Achondrite meteorites

Achondrite meteorites are stony meteorites lacking chondritic texture; that is, they lack the spherical chondrules that are the defining characteristic of chondrites. As originally defined in the Rose—Tschermak—Brezina classification system, achondrites were also distinct from chondritic meteorites in composition. In particular, the original achondrite groups were poor in iron—nickel metal, and many had broadly basic compositions, rather than the ultrabasic compositions typical of chondrites. Hence, the chemical compositions of most achondrites are significantly different from that of material formed directly from the solar nebula. However, recent achondrite falls and finds have blurred the boundary between achondritic and chondritic compositions. A subset of the achondrites, often referred to as the primitive achondrites, are rocks with broadly chondritic compositions, but with achondritic textures. Almost all achondrites have ages of about 4.6 Ga, essentially identical to the ages of chondrites. Table A1 summarizes the major groups of achondritic meteorites.

Table A1 The major achondrite meteorite groups

Most achondrites are igneous rocks and were therefore formed by planetary processes. The most common types of igneous achondrites are basalts similar to basalts from the Earth and Moon. The largest group of achondrites is the basaltic achondrite group, commonly known as the HEDs for the three types; howardites, eucrites and diogenites (see Basaltic achondrite meteorites). The HED meteorites are basalts, gabbros and orthopyroxenites similar to those occurring on Earth, plus breccias composed of these rock types. The next largest group of achondrites is the ureilites; rocks generally consisting of olivine and pigeonite (see Ureilite meteorites ). The ureilites exhibit a curious mixture of nebular and planetary features. The mineralogy, texture and lithophile element composition appear to be those of either a magmatic cumulate or partial melting residue, while their oxygen isotopic compositions show variable anomalous enrichments in ¹⁶O, which are typical of some nebular materials (Clayton and Mayeda, 1988). The aubrites are the third most abundant type of achondrite. They are highly reduced achondrites composed of coarsegrained, nearly pure enstatite. They are believed to be cumulates from a differentiated asteroid (Okada et al., 1988). Two small but diverse groups of igneous achondrites are distinguished from other achondrites by their younger ages (among other things). Lunar meteorites are meteorites from a known parent body. Their lunar origin was established by their similarity to samples returned by the Apollo and Luna missions. Most lunar meteorites are breccias of igneous rocks from the mare and/or highlands regions and have ages between 3–4 Ga (see Lunar meteorites). The other small, diverse group of achondrites is commonly referred to as the SNC group after the principal members; shergottites, nakhlites and the meteorite Chassigny (see SNC meteorites). This group includes basalts, clinopyroxenites, plagioclase-harzburgites and a dunite. The SNCs have ages < 1.3 Ga, and are widely believed to be fragments of the Martian crust. The smallest group of igneous achondrites is the angrites. These rocks are broadly equivalent to alkali-olivine basalts on Earth in that they are critically silica undersaturated. The angrites are very poor in the volatile elements, however, including the alkali elements (Mittlefehldt and Lindstrom, 1990). The angrites are composed of fassaitic pyroxene and Ca-rich olivine, with or without pure anorthite.

The primitive achondrites consist of Acapulco-like, Brachina-like, Lodran-like and Winona-like achondrites. All these achondrite types exhibit granulitic texture and have broadly chondritic composition. The Acapulco-like and Londran-like achondrites are closely related achondrites. They are distinct from either the Brachina-like or Winona-like achondrites in oxygen isotopic composition and in the amount of FeO in the silicates (e.g. Nehru et al., 1992). The Lodran-like achondrites were originally classified as stony-iron meteorites in the Rose—Tschermak—Brezina classification system because the only member know at the time, Lodran, contained about 25% metal plus troilite by volume. Recently recovered Lodran-like achondrites have metal contents that vary from about 3 to 30%, and petrologic and isotopic work on them suggest that they are closely related to the Acapulco-like achondrites (McCoy et al., 1992). Therefore, it is better to classify the Lodran-like meteorites with the achondrites, rather than the stony-irons. The Acapulco-like and Lodran-like achondrites are believe to be chondritic materials that have experienced extensive metamorphism, perhaps up to temperatures where melting initiated. This metamorphism totally recrystallized the rocks, completely wiping out their presumed original chondritic texture. The Brachina-like achondrites are dunitic—wehrlitic meteorites and may be either ultra-metamorphosed chondrites (Nehru et al., 1992) or igneous rocks (Warren and Kallemeyn, 1989). The Winona-like achondrites are compositionally similar to the chondritic silicates from IAB iron meteorites and appear to be lithic fragments of the same parent body (Bild, 1977; and see Iron meteorites).