorite samples. Deep appreciation is extended to the National Science Foundation which supported this work (contract grant No. EAR 78-22440), and to the Physical Research Laboratory in Ahmedabad, India, where this paper was written. This paper benefitted from the useful reviews of Tim Benjamin and G. J. Taylor.

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