Ordering Carbonaceous chondrites

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

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). 

The elemental abundance of the lunar mare rocks as compared to that of carbonaceous chondrites vary up to 6 orders of magnitude (Fig. 3a). The strongly siderophile elements and the very volatile elements are highly depleted, while the refractory elements Al, Ca, Ti, REE, Th, U. etc. are enriched. Hence, it is rather difficult to explain the fractionation of the lunar mare basalts by... 

Figure 11 Compositions of olivine in AO As from the reduced CV chondrites Vigarano (a), Leoville (b), Efremovka (c), oxidized CV chondrite Allende (d), CR chondrites (e), CH and CB chondrites (f), unique carbonaceous chondrites Adelaide (g), and Acfer 094 (h). Olivines in CRs, Adelaide, and Acfer 094 are magnesium-rich compared to olivines in CV AO As. Fayalite contents in olivines from CV AO As increase in the order Leoville and Vigarano, Efremovka, Allende this is correlated with the degree of secondary alteration and thermal metamorphism experienced by CV chondrites. Data for Allende AO As are from Hashimoto and Grossman (1987) and for Efremovka, Leoville, and Vigarano AOAs from Komatsu et al (2001).

Mazor et al. (1970) carried out the first extensive set of CRE ages for carbonaceous chondrites, based mainly on Ne. Figure 1 presents Ne exposure ages for carbonaceous chondrites calculated from the compilation of (Schultz and Franke, 2002), assuming that the neon was in each case a mixture of solar and cosmogenic gas. Some cautions are in order for Cl and CM chondrites in particular. 

To first order, the larger data set now available extends and confirms the general assessments of MacPherson et al. (1995), albeit with some modifications and enhancements. The distribution of inferred initial Al/ Al in CAIs is bimodal (Figure 3(a)), with the dominant peak at the so-called canonical value of 4.5 X 10, and a second peak at dead aluminum (i.e., Al/ Al = 0). MacPherson et al. (1995) demonstrated that this pattern applied to all classes of carbonaceous chondrites, although the relative... 

Figure 6 Volatile/refractory element ratio-ratio plots for chondrites and the silicate Earth. The correlations for carbonaceous chondrites can be used to define the composition of the Earth, the Rb/Sr ratio of which is well known, because the strontium isotopic composition of the BSE represents the time-integrated Rb/Sr. The BSE inventories of volatile siderophile elements carbon, sulfur, and lead are depleted by more than one order of magnitude because of core formation. The values for Theia are time-integrated compositions, assuming time-integrated Rb/Sr deduced from the strontium isotopic composition of the Moon (Figure 8) can be used to calculate other chemical compositions from the correlations in carbonaceous chondrites (Halliday and Porcelli, 2001). Other data are from Newsom (1995).

Carbonaceous material is widespread throughout CP IDPs both as discrete inclusions of noncrystalline material and as a semicontinuous matrix with embedded mineral grains. Often it has a vesiculated appearance consistent with an organic component (Figure 7(a)). The bulk abundance of carbon in CP IDPs varies from —4% to 45% with an average of 13% (Keller et al., 1994). In contrast to the fine-grained matrices of carbonaceous chondrites, ordered... 

After the accretionary event in which the Earth acquired its volatiles, other processes took place which caused it to lose them. There are two lines of evidence which tell us about the early Earth s loss of volatiles. The first comes from a comparison between the volatile concentrations in the outer Earth and those of carbonaceous chondrite meteorites (the most primitive and most volatile-rich of all the meteorite groups). It is clear from Fig. 5.6 that the Outer Earth Reservoir has two to three orders of magnitude less volatiles than carbonaceous chondrites. In addition it is evident that the lighter major elements are more depleted than the heavy ones. 

The analysis of the insoluble organic component (lOM) in carbonaceous chondrites is based on the techniques developed for coal, oil shales and petroleum source rocks. In order to analytically access the organic material the mineral phase of the meteorite is dissolved using a mixture of HCl and HF (9), leaving behind an organic residue that can be either processed further or used directly for analysis. 

Figure 1. Basic planetary patterns as observed in various meteorites and the atmospheres of the Earth and Mars. Shown are abundance ratios relative to Xe of " He, °Ne, Ar and " Kr normalized to the solar ratios (Wieler 2002). In order to be able to show data for " He with minimal radiogenic contribution, for the carbonaceous chondrite Allende data for a carbon-rich acid residue (BAl, Ott et al. 1981) are shown, for the ureilite Haverd data for a handpicked carbon-rich sample (B-18-1, Gobel et al. 1978). Enstatite data (South Oman) are from Crabb and Anders (1981), those for Mars from Pepin (1991).

Earth and its reservoirs, during the lifetime of i82Hf HfAV ratio of the silicate Earth is considered to be in the range of 10-40 as a result of an intensive study by Newsom et al (1996). This is an order of magnitude higher than in carbonaceous and ordinary chondrites and a consequence of terrestrial core formation. More recent estimates are provided in Walter et al (2000). 

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