CONCENTRATIONS OF 17 ELEMENTS IN 36 CHONDRULES FROM ALLENDE

Transcription

1 Manuscript - unpublished CONCENTRATIONS OF 17 ELEMENTS IN 36 CHONDRULES FROM ALLENDE C. MENNINGA 1 1 Geology, Geography, and Environmental Science, Calvin College, 1740 Knollcrest Circle, Grand Rapids, MI , USA Abstract Thirty-six chondrules and four matrix samples from fragments of the meteorite Allende were analyzed by non-destructive instrumental neutron activation for Al, Au, Ca, Co, Cr, Eu, Fe, Ir, Mg, Mn, Na, Ni, Sc, Si, Ti, V, and Zn. Statistical evaluation of data provides information on correlation relationships among those elements. Introduction Meteorites have long been of interest as sources of information about the early history of the solar system since they are the only samples available to us that present evidence from which the processes of that early formation may be inferred with some confidence [1, 2]. While many meteorites exhibit evidence of metamorphic processes in their histories, the C3 chondrites appear to be among the least affected by such metamorphism [3]. Consequently, the C3 chondrite Allende, with the large amount of material recovered from that fall, has received a lot of attention. Many compositional analyses of whole meteorites have been published [4-10]. Some studies have also been done on chemical and mineralogical characteristics of individual chondrules [11-13]. Since many puzzles regarding the early history of the solar system remain unsolved, additional observational data are always welcomed. This paper reports the concentrations of 17 elements in 36 chondrules from the meteorite Allende by non-destructive instrumental neutron activation.

2 Macrozoom photos of a few of the chondrules at various magnifications are shown in Fig. 1. Photos of chondrules #2-#35 can be found at Fig. 1 Three chondrules in reflected light (Graduations on the scale are 0.01 in.) Note: The irradiations and measurements for the elemental analyses reported in this paper were done during the span of years , and the results lay on the shelf until now, with publication delayed largely for personal reasons. Experimental A. Sample preparation The chondrules in this study were obtained from fragments of the Allende meteorite that were supplied by the Smithsonian Institution. Most of the chondrules were taken from a single fragment that had been picked up in Field No. 22, and had been purchased by the Smithsonian in Santa Ana; the others were obtained from uncataloged fragments. Chondrules were picked from a freshly broken surface of the meteorite with the tip of a scalpel in a laminar-flow clean hood, and handled with Teflon-coated tweezers. Samples were weighed on a Mettler Microgram balance and numbered in order of increasing weight. Matrix samples were scraped from the region surrounding each of four of the chondrules, weighed, and sealed in polyethylene containers for analysis. Great care was exercised to avoid contamination with any of the materials in the environment in which the analyses were performed. (Chondrules #1-35 remain in the possession of the author.) B. Irradiations and counting procedures Samples were irradiated in the Hanford K-East plutonium production reactor in 1969 for two short irradiations (1-6 minutes) to measure the concentrations of Al, Mg, Mn, Na, Ti, and V. Counting was done promptly with a Ge(Li) detector system with 3% relative efficiency. After some elapsed time, samples were also counted in a lowbackground anticoincidence-shielded multi-parameter gamma-ray spectrometer at Battelle Pacific Northwest Laboratories [14, 15] to verify the measurement of Mn and Na.

