A test of the smoothness of the elemental abundances of carbonaceous chondrites

Geochimica et Co~mochimica Acta Vol. 53. pp. 471--481 Copyrighl © 1989 Pergamon Press plc. Printed in U.S.A.

0016-7037/89/$3.00 + .00

A test of the smoothness of the elemental abundances of carbonaceous chondrites* D. S. BURNETT I, D. S. WOOLUM 2, T. M. BENJAMIN 3, P. S. Z. ROGERS 3, C. J. DUFFY 3 and C. MAGGIORE 3 aDivisionof Geological and Planetary Sciences. California Institute of Technolngy, Pasadena, CA 91125, U.S.A. 2Department of Physics, California State University, Fullerton, CA 92634, U.S.A. 3Los Aiamos National Laboratory, Los Alamos, NM 87545, U.S.A.

(Received May 20, 1988; accepted in revisedform November 14, 1988) AlntmctDThe identification ofCl chondrite concentrations with average solar system abundances for heavy elements is based primarily on the smoothness of the CI abundance curves for odd mass nuclei. A good test of smoothness is measurement ofall elements in a given mass range in the same sample with the same technique. High preci~on protoninduced X-ray spectra ofCl chondrites yieldedanalysesof 17 elements (Ni through Ru, plus Fe and Pb) with precisions better than 10% for all except As, Pb, Nb, and Ru. Excellenttheoretical descriptions of the spectra were obtained. Two independent estimates of precision agree well, giving confidence in the quoted errors. Intersample differences are the largest source of variability. Within these limits good agreement with literature results are obtained, except for As and Y. Although our Y values are 10 to 30% lower than previously adopted, a monoelemental s-process peak in the abundance curve at Y is still necessary. Except for Br (higher by 59% in Ivuna), there are no significantconcentration differences between Orgueil and Ivuna. In general, our results confirm previous abundance curves. The abundances are exceptionally smooth and strongly decreasing in the mass 60-75 region. From mass 75-101 a smooth curve can be drawn, within limits of intersample variability,except for the Y peak. Over the whole periodic table a large number of peaks of probable nucleosyntheticorigin can be identified,some understood, some not. These smoothness deviations are 10 to 30% and set an overall limit to the smoothness argument alone in justifying using CI abundances as average solar system values. INTRODUCTION

10% level, entirely on a CI-normalized basis. Thus, it is important to determine the limits of validity of the CI concentrations as average solar system abundances, i.e. the limits on the concept of CI abundance smoothness. Significant new conclusions on solar nebula chemical fractionation processes could be lurking in relatively small non-smooth features in the CI abundance curves. We have performed analyses of carbonaceous chondrites using proton-induced X-rays (PIXE) for mass numbers (A) of 56-100, i.e. for elements from Fe through Ru. All the elements (except Co) in this mass range were measured at the same time in the same sample. Any non-smooth feature in present abundance compilations due to a systematic error in a specific analytical procedure or due to an anomalous sample could be recognized, although we had no reason to believe that such biases were present. The precision of the relative concentrations of these elements is most important. In our case, corrections are required to convert the relative X-ray peak intensities to relative concentrations: for X-ray production, for X-ray absorption both in the sample and in the absorber foils over the Si(Li) detector. These corrections can be made with reasonable accuracy. Tabulations of the required atomic data are stated to be accurate to a few percent or better, but the important point is that these corrections are smooth functions of atomic number to a high degree of accuracy. The essential issue, the smoothness of the abundances of the odd mass isotopes of the elements measured, is independent of these corrections. In other words, what is most difficult to measure is the absolute concentrations; the relative concentrations are better measured, but elemental smoothness should be measured best of all. Our goal was to be able to see 10% deviations in the abundance of a given element from a smooth trend, at least for the more abundant even atomic number (Z) elements. Our analytical objective was to test the accuracy of our

THIS STUDYHAD both scientific and analytical objectives. Our science objective was to investigate the smoothness of CI chondrite abundances at as high a level of precision as possible. CI chondrite composition is widely accepted as being identical to that of the average solar system composition for non-volatile elements. There is no a priori reason that a rock falling from the sky should have the average composition of the solar system. The basis for the above identification is empirical but based on two strong arguments, traceable at least to SUESS (1947) and S u ~ s and UgEY (1956): (1) the agreement of Si-normalized abundances between chondrites (restricted to CI chondrites at present) and the solar photosphere, within the error limits of the latter, and (2) the smoothness of CI abundances of odd-mass nuclei, plotted as a function of mass number. Recent reviews of CI abundances by ANDERSand EBIHARA(1982) and ANDERSand GREVESSE (1989) show that, as both solar and meteoritic data have improved, these arguments have been strongly supported. The limitation of argument (1) has always been the accuracy of the photospheric abundances. Quoted error limits at present ale low (see, e.g. GREVESSE, 1984, or ANDERSand GREVESSE, 1989), especially for major elements lighter than Fe. This is fortunate, because elemental abundances below Fe are not especially smooth. The accuracy of the heavier element photospheric abundances is possibly still a debatable issue, thus the validity of CI heavy element abundances rests on the smoothness argument. Essentially all solar system abundance compilations have adopted CI abundances for non-volatile elements heavier than Ne. The smoothness issue is also important for meteoritics. Meteoritic abundance data are now being interpreted at the * Contribution number 4631. 471

472

D.S. Burnett et aL

PIXE techniques. Although not a true standard, we accepted that CI abundances are relatively well known. The presence of all elements in this mass range in measurable abundances and the variations in abundances due to the even-odd effect provide a good but realistic challenge to our spectral data processing techniques. Only in the cases of Br and Y did we believe that we could possibly improve the absolute abundances, as there were deviations from a smooth abundance curve for these elements in the Anders-Ebihara compilation. The situation for the Br abundance is discussed by ANDERS and EBIHARA (1982), and new measurements seemed justified. Taken literally, the Anders-Ebihara Y abundance implied a single element s-process abundance peak for the N ffi 50 neutron shell, which was a conclusion of sufficient importance to warrant special attention. Consequently, emphasis has been given to obtaining good data for these two elements.

ground, (i.e. primarily on concentration). Concentrations are not a smooth function of Z but oscillate because of the pronounced oddeven atomic number effect.This precisiongoverns our abilityto assess abundance smoothness. In allcases except Nb, M o and Ru, the error

in fitting the background spectrum is larger than counting statistics. An estimate of precision is provided by the standard deviations of the least squares fit calculated by the DUFFY et aL prosmm (lst column, Table l). This allows for counting statistics and also propogates errors from the background correction, and from any required peak deconvolutions. An alternative estimate of precision has been made by varying the parameters which are fixed in the least squares fit, specifically the order of the polynomial background shape and the energy range. The quality of fit was critically assessed by visual inspection, and only the results of acceptable fits (those which described the total peak and background spectrum within counting statisti~ errors) were averaged to obtain concentrations. The variation (standard deviation) of the results from different fits was taken as a measure of precision. The second column of Table I summarizes our estimated precisions for a single analysis based on variations in fit parameters. Comparison of the first and second columns in Table l shows that the agreement between the two independent estimates of precision is good. For Mo and Ru a lot of trials in fitting were not run, so for these elements the least squares estimates of precision (column 1) are better. We emphasize that this precision is what is relevant in assessing smoothness. Additional systematic errors are possible in comparisons of relative or absolute concentrations. These precisions oscillate in accord with the odd-even effect in solar system abundances, being lower for odd Z elements. However, even for odd Z elements there is considerable variation, with the relatively high abundances and simpler spectra making analyses of Br and Rb good and, conversely, for As and Pb, relatively poor. This agreement gives us considerable confidence in our precision estimates. For comparison the counting statistics uncertainties in the background corrected peaks tabulated in column 3 are distinctly less, showing that for all elements tabulated except Ru, the major source of error arises in locating the background. Additional discusdon ofprechion is given in BURNETTeta/. (1988).

