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Critique of “nebular condensation of moderately volatile elements and their abundances in ordinary chondrites” by Chien M. Wai and John T. Wasson
Earth and Planetary Science Letters, 36 (1977) 14-20 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
14
[4]
CRITIQUE O F "NEBULAR CONDENSATION O F MODERATELY VOLATILE ELEMENTS AND THEIR ABUNDANCES IN ORDINARY CHONDRITES" BY CHIEN M. WAI AND JOHN T. WASSON EDWARD ANDERS
Enrico Fermi Institute and Department of Chemistry, University of Chicago, Chicago, I11. 60637 (USA)
The abundances of 17 moderately volatile elements in ordinary chondrites show three discrete groupings, significant at the 0.1% level. They reflect mainly condensation on matrix and volatilization from chondrules and coarse metal grains. Elements of high condensation temperature (1300-1000 K) such as Na or Mn are fully condensed on matrix and only partially lost from chondrules and therefore have abundances of 0.70.9 the C1 chondrite value. Elements of lower condensation temperature (1000-600 K) such as Ga and Se are fully condensed on matrix but axe largely lost from chondrules; they have abundances of 0.4-0.2. Elements of still lower condensation temperature such as Bi and T1 are incompletely condensed on matrix and are lost from ehondrules; they have abundances ranging from 0.1 to <0.001. Gas-dust fractionation may have produced second-order effects, but it cannot be the sole mechanism, contrary to claims by Wai and Wasson. There is much experimental evidence for depletion of volatiles from chondrules, and since chondrules comprise 3/4 of the meteorite, Wai and Wasson are not justified in ignoring their contribution to the volatileelement budget.
1. lntroauction Wai and Wasson [1] are to be commended for their efforts to recalculate Larimer's [2] condensation temperatures o f volatile elements in the light o f newer thermodynamic data. Unfortunately, they take considerable liberties with the facts in their attempt to prove that the evidence favors the Wasson-Chou [3] over the Larimer-Anders [4] model, I shall briefly comment on the principal flaws in their argument, expanding on m y earlier criticisms [5]. Wasson has
written a reply [6] which cries out for a rebuttal However, mindful o f Urey's [ 7] remark that such exchanges must not become a divergent series, I have limited myself to a f e w responses to the most provocative o f Wasson's statements. Such responses are given in italics.
2. Depletion pattern of ordinary chondrites Wai and Wasson [1] regard the pattern in their Fig. 1 (typified by H5,6 chondrites) as a single, continuous sequence, whereas Larimer and I [4] have
perceived it as a terraced landscape where gently sloping plateaus alternate with steep declines, at Cu and Te. The difference in mean plateau elevations has persisted since 1967 (Table 1). And so has the location o f the steps at Cu and Te (see Fig. 1 o f Larimer and Anders [4]). Wai and Wasson's perception o f the topography can be submitted to objective tests. The means o f the three groups differ from each other by several standard deviations (Table 1), and do not form a single continuum. The reality o f the steps at Cu and Te can be confirmed by plotting the first derivative o f the distribution in Fig. 1. And finally, plateaus can be seen even in Wai and Wasson's own Figs. 2 and 3, if one ignores the Na point which they themselves disown, and allows for a higher condensation temperature o f As ('r should be smaller than 0.1, in view o f the known stability o f iron and nickel arsenides).
Wasson [6] now attempts to show by X2 tests that the plateaus are statistically not significant at the 5% level However, he stacks the deck by choosing improper intervals, and by averaging H and L chondrites, thus confusing volatile~element with metal-
15 TABLE 1 Mean abundances of volatile elements in ordinary chondrites Elements
Mean abundance, relative to C1 chondrites * Wasson and Chou [31
Larimer and Anders [4]
K, Au, Rb, Li, Mn, Na, P, As **
0.79 -+ 0.09
0.81 -+ 0.18
Cu, Sb, F, Ga, Ag, Se, Ge, S, Sn
0.29 +_0.08
0.22 + 0.07
Te, Zn
0.11 +- 0.01
0.10 -+ 0.02
* Errors are standard deviations of a single value, not of the mean. Wasson and Chou values are for H5, 6 chondrites only, whereas Latimer and Anders values are for all ordinary chondrites. ** Larimer and Anders' compilation does not include Li, P, and As.
explain and the Wai-Wasson model must be interred alongside the Larimer-Anders model! It may be a coincidence, but a simple mirror inversion o f the right-hand side o f Wasson's [6] Fig. 1. pro. duces five little boomerangs.
silicate fractionation. Because the central question is the reality o f the Cu-Sn plateau, the proper intervals are the plateau itself, the two empty regions above and below it, and the two populated regions beyond. The results, for H chondrites only, are given in Table 2. The ×z value for these data is 20.96, not 5.54, and it is significant at the 0.1%, not >5% level Wasson 's attempt to match the chondrite distribution by sets o f random numbers is entertaining and illuminating, but not in the way he intended. Once again he has stacked the deck, first by restricting the random numbers to the range 0.1-0.9, thus guaranteeing a fit at the ends o f the distribution, and second by averaging H and L chondrites, thus mixing up volatile-element and metal-silicate fractionation. But suppose Wasson is right, and the chondrite pattern is truly random. Then there is nothing left for him to
3. "Anders... argued that the elemental ordering in Fig. 1 was not a volatility s e q u e n c e . . . " I n d e e d it isn't. Wasson a n d C h o u [3] t h e m s e l v e s a d m i t : " T h e a b s e n c e o f a p l a t e a u o n o u r p l o t reflects m a i n l y t h e i n c l u s i o n o f seven a d d i t i o n a l e l e m e n t s a n d o u r deliberate arrangement o f the elements in order o f decreasing abundance ratio" ( m y italics). O n e c o u l d equally well p r o v e t h a t t h e v e l o c i t y o f light is n o t c o n s t a n t , b y t a k i n g 18 e x p e r i m e n t a l d e t e r m i n a t i o n s a n d arranging t h e m in d e s c e n d i n g order.
TABLE 2 Depletion factors of 19 volatile elements in H chondrites: x 2 test for randomness *
Depletion factor intervals 0.100-0.115
0.115-0.210
0.210-0.426
0.426-0.683
0.683-1.00
Elements
Zn, Te
none
Sn, S, Ge, Se, Ag, Ga, F, Sb, Cu
none
Na, As, P, Mn, Li, Rb, K, Au
Observed
2
0
9
0
8
Expected
0.32
2.15
4.41
5.72
6.40
* x 2 = 20.96. For 4 degrees of freedom and F = 0.001, x 2 is only 18.465.
