AUBRITE
Aubrites are rare, asteroidal pyroxenites. About 66 aubrites have been recorded so
far. Pyroxenite is a >90% pyroxene-bearing, ultramafic intrusive
igneous rock. The pyroxene in aubrites is mostly in the form of enstatite
(MgSiO3), but there's a little diopside as well. The most
remarkable aspects of aubrite meteorites are their pale to whitish color and
their remarkable fragility.
The aubrite shown below is a 2.4 gram fragment of the
Bishopville Meteorite, a 13 pound rock which impacted on 25 March 1843 in
Bishopville, Lee County, South Carolina, USA. The Bishopville Aubrite is
a fragmental breccia apparently derived from near the surface of its parent
body.
Aubrites are dominated by enstatite, but their minor
and trace mineralogy typically includes forsterite olivine, plagioclase
feldspar, diopside, Fe-Ni metal, Ti-troilite (or Ti-pyrrhotite), schreibersite,
alabandite, oldhamite, schllhornite, etc. Bishopville has a very high
plagioclase feldspar component for an aubrite - 16% of its volume is plagioclase.
Based on CRE (cosmic ray exposure) age determinations
of various aubrites, the parent body or bodies (maybe up to 4 separate bodies?)
have been subjected to several major impacts or breakup events. It's
suspected that the ~near-Earth orbiting, E-class asteroid named Eger (or
some other E-class asteroid, such as Nysa) is the likely source of most aubrite
meteorites. Bishopville has a CRE date of 52 to 57 million years (= how
long ago the rock was ejected from its parent body).
Aubrite crystallization ages go back to the beginning
of the solar system, as do almost all asteroidal rocks. Available age
information shows that the Bishopville Aubrite is 4.547 to 4.553 billion years
old.
Aubrite
(2.2 cm across) - fragment of the Bishopville Meteorite.
(More info. on the
Bishopville Aubrite)
Synthesized from various sources, principally Floss et
al. (1990), Weber & Bischoff (1999), Hutchison (2004), Welten et al.
(2004), Shukolyukov & Lugmair (2004), Skala & Drabek (2005), and Miura
et al. (2007).