EXPERIMENTAL The experimental techniques, data analysis, and additional discussion of precision are given in BURNETTel al. (1988). PIXE analysis with 3 MeV protons samples approximately the outer 30 microns; consequently, to minimize sampling variations, materials were homogenized prior to analysis. Museum-interior samples (about 50 nag) were excavated using stainless steel tools from single 2 gram specimens of Otgueil (USNM 2216) and Ivuna (USNM 2478). Our samples for a given meteorite thus correspond to mm separations on the CI parent body. Samples were broken and mixed, usually in a steel ball mill with a polyethylene ball, and pressed into a 5 ram diameter pellet using a stainless steel die. Some of the pellets had bern used previously (STAPANIAN,1981; WELLERet al., 1978). X-ray spectra were decomposed with a slightly modified version (BURNETT et al., 1988) of the constrained least squares procedures of DUFFY et al. (1987). Compositional homogeneity is assumed in the data analysis. The meteorites are a mixture of phases, but, because of the mechanical homogenization and because of the inherent micron to sub-micron grain size,the uniform composition assumption should be good. Each X-ray passes through typically 20 to 200 grains. The major issue in spectral deconvolution is the precision of the background subtraction. This source of error is not a smooth function of atomic number (Z) but instead is dependent on signal to backTable 1.

Absolute concentrations

Our present PIXE analysis procedures are based on an "internal standard", with absolute concentrations calculated by normali~ng to a known, usually major, element concentration. In these spectra, possible normalization elements are Fe, Ni, or Sr. All of these have

Uncertainty estimates (%). SlnKle analysis standar~ deviations Least Visual Countin s squares fit statistics (a) (b) (c)

Fe Ni Cu Zn Ga Ge As Pb Se BP Rb Sr Y 2r Mo Eu

1.2 1.4 8 2.0 9 1.3 14 14 0.8 2.9 6 1.5 9 3.0 9 15

1.6 1.O 14 3 10 1,3 13 13 1.0 3 7 2.0 11 ~ 5 (5)

0.07 0.29 2.7 0.53 4.4 0.9 }6 0.7 2.0 3.0 1.2 4 1.4 4 17

Intersample standard deviations Orguell only

Ivuna only

2.3 7.9 3 (d) 23 4.0 3.3 20 36 2.2 10 25 10 14 5.7 12 47

6.1 7.4 11 14 15 2.6 25 21 4.2 19 19 2.0 11 6.4 14 (d) 37

All samples 4.4 8.5 8.1 (d) 17 10 2.7 21 27 4.1 30 20 6.5 14 6.0 9 (d) 38

(a) Average of statistical standard deviations of least squares fits as calculated by a n a l y s i s program of Duffy et al. (1987). (b) Estimated frOm range of values from visually acceptable fits. Number of cases for Ru too small for reliable estimate. (c) Countlng statistics for hand background-corrected peaks for sample 627. Samples 627-631 have comparable numbers o f counts. Counts f o r spectrum 626 are lower by a factor of approximately 1.7. Single entry for As and Pb since peaks are totally unresolved. (d) Cu analysis o f sample 629 and Ho analysis o f sample 628 have been excluded.

Chemical composition of carbonaceous chondrites clean, high intensity peaks. However, the detector foil absorption corrections for Fe and Ni are large, and these elements are at the low end of the mass range under consideration. Srisa better normalization element, as it is near the middle of the mass range under consideration with the overall corrections between intensity ratios and concentration ratios to many other elements being relatively small. Also, accurate isotopic dilution measurements for Sr (MURTHYand COMPSTON, 1965; KAUSHAL and WETHERILL, 1970; MITTLEFEHLDT a n d WETHERILL,1979) agree reasonably well with each other (6% intersample standard deviation for Orgueil; 165 for lvuna) and show no systematic differencesbetween Orgueil and Ivunm Consequently, our absolute concentrations are based on an average Sr content of 7.91 ppm as compiled for Orguefi by ANDER8and EBIHARA(1982). (For comparison, the average of three Ivuna Sr measurements in the literature is 7.81 ppm; the more recent ANDERSand GREVESSE, 1989, compilation gives 7.80 for a mean CI chondrite.) There are definite intersample variations in the literature Sr data, and probably in our samples as well. However, it is not necessary to normalize the Sr in each sample to 7.91 ppm, because the relative Sr concentrations among the six samples can be calculated using the relative integrated proton beam currents. Consequently, we normalized the average of Sr contents for all six samples to 7.91 ppm. As a check, we compared concentrations calculated in this way with concentrations calculated by normalizing the Sr of each sample to 7.91 ppm and also with normalization of each spectrum to the Orgueil Fe or Ni concentrations compiled by ANDERSand EelHARA (1982). There is no objective way to evaluate which normalization scheme gives "better" results; however, the variations in concentrations for a given sample among the different normalizations are usually less than the intersample variations. Further, the intersample variations for most elements are less when the "all-sample" Sr normalization described above is used than with the alternative normalization schemes. In other words, the a priori better way of normalizing gives an overall data set with less scatter.

RESULTS

Our analytical results are given in Table 2. The first column is the single analysis standard deviation, based on the higher of the two estimates from columns 1 or 2 of Table 1. The "visual fit" entry in Table 1 does not allow for counting statistics, so when this entry was used for Table 2, a small contribution from counting statistics to the total error was included. Table 2.

Element

Y Zr

Two pairs of analyses (Ivuna 628-631 and Orgueil 626629) refer to the opposite faces of the same meteorite pellet. Ideally these should agree independent of any sampling problems; however, they do not consistently show better agreement than interpellet comparisons. The Orgueil 626629 results are more similar to each other than either compared to 630, but this is not true for the Ivuna samples. For both 628 vs. 631 and 626 vs. 629, over half of the elements show differences greater than twice the single analysis standard deviation. It appears that we did not achieve complete homogenization; consequently, we regard each analysis as an independent sample. Two high analyses have been rejected in calc~ating averages: Orguei! 629 Cu and Ivuna 628 Mo. An advantage of the instrumental nature of PIXE is important here. Visual inspection of the spectra unambiguously shows that these elements are enriched in these cases. High Cu might reflect contamination. Commercial AI with high Cu content is extensively used in the experimental area. High Mo is less easily explained and may be real, hut high analyses are always suggestive of contamination (MoS2 lubricants are not used). Although Ph is highly sensitive to contaminatio n, our Pb concentrations look reasonable relative to the literature. This agreement indicates that, overall, contamination is not serious. The Ru concentration for Orgueil 626 is anomalously low, also evident by inspection of the spectrum, but we have no reason for rejecting this analysis. The theoretical relative intensities used to convert counts to concentrations depend on knowing the bulk absorption coefficient, which in turn requires knowledge of the bulk composition, especially Fe. Initial data processing, based on the Orgueil Fe wt% compiled by ANDERS and EBIHARA (1982), was iterated using absorption corrections based on the Fe concentrations obtained in the first calculation. The changes in concentration for all elements by this iteration were less than 2%.

