Chondrite petrologic type

The least metamorphosed ordinary chondrites (petrologic types 3.0-3.1) show evidence of minor aqueous alteration that primarily affected the matrix, but in some cases also affected chondrule mesostasis. In these meteorites, the amount of aqueous fluid was very small, limiting the degree of alteration. 

Figure 20. Lithium isotopic compositions of chondritic meteorites relative to their petrologic type (James and Palmer 2000a McDonough et al. 2003). The extant data indicate lighter and more variable isotopic compositions in higher petrologic types of meteorites, those with the least record of hydrous parent body alteration.

Huss, G. R. and Lewis, R. S. (1995) Presolar diamond, SiC, and graphite in primitive chondrites Abundances as a function of meteorite class and petrologic type. Geochimica et Cosmochimica Acta, 59, 115—160. 

Classification system for chondrites, adapted from Van Schmus and Wood (1967). A meteorite is classified by identifying its chemical group and petrologic type. Approximate temperatures for metamorphism or alteration are shown at the bottom. The relative abundances of meteorites assigned to various petrologic types are indicated by the shaded proportion of each box (data from Scott and Krot, 2004). 

Table 6.1 Criteria for classifying chondrites according to petrologic type (Weisberg et al., ) ... 

As seen in Figure 6.3, the taxonomy used for chondrites is a combination of chemical group and petrologic type, as in H4 or 2. This system avoids the problems of previous chondrite classifications, by cleanly separating primary and secondary characteristics. 

Bonal, L., Quirico, E., Bourot-Denise, M. and Montagnac, G. (2006) Determination of the petrologic type of CV3 chondrites by Raman spectroscopy of included organic matter. Geochimica et Cosmochimica Acta, 70, 1849—1863. 

For F and FF chondrites, the isotopic data do not correlate as well with the petrologic type as they do for H chondrites. This could reflect earlier disruption of the parent asteroids or the subsequent shock history or both. In general, the ages of the L chondrites tend to be younger than for the H group, but the overall timescales are similar for H, L, and LL chondrites. 

Sketches of the interiors of chondritic asteroids, illustrating (a) onion shell and (b) rubble pile structures. The numbers and shadings identify petrologic types. 

Cooling rates for H chondrites of different petrologic types can be estimated from the blocking temperatures for diffusion in various radioisotope systems. The age measured by each radioisotope indicates the time the meteorite passed through that blocking temperature. Slower cooling rates for more highly metamorphosed H5 and H6 chondrites are evidence that the parent body had an onion shell structure. 

The diamonds first recognized by Lewis et al. (1987) have now been traced to every class of chrondritic meteorite (Alexander et al. 1990 Huss 1990 Russell et al. 1991) and are known to occur as a component within the matrix. The amounts available for extraction reach a maximum of 900 ppm but decline to nothing in higher petrologic grades. The cut-off point in unequilibrated ordinary chondrites is about type 3.6 but for enstatite chondrites the type 4s still contain diamond. Progressive... 

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-... 

Van Schmus and Wood used the relative abundance of glass and feldspar as an important criterion in establishing the petrologic types (5). In type 3 chondrites, there appears to be no feldspar, but instead there exists a glass of, approximately, feldspathic composition. In type... 

Figure 1. Plot of thermoluminescence sensitivity against petrologic type (5,6), which reflects the degree of metamorphic alteration 3, least metamorphosed 6, most metamorphosed. The feldspar scale is calculated on the assumption that the TL sensitivity is directly proportional to feldspar abundance and type 5,6 chondrites typically contain about 8% of this mineral. The cross-hatched region refers to TL sensitivity levels at which the feldspar is thought to be in the low-temperature ordered form (see Figure 13). The symbols refer to the three data sources (6,8,14). (Reprinted by permission from Ref. 31.

Figure 3. Peak temperature against peak width for type 3.2—3.9 ordinary chondrites. The petrologic types are indicated by the symbols. (Reprinted with permission from ref. 9. Copyright 1985 Pergamon.)...

Figure 6. Photomosaics of the cathodoluminescence of four meteorites of types 3.7-5 (a) Hedjaz (type 3.7, top right), (b) Dhajala (type 3.8, center right), (c) Bremervorde (type 4, center left), and (d) Barwell (type 5, bottom). Scale bar, as in Figure 5. These relatively high petrologic type chondrites show a uniform blue feldspar CL, with only an occasional phosphate grain with red CL.

The type-3 ordinary, CO, and CV chondrites are commonly subdivided into 10 subtypes (3.0-3.9), of which 3.0 is the least metamorphosed (e.g.. Sears et al., 1991). Table 3 provides a summary of the criteria used to dehne petrologic types. 

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