Achondrites aubrites

Burbine et al. (2002) tested an extreme case of a possible composition for the surface of Mercury. They made spectral observations of enstatite achondrites (igneous meteorites composed almost entirely of pure MgSi03, with some accessory minerals and essentially no FeO). The spectral features of enstatite achondrites (aubrites see Chapter 1.05) are similar to those for Mercury, but lack the spectral reddening observed in spectra of Mercury and have an additional feature at 0.5 p.m caused by troilite (FeS). This reddening (visible to UV ratio) is the result of space weathering, in which FeO is reduced to very small grains of metallic iron. Thus, the reddening indicates that some FeO must be present on Mercury to produce the nanophase iron. Alternatively,... 

Magmatic achondrites - crystallized from silicate magma Aubrites... 

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. 

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. 

Differentiated (planetary) Achondrites Angrites Aubrites Brachinites HED meteorites Eucrites Howardites Diogenites Ureilites Stony-irons Pallasites... 

Okada A., Keil K., Taylor G. J., and Newsom H. (1988) Igneous history of the aubrite parent asteroid evidence from the Norton County enstatite achondrite. Meteoritics 23, 59-74. 

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. 

Figure 1 Na/Al versus FeO/MnO for achondrites. The FeO/MnO for acapulcoite-lodranite clan (ale), wino-naite-IAB-iron silicate inclusion clan (wic) and aubrites are for orthopyroxene. All other data are bulk rock values. Meteorite data are from sources listed in the text, while planet data are from McDonough and Sun (1995), Taylor (1982), and Wanke and Dreibus (1994).

Figure 8 REE diagram for representative ultramafic differentiated achondrites. Kenna wr and Kenna res are whole-rock and acid-leached residues, illustrating the easily removed LREE-enriched component found in many ureilites. KTG is a sulfide-rich dark clast from Khor Temiki, illustrating that REEs are concentrated in sulfides in aubrites. Kenna, Havero, and EET 83309 are ureilites, the others are aubrites. Data are taken from sources listed...

Figure 9. Exposure age distributions of HED meteorites (howardites, eucrites, diogenites) and aubrites or enstatite achondrites. The HED data are from the compilation by Welten et al. (1997), with only the (shielding corrected) Kr-Kr ages being displayed for the eucrites. Included are 4 new diogenite ages from Welten et al. (2001b). In addition, the -3 Myr age of the howardite Kapoeta has been added (Caffee and Nishiizumi 2001). For data sources of aubrites see text. Note the different abscissa scales between the lowermost panel and the others.

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