3 Samples were exposed in a longer irradiation in the Hanford K-East plutonium production reactor in 1970 to a time-integrated flux (fluence) of about neutrons/cm 2 for the measurement of Au, Ca, Co, Cr, Eu, Fe, Ir, Na, Sc, and Zn. After some elapsed time for the decay of short-lived isotopes, samples were counted with a Ge(Li) detector system with 3% relative efficiency, and with a low-background anticoincidence-shielded gamma-ray spectrometer. [16] Samples were counted again in 1971 (one year after irradiation) with the low-background anticoincidence spectrometer for measuring Eu and Zn. Silicon and nickel content were measured in 1974 by irradiation with 14-MeV neutrons from a Kaman Nuclear Model A-711 fast neutron generator with an output of 4 x neutrons/sec. Samples were counted with a Ge(Li) detector with 26% efficiency. Additional irradiations of several samples were carried out at the Washington State University Nuclear Radiation Center in 1974 to double-check earlier results. Nickel content was measured by fast neutron activation in a Cd-shielded facility in the 1-MW Triga reactor at WSU-NRC. Samples were counted with an anticoincidence-shielded gamma ray spectrometer incorporating a 14% Ge(Li) detector surrounded by NaI(Tl). A 24-hour irradiation with thermal neutrons was also carried out to double-check results for some isotopes, with counting done by instruments at Battelle Pacific Northwest laboratories. No significant differences from earlier results were found. The output from all counting systems was processed by 2048-channel analyzer calibrated so that the channel number equals the gamma ray energy in kev, and printed on paper strip. Sums of events under each gamma-ray peak of interest were taken manually with the help of a multi-function electronic calculator. All calculations were also performed manually with the help of a multi-function electronic calculator. Nuclear data for all nuclides used in this study are presented in Table 1. Table 1. Nuclear Data for isotopes used in this study Element Target Abundance(%) Reaction Activated Half-life Gamma(s) (kev) Al Al n,γ Al m Au Au n,γ Au d Ca 1 Ca n,γ Ca d Sc d Co Co n,γ Co y Cr Cr n,γ Cr d Eu 2 Eu n,γ Eu y Eu n,γ Eu y Fe Fe n,γ Fe d Ir Ir n,γ Ir d Mg Mg n,γ Mg m Mn Mn n,γ Mn h Na Na n,γ Na h

4 Ni Ni *n,α Fe d Sc Sc n,γ Sc d Si Si *n,p Al m Ti Ti n,γ Ti m V V n,γ V m Zn 3 Zn n,γ Zn d * 14-MeV neutrons; all others thermal neutrons 1 Ca concentrations determined through Sc-47, the radioactive daughter of Ca-47 2 Counts in composite peak consist of 95% Eu-152 and 5% Eu-154, effective t½ = 12.9y 3 Zn corrected for partial overlap with kev by excluding the count in the channel in the valley between peaks from the Zn count, and using the valley count as the background for the adjacent channel on the shoulder of the Zn peak Blanks consisting of the packaging materials for all samples and standards were run along with the samples and standards in all of the irradiations and counting procedures in order to insure that the packaging materials did not contain any impurities that would affect the results attributed to the chondrules. Copper discs or high-purity iron wires were used to measure relative timeintegrated neutron flux with all sample irradiations. Corrections for decay during irradiation and counting were made by the method of Hoffman [17]. C. Standards and data handling The composition of samples was determined by comparison with known standards. Standards in the form of the high-purity element were used for measuring concentrations of Al, Fe, Mg, Ni, Si, and Ti. Standard solutions were prepared from reagent grade compounds in ionized water for measuring concentrations of Ca, Co, Cr, Eu, Ir, Na, Sc, and Zn. Aliquots of standard solutions were absorbed in filter paper and dried in a desiccator at room temperature; the impregnated papers were then folded compactly and sealed in envelopes of polyethylene foil and heat sealed at all edges. U.S.G.S. rock sample W-1 was used as a standard for V [18], and the Au concentration was determined by the published production cross-section and decay characteristics relative to those of Co, Fe, and Sc. The possibility of interference from reactions other than neutron capture producing the nuclide of interest was investigated by irradiation of samples of Al and Mg and determination of the amount of Na-24 produced by fast neutron reactions Al-27 (n,α) Na-24 and Mg-24 (n,p) Na-24. The results were applied to the analysis of chondrule #2 (high Al), and found that the interference from fast neutrons amounted to 7 counts of Na- 24 in 100,000 total counts of Na-24 following a 2-minute irradiation, and to chondrule #23 (low Na), and found that the interference amounted to 25 counts of Na-24 in 10,000