16 Larimer showed ten years ago that abundance correlated fairly well with volatility (see Fig. 7 of Larirner [2]) and so it was a foregone conclusion that an abundance sequence would roughly correspond to a volatility sequence. What I objected to was Wasson and Chou's tactic of rearranging the data into an arbitrary sequence of maximum slope, and then using this pretty but contrived sequence to "prove" the absence of a plateau and to tout a model that predicts a steep slope. It does not matter that this sequence shows a vestigial correlation with volatility. For both the above purposes, only an honest, undoctored volatility sequence will do. Now that Wai and Wasson have finally replotted the data against volatility (their Fig. 3), we can see how close their arbitrary sequence (Fig. 1) came to being a volatility sequence: Rank by:
K Au Rb Li Mn Na P As Cu Sb F Ga Ag Se Ge S Sn Te Zn
abundance [1, Fig. 1]
volatility [1, Fig. 3bi
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
(>8) 2 (>8) 3 4 8 1 5 6 10 11 7 9 13 12 14 15 17 16
Let Wai and Wasson have the last word on the question whether Fig. 1 is a volatility sequence: " . . . Fig. 3 demonstrates quite clearly that it is".
4. Constancy of abundances
Wai and Wasson set up a straw man by implying that the two-component model requires all elements
of the Cu-Sn group to have identical abundances of 0.25. The original 1967 data showed a nearly twofold spread which we explicitly attributed in part to analytical error and in part to incomplete loss o f the less volatile elements during chondrule formation (see second footnote on p. 1243 of Larimer and Anders [4], and Anders [5] for a more recent discussion of the problem). I shall have more to say about these points in the next two sections of this paper.
5. "Anders' second point is not well taken"
Wai and Wasson set up a second straw man by noting that Zn and Te which they classify as moderately volatile elements fall significantly below the Cu-Sn plateau. But this classification is man-made, not Godgiven. Latimer and I have never included Zn and Te among the moderately volatile elements, precisely because they fell below the plateau, along with 10 other, highly volatile elements. We attribute the depletion of these 12 elements to incomplete condensation [4,5,8-11 ], and though Wai and Wasson do not accept this particular mechanism, they also have chosen to exclude 10 of these elements from their own paper. It is disingenuous of them to sneak the remaining two elements into the Cu-Sn group, and then complain that they do not fit. Ironically, later on Wai and Wasson themselves have trouble accounting for Zn and Te, and resort to various ad hoc assumptions. Indeed, their calculations confirm that these element fall at the boundary between the moderately and highly volatile elements. Thus each author has the right to classify them with one or the other group, depending on the capabilities of his fractionation mechanisms. Wai and Wasson also express the belief that new analyses are as likely to steepen as to flatten the plateau in the Cu-Sn region. But past experience does not support this expectation. When a large group of volatile elements was analyzed simultaneously in a single laboratory, the mean abundances became more, not less constant. For 12 elements in C2 chondrites, the standard deviation fell from 35% to 8% [12], and for 7 elements in C3V chondrites, from 23% to 14% [ 11 ]. Even for ordinary chondrites, where no comprehensive study from a single laboratory is yet available, the standard deviation of the disputed Cu-Sn
17 group fell from 33% in the Larimer-Anders review [4] to 26% in the Wasson-Chou review [3] seven years later.
TABLE 3 Depletion of Na, Mn, and Cu from chondrules Meteorite
Class
6. Chondrules The most glaring defect of the Wasson-Chou model is its disregard of volatile loss from chondrules. Wasson and Chou accept the prevailing view that chondrules (and the genetically associated coarsegrained metal) were made by remelting of a finegrained primary condensate, similar to the matrix of unequilibrated chondrites. But they contend that no volatiles were lost during this process, citing both theoretical and experimental arguments. The theoretical argument, that chondrules would freeze in 1 0 - 3 second whereas loss of volatiles by diffusion would take 1 second, is not borne out by experimental data of Keil et al. [ 13]. A laserproduced AI20 3 chondrule superheated 100 ° above its melting point took 0.7 second to start crystallizing, though it was efficiently cooled by free fall through cold 02 gas at 625 torr. Moreover, Keil et al. [ 13] concluded from textural evidence that meteoritic chondrules had cooled still more slowly than the synthetic chondrules. Apparently freezing times were at least 3 orders of magnitude longer than estimated by Wasson and Chou, and thus more than ample to permit volatile loss by diffusion. That such loss actually occurred is shown by analyses of volatile elements in separated chondrules. The chondruies are consistently depleted relative to the whole rock (Table 3). Since the rock itself consists mainly of chondrules, the depletion relative to matrix is still greater. Wasson and Chou [3] claim that Tieschitz and Chainpur show no depletion except for Cu, but they had neglected to normalize to Si. And they made no mention o f Schmitt's data on C2 chondrites [ 15], which had been in the literature since 1965 and had been quoted by Larimer and Anders. Wasson's [6] attempts to discredit the data in Table 3 do not stand up to scrutiny. First, even though some individual ratios may be uncertain by +-20%, all 15 ratios are less than unity, which can hardly be due to chance. Second, Wasson is not ]usti-
Chondrules relative to whole rock *
Reference
Na
Mn
Cu
H3
0.82
0.94
0.44
Chainpur
LL3
0.66
0.84
0.20
[ 14]
A1Rais
C2
0.19 0.29
0.75 1.15
0.44 0.67
[15]
Murray
C2
0.30 0.42
0.54 O.75
0.58 0.81
[15 i
Santa Cruz
C2
0.15 0.20
0.48 0.65
0.33 0.44
[15]
Tieschitz
[ 14]
* Normalized to Si, where data were available. For the C2 chondrites, data were normalized to Sc, in order to compensate for the high water and carbon content of the matrix. However, even the raw weight ratios (italics) generally show a substantial depletion, contrary to the claims of Wasson [6].
