The behavior of actinides, phosphorus, and rare earth elements during chondrite metamorphism

0016-7037/83/l

Gewhrmrca CI Cosmochumca Acro Vol. 41. pp. 1999-2014 @I Pergamon Press Ltd. 1983. Printed in U.S.A.

Il999-16$03.00/0

The behavior of a~tinides, phosphorus, and rare earth elements during chondrite metamorphism M. T. MURRELL and D. S. BURNETT Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 9 1125, U.S.A. (Received January 26, 1983; accepted in revisedform August 11, 1983) Abstract-New dataon the U, Pu, and P distributions in less metamorphosed H-chondrites (type 3-S), coupled with literature results, permit a provisional picture to be assembled of the chemistry of these elements and for the rare earth elements in ordinary chondrites and the changes brought about by chondritic me~mo~hism. Preferential associations of phosphates with metals and/or sulfides in all chondrites strongly indicate an “initially” siderophile or conceivably chalcophile character for P in ordinary chondrite precursor materials with phosphate subsequently formed by oxidation. This oxidation occurred prior to or during chondritic metaI-silicate fractionation. Uranium is initially concentrated in chondrule glass at - 100 ppb levels with phosphates (primarily merrillite) in H-3 chondrites being essentially U-free (~20 ppb). As chondrule glass devitrified during metamorphism, U migrated into phosphates reaching - 50 oclb in Nadiabondi (H-51 merrillite and 200-300 oub in merrillite from eauilibrated chondrites but”frozeb;t” before total concentration in phosphates &&red. Relative 2MP~ fission track densities in the outer 5 pm of olivine and pyroxene grams in contact with merrillite and with chondrule mesostasis in Bremervorde (H-3) give Pu(mesostasis)/Pu(metillite) ~0.01, implying total concentration ofPu in phosphates. Similarly, no detectable Pu (CO. i ppb) was found in chondrule mesostasis in Tieschitz and Sharps; whereas, direc+, measurements of tracks in phosphates in H-3 chondrites are consistent with high (Z 10 ppb) Pu concentrations, Thus, a strong Pu-P correlation is indicated for ordinary chondrites. There is variable Pu/U f~~io~a~~n in all phonetic phosphates reaching an extreme degree in the unequiiibmt~ chondrites; therefore, the PufU ratio in phosphates appears relatively useless for relative meteorite chronology. Literature data indicate that the REE are located in chondruies in unequilibrated chondrites, most likely in s\arr; thus there may also be strong Pu/Nd fractionation within these meteorites. Like U, the REE migrate into phosphates during metamorphism but, unlike U, appear to be quantitatively concentrated in phosphates in equilibrated chondrites. Thus relative ages, based on Pu/Nd, may he

possible for equilibrated chondrites, but the same chronological conclusions are probably obtainable from Pu concentrations in phosphates, i.e., on the Pu/P ratio. However, Pu/P chronology is possible only for ordinary chondrites, so there appears to be no universal reference element to cancel the effects of Pu chemical fractionation in all meteorites, Available data are consistent with-but certainly do not provethat variations in Pu/P represent age differences, but if these age differences do not exist, then it is conceivable that the solar system 2”Pu/23*U ratio, important for cosmochronology, is still lower than the presently accepted value of 0.007.

THE SOLAR SYSTEM 244Pu/23sU ratio is an important

quantity for cosmochronology. Although more data are needed, it appears that the Pu/Nd value in several meteorites, even highly differentiated objects such as Angra dos Reis, is essentially constant at 1.5 X 10e4 (by weight) (MARTI et al., 1977; JONES,1982). This implies that the solar system (chondritic) Pu/U can be. indirectly determined by multiplying a presumably ubiquitous Pu/Nd by the carbonaceous chondrite Nd/ U ratio leading to Pu/U = 0.004~.~5. However, we believe that, if at all possible, the solar system Pu/U should be determined directly from data on chondritic meteorites. A recent remeasurement of total chondrite z44Pu fission Xe from the St. Severin chondtite (HUDSONet al., 1983) yields Pu/U = 0.007 rt 0.002. Precise data for chondrites have been obtained by fission Xe analyses on phosphate separates (WASSERBURGei al., 1969; LEWIS, 1975; KIRSTEN et al., 1978). In order to utilize Pu/U values determined directly in merrillite (meteoritic whitlockite, essentially Ca3(PO&) or bu!k phosphates (merriliite plus apatite), the well established Pu/U enrichment (PODOSEK, 1970; CROZAZ, 1974;

BENJAMINet al., 1978) in merrillite must be considered. There appear to be at least three approaches to this problem: 1) Correct Pu/U from phosphates for fractionation in order to calculate a whole rock value (BURNETT et al., 1982). This requires knowing the other actinide bearing phases and assuming that there is an equilibrium distribution of Pu and U between these phases and phosphate. 2) Assume that all chondritic merrillite may have fractionated Pu and U to the same degree (PELtAS ef al., 1979) which gives chrono~o~~~ significance to the relative Pu/U for merrillite from different meteorites. The Pu/U ratio in merrillite at a particular time is then a function of the 244Pu and 238U half-lives and the initial (assumed constant) Pu/U for merrillite. However, it has been noted that there is an inverse correlation between the Pu/U ratio and U content of merrillite grains from different meteorites (MOLD et al., 198 1) or within the same meteorite (KOTHARI and RAJAN, 1982) which

implies ative to tration. ignored

1999

that U has fractionated in varying degrees relan approximately constant initial Pu concenThis observation suggests that U should be and leads to the third approach. 3) Assume

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M. T. Murrell and D. S. Burnett

the initial “*Pu content, as opposed to Pu/U, of all chondritic merrillite to be constant (MOLD et al., t 98 11, Differences in measured Pu contents in merrillite from different chondrites are then interpreted as being due only to the decay of 244Pu and differences in the time of fission Xe or fission track retention among chondrites. This model also must assume that the formation of chondrhic materials (and presumably parent hodies) occurred within a time interval short compared to the 244Pu half-life (82 my). All models must deal with coexisting apatite, when present. Depending on the initial assumptions, the order of magnitude variation in merrillite Pu/U reported for a variety of chondrites can be interpreted as due to ‘%I decay and/or variable U or Pu fmctionation. The highest reported merrillite ratio (0.12) was observed in Naomi (H-5) (KIRSTEN et af.,1978). This large pU/U enrichment appears to be related to the low U content of the merrillite (52 ppb, CHEN and WASSERBURG, 1981a). In an effort to understand better the cause of this high ratio, we have determined the mineralogical distribution of U in Nadiabondi. In addition, we report here our U and Pu results for a group of unequilibrated H-chondrites. Unequilibrated chondrites have not been useful for Pu-Xe primarily due to a large trapped Xe component and the small phosphate grain size; however, these objects can provide additional info~ation on act&&de chemistry during and, possibly, preceding chondrite metamorphism, especially in regard to Pu-U fractionation in phosphates. EXPERIMENTAL The d~~bu~o~

and concentration of U in the samples

were determined using fission track radiography (BURN& and WOOLUM, 19831. Polished sections. mounted on Al posts, were prepared using 100% ethanol: Low U mica detectors were clamped to the samples which were then irradiated at the University of Missouri reactor. The Nadiabondi samples received a neutron flucnce of2.0 X 1Oi8rz/ cm2 and the unequilibrated H-chondrites received 3.6 X 10” n/cm? Uranium fission tracks mrded in the mica detectors were counted either optically or USiDg a scanning electron microscope (SEM) (for p b 10’ tracks/cm2). Absoiute U inanitions were determined using a @ass standard containing 368 + 6 ppb LJ. Fossil tracks were etched in Ca-phosphates from polished, m&radiated sections and grains mounts using 0.23% HNOs for 30 to 60 seconds. At longer etching tima these grains dissolved. A Nadiabondi grain mount prepared from the -7% merrillite separate described in ChEN and WASSERBURG (198la) was made available to us by G. J. Wasserburg. Replicas (CROZAZand TASKER, 198 t f of the etched phosphates were pnparcd using a&yl~&lulose film with methyl acetate as the solvent. After gold coating, the track mplicas were counted using the SEM. After the phosphate track counting was completed, tracks in olivine were etched. This was accomplished by first dissolving out the phosphate grains (Z-5 min. 0.25% H?+JO*Iand then im meauatina the holes with epoxy to hold th;*section toge&er>ur& the olivine WN etch (2-3 hours; KRISHNASWAMI et al.,1971). Followin the track counting in olivine, the section was IEimpregnated with epoxy and etched in boiling NaOH sofution (6 g NaOH, 4 ml H20) for 45 minutes &AL et al., 1968)to reveal tracks in pyroxene. The etched samples were

examined for tracks both in transmitted light and on the SEM. Mineral phases were identified using energy-dispersive X-ray analysis on the SEM. G-phosphate and chondrule compositions were determined using the Caltech ekctron microprobe. The meteorites examined in this way are listed in Table 1.

