Further studies of trace elements in C3 chondrites

Geochimbca

et Cosmochlmlca

Acta.1978,

Vol

42, pp

97 to

106

Pergamon Press.Printed in GreatBrltam

Further studies of trace elements in C3 chondrites H.

TAKAHASHI,* MARIE-JO&E JANSSENS,? JOHN W.

MORGAN$

and

EDWARD ANDERS

Enrico

Fermi

Institute

12 July

(Receioed

and Department of Chemistry, Chicago, IL 60637, U.S.A. 1977; accepted

irz recked

form

University

1 September

of Chicago.

1977)

Abstract-Five carbonaceous chondrites (Renazro C2V. Allende C3V, Ornans C30, Warrenton C30. and Orgueil Cl) were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd. Cs. Ge, In. Ir, Ni, OS. Pd. Rb, Re, Sb, Se. Te. T1, U and Zn. These data, together with earlier measurements on seven additional C3 s, are interpreted in the light of petrographic studies by MCSWEEN (1977a, b) and revised condensation temperatures (WAI and WASSON. 1977). Elements condensing between - 700 and 420 K (Se, Zn, S, Te, Br, In, Bi. Tl) are systematically more depleted than those condensing between 1000 and 900 K (Ge, Ag, Rb), by factors of I .3 to 2, and the depletion correlates inversely with matrix content and directly with degree of metamorphism. The most plausible explanation appears to be a gas-dust fractionation during condensation, by settling of dust to the median plane of the nebula. In this model, gas/dust ratios relative to the cosmic ratio ranged from 0.7 at lOOOK to 0.5 at 700K for those C30 s that accreted first (Ornans, Warrenton) and from 1.3 to 0.6 for the last (Kainsaz). There appears to have been no further gas/dust fractionation below 700 K. Abundances of Sb, Au and Cd follow earlier trends. Depletion of Sb and Au correlates with abundance of Fe-poor olivine and seems to reflect greater volatilization upon more prolonged or intense heating during chondrule formation. The 50-IOO-fold depletion of Cd in C30 s compared to C3V s suggests condensation in a region where enough Fe was present to buffer the HQS pressure. 1. INTRODUCTION IN

OUR

previous

(ANDERS

study

of

seven

C3

chondrites

et al., 1976) we reported

several new trends, including some systematic differences between the Ornans (C30) and Vigarano (C3V) subclasses (VAN SCHMUS and HAYES, 1974). The extreme depletion of Cd in C30 s implied condensation at lower H,S fugacities, and the slight but consistent underabundance of eight highly volatile elements relative to Ge, Ag, Rb suggested a gas-dust fractionation. To check these trends and interpretations, we have measured five additional meteorites: Warrenton C30, Ornans C30, Renazzo C2V, Allende C3V, and Orgueil

Cl.

(Several

of the six Allende

analyses

have

* Present address: Centre des Faibles Radioactivites. Laboratoire Mixte C.N.R.S.X.E.A.. 91190 Gif sur Yvette. France. t Present address: Instituut voor Nucleaire Wetenschappen, Riiksuniversiteit Gent. B-9000 Gent. Beleium. : Present address: U.S. Geological Survey. Stop-923, National Center. Reston. VA 22092. U.S.A. $ Of the 198 Chicago values, we have omitted 5 atypically low ones from the average: Bi and Tl for Warrenton and Ornans (Fig. 2) and Tl for Kaba (Fig. 1). These two elements are near the end of the condensation sequence, and tend to be depleted in meteorites of higher petrologic types. This depletion has been attributed either to incomplete condensation (LARIMER and ANDERS, 1967) or to metamorphic loss (WASSON and CHOU, 1974), but whatever the cause, all parties agree that such sporadically low values do not give a meaningful estimate of the limiting abundance corresponding to complete condensation. [For example, WASSON and CHOU (1974) omitted these elements from their treatment of ordinary chondrites]. Of the 22 Bi and Tl values in Figs. 1 and 2, 17 fall in the range of the remaining got&420 K elements, and thus probably also represent complete condensation. We have therefore based our Bi and Tl averages on these 17 points.

already been published elsewhere, but have never been discussed in the context of other C3 chondrites.) Though few in number, these samples were of unusual interest. Warrenton is the most metamorphosed meteorite among C30 s, whereas Omans is atypical (MCSWEEN,

1977a).

Renazzo

and

Allende

also

are

in many respects (WOOD, 1967a; VAN SCHMUS and HAYES, 1974). The new data fit well into the previous trends. The present paper has therefore been written as a sequel rather than as a successor of the earlier one, with a minimum of duplication. unique

2. MATERIALS,

METHODS

AND

RESULTS

Samples were treated as in KR.&HENB~~HL rt al. (1973), and were analyzed according to the procedure of KEAYS et al. (1974). revised and expanded to include OS and Pd (GROS et al., 1976). Monitors were included for all elements. except U, where a secondary standard was used (BCR-I = 1750 ppb U for Allende. Orgueil = 8.2 ppb U for all other meteorites). Samples ranged from 29 to 96 mg for Allende, and from 59 to 165 mg for the other meteorites. Their sources are given in Table 1. The Orgueil sample was a powdered aliquot of sample M9418B analyzed by KRKHENB~JHL et al. (1973). Data are shown in Table 1. An indication of the precision and sampling error may be obtained by comparison of the two Orgueil and six Allende analyses. 3. DISCUSSION meteorites are shown in Figs. 1 and 2. The new analyses are indicated by filled symbols. Elements are arranged primarily by decreasing volatility, and secondarily by cosmochemical character. Mean abundances for the two classes as a function of condensation temperature are shown in Fig. 3.5 Data

for individual

YX

H. TAKAHASHI et al. Table 1. Trace element abundances Source* and number

Orgueil Cl

U (ppb)

M9418B = 8.2 M9418B 8.68 Ornans C30 P 2688 16 Warrenton C30 R 11.6 Renarzo C2V U2196 10.8 Allende C3V U/l 5,l231- 13.4 Allende C3V U/I 5/23t 15.3 Allende C3V Ui’l5,‘23t Allende C3V U/15/23+ Allende C3V U/l s/231Allende C3V u/15/231Allende C3V Mean 14.4 C3V Averagei/ 1.17 C30 Average11 1.08

