THE COMPOSITION OF THE CHASSIGNY METEORITE

Transcription

1 THE COMPOSITION OF THE CHASSIGNY METEORITE Brian Mason, J.A. Nelen Smithsonian Institution Wadlington, DC P. Muir, S.R. Taylor Australian National University Canberra, A.C.T. 2600, Australia Spark source mass spectrometric analysis of the Chassigny meteorite has given the following data (in ppm): Rb 0.4, Sr 7.2, Y 0.64, Zr 1.5, Nb 0.32, Ba 7.1, La 0.39, Ce 1.12, Pr 0.13, Nd 0.54, Sm 0.11, Eu 0.038, Gd 0.11, Tb 0.02,Dy 0.12, Ho 0.03, Er 0.09, Yb 0.10, Pb 1.0, Th 0.057, U These data, in conjunction with major element composition and mineralogical and textural features, indicate that this meteorite is an olivinerich cumulate, possibly genetically related to the nakhlites. INTRODUCTION The Chassigny meteorite fell on October 3, 1815, near the village of Chassigny on the plateau of Langres in the province of Haute-Marne, France. One stone (or possibly more) was recovered. It had broken into numerous fragments on impact; the total weight collected was about 4 kg, the largest fragment being about 1 kg. The largest piece now known is one of 246 g in the Museum d'histoire Naturelle, Paris; small pieces are widely distributed in collections. Chassigny is a unique meteorite, an olivine achondrite showing no close relationship to any other achondrite. Although its major element composition has been established by several analyses, there are few published data on minor and trace elements. To fill this gap, and possibly to shed some light on the relationship of Chassigny to other meteorites, we obtained material from the specimen in the Smithsonian Institution (USNM 624). MAJOR ELEMENT COMPOSITION Chassigny was first analysed by Vauquelin in This analysis, while incomplete by modern standards, was adequate to establish that Chassigny consisted largely of iron-rich olivine. Five analyses from the literature are given in Table I, along with the composition calculated from a modal analysis and the composition of the individual minerals. The model analysis was published by Prinz et al. (1974) in volume percent (olivine 91.6, pyroxene 5.0, feldspar 1.7, chromite 1.4, melt inclusions 0.3), and was converted to weight percent by using the appropriate densities for the individual minerals. The pyroxene has been arbitrarily divided equally between clinopyroxene and orthopyroxene, and the 0.3% melt inclusions were omitted from the calculation. The mineral compositions are given in Table 2. Meteoritics, Vol. 11, No.1, March 31,

2 Table 1 Chemical analyses of the Chassigny meteorite Si Ti Alz CrZ FeO MoO MgO CaO NazO K P 2 0 S Sum Q m* * 1OOMg(Mg+Mn+Fe) 1. Damour, 1862; the total includes 3.77% insoluble, which Damour identified as chromite and pyroxene. 2. Dyakonova and Kharitonova, 1960; they report no FeS, and 0.06% Ni, 3. Jerernine et at., 1962; they report 0.60% FeS, 0.24% H 20-, and no Ni. 4. Jerome, McCarthy et al., 1974; the analysis was given as elements and has been converted to oxides to facilitate comparison with the other analyses. 6. Calculated from the modal analysis (Prinz et al., 1974) and mineral compositions (Table 2). Table 2 Mineral compositions in the Chassigny meteorite, by microprobe analysis. Olivine Clinopyroxene Orthopyroxene Plagioclase Chromite Si0 2 Ti0 2 Ah03 CrZ03 FeO MnO MgO CaO NazO KzO Sum < < < 0.D <

3 All five analyses show good agreement for the major components Si0 2, FeO, and MgO, and the figures are consistent with those derived from the mode. Damour's analysis is essentially that of the olivine, since he analysed the portion soluble in RN0 3, and reported the remainder as insoluble, commenting that it consisted of chromite and pyroxene. His value for Kp is clearly erroneous, possibly introduced from his reagents, and the presence of Cr203 in the acid-soluble material is puzzling, since the olivine contains only traces of this component. The values for some of the minor components are fairly consistent for all analyses, but for a few there are notable discrepancies. These discrepancies are not likely to be due to sampling difficulties, since Chassigny is a relatively fine-grained meteorite and even a small sample should be adequate. The most notable discrepancies are in AIP3' Here we see the inadequacy of the older wet-chemical determination of small amounts of this component, analyses 2 and 3, against more recent determinations, analyses 4 and 5. Analyses 4 and 5 give lower Ah03 values than those determined from the mode, but the difference is small and may be due to a slight overestimate of plagioclase in the mode. The alkali figures determined from the mode are consistent with the lowest figures for these components in the chemical analyses. Analysis 5 is notably discrepant in CaO content from the other determinations and is certainly erroneous for this component. The small amount of P20S is consistent with the presence of accessory chlorapatite reported by Prinz et al. (1974). Jeremine et al. (1962) reported 0.60% FeS, whereas Dyakonova and Kharitonova (1960) reported the absence of FeS. The truth lies somewhere in between; examination of a polished thin section shows small amounts of sulfides, considerably less than 0.6%. Prinz et al. (1974) record troilite, pyrite, and pentlandite as minor minerals. The 0.06% Ni reported in analysis 2 is confirmed by Jerome (1970), who found 475 ppm Ni (by emission spectrometry); this nickel evidently accounts for the pentlandite, but whether some of this element is also present in olivine is not known. TRACE ELEMENTS Trace elements were determined by spark source mass spectrography, using the technique described by Taylor (1965, 1971). This technique uses the rare earth element Lu as an internal standard, so its abundance cannot be directly determined. The rare earth Tm suffers a serious interference from a multiple carbon molecule, so its abundance is also not determined. Abundances of both Tm and Lu can be estimated from the rare earth pattern. Accuracy and precision of the method are dependent on the total number of exposures used to calculate the abundance of each element, as well as other factors. For this study four photoplates were exposed, each with about 15 exposures, and determinations for many elemen15 were based on more than one isotope. This resulted in the measured abundance of each element being based on 8 to 30 determinations. The precision obtained for all elements was 23