RESULTS

The U results for Ca-phosphates are summarized in Table 1. For comparison, the table begins with average U values for H-6 phosphates taken from PJZLLAS and STORZER (198 I). The phosphate abundance in Nadiatxmdiis variable (see PELL+LS et al., 1979) but, bass3on SEM X-ray identification, our sections average -0.5% (weight) phosphates with -85% menillite. Microprobe analyses of apatite indicate a high F/Cl with -3%F and only 0.6%Cl which is unusual as chloroapatite is most often observed in chondrites. As expected, apatite has the highest U content of any phase in Nadiabondi at 2.2 ppm. This result is in good agreement with the value of 2.1 ppm U obtained by PELw\S and STORZER (198 t1, but it is somewhat low compared to an average H-6 apatite value of 3.3 ppm. The U content of Nadiabondi merriliite at 49 f 2 ppb is quite knv compared with the H6 average, but this result is in excellent agreement with the value of 52 ppb obtained by isotopic dilution on a - 100% merrillite separate from Nadiabondi K.~HEN and WASSERBURG, 1981;). We found a meniIii&‘grain within a chondrule which had a higher U content of 95 + 13 ppb (not included in Tabk 1) which is more Eke the 80 ppb tl result for Nadiabndi merriUite report& by PRLLASand STORZER ( 198 1). However, in their examination of Nadiabondi, PELLA.5et of.. (1979) rqmted hvo very different specimens which have different phosphate abundances, apatite/merri&te ratios, and dif?&nt U contents in the phosphates; therefore, the U content of Nadiabondi merriliite will be sensitive to sampling. Track scanning random tips of our sections yields a whole rock U content of 11 ppb which is typical for ordinary chondrites. The contribution from Caphosphates to the whole rock U value for Nadiabondi ap peais to be at most - 17% with the remainder lying elsewhere, again typical of chondrites (JONES and BURNETT, 1979). The U content of Ca~phosphates in the unequilibrated H-chondrites is also quite low (Table 1) which, in combination with the sma&er average grain size, complicates their study. We did find three apatites in Dhajala which are -200 pm in size, but the remainder of the phosphate grains making up Table I are in the 20-50 rrn range. In most cases, phosphate grains axe found in the matrix in association with metal or troiiite, Such an association in DhajaIa is shown in Fig. I. Unfortunatefy, the merrillite @ains found in Ties-

lrr*rrorde

-w

Ti~dIirr

*.J’ n-3

n-3

200 f JO n.f .e) U.f.f)

iii

Cl? <3

D...

(1) 4) 6)

Actinides, P and REE in chondrites

FIG. 1. A large apatite grain (Ap) in Dhajala bounded by FeNi metal below and to the right and by troilite (Tr) above. In the middle of the apatite lies a troilite grain. Ph~phate associated with metal or troilite is typical for the meteorites of this study and chondrites in general. (Backscattered SEM photo).

2001

tents in mesostasis within a single U-rich chondrule. Uranium enrichment in mesostasis appears common to all chondrule types; however, there is some suggestion of an inverse relationship between U content and the extent of de~t~fi~tion or the Na and K content of the glass. With regard to a U whole rock material balance, the negligible U contribution of Ca-phosphates was noted in the preceding section. Assuming that chondrtdes, on the average, consist of - 10% glassy mesostasis which has a U content of - 100 ppb, then chond~l~ should contain U at CI levels. Consequently, comparable U ~on~ntmtions are also expected in the matrix and, although matrix regions large enough for analysis are hard to find, this appears to be ap proximately true; e.g., we find 9 ppb U in Sharps matrix. In addition to the general U enrichments in chondrule mesostasis just discussed, other more unusual U-enriched phases were located. Figure 4 shows a ZOOpm area in Dhajala (area directly above apatite) which contains up to 560 ppb U in some regions and also displays an unusual chemistry consisting of silicate phases (&ss?) high in Al and Ca along with major amounts of Cl, Ti, Cr, and Fe. In Tieschitz,

chitz were all less than 10 am in size; however, qualitatively their U content also appears very low. We found no apatite in Sharps or Tieschitz, the least equilibrated meteorites of this study (SEARS eral.,1980). The irradiated Bremerviirde section contains four merriilite grains of 30-50 pm in size, none of which correspond to any noticeable fission track localization. The limit given in Table 1 corresponds to one grain for which the location was carefully mapped out. AS can be seen from Table 1, the U contents of Ca-phosphates from unequilib~ted H-chondrites are much lower than both the H-6 average and Nadiabondi. The menillite values are upper limits; the actual U content may be near-zero. It appears from these data that the U content of phosphate in the Hthondrites is related to petrologic type: the mom ~uil~bmt~ the meteorite, the higher the U content of the phosphate. A similar trend among type 4-6 meteorites was noted previously by PELLAS and STORZER (1975). The abundances of C&phosphate in our samples of the unequilibmt~ chondrites are not known; however, given that the P content of equilibrated and unequilibrated chondrites are similar (VON MICHAELIS et al., 1969), the low U concentrations in phosphates cannot be explained by greater abundance of phosphate in the unequilibrated chondrites. Essentially none of the U in unequilibrated H-chondrites is found in Ca-phosphates.

U in chondmle mesastasis Despite its H-5 classification, Nadiabondi has numerous distinct chondrules which contain variable amounts of NaAl-Ca-rich interstitial material, probably devitrified glass. (In general, our microprobe analyses of Nadiabondi chondrules show glass-compositions which are in the range of those found by K~MURA and YACI (1980) in a study of an L-3 chondrite.) This interstitial material is enriched in U. An example of this en~chment is shown in Figs. 2a and 2b which compare a reflected light photo of a Nadia~ndi chondrule with a drawing of the U track pattern from the mica detector over the same region. As can be seen from the track pattern, the large patches of chondrule mesastasis (dark prey) are enriched in U ( 160 ppb). Uranium-rich chondrule mesostasis is common to ail the H-chondrites of this study. These results are summarized in Fig. 3 which shows the U content of a35 pm patches of mesostasis within individual chondrules from H-chondrites. The variations are significant, with numbers ranging from 40 to 600 ppb U. Average values (marked by arrows) are u 100 ppb. The Tieschitz points labeled T-l illustrate the range in U con-

FIG. 2. A reflected light photomicrograph of a typical chondrule from Nad~a~ndi is shown in 2a. The large patches of darker grey material in the center of the chondrule are mesostasis. 2b is a drawing of the corresponding U track pattern from the detector mica which shows the mesostasis to be enriched in U (160 ppb). Uranium-rich chondruk mesostasis is common to all the meteorites of this study.

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M. T. Murrell and D. S. Burnett

kb-

.

500-

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300-

.

200-

i lOO-

.

. .

.

. .

5040-

l

30ZO-

Uranium Content of Chondrule Mesostasis (- = AVERAGE)

to-

Nodiibandi

DhJo~o

Bremerviirde

Tieschiiz

Sharps

H3

H3

H3

H5

FIG. 3. The U content of mesostasis within individual chondrules. The averages of these analyses in each meteorite are denoted bv arrows. The Tieschitz points labeled T-l illustrate a range of U within a single chondrule.

small track stars representing U contents in the 650-2200 ppb range appear to correspond to - 1-2 pm Ca-phosphate (?) grains in association with ilmenite within a &o&rule fragment. In sharps, an olivineflow Ca-pyroxene chondrule was found which has bulk U of -700 ppb and small unidentified stars representing U up to 2300 ppb. In Nadiabondi, a few fragments of &rich pyroxene composition (as determined by electron microprobe analysis) were also found to be emiched in U (100-200 ppb); these objects are not obvious chondrule liagments. They are texturally homogeneous even to the SEM. Comparison with a thin section of Nadiabondi shows that the only objects with similar textures are very fine-grained (crypto-crystalline) chondrules. There are also some lower U areas observed in what initially appeared to be typical chondrule mesostasis but turned out to be crystalline SiOl, (BRIGHAM el d, 1982). These silicarich chondrules have bulk U contents in the 6-10 ppb range.

The majority of the U in these chondrules is located in trace amounts of glass, clinopyroxene, and in one case a glassmerrillite association. No U was detected in the silica phase itself.

