C-chondrites

Carbonaceous chondrites (C-chondrites) account for only 2-3% of the meteorites so far found, but the amount of research carried out on them is considerable. C-chondrites contain carbon both in elemental form and as compounds. They are without doubt the oldest relicts of primeval solar matter, which has been changed only slightly or not at all by metamorphosis. C-chondrites contain all the components of the primeval solar nebula, apart from those which are volatile they are often referred to as primitive meteorites . 

Now and then, projectiles from outer space cause excitement and surprises, as in January 2000, when a meteorite impacted the frozen surface of Lake Targish in Canada. It was a new type of C-chondrite with a carbon concentration of 4-5%, and probably came from a D-type asteroid (Hiroi et al., 2001). More exact analysis of the Targish meteorite showed the presence of a series of mono- and dicarboxylic acids as well as aliphatic and aromatic hydrocarbons (Pizzarello et al., 2001). Aromatic compounds and fullerenes were detected in the insoluble fraction from the extraction this contained planetary helium and argon, i.e., the 3He/36Ar ratio was... 

Bulk rock analyses of carbonaceous chondrites exhibit a Cr excess from 1 to 2 s (Rotaru et al. 1992 Shukolyukov et al. 2003). The carbonaceous chondrites are not exactly solar in their Cr isotopic bulk composition, but taking into account that the components are more than 220 e apart, the match is very close and the idea that C chondrites are a fair representation of the solar system average is still reasonable. 

Matrix of ungrouped C chondrites, Acfer 094, and Adelaide H-L-LL3 chondrite matrices KS chondrite matrix (Kakangari)... 

So what is the relationship, if any, among comets, asteroids, and meteorites Part of the answer to that question is fairly clear It appears nearly certain that the vast majority of meteorites are remnants of bodies that originated in the asteroid belt. There is now enough evidence to say with some certainty that carbonaceous chondrites came from C chondritic asteroids, iron meteorites came from M asteroids, enstatite achondrites came from E asteroids, and so on. It can also he said with some assurance that no known meteorites came from a comet. Many astronomers are willing to agree that comets may he the source of meteorites and that a cometary meteorite may someday he found, hut that day has not yet arrived. 

Workers often speak of chondritic compositional models for the terrestrial planets, but some use the phrase to denote C chondrites and some take it to mean a mix of chondrite classes or material mineralogically similar to chondrites, but not exactly the same as any single class . This confusion is a perennial source of confusion. Caveat lector. 

Chondrite classes are also distinguished by their abundances of both volatile and refractory elements (3). For volatile elements the variation among groups results from incomplete condensation of these elements into soHd grains that accrete to form meteorite parent bodies. Volatile elements such as C,... 

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

Fig. 2.34. (A) Chondrite-normalized REE pattern of fresh basaltic andesite. (B) Chondrite-normalized REE pattern of altered basaltic andesite. (C) Ratios of REE content in the altered rock normalized to the fresh basaltic andesite (298-R-02). The dashed line is the ratio line of one. (D) Chondrite-normalized REE patterns of hydrothermal fluids from Vienna Wood, Pacmanus and Desmos, Tamagawa and Kusatsu-Yubatake (Gena et al., 2001).

Now, apart from the planets, many meteorites were formed, moving in quite different orbits and of quite different chemical composition. In particular, the so-called C-l meteorites composed of carbonaceous chondrites have a composition of elements much closer to that of the Sun. It is proposed (see for example Harder and also Robert in Further Reading) that many of these meteorites collided with very early Earth and became incorporated in it, so that eventually some 15% of Earth came from this material (see Section 1.11). Other planets such as Mars and the Moon could have had similar histories, but the remote planets and Venus are very different. 

Each abundance was divided by the abundance of that element (except for Rh) in Type / carbonaceous chondrites. Rh abundances were divided by Rh abundances in other types of chondrites as Cl values were not available. Errors in the LBL measurements reflect 1 a values of the counting errors, except for the Au error. The latter is the root-mean-square deviation of six measurements, because the six values were not consistent within counting errors. The Os measurement was on a HNO,-insoluble residue that had been fired to 800°C. Key , this work and O, previous work of Ganapathy. 

If the C-T boundary includes an asteroid component with chondritic abundances, the ratios of the abundances of various elements to Ir should be larger than chondritic because of terrestrial contributions. 

Fig. 3. Total alkalis versus silica (TAS) and (b) AFM plot of Irvine and Baragar (1971), (c) modified Zr/Ti02-Nb/Y plot (Pearce, 1996) of Winchester and Floyd (1977), (d) Rock/chondrite-normalized REE diagram for rocks of amli-llica pluton and (e) rock/MORB-normalized spidergrams, (f) Th/Yb vsTa/Yb diagram.

Figure 8. Figure (a) after Clayton et al. (1976, 1977). The scales are as in Figure 1. The O isotopic compositions of the different meteorite classes are represented ordinary chondrites (H, L, LL), enstatite chondrites (EFl, EL), differentiated meteorites (eucrites, lAB irons, SNCs) and some components of the carbonaceous chondrites. As the different areas do not overlap, a classification of the meteorites can be drawn based on O isotopes. Cr (b) and Mo (c) isotope compositions obtained by stepwise dissolution of the Cl carbonaceous chondrite Orgueil (Rotaru et al. 1992 Dauphas et al. 2002), are plotted as deviations relative to the terrestrial composition in 8 units. Isotopes are labeled according to their primary nucleosynthetic sources. ExpSi is for explosive Si burning and n-eq is for neutron-rich nuclear statistical equilibrium. The open squares represent a HNOj 4 N leachate at room temperature. The filled square correspond to the dissolution of the main silicate phase in a HCl-EIF mix. The M pattern for Mo in the silicates is similar to the s-process component found in micron-size SiC presolar grains as shown in Figure 7.

Gopel C, Manhes G, Allegre CJ (1994) U-Ph systematics of phosphates from equilibrated ordinary chondrites. Earth Planet Sci Lett 121 153-171... 

Virag A, Zinner E, Lewis RS, Tang M (1989) Isotopic compositions of H, C, and N in C8 diamonds from the Allende and Murray carbonaceous chondrites. Lunar Planet Sci XX 1158-1159 Volkening J, Papanastassiou DA (1989) Iron isotope anomalies. Astrophys J 347 L43-L46 Volkening J, Papanastassiou DA (1990) Zinc isotope anomalies. Astrophys J 358 L29-L32 Wadhwa M, Zinner EK, Crozaz G (1997) Manganese-chromium systematics in sulfides of unequilibrated enstatite chondrites. Meteorit Planet Sci 32 281-292... 

Zitmer E (1997) Presolar material in meteorites an overview. In Astrophysical Implications of the I aboratory Study of Presolar Materials. Bematowicz TJ and Zinner E (eds) AIP, New York, p 3-26 Zinner EK, Gopel C (2002) Aluminium-26 in H4 chondrites implications for its production and its usefulness as a fine-scale chronometer for early solar system events. Meteorit Planet Sci 37 1001-1013 Zinner E, Amari S, Guitmess R, Nguyen A, Stadermann FJ, Walker RM, I wis RS (2003) Presolar spinel grains from the Murray and Murchison carbonaceous chondrites. Geochim Cosmochim Acta 67 5083-5095... 

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