fled in normalizing Cu to Ni, on the grounds o f its "siderophile character". Schmitt et al. [15] have concluded from the poor coherence o f Cu with siderophile Co that it occurs in a minor phase rather than in nickel-iron, and this is supported by the Tieschitz data [14]. Whereas Cu in Tieschitz chondrules is depleted by only a factor o f 2 relative to the bulk meteorite (48 vs. 93 ppm), Co is depleted by a factor o f 8 (84 vs. 710 ppm). Wasson failed to find a difference because he inexplicably chose Ni (3 analyses) rather than Co (25 analyses) as the normalizing element. Third, Wasson's contention that the Sc-normalized C2 chondrite data in Table 3 could represent "'either refractory enrichment or volatile depletion" is a red herring (Clupea harengus rubens). Even the raw weight ratios (italicized values in Table 3) show a substantial depletion. These ratios need to be corrected downward by some 30-60%, to compensate for the high H20, C, S, and FeO contents o f the bulk meteorites, but it makes little difference whether this correction is based on the actual H20, etc., contents[16] or on some non-volatile element. In particular, Fig. 8 o f Schmitt et al. [15] shows clearly that the correction factors based on refractory Sc and non-refrac-
18 tory Cr are essentially the same for the three meteorites: AI Rais 1.53 and 1.60;Murray 1.40 and 1.36, Santa Cruz 1.34 and 1.36. The reader is invited to cast another glance at the Na column o f Table 3, in view o f ICasson's statement that "chondrule/whole rock ratios o f highly mobile elements such as Na are not significantly different from u n i t y . . . '"
Coarse nickel-iron, which is the metallic counterpart of chondrules, is similarly depleted in the volatile metals Ga and Ge, as shown, ironically, by the junior author of the Wasson-Chou model [ 17]. Metal from Hallingeberg (L3) and Parnallee (LL3) has only 7.1% and 5.2% the Ga content of metal from L5,6 or LL5,6 chondrites. Evidently chondrules (and coarse metal) have lost substantial amounts of at least 5 moderately volatile elements: Mn, Na, Cu, Ga, and Ge. Since chondrulesplus-metal comprise 50-80% of C2 or ordinary chondrites, they play a major role in the volatile-element budget of chondrites. The Wai-Wasson treatment, which ignores chondrules, is grossly unrealistic. Wasson [6] latest argument is that "chondrules (according to Grossman and Olsen [18]) account for less than 5% o f the non-matrix, high-temperature f r a c t i o n " o f C2 chondrites, and are hence irrelevant to the volatile-element budget. This is a deft attempt to hide scientific facts behind a semantic smokescreen. Grossman and Olsen [18] introduced a new and very restrictive definition o f chondrules, requiring them to have "interstitial glass and/or barred and/or radial crystalline texture". They then classified the remaining high-temperature components as "white inclusions" or "single grains and grain fragments", together accounting for 48% o f the meteorite. But Schmitt et al. [15] evidently used a much less restrictive definition o f chondrules, as shown by descriptions such as "abundant", "granular" "blocky'" "platy '" "sugary ", or "[apparently] composed o f sheets o f glass". Thus they must have included many objects which Grossman and Olsen would have classified as "white inclusions". Moreover, McSween [19] has shown in a petrographic study that the single olivine grains and grain fragments in C3 chondrites (and possible also C2 chondrites) are derived from the same melts as the
chondrules themselves. Thus it seems that the less photogenic but more abundant components o f the high-temperature fraction are chemically and petrographically related to chondrules sensu strictu. In any event, since the volatile depletion in Table 3 was established on a very broadly defined population o f chondrules [15], the much narrower definition o f Grossman and Olsen is not relevant.
7. " A b u n d a n c e [ s in C2 c h o n d r i t e s ] fall m o n o t o n i cally as a f u n c t i o n o f c o n d e n s a t i o n temperature..."
Wai and Wasson claim that even C2 chondrites show no plateau; only a "distinct negative correlation . . . . significant at the >99.9% level". But for reasons best known to them, they omit all elements condensing below 600 K, though these elements in C2 chondrites fit smoothly into the general abundance trend [4,12]. I have therefore provided a duly expanded version of their graph (Fig. 1). For consistency, I used only analyses of Mighei, Murchison, and Murray, but this actually make little difference. Arrows indicate suspected errors in the Wai-Wasson condensation temperatures (or upper limits, for Rb
1.0
I
I
I
I
~0.6
o
~-~-~m
?
~ 0.4
0.2
~[]
I IlO0
C2 CHONDRITES, 10-6olm • Kr~he~hl et el. (1973) [ ] Other I I I I {(XX) 900 8(3O 700 50% Cond~flm Tempe,otufe(°K)
I 600
U
~'~
..... C
{ 500
4OO
Fig. 1. Elements condensing near 400 K are no less abundant than those condensing near 900 K, and so abundances do not "fall monotonically as a function of condensation temperature", as contended by Wai and Wasson. In terms of the two-component model, elements condensing between 950 and 400 K are quantitatively condensed on matrix (52% of the meteorite) and almost quantitatively lost from chond~les. Elements of higher condensation temperature are partially retained in chondmles, and hence are more abundant. Data from sources in Wai and Wasson [1] except P [16] and alkalis [23].
19 and Cs). Following the example of Wai and Wasson, I have calculated a least-squares line representing their "negative correlation"; it was fitted to their Fig. 4b. The data below 600 K certainly do not conform to the Wai-Wasson line. I wonder why Wai and Wasson chose to omit this vital piece of evidence, though it had been known for more than ten years [4,5,12]. To my prejudiced eye, the elements divide into two groups. Those condensing above ~1000 K are depleted to 0.7-0.9 of the C1 value, whereas those condensing below 950 K are depleted to an assentiaUy constant plateau level of 0.52 -+ 0.06. These two groups apparently represent partial and complete loss from chondrules. Indeed, the first group contains elements (Au, Mn) known to be partially retained in chondrules, and the second group has a mean abundance just equal to the planimetrically determined abundance of matrix in five C2 chondrites, 0.52 + 0.03 [18]. By Occam's Razor, one is led to the conclusion that C2 chondrites are a mixture of 52% matrix of C 1 chondrite composition with 48% chondrules that had partially retained some of the less volatile elements in Fig. 1, but had lost all others.
two-component model explains this, ad hoc, by a gasdust fractionation [ 11 ], and since Wai and Wasson also resort to ad hoc explanations, they should perhaps abide by the glass house metaphor. (3) "It is kinetically difficult to thoroughly [sic] outgas chondrules during brief condrule forming events." This is an armchair argument, thoroughly refuted by experimental data on meteoritic [14,15, 17] and synthetic [13] chondrules. (4) "An ad hoc mechanism is needed to prevent the recondensation of volatiles.., from [chondrules of higher petrologic t y p e . . , on] matrix of lower petrologic t y p e s . . . " . True, but there is no lack of such mechanisms. Latimer and I [4, p. 1259] proposed three: (a) exhaustion of nucleation sites for siderophiles, by conversion of all metal to FeS; (b) production of chondrules above the median plane of the nebula; (c) preferential accretion of chondrules. Of these, (a) now looks least promising [11], (b) is viable but unproven, and (c) has been strengthened by Whipple's work [20] showing that such preferential accretion is to be expected on aerodynamic grounds.