P]XE a n a l y t i c a l r e s u l t s f o r CI chondrites.

oo Precision (%)

Fe (g) NI (%) Cu (ppm) Zn Ga Ge As Pb 5e Br Rb

Sr

473

Anders &

Orguell

Ivuna 627

Ivuna

]vuna

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average

Total

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629

Orguell 630

628

631

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(b)

average

(e)

17.9 0.99 256 310 9.1 35.9 1.14 3.14 19.3 3.16 2.57

t8.7 1.10 114 196 8.7 33.6 1.71 1.45 19.7 2.75 1.59

19.9 1.20 96 235 8.3 35.4 1.09 2.36 2t.6 3.98 1.80

18.9 1.11 119 304 10.4 35.3 1.70 2.71 20.6 4.97 2.19

17.6 1.03 109 299 7.9 33.8 1.18 3.51 19.9 5.89 2.64

18.5 1.10 112 308 10.1 32.2 1.91 2.43 18.2 3.56 2.30

18.5 1.06 110 272 9.0 3q.7 1.LIO 2.65 20.2 4,02 2.23

1.2 9 0.6 6 1.0 2.1 1.5 1.9 3 9 2.5

7.97

7.06

8.64

7.95

7.75

8.07

7.91

7.89

=-7.91(£)

2.6

1.09 3.94

0.99 3.82

1.29 4.26

1.22 3.85

1.47 4.05

1.23 3.56

1.50 3.69

1.12 4.01 1.08 0.56

1.21 3.91 1.08(c) 0.62(d)

1.2 1.5 1.0 2.5

Orguell 626(a)

Orguell

1.6 1.4 14 3 10 1.3 14 14 1.2 3.5 8

18.1 0.95 112 291 9.5 34.3 1.57 2.74 20.2 3.38 2.58

2.5 12 4

Ho

9

0.93

1.16

1.15

1.05

3.15

1.12

0.92

Ru

15

0.26

0.69

0.74

0.53

0.95

0.52

0.71

18.2 1.01 113 (b) 265 9.1 34.6 1.48 2.45 19.7 3.10 2.25

(a) Counting t l c e on 8ample 626 Is about 1/2 that o f remaining samples; t h i s a f f e c t s the preeision estimate only f o r Mo and and Ru (compare Table 1). (b) Excluding analysis o f sample 629. (c) Excluding analy818 o f sample 628. (d) Average includes 628 analysis; I f omitted, average is 0.69. (e) Ols ~ lntersample standard deviation (Table 1); o0 is precision trois f i r s t column. ( f ) Total average Sr normalized to Anders and Eblhara (1982).

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D.S. Burnett et al.

Nb abundance

The precision for individual Nb analyses is low for this element (18-50%), and they are not tabulated. Combining all data gives 0.34 _+0.06 ppm, where the quoted error is the intersample standard deviation of the mean, in acceptable agreement with the more precise value for Orgueil of 0.246 ppm from JOCHUM et aL (1986). Intersample variations

The final column in Table 2 is the ratio of the intersample standard deviation to the single analysis precision. For all elements except Cu this ratio is larger than 1 and is greater than 2 for 8/16 elements, indicating that, overall, inhomogeneities are the largest source of variability in our data. Comparisons with the literature

For comparison, Table 2 includes the Orgueil concentrations from ANDERSand EmHARA (1982), the average of our Orgueil analyses, and our total sample average concentrations. (The Orgueil concentrations in the more recent ANDERSand GREVESSE, 1989, compilation differ insignificantly for these elements.) Overall agreement is reasonably good. The comparison is better seen in Fig. 1, which shows the percent deviation between the literature and our Orgueil and Ivuna analyses separately. For Ivuna we appear to have made the only Pb analysis and the only modern Zr analysis. There is no evidence for systematic errors arising from our adopted X-ray production cross sections and absorption coefficients; differences with the literature are both positive and negative. Including both meteorites, (10/28, 19/28) comparisons (omitting Sr) agree within (1, 2) standard deviations. Differences outside 2 standard deviations occur for Orgueil (Ga, Ge, As, Se, Y) and for Ivuna (Ge, As, Se, Y). Our Ga intersample variation for Orgueil is probably fortuitously low, but the other differences are worth discussing in more detail. Zn, Ge, Se abundance

The best comparisons with the literature are made with the most precise data, in our case Zn, Ge, and Se. Figure 2 compares our individual sample analyses with two modern activation analysis studies (EBIHARAet al., 1982, using 50100 nag samples, and KALLEMEYNand WASSON, 1981, using 100-500 mg samples). Note the linear scales and offset zeros for Fig. 2; we are considering a finer scale of variation than in most geochemical discussions. Both activation analysis studies showed little variation in Zn. Four of our samples agree closely with the neutron activation Zn values, but one Orgueil and one Ivuna sample are distinctly lower. There are potentially serious Zn contamination problems, so excursions on the high side could be interpreted, but low excursions are more difficult to explain. A large number of high results could indicate pervasive contamination, but this seems implausible because a lot of scatter would also be expected, and the sharp clustering of most of the analyses must be respected. At 300 ppm, CI materials are probably relatively insensitive to Zn contamination. The opposite trends from Zn are true for Ge and Se in Fig. 2; our concentrations show relatively little variation

compared with those in the literature. Further, our results are dose to mid-range of the literature concentrations for both elements. The Se concentrations of KALLEMEYNand WASSON (1981) are systematically about 20% higher than those of EBIHARAel al. (1982), and there is a hint of a similar systematic offset for Ge, although it is less clear. Taking all data at face value, there is a suggestion of a correlation of Ge and Se, but interpreting 20% differences in data from different laboratories is still probably not warranted. At least part of the spread may be analytical, as comparison of the analyses of the USNM Allende reference powder shows the KALLEMEYN and WASSON(1981) concentrations of Ge and Se to be higher than EmHARA et al. (1982) by 13 and 11%, respectively. With respect to PIXE data, the literature comparisons of the high precision data give no evidence for systematic inaccuracies beyond the 10% level, although CI ehondrites are not the optimum samples for this comparison. This discussion illustrates the value, from the point of view of assessing abundance smoothness, of having a data set with all elements measured by a single technique with good precision over a broad mass range. As abundance

Four analyses of As in Orgueil from KALLEMEYNand

WASSON(1981) range from 1.75 to 2.05 ppm. FOUCHE and SMALES(1967) report 2.1 ppm for Orgueil and 1.9 for Ivuna. Our As concentrations are lower, ranging from 1.1 to 1.7 ppm. Our Orgueil average is 22% lower than that adopted by ANDERSand EBIHARA(1982), and our Ivuna value is 31% lower than FOUCHEand SMALES(1967). Arsenic is precisely determined by neutron activation, whereas it is difficult for us because of the blending of the As K-alpha with the Pb Lalpha peaks (BURNETTel al., 1988). If our As analyses are low, it might be expected that our Pb concentrations would be high because of the blending. However, our average Pb for Orgueil is within 1% of that given by ANDERS and EmHARA(1982). Although not apparent in expanded scale plots of the deconvolutions, it is conceivable that the best fit hackground spectra are systematically high for As, possibly because As is near the broad background maximum (BURNETTet al., 1988), which is the most difficult part of the background spectrum to define precisely. However, to the extent that the Pb L-alpha contributions to the blended line are fixed by the Pb L-beta intensity, then all background errors must be absorbed by As, explaining why As might be low but Pb not high. Solar system Y abundance

Our analyses for Y are systematically lower than the literature. However, it is important to emphasize that, even with our lower Y, it is still necessary to have a single element peak in the odd A solar system abundance curve at mass 89. This decreases the importance of this difference between us and the literature for issues of average solar system abundances and nuclcosynthesis. However, as discussed below, there are interesting geo- and cosmochemical questions associated with this difference.