Orgueil Cl

(p% 35.3 35.7 63.3 60.8 52.8 69.2 15.4

64.6 64.4 68.4 1.11 1.09

(p;b)

(Popi)

(pppdb)

(G

462 500 733 709 662 779 806 792 711 745 824 776 0.893 0.931

488

534

1.12

751 745 720 925 868 854 757 759

705 701 690 694 841 664 611 708

1.20 1.42

883 0.962 0.926

705 0.856 0.862

1.44 1.31 1.51 1.42 1.38 1.45 1.44 1.42 0.900 0.885

(2)

(pspbb)

173 149 182 176 161 136 145 140 121

118 124 107 79 67 80 68 74

144 137 0.709 0.798

72 74 0.456 0.65 1

(PZ) 28.5 34.9 16.3 15.7 11.9 29.14 13.8 13.6 16.1 14.4 18.6 15.3 0.398 0.428

*The abbreviations stand for the following institutions and donors, to whom we express our sincere gratitude. M = Musee d’Histoire Naturelle, Montauban (A. Cavaille); P = Museum National d’Histoire Naturelle. Paris (J. Orcel); R = G. W. Reed, Chicago; U = U.S. National Museum, Washington (R. S. Clarke, Jr.). t U./l5/23 = Split 15, position 23 of USNM standard powder. 3 (a) KIGHENBUHL rt al. (1973); (b) ANDERSet al. (1975); (c) HERT~GENet al. (1977); (d) MORGANet al. (1977); (e) JANSSENSet al. (1977); (ft PALMEet al. (1977); (g) MORGANet al. (1975).

drules, in turn, are made by brief high-temperature Error bars represent 80% confidence limits of the mean, following the practice of WASWN and CHOU events: either remelting of matrix or a similar, volatile- and Fe’+-rich condensate (WHIPPLE, 1966; (1974). In addition to our 20 elements, we have inCAMERON,1966; WOOD, 1967a; KIEFFER, 1975) or cluded seven important volatiles measured by others: local condensation (WOODand MCSWEEN,1977). The Li, Na, K (NICHIPORUKand MOORE.1974), Mn and Cu (SCHMITTet al., 1972) and S, C (see VAN SCHMUS 1800-1200 K elements are retained by both chondrules and matrix, and are therefore present in essenand HAYES, 1974, for original references).* Our choices of condensation temperatures are listed in the tially Cl chondrite proportions. The 80@420K elements are completely volatilized from chondrules Appendix ; less certain or interpolated values are indicated by short horizontal bars. All values were recal- (and the genetically equivalent coarse metal), but are culated to 50% condensation at a total nebular pres- fully condensed on matrix, whereas the still more volatile elements are only partially condensed on sure of 10-j atm. matrix. The 1200-800 K elements are a mixed group, Both classes show a typical chondritic step pattern: flat at approximately Cl chondrite levels from 1800 being fully condensed on matrix but often only parto 1200 K, declining from - 1200 to - 800 K, flat tially lost from chondrules, and, moreover, affected by a dust-gas fractionation (ANDERS et al., 1976). again from - 800 to -420 K, and declining once more below 420K. In the conventional two-component model, these trends are explained by loss of 3.1. 1800-1200 K elements volatiles from chondrulesf and retention by matrix Four of the five siderophiles in this group (OS, Ir, (ANDERS,1964; LARIMERand ANDERS,1967). ChonNi, Pd) are slightly depleted relative to Cl levels. This depletion parallels the analogous depletion in Fe, *One reviewer of this paper asked whether we had which apparently represents loss of metal phase (LARchosen a reciprocal temperature scale in Fig. 3 instead of IMERand ANDERS,1970). a linear scale (e.g. Fig. I of ANDERS, 1977), in order to “produce” a plateau among the volatiles. The answer is no. The present data cover twice as wide a temperature interval (I 500 K vs 700 K), and hence would have overlapped extensively at the low end of a linear plot. The I/T scale has the advantage of compressing the 1 IO&l900 K region, which is of little interest to us, while expanding the 40@-500K region where the two classes show small but important differences in condensation temperature. Moreover, the plateau follows inevitably from the constancy of the ordinate between 700 and 400K (Fig. 3) and cannot be “produced” by any manipulation of the ahccisstr. Horli wit qui ma1 y pensr. ?- When we say “chondrules” we often mean “chondrules and associated coarse metal”.

Elements OS, Ir, Ni, Pd Fe

Abundance relative to Cl c3v c30 0.903 0.869

0.901 0.909

Among the 11 C3 chondrites analyzed at Chicago, Allende has the highest abundance of the refractory siderophiles Re, Ir and OS (Figs. 1. 2 and Table 1). This may reflect its high content of Ca, Al-rich inclusions, which are known to be enriched in these ele-

Further in carbonaceous

studies

of trace elements

09

in C3 chondrites

chondrites

Rb

cs

Bi

Tl

Br

Se

Te

Zn

(p%)

(ppb)

(PPb)

(Ppb)

(ppb)

(ppb)

(ppm)

(ppb)

(ppm)

225 1x5 x4 78 76 90 93 x9

2310 1420 1140 I070 I070 990 1100

176 182 89 X6 83 82 102

137 137 15.7 20 40 58 60

3670

19.1 19.2 x.4 7. I x.2 7.4 X.0 8. I

2680 3070 1020 940 990

325 30X 104 90 98

IO00

II2

930

114 113

91 0.396 0.355

1040 0.412 0.430

113 103 10.1 19.3 35 43 46

44X)$ 1120 1130 1480 1550

(d;b) 74 79 26 1x.5 23 41 20

($) 670 677 7.4 1.87 215

Ref.:

(a)

464

(b)

475 484

$1

8.8 92 0.304 0.284

44.5 0.304 0.277

1520 0.260 0.228

59 0.256 0.272

8.1 0.299 0.255

960 0.232 0.199

113 0.233 0.214

30 0.277 0.195

0.3 I5 0.0059

# Doubtful, owing to very low chemical yield (Ge) or suspected contamination (Br). (/Normalized to Cl chondrites and Si, and including the analyses from ANDERS et al. (1976). Cl chondrite were from KRXHEN~OHL et al. (1973). except OS = 540 ppb, Pd = 545 ppb. Br = 3990 ppb. The following values omitted from the average: Tl: Kaba, Ornans. Warrenton; Bi: Ornans, Warrenton; Br: Ornans.

