Carbonaceous chondrites elemental composition

Figure 13 A 5 jjim interplanetary dust particle. This is a carbon-rich particle with chondritic elemental composition. It is porous and entirely composed of anhydrous phases. This 10 ° g particle is an aggregate of >10 umelated and unequilibrated grains. The smooth grains are t5fpically single mineral grains such as Fo, En, or pyrrhotite or carbonaceous material, and the <0.5 p,m lumpy grains are usually GEMSs. This is a relatively t5fpical example of the particles that have entry speeds consistent with cometary origin.

Percentage of meteorites seen to fall. Chondrites. Over 90% of meteorites that are observed to fall out of the sky are classified as chondrites, samples that are distinguished from terrestrial rocks in many ways (3). One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Elemental composition is also a property that distinguishes chondrites from all other terrestrial and extraterrestrial samples. Chondrites basically have undifferentiated elemental compositions for most nonvolatile elements and match solar abundances except for moderately volatile elements. The most compositionaHy primitive chondrites are members of the type 1 carbonaceous (Cl) class. The analyses of the small number of existing samples of this rare class most closely match estimates of solar compositions (5) and in fact are primary source solar or cosmic abundances data for the elements that cannot be accurately determined by analysis of lines in the solar spectmm (Table 2). Table 2. Solar System Abundances of the Elements ... 

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. 

Refractory materials in primitive meteorites were investigated first as they have the best chance of escaping homogenization in the early solar system. Inclusions in C3 carbonaceous chondrites exhibit widespread anomalies for oxygen and the iron group elements. Only a few members, dubbed FUN (for Fractionated and Unknown Nuclear effects), also display anomalous compositions for the heavy elements. Anomalies in inclusions have generally been connected with explosive or supernova nucleosynthesis. 

Almost all of the elements heavier than He are synthesized in the interiors of stars. The work of Burbidge et al. (1957) gives the theoretical framework for the synthesis of the elements. The experimental evidence of active nucleosynthesis came from the discovery of the unstable nuclei of technetium in the spectra of red giants (Merrill 1952). The solar elemental and isotopic abundances which are taken from the primitive carbonaceous chondrites constitute the guidelines for testing such models (Anders and Grevesse 1989). A minimum of eight basic processes are required to reproduce the observed compositions. Nucleosynthetic... 

Weber D, Zinner E, Bischoff A (1995) Trace element abundances and magnesium, calcium, and titanium isotopic compositions of grossite-containing inclusions from the carbonaceous chondrite Acfer 182. Geochim Cosmochim Acta 59 803-823... 

The carbonaceous chondrites, which constitute a tiny proportion of the matter within the Solar System, do conserve within them the original composition of the Solar System. If we exclude the volatile elements mentioned above, these rare meteorites have hardly been affected by the subsequent metamorphism of our planetary system. 

Compositional variations among chondrites, (a) Lithophile and (b) siderophile and chalcophile elements in ordinary (H, L, LL), enstatite (EH, EL), R, and chondrites. In (c) and (d), the same data are shown for anhydrous carbonaceous chondrite groups. Elements are plotted from left to right in order of increasing volatility. Lithophile elements are normalized to Cl chondrites and Mg, siderophile and chalcophile elements are normalized to Cl chondrites. Modified from Krot et al. (2003). 

As implied by their names, chondritic IDPs have roughly cosmic bulk compositions. Element ratios for hundreds of analyzed particles are roughly chondritic (data for CP IDPs are shown in Fig. 12.7) (Schramm et al., 1989). An exception, though, is carbon, which is significantly more abundant in IDPs. The mean carbon content of I DPs is 10 wt.%, relative to 3.2 wt.% for Cl chondrites (Bradley, 2004). The abundances of trace elements in bulk IDPs scatter from 0.3 to 3 times Cl, and volatile elements especially tend to be enriched (Flynn and Sutton, 1992). Higher abundances of carbon and of volatile elements, relative to the most solar-like carbonaceous chondrites, support the contention that IDPs are among the most primitive materials known. 

Major-element compositions (weight ratios of Mg/Si and Al/Si) for mantle rocks (peridotites) and estimates of the primitive mantle composition of the Earth compared with various groups of chondrites and the Sun. No mixture of chondrite types provides an exact match to the primitive mantle composition, although some carbonaceous chondrites provide the closest match. Modified from Righter et al. (2006). 

Chondrites. Over 909, of the meteorites lhat are observed lo fall out of the sky are classified as chondrites, samples lhat are distinguished from terrestrial rocks in many ways. One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Chondrites also have basically undifferentiated elemental compositions lor most nonvolatile elements and match solar abundances except for moderately volatile elements. The imtsl cunipositionally primitive chondrites are members for the type I carbonaceous f Cl I class. 

The classification of chondrites on the basis of their elemental composition is recent with respect to the more traditional classification based on textural and mineralogical differences 6). In the case of carbonaceous chondrites this classification was revised by Wasson in 19747). The petrological type (from 1 to 6 even if some authors also use 7) is intended to indicate the degree of equilibration and metamor-phical recrystallisation. So 1 indicates the least-equilibrated and 6 the most-... 

At the end of this section dealing with the question What are carbonaceous chondrites and before trying to answer the question Where do they come from and what is their age , it seems necessary to emphasise the fact that the carbon present in the Cl and CM and to a lesser extent in the CO and CV chondrites exists as elemental carbon (Cl, CM, CO, CV), organic molecules (Cl and CM) and an ill-defined macromolecular compound (Cl, CM, CO, CV). The exact composition of this complex mixture will be discussed below. The only point that we would like to stress in this section is that organic matter is always found intimately mixed with mineral matrices. 

The variation of CL intensity and its relation to the growth of forsterites in meteorites requires a detailed study of the chemical variation and the observed CL. Numerous line scans were described (Steele, I.M. Geochim. Cosmochim. Acta, accepted for publication) where spectra and composition were obtained at the same point for forsterites in the Ornans (C3) carbonaceous chondrite. Interpretation is complicated by the fact that some elements are strongly correlated and the effect on CL can not be attributed to any one element. The most important influence is found to be Fe which had different quenching effects on the blue and infrared CL emissions. The blue emission is effectively quenched for Fe concentrations above 0.75 wt.% FeO while the infrared emission is... 

For the siderophile elements, the metal particles contribute between 50 and 90% to the bulk composition of the soil samples. As carbonaceous chondrites do not contain metal, reduction and equilibration is required to explain the high contribution of the metal particles. 

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). 

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