Meteorites, types

McSween HY, Taylor LA, Stolper EM (1979) Allan HrUs 77005 a new meteorite type found in Antarctica. Science 204 1201-1203... 

Ejection ages (sum of cosmic-ray exposure age + terrestrial age) for Martian meteorites. The ages cluster by meteorite type, suggesting that each cluster represents a distinct impact (ejection) event. The only outliers are the EETA 79001 and Dhofar 019 shergottites and ALHA84001. Modified from McSween (2008). 

As already noted, spectral similarities between the various asteroid classes and specific types of meteorites provide a way to identify possible meteorite parent bodies. The Tholen and Barucci (1989) asteroid taxonomy has been interpreted as representing the types of meteorites shown in Table 11.1. Using the Bus et al. (2002) taxonomy, the C-complex asteroids are probably hydrated carbonaceous chondrites (e.g. Cl or CM). These carbonaceous chondrite asteroids probably accreted with ices and will be considered in Chapter 12. Some S-complex asteroids are ordinary chondrite parent bodies, but this superclass is very diverse and includes many other meteorite types as well. The X-complex includes objects with spectra that resemble enstatite chondrites and aubrites, and some irons and stony irons, although other X-complex asteroids are unlike known meteorite types. A few asteroid spectra are unique and provide more definitive connections, such as between 4 Vesta and... 

Anhydrous planetesimals, and especially the meteorites derived from them, provide crucial cosmochemical data. Spectroscopic studies of asteroids do not provide chemical analyses, but the spectral similarities of several asteroid classes to known meteorite types provide indirect evidence of their compositions. The few chemical analyses of asteroids by spacecraft are consistent with ordinary chondrite or primitive achondrite compositions. Laboratory analyses of anhydrous meteorites - chondrites, achondrites, irons, and stony irons - allow us to study important chemical fractionations in early solar system bodies. Fractionations among chondrites occur mostly in elements with higher volatility, reflecting the accretion of various components whose compositions were determined by high- and low-temperature processes such as condensation and evaporation. Fractionations among achondrites and irons are more complex and involve partitioning of elements between melts and crystals during differentiation. 

Figure 1. Nitrogen abundance in diamonds from various meteorite types.

Figure 9. Glow curves for the Sharps meteorite (type 3.4) before and after annealing at 755-855°C and 0.77-1 kbar for 168-174 h in the presence of water and sodium disilicate. (Reprinted by permission from Ref. 31. Copyright 1986 American Geophysical Union.)...

Figure 16. Plot of TL sensitivity against peak temperature and peak width for chondrules separated from the Dhajala meteorite (type 3.8). Because the chondrule masses vary over several orders of magnitude, their TL data have been divided by mass. (Reprinted with permission from Ref. 26. Copyright 1984 Pergamon Press.)...

Spectral reflectance curves for the range of meteorite types are illustrated in fig. 10.12. These spectra demonstrate the diagnostic features of the various... 

McFadden, L. A., Gaffey, M. J., Takeda, H., Jackowski, T. L. Reed, K. L. (1982) Reflectance spectroscopy of diogenite meteorite types from Antarctica and their relationship to asteroids. Mem. Nat. Inst. Planet. Res., 25,188-206. 

In Fig. 4.6 the oxygen isotopic composition of these two meteoritic types are compared. The bulk oxygen isotopic composition of various groups of achondrites... 

In this model, large gravitationally bound clumps form only occasionally, which would explain why planetesimal formation in the Asteroid Belt continued for several million years. This mechanism also reproduces the narrow size distribution of chondrules seen in chondritic meteorites (Cuzzi et al. 2001). While the mean chon-drule size differs from one meteorite type to another, the size distributions closely... 

An interesting consequence of this model is that bodies from the terrestrial-planet region may be scattered outwards while the planets are forming and implanted in the Asteroid Belt. With dynamical and collisional modeling of this process, Bottke et al. (2006) find that this may be the origin of most iron meteorites. This would explain the diversity of iron-meteorite types, why there is little observational or meteoritical evidence of mantle material from differentiated bodies in the Asteroid Belt, and the fact that most iron-meteorite parent bodies appear to have formed >1 Myr before the parent bodies of the chondritic meteorites (Kleine et al. 2005). 

Achondrites as a whole show one curious property when compared to the total range of meteorite types—chondrites, achondrites, stony irons, and irons. Many of the achondrite types... 

There is a wide range of meteorite types, which are readily divided into three main groups the irons, the stony irons and the stones (see also Volume 1 of the Treatise). With this simple classification, we obtain our first insights into planetary dilferentiation. All stony irons and irons are differentiated meteorites. Most stony meteorites are chondrites, undifferentiated meteorites, although lesser amounts are achondrites, differentiated stony meteorites. The achondrites make up —4% of all meteorites, and <5% of the stony meteorites. A planetary bulk composition is analogous to that of a chondrite, and the differentiated portions of a planet—the core, mantle, and crust— have compositional analogues in the irons, stony irons (for core-mantle boundary regions), and achondrites (for mantle and crust). 

The primitive carbonaceous meteorites, which include the hydrated CI and CM meteorites and mostly-anhydrous meteorites such as the Allende CV meteorite [69], reach Earth from the asteroid belt between Mars and Jupiter. Asteroid reflectance properties display a remarkably systematic distribution as a function of heliocentric distance for asteroids in this belt, and hence meteorite types, with the most primitive ones located farthest from the sun. Asteroid hydration occurred when internal heating melted (water) ice that had co-accreted with dust, chondrules and refractory inclusions in the solar nebula. These asteroids form the IR spectroscopic C-class with clays, carbon and organics at the surface similar to CI and CM meteorite parent bodies [70]. They and the Allende CV parent body, which apparently did not accrete (much) ice, are from the same zone of the asteroid belt. Even more primitive asteroids closer to Jupiter still contain co-accreted ices, organic materials and silicate dust. They define the IR spectroscopic primitive (P)-and dark (D)-class [70] bodies that include comet nuclei and many near-Earth asteroids [10]. 

Chondrites. Chondrites are stony meteorites and are the most abundant meteorite type (87% of all meteorites). Their radiometric ages are around 4.56 Ga and these ages are thought to define the time when the solar system formed. Chemically their element abundance patterns, apart from the very light and/or volatile elements, are the same as that of the sun and other stars, and for this reason they are thought to represent undifferentiated cosmic matter. Chondrites therefore are thought to represent the most primitive material in the solar system. They are the "stuff" from which all other rocky materials were built. 

Chondrites are subdivided into carbonaceous (C), ordinary (O), and enstatite (E) varieties (Fig. 2.8). Carbonaceous chondrites are volatile rich and contain abundant carbon in their matrix. Because they have a high volatile content they are thought to be the most primitive of all chondrites. Within this group there are a number of varieties named after type specimens designated Cl, CM, CV, etc. An earlier classification used Cl to C3. Cl chondrites are the most primitive meteorites within the carbonaceous chondrite groups and the most primitive of all meteorite types. They are the least chemically fractionated and have the highest volatile content. Ordinary chondrites, as their name implies are the most abundant... 

As an aside, it is also worth noting that a close look at the isotopic evolution of the Earth s mantle challenges the chondritic model for the Earth. This is particularly clear for the Lu-Hf and Re-Os isotope systems, where there is uncertainty about which meteorite type is the most appropriate starting point for the isotopic evolution of the Earth. Recent studies of chondritic Sm-Nd ratios by Boyet and Carlson (2005) further support the nonchondritic view of the bulk Earth. 

Plate 9.1. Common meteorite types (approximate longest dimensions) ... 

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