8. "Chief arguments against the two-component model"
9. Gas-dust separation
Wai and Wasson [1] list four such arguments in the final section of their paper. All are refutable, as I shall attempt to show. (1) "There is no particular tendency of abundance ratio [of the Cu-Sn group] . . . to cluster near 0.25 (Fig. 1)." This subjective opinion is not borne out by objective tests, as shown above. The trend in their Fig. 1 drops steeply just before and after the Cu-Sn group, the mean abundances of the three groups differ by several standard deviations (Table 1), and a X2 test shows that the distribution is non-random at the 0.1% level (Table 2). (2) "The accurately determined Zn abundance ratio of 0.106 is not explained by the two-component model." This is not correct. In the framework of the two-component model, any abundance below the plateau value of ~0.2, be it 0.106 or 0.0106, can be attributed to incomplete condensation. The problem with Zn (and Te) is not the magnitude of the abundance, but the small amplitude of its variation. The
It has long been recognized that gas-dust separation played a role in the formation of meteorites, and influenced their chemistry [21,22]. I myself have recently suggested that some subtle details in the abundance patterns of C3 and ordinary chondrites may reflect gas-dust fractionations by factors of up to 3 [ 11 ]. To this extent, I find myself on common ground with Wasson and Chou. Where we differ, is on the role of chondrules. Wasson and Chou try to explain the abundance of volatiles by gas-dust fractionation alone, whereas I attribute it mainly to volatile loss from chondrules and only secondarily to gas-dust fractionation. Since the depletion of volatiles in chondrules has been repeatedly confirmed in the last 11 years, it would seem that a model totally ignoring this fact is unrealistic.
Acknowledgement This work was supported in part by NASA Grant NGL-14-O01-O10.
20
References 1 C.M. Wai and J.T. Wasson, Nebular condensation of moderately volatile elements and their abundances in ordinary chondtites, Earth Planet. Sci. Lett. 36 (1977) 1-13. 2 J.W. Latimer, Chemical fractionations in meteorities, I. Condensation of the elements, Geochim. Cosmochim. Acta 31 (1967) 1215-1238. 3 J.T. Wasson and C.-L. Chou, Fractionation of moderately volatile elements in ordinary chondrites, Meteoritics 9 (1974) 6 9 - 8 4 . 4 J.W. Latimer and E. Anders, Chemical fractionations in meteorites, II. Abundance patterns and their interpretation, Geochlm. Cosmochim. Acta 31 (1967) 1 2 3 9 - 1 2 7 0 5 E. Anders, On the depletion of moderately volatile elements in ordinary chondrites, Meteoritics 10 (1975) 283-286. 6 J.T.Wasson, Reply to Edward Anders: a discussionof alternativemodels for explainingthe distributionof moderately volatile elements in ordinary chondrites, Earth Planet. Sci. Lett. 36 (1977) 2 1 - 2 8 . 7 H.C. Urey, A typical parent meteorite body - a reply to Dr. J.F. Lovering, Geochim. CUsmochim. Acta 16 (1959) 194-196. 8 R.R. Keays, R. Ganapathy and F. Anders, Chemical fractionations in meteorites, IV. Abundances of fourteen trace elements in L-chondrites; implications for cosmothermometry, Geochim. Cosmochim. Acta 35 (1971) 337-363. 9 J.C.Laul, R. Ganapathy, E. Anders and J.W. Morgan, Chemical fractionationsin meteorites,VI. Accretion temperatures of H-, LL-, and E-chondrites,'from abundance of volatiletrace elements,Geochim. Cosmochim. Acta 35 (1973) 329-357. I0 J.W. Larimer, Chemical fractionationsin meteorites,VII. Cosmothermometry and cosmobarometry, Geochim. Cosmochim. Acta 37 (1973) 1603-1623. I I E. Anders, H. Higuchi, R. Ganapathy and J.W. Morgan, Chemical fractionationsin meteorites,IX. C3 chondrites, Geochim. Cosmochim. Acta 40 (1976) 1131-1139. 12 U. Kr~qqenbiihl,I.W. Morgan, R. Ganapathy and E.
13
14
15
16 17
18
19
20
21
22
23
Anders, Abundance of 17 trace elements in carbonaceous chondrites, Geochim. Cosmochim. Acta 37 (1973) 1353-1370. K. Keil, M. Prinz, H.N. Planner, S.R. Skaggs, E. Dowty, L.S. Nelson, N.L. Richardson and M. Blander, A qualitative comparison of textures in lunar chondmles and CO 2 laser-formed synthetic chondrule-like spherules, Spec. Publ. No. 7, Univ. of N. Mexico, Inst. of Meteoritics (1973) 15 pp. T,W. Osborn, R.H. Smith and R.A. Schmitt, Elemental composition of individualchondrules from ordinary chondrites,Geochim. Cosmochim. Acta 37 (1973) 1909-1942. R.A. Schmitt, R.H. Smith and G.G. Goles, Abundances of Na, Sc, Cr, Mn, Fe, Co, and Cu in 218 individual meteoriticchondrules via activationanalysis,I, J. Geophys. Res. 70 (1965) 2419-2444. B. Mason, The carbonaceous chondrites,Space Sci. Rev. I (1963) 621-646. C.-L.Chou and AJ. Cohen, Gallium and germanium in the metal and silicatesof L- and LL- chondrites, Geochim. Cosmochim. Acta 37 (1973) 315-327. L. Grossman and E. Olsen, Origin of the high-temperature fractionof C2 chondrites,Geochim. Cosmochlm. Acta 38 (1974) 173-187. H.Y. McSween, Jr.,O n the nature and originof isolated olivinegrainsin carbonaceous chondrites,Geochim. Cosmochlm. Acta 41 (1977) 411-418. F.L. Whipple. O n certainaerodynamic processesfor asteroidsand comets, in: Nobel Symposium 2 I, From Plasma to Planet, A Elvius, ed. (Almqvist and Wiksell, Stockholm, 1972) 2 1 1 - 2 3 2 . H. Suess, Chemical evidence bearing on the origin of the solar system, Annu. Rev. Astron. Astrophys. 3 (1965) 217-234. J.A. Wood, Olivine and pyroxene compositions in Type II carbonaceous chondrites, Geochim. Cosmochim. Acta 31 (1967) 2095-2108. W. Nichiporuk and C.B. Moore, Lithium, sodium and potassium abundances in carbonaceous chondrites, Geochim. Cosmochim. Acta 38 (1974) 1691-1701.