244P~distribution To see if Pu, like U, was concentrated in chondrule mesostasis, olivine-mesostasis contacts and olivinepyroxene-phosphate contacts in polished thin sections were checked for excess tracks after etching (HAINES et al., 1975). Tracks were counted within 5 rm-wide bands along the length of the contacts. The expected *“Pu track density can be calculated the following equation: p = [2uPu]fRc where

with

(It

P= 2uPu fission track density [*%I] = The number of 2uPu atoms per milligram at the time of track retention /= Spontaneous fission branching ratio ( 1.25 X IO-’ for auPu) R = Mean rangecalculated for a single fission fragment in the Pu host phase (3.46 mg/cm’ in phosphate; 3.25 mg/cm* in glass) c = Track detection efficiency

FIG. 4. A low U (140 ppb) apatite grain (Ap) in Dhajaia indircctcontactwithahi%U(560ppb)bGaringphase(di-

nctly above). The bulk U content of the chondrule below is 270 Ppb. Over a 100 H scale, U appears to be unequilibrated. (Backscattered SEM photo).

The e&ciency factor is a property of the detector and the geometry of the detector relative to the Pu host phase. It in&ides the eBects of incomplete mgiatration of low energy hssion fragments, incomplete etching, and incomplete counting of small tracks. For a perfect track detector and 4~ geometry, c = 1. As de&@ it doea not include any e!Ibcts due to annealing For olivine grains in 4r contact ge5meti-y (5 pm wide strip), we measured c = 0.10 * .02 for a reactor-irradiated synthetic sample of olivine, suspended in a U-spiked silicate melt. In general, c may vary with crystaBographic orientation; we assume that the same distribution of olivine ctystal faces was sampled in the meteoritic and in the synthetic samples. This equation assumes that there is a well-defined time/temperature of fission track

2003

Actinides, P and REE in chondrites retention such that at later times/lower temperatures fossil fission tracks are recorded with the same efficiency as lab oratory-induced fresh tracks but no tracks are recorded earlier. To the extent that this model is inaccurate, the calculated [z”Pu] concentration will be an effective value. If the average U content of chondrule mesostasis is assumed to be 100 ppb and if Pu-U is unfractionated (Pu/U = 0.007), then the initial Pu content of chondrule glass would be 1.4 ppb, the *%I fission track density would then be 1.3 X 106/cmZ within a 5 pm-wide band along adjacent olivine contacts. This is an upper limit in the sense that negligible track annealing has been assumed. In Bremervbrde, Sharps, and Tieschitz many excellent olivine-mesostasis contacts were observed in etched sections with the results summarized in Table 2. Tracks were counted optically in transmitted light in olivine grains which were in direct contact with chondrule mesostasis. When the track density along the contacts is compared to that measured in the same olivine grain but well away from the contact (bulk or cosmic ray background track density), there appears to be no significant track excess. This is best illustrated in Sharps because of its lower cosmic ray track density, but in none of these meteorites were excess tracks ever observed along any olivine-mesostasis contact. Thus, it would appear that at the time of track retention in olivine, chondrule mesost&s contained ~0.1 ppb (based on Sharps), significantly less than expected on the basis of the measured U concentrations. Because of the small phosphate. grain size and the dissolution ofCa-phosphatewhen tracks are etched in olivine or pyroxene, adjacent grain track measurements for phosphate contacts in the unequilibrated H-chondrites are difficult. However, in Bremetvorde, olivine and pyroxene contacts with phosphates were located which survived etching. Although tracks along the contacts could be observed optically, the track densities could be. counted more reliably in the SEM (Table 3). All tracks within a 5 pm-wide band along the length of the phosphate contact were counted. Given the poor statistics and difficulty in counting, especially in pyroxene, the difference in olivine and pyroxene track density is not significant, and the uncertainties are probably larger than the statistical errors given in Table 3. Also, the total number of contacts and tracks observed was not great; nevertheless, there is a definite excess of tracks in the contact zone which indicates the presence of 244Puin Bremervorde phosphate. The olivine data of Tables 2 and 3 indicate that [Pu] mw/[Ptt],,tirrit < 0.01. This result is independent of track annealing. The partitioning ratio, Pu(menillite)/Pu(apatite), inferred from the olivine contacts is 3 * 1, which is in reasonable agreement with the value of -4.6 observed by F%LLAS and STORZER(1981) in phosphates from equilibrated H-chondrites. The interpretation of merrillite fossil track densities in terms of Pu concentrations is complicated, nevertheless, as discussed below, these data support the conclusion from the contact grains that merrillite is a major Pu host phase for the H-3 chondrites. Before etching for olivine and pyroxene, the phosphate grains themselves were first etched and examined for tracks

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tr.ck. (optical) within . 5 urnbsod slang the cont.ct. ")nu.bcr. in p.r.nrh.... indicsce nu.ber of chondrulca studied.

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tr.ek. riChin s 5 ,m band .low Lhc contsct. bkorrectad for comic rsy tr.ck.; co..ic r.y trsck. sr. sssurd to rsgister 2.4 ri... or. efficiently in pyr0x.n. al.. ia o1ivinc (P.11.. lf .l., 1983). +'.l". t.bu1.L.d is for gr.ia. with st 1es.t 1 Lrsck; upper lirit because there till b. som. olivine grsin. without trsck..

in three ways: 1) optically in transmitted light, 2) SEM surface pit counting, and 3) Au-coated plastic replicas using the SEM. Methods 1) and 3) allow discrimination of tracks of different length. Our results for fossil tracks in Ca-phosphates from St. Severin and three H-chondrites are shown in Table 4. We find that phosphates from the unequilibrated H-chondrites begin to dissolve after as little as 30-60 seconds in the 0.25% HNOr etch solution. But with such short etch times, the tracks are not fully developed. This effect is clearly seen with the St. Severin data. A St. Severin phosphate grain mount from the study Of WASSERBURG et a/., (1969) was reexamined by us (Table 4). Our optical result agrees well with the replica data and the literature values (CANTELAUBEet al., 1967; WASSERBURGet al., 1969). (The density of surface pits observed on the SEM is much higher and will be discussed later.) After the initial examination, the grain mount was repolished and re-etched. After 30 seconds of etching, tracks were quite faint and not suitable for counting, but after a 60-second etch, tracks were more easily observed and gave optical results consistent with the original etch. Nevertheless, the tracks after the 60-second etch are clearly fainter and shorter than those present with more extensive etching. Based on these observations, our 30-second etch track results for H-chondrite phosphates are lower limits, and while a 60-second etch yields somewhat higher track densities, tracks are probably not completely etched. The surface pit density we observed for Nadiabondi merrillite is 23 X IO’ t/cm2. Cosmic ray and “*U fission track contributions are negligible. This result is about 1.5 times that expected based on the Pu-Xe data (PELLASand STORZER, 1981). Because of annealing, track retention begins at a lower temperature than Xe retention. Thus, Pu contents from fission track densities are generally lower, i.e., [P~I],~~~;, < [PI&_,, (e.g. St. Severin, WASSERBURG et al., 1969). Either 2‘?u was higher, at the time of track retention, in menillite from our Nadiabondi specimen or not all of these surface etch pits are 2MP~fission tracks. PELLASand STORZER(198 1) report track densities of I I X IO’ t/cm* for Nadiabondi mer-

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3.

4.

Tack.

