Chondrite normalization chondritic meteorite

Table 9.1. Abundance of REE in a chondrite meteorite (ppm) used for normalization of REE data (reported in Henderson 1984).

The relative distributions of REE in geological materials are often represented by plotting the normalized REE concentration (concentration of element in the rock divided by the average concentration of that element in chondritic meteorites) as a function of atomic number (which is inversely proportional to the radius of the 3 + ion) as shown in Figure 3. 

REE are normalized to their concentration in chondritic meteorites. Fe concentration has been calculated as % Fe203. Other elements are reported as ppm (weight basis). 

The most ancient organic molecules available for study in the laboratory are those carried to Earth by infalling carbonaceous chondrite meteorites. All the classes of compounds normally considered to be of biological origin are represented in carbonaceous meteorites and, aside from some terrestrial contamination it is safe to assume that these organic species were produced by nonbiolo-gical methods of synthesis. In effect, carbonaceous chondrites are a namral laboratory containing... 

Figure 10 Normalized reflectance versus wavelength ( jLm) for M-type 16 Psyche versus iron and enstatite chondrite meteorites (lines) (Gaffey, 1976 Bell et al., 1988 Bus, 1999). All spectra are normalized to unity at 0.55 xm. Spectra for all three are relatively featureless with red spectral slopes and moderate albedos. While some M-class asteroids may be metallic core material, existing spectral and density data are inconsistent with this explanation for all M asteroids.

Figure 7 shows the abundances of the four refractory lithophile elements—aluminum, calcium, scandium, and vanadium—in several groups of undilferentiated meteorites, the Earth s upper mantle and the Sun. The RLE abundances are divided by magnesium and this ratio is then normalized to the same ratio in Cl-chondrites. These (RLE/Mg)N ratios are plotted in Figure 7 (see also Figure 1). The level of refractory element abundances in bulk chondritic meteorites varies by less than a factor of 2. Carbonaceous chondrites have either Cl-chondritic or higher Al/Mg ratios (and other RLE/Mg ratios), while rumurutiites (highly oxidized chondritic meteorites), ordinary chondrites, acapulcoites, and enstatite chondrites are depleted in refractory elements. The (RLE/Mg)N ratio in the mantle of the Earth is within the range of carbonaceous chondrites.

Figure 7 Element/Mg ratios normalized to Cl-chondrites of RLEs in various groups of chondritic meteorites. The carbonaceous chondrites are enriched in refractory elements other groups of chondritic meteorites are depleted. The PM has enrichments in the range of carbonaceous chondrites. The low V reflects removal of V into the core...

Chondritic relative abundances of strongly incompatible RLEs (lanthanum, niobium, tantalum, uranium, thorium) and their ratios to compatible RLEs in the Earth s mantle are more difficult to test. The smooth and complementary patterns of REEs in the continental crust and the residual depleted mantle are consistent with a bulk REE pattern that is flat, i.e., unfractionated when normalized to chondritic abundances. As mentioned earlier, the isotopic compositions of neodymium and hafnium are consistent with chondritic Sm/Nd and Lu/Hf ratios for bulk Earth. Most authors, however, assume that RLEs occur in chondritic relative abundances in the Earth s mantle. However, the uncertainties of RLE ratios in Cl-meteorites do exceed 10% in some cases (see Table 4) and the uncertainties of the corresponding ratios in the Earth are in same range (Jochum et ai, 1989 W eyer et ai, 2002). Minor differences (even in the percent range) in RLE ratios between the Earth and chondritic meteorites cannot be excluded, with the apparent exception of Sm/Nd and Lu/Hf ratios (Blicher-Toft and Albarede, 1997). 

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... 

Blichert-Toft and Arndt (1999) proposed an alternative different set of reference values for the BSE. If these were to be adopted, then it is necessary that either the Earth accreted at a different time from the normally accepted 4.57 Ga or from a bulk composition different from that of Cl chondrites. This latter option was explored by Patchett et al. (2004) who found significant variation in the Hf-isotopic composition of chondritic meteorites, indicating that the future resolution of terrestrial Hf-isotope systematics is likely to be found in an appropriate choice of meteoritic parent for the Earth. 

Examples of the isotopes of Nd and Be as water mass labels are shown in Figure la and b. The isotope variations of Nd are so small that the ratio is reported normalized to a ratio typically found in chondritic meteorites ( CFIUR ) ... 

Rare earth element concentrations in rocks are usi lly normalized to a common reference standard, which most commonly comprises the values for chondritic meteorites. Chondritic meteorites were chosen because they are thought to be relatively unfractionated samples of the solar system dating from, the original nucleosynthesis. However, the concentrations of the RZE in the solar system are very variable because of the different stabilities of the atomic nuclei. REE with even " atomic numbers are more stable (and therefore more abundant) than REE with odd atomic numbers, producing a zig-zag pattern bn a composition-abundance diagram (Figure 4.19). This pattern of abundances is also found in natural samples. 

The platinum group elements may be presented in the same way as are the REE and incompatible elements and normalized to either chondrite meteorites or to the primitive mantle.. . v... 

Fig. 2. The plot of total reduced iron, Fe, and oxidized iron, Fe, normalized to Si abundance shows how the chondrite classes fall into groups distinguished by oxidation state and total Fe Si ratio. The soHd diagonal lines delineate compositions having constant total Fe Si ratios of 0.6 and 0.8. The fractionation of total Fe Si is likely the result of the relative efficiencies of accumulation of metal and siUcate materials into the meteorite parent bodies. The variation in oxidation state is the result of conditions in the solar nebula when the soHds last reacted with gas. Terms are defined in Table 1 (3).

The number of scientific articles published on meteorites has increased dramatically in the last few years few of these, however, concern themselves with small meteorites, the size of which lies between that of the normal meteorites (from centimetres to metres in size) and that of interplanetary dust particles. In the course of an Antarctic expedition, scientists (mainly from French institutions) collected micrometeorites from 100 tons of Antarctic blue ice (Maurette et al 1991). These micrometeorites were only 100 400 pm in size five samples, each consisting of 30-35 particles, were studied to determine the amount of the extraterrestrial amino acids a-aminoisobutyric acid (AIBS) and isovaline—both of which are extremely rare on Earth—which they contained. The analysis was carried out using a well-tested and extremely sensitive HPLC system at the Scripps Institute, La Jolla. Although the micrometeorites came from an extremely clean environment, the samples must have been contaminated, as they all showed traces of L-amino acids. Only one sample showed a significantly higher concentration of AIBS (about 280 ppm). The AIBS/isovaline ratio in the samples also lay considerably above that previously found in CM-chondrites. 

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