I

I

I

I

C3V C hondri tes I

E ,o ” Z ,o 2 % 5

I

I

_

J

0.2 1800-I2OOK U ,Re,Ir,Nl

0.1 O.O& 5 0 Kobo

Au,Sb Foio+ 146 %

-

Fa 4 r 95 % FQC93%

1 m Ornans

FaZOf 68%

I<800 h Mcophtle IBr ,Se, Te_.1Jr & Figs. I and 2. Elements condensing below 1200 K are depleted by similar factors, owing to volatilization from chondrules. Exceptions are Au and Sb (incomplete, often variable loss), Cd (incomplete condensation on C30 matrix). and Bi, Tl in Ornans and Warrenton (incomplete condensation, caused by higher accretion temperature). Except for Warrenton, there is little correlation between abundance and metamorphism, as expressed by composition of chondrule olivine (mean fayalite content and its mean deviation). Low abundances in Renazzo reflect the low matrix content of this meteorite (MCSWEEN. 1977bt 0.002 *

Warrenton Faj5t 21%

IZOO-800K Ge,Ag,Rb


data were

100

H. TAKAHASHIet al. 0.5

i.0

1.5

2.0

1000/T

shown by the low PMD (percent mean deviation) of its olivine (21%) and its pfacement in MCSWEJZX’S (1977a) grade III], its original content of Fe-poor olivine must have been higher. The C5 chondrite Karoonda (TAKAHASHIet al., 1977) is an even more striking case. With Sb = 0.20 and Au = 0.56, it actually falls at the head of the list in terms of depletion, although, being equilibrated, it naturally contains no Fe-poor olivine at all. Apparently Sb abundance provides a more durable record of chondrule formation conditions than does the com~sition of ohvine. By the same token, tests of the Sb-Fa correlation must be limited to meteorites less metamorphosed than Warrenton. Now that data for 11 meteorites are available, it appears that Sb, Au, Cu depletion correlates with _ oi89‘019 I,, I, I class. C3V s fall mainly in the top half of Table 2, 0.1 “I’ ’ 400 I500 1000 800 600 500 and C30 s, in the bottom half. VAN SCHMUS and 50% Condensallon Temperature ioK1 al Id50tm HAYES(1974) had previously recognized this trend for Cu. On the other hand, litkopkile elements of similar Fig. 3. Mean abundances vary with condensation temperature. Elements condensing between 800 and 420 K are devolatility correlate only weakly (Na, K) or not at all pleted by constant factors of about 0.25, due to complete (Li, Mn) with class. Sb depletion, or olivine composiloss from chondrules. Their mean abundance in C3V s and tion (Fig. 3; S~H~TT t-‘t al., 1972; NICHIPOR~K and C30 s is less than the mean matrix contents of 0.43 and MOORE, 1974). A related puzzle is the great range in 0.37 (JLIcSWEEN,1977a. b), which suggests that the formaSb contents, which exceeds that of Cu, Au, and other tion region of these meteorites became enriched in dust after the temperature had fallen below h 900 K. Vertical elements of similar volatility (Table 2; Figs. l-3). error bars represent 80:/, confidence limits of the mean; These differences cannot be blamed on metalhorizontal error bars indicate qualitatively that the consilicate fractionation, because Fe and non-volatile densation temperature is not well known. siderophiles have virtually the same abundance in C3V s and C3O s (see Fig. 3 and unnumbered table at the start of Sec. 3.1). Instead, these trends may ments (GROSSMAN,1973; GROSSMANand GANAPATNY, reflect differences in behavior of silicate, metal, and 1976). troilite during chondrule formation. Whereas the decreasing Fe’ ‘- content of the silicate should have little 3.2. 1200-1000 K elements effect on the activity coefficients of lithophile trace These elements, typified by Sb and Au in the elements, the increasing Fe-content of the metal probpresent study, show transitional behaviour, being un- ably has a larger effect on the siderophiles. Dilution depleted in some meteorites but appreciably depleted of the alloy by the added Fe w~illreduce the concentration of ail trace elements, but for those elements in others (Figs. 1, 2). In terms of the two-component model, this implies variable and generally incomplete loss from chondrules (ANDERS, 1968, 1971; KURIMOTO Table 2. Correlation between depletion of Sb, Au. Cu and et al., 1973). Such loss has actually been confirmed abundance of iron-poor olivine for Na, Mn and Cu in separated chondrules (SCHMITT , et al., 1965; OSBORN et al., 1973). As pointed out by grains of ! fnyalltc ANDERS et ul. (1976), under non-equilibrium condii:as5* I % ’ tions this loss will depend on chondrule size, peak temperature, and time spent in the molten state, and should therefore be inversely correlated with the Fe2+ content of the chondr~~les, which is controlled by the reaction Fe0 + H, 2 Fe + H20. Because this equilibrium shifts to the right at higher temperatures, the fayalite (Fa) content of the olivine falls with more prolonged or intense heating, and eventually approaches the equilibrium value of Pa,, (WOOD, * W. R. VAN SCHMIJS,private communication; WOOD 1967aj. (1967a); CLARKEet al. (1970). These values refer to the As in our earlier study, there indeed is some correlarger grains used for the microprobe analyses, and may lation between the depletion of Sb and Au and the not be truly representative of the entire coarse-grained fraction, proportion of Fe-poor olivine (Table 2). This correla7 SCHMITTet aE. (1972). tion also extends to Cu. Warrenton falls out of line, $ Low due to metamorphism, as evidenced by the high but since it is substantially more metamorpho~d [as mean fayalite content and the PMD of 21. 0

101

Further studies of trace elements in C3 chondrites having a higher affinity for Ni than for Fe, this effect will be counteracted by an increase in activity coefficients. Thus the generally greater volatilization with increasing equilibration and reduction may be lessened for some siderophiles but enhanced for others. The greater depletion of Au and Cu in C3V s, whose chondrules (+ coarse metal) are more reduced (Table 2) may well reflect such an increase in activity coefficient, because both elements have only limited solid solubility in Fe but not Ni (HANSEN, 1958), and might therefore show higher volatility with rising Fe content of the metal. For Sb, some additional factors must be invoked. to explain its greater range of variation (Figs. 1, 2; Table 2). One possible reason is compound formation. Whereas Au and Cu merely form solid solutions with Ni, Sb forms stable intermetallic compounds with Fe and especially Ni (HANSEN, 1958). This should give rise to smaller activity coefficients [we assumed 10m3 for Sb, vs 5 for Cu and Au (WAI and WASSON, 1977)], and to a greater dependence of activity on the mole fraction of Ni. Another, less likely reason is the somewhat chalcophile character of Sb (FOUCH~ and SMALES, 1967). Chalcophile volatiles would fare especially badly during chondrule formation, because their host phase FeS reverts entirely to Fe if equilibrium is reached. It is not clear to what extent this actually happened. Chondrules of C3 chondrites often have troilite rims and inclusions (WOOD, 1967b; CLARKE et al., 1970; MCSWEEN, 1977a), but one cannot tell from the published information how much of this is surviving primary troilite and how much is a secondary product regenerated on cooling.