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In

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2004

M. 1. Murreif and D. S. Burnett

rillite; however, they suggest that this track density may be high due to dislocations. They report 9 X IO’ t/cm* (4~ corrected) for pyroxene grains in contact with m&l&e and, based on this value, estimate a track density for Nadiabondi merrillite, corrected for dislocations, of 7.5 X 10’ t/cm’. In our sample of Nadiabondi, we find that the majority of surface pits in merrillite are randomly distributed, however, there are some examples of etched holes which roughly line up in rows. AS theseare possiblyan indication of dislocations, our SEM results should be taken as an upper limit for the *%I fission track density, even though such aligned pits were not counted. Further, examination of Nadiabondi merritlite grains in transmitted l&t or ofnplicas indicates that the great majority of these surface etch pits are quite shallow (< 1pm); however, this also is true of our St. Severin data (compareoptical and SEM data in Table 4). It should be made clear at this point that the presence of 2uPu in Nadiabondi phosphate is not in question here as the fission Xe data clearly show that Nadiabondi merrillite had a ?%I content of 19 ppb which is similar to that of merrillite from other chondrites. It is the source of the short “tracks” which is uncertain. The track resulta for - 30- 100 pm phosphatea from Bremervtirde and Tieschitz are shown in Table 4. On the whole our sections have typical unequilibratti chondrite textures. The phosphate gtains studied are not from obvious clasts of diffennt petrolo8ic types, Mean merrillite surf&e pit densities are 6-9 X IO’ cm-l; however, the optical and repIica results indicate that the majority of the tracks are short, similiar to what we observe for St. Severin and Nadiabondi. Because of the hi surf&e pit density and the fact fhat many of the phosphate @as are opaque due to the preferential phosphate-metal-sul&e association, opt&I track counting is difficult even for longer tracks. Consequently, the optical track densities for the H-chondrites in Table 4 are relatively uncertain, but the short length distribution of the surface pits is clearly demonstrated by the iarge d&remxs in SEM and optical counts. As discussed above, the short track lengths must in part represent incomplete etching. For St. Severin and Nadiabondi, many of the surface pits could be annealed 2u~ fission tracka due to slow cooling (~ELLAS and STORZER, 1981). Extension of this interpretation to Bremervorde and Tieschitz requires further discussion. In the H-3 chondrites, there is no evidence that these pits are dislocations as their orientations and distribution appear totally random. Contributions of fission tracks from spm~taneous U fission are near-zero; and the cosmic ray track densities in Bremerv&de and Tieschitz chondrules (see “bulk” Table 2) are also low. In addition, the ~~~(~)~ partition coefficient in Bremervbde inferredfrom the SEM surf&x pits is 4.5 which agrees with the contact value and the work of Peiles and Stoner ( 198 1) and would support the idea that the pits are related to the ‘%J content. A pos&bIe alternative to 2?u fission tracks for the phosphate surface pits is spallation recoils (CROZAZand TASKER, 198 1). The recoil track production rates are not well known but could be important. However for Angra dos R.&s, which had a 55 my exposure age (LUGMAIR and MART& i977), much longer than most chondxites, the merrillite track density agrees with that expected for fission tracks fiom fddspar and pyroxene contacts and with the track density calculated from the z”~ fission Xe (STORZERand PELLA.5,1977’). Thus, significantnumbers of merrillite spallation recoils are not observedfor Angra dos Reis, posaibiy due to preferential annealing (PF,LLAS,private commun.). Unless the H-3 chondrites studied have had significantly lower oribital temperatures, spallation recoils cannot account for the surfsce pit densities given in Table 4. The surfacepit densities appear to be 2”Pu fission tracks. The average pit density is equivalent to - 10 ppb Fu, in agnement with that which would be calculated from the olivine-~oxene-ph~p~te contact data (discussed above). Thus, merrillite is a major host phase for Pu in unequili-

brated H chondrites. The 10 ppb is a lower limit to the Pu concentration due to incomplete track etching or, possibly, to some track annealing, Either of these effects could explain why only short tracks are observed. Thus, it is quite possible that merrillite contains all the Pu in unequilibrated chondrites. As seen in Table 2, the cosmic ray track density observed in olivine from Breme&rde chondrules is generally ~8 X I@ f/cm2. OccaaionaIly, however, track-rich chondrules were observed which appear similar to those Seen in the gasrich meteorites. These chondrules have high track densities at the surface with steep 8radienta towards the interior indicatin8 solar flare irradiation. There is evidence that Bremerv&de is a gas-rich meteorite (HEYMANN and MAZBR, 1968);so the observation of track-rich chondrules may not be surprisin8. The track-rich chondndes raise the possibility that some merrillite track densities could be h&h due to solar tlare in&&on. However, fm phosphates ase in chondrules and, most importantly, it seems very unnzasonable that all of the h&h merrillite track densities (speciftcally the surface pita) could be due to solar flare irradiation. This would require solar Bare irradiation for alf phosphates but only a few chondruies. DISCUSSION In the following discussion we assume that H-6 chondrites were derived by thermal metamorphism from material having the actinide and REE distribution that we observe/infer for H-3 chondrites. AIthough plausible, this is an assumption because, although the pre-metamorphism H-6 parent material unqu~tionabiy had chondritic texture (i.e., chondrules), one cannot prove that the detailed mineral chemistry discussed here was the same.

The chemistry of the actinides and lanthanides in ordinary chondzites is closely coupled to that of phos-

phorus. A strong preferential association of Ca-phosphates (both apatite and merrillite) with metal or FeS grains is present in all ordinary chondrites we have examined and has also been noted by other authors (MANHESet al., 1978; JONESand BURNETT, 1979; HEUSER et al., 1980; RAMBALDI et al., 1980; RAMBALDI and WASSON, 198 1; WOOLUM and BURNETT, 198 I; RAMBALDI and RAJAN, 1982). This and the occurrence of small amounts of schreibersite @AMDOHR, quoted in FUCHS, 1968; RAMBALDI and WASSON, 198 1) in unequilibrated chondrites and P-rich metaf in C-chondrites (GROSSMAN and OLSEN, 1974; GROSSMANet al., 1979) are strong arguments that “initially” P was alloyed in metal, or possibly sulfide phases. However, the preponderance of chondritic phosphors is now in the oxidized form. This is URquestionably true in the equilibrated chondrites; our observations and those of RAMBALDI (private comnun.) indicate that this is also true of the unequilibrated chondrites. Equilibrium condensation caiculations for a gas of solar composition predict P condensation in FeNi metal (GR~SSMAH and OL.%N, 1974; WAI and WASSON, 1977) or in schreibersite (FEGLEY and LEWIS, 1980). The Ca-phosphates presumably

Actinides, P and REE in chondrites formed sub~quently by oxidation, as was suggested originally for mesosiderites by FUCHS ( 1968) and more recently by HARLOW et al. (1982) and suggested for chondrites by AHRENS (1970). The oxidation of Prich metal to form phosphates has also been discussed by FRIEL and GOLDSTEIN (1977) for lunar rocks. RAMBALDIand RAJAN (1982) report relatively high Ca in FeS from unequilibrated chondrites, so conceivably an external Ca source may not be needed. However, glass and pyroxene can also provide the required CaO. Oxygen can be provided by Fe0 with the formation of additional Fe metal. Local depletion of Fe0 and CaO would tend to produce an SiOZ excess which would then combine with olivine to form pyroxene, and this may explain the lower frequency of phosphate-olivine binary grains compared to phosphate-pyroxene (PELLASand STORZER, 198 1; PELLAS, 198 1). (Olivine-phosphate contacts in Bremervijrde were also more difficult to find than phosphate-pyroxene.) Alternatively, if oxidation occurred in the solar nebula (FEGLEY and LEWIS, 1980), HZ0 could provide the required oxygen. As pointed out to us by H. Palme, it is interesting to compare the proposed behavior of P with that observed for W. During nebular condensation, W is predicted to condense as metal at high temperatures (GROSSMAN,1973; PALMEand WLOTZKA, 1976), and W is observed in refractory metal in Ca-Al-rich inclusions from Allende (PALME and WLOTZKA, 1976; WARK and LOVERING, 1978. In type 3 ordinary chond&es, the majority of the W is found in silicate (RAMBALDIet al., 1979). Because of the increase in the bulk W/h between H and LL chondrites, RAMBALDIel al. (1979) proposed that oxidized W was present at the time of metal-silicate fractionation (LARIMERand ANDERS, 1970). As was just discussed, P is also predicted to initially condense in a reduced form to be later oxidized. The P/b ratio also increases from H to LL chondrites while the P/W ratio holds relatively constant (data from ‘MASON, 1979), which suggests that oxidized P was also present at the time of metal-silicate fractionation. Thus, the oxidation of phosphorus occurred before or during the accretion of the H-chondrite parent body(s). However, phosphate-metal-sulfide grains are a preferential association in all ordinary chondrites. Even if oxidized, why was not more of the phosphate lost with the metal than indicated by the 14% difference between the P content of L and H chondrites (VON MICHAELISet al., 1969)? It may be that (a) the primary association is phosphate-sulfide or (b) the form of the reduced phosphors was schreibersite, not P dissolved in metal, or (c) the phosphate-metal grains were preferentially retained. All of these alternatives are interesting. In the equilibrated chondrites, W is associated with the metal fraction (RAMBALDI et al., 1979) which indicates that during me~mo~hism W pa~itioned into metal. This is the opposite of the behavior for P and probably requires that, during metamorphism, .fO, became reducing enough to reduce W but not

200s

P. From the~odynamic data for the redox reactions of W and P such a situation appears reasonable. It thus appears likely that merrillite is a primary (as opposed to metamorphic) mineral in chondrites. The situation with apatite is less clear. The phosphates are not inert during me~mo~hism, however. There is a pronounced increase in grain size between equilibrated and unequilibrated chondrites.

