Carbonaceous chondrites volatile element depletion

Volatile-element depletion patterns in other (e.g., CV, CM, or CO) carbonaceous chondrites (Larimer and Anders, 1970 Palme et al., 1988 ... 

Copper in meteorites is depleted in the heavier 65 isotope with respeet to the Earth (Luek et al. 2003 Russell et al. 2003). Luck et al. s (2003) study of the four main groups of carbonaceous chondrites CI-CM-CO-CV showed that Cu depletion is maximum (-1.5%o) for the C V chondrites (e.g., Allende) for which the depletion of volatile elements is strongest, which indicates that volatilization does not accormt for the observed isotopic heterogeneity (Fig. 4). Luck et al. (2003) found that 8 Cu in CI-CM-CO classes correlates with O excess, but this does not seems to be the case for CV (Luck et al. 2003) nor for the CR, CB, and the particularly Cu-depleted CH-like classes (Russell et al. 2003). In contrast, chondritic Zn is relatively heavy with 8 Zn up to 1 %o (Luck et al. 2001). The rather high 5 Zn values of iron meteorites (up to 4%o)is reminiscent of a similar fractionation of Fe isotopes between metal and silicates (Zhu et al. 2002). 

Figure 12.17a shows lithophile element abundances, and Figure 12.17b shows sid-erophile and chalcophile element abundances in CM chondrites, normalized to Cl chondrites. Illustrated for comparison are the abundances in CO chondrites, which are the anhydrous carbonaceous chondrite group most closely allied to CM chondrites. As in other chondrites, the greatest differences are in volatile elements. The volatile and moderately volatile elements in CM chondrites are present at 50-60% of the abundances of the refractory elements. The volatile elements are primarily located in the matrix, and the matrix comprises 50-60% of CM chondrites. This implies that the matrix has essentially Cl abundances of all elements, while the chondrules and refractory inclusions have Cl relative abundances of refractory elements but are highly depleted in the volatile elements. The sloping transition in the region of moderately volatile elements indicates... 

Laboratory study of the first lunar samples brought to Earth by Apollo 11 ruled out all ideas that the Moon might be a primitive object, i.e. an object that had remained rather cool after its accumulation with only minor melting processes induced by large impacts on the surface. On the contrary, it became evident from the chemical analysis that the Moon, like the Earth, is a highly differentiated object. On the Moon many chemical elements are strongly enriched or depleted as compared with their abundances in carbonaceous chondrites which apart from the most volatile elements, are believed to be most representative of solar matter. Thus it became clear that, at least in the upper 200 km, extensive melting processes must have occurred. 

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 4 Abundances of refractory and moderately volatile elements in various groups of carbonaceous chondrites, normalized to Cl and Mg. Refractory elements increase from Cl to CV3 chondrites while Mg/Si ratios are constant in all groups of carbonaceous chondrites. Although the elements Cr, Fe, and P are significantly less depleted than Mn and Zn, they show a similar behavior, suggesting volatility related depletions of Cr, Fe, and P in carbonaceous chondrites of higher metamorphic grades (source Wolf and Palme, 2001).

Figure 23 Bulk concentrations of lithophile elements normalized to Cl chondrites and silicon in skeletal-olivine and cryptocrystalline chondmles including cryptocrystalline inclusions in metallic Fe,Ni in two CBb chondrites (HH 237 and QUE 94411). Shaded regions show compositional range of chondmles in other carbonaceous chondrites, excluding the CH group chondmles that are closely related to CB chondmles. The wide range of refractory abundances in CBb chondmles ((0.02-3) X Cl levels) appears to reflect fractional condensation with skeletal-ohvine chondmles condensing at higher temperatures than cryptocrystalline chondmles. Both types are more depleted in moderately volatile elements than other carbonaceous chondrites (after Krot et ah, 2002a).

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).

The concentrations of four typical moderately volatile elements—manganese, sodium, selenium, and zinc—in the various classes of chondritic meteorites are shown in Figure 12, where elements are normalized to magnesium and CI-chondrites. Again there is excellent agreement between solar abundances and Cl-meteorites. A characteristic feature of the chemistry of carbonaceous chondrites is the simultaneous depletion of sodium and manganese in all types of carbonaceous chondrites, except Cl. Ordinary and enstatite chondrites are not or only slightly... 

Figure 12 Moderately volatile/Mg ratios in various types of chondritic meteorites. All groups of chondritic meteorites are depleted in moderately volatile elements, none is enriched. The two elements Mn and Na are depleted in carbonaceous chondrites and in the Earth but not in ordinary and enstatite chondrites. The Earth is also depleted...

In summary, it appears from Figure 17 that the bulk composition of the Earth is related to the carbonaceous chondrites, suggesting the same event of manganese depletion for carbonaceous chondrites and the Earth. This supports the hypothesis that the depletion of volatile elements in the various solar system materials is a nebular event at the beginning of formation of the solar system as suggested by Palme et al. (1988), Humayun and Cassen (2000), and Nyquist et al. (2001). 

Potassium is a moderately volatile element and is depleted by a factor of —8 in the bulk silicate Earth compared to Cl chondrites, but a precise and unambiguous concentration is difficult to obtain. Estimates have been made by comparison with uranium, which like potassium is highly incompatible during melting and so is not readily fractionated between MORE and the upper mantle. There is little debate regarding the concentration of uranium, which is obtained from concentration in carbonaceous chondrites and, by assuming that refractory elements (e.g., calcium, uranium, thorium) are unfractionated from solar values... 

FIGURE 5.6 Major volatile element and noble gas abundances in the outer Earth reservoir of Kramers (2003) and in carbonaceous chondrites relative to Al and solar abundances. The data show that apart from xenon the Earth and chondritic meteorites have similar element distribution patterns and that both are strongly depleted in the noble gases and in H, C, and N relative to solar abundances. 

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. 

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