Introduction to chondritic meteorites compositions and taxonomy

The most primitive meteorites are aggregates consisting of a fine-grained heterogeneous matrix, in which are embedded small (generally from 0.2 to 1.0 mm) droplet-like silicate particles termed chondrules, which have given the name to this meteorite class, chondrites. The chondrules and the matrix comprise distinct and different assemblies of chemical elements, e.g. volatile-depleted chondrules coexist with a volatile-enriched matrix. Some chondrites also contain metal grains. The third component of chondrites, Ca-Al-rich inclusions, was discussed in Chapter 10.

Chondrites preserve a unique record of processes and conditions in the early nebula. Although never molten after their agglomeration, chondrites have variably undergone metamorphism that has in some cases altered their mineralogy and composition. The relative abundances of involatile major and trace elements in bulk chondrites are generally similar to the solar abundances (see Section 3.4). However, in sharp contrast with the solar composition, only four elements, oxygen, silicon, magnesium and iron, are the major chondritic components, contributing almost 90% by mass, and these same four are also the major constituents of the terrestrial planets (Tables 11.1 and 17.1). There are clear reasons for the high contributions of these elements to the meteoritic and planetary compositions. First of all, the most abundant isotopes of O, Mg and Si are the so-called a- or primary isotopes (16O, 24Mg and 28Si), characterized by a high binding energy per nucleon (Chapter 1). Therefore these isotopes are produced preferentially to other nuclides in explosive SNe II nucleosynthesis, which is the dominant source of heavier-than-carbon elements in the Galaxy (Chapter 8). Iron-peak elements having the highest binding energy per nucleon are the final products of nuclear fusion and are abundant in both SNe II and SNe I ejecta (Table 8.1). Another important reason for the exceptionally high abundances of these four elements is that the oxides of these

Table 11.1 Abundances of major elements in chondrite groups (the abundance ratios are in atomic %, excepting the last column, which is in mol %)a

Table 11.1 Abundances of major elements in chondrite groups (the abundance ratios are in atomic %, excepting the last column, which is in mol %)a

Clan

Group

Si, wt %

Al/Si

Ca/Si

Mg/Si

Fe/Si

Si

+MgO)

refractory-rich

CVb

15.6

11.6

8.4

108

76

0.6-19

35

mini-chondrule

COc

15.9

9.3

6.9

91

78

2.3-15

35

CMd

12.9

9.7

6.8

91

80

0.1-0.5

43

volatile-rich

C1e

10.6

8.6

6.2

92

86

< 0.1

45

ordinary

LL f

18.9

6.5

4.7

81

53

2.7-22

27

Lg

18.5

6.6

4.7

80

57

17-22

22

Hh

16.9

6.8

4.9

83

80

46-52

17

enstatite

EL1'

18.6

5.4

3.6

76

65

47-57

0.05

EHj

16.7

5.1

3.6

63

97

68-72

0.05

a After Kallemeyn and Wasson (1981), Wasson (1985), Wasson and Kallemeyn (1988), Rubin (1997).

b CV, large-chondrule-bearing, abundant CAIs, volatile-poor (relative to other groups), partially aqueously altered. c CO, mini-chondrule-bearing, metal-bearing. d CM, mini-chondrule-bearing, aqueously altered. e C1, chondrule-free, volatile-rich, aqueously altered. f LL, low total iron, low metallic iron. g L, low total iron. h H, high total iron.

' EL, low total iron, highly reduced, moderately sized chondrules. j EH, high total iron, highly reduced, mini-chondrule-bearing.

a After Kallemeyn and Wasson (1981), Wasson (1985), Wasson and Kallemeyn (1988), Rubin (1997).

b CV, large-chondrule-bearing, abundant CAIs, volatile-poor (relative to other groups), partially aqueously altered. c CO, mini-chondrule-bearing, metal-bearing. d CM, mini-chondrule-bearing, aqueously altered. e C1, chondrule-free, volatile-rich, aqueously altered. f LL, low total iron, low metallic iron. g L, low total iron. h H, high total iron.

' EL, low total iron, highly reduced, moderately sized chondrules. j EH, high total iron, highly reduced, mini-chondrule-bearing.

metals as well as of metallic iron are involatile and condense at high temperatures between 1300 and 1600 K, thus forming solid grains.

On the basis of textural, chemical and mineralogical criteria, three major classes of chondritic meteorites are distinguished: carbonaceous (CC), enstatite (EC) and ordinary (OC) chondrites, which are in turn subdivided into subgroups (Table 11.1). Carbonaceous chondrites are so named because they contain organic matter; these meteorites constitute the least metamorphosed group even though most underwent hydrous alteration (Kallemeyn and Wasson, 1981). They play a central role in understanding the early evolution of the solar system and are therefore the focus of our discussion. Ordinary chondrites are the most common "falls" and "finds". However, spectral analysis of the light reflected by objects in the asteroid belt shows that the majority of these resemble carbonaceous chondrites (Meibom and Clark, 1999).

Abundances of the major elements vary slightly from one group of chondrite meteorites to another and also within a group, giving rise to a more detailed classification. Thus, the total Fe/Si ratio is highest in carbonaceous C1 chondrites,

Fig. 11.1 Atomic Fe/Si ratios in meteorite classes depending on oxidized/redox states of iron. After Kerridge (1993) and McDonough and Sun (1995).

intermediate in high-metal (H) and low-metal (L) chondrites and lowest in the very-low-metal chondrites LL (Table 11.1, Fig. 11.1). The proportion of metallic to total [Fe] also varies, increasing from ~ 0 in the highly oxidized C1 (all the Fe is present as oxide or silicate minerals) to ~ 1.0 in the highly reduced EH (almost all the Fe is present as metal).

Refractory lithophile-element abundances in chondrites clearly show flat patterns when normalized to C1, exceeding the C1 abundances by ~ 15% for the CM and CO groups and by as much as ~ 30% for the CV groups. These differences are for the most part due to a variable depletion in the volatile elements and the proportion of major refractory compounds.

Even though carbonaceous chondrites are considered the most primitive group, there are substantial differences between them, related to formation processes, aqueous alteration and the extent of parent body metamorphism. Compared with C1, moderately volatile elements are depleted in other CC groups in order of decreasing condensation temperature (Fig. 11.2, Table 11.2). This pattern is even more pronounced for ordinary chondrites, so that OC/C1 abundance ratios are below

1300 1200 1100 1000 900 800 700 600

Condensation temperature (K)

Fig. 11.2 Abundances of major and trace elements in chondrite meteorites. Plotted is the ratio for a given class of meteorite of the average concentration of element X and the average concentration of Si, divided by the same ratio for C1 meteorites, as a function of condensation temperature. From Cassen (1996), © Meteoritical Society 1996, reproduced by permission.

1300 1200 1100 1000 900 800 700 600

Condensation temperature (K)

Fig. 11.2 Abundances of major and trace elements in chondrite meteorites. Plotted is the ratio for a given class of meteorite of the average concentration of element X and the average concentration of Si, divided by the same ratio for C1 meteorites, as a function of condensation temperature. From Cassen (1996), © Meteoritical Society 1996, reproduced by permission.

~ 0.1 for Tl, Bi, Pb and several other highly volatile elements (Wasson and Kalle-meyn, 1988). The abundance patterns seen in Fig. 11.2 emphasize specific features of the loss process and the environment in which it occurred, e.g. enhanced abundances of K and Nain H-chondrites most probably reflect highly reduced conditions, at which alkali metals are less volatile.

The matrix and chondrules of meteorites record nebula conditions and their interrelationships are of prime importance for understanding nebula processes.

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