B. W chemistry in chondrites Our results show that in the H-3 chondrites, chondrule mesostasis is the only conspicuous U host phase; phosphates have negligible U concentrations (~20 ppb). High U “glass” is found in contact with low U phosphates (Fig. 4f, indicting a Iack of U migration on a IOO-micron scale. It has not yet been proved, but it is likely that Th is similarly distributed. In equilibrated chondrites, phosphates are known to be major U, Th host phases, although JONES and BURNETT (1979) showed that ~30% of the U in St. Severin could be accounted for by phosphates, with the remainder being interstitial. During metamorphism, U has migrated out of chondrule glass into phosphates reaching 200-300 ppb levels in merrillite in type 6 chondrites. Nadiabondi represents an intermediate state with -50 ppb U in merrillite, and approximately 17% of the U in phosphates. It is striking that merrillite U contents in equilibrated chondrites never rise above about 300 ppb; whereas much higher concentrations, >800 ppb, should be possible, even allowing for the presence of apatite. (Shaw has higher merrihite U con~n~tions (KIRSTEN et al., 1978; HEUSERet al., 1980); but, because of the complications due to partial melting, we have not included Shaw in any of our discussions.) The observed, primarily interstitial, distribution in equilibrated chondrites (JONES and BURNETT, 1979) is probably not in equilib~um. (An ~u~b~urn model is discussed by JONES and BURNETT (1979) and BURNETT et al. (1982) in which the “interstitial” U is contained in submicron apatite grains, undetectable by fission track radiography. However, the number density of these grains is required to be very large, and a high magnification SEM search on the surfaces of crushed St. Severin material has failed to find the required density of small apatites.) It may be that chondritic phosphate U concentrations are dynamically controlled by peak metamorphic conditions such that, had there been higher tem~ratures andfor longer times at peak temperature, all of the U would be in phosphates. However, it is still possible that over some period, where both phosphates and primary (not devitrified) chondr-ule glass co-existed, the U distribution reflected equilibrium partitioning between these phases. Here the assumptions are that chondrule glass behaved as a liquid and that phosphate and glass continued to partition U down to some minimum temperature. Below this temperature, phosphate and glass were no longer able

2006

M. T. Murrell and D. S. Burnett

to “communicate” and the U contents of the phosphates were “frozen.” Subsequently, as glass continued to devitrify, the remaining U was forced to local interstitial sites without affecting the phosphate U concentration. This “transient equilibrium” model is creditable to the extent that, for St. Severin, the calculated ratio of Th/U between merrillite and “glass” (by model assumption, everything in the total rock except phosphates) is 2.6-4 (depending on what apatite concentration is assumed) which is similar to the ratio of laboratory whitlockite/silicate liquid partition coefficients, 2.4 (BENJAMIN et al., 1980). Nadiabondi can be understood in this model since the U concentration ratio, merrillite/mesostasis, is -0.5, agreeing with the laboratory U whitlockite/liquid partition coefficient. Different degrees of glass devittification prior to “U freeze-out” could give rise to variations in U merrillite concentrations between chondrites. Alternatively, these variations could represent temperature variations of the U glass-men-ill&e partition coefficient. The laboratory whitlockite/liquid partition coefficient of Pu is 3.6, but there is much more glass than phosphate; thus, to account for apparently complete concentration of Pu in phosphates (Section C) with this model, it is necessary to assume that Pu is always in equilibrium, i.e., that it is much more mobile than U and continues to equilibrate until the majority of chondrule glass has been devitrified. This is probably unreasonable, and it is probably best to regard phosphate U concentrations as dynamically controlled and not easy to interpret quantitatively. Coupling our observations with the small differences in 206Pb-z”‘Pb ages between St. Severin merrillite, Angra dos Reis, and an Allende coarse-grained Ca-Al-rich inclusion (CHEN and WASSERBURG, I98 1b), we conclude, essentially independent of all models, that U redistribution on a mm and smaller scale possibly ceased very early in the metamorphic history of equilibrated chondrites. It is possible that the U concentration in individual phosphates is a qualitative measure of peak metamorphic temperature.

C. Pu chemistry in chondrites Menillite appears to be the dominant host phase for Pu in equilibrated chondrites (see also ALAERTS eI al., 1979; JONES, 1982). (Apatite is much less important; PELLASand STORZER, 198 1.) This is best documented for St. Severin: for 26 ppb z44Pu in merrillite (WASSERBURGet al., 1969; LEWIS, 1975), negligible Proportions of apatite relative to whit&kite, and a merrillite abundance of 5 X 10d3 (from the total P content, ORCEL~~al., 1967; JAROSEWICHand MASON, 1969; MOLLER, 1968) a total rock Pu content of 0. I 3 ppb is calculated on the assumption of no other significant host phases. From the new total rock Pu/U = 0.007 +- 2 (HUDSON et al., 1983) and a U content of 11 +- 3 ppb, today (based on data compiled in JONES

and BURNETT, 1979 and HUDSON er al.), 0.14 + .05 ppb is calculated. Also, using the recent HUDXIN e/ al. remeasurement of *@Puin St. Severin, a Pu material balance based on the ‘36Xecontent of their merrillite separate is again consistent with all Pu in merrillite. However, HUDSON et al. find that phosphate-depleted separates (residues left after merrillite separation and acid leaching) still contain about 50% of the bulk [‘36Xe]h, but it is not known what fraction of phosphate may have survived the separation procedure, perhaps protected from the acid by a metal or silicate layer. Thus, 50% should be regarded as the minimum fraction of Pu in merrillite for St. Severin. Based on all the available data (compare JONES, 1982), the majority of the Pu in St. Severin appears to be concentrated in merrillite; however, given the relatively large uncertainties in the material balance, a significant fraction of the Pu, say 30% could in fact be elsewhere. Because of the previously discussed complications with the fossil track data, we are unable to calculate accurately the Pu concentrations in phosphates from the H-3 chondrites; however, Pu concentrations of at least 10 ppb are indicated. However, it is not necessary to interpret absolute track densities in phosphates. Based on the tracks in olivine and pyroxene contacts with phosphate and with chondrite mesostasis in Bremervbrde, we infer that the Pu concentration is at least 100 times higher in menillite than in mesostasis. Similarly, 244Pu was not detected in Tieschitz and Sharps mesostasis. Unless there are significant Pu host phases which we have missed, we must conclude that most of the Pu in the H-3 chondrites is contained in phosphates. Fission Xe data for Nadiabondi merrillite (summarized in Table 5) indicate a 2”Pu content close to St. Severin, showing that for the intermediate stage of metamorphism rep resented by Nadiabondi, 2”Pu is also highly, if not totally, concentrated in merrillite, although U is only - ‘/athe level it achievesin the equilibrated chondrites. Thus, Pu is concentrated in phosphates in even the least metamorphosed chondrites; whereas U migrates into phosphates relatively slowly as chondrule glass devitrifies during chondrite metamorphism. There is Pu-U fractionation in all chondrites, but this reaches an extreme degree in unequilibrated chondrites.

D. Rare earth element chemistry in chondrites Table 5 gives Nd concentrations for merrillite separates from St. Severin and Nadiabondi (CHEN and WASSERBURG, 198 1a). Comparison with measured or estimated whole rock Nd concentrations leads to the conclusion that, like Pu, the Nd, and possibly ah other trivalent light REE, in equilibrated chondrites are totally concentrated in phosphates (MASON and GRAHAM, 1970; JONES, 1982). HASKIN et al. ( 197 1) report surprisingly high light REE concentrations in an olivine separate from Bruderheim (L-6) with -70% of the Ia in this separate. We find it unrea-

Actinides, P and REE in chondrites

St.

Nd (Pd

eu (PPb) U (ppb) PulNd (x lo41

Severin

l&6*) 26b) 276d) I.8

Nadi.bondi

LOW) 19c) 49 1.8

2007

high Sm chondrules, no phosphates are found within the chondrules themselves. However, small spots (usually 51 p) which give X-ray spectra consistent with menillite are found in matrix material adhering to these chondrules. The volume percent of menillite due to these areas is negligible. The amount and chemistry of the mesostasis varies among these chondrules, but this phase is rich in Ca and Al and low in alkalis. The presence of significant amounts of high Ca pyroxene is common to all three and relatively uncommon for chondrules in general. Section 20, a low Sm chondrule, probably contains merrillite as it has submicron areas of Ca and P located within glass regions. This glass is enriched in the volatiles Na, K, and Cl; in our U studies, glass of such composition was generally observed to be lower in U. No appreciable amount of phosphate minerals are found in any of these four chondrules. It, therefore, appears very unlikely that the REE carrier in these chondrules is phosphate; glass or possibly clinopyroxene appear more likely. It seems likely that in the unequilibrated chon-

