Geology Reference
In-Depth Information
chemically, and mineralogically complex set of metal-sulfide
veins. These veins, particularly when compared to smaller
metal particles, record a complex history of melting, melt
migration, carbon incorporation, and isotopic homoge-
nization and oxidation-reduction reactions [
McCoy et al.
,
2006]. While recent recoveries from Antarctica continue
to provide greater sampling of this group, an obvious gap
in our sampling that might be rectified by future recov-
eries is a regolith breccia that samples the full range of
lithologies on the acapulcoite-lodranite parent body.
share a common oxygen isotopic signature with group
IVA, but whose high-Ni composition and rapid cooling
rate argue for formation in a separate parent body
[
McCoy et al.
, 2011]. Interestingly, some meteorites origi-
nally listed as ungrouped irons (e.g., the sulfide-rich
HOW 88403) are more likely segregated metallic impact
melts from a chondritic parent body, even though the
exact parent body is currently unrecognized [
Schrader
et al.
, 2010].
Wasson
[1990] argued that smaller fragments of
asteroids tended to be ejected at higher velocities than
larger fragments, and these smaller fragments experi-
enced more changes in orbital velocity through
subsequent collisional or gravitational interactions.
The average mass of Antarctic irons is only about
1/100th that of non-Antarctic irons. Thus, these small
irons may represent a broader sampling of the asteroid
belt than their larger counterparts.
Burbine et al.
[2002]
extended this argument, suggesting that our entire
meteorite collection might sample ~100 distinct mete-
orite parent bodies, with ungrouped irons representing
perhaps half of these. Thus, small ungrouped irons
from Antarctica have a significant potential to reveal
the full diversity of materials and conditions present
(at least in the cores of differentiated bodies) in the
early solar system.
5.3. ASTEROIDAl CORES: UNgROUPED IRON
METEORITES
In general, iron meteorites are severely underrepre-
sented in the Antarctic population relative to either
observed falls or non-Antarctic meteorites. Although
compiled several years after the start of the Antarctic col-
lection effort,
Graham et al.
[1985] reported 4.6% of
observed falls (42 of 905), and 40% of non-Antarctic
finds were iron meteorites. The greater resistance to
weathering and higher probability of recognition of
irons after long residence times on the ground is
responsible [
Clarke
, 1986]. In contrast, iron meteor-
ites represent ~0.5% of U.S. Antarctic meteorites
(~111/20,000). The reason for this discrepancy is
unclear, but Antarctic iron meteorites skew toward
lower masses than their non-Antarctic counterparts
and differences in transport and exposure of dense,
heat-conducting iron meteorites in Antarctica may
produce much of the difference.
While Antarctic irons compose a smaller fraction of
the total Antarctic meteorite population, ungrouped
irons compose a substantially larger fraction of those
irons. Iron meteorites are classified based on their struc-
ture and chemical composition. Among non-Antarctic
iron meteorites, 85% of irons can be classified into 13
well-defined groups. The remaining 15% fall outside these
groups and are termed ungrouped.
Clarke
[1986] first rec-
ognized that a higher proportion of iron meteorites from
Antarctica are ungrouped, an idea confirmed by
Wasson
et al.
[1989] and expanded upon by
Wasson
[1990]. In this
latter paper, 12 of 31 irons (39%) were recognized as
ungrouped.
Among the ungrouped irons are a number of inter-
esting samples. lewis Cliffs (lEW) 85369 contains Si dis-
solved in the metal, indicating unusually reducing
conditions. It is compositionally similar to the ungrouped
iron Horse Creek. The iron sulfide troilite makes up
nearly two-thirds the volume of lEW 86211 (Figure 5.3).
Troilite is more commonly underrepresented in iron mete-
orites compared to what is expected based on estimated
parent magma compositions. Mount Howe (HOW)
88403 (Plate 80) is a Ni-rich iron whose phosphates
Figure 5.3.
lewis Cliff (lEW) 86211 is an ungrouped iron mete-
orite with silicate inclusions. Yellowish troilite includes irregular
to dendritic blebs of white metal that sometimes rim dark sili-
cate inclusions. Troilite-rich iron meteorites like lEW 86211 are
rare and often originate from early cotectic Fe,Ni-FeS partial
melts or late cotectic residual melts during asteroidal melting or
core solidification. The section is 26 mm across horizontally at
the widest point.