14
[4]
CRITIQUE O F "NEBULAR CONDENSATION O F MODERATELY VOLATILE ELEMENTS AND THEIR ABUNDANCES IN ORDINARY CHONDRITES" BY CHIEN M. WAI AND JOHN T. WASSON EDWARD ANDERS
Enrico Fermi Institute and Department of Chemistry, University of Chicago, Chicago, I11. 60637 (USA)
The abundances of 17 moderately volatile elements in ordinary chondrites show three discrete groupings, significant at the 0.1% level. They reflect mainly condensation on matrix and volatilization from chondrules and coarse metal grains. Elements of high condensation temperature (1300-1000 K) such as Na or Mn are fully condensed on matrix and only partially lost from chondrules and therefore have abundances of 0.70.9 the C1 chondrite value. Elements of lower condensation temperature (1000-600 K) such as Ga and Se are fully condensed on matrix but axe largely lost from chondrules; they have abundances of 0.4-0.2. Elements of still lower condensation temperature such as Bi and T1 are incompletely condensed on matrix and are lost from ehondrules; they have abundances ranging from 0.1 to <0.001. Gas-dust fractionation may have produced second-order effects, but it cannot be the sole mechanism, contrary to claims by Wai and Wasson. There is much experimental evidence for depletion of volatiles from chondrules, and since chondrules comprise 3/4 of the meteorite, Wai and Wasson are not justified in ignoring their contribution to the volatileelement budget.
1. lntroauction Wai and Wasson [1] are to be commended for their efforts to recalculate Larimer's [2] condensation temperatures o f volatile elements in the light o f newer thermodynamic data. Unfortunately, they take considerable liberties with the facts in their attempt to prove that the evidence favors the Wasson-Chou [3] over the Larimer-Anders [4] model, I shall briefly comment on the principal flaws in their argument, expanding on m y earlier criticisms [5]. Wasson has
written a reply [6] which cries out for a rebuttal However, mindful o f Urey's [ 7] remark that such exchanges must not become a divergent series, I have limited myself to a f e w responses to the most provocative o f Wasson's statements. Such responses are given in italics.
2. Depletion pattern of ordinary chondrites Wai and Wasson [1] regard the pattern in their Fig. 1 (typified by H5,6 chondrites) as a single, continuous sequence, whereas Larimer and I [4] have
perceived it as a terraced landscape where gently sloping plateaus alternate with steep declines, at Cu and Te. The difference in mean plateau elevations has persisted since 1967 (Table 1). And so has the location o f the steps at Cu and Te (see Fig. 1 o f Larimer and Anders [4]). Wai and Wasson's perception o f the topography can be submitted to objective tests. The means o f the three groups differ from each other by several standard deviations (Table 1), and do not form a single continuum. The reality o f the steps at Cu and Te can be confirmed by plotting the first derivative o f the distribution in Fig. 1. And finally, plateaus can be seen even in Wai and Wasson's own Figs. 2 and 3, if one ignores the Na point which they themselves disown, and allows for a higher condensation temperature o f As ('r should be smaller than 0.1, in view o f the known stability o f iron and nickel arsenides).
Wasson [6] now attempts to show by X2 tests that the plateaus are statistically not significant at the 5% level However, he stacks the deck by choosing improper intervals, and by averaging H and L chondrites, thus confusing volatile~element with metal-
15 TABLE 1 Mean abundances of volatile elements in ordinary chondrites Elements
Mean abundance, relative to C1 chondrites * Wasson and Chou [31
Larimer and Anders [4]
K, Au, Rb, Li, Mn, Na, P, As **
0.79 -+ 0.09
0.81 -+ 0.18
Cu, Sb, F, Ga, Ag, Se, Ge, S, Sn
0.29 +_0.08
0.22 + 0.07
Te, Zn
0.11 +- 0.01
0.10 -+ 0.02
* Errors are standard deviations of a single value, not of the mean. Wasson and Chou values are for H5, 6 chondrites only, whereas Latimer and Anders values are for all ordinary chondrites. ** Larimer and Anders' compilation does not include Li, P, and As.
explain and the Wai-Wasson model must be interred alongside the Larimer-Anders model! It may be a coincidence, but a simple mirror inversion o f the right-hand side o f Wasson's [6] Fig. 1. pro. duces five little boomerangs.
silicate fractionation. Because the central question is the reality o f the Cu-Sn plateau, the proper intervals are the plateau itself, the two empty regions above and below it, and the two populated regions beyond. The results, for H chondrites only, are given in Table 2. The ×z value for these data is 20.96, not 5.54, and it is significant at the 0.1%, not >5% level Wasson 's attempt to match the chondrite distribution by sets o f random numbers is entertaining and illuminating, but not in the way he intended. Once again he has stacked the deck, first by restricting the random numbers to the range 0.1-0.9, thus guaranteeing a fit at the ends o f the distribution, and second by averaging H and L chondrites, thus mixing up volatile-element and metal-silicate fractionation. But suppose Wasson is right, and the chondrite pattern is truly random. Then there is nothing left for him to
3. "Anders... argued that the elemental ordering in Fig. 1 was not a volatility s e q u e n c e . . . " I n d e e d it isn't. Wasson a n d C h o u [3] t h e m s e l v e s a d m i t : " T h e a b s e n c e o f a p l a t e a u o n o u r p l o t reflects m a i n l y t h e i n c l u s i o n o f seven a d d i t i o n a l e l e m e n t s a n d o u r deliberate arrangement o f the elements in order o f decreasing abundance ratio" ( m y italics). O n e c o u l d equally well p r o v e t h a t t h e v e l o c i t y o f light is n o t c o n s t a n t , b y t a k i n g 18 e x p e r i m e n t a l d e t e r m i n a t i o n s a n d arranging t h e m in d e s c e n d i n g order.
TABLE 2 Depletion factors of 19 volatile elements in H chondrites: x 2 test for randomness *
Depletion factor intervals 0.100-0.115
0.115-0.210
0.210-0.426
0.426-0.683
0.683-1.00
Elements
Zn, Te
none
Sn, S, Ge, Se, Ag, Ga, F, Sb, Cu
none
Na, As, P, Mn, Li, Rb, K, Au
Observed
2
0
9
0
8
Expected
0.32
2.15
4.41
5.72
6.40
* x 2 = 20.96. For 4 degrees of freedom and F = 0.001, x 2 is only 18.465.