5 total counts of Na-24 following a long irradiation of 24 hours. These results confirm the assurance that this author received from workers at Battelle National Laboratory that fast neutron reactions need not be considered from irradiations in the Hanford K-East reactor facility in this study. Samples of U.S.G.S. rock standards G-1, W-1, BCR-1, and DTS-1 and a sample of whole rock Allende reference material were irradiated and counted along with the chondrules. The results show a few cases where the spread of values for multiple samples of the same reference material was greater than would be expected, but the overall pattern was acceptably close to the published composition of those standard materials. Those results are available in tabular format at Results and discussion A. Results The measured concentrations of 17 elements in 36 chondrules and 4 matrix samples are presented in Table 2. Uncertainties listed are due to counting statistics only. Table 2. Elemental composition of Allende chondrules chon. mass (mg) Si (%) Mg (%) Fe (%) Al (%) Ca (%) Na (%) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.11

6 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.04 Matrix 10X ± ± ± ± ± ± X ± ± ± ± ± ± X ± ± ± ± ± ± X ± ± ± ± ± ± Table 2. (continued) mass Sc V chon. (mg) Ni (%) Ti (%) Cr (%) Mn (%) (mg/kg) (mg/kg) ± 0.04 TLD ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.05 TLD ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.04 TLD ± ± ± ± 12

7 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.03 TLD ± ± ± ± ± ± ± ± ± ± ± 0.02 TLD ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 6 Matrix 10X ± ± ± ± ± ± 5 17X ± 0.04 TLD ± ± ± ± 9 27X ± 0.08 TLD ± ± ± ± 6 34X ± ± ± ± ± ± 8 TLD = too low to be determined accurately Table 2. (continued) chon. mass (mg) Co (mg/kg) Zn (mg/kg) Eu (mg/kg) Ir (mg/kg) Au (mg/kg) ± 6 37 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 9 42 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 4 19 ± ± ± ± ± ± ± ± ± ± 9 38 ± ± ± ± ± 6 33 ± ± ± ± ± 2 22 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.010

8 ± ± ± ± ± ± 5 43 ± ± ± ± ± 6 20 ± ± ± ± ± 1 39 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 2 24 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 7 43 ± ± ± ± ±20 67 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 7 50 ± ± ± ± ± 3 34 ± ± ± ± ± 2 28 ± ± ± ± ± 9 35 ± ± ± ± ± 8 41 ± ± ± ± ± 8 48 ± ± ± ± ± 4 34 ± ± ± ± ± 2 22 ± ± ± ± Matrix 10X ± ± ± ± ± X ± ± ± ± X ± ± ± ± X ± ± ± ± Note: The high Al and Ca in sample 2 and the high Al in sample 19 might lead one to suspect that they are CAI s in whole or in part, except for the fact that the external appearance of those samples is not different from that of most of the other samples. The ranges of the concentrations of these elements relative to CI (C1) chondrites are presented in Fig. 2. (CI concentrations of Ir and Au from Lodders [19], others from Barrat [20]).

9 Fig. 2 Concentrations of 17 elements in Allende chondrules relative to CI chondrites A casual review of the results finds some values for several of the elements involved in this study that appear to be outliers. The data for each element were evaluated by iterative Grubb s test, and were also checked by the ROUT test that is included in the Graphpad Prism 6 software that was used for statistical calculations in this study; the two tests yielded the same list of values identified as outliers. Therefore, all statistical analyses in this study were performed on the full set of data, as well as on sets of data from which one or more identified outliers have been excluded. Differences among these analyses will be the subject of some later discussion. B. Correlation Coefficients Statistical analysis calculations were performed with Graphpad Prism 6.04 software for Windows from Graphpad Software, LaJolla, California, Possible correlations among the elements in this study were sought by correlation coefficient calculations. The analyses included chondrules only; matrix samples were not included since they represent a different population. The correlation coefficients for the full set of data, as well as for sets of data from which one or more outliers were excluded, are available at