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Chemical composition of carbonaceous chondrites

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FIG. 1. The upper panel is the percentage difference between our analyses of Orgneil and literature values as compiled by ANDERSand EmHARA(1982). Error bars arc the standard deviation of the mean based on the intersample variations. The middle panel is the same for Ivuna with data taken from references given in ANDERSand EmHARA(1982). NO appropriate comparisons are possible for Pb or Zr. The bottom panel is the percentage difference and the standard deviation of the difference between our Ivuna and Orgneil analyses. Significant deviations arc rare; exceptions are discussed in the text. SCHMITT ely a/. (1964) report 1.44 ± . 16 ppm Y for Orgueil and 1.69 ± .19 ppm for Ivuna, while JOCHUM et aL (1986) report 1.57 ± .04 ppm for Orgueil. Our mean for Orgueil is 29 ± 5% lower than JOCHUM ely aL For Ivuna we are 22 ± 5% lower than SCHMITT ely •l. Y is an important element in the smoothness discussion and was a target element for our study, with experimental conditions chosen to give good precision. Also, a considerable amount of effort was put into evaluating the precisiOn. The largest source of error is the background correction, but we arc unable to fit background equations which give concentrations outside the range expected from the precision estimate in Table 2. The situation for Y is far more favorable than for As, and even though our results may be low for As, we see no parallel with Y. Given that all our analyses are on material from single gram-sized specimens, Y data are available on only three samples of Orgueil and two of lvuna; consequently, there is a finite probability that the differences are coincidental. Giving each specimen analyzed (our two plus literature) equal weight gives an average CI Y abundance of 1.43 ppm, about 8% lower than the mean CI chondrite value tabulated by ANDERS and GREVESSE (1989). Nevertheless, it is of interest to see if other data favor the higher literature or our lower Y abundance. According to GREVESSE (1984) or ANDERS and GREVESSE (1989), Y is one of the more-precisely determined elements in the solar photosphere (±7% relative to H). For a relatively high precision comparison, we need another element as a photospheric reference element which is known to at least comparable precision. Other than Y the smallest errors quoted by ANDERS and GREVESSE (1989) are for Ca and Ti, which, cosmochemically, are good comparison elements for Y. The quoted errors in the photospheric Y/Ca and Y/Ti are ±8% and agree within these limits with the higher literature CI abundance and do not support our lower Y abundance.

One can also attempt to use planetary refractory lithophile element abundance ratios involving Y as an additional constraint. Because of the identity of charge and ionic radius, the Y/Ho ratio is expected to be highly invariant with respect to a great many possible fractionation processes. Although there are differences in other properties, this ratio might be regarded as the ultimate planetary constant. However, in terms of volatility, Y is more refractory than Ho and, more importantly, is predicted to condense as the oxide before pcrovskite condensation, unlike Ho (KORNACKI and FEGLEY, 1986). Thus cosmochemical separation is possible, but fractionation in planetary processes appears more difficult, making this ratio of considerable interest. Since cosmochemical fractionations are possible, only materials with smooth, ideally unfractionated, heavy REE patterns are suitable for this discussion. Thus, in general Ca-Al-rich inclusions might be relatively unsuitable for evaluations of the solar system Y/ Ho, althOUgh precise measurements are possible because of high refractory element enrichments. However, planetary crustal materials in which Y and the REE have been Substantially enriched by igneous differentiation meet the criterion. Figure 3 is a histogram of Y/Ho for samples on which both elements were measured simultaneously. Relatively few data are available, because neither element is measured routinely in modern activation analysis schemes designed primarily for rapid sample throughput. Our Y abundance and the Ho abundance from ANDERS and EmHARA (1982) correspond to a weight Y/Ho ratio of 22, which is closer to the most probable value of the lunar distribution than to 28, based on ANDERS and EBIHARA (1982) or the more recent work of JOCHUM et al. (1986). Y/Ho ratios for C2 and C3 chondrites by JOCHUM et al. (1986) are quite close to their Orgueil result (range of 26 tO 32; mean ffi 29 ± 2). There is an interesting high ratio tail to the lunar distribution based almost entirely on Apollo 15 samples. Except for one Apollo

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476

D.S. Burnett et al.

17 sample, all lunar samples with Y/Ho > 26 are from Apollo 15, with the highest samples being the green glass. Terrestrial volcanic rock and crustal sediment data can be found consistent with either 22 or 28 (Fig. 3, top), although the mean is close to 28. It may be that at least the terrestrial data, and possibly all data, are not sufficiently precise for this comparison. Overall, there is a total spread of a factor of 2 for terrestrial Y]Ho. This is typical or less than other lithophile element ratio variations, but it is not obvious that the spread should be this large for this element ratio. In summary, consideration of Y/Ho is inconclusive and does not resolve the minor discrepancy in the solar system Y abundance, but does indicate that further study of Y/Ho would be interesting, possibly important. Comparison with photospheric abundances does not support our lower Y abundance. Inter-element correlations We have systematically searched for correlations among the six CI samples analyzed. The principal conclusion, which is possibly important and will be discussed in detail later, is that there are essentially none, even though there are intersample variations well beyond the single analysis precision (Table 2). The best correlations among our analyses, between Zn, Pb, and Rb, are shown in Fig. 4. The correlations involving Zn are produced by the two low Zn concentrations

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The single analysis precisions given in Table 2 permit a relatively high resolution test of compositional differences between Orgueil and Ivuna (bottom panel, Fig. 1). This is an important comparison because there is no a priori way to tell which of the two widely available CI meteorites better approximates average solar system abundances if the two have different comi~sitions. It is remarkable that only Br shows a difference greater than 2 standard deviations. In 11/16 cases Ivuna is higher, but only five elements are significantly different at greater than one standard deviation. Only Pb, Y, and Ru show apparent differences between 10 and 20%. In strong contrast, Br is 59% higher in Ivuna, This was previously noted by ANDERS and EaIHARA (1982), who based their solar system Br abundance on Br/In and Br/Cd from C2 and C3 chondrites.

Abundance smoothness in the mass 60-100 region

III',

18 20 22 24 26 28 30 32 54 36

Orgueil-Ivuna differences

DISCUSSION

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,o

,

discussed in relation to Fig. 2 and could be fortuitous. The Zn-Rb correlation is not present in the data from EBIHARA et al. (1982). Essentially, Fig. 4 shows that the Orguefl (630) and Ivuna (627) samples that are lowest in Zn are also lowest in Rb and that the Orgueil 630 sample is also lowest in Pb. Inspection of Table 2 shows that these two samples have the lowest Br among Orgueil and Ivuna separately. These (Zn, Pb, Rb, Br) are relatively volatile elements, so it might be that these samples are depleted in a volatile-rich nebular component. (It does not seem reasonable that the low concentrations were produced by proton beam heating, as all samples except Orgueil 626 had essentially the same beam currents and irradiation times. All samples were mounted the same way and should also have had the same conductive cooling.) The volatility correlation is not perfect in that these samples are not depleted in Se. Alternatively, the common property might be their solubility of these elements under CI parent body hydrothermal conditions (E. ANDERS, priv. commun.). Overall, however, it is perhaps best to regard the CI intersample correlations in Fig. 4 as fortuitous.