4 about ± 5%. Results obtained by this technique on lunar samples agree well with results obtained on the same samples by other methods (Taylor et al., 1973). This and other comparisons indicate that the accuracy of the method is about ± 10%. Quantitative abundance data were obtained for the following elements (in ppm): Rb 004, Sr 7.2, Y 0.64, Zr 1.5, Nb 0.32, Ba 7.1, La 0.39, Ce 1.12, Pr 0.13, Nd 0.54, Sm 0.11, Eu 0.038, Gd 0.11, Tb 0.02, Dy 0.12, Ho 0.03, Er 0.09, Yb 0.10, Pb 1.0, U 0.021, Th Additional trace element data are given by Jerome (1970); he reported Ba 5, Sc 8, V 50 (by emission spectrometry), and Co 140.6, Sc 5.58 (by INAA). DISCUSSION As mentioned previously, Chassigny is a unique meteorite, showing no close relationship to any other achondrite. Mineralogically it shows a qualitative resemblance to the chondrites, with dominant olivine and minor pyroxene, plagioclase, and chromite. The olivine composition (Fa32) is similar to that of the most iron-rich olivine in common chondrites; clinopyroxene is lower in CaO and orthopyroxene higher in CaO than most pyroxene in chondrites (Mason, 1968), but are comparable to pyroxene compositions in the Shaw chondrite (Fredriksson and Mason, 1967); plagioclase is sodiumrich, similar to that in chondrites and unlike the calcium-rich plagioclase typical of most achondrites. For Chassigny the major mineralogical differences from chondrites are the absence and near-absence of nickel-iron and troilite. Texturally Chassigny has always been classed as an achondrite. However, Jeremine et al. (1962) recognized what they described as "chondres naissants," small spherical aggregates consisting mainly of pyroxene. In a thin section of Chassigny in the Smithsonian collection we have recognized structures similar to those described by Jerernine et al. but are unde cided on their interpretation. Whether they are truly incipient chondrules, or chondrules which have been almost erased by recrystallization, or some unrelated structures is difficult to determine from the limited evidence. The texture is that of an aggregate of euhedral to subhedral olivine crystals (size range 0.5 to 1.8 mm) with minor pyroxene and a little interstitial plagioclase; minute chromite grains are included in the olivine, and larger chromite grains are interstitial to the olivine and pyroxene. The texture strongly suggests a cumulate origin. The compositional relationship between Chassigny and the average chondrite is illustrated in Fig. 1. This shows that the major elements Mg, Fe, and Si are similar in abundance, with Mg showing minor enrichment in Chassigny. The only other elements showing relative enrichment in Chassigny are Cr, Mn, and Ba, and La, Ce, and Pr, illustrated in Fig. 2. The enrichment in Cr is explained by the relatively increased chromite content of Chassigny; the Mn enrichment is probably due to the high olivine content, but the source of the Ba enrichment is not immediately apparent. Relative to chondrites, 24

5 10 Fig. 1 I'I'.sa " ".Sr Sc lizr C {p K " ()3 1()4 loll AVERAGE CHONDRITE Plot (logarithmic scale) of elemental abundances (in ppm) in the Chassigny meteorite versus average chondritic abundances. The upper broken line represents 5 times, the lower broken line 0.2 times chondritic abundances. " F. Mg Si.Co AI.Na Mn.Ti 20 s 15 G >-.0 iii G0.7 Q: 0.5 f- %!2 0.4 OIl 0.3 La c. Pr Nd (Pm) Sm Eu Gd Tb Dy Ha Er Tm Vb Fig. 2 REE abundances, normalized to average chondritic abundances, in the Chassigny meteorite. 25