3.3. 1000-800 K elements In our previous study, we found that C3 s, in contrast to C2 s (KRXHENBLJHLet al., 1973), showed a distinctive drop in abundance for the nine most volatile elements following Ge, Ag, Rb. This trend has persisted (Fig. 3): mean abundances of Ge, Ag, Rb in C3V s and C30 s are 0.402 f 0.009 and 0.404 k 0.043, compared with 0.267 &- 0.030 and 0.238 _t 0.034 for the next nine elements (Cs, Se, Zn. S, Te, Br, Bi, In, Tl). (Errors given are standard deviations of the distribution, not of the mean.) Of three possible explanations (incomplete loss of volatiles from chondrules, incomplete condensation due to progressive agglomeration of dust, and gasdust fractionation) the first two seemed distinctly less plausible, and so the third was accepted by default. Arguments against the first two remain unchanged, but the case for the third can now be re-examined, because a crucial parameter, matrix content, has since become available (MCSWEEN, 1977a, b). For a volatile element E that is completely lost from chondrules but is fully condensed on matrix, the abundance E, in the hulk ckondrite, relative to the Si-normalized abundance E, in Cl chondrites,

Table 3. Matrix content and abundance of volatiles Meteorite

Metamorphic gradr’ and FM”

* MCSWEEN (1977a. b). t R1 = mean abundance of Ge. Ag, Rb. normahzed to Cl chondrites and Si. Omitted from average: Ag in Mokoia. R, = mean abundances of Cs, Se, Zn, S. Te, Br. Bi, In, Tl, normalized to Cl chondrites and Si. Omitted from average: Bi in Ornans and Warrenton and Tl in Ornans. Warrenton. and Lance. $ B = gas/dust ratio, relative to cosmic ratio; y = mean abundance of volatile elements in gas (= E;Hz, relative to cosmic ratio); f = fraction of unaccreted dust fine enough to collect volatiles. Subscripts I and 2 refer to the two element groups given in the preceding footnote.

equals EJE,

= R =

/$;,f“:‘f

where fi = ratio of gas to unaccreted dust. relative to the cosmic ratio, 7 = condensation index, defined as E/Hz in the gas relative to the cosmic r&o, f“ = mass fraction of matrix in the chondrite and j=f rat t’ton of unaccreted dust fine enough to condense volatiles. This equation is similar to earlier versions (ANDERS, 1972; LAUL et al., 1973; ANDERS et al., 1976). except for the explicit inclusion of the condensation index, y, which figured in discussions of the model since 1967 (LARIMER and ANIXRS, 1967). This factor reflects the gradual accumulation in the gas of volatiles left behind by earlier generations of chond&es, both uncondensed on matrix and expelled from chondrules. Because y falls as b rises, the product 8~ remains close to 1 as long as only small fractions of the dust change into chondrules (.f’: 1) and accrete (fl z l), in which case R = ,f” (p. 1259 of LARIMER and ANDERS, 1967). However. for any volatiles that are still largely uncondensed toward the end of accretion (e.g. Bi, Tl. In, Cd in ordinary chondrites), the fall in y no longer offsets the rise in p. The product jy becomes larger than unity. causing these elements to be greatly enriched in late condensates. e.g. uncquilibrated ordinary chondrites (TANDONand WASSON, 1968; RIEDER and W;~NKE, 1969 ; LAUL ct al., 1970. 1973) or “mysterite” (LAUL et al.. 1973; HIGUCHI et al.. 1977). Of the parameters appearing in eq. (I ). R is known from Figs. l-3, whereasf; though not directly measurable, probably was between 0.9 and 1 (LAUL et a[., 1973). Thus, if the matrix fraction .f” is known. the equation can be solved for the product of gas/dust ratio /I and condensation index y. With additional assumptions. these two can be disentangled.

H. TAKAHASHI et al.

I 02

MCSWEEN(1977a,b) has measured the volume fraction of matrix in most C30 and C3V chondrites. His values (including troilite, which belongs to the lowtemperature fraction on the two-component model), are shown in Table 3, along with mean abundances R, and Rx of two groups of volatiles: GeAgRb and CsSeZnSTeBrBiInTl. Meteorites are arranged by metamorphic grade where known. otherwise by PMD.

