Primitive Achondrites


Primitive Achondrites

Although the origins of differentiated achondrites are fairly straightforward, i.e., basalts and cumulate rocks, the origins of primitive achondrites are less obvious. Recently, several research groups have suggested that many of the primitive achondrites are residues from the partial melting of chondrites, but at temperatures too low to promote much melting. Temperatures were sufficient to metamorphose the chondritic rocks to the degree that little evidence of the parent exists. The fate of small amount of magma that was produced is unknown. This section provides descriptions of primitive achondrites that formed as melt residues (acapulcoites, lodranites, brachinites, and bencubbinites). A summary of the diagnostic mineralogic and chemical characteristics of primitive achondrites is given in the table below.

Acapulcoite & Lodranite1AB iron silicate & WinonaiteUreiliteBrachinite
TextureACA: fine; LOD: coarsefine to mediumcoarseequigranular (triple junctions)
Olivine/Pyroxene≤1<1>1>>1
OlivineFa3-14Fa1-8Fa2-26Fa30-35
Olivine FeO/MnO24-301817-2250-60
Low-Ca pyroxeneFs1-9Fs1-9Fs13-25trace
Low-Ca pyroxene FeO/MnO13-1715-40
Ca-pyroxeneFs46-50Wo43-46Fs2-4Wo44-45Fs13-32Wo2-16 Fs10-13Wo38-47 (up to 5% TiO2 and 12% Al2O3)
PlagioclaseAn12-31An11-22rare to absent An22-39
Silicaabsentabsentabsentabsent
Kamaciteminorminorminorabsent
Taeniteminorminor-trace
Troiliteminorminortraceminor
Other mineralsphosphates, spinel, graphitedaubreelite, schreibersite, graphiteaugite, graphiteoxides

Notes. minor, <5 vol. %; trace, <0.5 vol. %

Primitive achondrites are those meteorites whose bulk compositions are approximately chondritic, but have been texturally modified by partial melting or metamorphic recrystallization. Because primitive achondrites are very different from other meteorites, but have similarities within themselves, they will be discussed here as a diverse group. A fundamental distinction between the acapulcoite and lodranite classes is that lodranites are relatively coarse-grained (0.5 to 1.0 mm) and acapulcoites are finer-grained (0.2 to 0.4). However, they are not easily distinguishable on casual observation; nor are they distinguishable using oxygen isotopes (figure below).

Olivine and oxygen isotopic compositions for primitive achondrites and related chondrites (circles). Numbers refer to NWA specimens.

Both typically contain olivine, orthopyroxene, clinopyroxene., chromite, troilite, metal and phosphates; plagioclase is present in only a few examples of both types. Acapulcoites commonly have orthopyroxene as the dominant silicate and olivine is dominant in lodranites, although there are exceptions and mineral abundances can vary considerably between the two classes. Lodranites can also have large amounts of metal and were originally confused with stony-irons. The type example for acapulcoites, Acapulco, Mexico, fell in 1976. Lodran, the type example for lodranites, fell in Pakistan in 1868.

Lodran. Specimen of is 2.7 mm across the base. Photo courtesy of Bethany Sciences.

Antarctic lodranite MAC 88177. Recrystallized assemblage of olivine, pyroxenes, metal and troilite in XPL. Base width is 5 mm.

Acapulcoites and lodranites appear to have originated from the same parent body. Although they have similar mineralogy, mineral abundances are different, which may be attributed to different degrees of partial melting. Because of the higher temperatures reached during partial melting of precursor rocks, some of the melt migrated away from the source areas, which left residues that formed lodranites. Acapulcoites were heated to lower temperatures causing less melt to form with little or no migration.

Few brachinite specimens have been found and constitute one of the rarest of achondrite classes. The type example, Brachina, was found in Australia in 1974. Brachinites are the most olivine-rich of all asteroidal meteorites with olivine contents up to 93 vol. %. Only Chassigny, a Martian meteorite, contains more olivine. In addition to olivine, they also contain augite pyroxene, chromite, sulfides, minor phosphates and metal. Of the 6 specimens known to date, 3 contain plagioclase.

All of this talk about melting, differentiation, residues, etc. is probably confusing. Consider that achondrite parent bodies were initially composed of accreted chondrules. The HED parent body, Vesta, had accreted to such a large size (~500 km diameter) that extensive melting and differentiation took place, which erased evidence of the chondritic parent. Primitive achondrite parent bodies were much smaller and thermal activity was less pronounced therefore, most of the distinction between the chondritic parent and the achondritic offsprings, is texture, i. e., thermal destruction of chondrule textures, replaced by igneous and metamorphic textures with retention of most chondritic composition.