Achondrite

Acheson process Achlorhydria Achondrites Achromatic hologram Achromobacter sp. Achromycin Achromycin [60-54-8]... 

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... 

Wadhwa M, Foley CN, Janney PE (2003) High precision Mg isotopic analyses of achondrites is the " Al- Mg chronometer concordant with other high resolution chronometers Geochim Cosmochim Acta Suppl 67 A517... 

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. 

Clayton RN, Mayeda TK (1963) The use of bromine pentafluoride in the extraction of oxygen from oxides and sihcates for isotopic analysis. Geochim Cosmochim Acta 27 43-52 Clayton RN, Mayeda TK (1996) Oxygen isotope studies of achondrites. Geochim Cosmochim... 

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... 

Magmatic achondrites - crystallized from silicate magma Aubrites... 

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

The aubrites are the most reduced achondrites (Keil et al., 1989). Their silicates are essentially free of iron, and they contain minor metallic iron. A variety of unusual sulfides of calcium, chromium, manganese, titanium, and sodium - all usually lithophile elements -occur in aubrites. These unusual sulfides also characterize the highly reduced enstatite chondrites, which may have been precursors for these rocks. 

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. 

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. 

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

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). 

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). 

Crystallization of melts provides another way to fractionate chemical elements. Most crystals are denser than the melt from which they form, so crystal settling can separate the phases. Convection currents in hot magma can also entrain crystals and carry them to the bottom of a magma chamber. Accumulations of crystals are a common occurrence on the floors of terrestrial plutons, and many achondrites are cumulates formed in an analogous way in asteroids. Cumulates and the complementary melt have different elemental compositions. 

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