the same. The simplest explanation is that y remained near I throughout the C30 accretion sequence, in which case the & values in Table 3 are essentially equal to p alone. On this interpretation, Warrenton and Ornans were located in a slightly dust-enriched region (/r 2 0.7) during condensation of Ge. Ag, Rb, whereas the remaining meteorites were in a slightly dust-depleted region (fl = 1&l.3) at that time. By the time the 8~20 K elements condensed. B had fallen to 0.5 and 0.6 in the two regions. c30 s This trend is consistent with the conventional picture of the solar nebula, according to which dust setThere is a distinct inverse correlation between tled continually toward midplane, where accretion matrix content and the abundance of the two groups of volatiles (Table 3). MCSWEEN (1977a), who first took place. Warrenton and Ornans, the first meteorites to accrete, must have been closest to midplane, noticed this correlation, thought that it contradicted the two-component model, but it does not. Let US and hence had the lowest gas-dust ratios (PI z 0.7, fi2 % 0.5). Kainsaz, the last meteorite to accrete, must set f= 0.9 for the sake of definiteness, and then use the matrix contents to solve for fly (neglecting the have been at higher, less dusty latitudes while condensing its volatiles, and consequently saw higher p small and nearly constant difference between volume values (j$ z 1.3. fi2 2 0.6). fraction and mass fraction of matrix). Though these absolute values of j? depend on our For both groups of volatiles, &J falls with increaschoices of .f(= 0.9) and y (= 1). and hence may be ing degree of me~morphism, which in turn probably in error by some tens of percent, the relative values corresponds to increasing depth in the parent body should be accurate. Thus it seems rather certain that and hence earlier accretion. Warrenton, most metathe first-accreted C30 s formed in the dustiest morphosed and hence first to accrete, has the lowest regions. meteorite, /$J> whereas the least metamorphosed Our analysis is predicated on the assumption that Kainsaz, has the highest. The other three meteorites the measured GeAgRb reside entirely in matrix. If fall in the order of McSween’s metamorphic sequence: some fraction E instead were retained in chondrulesFelix z Lanck, followed by Ornans which “belongs in a higher metamorphic grade on the basis of its plus-metal, then fi would be reduced by the factor (1 - E). ANDERS rt al. (1976) argued against such chemical properties than on the basis of its petroretention on the grounds that GeAgRb did not show graphic properties” (MCSWEEN,1977a). the variable and often high abundances that characFor a physical interpre~tion of this trend, we need terize “partly depleted” elements such as Sb and Au. to disentangle B and y. Here the systematic difference between the two groups of volatiles (i.e. plyI > p2y2) In principle, this point could be checked directly by measuring these elements in separated chondrules and provides at least a qualitative clue. This drop cannot coarse metal. However, since all C30 s have been be due to 7, because there is no way yz can be smaller metamorphosed sufficiently to redistribute Ni and at than yl. Because the second group of elements conleast some Fe2+ (Woou, 1967a. b; MCSWEEN. l977a), denses less readily than the first, its abundance in this experiment may not give meaningful results. But the gas phase must be at least as great, and possibly now that matrix contents are known, there is another greater: p, < y2. Thus the inequality fi,yl > &y2 imargument against such retention. For 7 of the 13 plies p1 > pz, i.e. the gas/dust ratio fell after condenmeteorites in Table 3, the mean abundance of sation of the first group of volatiles. This conclusion GeAgRb (= R, ) is lrss than the matrix fraction ,f’. was already reached by ANDERSet al. (1976). Though one cannot rule out a systematic overestiWe can carry the analysis a step further, by trying to constrain y. It seems likely that y for the highly mate of the matrix content in McSween’s work, or volatile elements Tl and Bi was close to 1. These ele- other, more contrived explanations, the most straightments are incompletely condensed in two of the five forward interpretation is that GeAgRb were quantilost from chondrules-plus-coarse-metal C30 s (Fig. 2), and hence presumably were even less tatively condensed in any C40 s to C60 s that preceded them. (though they may have been reintroduced from Whether or not such meteorites ever existed*, they matrix during metamorphism). would have left most of their complementary Bi and Tl in the gas. so that l/‘T,,si% 1. But since R for the C3V s These meteorites, except Allende, have not been other 6-7 elements in this group is essentially the same as for Tl, Bi (Fig. 3), fly must also have been appreciably metamorphosed (MCSWEEN. 1977b), and we have therefore arranged them by PMD for want *The C4 or CS chondrite Karoonda has been tentatively of a better criterion for their order of accretion. C3V s assigned to the Omans type by some authors (MCSWEES, 1977a). but was left unclassified by others (VAN SCHMUS are known to be a more heterogeneous group than C3Os (VAN SCHMUS and HAYES, 1974; MCSWEEN, and HAYES.1974). We regard it as a Vigarano type because of its high Cd content

(TAKAHASHIet a/.. 1977).

1977b). and so it is not too surprising

that the matrix

Further studies of trace elements in C3 chondrites contents are more variable than those of C30 s and show no inverse correlation with PMD or R,. However, most of them (Kaba, Renazzo, Mokoia, Vigarano) are numerically close to R,, whicch suggests that Ply& z 1. The most straightforward interpretation is that pl, yl, and f each were close to 1 when Ge, Ag, and Rb condensed, as in the simplest form of the two~omponent model. The plyI values, calculated with our nominal assumption that .f = 0.9, accordingly cluster near 0.9. The dispersion in fil;jl is only half as great as for C30 s (& 12% vs 25%), and there is hardly any trend, except that Allende (most metamorphosed) and Grosnaja (low PMD) have the lowest two values. Arguing as before, we might infer that these meteorites accreted in regions slightly enriched in dust, /l z 0.7--O.&

3.4. 800-420 K eletllents Mean abundances of the 9-10 elements in this interval are essentially constant, at 0.267 & 0.030 and 0.238 & 0.034 for C3V s and C30 s (Fig. 3). In terms of the two-component model, this implies that pY/f remained constant during condensation of these elements. There is no rigorous way of deciding whether all three parameters remained constant (as assumed above) or whether they merely varied in complementary fashion, but the approximate constancy of R for individual meteorites (Figs. 1, 2) favors the first alternative. Accepting this interpretation, we can consider the implications of constant fi, y and J: A constant fl implies that no further settling of dust occurred while the gas cooled from 700 to 420 K, although some such settling must have occurred between 900 and 700K. after condensation of Ag but before condensation of Se. (We are ignoring Cs, whose condensation temperature is ill-determined). It is easy to explain why dust did lmt settle, because the Stokes l/e settling time (calculated according to CAMERON, 1973) is no less than 10” yr for 1 pm particles* at 430 K and IO-’ atm. The problem then is, to explain why dust

did settle between 900 and 700 K, but not in the next 300” interval, though the time was almost certainly longer. CAMERON (1973, 1975) has found it necessary to postulate formation of decimeter-sized dust balls, to obtain settling times short compared to the estimated *A skeptical referee asked what relevance 1pm grains have in this context, because “by the time the nebula had cooled to 430 K, significant grain growth must surely have occurred”. There are two simple reasons. First, under equilibrium conditions 90% of all potentially condensable material has condensed by - 1100 IS, so even if no further nucleation takes place, grains can thereafter grow by only 1I‘;/, in mass or -3.6% in radius. Second, high-voltage electron microscopy of matrix from the LL3 chondrites Chainpur and Parnailee shows modal diameters of 0.05 and 0.17 pm and maximum diameters of 1.2 and 2.2 pm (ASHWOR~,1977)so the assumed radius of 1 pm is, if anything, too large.

