Murchison chondrite composition

The Murchison meteorite shown in Figure 6.7, like all meteorites, is named after the place from which it was recovered and in this case it is the town of Murchison, Victoria in Australia about 100 km north of Melbourne. The fall occurred in 1969 and was followed by an analysis of the chemical composition in some considerable detail. The Murchison meteorite is a carbonaceous chondrite containing about 2 per cent carbon, some as inorganic carbonates, and some as soluble compounds such as amino acids but the bulk as a macromolecular heterogeneous material referred to as kerogen. 

Hoppe P, Amari S, Zinner E, Ireland T, Lewis RS (1994) Carbon, nitrogen, magnesium, silicon and titanium isotopic compositions of single interstellar silicon carbide grains from the Murchison carbonaceous chondrite. Astrophys J 430 870-890... 

Kerridge JF, Chang S, Shipp R (1987) Isotopic characterization of kerogen-hke material in the Murchison carbonaceous chondrite. Geochim Cosmochim Acta 51 2527-2540 Kharaka YK, Berry FAF, Friedman I (1974) Isotopic composition of oil-field brines from Kettle-man North Dome, California and their geologic implications, Geochim Cosmochim Acta 37 1899-1908... 

The degree of equilibrium isotopic fractionation among phases depends on temperature, so the isotopic compositions of co-existing phases can be used for thermometry. Oxygen is widely used in this way. For example, Clayton and Mayeda (1984) found that the oxygen isotopic compositions of calcite and phyllosilicates from Murchison lie on a mass-dependent fractionation line and differ in 6180 by 22%o. This difference requires a temperature of around 0 °C, which is interpreted to be the temperature of aqueous alteration on the Murchison parent asteroid. Similar measurements for Cl chondrites indicate that aqueous alteration for these meteorites occurred at higher temperature, 50-150 °C (Clayton and Mayeda, 1999). 

Soluble organic compounds in chondrites are highly varied in structure, composition, and size (molecular weight). Table 10.1 lists the groups of extractable organics in Murchison ( 2) and Tagish Lake (another CM-like chondrite). These two falls are the most... 

About 70-95% of the organic matter in carbonaceous chondrites consists of an ill-defined, insoluble macromolecular material, often referred to as polymer or kerogen . A typical elemental composition for Murchison polymer (Hayatsu et al., 1980a), on a dry, ash-free basis, is C 76.5%, H 4.5%, N 2.4%, S 4.3%, and O 12.4% (by difference). 

The compositions of individual components of CM chondrites suggest a complex alteration history. Calcite and phyllosilicates in Murchison fall on a mass-dependent fractionation line with A O=—1.4%o (Clayton and Mayeda, 1984), indicating that they formed at equilibrium from the same aqueous reservoir. Clayton and Mayeda (1984, 1999) have argued that this was most likely to have occurred within an asteroidal parent body consistent with the very high of carbonates in CM chondrites (-h40%o to -h70%o) (Clayton, 1963 Halbout etai, 1986). Analyses of carbonate from four splits from Murchison (Benedix et al., 2003) show a range of values... 

Figure 3 Oxygen-isotopic compositions of CM2 carbonaceous chondrites plotted in a three-isotope diagram. The CM2s form a linear array with a slope of 0.7, with enrichments in the heavy isotopes that show a general increase as a function of increasing alteration. Nogoya (NG) and Cold Bokkeveld (CB) are the most heavily altered CMs and Murchison (MC) is one of the least altered. Chondrites with intermediate degrees of alteration are not well resolved in this diagram (after Browning et al., 1996 source Clayton, 1993).

Figure 3 Carbon stable-isotope compositions of solvent extractable aromatic and PAHs plotted against carbon number from the Murchison and Asuka-881458 CM2 carhonaceous chondrites (sources Yuen et al., 1984 Gilmour and Pillinger, 1994 Sephton et al., 1998 ...

Figure 4 Carbon stable-isotope compositions of solvent extractable n-alkanes from the Orgueil (Cl), Cold Bokkeveld (CM2), Murchison (CM2), Vigarano (CV3), Ornans (CO), and Tagish Lake carbonaceous chondrites plotted against carbon number (sources Sephton et al., 2001 PizzareUo et al, 2001).

Figure 6 Carbon and hydrogen stable-isotope compositions of discrete moieties identified using stepped combustion of macromolecular material from the Murchison (CM2) chondrite (source Kerridge et al., 1987).

Mercury abundances and isotopic compositions in the Murchison (CM) and Allende (CV) carbonaceous chondrites. Geochim. Cosmochim. Acta 65(16), 2807—2818. 

PAHs found in carbonaceous chondrites show a compositional difference from the shock-synthesized PAHs in this study. The former are predominantly condensed ring compounds, whereas the latter are dominated by polyphenyl compounds. However, many PAHs reported to be present in carbonaceous chondrites could be produced by the shock reaction from benzene. Major species of PAHs in carbonaceous chondrites such as naphthalene, biphenyl, and phenanthrene were formed abundantly in this study. Furthermore, the mutual ratios of structural isomers in the Murchison meteorite [149], the Yamato-791198 meteorite [150], and the Yamato-74662 meteorite [151] resemble those of the shock products in our experiments in particular, the coincidence in the ratios of 2-MeNap/l-MeNap and fluoranthene/pyrene is striking. This implies a genetic connection between the shock products and the organic materials in carbonaceous chondrites. 

Figure 10.5 Variations in Zr isotope compositions illustrated in (a) 8 Zr and (b) e Zr versus 8 Zr diagrams for leachates of the carbonaceous chondrites Orgueil (Cl), Murchison (CM), and Allende (CV) (Table 10.1). 8 Zr denotes variations in the Zr/ r ratio, given in parts per 10 000. The leachates were obtained by treatment of bulk rock samples with sequentially stronger mineral acids 50% HAc, 4M HNOj, 6M HCI, 13.5M HF + 3M HCI, followed by dissolution in a Parr bomb with HNO3-HF. The dashed line denotes the Zr isotopic composition of average solar system material. Adapted from [64].

Direct evidence for the abiotic presence of fatty acids comes from the detection of fatty acids in the interior of the Murray and Murchison carbonaceous chondrite meteorites from Australia (up to C8), as well as an Asuka carbonaceous chondrite meteorite (A-881458) from Antarctica (up to C12) [84-87]. Fatty acids are relatively abundant in these meteorites, being 20 times more abundant than amino acids in the organic extract of A-881458. Indeed, organic extracts from the Murchison meteorite form boundary membranes when rehydrated [88, 89]. The presence of fatty acids is particularly suggestive because the chemical composition of these meteorites is believed to resemble that of the early solar system. 

The isotopic anomalies detected in the iron peak elements are extremely small (Figure 4), since the anomalous material is diluted with material with a terrestrial isotopic composition. This contamination results from the fact that the meteorite inclusion is taken into solution before chemically extracting the relevant element in a form suitable for conventional TIMS analysis. However, ion microprobe mass spectrometry can be used to analyse small meteoritic inclusions in situ without the need of chemical processing. This enables single inclusions to be analysed for a variety of elements, whilst maintaining the petrographic context of the sample. The carbonaceous chondrites Murchison and Murray also contain refractory inclusions such as corundum and hibonite, but they are invariably small and difficult... 

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