Geology Reference
In-Depth Information
Unbrecciated eucrite
52.93 g
Found January 7, 1982
4.5 × 4.0 × 4.5 cm
Weathering = Ae
Eucrites are basaltic igneous rocks that comprise a range of textural types
from ophitic to subophitic to variolitic, and contain plagioclase feldspar
and pyroxene. The range of textures indicates formation conditions from
shallow intrusive to extrusive. There are even a few examples of vesicular
basalt. Two compositional types may define an equilibrium melting trend
and a fractional crystallization trend. Eucrites are thought to be from the
asteroid 4 Vesta, which has a similar surficial composition and mineralogy.
1
0.9
0.8
Serra de Magé
0.7
0.6
Chervony Kut
0.5
0.4
PCA 91078
0.3
ALH A81001
0.2
0.1
0
0.3
0.45
Wavelength (
µ
m)
0.6
0.75
0.9
Plate 56
MiNERALogy
SigNiFiCANCE
Allan Hills A81001 is an unbrecciated eucrite with very
fine grained, variolitic-spherulitic texture, and also
contains rare elongate microphenocrysts of pyroxene.
Plagioclase (An
96
) is approximately 61%, and pyroxene is
39%; orthopyroxene has rather uniform composition,
averaging Wo
1.6
En
40
Fs
59
, and minor clinopyroxene has
Wo
43.7
En
31.4
Fs
24.8
.
ALH A81001 has the appearance of a quench-textured
melt, and thus its origin has been proposed to be that of
an impact melt or from the quenched portion of an early
magma ocean. The latter hypothesis is based on the high
Cr content of the pyroxenes, which may indicate a more
primitive composition, coupled with the evidence for a
0.6 micron absorption feature of iR spectra associated
with the high Cr (right, [410]). Studies of 4 Vesta by the
Dawn
spacecraft may help evaluate this idea.
References [408-412]
