Geology Reference
In-Depth Information
less certain is whether GRA 06128/129 was derived solely
from crystallization of a low-degree partial melt, as
has been argued by, for example,
Day et al
. [2012].
Alternatively, mineral accumulation akin to the plagio-
clase flotation that formed the lunar highlands crust or
crystal segregation in a magma chamber may be res-
ponsible for the extraordinarily high plagioclase
concentration, as argued by
Shearer et al
. [2010]. If pla-
gioclase accumulation is required, GRA 06128/129 sam-
ples a process that was previously only known from larger
planetary bodies. It seems clear that existing experimental
studies of partial melting of oxidized chondritic meteor-
ites are insufficient in number or range of conditions to
fully decide between these two possible models.
8.8. SPACECRAFT MISSIONS
Among the rarest opportunities afforded to a planetary
scientist is that of visiting, for the first time, a planetary
body from which we have meteorites samples. These
encounters, even if only brief flybys, allow us to test par-
adigms established in the laboratory and, in most cases,
realize that our understanding is both shallow and incom-
plete. Such an opportunity has recently been afforded by
the visit of the Dawn spacecraft to asteroid 4 Vesta. A
variety of arguments have been presented that the HED
meteorites originate from this asteroid [see
Mittlefehldt
and McCoy
, 2014 (this volume)]. As such, we have a
chance to evaluate our models for the formation of the
most abundant group of igneous meteorites, the HEDs.
One of the major surprises of Dawn's encounter with 4
Vesta was the presence of not just the single southern
polar basin imaged by Hubble [
Thomas et al
., 1997] but
two major basins, the formation of which appear to have
dominated the geological history of the asteroid since its
crystallization and cooling [
Jaumann et al
., 2012]. This
finding presents an opportunity to revisit two of the more
interesting Antarctic meteorites from the HED group.
Grosvenor Mountains (GRO) 95555 (Figure 8.4; Plate 59)
is an unbrecciated diogenite with a polygonal-granular
texture and an orthopyroxene grain size of ~1 mm [
Papike
et al
., 2000]. MIL 07001, a harzburgitic diogenite, exhibits
a similarly equigranular texture, but with ~10 vol. % small
olivines interspersed [
Beck et al
. 2012].
Papike et al
. [2000]
argued that GRO 95555 formed at sufficient depth to
escape brecciation where high-grade thermal metamor-
phism might have occurred. In addition, a significant
percentage of eucrites within the Antarctic population
appear to contain disparate textures or lithologies without
sharp delineation of boundaries between these areas, sug-
gestive of metamorphism after brecciation. Our new
understanding of the geology of Vesta challenges us
to place these metamorphosed meteorites in context.
Did the metamorphism result from impact? Were the
7mm
Figure 8.3.
Mineral map derived from composite elemental
maps of the Graves Nunataks 06128 meteorite. The high abun-
dance of sodic feldspar (green) and clastlike appearance of
olivine and pyroxene (brown), phosphates (purple), and sulfides
(yellow) suggest the formation of a lithology significantly
enriched in feldspar. Whether such a lithology derived by partial
melting or required plagioclase fractionation akin to the lunar
magma ocean is unresolved.
scale of tens of micrometers.
Righter et al
. [2008] applied
this technique to QUE 93148 but were limited by the
availability of a sample with minimal metal. Further,
other olivine-bearing rocks from the HED parent body,
namely the harzburgitic diogenites, are now known [
Beck
and McSween
, 2010]. A search for metal within the harz-
burgitic diogenites and a subsequent comparison of com-
positions to metal in QUE 93148 may yield new insights
into the origin of that meteorite.
In other cases, some meteorites remain anomalous not
because of the availability of material but because ancil-
lary studies cannot fully distinguish between two com-
peting models. Graves Nunataks (GRA) 06128 and 06129
(Figure 8.3) are two paired, relatively large (~0.65 kg
total) meteorites that are exceptional in containing a
highly oxidized mafic assemblage with ~85 vol.% sodic
plagioclase (Plate 49). A relationship to the brachinites
seems certain, although brachinites are olivine-domi-
nated meteorites. In most respects, GRA 06128/129
appear to represent the complementary partial melts
removed during relatively low degrees of partial melting,
leaving the brachinites as residues. Complementary
basaltic partial melts are known from a number of other
meteorite groups, including silicate-bearing IAB irons,
aubrites, ureilites and acapulcoites-lodranites. What is far