38

Y/H0 FIG. 3. Y and H o should be difficult elements to fracfionat¢ geochemically, and it is o f interest to compare Y / H o ratios for planetary

crustal materials with the CI ratio either based on the Y abundance in this work or, altexnafively,from JOCHUMet al. (1986), as indicated by vertical dashed lines in the bottom panel. For both CI values the Ho abundance for ANDERSand EBIHARA(1982) is used. Lunar and terrestrial data are primarily from the Mainz (H. Wttnke) and Canberra (S. R. Taylor) laboratories. Lunar data compiled from the Proceedings of the Lunar and Planetary Science conferences. Terrestrial volcanic rock d~ta from BASALTICVOLCANISM STUDY PROJECT ( 1981) and PERFITet al. (1983). Crustal sediment data from TAYLOR and MCLENNAN0985).

Abundances in the mass 60-100 range cover 4 orders of magnitude. A detailed study of smoothness requires examining the data on a finer scale. Figure 5 shows our average odd A abundances for the Ni-As region. The discussions of solar system abundances by SUESS (1947) and SUESS and UREY (1956) were unusual in that the local slope of the abundance curve was well known from the isotopic abundances of those elements which had two odd A stable isotopes, but the absolute abundances were poorly known. This important concept--along with the bold assumption that the abundance curves were indeed smoothmwas extensively used by SUESS and UREY (1956). The abundances shown in Fig. 5 are exceptionally smooth. Today, presumably both the slopes and the values of the abundances are known. The slopes of the curve in Fig. 5 at masses 61-63 and 69-71, dictated by the isotopic abundances of Cu and Ga, are quite consistent with possible smooth curves through the data points ignoring

Chemical composition of carbonaceous chondrites i

i

477 I

i

CI Chondrites

•

30o

~

: ~-: N~i\\ Cu

N 25C 20C

x z I

~2

, •

\ I

\

I

\

3.~ Zn ~'.~

-~- 3.0 2.5 2.0

i t

15

l PIXE EWA AE • Orgueil • ,~ • [vuno A I

/

20 25 Rb(ppm)

FIG. 4. The only significantinterelement correlations in our data are for Zn, Pb, and Rb, and these are possibly fortuitous. There is no Zn-Rb correlation in the data of EBIHARAel a[. (1982; EWA). Averase Orgueil concentrations from ANDERSand EmHARA(1982) are shown as stars.

the isotopic constraints. Both our abundances and those of ANDERS and EBIHARA (1982) or ANDERS and GREVESSE (1989) show a small (roughly 10%) excess at Cu relative to the smooth trend in this mass region. This could be the residual effect of Cu contamination, or it may represent an actual smoothness deviation in the abundance curve. In the mass range 75-91 the abundances are sufficiently slowly varying that a small-scale linear plot (Fig. 6) can be made, which is optimum for discussing abundance smoothness. The uncertainties associated with intersample variability can be illustrated on this scale. The magnitude and slope of the curve on Fig. 6 have been drawn to be consistent with Fig. 5. The Br isotopic abundance is a major constraint, although, in view of the Ivuna-Orgueil Br concentration difference, some freedom exists in drawing the curve between masses 79 and 83. We have drawn a curve through the lower bound of the error bar on 'SRb. With the constraint of the Br isotopic abundance, a Se abundance which is 4 to 5% higher than what we find is required, but this is quite plausible within the overall chemical heterogeneity in CI material (compare Fig. 2). Figure 6 shows that the Orgueil Br abundance is more consistent with this smooth abundance curve than Ivuna. Similarly, ANDERS and EBIHARA(1982) found that the Orgueil Br concentration agreed with their Br abundance estimated from Br/In and Br/Cd for C2 and C3 chondrites. In this mass region a very different perspective on solar system abundances would result if only Ivuna were available for study. A mono-elemental Br abundance peak would result which, in fact, could be plausibly interpreted as an r-process feature. Only the arguments based on Br/In and Br/Cd from ANDERS and EBIHARA (1982) support the curve drawn on Fig. 6, as opposed to one drawn through the Ivuna points. The necessity for a single element mass 89 peak is clear from Fig. 6. The sharp decline in abundance beyond the s9y peak is clearly defined on Fig. 6 and appears quite smooth. An abundance m i n i m u m at mass 97-99 is required by the

Asf

I

r

1.5

60

65

Mossnumber,A

RG. 5. Our average CI concentrations have been converted to atomic abundances for the odd A isotopes of the elements measured using the equation in Table I of ANDERSand EmHARA(1982). Error bars are intenample standard deviation. The data define a smooth curve with respect to mass number. The isotopic compositions of Cu and Ga define the local slope of the curve, as shown by the solid segments. A 10 to 20% lowerCu abundance would be in better accord with the curve drawn through the other elements. Our As abundance may be somewhat low in comparison with literature data (see text).

isotopic abundances of Mo and Ru and is consistent with our abundance data. Our interpretations of the N = 50 neutron shell abundances will be discussed elsewhere. Unlike the resolved s- and rprocess peaks associated with N : 82 and 126 (see following

i

As

Rb\\ RI)+S~r/'~ Y I I L

\

\ 1".

90 Moss number,A

Mu 95

Ru 100

RG. 6. The solar system abundance curve for the mass 75-100 region, constructed as in Fig. 5, except that a linear scale is used here. For Br the data for Orgueil (solid square) and lvuna (open triangle) are shown separately. This is the only element for which this distinction is necessary. The local slope at mass 79-81, given by the Br isotopic composition, is shown as a solid line. Our Y abundance is about 20% lower than literature values, but a monoelemental peak in the abundance curve at mass 89 is still necessary.

478

D.S. Burnett et al.

section), there is a sharp s-process peak and a broad, possibly fiat, "non-s" peak which is obscured by the abundance rise below mass 75.

Abundance smoothness in other mass regions An overall discussion of the issue of abundance smoothness requires a definition of what is meant by smoothness. It is well known that the odd A abundance curve has structure: (1) abundance peaks due to the s-process at nuclear shell closures: A = 119 (Z = 50 shell), A = 137 (N = 82 shell) and A = 207 (N = 126 and Z = 82 shells); (2) peaks at A = 129 and A = 195, traditionally interpreted (BURBIDGEet al., 1957) as the influence of the N = 82 and 126 neutron shells on the r-process. (see e.g. WOOLUM, 1988, for recent figures and references). The abundance data defining these peaks are in general quite smooth, although at mass 147-149 Sm appears about 20 to 30% low compared to the decline of the A = 137 peak defined by the Nd isotopic abundances (ANDERS and EBIHARA, 1982). A useful operational definition of a smoothness deviation is a feature deviating from a curve with single minima between these five well-defined peaks. Inspection of the abundance curve shows six deviations in addition to the small ones at Cu and Nd-Sm discussed already. It is unlikely that any of these deviations is due to errors in CI analyses. The larger deviations, e.g. in the heavy REE region, are defined by isotopic abundances and are likely of nucleosynthetic origin, but detailed understanding is lacking. The smaller ones may be also, but until nucleosynthetic explanations are forthcoming, the small 10 to 30% features define a limit to the validity of the smoothness argument alone as a justification for adopting CI concentrations as the average solar system heavy element abundances. If we accept that there are ten abundance peaks of nucleosynthetie origin between mass 60 and 210, this in itself is a barrier to smoothness assessment. However, if good nucleosynthetic explanations, even if only qualitative, can be found for all of these, this could justify the association of CI and solar system abundances more strongly than the smoothness argument ever could.