10 There are two main clusters of positively correlated elements that merit attention: 1. The cluster that includes iron, nickel, and cobalt, shown in Table 3. Table 3. Correlations among siderophile elements Ni Cr Mn Co Zn Ir Au Fe x x-2, x-2,19,24, Ni x x-2, x-2,19,24, Cr x x-2, x-2,19,24, Mn x x-2, x-2,19,24, Co x x-2, Zn x x-2, Ir 0.20 x x-2, Zinc, though not classified as siderophile by Goldschmidt [21], is strongly correlated with Fe, Ni, and Co if the Zn outlier in sample #2 is excluded. Manganese shows barely significant correlation with iron only if the Mn outliers in samples #24 and #36 are excluded. Chromium shows positive correlation of low significance with manganese, but no correlation of significance with other elements in the iron cluster. Iridium shows barely significant correlation with Fe, Ni and Co only if the Ir outlier in sample #19 is excluded, and with Zn if the outlier in Zn is also excluded.

11 1. The cluster that includes aluminum, calcium, sodium, scandium, and europium, shown in Table 4. Table 4. Correlations among lithophile elements Al Ca Na Ti Sc V Eu Zn?? Mg x x-2, Al x x-2, Ca x x-2, Na x x-2, Ti x x-2, Sc x x-2, V x x-2, Eu 0.75 x x-2, Titanium, vanadium and zinc appear to be strongly correlated with aluminum when all data are included; the correlation of those elements with aluminum is very weak when the outliers in each of those elements and those in aluminum are excluded. Only a few anti-correlations in this study have coefficients of greater significance than -0.50, and none of greater significance than Therefore, those relationships have not received any detailed consideration in this study.

12 These correlation results are significantly different from those reported by Osborn [11]. The differences are most notable when the correlation coefficients have been calculated with sets of data from which outliers have been excluded, but some differences remain even when the full set of data is used in the calculations. The differences when the full set of data is used are likely due, at least in part, to the variability of elemental composition from one chondrule to another, considering that each collection of randomly selected chondrules examined was necessarily different from other collections, and no collection encompasses all of the population. C. Regression Analysis Regression analysis was performed with the procedures of weighted Deming regression in Graphpad Prism 6, providing data on slope and intercepts of the regression lines, and graphs of these data. Regression analysis was performed for all pairs of elements that showed any significant correlation, both with outlier data points included, and also with one or more outlier data points excluded. The summary statistical data are available at The graphical representations for those results are instructive for the consideration of the appropriate treatment of outliers in this study. A few examples are presented to demonstrate some undue influence of one or more outliers on the correlation coefficients and the regression lines. (Note differences in scale among the graphs presented.) 1. In some cases an outlier has such a strong influence that the regression line is far from the trend that is obvious in the remaining 35 or 34 samples, and the correlation coefficient with the full set of data is near zero. Excluding the outlier(s) results in a regression line that follows the remaining trend, and reveals a strong positive correlation. An example is provided by Fe vs. Zn in Fig. 3. Fig. 3 Regression graphs of Fe vs Zn Similar patterns are found in Ni vs Zn and Co vs Zn. Also, similar patterns are found in Fe vs Ir, Ni vs Ir, and Co vs Ir in comparing the correlation coefficients and

13 trend in the graph for the full set of data with the remaining set after excluding the Ir outlier in sample # In some cases one or two outliers lie far from a cluster of the remaining 35 or 34 points, controlling the regression line and indicating a strong correlation of the two elements involved. Excluding the outlier(s) leaves widely scattered points with very low correlation coefficient. The plot of Al vs V shown in Fig. 4 provides an example. Fig. 4 Regression graphs of Al vs V Similar patterns are shown by Al vs Zn, Sc vs Ir, V vs Zn, Zn vs Eu, Ca vs Zn, Ca vs V, and V vs Eu. 3. In some cases there are two outlier points on the graph, quite far apart, strongly influencing the regression line to be drawn roughly midway between them, and the remainder of the samples in a cluster quite separated from the two outlier data points. Exclusion of one of the outlier points may or may not alter the correlation coefficient, but exclusion of both outlier points leaves the remaining points widely scattered, and the correlation coefficient near zero. The example of Al vs Ir is shown in Fig. 5. Fig. 5 Regression graphs of Al vs Ir

