Pecora Escarpment 82506 meteorite, Pecora Escarpment, Eastern Antarctica, Antarcticai

Regional Level Types
Pecora Escarpment 82506 meteorite	Meteorite Fall Location
Pecora Escarpment	- not defined -
Eastern Antarctica	Region
Antarctica	Continent

Ureilite (olivine-pigeonite subgroup)
(Find, 1982; 5.32 kg)

When discovered on the Antarctic ice, the moderately large Pecora Escarpment 82506 (PCA 82506) meteorite [dimensions, 22 x 16 x 9 cm] retained a patchy fusion crust covering ~75% of the surface. Beneath the surface small grains of olivine (Fa21) and pigeonite (Wo6Fs18En76) of uniform composition dominated the meteorite's volume. Many individual grains were rimmed by carbonaceous material — primary graphite accompanied by Fe-rich metal, troilite, occasional cohenite, and other phases. The olivine is Ca-rich as is characteristic for ureilites with some grains exhibiting shock-produced undulose extinction (albeit, for a ureilite, relatively mild). The very small amounts of original metal have largely been converted into limonite during the meteorite's ~12,000 year residence on (or under?) the Antarctic ice. Sm-Nd model ages suggest that most of the meteorite's components were accumulated ~4.55 billion years ago. During this epoch of aggregation, the pigeonite was produced during a short period of very rapid cooling. In the main PCA 82506's general characteristics conform to those of most ureilites, but the presence of magmatic carbon has generated perhaps the most sustained topic of inquiry.

Ureilites are olivine-pyroxene cumulates that contain interstitial carbon. Ureilites are the second largest group of achondrites or differentiated stony meteorites (only the HED meteorites, largely derived from Vesta are more numerous). PCA 82506 is among the more massive of theses unusual meteorites, ranking 8th out of the 400+ ureilite specimens listed at the 'Meteoritical Bulletin Database' as of mid-July 2015. Indeed, PCA 82506 was the second largest ureilite recovered before 2000 (6 larger ones, 5 from African deserts have since been recovered). It is somewhat surprising that such a large meteorite has not been given more attention than it has. It may be that untangling the details of an early episode(s) of reduction found on the rims of some grains has been obscured by the slow, but relentless oxidation produced by twelve millennia of exposure to the earth's air and ice.

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References