16 Larimer showed ten years ago that abundance correlated fairly well with volatility (see Fig. 7 of Larirner [2]) and so it was a foregone conclusion that an abundance sequence would roughly correspond to a volatility sequence. What I objected to was Wasson and Chou's tactic of rearranging the data into an arbitrary sequence of maximum slope, and then using this pretty but contrived sequence to "prove" the absence of a plateau and to tout a model that predicts a steep slope. It does not matter that this sequence shows a vestigial correlation with volatility. For both the above purposes, only an honest, undoctored volatility sequence will do. Now that Wai and Wasson have finally replotted the data against volatility (their Fig. 3), we can see how close their arbitrary sequence (Fig. 1) came to being a volatility sequence: Rank by:
K Au Rb Li Mn Na P As Cu Sb F Ga Ag Se Ge S Sn Te Zn
abundance [1, Fig. 1]
volatility [1, Fig. 3bi
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
(>8) 2 (>8) 3 4 8 1 5 6 10 11 7 9 13 12 14 15 17 16
Let Wai and Wasson have the last word on the question whether Fig. 1 is a volatility sequence: " . . . Fig. 3 demonstrates quite clearly that it is".
4. Constancy of abundances
Wai and Wasson set up a straw man by implying that the two-component model requires all elements
of the Cu-Sn group to have identical abundances of 0.25. The original 1967 data showed a nearly twofold spread which we explicitly attributed in part to analytical error and in part to incomplete loss o f the less volatile elements during chondrule formation (see second footnote on p. 1243 of Larimer and Anders [4], and Anders [5] for a more recent discussion of the problem). I shall have more to say about these points in the next two sections of this paper.
5. "Anders' second point is not well taken"
Wai and Wasson set up a second straw man by noting that Zn and Te which they classify as moderately volatile elements fall significantly below the Cu-Sn plateau. But this classification is man-made, not Godgiven. Latimer and I have never included Zn and Te among the moderately volatile elements, precisely because they fell below the plateau, along with 10 other, highly volatile elements. We attribute the depletion of these 12 elements to incomplete condensation [4,5,8-11 ], and though Wai and Wasson do not accept this particular mechanism, they also have chosen to exclude 10 of these elements from their own paper. It is disingenuous of them to sneak the remaining two elements into the Cu-Sn group, and then complain that they do not fit. Ironically, later on Wai and Wasson themselves have trouble accounting for Zn and Te, and resort to various ad hoc assumptions. Indeed, their calculations confirm that these element fall at the boundary between the moderately and highly volatile elements. Thus each author has the right to classify them with one or the other group, depending on the capabilities of his fractionation mechanisms. Wai and Wasson also express the belief that new analyses are as likely to steepen as to flatten the plateau in the Cu-Sn region. But past experience does not support this expectation. When a large group of volatile elements was analyzed simultaneously in a single laboratory, the mean abundances became more, not less constant. For 12 elements in C2 chondrites, the standard deviation fell from 35% to 8% [12], and for 7 elements in C3V chondrites, from 23% to 14% [ 11 ]. Even for ordinary chondrites, where no comprehensive study from a single laboratory is yet available, the standard deviation of the disputed Cu-Sn
17 group fell from 33% in the Larimer-Anders review [4] to 26% in the Wasson-Chou review [3] seven years later.
TABLE 3 Depletion of Na, Mn, and Cu from chondrules Meteorite
Class
6. Chondrules The most glaring defect of the Wasson-Chou model is its disregard of volatile loss from chondrules. Wasson and Chou accept the prevailing view that chondrules (and the genetically associated coarsegrained metal) were made by remelting of a finegrained primary condensate, similar to the matrix of unequilibrated chondrites. But they contend that no volatiles were lost during this process, citing both theoretical and experimental arguments. The theoretical argument, that chondrules would freeze in 1 0 - 3 second whereas loss of volatiles by diffusion would take 1 second, is not borne out by experimental data of Keil et al. [ 13]. A laserproduced AI20 3 chondrule superheated 100 ° above its melting point took 0.7 second to start crystallizing, though it was efficiently cooled by free fall through cold 02 gas at 625 torr. Moreover, Keil et al. [ 13] concluded from textural evidence that meteoritic chondrules had cooled still more slowly than the synthetic chondrules. Apparently freezing times were at least 3 orders of magnitude longer than estimated by Wasson and Chou, and thus more than ample to permit volatile loss by diffusion. That such loss actually occurred is shown by analyses of volatile elements in separated chondrules. The chondruies are consistently depleted relative to the whole rock (Table 3). Since the rock itself consists mainly of chondrules, the depletion relative to matrix is still greater. Wasson and Chou [3] claim that Tieschitz and Chainpur show no depletion except for Cu, but they had neglected to normalize to Si. And they made no mention o f Schmitt's data on C2 chondrites [ 15], which had been in the literature since 1965 and had been quoted by Larimer and Anders. Wasson's [6] attempts to discredit the data in Table 3 do not stand up to scrutiny. First, even though some individual ratios may be uncertain by +-20%, all 15 ratios are less than unity, which can hardly be due to chance. Second, Wasson is not ]usti-
Chondrules relative to whole rock *
Reference
Na
Mn
Cu
H3
0.82
0.94
0.44
Chainpur
LL3
0.66
0.84
0.20
[ 14]
A1Rais
C2
0.19 0.29
0.75 1.15
0.44 0.67
[15]
Murray
C2
0.30 0.42
0.54 O.75
0.58 0.81
[15 i
Santa Cruz
C2
0.15 0.20
0.48 0.65
0.33 0.44
[15]
Tieschitz
[ 14]
* Normalized to Si, where data were available. For the C2 chondrites, data were normalized to Sc, in order to compensate for the high water and carbon content of the matrix. However, even the raw weight ratios (italics) generally show a substantial depletion, contrary to the claims of Wasson [6].
