Meteorites achondrites

Until recently, it was generally believed that the chondrites, which consist of relatively unaltered nebular material and thus are primitive objects, accreted early, whereas the differentiated meteorites (achondrites, irons, pallasites), which have had a history of melting... 

Chondrites Achondrites Stony iron meteorites Iron meteorites... 

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

Achondrite A meteorite subclass that does not contain chondrules - small globules of once-molten mineral. 

Fig. 10.1. Rb-Sr isochrone measured from separated components of the stony meteorite Guarena. The initial 87Sr/86Sr ratio is slightly higher than that inferred in basaltic achondrites (BABI) because of a period of metamorphism. After Wasserburg, Papanastassiou and Sanz (1969), with permission. Courtesy G.J. Wasserburg.

Swindle TD, Podosek FA (1988) Iodine-Xenon dating. In Meteorites and the Early Solar System. Kerridge JF and Matthews MS (eds) University of Arizona Press, Tucson, p 1114-1146 Tang M, Lewis RS, Anders E (1988) Isotopic anomalies of Ne, Xe, and C in meteorites. I. Separation of carriers by density and chemical resistance. Geochim Cosmochim Acta 52 1221-1234 Tera F, Eugster O, Burnett DS, Wasserburg GJ (1970) Comparative study of Li, Na, K, Rb, Cs, Ca, Sr and Ba abundances in achondrites and in Apollo 11 lunar samples. Geochim Cosmochim Acta Suppl 1 1637-1657... 

Evolved extraterrestrial materials are generally igneous rocks, which according to their thermal history can be discnssed analogonsly to terrestrial samples. To this category belong planetary bodies, differentiated asteroids, and achondritic meteorites. 

Primitive achondrites exhibit metamorphic textures, as appropriate for the solid residues from which melts were extracted. In effect, these meteorites represent an extension of the highly metamorphosed type 6 chondrites, which show no eutectic melting of metal and sulfide. In a few cases, some primitive achondrites have recognizable chondritic textures, but often they are so thoroughly recrystallized that chondrules are not identifiable. 

Iron meteorites offer the unique opportunity to examine metallic cores from deep within differentiated bodies. Most of these samples were exposed and dislodged when asteroids collided and fragmented. Although irons constitute only about 6% of meteorite falls, they are well represented in museum collections. Most iron meteorites show wide variations in siderophile-element abundances, which can be explained by processes like fractional crystallization in cores that mimic those in achondrites. However, some show perplexing chemical trends that may be inconsistent with their formation as asteroid cores. 

How are oxygen isotopes in chondrites and achondrites illustrated graphically, and how are the isotopic compositions useful in classifying meteorites and recognizing relationships between them ... 

Goodrich, C. A. and Delaney, J. S. (2000) Fe/Mg-Fe/Mn relations of meteorites and primary heterogeneity of primitive achondrite parent bodies. Geochimica et Cosmochimica Acta, 64, 149-160. 

Plots of uranium versus lanthanum (two refractory elements), and potassium versus lanthanum (a volatile element and a refractory element) for terrestrial and lunar basalts, HED achondrites (Vesta), and Martian meteorites. All three elements are incompatible elements and thus fractionate together, so their ratios remain constant. However, ratios of incompatible elements with different volatilities ( /La) reveal different degrees of volatile element depletion in differentiated bodies. After Wanke and Dreibus (1988). 

The U-Pb system has been a chronometer of choice for the Earth s age since the pioneering study of Clair Patterson in 1956, as discussed in Chapter 8. Virtually all U-Pb model ages of the Earth (reviewed by Allegre et al., 1995) are younger than the ages of chondritic and achondritic meteorites. The oldest Pb-Pb age, based on ancient terrestrial... 

Cosmic-ray exposure ages of differentiated meteorites, (a) HED achondrites, and (b) iron meteorites. Modified from Eugster et al. (2006). 

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. 

Mittlefehldt, D. W. (2004) Achondrites. In Treatise on Geochemistry, Vol. 1. Meteorites, Comets, and Planets, ed. Davis, A. M. Oxford Elsevier, pp. 291-324. A superb review of achondritic meteorites, containing many high-quality analyses. 

Chondrules comprise the major portion of most chondrites, the most abundant type of meteorites. If the achondrites and terrestrial planets formed from chondrite-like precursors, then much, perhaps most of the solid matter in the inner solar system once existed as chondrules. Even if chondrules were restricted to the chondrites, the process that formed them was important in that region. The origin of chondrules is an important unsolved problem in cosmochemistry. Chondrules formed in the Sun s accretion disk through some sort of transient flash-heating event(s). Some CAIs apparently also were melted in the disk. What was the process (or processes) that melted the chondrules and CAIs Whatever it was, it dominated the disk for at least a few million years. 

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