Primitive achondrites

Distinguish between the origins of magmatic achondrites, primitive achondrites, and irons. 

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. 

Primitive achondrites - residues from partial melting... 

Here we describe three important groups of primitive achondrites, omitting other less important primitive achondrites identified in Table 6.2. 

Oxygen isotopes in achondrites (above) and primitive achondrites (below). The 8 notation and units are explained in the caption for Figure 6.4. Most achondrites define mass fractionation lines parallel to, but slightly offset from the terrestrial line. Aubrites and lunar samples plot squarely on the terrestrial line. Primitive achondrites generally do not define oxygen mass fractionation lines, but are scattered and resemble their chondrite precursors. 

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. 

Low-degree partial melts of undifferentiated chondritic material generate a range of different compositions. The first melt from a chondritic precursor appears at 950 °C and forms by melting of FeS, Fe-Ni metal, and phosphates. Melts of this composition are observed in primitive achondrites (acapulcoites and lodranites). As the temperature increases, silicates begin to melt. At 5 to 10% partial melting, the melt is basaltic in composition (McCoy et al., 1997). 

It was soon discovered that there were other solar system objects that were older and had more primitive strontium than the basaltic achondrites. Table 8.3 compares the (87Sr/86Sr)o value determined for BABI with those from some other important samples from the early solar system. If Allende, Angra dos Reis, and the basaltic achondrites formed directly from the solar nebula, then the time intervals shown in the right-hand column of Table 8.3 are valid. Note that the uncertainties on these time intervals, a few million years, are much smaller than the uncertainty in the eucrite isochron shown in Figure 8.9 ( 260 Myr). However, the validity of the time intervals determined from the initial ratios depends completely on the validity of the idea that they all formed directly from the bulk material of the solar nebula. 

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... 

Table 11.4 Representative chemical analyses of primitive achondrites...

Elemental abundances, normalized to Mg and Cl chondrites, for two groups of primitive achondrites (acapulcoites and winonaites) that experienced low degrees of partial melting. These abundances are similar to chondritic abundances (average H-chondrite composition is illustrated). Modified from Mittlefehldt (2004). 

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. 

Meteorites provide perhaps the best record of the chemical evolution of small bodies in the Solar System, and this record is supplemented by asteroidal spectroscopy. Meteorites show progressive degrees of thermal processing on their parent asteroids, from primitive carbonaceous chondrites that contain percent-level quantities of water, through ordinary chondrites that show a wide range of degree of thermal metamorphism, to the achondrites that have been melted and differentiated. 

The Apollo 11 rocks contain large amounts of ilmenite, as can be seen from Tables 2 and 3 (high titanium content). We have plotted the chemical composition of rock sample 12018 in Fig. 3a vs. that of the carbonaceous chondrites (the most primitive of all meteorites), in Fig. 3b vs. the basaltic achondrite (eucrite) Juvinas (a class of meteorites which have undergone magmatic differentiation) and in Fig. 4 vs. the average composition of the Earth s... 

Based on their bulk compositions and textures, meteorites can be divided into two major categories, chondrites and nonchondritic meteorites, the latter include the primitive achondrites and igneously differentiated meteorites (Figure 1). 

Figure 17 Bulk oxygen isotopic compositions of primitive achondrites and differentiated meteorites (sources...

Divnoe 01 (Fa20-28)> Px (FS20-28WOo.5 2.5)> PI (An45 32) Ol-rich primitive achondrite [4-6]... 

Kimura M., Tsuchiayama A., Fukuoka T., and limura Y. (1992) Antarctic primitive achondrites, Yamato-74025, -75300, and -75305 their mineralogy, thermal history, and the relevance to winonaites. Proc. NIPR Symp. Antarct. Meteorit. 5, 165-190. 

Yugami K., Takeda H., Kojima H., and Miyamoto M. (1997) Modal abundances of primitive achondrites and the end-member mineral assemblage of the differentiated trend. Symp. Antarct. Meteorit. 22, 220-222. 

The achondritic meteorites can be subdivided into the differentiated achondrites igneous rocks from parent bodies that were extensively melted, and the undifferentiated, or primitive, achondrites from parent bodies that underwent little melting. 

Figure 11 Oxygen isotopic compositions of whole-rock primitive achondrites. Data for a given class are much more scattered than data for differentiated achondrites shown in Figure 10, as a result of incomplete melting and homogenization. Several genetic associations are implied by the data (i) aubrites and enstatite chondrites (ii) acapulcoites and lodranites (iii) lAB irons and winonaites and (iv) ureilites and dark inclusions in carbonaceous chondrites. This and Figure 10 are drawn to the same scale for comparison (source Clayton and Mayeda, 1996).

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