that olivine is the actual light REE carrier phase; it is more likely that the olivine separate contained adhering crystallized relict chondrule mesostasis containing high REE concentrations (see below). The Pu/Nd ratios indicated by Table 5 for St. Severin and Nadiabondi merrillite are the same, and they are also similar to those from the achondrite Angra dos Reis, melilite from an Allende coarse-grained Ca-Alrich inclusion, and the Juvinas eucrite (MARTI et al.. drites the REE are still in the chondrules with U, while 1977; JONES, 1982). These observations, coupled with Pu is already in &phosphates. For the temperature/ the recognition that Pu should be trivalent under meteoritic conditions (BENJAMINet al., 1978; BOYNTON, time conditions experienced by more equilibrated chondrites such as Nadiabondi or St. Severin, the REE 1978) provide the arguments for Pu-light REE cohave followed Pu into phosphate, but have migrated herence and Pu-Nd chronology. However, REE analmore rapidly than U. In a study of chondrules from yses on separated chondrules indicate a very different Richardton (H-5) EVENSONet al. (1979) report five distribution for the REE in unequilibrated chondrites of nine chondrules to be depleted in REE relative to than we have inferred for Pu. CI values. The REE patterns in four of these five deChondrules separated from unequilibrated chonpleted chondrules have positive Eu anomalies and an drites generally show enrichments in the REE over Cl enrichment in the heavy REE. These authors suggest levels by factors of up to 2 (GOODING et al., 1980; GOODINC and FUKUOKA, 1982; GROSSMAN and that this REE pattern is complementary to that found in chondritic phosphate and indicates REE migration WASSON, 1982, 1983). The mineralogical siting of the REE in the chondrules is unknown; however, GROSS- across chondrule boundaries into phosphates in agreement with our evaluation above. The REE contents MAN and WASSON (1982, 1983) and GOODING and of the remaining four chondrules of the EVENSONet FUKUOKA (1982) found REE abundances in chonal. study were either at or above Cl values. The authors drules correlated with major element refractory lithinterpret this result as possibly being due to phosphate ophiles, such as Ca and Al, which suggests that the grains in these chondrules; however, they analyzed a REE may be found with U in chondrule glass, since fused sample of one of the high REE chondrules using this is the major Ca and Al host phase in chondrules. an electron microprobe and found no phosphorus. It Indeed, GOODING and FUKUOKA find the total REE would appear that the REE are in relict mesostasis in content in porphyritic chondrules to be weakly to these enriched chondrules and that migration from moderately correlated with the abundance of chondrule chondrule to phosphate did not go to completion in mesostasis. An alternative location for the REE within Richardton. In their study of REE elements in unchondrules might be in Ca-phosphates; however, our equilibrated chondrules, G~ODING and F~JKUOKA observations indicate few such phosphates. Also, (1982) find both positive and negative Eu anomalies phosphates within chondrules must be rare given the among chondrules from the same meteorite; however, association of metal and phosphate in the matrix disthe mean patterns from chondrules from more equilcussed in Section A and the general depletion of metal ibrated meteorites have small positive Eu anomalies (e.g., ARAT-~ALABand WASSON, 1980) and probably and enrichments in the heavy REE. These authors also P (EVENSENel al., 1979; Lux ef al., 198 1) in chonsuggest that such patterns may indicate the redistridrules. GOODING and FUKUOKA find the mesostasis bution of REE during equilibration. The above disportions of REE-rich porphyritic chondrules to be encussion indicates that during metamorphism the acriched in Ca but not P. It thus appears that the majority tinides and REE approach equilibrium concentrations of the Ca-phosphates lie outside the chondrules. in Ca-phosphate in the following order: Pu > REE In order to study possible REE carriers in chonz Th 2 l-J. 11 would appear that large Pu-REE fracdrules, we obtained four thin sections (sample numbers: tionation might be expected in chondrule or phosphate 7, 20, 25, 36) of Chainpur chondrules from the study by GROSSMANand WASSON (1982). In 7,25, and 36, separates from the unequilibrated chondrites. sonable

2008

M. T. Murreliand D. S. Burnett

BENJAMIN et al. (1978) proposed a “bracketing theorem” in that meteoritic materials which showed unfractionated (relative to CI chondrites) Nd/U (or Sm/U) and Th/U ratios would have unfractionated Pu/U also. This postulate was based on crystal-liquid partitioning studies which indicated that Pu was “chemically intermediate” between U or Th and middle-weight REE such as Nd or Sm. Our data on unequilibxated chondrites seem to violate this theorem and, in general, may represent a serious complication for *“PII chronology, as discussed in the following sections.

tact track densities. Once devitrification has occurred, it is conceivable that grain boundary diffusion of Pu and REE would be rapid and preferential from the original glass-olivine interface. However, significant REE depletion in chondrules should result which does not appear to be observed. Devitrification textures are present in some of the chondrules studied; however, in Sharps a special effort was made to seek out clear glass-olivine contacts. Such cbondrules showed no significant excess track density at the contact with quantitative limits in agreement with those given in Table 2. 3) Although REE and Pu studies have not been made on the same chondrules (a study which should be done for completeness), we consider F. Alternative interpretations it very unlikely that we have, by accident, selected chondrules for which both the trivalent REE and Pu Our results appear to indicate that Pu may have a relatively unique cosmochemistry. This is an imhave migrated into phosphates. A comparison for the same meteorite is possible for Tieschitz chondrules portant result, although unfortunately a complication for *“‘PII chronology; consequently, it is im(GOODING et al., 1980) which exhibit REE enrichments on the average. A more plausible type of sampodnt to discuss possible ways that we may have pling bias might result from the fact that we select misinterpreted our data such that the bulk of the chondrules with clear olivine and abundant olivine*?u in H-3 chondrites is in chondrule mesostasis mesostasis contacts. There are many chondrules along with the REE and U. Three possibilities deserve which have an obvious formation sequence: olivinediscussion, although all appear unlikely. 1) A factor of 3 or larger attenuation in *%I fission tracks in pyroxene-mesostasis with primarily pyroxene-mesostasis contacts. There is also a tendency for Ca-rich, olivine due to thermal annealing could leave tracks in mesostasis-olivine contacts unrecognizable above alkali-poor glass to have primarily pyroxene contacts and to be more devitrified. If these chondrites have cosmic ray background. However, to accept this alternative it is necessary that the identification of surPu-rich mesostasis, they would be missed in our face pits in phosphates as fission tracks be incorrect study. In summary, although there are conceivable alterand that the regular trend in cooling rates with petrologic type for H chondrites established by F’ELLAS natives (particularly 3), we believe these are less plauand STORZER(198 1) not apply to the H-3 chondrites sible than the interpretation that Pu and the light studied here. The Pellas-Storzer trend would indicate trivalent REE are sited differently in H-3 chondrites. rapid cooling and negligible track annealing for H-3 We assume this in the remainder of the discussion. chondrites. If these two conditions are met, olivine track annealing is possible. For the metallographic G. Mechanism of Pu-REE fractionation cooling rate of 1 deg/my obtained by WOOD (1967) From a geochemical point of view, the coherence for Ties&i& track annealing in olivine by a factor of Pu with the light REE is plausible because two of S-30 would be expected. The relatively high obelements of the same valence and ionic size might be served ph+phate track density for Ties&& is not expected to behave identically. Cosmochemical procompatible with the low metallographic cooling rate, cesses offer more variety, e.g., large irregular fractiona result observed in many other 2uPu track studies ations of the heavy REE, presumably due to small (see e.g., PELLAS and STORZER,1981; CROZAZ and volatility differences, are observed in fine-grained CaTASKER, 1981; PELLAS et al., 1983). Our BremerAl-rich inclusions from Allende (BOYNTON, 1975; vi)rde data, given the relative large errors, are comDAVISand GRO.%SMAN, 1979).Trivalent Pu has a conpatible with cooling rates 23 degjmy. Even if olivine densation temperature within the range of the light track annealing were important for Sharps and TiesREE (BOYNTON, 1978); so this does not appear to be chitz, a special explanation would be necessary for BremervGrde where olivine contacts give a viable mechanism of Pu-REE fractionation, although a re-examination of this question may be appropriate. Pu(me%Mtasis)/Pu(menillite) ~0.0 1 directly, independent of olivine track annealing. Alternatively, if There is also no obvious explanation of the Pu-P corit is assumed that phosphates and chondrules had relation in terms of condensation processes. In general, we have no good explanation for the observed Puindependent thermal histories in all three meteorites, REE chemical differences. There are two, rather specolivine track annealing is possible. 2) Glass devitriulative, mechanisms worth further consideration: 1) fication could, in principle, selectively exclude Pu Pu was “initially” in chondrule glass along with U from olivine contacts, although one would expect and the REE, but, even with small degrees of metaonly micron scale migration of Pu to local grain morphism, migrated more rapidly to phosphates than boundaries, which should not grossly affect the con-

