Geology Reference
In-Depth Information
Figure 5.10.
MIl 03443 is a dunitic diogenite, extending the diogenite suite from orthopyroxene cumulates to olivine cumulates.
Prior to the recovery of this meteorite, dunites were unknown from the HED meteorite suite. Image is 2.5 mm across.
Unbrecciated basaltic eucrite AlH A81001 (Plate 56)
has provided evidence that Vesta had a liquid metallic
core early in its history that generated a magnetic field,
but the evidence is indirect.
Fu et al.
[2012] measured
natural remanent magnetization of this meteorite. Their
data support a thermoremanent magnetization of the
rock that was acquired as a result of cooling in a magnetic
field. They also measured an Ar-Ar age of 3.69 ga, which
dates the time the thermoremanent magnetization was
acquired. This is far too late in Vestan history for the core
to have been still molten, and
Fu et al.
[2012] concluded
that the Vestan crust contains remanent magnetization
from when the core was producing a magnetic field, and
that AlH A81001 acquired its thermoremanent magneti-
zation from the crust.
The HED suite and therefore Vesta has been consid-
ered a volatile-poor and essentially anhydrous body.
However, petrographic study of a clast in ANSMET
howardite EET 92014 showed evidence for interaction
with an FeO-rich metasomatizing fluid [
Mittlefehldt and
Lindstrom
, 1997]. Additional work has provided more
evidence for interactions between eucrites and late-stage
aqueous fluids [see
Barrat et al.
, 2011, and references
therein]. Analyses of apatite grains in eucrites has shown
that while most contain F as the dominant volatile anion
species, small amounts of OH are present in some, up to
~0.8 wt.% [
Sarafian et al.
, 2013]. Further, the unusual
metamorphosed basaltic eucrite gRA 98098 contains apa-
tite with unusually high Cl contents [
Sarafian et al.
, 2013].
Thus, although Vesta is not awash in volatile elements by
any means, recent studies of basaltic eucrites have shown
that it is not an anhydrous body as previously thought.
Water in even small quantities changes the melting
behavior of silicates, and continuing research into the
H
2
O content of Vesta through studies of HEDs may
require refining models of Vestan geologic evolution.
Our understanding of the genesis of HED meteorites
and the evolution of their parent asteroid were already
very well developed before systematic collection of mete-
orites in Antarctica began. Nevertheless, studies heavily
influenced by Antarctic HEDs have made important con-
tributions to our knowledge of regolith processes, basaltic
magmatism, cumulate formation, and the differentiation
history of large asteroids. Our understanding of these
processes has been used in turn to constrain interpreta-
tions of data from the Dawn mission to Vesta.
5.6.2. Angrites
The angrite group is small, and at the start of systematic
meteorite searches in Antarctica, only one, Angra dos
Reis, was known. This rock is so unusual that a con-
sortium was organized to study it [
Keil
, 1977]. The results
of those consortium studies highlighted the importance
of Angra dos Reis for understanding asteroidal mag-
matic processes in the earliest history of the solar system.
Yet because it is a one-off meteorite, only limited progress
could be made in developing an overall understanding of
