Achondrites oxygen isotope composition

The oxygen isotopes of terrestrial materials mostly fall along with what is termed a mass-fractionation line (MFL). The MFL is defined by a slope of 0.5 passing through VSMOW as shown in Fig. 4.5. The oxygen isotopic composition is unique for different types of meteorites, which are broadly classified as chondrites and achondrites. 

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... 

Figure 4.6 Bulk oxygen isotopic compositions of (a) achondrites and meteorites from Mars, the Moon, and Vesta (b) chondrites (after Yurimoto el al. 2006).

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

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).

The aubrites (enstatite achondrites) are clearly closely related to the enstatite chondrites, discussed earher. They also share the property of being highly reduced, and have identical oxygen isotopic compositions (Clayton et al., 1984). Experimental studies by McCoy et al. (1999) show that partial melting of an E-chondrite can yield aubiitic material by removal of a basaltic melt and a metal-sulfide melt. This is the best known instance of a genetic connection between chondrites and achondrites. 

The brachinites are a small group of olivine-rich residues, with oxygen isotopic compositions overlapping those of the HED achondrites, although no genetic association has been proposed (Clayton and Mayeda, 1996). 

The ureilites are the most remarkable of the primitive achondrites. They are coarse-grained, carbon-bearing ultramafic rocks, and are residues of extensive partial melting that has removed a basaltic component. Their oxygen isotopic compositions are highly variable, and extend along an extrapolation of the CCAM line. The isotopic variability is probably inherited from heterogeneous carbonaceous chondrite precursors (Clayton and Mayeda, 1988). 

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 ... 

Clayton RN, Mayeda TK (1986) Oxygen isotopes in Shergotty. Geochim Cosmochim Acta 50 979-982 Clayton RN, Mayeda TK (1988) Isotopic composition of carbonate in BETA 79001 and its relation to parent body volatiles. Geochim Cosmochim Acta 52 925-927 Clayton RN, Mayeda TK (1996) Oxygen isotope studies of achondrites. Geochim Cosmochim Acta 60 1999-2017... 

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. 

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