Critical junctions ANDERS ( 1971) has argued that there are special places on the abundance curve which are optimum for smoothness assessment because of changes in cosmochemical character. This is a generally valid argument, although there are complications in practical application (see ANDERSand EBIHARA, 1982; ANDERS and GREVESSE, 1989). One complication is how fine a cosmochemical elemental subdivision to apply, especially since meteoritic materials in general exhibit an impressive variety of fractionations, e.g. among nominally refractory lithophile elements in Ca-Al-rich inclusions. Thus, in our data the small Cu irregularity noted above, if cosmochemical, would have to be unique to Cu relative to Ni or Zn, because the irregularity offsets Cu from these other elements. As noted by ANDERS and EBmARA (1982), the GeAs junction is quite smooth, but would not have been for many kinds ofsiderophile, chalcophile element fractionations. Other junctions are more equivocal and might be consistent with small (10-30%) irregularities. Pd-Ag is discussed by ANDERS and EBIHARA(1982); others are Zr-Nb-Mo and Ta-W.

In summary, a consideration of critical junctions reinforces the conclusion drawn from general arguments on smoothness. The limit to which CI heavy element concentrations can be identified with average solar system composition is 10 to 30%, depending on exactly which portion of the abundance curve is considered. This is quite good, but, as discussed above, might even be made better with detailed understanding of the abundance curve fine structure.

Intersample variability as a limit to the validity of CI abundances We have analyzed our data along with that of KALLEMEYN and WASSON (1981) and EBmARA et al. (1982) in terms of intra- and intermeteorite CI compositional variations. Although apparently smaller than the variations due to smoothness deviations, intersample and intermeteorite variability are ultimate limits to the validity of CI abundances. There are two issues here: (1) how large is this variability, and (2) does it average out? Abundance compilations have at least implicitly recognized the variability, but the convergence of the mean has been assumed. Intermeteorite variability is more significant because it may represent at least meter- to kilometer-scale transport on the CI parent body, and it is less obvious that averaging the concentrations from different meteorites gives the average solar system abundance. In our study, except for Br, differences between Orgueil and Ivuna were typically less than 10% (Fig. 1). K A L L E M ~ and WASSON(1981) measured Alais (2 samples) and Orgueil (4 samples); EmHAP,A e t al. (1982) measured a sample of Alais and Ivuna plus four samples of Orgueil. A wide variety of comparisons are possible. We have chmen intermeteorite variability relative to Orgucil, the best-studied CI meteorite. (Intersample standard deviations for all elements are given by ANDERS and GREVESS~ (1989) for the analyses they selected for compilation.) There are two issues: whether the variability is large, and whether it is significant. These issues are correlated but are not exactly the same. In summarizing the data from these three studies in Table 3 we have given more weight to large differences and listed elements which met one of four criteria: (A) Orgueil intersample standard deviation for one of the three studies greater than 10%, (B) difference between Alais and Orgueil greater than 10%, (C) difference between Orgueil and Ivuna greater than 10%, (D) siderophile element. Criterion D was included because EBIHARA et al. (1982) reported correlated variations among siderophile elements. The Orgueil intersample variations (A) include both analytical precision and Orgueil heterogeneity. No attempt to resolve these is made here, because no comparisons are attempted for data between different studies. The most striking feature on Table 3 is that there are relatively few two-digit entries. The three CI meteorites are relatively homogeneous materials. Only 10/39 elements involved in the three studies combined show > 10% variation in Orgueil in any of the studies, and only 13/39 show any intermeteorite difference > 10% relative to Orgueil. Again using Orgucil as a reference, a significant intermeteorite difference requires that an entry in any row in II or III in Table 3 be greater than twice the equivalent entry for the same study in I. We have highlighted the elements with significant intermeteorite differences. This is not a perfect

479

Chemical compo~fion of carbonaceous chondntes Table 3.

Compositional v a r i a t i o n s among Cl meteorltes ( a l l e n t r i e s in g)

$ i d e r o p h l l e elements Ref I.

0rgueil,

Ois

~

e II.

Orgueil-Alals

I11. O r g u e i l - l v u n a

a b a c

References: a) gblhara e t a l . f o r Orguell analyses. *

Re

Os

Pd

Ni

11 9 5 6 . . . . 9 -I 3 -- 0.3 . . . . . . . . . . . . 46 12 --

2~I --

6 8"

Ir

-2 4

Pt

Ru

Mo

. . . . . . . 4 -81 . . . .

3 9 -3 -20" . . . . . 2 . . . . . . . . . 19 0 . .

.

Co

Au

Sb

Ge

Ga

Br

Rb

6 6 8

19 16 -2 8 8 . . . . . .

4 11 3

"5 --

25 2 10

5 8

-3

15 29'

3 -4

-3

6 . .

-. .

13 16 . . .

II* 1

-3

27 38"

.

(1982); (b) Kallemeyn and Wasson (1981); (c) t h i s work.

V

Y

Te

Zn

T1

18 -25

. . . . 4 3 . . . . -14 --

3 10 23

5 ---

30 35 e

18 --

.... 13

2~"

0 12

I0 e --

-561 -59"

10 1

.... --17

0 --

2 -5

16i --

. . . .

Ois : intersample standard d e v i a t i o n

Elements showing significant intermeteorite variation relative to the intersample variation observed in Orgueil.

O t h e r e l e m e n t s s h o w i n g no s i g n i f i c a n t

intermeteorite

variation

or ois(Orgueil)

> 10~:

Ref.

( a ) Ca, Se, $ n ,

In,

Cd, Ag; ( b ) Na,

Mg, AI, K, Ca, Se, Cr, Mn, Fe, As, Se, Cd; (c) Fe, Cu, Se, Sr, Zr.

