Geology Reference
In-Depth Information
Polymict ureilite
60.76 g
Found December 27, 1983
4.0 × 4.0 × 2.5 cm
Weathering = C
A rarer kind of ureilite is a polymict ureilite, which, like howardites or
polymict eucrites, contain lithic clasts of at least two kinds of meteorites.
A stunning case of a polymict ureilite is EET 83309, in which are found
clasts of R chondrites, identified as such based on oxygen isotopic mea-
surements and mineral compositions of silicates. These observations show
that an asteroid's regolith can be a mixture of quite diverse materials.
EET83309,0
Gr16
U-pig
EET 83309,51
1mm
Large
ferroan
clast (RC)
Gr17
U-pig
Gr19
U-ol
Gr20
Gr14
U-ol
Gr1
non U
enstatite
Gr4
U-ol
Gr36
non U-ol
Gr35
U-pig
Gr24
U-ol
Gr26
U-ol
Plate 46
MiNERALogy
SigNiFiCANCE
Most polymict ureilites contain olivine grains that span
the range of olivine seen in monomict ureilites, but the
range in EET 83309 is wider, from Fo
62
to Fo
98
. EET
83309 also contains plagioclase feldspar, and clasts com-
posed of pyroxene, feldspar, and ulvöspinel. There are
also commonly clasts of R chondrite material in EET
83309.
Careful petrographic and geochemical studies of EET
83309 have revealed that it contains multiple lithologies
in addition to the purely ureilitic clasts. The other “exotic”
clasts indicate that the ureilite parent body had a hetero-
geneous regolith containing pieces of R chondrites (left,
[331]), possibly enstatite chondrites (or achondrites), and
perhaps even some material from unknown meteorite
groups. This rarer kind of ureilite can provide information
about impact gardening and regolith development on
asteroids.
References [330-342]