Actinides. P and REE in chondrltes the REE. The three H-3 chondrites, Bremervorde, Sharps and Ties&&z, which we studied, generahy cover the range of metamo~hi~ intensity proposed for type 3 chondrites (see far example SEARSet al., 1980; HUSS et al., 1981); however, it is possible that more primitive chondrites will show Pu in chondrule glass; this should be checked. 2) P was siderophile or, conceivably, chalcophile in the precursor materials of the H-3 chondrites. Pu is strongly correlated with P. Could Pu possibly be siderophile also? JONES and BURNETT ( 1980) showed that, under highly reducing conditions, Pu and U, but not Sm, could be reduced from a silicate liquid, alloying with Pt-Si liquid metal alloys. This alloying did not occur with F&i liquids. Possible actinide-REE ~ctiona~ons via refractory noble element alloys have recently been discussed by FEGLEY and KORNACKI (1983). The Pt-metal grains found in coarse-grained, Ca-Al-rich inclusions (WARK and LOVERING, 1978; EL GORESY etal., 1978; BLANDER et al,, 1980) could have been formed among the precursor materials of H chondrites during a high temperature stage. Pu may have alloyed with the P&metals but not U because of the greater volatility of U (BOYNTON, 1978). Subsequently, at lower temperatures the Pt-metals may have served as nucleation or recrystallization sites for the other siderophile elements including P and S. The Pt group elements are concentrated in Fe-Ni metals in all ordinary chondrites (RAMBALDIet al., 1978). As the alloys became more Fe-Ni rich, Pu would presumably be less stable but, if phosphate formation occurred more or less simultaneously, then the Pu would be stabilized (see Section A). There is no evidence that Pu is concentrated in Pt-metal grams in Ca-Al-rich inclusions. SHIRCK(1974) showed that both meiilite and chnopyroxene in an Allende type B inclusion contained ‘?u fission tracks. (However, the dis~bution was quite irregular and, conceivably, could represent Pu in the process of redistributing from P&metals into silicate phases during secondary alteration processes.) Also, fine-grained Allende inclusions are depleted (relative to CI chondrites) in refractory metals (GROSSMAN and GANAPATHY, 1976); yet the 2”Pu contents of fine-grained inclusions appear to be about 50% higher than that observed in a coarse-grained inclusion (see HUDSON et al., 1983). Therefore, model 2) is quite speculative, but we consider it more plausible than 1).

2009

depend on proximity of chondrule glass, local chemistry, etc. consequently, we feei that it will be very difficult to obtain quanti~tive chronolo~~l information from chondritic merrillite Pu/U ratios (compare PELtAser al., 1979; KOTHARI and RAJAN, l982). It should be emphasized that the “adjacent grain” fission track studies of PELLASand STORZER(e.g. 198 1) are based on Pu, not Pu/U, and require only that the merrillite Pu concentrations vary solely due to radioactive decay. This appears valid for the meteorites of this study. In particular, our results support a major assumption in the Pellas-Storzer studies, viz. that the 244Pu concentrations in chondritic merriliite are estabhshed early in the thermal history prior to track re~stmtion in adjacent mineral grains. I. Pu-Nd chronology?

The basic assumption of Pu-Nd chronology is that, because of identical trivalent states and similar ionic sizes, any variation in the relative abundance of 244Pu to a trivalent light rare earth element (Ce, Pr, Nd) will be due only to **Pu decay prior to fission Xe or fission track retention. For chondrites with a degree of me~mo~hism somewhat greater than Nadiabondi, it appears that both Pu and the light REE have totally concentrated in phosphates. However, unlike Pu, Nd must move (or is moving more slowly than Pu) from chondrule glass to phosphate during chondrite metamorphism. It is always possible that this migration will not be complete and that variations in phosphate Pu/Nd will not be solely due to 244Pu decay. However, the situation is asymmetric: a higher Pu/Nd (e.g., relative to St. Severin) would definitely be suspicious, but a lower Pu/Nd might still indicate an age difference. HUDSON et al. (1983) have questioned Pu-Nd coherence, arguing that the - 50% spread in the ‘“Pu/Nd data among meteorites is larger than that expected from analytical uncertainty, sampling heterogeneity, or differences in formation ages (compare JONES, 1982). Both Pu and Nd are somewhat lower in Nadiabondi merrillite than in St. Severin (although the Pu/Nd ratio is identical; see Table 5). This may reflect the generally lower (- 14%) P and higher (-8%) refractory element contents of L or LL vs. H-chondrites (data from MASON, 1979). However, the P content of St. Severin (ORCEL et al., 1967; JAROSEWICH and MASON, 1969; MILLER, 1968) isin fact nearly the same (-3% lower) as the H. 244Pu/z38U~hrono~ogy~ H-chondrite average, thus the relatively small H-LL differences implied by the average group abundances With the available information, it appears clear that tabulated by Mason may not be applicable. The lower 244Pu/238U measured in chondritic merrillite is not Pu and Nd in Nadiabondi merrillite may also be due solely a function of radioactive decay, and that the to some Pu and Nd residing in chondrule glass, or Pu/U ratio will be sensitive to the details of the thermal it may all just be a numerical coincidence reflecting history, most likely the peak metamorphic temperature. Given the reported variations in Pu/U (KOTHARI incomplete Nd incorporation and a slightly younger age for Nadiabondi. An additional complication is and RA.tAN, 1982) for phosphate grains within a single the presence of two phosphate minerals, apatite and chondrite (due almost entirely to U variations in this merrillite, because there is evidence that the particase), the Pu/U value of a particular grain may also

2010

M. T. Murrell and D. S. Burnett

tioning of Pu and Nd between these two phases is not the same (EBIHARAand HONDA, 1982). Overall, for ordinary cho~r~tes, it is not ciear that anything is gained by normalizing Pu data to Nd or any other REE. As discmsed in the next section, any available chronological information may be contained in the Pu ~o~e~tr~~on in phosphates.

Pu concentrolions

20

10

Although not completely es~b~ish~ for all unequilibrated chondrites by our results, our best interpretation is that Pu is totally concentrated in phosphates in all ordinary chondrites. This suggests that the abundance ratio best preserved during the formation and metamorphism of these meteorites is Pu/ P and that variations in Pu/P among ordinary chondrites may represent differences in phosphate fission Xe or fission track retention times. This is best regarded as a hypothesis to be tested. The alternative is that there are variations in initial Pu/P among ordinary chondrites which have nothing to do with age differences. An important point to be emphasized is that conservation of Pu/P is clearly not valid for meteorites in general but only possibly for ordinary chondrites. For example, Ca-Al-rich incfusions are clearly enriched in Pu (PODOSEK and LEWIS, 1972; SHIRCK, 1974; DROZD et al., 1977) but not in P, presumably reflecting the volatility differences of these two elements. An important conclusion is suggested: There is no “universal” reference element for all meteorites for which “‘Pu data can be normalized to correct for chemical fra~tionations, permitting unambi~o~ 244Puchro~o~o~. The hypothesis of Pu/P chronology can be tested against literature data for Pu concentrations in chondritic phosphates (Fig. 5) (KIKSTENet al., 1978; PELLAS, 198 1; PALME etal., 198 I ). Data have been plotted for fission Xe measurements or for fission track data from meteorites which show no evidence for track annealing. The presence of both apatite and merrillite in variable proportions is a problem, both conceptually and analytically, because Pu is peferentially concentrated in merrillite. We find no apatite in Sharps or Tieschitz, the feast ~~~~ meteorites in our study; RAMBALDI and RAJAN (1982) find none in Chainpur (LL-3) and Kymka (L-3). Although more study is required for a firm conclusion, it appears that apatite may be formed from merrillite during chondritic metamorphism and that this is accompanied by Pu concentration in merriilite (and preferential U concentration in apatite). Independent of specific models, the phosphorus contents among I-i or L chondrites are constant (within +S%) although the average Ii chondrite concentration is about 14% higher than the L value (VON MICHAELIS etal., 1969). This un~fo~ity is hard to understand if merriliite and apatite were formed separately and mixed in diffmnt proportions into different chondrite parent materials. Conse-

StSeverin fLL6)

l-w

~enow~4~

t--.-i k-+-i &w--i

Ambapur Nagto lH61 Guare?fa(tl6~

I--W k-e-l

Estacodo (H6)

t

Fe& iL6)

I

Leedey R.61

=

4

-

t t

Alfianella (t6)

Mocs (16)

k-++

Tothlith (16)

l-k+

lillaberi (I.61

Phosphates

30 I

f

Nadiabondi H5)