6 Chassigny is notably depleted in AI, Na, K (and Rb), P, Co, Ni, Zr, and to a lesser extent in Ca and Ti; 8 is also strongly depleted. This suggests that if Chassigny was derived from material of chondritic composition, this derivation included the elimination of most of the metal, sulfide, feldspar, and phosphate components. Binz et al. (1975) have pointed out that Chassigny shows compositional similarities to the ureilites; this is certainly true for most ofthe major components, although the FeOMgO ratio for the urelites is consistently lower than this ratio for Chassigny. The urelites are quite different from Chassigny in texture, and Chassigny lacks the carbonaceous matter characteristic of the ureilites. The rare earth distribution, Fig. 2, is an intriguing one. La and Ce are enhanced at a level of 1.3 times chondrites, and the relative concentration falls off rapidly to Sm at 0.5 times chondrites and then remains relatively uniform at this level; there may be a very slight positive Eu anomaly. The distribution pattern resembles that of terrestrial lherzolite and dunite xenoliths in basalts, as exemplified by those described by Frey and Green (1974); these are enriched in La and Ce at 2 to five times chondrites, with rapidly diminishing enrichment to Sm and Eu, and rather uniform concentration of heavier REE at 0.2 to 0.6 times chondrites. This distribution pattern is not controlled by the REE abundances in the dominant mineral, olivine; olivine, both in meteorites (Masuda, 1968) and in terrestrial dunites (Philpotts et al., 1972), has extremely low REE abundances, for most elements at less than 0.1 times chondrites. The distribution pattern in Chassigny must therefore represent a composite of the minor and trace constituents - pyroxene, plagioclase, apatite, and the 0.3% melt inclusions described by Prinz et al. (1974). Of particular significance is the absence of any marked Eu anomaly. If Chassigny were a cumulate from a melt of chondritic composition, with the elimination of most ofthe feldspar, then this meteorite should show a strong negative Eu anomaly, since meteoritic plagioclase is characterized by a strong positive Eu anomaly. The absence of this anomaly indicates that the REE distribution pattern in Chassigny cannot be explained by the simple subtraction of plagioclase (and phosphate) from material of chondritic composition. A reviewer has pointed out that the REE distribution in Chassigny is similar to that in the nakhlites (Schmitt and Smith, 1963), but at much lower concentrations, about 0.2 times those in the nakhlites. This suggests the possibility of a relationship between Chassigny and the nakhlites. The nakhlites have a much higher CaO content and FeOMgO ratio than Chassigny (mineralogically expressed by their consisting largely of diopsidic pyroxene with a minor amount of iron-rich olivine). If Chassigny is genetically related to the nakhlites, it probably represents an early cumulate from a common parent liquid. 26

7 REFERENCES Binz, CM., M. Ikramuddin and M.E. Lipschutz, Contents of eleven trace elements in ureilite achondrites. Geochim. Cosmochim. Acta 39, Damour, A., Note sur la pierre meteoritique de Chassigny. Compt. Rend. Acad. Sci Paris 55, Dyakonova, M.1. and V.Y. Kharitonova, Results of the chemical analyses of some stony and iron meteorites in the collection of the Academy ofscience ofthe U.S.S.R. Meteoritika 18, Fredriksson, K. and B. Mason, The Shaw meteorite. Geochim. Cosmochim.Acta 31, Frey, F.A. and D.H. Green, The mineralogy, geochemistry and origin of lherzolite inclusions in Victorian basanites. Geochim: Cosmochim: Acta 38, Jeremine, E., J. Orcel and A. Sandrea, Etude mineralogique et structurale de la meteorite de Chassigny. Bull. Soc. franc. Mineral. CristaL 85, Jerome, D.Y., Composition and origin of some achondritic meteorites. Dissertation, University oforegon, 113 pp, Mason, B., Pyroxenes in meteorites. Lithos 1, Masuda, A., Lanthanide concentrations in the olivine phase of the Brenham pallasite. Earth Planet. Sci. Lett. 5, McCarthy, T.S., A.J. Erlank, J.P. Willisand L.B. Ahrens, New chemical analyses of six achondrites and one chondrite. Meteoritics 9, Philpotts, J.A., C.C. Schnetzler and H.B. Thomas, Petrogenetic implications of some new geochemical data on eclogitic and ultrabasic inclusions. Geochim. Cosmochim. Acta 36, Prinz, M., P.H. Hlava and K. Keil, The Chassigny meteorite: a relatively iron-rich cumulate dunite.metevritics 9, Taylor, S.R., Geochemical analysis by spark source mass spectrography. Geochim. Cosmochim. Acta 29, Taylor, S.R., Geochemical application of spark source mass spectrography 11. Photoplate data processing. Geochim. Cosmochim. Acta 35, Taylor, S.R., M.P. Gorton, P. Muir, W.B. Nance, R. Rudowski and N. Ware, Composition of the Descartes region, lunar highlands. Geochim. Cosmochim. Acta 37, Manuscript received