IO.3

lifetime of the nebula, _ lo4 yr. This mechanism could certainly account for the settling between 900 and 700K. To prevent further settling, one can invoke turbulence or fragmentation of dust balls by collisions (HARTMANN,1977). Constancy of ; implies that only a small fraction of the potentially accretable matter actually accreted in the region of the C3 chondrites, so that the complementary volatiles left behind did not significantly enrich the gas. This is a familiar and plausible conclusion (LARIMERand ANDERS,19673, given the fact that the asteroid belt contains only a tiny fraction of the mass expected from the radial density distribution of the solar nebula, as inferred from planetary masses or astrophysical models (e.g. KUIPER, 1951; CAMERON 1962). Constancy of .f; the mass fraction of fine-grained dust, implies that dust always predominated over chondrules. but does not preclude a secular increase in the mass fraction of chondrules, 1-f: As mentioned above, chondrules apparently were rare in the nebula, but accreted preferentially (WHIPPLE,19721. The data in Fig. 3 also have a bearing on the Wasson-Chou model for the depletion of volatiles in chondrites (WASSONand CHOU. 1974; WASXN, 1974, 1977; WAI and WASSON,1977). In the view of these authors, mean abundances of volatiles in ordinary and C2 chondrites fall conrinuously with condensation temperature, and show none of the plateaus which the two-component model tries to explain by loss from chondrules but retention by matrix. In the Wasson-Chou model. this “continuous negative slope” is attributed to gradual dissipation of the nebular gas, and to loss of fine-gr~~ined, vo~~ile-rich phases. Volatiles either are not lost from chondruies, or are recondensed on matrix, so that no net depletion results from chondrule formation. Detailed critiques of the Wasson-Chou model have been presented elsewhere (ANDERS, 1975. 1977). and will not be repeated here. Suffice it to say that the data for C3V and C30 chondrites (Fig. 3) show a plateau between 800 and 420K, not a “continuous negative slope”.

The new Cd data follow the previous trend: high in C3V s (except Vigarano) and low in C30 s (Figs. 1, 2). The exceptional behavior of Vigarano is no longer an embarrassment, because MCSWEEN (1977b) has found this meteorite to resemble C30 s in some characteristics, such as metal content and sulfide composition. As argued by ANDERS et ~1.(1976), the key

parameter probably was the H,S/H, ratio in the nebula, which is controlled by the equalibrium Fe + H,S $ FeS i- Hz. Because Cd condenses as CdS, its condensation temperature depends critically on sulfur fugacity. C30 chondrites formed in a region where metallic Fe was in excess over S. so that the H2S fUgdCity was kept low by the Fe- FeS buffer

I 04

H. TAKAHASHI et al.

system*. C3V chondrites, on the other hand, formed under conditions where S was in excess over Fe, causing the HIS fugacity to remain high. Bismuth- and thallium are depleted in Warrenton and Ornans, relative to other volatiles or to other C3Os. We do not with to repeat earlier arguments about whether this depletion is caused by incomplete condensation from the nebula (LAUL et al., 1973; HIGUCHIet al., 1977) or by volatilization during metamorphism (DODD, 1969; WASSON,1974; IKRAMUDDIN and LIPSCHUTZ,1975). However, we note that Ornans, though less metamorphosed than Warrenton, is more depleted in Bi and Tl (Fig. 2). MCSWEEN(1977a) had previously noticed a similar trend for noble gases (MAZOR et al., 1970) and Bi (SANTOLIQUIDOand EHMANN,1972), and concluded that these depletions were established during nebular condensation, not metamorphism. We agree. C and N are underabundant in C3Vs and especially C30 s, relative to Cl and C2 chondrites (GIBSONand MOORE,1971), and the fraction occurring in organic form is variable and often low. Condensation of organic compounds seems to require catalytically active minerals such as phyllosilicates or magnetite (ANDERSet al., 1973), which are rare or absent in C3 s. Equilibrium condensation of graphite and C in solid solution with Fe has been investigated by LEWIS et al. (1977), and can probably account for the inorganic carbon in C3 s. A detailed discussion does not seem profitable, in the absence of data on the distribution of C and N among different chemical states. are indebted to JAN HERTOGEN for criticism, to ALICIA WWDROW for assistance with the drawings and to HARRY Y. MCSWEENfor a constructive review. This work was supported in part by NASA Grant NGL-14-001-167. Acknowledgements-We

REFERENCES ANDERSE. (1964) Origin, age and composition of meteor-

ANDERSE. (1972) Physico-chemical processes in the solar nebula, as inferred from meteorites. In POriyine du SJJSthme Solaire (Editor H. Reeves), pp. 179.-201. C.N.R.S.. Paris. ANDERSE. (1975) On the depletion of moderately volatile elements in ordinary chondrites. Meteoritics 10. 283-286. ANDERSE. (1977) A critique of “Nebular condensation of moderately volatile elements and their abundances in ordinary chondrites”, by Chien M. Wai and John T. Wasson. Earth Planet. Sci. Lett. 36, 1420. ANDERSE., HAYATSUR. and STUDIERM. H. (1973) Organic compounds in meteorites. Science 182. 781-790. ANDERSE., HIGUCHI H., GANAPATHYR. and MORGAN J. W. (1976) Chemical fractionations in meteorites--IX. C3 chondrites. Geochim. Cosmochim. Acta 40, 1131-l 139. ANDERSE.. HIGLJCHIH., GROSJ., TAKAHASHI H. ‘and MORGAN J. W. (1975) Extinct superheavy element in the Allende meteorite. Science 190. 1262-1271. ASHWORTHJ. R. (1977) Matrix textures in unequilibrated ordinary chondrites. Eartk Planet. Sci Lett. 35. 2534. CAMERONA. G. W. (19621 The formation of the Sun and planets. Icarus 1. i3-64. CAMERON A. G. W. (1966) The accumulation of chondritic material. Earth Planet. Sci. Lett. 1. 93-96. CAMERONA. G. W. (19731 Accumulation orocesses in the primitive solar n&la. Icarus 18, 4074’50. CAMERONA. G. W. (1975) Clumping of interstellar grains during formation of the primitive solar nebula. Icarus 24, 128-l 33. CLARKER. S., JR., JAROSEWICH E., MACONB.. NELEN J., GOMEZM. and HYDE J. R. (1970) The Allende, Mexico, meteorite shower. Smithson. Contrih. Earth Sci. r5l. l-53. DODD R. T. (1969) Metamorphism of the ordin&; chondrites: a review. Geochim. Cosmochim. Acta 33, 161-203. FOUCH~:K. F. and SMALESA. A. (1967) The distribution of trace elements in chondritic meteorites--II. Antimony, arsenic, gold, palladium and rhenium. Chem. Geol. 2. 105-134. GIBSON E. K.,

MOCIREC. B. and LEWISC. F. (1971) Total nitrogen and carbon abundances in carbonaceous chondrites. Geochim. Cosmochim. Acta 35. 599-604. GROS J., TAKAHASHI H., HERTOGENJ., MORGAI\;J. W. and ANGERSE. (1976) Composition of the projectiles that bombarded the lunar highlands. Proc. 7th Lunar Sci. Conf, Geochim. Cosmochim. 2403-2435. Pergamon Press.