criterion, but applying it consistently shows 10/39 elements with significant intermeteorite differences relative to the heterogeneity observed in Orgueil. The special nature of Br stands out clearly in Table 3. Iodine is probably similar (ANDERS and EBIHARA, 1982); the situation for C1 is unclear. The only other element with any variation greater than 30% is Au, and these variations correlate with Br (EmHARA et al., 1982). Only six other elements show some kind of variation >20% (Re, Ru, Sb, Rb, Te, Zn). The available data are far from optimum for the type of comparisons being made, so a lot of scatter is expected. For example, Zn is entered on Table 3 only because of anomalously low samples in our study. In our case there is no question but that the Zn fluctuations are real, as is a single fluctuation for Ru. Such samples prove that fractionations have occurred, at least on a m m scale. The situation with the siderophile elements is complex. Some, like Au and Sb show both large and significant variations; others, e.g. Ir or Pd, show neither. EmHARA et al. (1982) propose differential mobility of siderophile elements during parent body hydrothermal activity. This is quite plausible, because this mechanism is responsible for isolation of extremely rare elements into impressively pure terrestrial ore bodies. ANDERS (priv. commun.) has suggested that It-metal nuggets might explain some of the siderophile element variability, e.g. the anomalously low Ru for our Orgneil 626 sample. The issue of whether mean CI concentrations converge to average solar system abundances is less serious in view of the overall relatively small amount of variability. This is fortunate because there is no totally objective way, other than by reference to abundance curve smoothness or photosphere comparisons, to address this question. If there were strong inter-element correlations in the observed variations, this might be a problem for convergence of the mean, but, as discussed below, this does not appear to be the case. In summary, the limits placed on the validity of CI abundances by intersample variability are typically 10% or less. Inter-element non-correlations and the origin o f CI material

Geoehemists are accustomed to chemical variability, so to find that CI meteorites are chemically heterogeneous on a

mm scale is no surprise. Terrestrial rocks are often heterogeneous, even on the outcrop scale, and "whole rock" compositions require discussion and interpretation for any sample size. Intersample variations do not occur by magical or incomprehensible mechanisms. For many materials, simple non-representative sampling of the constituent minerals is, at least implicitly, assumed as a trivial source of variability. Ideally, such variable sampling produces inter-elemental correlations among subsamples of a given rock. The presence of heterogeneous mineral compositions is a complication; nevertheless, coherent behavior of groups of trace elements should occur. The CI heterogeneity, although small, is analytically significant and may be an opportunity to learn something of CI mineral chemistry (which is otherwise difficult for such fine-grained materials) or possibly of the initial components that combined to form CI matter. Our data are primarily conspicious by the lack of interelement correlations. Based on 21 elements in six CI samples, EBIHARA et al. (1982) found a few "weak to moderate correlations" that made sense cosmochemically (compare their Fig. 4). We have systematically reviewed their data along with that of KALLEMEYN and WASSON (1981 6 CI samples, 29 elements) for inter-element correlations. The situation is complicated, and detailed discussion does not seem profitable. As with our data, there is an overall lack of correlations that make sense, with some exceptions. As would be suspected from Table 3, there are weak correlations between some, but not all, siderophile elements, the best between Ni and Os (EBIHARA et al, 1982). Other correlations, e.g. between K and Co (KALLEMEYNand WASSON, 1981 ) make no sense. The most striking correlation, between Au and Br (EBIHARA et al., 1982), would not have been predicted, a priori, although it can be rationalized in terms of halide (or cyanide, E. ANDERS, priv. commun.) complexing of Au during hydrothermal processes on the CI parent body. Why are these correlations weak? It may just simply be that the appropriate types of studies with adequate precision have not been made. A second alternative possibility is that all trace elements have several significant host phases with a lot of fractionation of trace elements between phases. Any two element plot is then a multi-component mixing diagram, dominated by scatter. It is true that CI material is character-

480

D.S. Burnett et al.

ized by many phases (phyllosilicates, magnetite, sulfides, carbonates, sulfates, carbon, rare mafic silicate grains, etc. (see, e.g., MCSWEEN, 1979; TOMEOKA and BUSECK, 1988), although magnetite does not appear to be a significant host phase for trace elements (KRXHENBOHL et al., 1973; KERRIDGE et al., 1979). Most of these phases will have variable major element composition, and hydrothermal fra~onations could produce very complex trace element distributions. This is plausible, but it is still perhaps surprising that little trace element coherence is observed. This topic is important because a third, speculative, possibility is that these heterogeneities are more fundamental in nature, representing compositional variation in the interstellar inputs to the solar nebula, with a large number of contributions of unusualmby solar system standards---compositions obscuring traditional inter-element correlations. The issue of inter-element non-correlations cannot be separated from the problem of the origin of CI materials in general. One point of view (see, e.g. WOLF et al., 1980; MCSWEEN, 1979) is that the average solar composition of CI material reflects complete condensation, down to relatively low temperatures (300 K), of a gas of average solar composition. However, the presence ofisotopically anomalous noble gases in CI materials (JUNGCK and EBERHARDT,1985) requires at least the admixture of carbonaceous and possibly other materials containing unreacted interstellar grains. Since some interstellar material is required, why not a lot? This should at least be kept in mind as an alternative to the condensation origin (ANDERS, 1987). There is no question that some meteoritic material has been processed at high temperatures, but there is no general constraint on quantitative proportions. The presence of veins (DUFRESNE and ANDERS, 1962; MCSWEEN, 1979) is evidence for fluid, presumably aqueous, CI parent body processing, and the sulfate and carbonate minerals were very likely formed by this process, possibly sulfides as well. A good correlation between Mg and Se in the CI data of KALLEMEYN and WASSON(1981) is suggestive that MgSO4 is the primary host phase for Se, so these parent body phases govern some aspects of trace element chemistry as well. Similarly, Br, I, and Te, appear to be concentrated in presumably secondary water-soluble phases (REED and ALLEN, 1966), although it is possible that the apparent solubility of these elements is due to neutron capture recoil (E. ANDERS, priv. eommun.) The phyllosilicates are very fine-grained materials, norreally regarded as alteration products under parent body conditions, but they are also similar to general expectations of interstellar materials. Could some or all of the phyllosilicares be interstellar or only slightly altered interstellar material (compare ANDERS, 1987)? The only sure way of recognizing interstellar material is by way of isotopic anomalies, and there is no question that individual stars will produce material of very anomalous isotopic compositions. On the 0.1-1 gram scale CI materials presumably have normal isotopic compositions for most elements. Nevertheless, it cannot be ruled out that interstellar processes, at least thorough mechanical mixing, has produced isotopic homogenization, at least on the 100 rag scale. (One nag of 1 micron particles contains 109 grains.) Isotopic measurements on small samples of CI material might be worthwhile. Rb-Sr isotopic studies of CI

material show considerable scatter (KAUSHALand WETHERILL, 1970; MACDOUGALLel al., 1984). This just may reflect highly mobile Rb-rich phases (suggested by the Rb variability shown in Table 3), but we should also entertain the possibility that this scatter represents initial S7Sr/S6Sr heterogeneity. Details aside, the issue of CI element non-correlation appears worthy of further study. Relationship o f C I and photospheric abundances

Our conclusion above was that smoothness alone limited the identification of CI and average solar system abundances to 10 to 30%. If the relatively small uncertainties assigned to photospheric abundances for many elements by GREVESSE (1984) or ANDERS and GREVESSE(1989) can be taken at face value, we are now at a point where the photospheric-CI comparison is the strongest constraint on the identification for all mass ranges. Overall, comparison of photospheric and CI abundances shows excellent agreement, always within the error bars in the photospheric abundances (ANDERS and GREVESSE, 1989). Perhaps we are ready for a radical change: shift from the CI standard to a set of solar system abundances based on the photosphere for any element whose abundance relative to H is known to _+20% or better? Until more confidence in the accuracy of the photospheric abundances is obtained, this shift is probably premature, but it is worth thinking about. Nothing would change at present but there are long-term advantages. It is possible to conceive of a CI chondrite slightly altered in composition from Orgneil. If such an object should fall, or be found in the Antarctic, it would perturb our confidence in CI average solar system abundances until the differences could be sorted out. But, using photospheric abundances, there would be less confusion, and any new meteorite composition would be studied for its own sake, as perturbed and unperturbed samples could be readily distingnished. AcknowledgementsmWe acknowledge valuable assistance with soft-

ware installation at CalTech by S. Spicklemeyer and with accelerator operation by M. Hollander and J. Tesmer. The manuscript was improved by reviews from E. Anders and J. Morgan. We thank R. Clarke for the meteorite samples. Support was by NASA grants NAG 9-94 (BurneR) and NAG 9-57 (Woolum). Editorial handling: S. R. Taylor

