Carbon meteorite evidence

ALHA 84001 is a unique Martian meteorite with an ancient age of 4.5 billion years. It consists mostly of orthopyroxene crystals that accumulated in basaltic magma. Its most distinctive feature, however, is the occurrence of small globules of carbonates with unusual compositions and textures. The controversial (now largely discredited) hypothesis that the carbonates contain evidence of extraterrestrial life (McKay et al., 1996) made this the most famous meteorite on Earth. ALHA 84001 has also experienced intense shock metamorphism. 

Smith, P. P. K. Buseck, P. R. 1981a Carbon in the Allende meteorite evidence for poorly graphitised carbon rather than carbyne. Proc. Lunar. Sci. Conf. 12b, 1167-1175. 

There are good reasons to assume that the core contains some amount of carbon, phosphorus, and sulfur. These three elements are among the 12 most common in the Earth that account for >99% of the total mass (Table 5), as based on geochemical, cosmochemical, and meteoritical evidence. Seven out of 12 of these elements (not including carbon, phosphorus, and sulfur) are either refractory or major component elements. 

But see T. Ferroir, L. Dubrovinsky, A. El Goresy, A. Srmiono-vici, T. Nakamura and P. Gillet (2010) Earth Plant. Sc. Lett., vol. 290, p. 150 - Carbon polymorphism in shocked meteorites Evidence for new natural ultrahard phases . 

As is the case for hydrogen, carbon isotope signatures in Martian meteorites present evidence for different carbon reservoirs. Wright et al. (1990) and Ro-manek et al. (1994) distingnished three carbon compounds one component released... 

Valley JW, EUer JM, Graham CM, Gibson EK, Romanek CS, Stolper EM (1997) Low temperature carbonate concretions in the martian meteorite ALH 84001 evidence from stable isotopes and mineralogy. Science 275 1633-1637... 

C60 has not yet been detected in primitive meteorites, a finding that could demonstrate its existence in the early solar nebular or as a component of presolar dust. However, other allotropes of carbon, diamond and graphite, have been isolated from numerous chondritic samples. Studies of the isotopic composition and trace element content and these forms of carbon suggest that they condensed in circumstellar environments. Diamond may also have been produced in the early solar nebula and meteorite parent bodies by both low-temperature-low-pressure processes and shock events. Evidence for the occurrence of another carbon allotrope, with sp hybridized bonding, commonly known as carbyne, is presented. 

Of the secondary processes that have affected chondritic meteorites, aqueous alteration is among the most widespread. Evidence of varying degrees of aqueous alteration is present in all the major chondrite groups, with the exception of the enstatite chondrites. This alteration is typically indicated by the presence of hydrous phyllosilicates (principally serpentines and smectite clays), often associated with carbonates, sulfates, oxides (magnetite), and secondary sulfides. The variable alteration assemblages present in different chondrite groups are principally the result of alteration under different conditions (P, T, fo, water/rock ratio) (e.g., Zolensky et al., 1993). 

Silicates provide further evidence for the unusual origin of lAB-IIICD. While differentiated silicates might be expected in association with iron meteorites, silicates in lAB-IIICD irons are broadly chondritic (Mittlefehldt et al., 1998 Benedix et al., 2000 see Chapter 1.11). Models for the origins of lAB-IIICD iron meteorites include crystallization of a sulfur- and carbon-rich core in a partially differentiated object (Kracher, 1985 McCoy et al., 1993), breakup and reassembly of a partially differentiated object at its peak temperature (Benedix et al., 2000), or crystal segregation in isolated impact melt pools on the surface of a porous chondritic body (Wasson and Kallemeyn, 2002). A recent compilation of the chemical compositions of lAB and IIICD iron meteorites may be found in Wasson and KaUemeyn (2002). 

The C/Si ratio in chondritic IDPs is systematically higher than all classes of chondritic meteorites. The mean carbon abundance is —10 wt.% versus 3.22 wt.% for Cl (see Chapter 1.03). Nitrogen has been detected in chondritic IDPs but as yet not quantified, although Keller et al. (1995) report that the C/N ratio is approximately chondritic. Electron energy-loss spectra show that nitrogen is carried in amorphous carbonaceous material and that it is heterogeneously distributed as hot spots. There is indirect evidence that the nitrogen is associated with polyaromatic hydrocarbons (Section 1.26.3.1). 

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