J. Pu/P chronology?

in Chondritic

_

i

f&ion fission

W

Xe *

Tracks it

Elenovka IL61 t-+-t earwell (L5) * t

Acapulco f t0

* i # 20 244Pu fppbf

1 30

FIG. 5. The Pu concentration of Q-phosphate separates from ordinary chondrites (URWEN etal., 1978; E%LAS, 198 1; PALME etal., 1981). Data 8rc from fission Xe meaSUremeats or fission track rn~~rncn~ from meteorites which show no evidence for track annealing. The data arc for total phasphates-not merrillite or apatite separately. See text for justification of this choice. The ermr brs are due to us and retlect primarily an assumed uncertainty of a f-or of 2 in the apatite/merriWe ratio in a mineral scpamte compared to the true value for the meteorite. In evaiuating the errors, Pu partitioning ratios (mtillite/apatite) of 5 (H chondritcs, PELLASand!$TORZER, 198 I), 3 (L chondrites, PELLAS,1981), and I (Acapulco, PALMEef al., 1981) were assumed.

quentiy, we conclude that it is the total phosphate Pu concentration (i.e., Pu/P) which may have chronological significance, not the Pu concentration in merrillite (or apatite) (cornpan? PEUAS m al., 1979; PELLAs 198 1; MOLD et al., 198 I), and this has been plotted in Fig. 5. Analytically, the presence of two phosphates is a complication because it is uniikeiy that any mineral separation procedure will sample the two phases in exactly the correct total rock proportion. Thus, we have included the effect of a factor of two variation in the ratio of apatite to me&l& in the error bars on Fig. 5. Even with this, the errors in Pu concentration are always less than lt25%, as pointed out previously by PELLAS(I98 1). If due to a di&rence in Xe or track retention time, the difference in phosphate Pu concentrations between Barwe and Menow corresponds

Actinides, P and REE in chondrites

2011

to 125 & 25 my. The difference in 40Ar-39Ar plateau ages between these two meteorites is 50 + 40 my (TURNER et al., 1978), which may be consistent. (Because of lower closure temperature for 40Ar than fission Xe, a larger difference might have been expected.) A similar comparison and conclusion results for Guamiia and Menow (TURNER et al., 1978) or Guareiia and St. Severin (FLOHS et al., 1979). No 40Ar-39Ar age difference between St. Severin and Estacado was found by FLOHS et al., in contrast to Fig. 5; however, the quoted rtl5 my error in relative age appears exceptionally small. High temperature Xe-I correlations give ‘291/‘271 ratios which correspond to ‘29Xe retention age differences for Nadiabondi and Peetz (relative to St. Severin) of 23 + 6 and 15 + 3 my (PELLAS et al., 1979). The corresponding apparent Pu/P age differences are 26 + 10 and 14 + 22 my which are in fortuitously good agreement with the I-Xe ages. With greater effort to define the apatite/merrillite ratio, greater precision (+ 10 my) is possible for the apparent Pu/P ages, In summary, interpretation of the variations in phosphate Pu concentrations in Fig. 5 as age differences in times of Xe retention is consistent with literature data except for St. Severin-Estacado. There is a suggestion that the model fission Xe ages for the more equilibrated H chondrites are younger. This correlates with the slower cooling rates calculated by PELLAS and STOFUER, (198 1) for those meteorites. The

by HUDSON et al. ( 1983) based on a correlated release of 244Puand “‘U n-induced fission Xe from a reactorirradiated whole rock sample, gives Pu/U = 0.007 + 0.002 which is in reasonable agreement with the 0.004-0.005 value inferred from meteoritic Pu/Nd systematics (MARTI .ef al., 1977; JONES, 1982). This is a strong argument for adopting Pu/U = 0.007 t 0.002. However, it would be premature to regard this issue settled, because a similar case (in hindsight due to a numerical coincidence) was made previously for 244Pu/238U= 0.0 15 based on POD~SEK (1970) and DROZD et al. (1977) (see JONES, 1982). JONES and BURNETT (1979) concluded that a significant fraction of the 244Puin St. Severin was located on grain boundaries, analogous to U. This no longer appears valid. The previous conclusion was in part due to adopting 244Pu/238U = 0.015, in which case the merrillite Pu fails to account for the total rock value in a material balance calculation as done in Section C. But the main reason for the Jones and Burnett conclusion was the U and Pu fission Xe correlation observed by PODOSEK (1970). However, in the HUDSONet al. remeasurement, significant (factor of two) variations in 244Pu/238Uare now observed during stepwise heating which indicate that Pu and U are really not closely correlated. It seems important to continue 244Pustudies on ordinary chondrites, although our work shows that Pu and U (or Th) are entirely different elements, and there is no

L-6 chonchites define two different apparent age groups

chemical reason to believe that the Pu/U in chondrites is any better conserved lhan any other whole-rock ratio of refractory elements. e.g., Pu/AI or Pu/Ir. This means

separated

by 80- 100 my, which may indicate a later heating event for some L-chondrites as discussed by PELLAS (198 l), or it may be due to two separate Lchondrite parent bodies. A possible complication in the interpretation of Fig. 5 involves the higher (- 14%) P and lower (-8%) refractory element contents of L or LL vs. Hchondrites (data from MASON, 1979) which could result in higher initial Pu/P ratios in these meteorites if Pu-P coherence is established after these L-H chemical fractionations took place. To “correct” for these differences (a different model than we have assumed) the Pu content ofthe L-chondrites relative to H shown in Fig. 5 could be lowered by as much as 20%. The only LL-chondrite (St. Severin) has a P content very similar to that observed in H-chondrites. In any case, the total spread in the phosphate Pu concentration is at least a factor of 3. As expected, this is less than the factor of 10 spread in Pu/U; however, differences in phosphate Pu concentrations are clearly resolved. K. The solar system Pu/U The previous discussion has focused on relative age measurements for meteorites based on 2”Pu. The other major issue is the solar system 244Pu/23*U,which is aI constraint on models of the timedependence of galactic r-process nucleosynthesis (SCHRAMM and WAS. SERBURG, 1970). The remeasurement of St. Severin

that a relatively large data base of whole rock Pu-Xe measurements, accompanied by rather complete petrographic and chemical controls, is required. We do not regard the 244Pu/238U question as satisfactorily solved. Possible roles for phosphate Pu data in this

problem were mentioned in the Introduction. Of the three alternatives mentioned, numbers 1 and 2, based on Pu/U, seem ruled out. Item 3), modified from MOLD et al. (198 1) to use the total phosphate (as opposed to merrillite) Pu concentration, as discussed in Section J above, needs to be considered. In this model Pu/U would be calculated as: [phosphate Pu] times [phosphate abundance] divided by whole rock U. With this calculation the total rock 244Pu/238U ratio (4.5 X 10’ yr ago) for St. Severin is 0.0044 f 0.0005 in reasonable agreement with HUDSONet al. (1983). The quoted errors are estimated analytical errors only; sampling errors are essentially impossible to estimate, but may be significant. A survey of isotopic dilution U concentrations from the literature indicates that U contents of H and L chondrites are quite similar at about 11 ppb, although real intermeteorite differences probably do exist. As discussed above, P contents are uniform to about 25% among H and L chondrites, separately, with H chondrites being slightly higher thus the spread in Fig. 5 will result in a corresponding spread in Pu/U with Barwell corresponding to Pu/U % 0.00 1. If the variations in Pu phosphate contents are due CO

2012

M. T. Murrell and D. S. Burnett

dtgerences ~n~ssion Xe retention time, then the highest value Qt. Severin) should be taken as the solar system value, but if this is not true, it is co~~ivab~e that the solar system Pu/U is still lower than the presentIy accepted 0.007, perhaps even down to 0.001, The implications of 2%t/238U = 0.007 for r-process nucleosynthesis have been discussed by FkmSON el al. (1983) (see also JONES, 1982). Essentially, a drop in 2*Pu/Z3BUto 0.007 or less from the previously-adopted 0.015 removes the necessity for an increase in the rate of supernovae in the gaiaxy in the IO’ yr period preceding the fo~ation of the solar system, (relative to the average rate for the time period back to the formation of the galaxy). A decrease in t4QP~/Z3sU,even to the 10-j level considered above, is still not suficiently low to indicate significant 244Pu production in the “last minute” nucleosynthetic event(s) (supernova?), which added 26A1, ‘O’Pd, and probably some “‘1 to the solar system at - 10m4levels compared to stable isotopes of these elements, as discussed by KELLY and WASSERBURG (1978). 244Pu synthesis must represent more ancient galactic events, unless the iast minute supernova produced debris very rich in actinides. However, the strong limits on z47Cm (CHEN and WASSERBURG, 198 1b) make the latter alternative unlikely. A 2erPu/238U as low as 0.001 would, however, probably indicate a “lull” in nucleosynthetic activity for our solar system material in the - 10’ yr period preceding the formation of the solar system. This would fit nicely with the general density wave concept for star formation (see e.g., TRIVEDI, 1977).

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