REFERENCES

ANDERSE. (1971) How well do we know cosmic abundances? Geochim. Cosmochim. Acta 35, 516-522. ANDERS, E. (1987) Local and exotic components of primitive meteorites and their origin. Phil. Trans. Roy. Soc. London A323, 287-304. ANDERSE. and EmHARAM. (1982) Solar system abundance of the elements. Geochim. Cosmochim. Acta 46, 2363-2380. ANDERSE., and GREVESSEN. (1989) Abundances of the elements: Meteoritic and solar. Geochim. Cosmochim. Acta 53, 197-214. Basaltic Volcanism Study Project (1981) Basaltic Volcanism on the Terrestrial Planets. Pergamon, New York BURBIDGEE. M., BURBIDGEG. R., FOWLERW. A., and HOYLE F. (1957) Synthesis of the elements in stars. Rev. Mod. Phys. 29, 547650. BURNETT D. S., WOOLUM D. S., BENJAMIN T. M., ROGERS P. S. Z., DUFFY,C. J, and MAGGIORE,C. (1988) High precision

Chemical composition of carbonaceous chondrites thick target analyses of carbonaceous meteorites. Nuclear Instr. Meth. (in press), DUFFY C. J., ROGERSP. S. Z., and BENJAMINT. M. (1987) The Los Alamos PIXE data reduction software. Nuclear Inst. Meth. B22, 91-95. DUFRESNE E. R. and ANDERSE. (1962) On the chemical evolution of the carbonaceous chondrites. Geochim. Cosmochim. Acta 26, 1085-1114. EBIHARA M., WOLF R., and ANDERS E. (1982) Are CI chondrites chemically fractionated? A trace element study. Geochim. Cosmochim. Acta 46, 1849-1862. FOUCHE, K. F. and SMALES,A. A. (1967) The distribution of trace elements in chondritic meteorites, 2. Chem. Geol. 2, 105-134. GREVESSE N. (1984) Accurate atomic data and solar photosphere spectroscopy. Physica Scripta 1"8, 49-58. GROSSMANL. and LARIMERJ. W. (1974) Early chemical history of the solar system. Rev. Geophys. Space Phys. 12, 71-102. JOCHUM K. P., SEUFERTH. M., SPETTELB., and PALMEH. (1986) The solar system abundances of Nb, Ta, and Y and the relative abundances of refractory lithophile elements in differentiated planetary bodies. Geochim. Cosmochim. Acta 50, 1173-1183. JUNGCK M. H. A. and EBERHARDT,P. (1985) Ne-E inclusions in apatite from Orgueil. Meteoritics 20, 677-678. KALLEMEYN G. and WASSONJ. T. (1981) The compositional classification of ehondrites: I. The carbonaceous chondrite groups. Geochim. Cosmochim. Acta 45, 1217-1230. KAUSHAL S. K. and WETHERILL G. W. (1970) Rb87-Sr87 age of carbonaceous chondrites. J. Geophys. Res. 75, 463--468. KERRtDGEJ. F., MACKAYA. L., and BOYNTON,W. V. (1979) Magnetite in CI meteorites: Origin by aqueous activity on a planetary surface. Science 205, 395-397. KORNACKiA. S. and Fr.GLEY,B. (1986) The abundance and volatility of refractory trace elements in Allende Ca-Al-rich inclusions: Implications for chemical and physical processes in the solar nebula. Earth Planet. Sci. Lett. 79, 217-234. KR~HENBf3HL U., MORGAN J. W., GANAPATHYR., and ANDERS E. (1973) Abundance of 17 trace elements in carbonaceous chondrites. Geochim. Cosmochim. Acta 37, 1353-1370. MACDOUGALLJ. D., LUGMAIRG. W., and KERRIDGEJ. F. (1984) Early solar system aqueous activity: Sr isotopic evidence from the Orgueil CI meteorite. Nature 307, 249-251.

481

MCSWEEN H. Y. (1979) Are carbonaceous chondrites primitive or processed? A review. Rev. Geophys. Space Phys. 17, 1059-1078. MITTLEFEHLDTD. W. and WETHERILLG. W. (1979) Rb-Sr studies of CI and CM chondrites. Geochim. Cosmochim. Acta 43, 201206. MURTHY V. R. and COMPSTONW. (1965) Rb-Sr ages of chondrules and carbonaceous chondrites. J. Geophys. Res. 70, 5297-5307. PERFIT M. R., FORNARI D. J., MALAHOFFA., and EMBLY R. W. (1983) Geochemical studies of abyssal lavas recovered by DSRV Alvin from Eastern Galapagos rift, Inca transform, and Ecuador rift: 3. Trace element abundances and petrogenesis. J. Geophys. Res. B88, 10551-10572. REED G. W. and ALLEN, R. O. (1966) Halogens in chondrites. Geochim. Cosmochim. Acta 30, 779-800. SCHMITTR. A., SMITH R. H., and OLEHY, D. A. (1964) Rare earth, Y, and Sc abundances in meteoritic and terrestrial matter. Geochim. Cosmochim. Acta 28, 67-86. SUESS H. (1947) Uber kosmische Kernh~ufigkeiten I. Mitteilung: Einige H~lufigkeitsregelnund ihre Anwendung bei der Abschatzung der Htiufigskeitwerte for die mittelsschweren und schweren Elemente. II. Mitteilung: Einzelheiten in der Htiufigskeitveneilung der mittelschweren und sehweren Kerne. Z. Natu~forsch. 2a, 311321,604-608. SUESSH. and UREY H. C. (1956) Abundances of the dements. Rev. Mod. Phys. 28, 53-74. STAPANIAN,M. I. ( 1981) Induced fission track measurements of carbouaceous chondrite Th/U ratios and Tla/U mierodistributions in Allende inclusions. Ph.D. thesis, California Institute of Technology. TOMEOKA K. and BUSECKP. R. (1988) Matrix mineralogy of the Orgueil CI chondrite. Geochim. Cosmochim. Acta 52, 1627-1640. TAYLORS. R. and MCLENNANS. M. (1985) The Continental Crust: Its Composition and Evolution. Blackwell, Palo Alto. WELLER M. R., FURST M., TOMBRELLOT. A., and BURNETTD. S. (1978) Boron concentrations in carbonaceous chondrites. Geochim. Cosmochim. Acta 42, 999-1009. WOLF R., PdCHTERG. R., WOODROWA. B., and ANDERSE. (1980) Chemical fractionations in meteorites XI. C2 ehondrites. Geochim. Cosmochim. Acta 44, 711-717. WOOLUM D. S. (1987) Elemental abundances and nucleosynthesis. In Meteorites and the Early Solar System (ed. J. KERRIDOE)Chap. 14.1. Univ. Arizona Press, Tuscon.