fled in normalizing Cu to Ni, on the grounds o f its "siderophile character". Schmitt et al. [15] have concluded from the poor coherence o f Cu with siderophile Co that it occurs in a minor phase rather than in nickel-iron, and this is supported by the Tieschitz data [14]. Whereas Cu in Tieschitz chondrules is depleted by only a factor o f 2 relative to the bulk meteorite (48 vs. 93 ppm), Co is depleted by a factor o f 8 (84 vs. 710 ppm). Wasson failed to find a difference because he inexplicably chose Ni (3 analyses) rather than Co (25 analyses) as the normalizing element. Third, Wasson's contention that the Sc-normalized C2 chondrite data in Table 3 could represent "'either refractory enrichment or volatile depletion" is a red herring (Clupea harengus rubens). Even the raw weight ratios (italicized values in Table 3) show a substantial depletion. These ratios need to be corrected downward by some 30-60%, to compensate for the high H20, C, S, and FeO contents o f the bulk meteorites, but it makes little difference whether this correction is based on the actual H20, etc., contents[16] or on some non-volatile element. In particular, Fig. 8 o f Schmitt et al. [15] shows clearly that the correction factors based on refractory Sc and non-refrac-
18 tory Cr are essentially the same for the three meteorites: AI Rais 1.53 and 1.60;Murray 1.40 and 1.36, Santa Cruz 1.34 and 1.36. The reader is invited to cast another glance at the Na column o f Table 3, in view o f ICasson's statement that "chondrule/whole rock ratios o f highly mobile elements such as Na are not significantly different from u n i t y . . . '"
Coarse nickel-iron, which is the metallic counterpart of chondrules, is similarly depleted in the volatile metals Ga and Ge, as shown, ironically, by the junior author of the Wasson-Chou model [ 17]. Metal from Hallingeberg (L3) and Parnallee (LL3) has only 7.1% and 5.2% the Ga content of metal from L5,6 or LL5,6 chondrites. Evidently chondrules (and coarse metal) have lost substantial amounts of at least 5 moderately volatile elements: Mn, Na, Cu, Ga, and Ge. Since chondrulesplus-metal comprise 50-80% of C2 or ordinary chondrites, they play a major role in the volatile-element budget of chondrites. The Wai-Wasson treatment, which ignores chondrules, is grossly unrealistic. Wasson [6] latest argument is that "chondrules (according to Grossman and Olsen [18]) account for less than 5% o f the non-matrix, high-temperature f r a c t i o n " o f C2 chondrites, and are hence irrelevant to the volatile-element budget. This is a deft attempt to hide scientific facts behind a semantic smokescreen. Grossman and Olsen [18] introduced a new and very restrictive definition o f chondrules, requiring them to have "interstitial glass and/or barred and/or radial crystalline texture". They then classified the remaining high-temperature components as "white inclusions" or "single grains and grain fragments", together accounting for 48% o f the meteorite. But Schmitt et al. [15] evidently used a much less restrictive definition o f chondrules, as shown by descriptions such as "abundant", "granular" "blocky'" "platy '" "sugary ", or "[apparently] composed o f sheets o f glass". Thus they must have included many objects which Grossman and Olsen would have classified as "white inclusions". Moreover, McSween [19] has shown in a petrographic study that the single olivine grains and grain fragments in C3 chondrites (and possible also C2 chondrites) are derived from the same melts as the
chondrules themselves. Thus it seems that the less photogenic but more abundant components o f the high-temperature fraction are chemically and petrographically related to chondrules sensu strictu. In any event, since the volatile depletion in Table 3 was established on a very broadly defined population o f chondrules [15], the much narrower definition o f Grossman and Olsen is not relevant.
7. " A b u n d a n c e [ s in C2 c h o n d r i t e s ] fall m o n o t o n i cally as a f u n c t i o n o f c o n d e n s a t i o n temperature..."
Wai and Wasson claim that even C2 chondrites show no plateau; only a "distinct negative correlation . . . . significant at the >99.9% level". But for reasons best known to them, they omit all elements condensing below 600 K, though these elements in C2 chondrites fit smoothly into the general abundance trend [4,12]. I have therefore provided a duly expanded version of their graph (Fig. 1). For consistency, I used only analyses of Mighei, Murchison, and Murray, but this actually make little difference. Arrows indicate suspected errors in the Wai-Wasson condensation temperatures (or upper limits, for Rb
1.0
I
I
I
I
~0.6
o
~-~-~m
?
~ 0.4
0.2
~[]
I IlO0
C2 CHONDRITES, 10-6olm • Kr~he~hl et el. (1973) [ ] Other I I I I {(XX) 900 8(3O 700 50% Cond~flm Tempe,otufe(°K)
I 600
U
~'~
..... C
{ 500
4OO
Fig. 1. Elements condensing near 400 K are no less abundant than those condensing near 900 K, and so abundances do not "fall monotonically as a function of condensation temperature", as contended by Wai and Wasson. In terms of the two-component model, elements condensing between 950 and 400 K are quantitatively condensed on matrix (52% of the meteorite) and almost quantitatively lost from chond~les. Elements of higher condensation temperature are partially retained in chondmles, and hence are more abundant. Data from sources in Wai and Wasson [1] except P [16] and alkalis [23].
19 and Cs). Following the example of Wai and Wasson, I have calculated a least-squares line representing their "negative correlation"; it was fitted to their Fig. 4b. The data below 600 K certainly do not conform to the Wai-Wasson line. I wonder why Wai and Wasson chose to omit this vital piece of evidence, though it had been known for more than ten years [4,5,12]. To my prejudiced eye, the elements divide into two groups. Those condensing above ~1000 K are depleted to 0.7-0.9 of the C1 value, whereas those condensing below 950 K are depleted to an assentiaUy constant plateau level of 0.52 -+ 0.06. These two groups apparently represent partial and complete loss from chondrules. Indeed, the first group contains elements (Au, Mn) known to be partially retained in chondrules, and the second group has a mean abundance just equal to the planimetrically determined abundance of matrix in five C2 chondrites, 0.52 + 0.03 [18]. By Occam's Razor, one is led to the conclusion that C2 chondrites are a mixture of 52% matrix of C 1 chondrite composition with 48% chondrules that had partially retained some of the less volatile elements in Fig. 1, but had lost all others.
two-component model explains this, ad hoc, by a gasdust fractionation [ 11 ], and since Wai and Wasson also resort to ad hoc explanations, they should perhaps abide by the glass house metaphor. (3) "It is kinetically difficult to thoroughly [sic] outgas chondrules during brief condrule forming events." This is an armchair argument, thoroughly refuted by experimental data on meteoritic [14,15, 17] and synthetic [13] chondrules. (4) "An ad hoc mechanism is needed to prevent the recondensation of volatiles.., from [chondrules of higher petrologic t y p e . . , on] matrix of lower petrologic t y p e s . . . " . True, but there is no lack of such mechanisms. Latimer and I [4, p. 1259] proposed three: (a) exhaustion of nucleation sites for siderophiles, by conversion of all metal to FeS; (b) production of chondrules above the median plane of the nebula; (c) preferential accretion of chondrules. Of these, (a) now looks least promising [11], (b) is viable but unproven, and (c) has been strengthened by Whipple's work [20] showing that such preferential accretion is to be expected on aerodynamic grounds.