14 Similar patterns are found in Ti vs Zn and Ti vs Ir. Very similar patterns are also found in Fe vs Mn and in Cr vs Mn by comparing the results for all data with the results when the Mn outliers in samples #24 and #36 are excluded. The regression line graphs of all pairs of elements with significant correlation coefficients can be found at So, what are we to make of the influence of the outliers on the regression lines and correlation coefficients? Which set of data is most meaningful with regard to the correlation of the elements under consideration in the environment of the early solar system in which the chondrules were being formed? To this author it would seem contrary to good sense to base conclusions regarding the formation history of chondrules on an apparently strong correlation (or anticorrelation) that is heavily influenced by one or two outlier data points when the remaining 35 or 34 data points do not support that apparent correlation, or to ignore an obvious correlation shown by 35 or 34 data points that is masked by the influence of one or two outlier data points. D. Effect of volatility on elemental composition of chondrules Studies have been done on the effects of volatility of elements under early solar system nebula conditions on the elemental composition of bulk chondrites of various classes [22]. In this study the arithmetic mean of the elemental composition of Allende chondrules, with outliers excluded, relative to CI chondrites and Mg has been plotted vs. 50% condensation temperatures [23] under likely early nebular conditions, shown in Fig. 6.

15 Fig. 6 Elemental composition of Allende chondrules vs. nebular condensation temperature The more refractory elements are indeed enriched in these samples, relative to CI chondrites and Mg, with the glaring exception of iridium. The data for the more volatile elements, however, are widely scattered and do not present a smooth pattern of variation with condensation temperature, such as was reported by Davis [22] for CV chondrites. Surely, factors other than volatility/condensation were predominant in producing the wide variation in elemental composition of the nebular materials from which these chondrules were formed. Conclusions The formation history of chondrules receives a lot of attention in attempts to gain understanding of early solar system history. Postulates regarding the processes of their formation include 1) direct condensation from the solar nebula [24, 25], 2) impact melting on the surfaces of large bodies [26-28], 3) impact melting by collisions of small bodies [29, 30] or dust grains [31], and 4) melting of condensed material by lightning-type discharges [32]. At the present time there is considerable consensus that chondrules were formed by an unexplained brief heating event that melted existing matter, probably aggregates of dust particles in that region of the nascent solar nebula, followed by rapid cooling [1, 33]. That consensus leaves many puzzles unsolved, of course. In Meteorites and their Parent Bodies, McSween [1] wrote (p. 56), thus far no consensus has emerged and Chondrules are as much a puzzle to us now as they were to Sorby [34]. The results reported here are an addition to the data to be accounted for in searching for solutions to those puzzles, and will hopefully aid in finding those solutions. Acknowledgements I am grateful to the U.S. Nuclear Regulatory Commission and to Washington State University Nuclear Research Center for providing access to neutron irradiation sources. I appreciate the generosity of the Radiochemistry Division of the Pacific Northwest National Laboratory (formerly the Battelle Pacific Northwest Laboratories) for providing access to their facilities and radioactivity counting equipment for carrying out the neutron activation analyses. I am grateful to the Smithsonian Institution for providing sample material. I am grateful for the support provided by the Northwest Colleges and Universities for Science and by a sabbatical leave from Calvin College. I thank Ned Wogman of Battelle Northwest for encouragement and smoothing my path to performing this research, and I am especially grateful to Louis Rancitelli for his invaluable help in obtaining the sample material and in gaining access to neutron irradiation facilities at the Hanford Works, and to the radioactivity counting equipment at the Battelle Laboratories.

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