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GROSSMANL. (1973) Refractory trace elements in Ca-Alrich inclusions in the Allende meteorite. Geockirn. Cosmochim. Acta 37. GROSSMAN L. and

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140.

GANAPATHYR. (1976) Trace elements in the Allende meteorite-l. Coarse-grained, Ca-rich inclusions. Geochim. Cosmochim. Acta 40. 33 l-344. GROSSMANL. and LARIMERJ. W. (1974) Early chemical history of the solar system. Rec. Geophys. Space Phys. 12,71-101. Ann Rev. Astron. Astrophys. 9. I-34. HANSENM. (1958) Constitution of Binary Alloys (second edition). McGraw-Hill. * Wasson has questioned whether this reaction will HARTMANNW. K. (1977) Planet formation: mechanism of occur at an appreciable rate after the Fe grains have early growth. Submitted to Icarus. become coated with FeS (WASSONand CHOU, 1974; WAI HAYC~CKE. W. (1959) High-temuerature sulfiding of iron and WASSON,1977), and this concern has been echoed by alloys in hydrogen sulfide-hydrogen mixtures. J. Electroa skeptical referee. However, as pointed out by LARIMER them. Sot. 106. 764771. and ANDERS(1967, p. 1254) kinetic data by HAYCOCK HERTOGEN J., TAKAHASHI H., JANSSENS M.-J. and MORGAN (1959) show that this reaction would be fairly rapid in J. W. (1977) Chemical fractionations in meteorites-XI. the solar nebula. At 700 K and lo-’ atm, a 1urn laver H and E chondrites revisited. Manuscript in preparation. of pure Fe would take 1.7 x lo3 yr to change to F&, HIGUCHIH., GANAPATHYR., MORGANJ. W. and ANDERS but judging from the data of HAYCOCK(1959). a Ni-bearine E. (1977) “Mysterite”: a late condensate from the solar alloy w&~lvdreact faster (e.g. 25 yr for a 304’stainless steel nebula. Geochim. Cosmochim. Acta 41, 843-852. with 18% Cr and 10% Ni). Nebular condensates might IKRAMUDDINM. and LIPSCHUTZM. D. (1975) Thermal be expected to react faster than well-annealed laboratory metamorphism of primitive meteorites- -1. Variation of alloys. Finally, it is perhaps not completely irrelevant that six trace elements in Allende carbonaceous chondrite ordinary, C3, and enstatite chondrites contain 2%70x of heated at 400-1000°C. Geochim. Cosmochim. Acta 39. their cosmic complement of sulfur in the form of _ 1Oilpm 363-375. FeS particles. ites. Space Sci. Reo. 3. 583-714. ANDERSE. (1968) Chemical processes in the early solar system, as inferred from meteorites. Act. Chem. Res. 1. 289-298. ANDERSE. (1971) Meteorites and the early solar system.