8. "Chief arguments against the two-component model"
9. Gas-dust separation
Wai and Wasson [1] list four such arguments in the final section of their paper. All are refutable, as I shall attempt to show. (1) "There is no particular tendency of abundance ratio [of the Cu-Sn group] . . . to cluster near 0.25 (Fig. 1)." This subjective opinion is not borne out by objective tests, as shown above. The trend in their Fig. 1 drops steeply just before and after the Cu-Sn group, the mean abundances of the three groups differ by several standard deviations (Table 1), and a X2 test shows that the distribution is non-random at the 0.1% level (Table 2). (2) "The accurately determined Zn abundance ratio of 0.106 is not explained by the two-component model." This is not correct. In the framework of the two-component model, any abundance below the plateau value of ~0.2, be it 0.106 or 0.0106, can be attributed to incomplete condensation. The problem with Zn (and Te) is not the magnitude of the abundance, but the small amplitude of its variation. The
It has long been recognized that gas-dust separation played a role in the formation of meteorites, and influenced their chemistry [21,22]. I myself have recently suggested that some subtle details in the abundance patterns of C3 and ordinary chondrites may reflect gas-dust fractionations by factors of up to 3 [ 11 ]. To this extent, I find myself on common ground with Wasson and Chou. Where we differ, is on the role of chondrules. Wasson and Chou try to explain the abundance of volatiles by gas-dust fractionation alone, whereas I attribute it mainly to volatile loss from chondrules and only secondarily to gas-dust fractionation. Since the depletion of volatiles in chondrules has been repeatedly confirmed in the last 11 years, it would seem that a model totally ignoring this fact is unrealistic.
Acknowledgement This work was supported in part by NASA Grant NGL-14-O01-O10.
20
References 1 C.M. Wai and J.T. Wasson, Nebular condensation of moderately volatile elements and their abundances in ordinary chondtites, Earth Planet. Sci. Lett. 36 (1977) 1-13. 2 J.W. Latimer, Chemical fractionations in meteorities, I. Condensation of the elements, Geochim. Cosmochim. Acta 31 (1967) 1215-1238. 3 J.T. Wasson and C.-L. Chou, Fractionation of moderately volatile elements in ordinary chondrites, Meteoritics 9 (1974) 6 9 - 8 4 . 4 J.W. Latimer and E. Anders, Chemical fractionations in meteorites, II. Abundance patterns and their interpretation, Geochlm. Cosmochim. Acta 31 (1967) 1 2 3 9 - 1 2 7 0 5 E. Anders, On the depletion of moderately volatile elements in ordinary chondrites, Meteoritics 10 (1975) 283-286. 6 J.T.Wasson, Reply to Edward Anders: a discussionof alternativemodels for explainingthe distributionof moderately volatile elements in ordinary chondrites, Earth Planet. Sci. Lett. 36 (1977) 2 1 - 2 8 . 7 H.C. Urey, A typical parent meteorite body - a reply to Dr. J.F. Lovering, Geochim. CUsmochim. Acta 16 (1959) 194-196. 8 R.R. Keays, R. Ganapathy and F. Anders, Chemical fractionations in meteorites, IV. Abundances of fourteen trace elements in L-chondrites; implications for cosmothermometry, Geochim. Cosmochim. Acta 35 (1971) 337-363. 9 J.C.Laul, R. Ganapathy, E. Anders and J.W. Morgan, Chemical fractionationsin meteorites,VI. Accretion temperatures of H-, LL-, and E-chondrites,'from abundance of volatiletrace elements,Geochim. Cosmochim. Acta 35 (1973) 329-357. I0 J.W. Larimer, Chemical fractionationsin meteorites,VII. Cosmothermometry and cosmobarometry, Geochim. Cosmochim. Acta 37 (1973) 1603-1623. I I E. Anders, H. Higuchi, R. Ganapathy and J.W. Morgan, Chemical fractionationsin meteorites,IX. C3 chondrites, Geochim. Cosmochim. Acta 40 (1976) 1131-1139. 12 U. Kr~qqenbiihl,I.W. Morgan, R. Ganapathy and E.
13
14
15
16 17
18
19
20
21
22
23
Anders, Abundance of 17 trace elements in carbonaceous chondrites, Geochim. Cosmochim. Acta 37 (1973) 1353-1370. K. Keil, M. Prinz, H.N. Planner, S.R. Skaggs, E. Dowty, L.S. Nelson, N.L. Richardson and M. Blander, A qualitative comparison of textures in lunar chondmles and CO 2 laser-formed synthetic chondrule-like spherules, Spec. Publ. No. 7, Univ. of N. Mexico, Inst. of Meteoritics (1973) 15 pp. T,W. Osborn, R.H. Smith and R.A. Schmitt, Elemental composition of individualchondrules from ordinary chondrites,Geochim. Cosmochim. Acta 37 (1973) 1909-1942. R.A. Schmitt, R.H. Smith and G.G. Goles, Abundances of Na, Sc, Cr, Mn, Fe, Co, and Cu in 218 individual meteoriticchondrules via activationanalysis,I, J. Geophys. Res. 70 (1965) 2419-2444. B. Mason, The carbonaceous chondrites,Space Sci. Rev. I (1963) 621-646. C.-L.Chou and AJ. Cohen, Gallium and germanium in the metal and silicatesof L- and LL- chondrites, Geochim. Cosmochim. Acta 37 (1973) 315-327. L. Grossman and E. Olsen, Origin of the high-temperature fractionof C2 chondrites,Geochim. Cosmochlm. Acta 38 (1974) 173-187. H.Y. McSween, Jr.,O n the nature and originof isolated olivinegrainsin carbonaceous chondrites,Geochim. Cosmochlm. Acta 41 (1977) 411-418. F.L. Whipple. O n certainaerodynamic processesfor asteroidsand comets, in: Nobel Symposium 2 I, From Plasma to Planet, A Elvius, ed. (Almqvist and Wiksell, Stockholm, 1972) 2 1 1 - 2 3 2 . H. Suess, Chemical evidence bearing on the origin of the solar system, Annu. Rev. Astron. Astrophys. 3 (1965) 217-234. J.A. Wood, Olivine and pyroxene compositions in Type II carbonaceous chondrites, Geochim. Cosmochim. Acta 31 (1967) 2095-2108. W. Nichiporuk and C.B. Moore, Lithium, sodium and potassium abundances in carbonaceous chondrites, Geochim. Cosmochim. Acta 38 (1974) 1691-1701.