Further studies of trace elements in C3 chondrites SANTOLIQU~OP. M. and EHMANN W. D. (1972) Bismuth in stony meteorites and standard rocks. Grochim. Cosmoand LAMRERTP. (1977) Rochechouart meteorite crater: chim. Acta 36. X97-r302. identification of projectile. J. Geophys. Res. 82. 750-758. KEAYS R. R., GANAPATHYR., LAUL J. C., K~~HENB~;‘HL SCHMITTR. A., GOLES G. G., SMITHR. H. and OS~ORN T. W. (1972) Elemental abundances in stone meteorites. U. and MORGANJ. W. (1974) The simultaneous deterMateoritics 7. 131-213. mination of 20 trace elements in terrestrial, lunar and SCHMITTR. A., SMITHR. H. and GOLESG. G. (1965) Abunmeteoritic material by radiochemical neutron activation dances of Na. SC, Cr, Mn, Fe. Co and Cu in 21X indivianalysis. A&. C&m.- Acta 72. l-29. dual meteoritic chondrules via activation analysis---I. J. KELI.YW. R. and LARIMERJ. W. (1977) Chemical fractionGqhys. Res. 7% 24 I9--2444. arions in meteorites-VIII. Iron meteorites and the cosTAKAHASHIH.. GROS J., HIC~UCHI H.. Momxu J. W. and mochemical history of the metal phase. Geochitn. Cosmrr cirinr. /tcra 41. 93-l Il. AKDERSE. (1977) Volatile elements in chondrites: metamorphism or nebular fractionation’? Manuscript in prepKtErFlR S. W. (1975) Droplet chondrules. Science 189, aration. 333.-340. KR~HENBCHLU., MORGAN J. W., GANAPAWY R. and TANDONS. N. and WAS~ONJ. T. (1968) Gallium, gerANI~I-RSE. (1973) Abundance of 17 trace elements in manium. indium and iridium variations in a smte of carbonaceous chondrites. Grochinr. Cosnzochim. Acca 37. L-group chondrites. Geochim. Cosmochim. Actn 32. 1353--l370. 1087-l 109. KUIPTR G. P. (1951) On the origin of the solar system. VAX SCHMUSW. R. and HAYESJ. M. (1974) Chemical and In ,4srropkvsic.s (Editor J. A. Hynek), pp. 357424. petrographic correlations among carbonaceous chonMcGraw-Hill. drites. Grochirn. Cosmochim. Acta 38. 47-64. K~~RIMOTO R. K.. PELLYI. Z., LAUL J. C. and LIP~CHUTZ WAI C. M. and WASSONJ. T. (1977) Nebular condensation M. E. (1973) Inter-element relationships between trace of moderately volatile elements and their abundances elements in primitive carbonaceous and unequilibrated in ordinary chondrites. Earth Planet. Sci. Len. 36, ordinary chondrites. Geochim. ~o.s~loc~Iim. Acta 37, l-13. 209 224. W~ssov J. T. (1974) ~~~r~o~i~~s.Springer. New York, 31b LARIM~KJ. W. and AE~UERSE. (1967) Chemical fractionPP. ations in meteorites~I1. Abundance patterns and their WASSONJ. T. (1977) Reply to Edward Anders: A discussion interpretation. Grochim. Co.w~ocizim.ilcru 31. 1239-l 270. of alternative models for explaining the distribution of LARIMERJ. W. and ANVERSE. (1970) Chemical fractionmoderately volatile elements in ordinary chondrites. ations in meteorites-III. Major element fractionations Earth Pl~wrt. sci. Len. 36, 21-28. in chondrites. Grochirn. Cosmochim Acta 34. 3677388. WASSON J. T. and Crrou C-L. (1974) Fractionation of LAUL J. C., CASKD. R., SCHMIDT-BLEEK F. and LIPSCHLITZ moderately volatile elements in ordinary chondritcs. M. E. (1970) Bismuth contents of chondrites. Geochim. .Metrwritic.s 9. 69984. Cosrnochinr. Acta 34, 89-108. WHIPPLEF. L. (1966) A suggestion as to the origin of chonLA~L J. C.. GANAPATHYR.. ANDERSE. and MORGAN J. W. drules. Science 153, 54-56. WHIPPLE F. L. (1972) On certain aerodynamic processes (1973) Chemical fractionations in meteorites---VI. Accrefor asteroids and comets. In iVvble Symposium 21, From tion temperatures of H-, LL- and E-chondrites. from Plasma to Pkmet (Editors A. Elvius. Almyvist and Wikabundance of volatile trace elements. Geoclrirn. Casinosell) Stockholm. 2 i l -232. dtim. Actu 37. 3299357. Woo11 J. A. (1967a) Olivine and pyroxene compositions Lriwrs J. W.. BARSHAV S. S. and NOYESB. (1977) Primordial in Type II carbonaceous chondrites. Gf,~~~~j~ft. Costnoretention of carbon by the terrestrial planets. Icarus, to c&m. Acfu 31. 2095-2108. be published. Wooo 3. A, (1967b) Chondrites: their metallic minerals. MAZOK E,, HEYMANND. and AND~RSE. (1970) Noble gases thermal histories and parent planets. Icarrrs 6. l-49. in carbonaceous chondrites. Geochim. Cosmochim. Acru Wool J. A. and MCSWFEE;H. Y.. JR. (1977) Chondrules 54. 7x I -x24. MCSWCENH. Y.. JR. (1977a) Carbonaceous chondrites of as condensation products. Proc. IAU Colloq. No. 30, to be published. the Ornans type: a metamorphic sequence. Grockim. Cosmochim. Acra 41, 477-49 1, MCSWEEN H. Y.. JR. (1977b) Petrographic variations among carbonaceous chondrites of the Vigarano type. APPENDIX Geochi?n. Cosmochim. Actu. 41. 1777-l 790. MOR~~AV J. W., GROS J.. TAKAHASHIH., H~RTOGENJ. and Condensation temperatures for Fig. 3 were taken from J~xssrus M.-J. (1977) Meteoritic material in the Ries the following sources. and recalculated. if needed, to 50% Crater. Manuscript in preparation. condensation at a total nebular pressure of IO-’ atm. MORGAN .J. W.. HICUCHIH.. GANAPATHY R. and ANDERS OS, Re, U, Ir: GRO~.%%AN and LAKrhlER (1974), on the E. (1975) Meteoritic material in four terrestrial meteorite assumption that U condenses with perovskite. craters. Proc. 6th Lunar Sei. Con& Geochim Cosmochim. Ni. Pd: KFLLY and LARIMER(1977). ,400 Suppl 7, pp. 160991623. Pergamon Press. Li. Au, Ag, Se, Zn. S. Te: WAI and WASSON (1977). NIC~UPOKUKW. and MOOREC. B. (1974) Lithium, sodium In, Bi. Tl, Cd: ANDERS et al. (1976). Two different values and potassium abundances in carbonaceous chondrites. of H,S/H, _ _ were used: 0.75 of the cosmic ratio for C3V s. Gcochirn. Cosmochim. Acfa 38, 1691-l 701. and the equilibrium ratio of the Fe,‘FeS buffer system for OSBOKNT. W.. SMITH R. H. and SCHMITTR. A. (1973) c30 s. Elemental composition of individual chondrules from Sb and Ge: WAI and WASSON(1977). but with smaller ordinary chondrites. Grochim. Cosmochim. Acta 37. activity coefficients: I x 10s3 for Sb and 3 x 10m3 for Ge. 1909-1942. Both elements form compounds with Fe and Ni, and so PALMSH., JANSSENSM.-J.. TAKAHASHIH. ANDERSE., and Wai and Wasson’s choice of r = 1 is not realistic. KELLY HER?UC~E?I J. (1977) Meteoritic material at five large imand LARIMEK(1977) used 3 x ‘10e4 for Cc. but this would pact craters. Geochini. Cosmichim. Actcc to be published, have made it condense at neatly the same temperature RIEDERR. and WKNK~ H. (1969) Study of trace element as Sb (1054 vs 1046K). whereas the abundances in Fies. & abundance in meteorites by neutron activation. In 1. 2 suggest a higher volatility. ~~~~~~~~~r(, R~,~~~l~~~~. (Editor P. M. Millman) pp. 75-86. Na. K. Rb. Cs: The Na value of 1000 K was a compromise Reidel. between the Wai-Wasson value of 925 K which they themJANSSCNSM.-J., HERTOGENJ., TAKAHASHIH., ANDERSE.

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H. TAKAHASHIet

selves consider too low and the Grossman-Larimer value of 1020K. Cs was placed between Ag and Se because it shows an intermediate degree of depletion. K and Rb were interpolated between Na and Cs. Br. C: Br in ordinary chondrites is less depleted than In

al

but more so than Te (HERTOGENet al., 1977). and was therefore interpolated between these elements. C was placed at 400 K, where the metastable formation of heavy hydrocarbons from CO and HZ becomes thermodynamitally feasible (ANDERSet al., 1973).