Geology Reference
In-Depth Information
icefields and moraines, some very highly localized and
others including shower falls of more than 1000 likely
specimens [
Harvey
, 2003]. Similarly, the Allan Hills (ALH)
meteorites come from several distinct icefields. While
“only” yielding 1628 meteorites to date, a wide variety of
interesting meteorites have been recovered, including the
first meteorites collected by ANSMET, the first lunar
meteorite found on Earth (ALH A81005; Plate 64), the
type CH chondrite (ALH 85085; Plate 24), and the only
known martian orthopyroxenite (ALH 84001; Plate 69).
The Allan Hills continues to be an important source of
meteorites, having been visited by organized meteorite col-
lection efforts, or other Antarctic science programs, during
20 of the 35+ seasons of ANSMET history (most recently
in 2010), many of which yielded samples.
Weather can also limit how many meteorites are col-
lected in a season. ANSMET teams do not work when
blowing or newly fallen snow limits visibility (Figure 10.3),
and the resulting number of “tent days” directly reduces
the number of meteorites recovered during any season.
For example, one of the lowest totals recorded by an
ANSMET field team was the 2012 reconnaissance team
working in the Graves Nunatak area where 63 meteorites
were recovered in the nine workable days Mother Nature
provided.
and 3400 meteorites (Queen Alexandra Range).
Essentially, field sites with 1000+ meteorites exhibit a
robust ratio of ordinary chondrites/other meteorite types,
suggesting that this ratio is likely ~90/10 for the entire
population of Antarctic meteorites. A corollary of this
convergence is that the assertion that one field site is pref-
erable to another for collection of unusual meteorites
likely reflects some combination of the statistics of small
numbers (e.g., the GRA meteorites), dominance by a
pairing group (e.g., the CMS pallasites; Table 10.4;
Plate 77), or the selection of an individual field site based
on the appearance that it would be more likely to yield a
certain type of meteorite (e.g., martian).
Looking at the ordinary chondrite abundances across
the field sites represented in Figure 10.4, it can be seen
that the proportions of each type (H, L, or LL) vary
widely across these sites. As noted earlier, the statistics
presented here have not been corrected for pairing. It is
likely that the varying proportions of H, L, and LL group
ordinary chondrites between field sites reflect the contri-
butions of large, unpaired meteorite showers.
The assertion that large, unpaired showers skew the
ordinary chondrite statistics is based on three main obser-
vations. First, as noted earlier, shower falls are much
more common than most realize; most meteorite speci-
mens are part of a shower, not solo falls [
Harvey and
Cassidy
, 1989]. A second key observation is made by
those involved in the recovery and classification of the
Antarctic meteorites. As noted earlier, most meteorites
tend to be whole, fusion-crusted stones, suggesting that
breakup during atmospheric passage dominates over
breakup in the ice in producing pairings. While proximal,
fragmental stones are certainly found having broken on
the Antarctic ice, they are rare and do not dominate pair-
ing groups. Indeed, obvious proximal fragments are typi-
cally collected and labeled as a single meteorite on the ice.
The third key observation is the lack of pairing of
equilibrated ordinary chondrites during classification.
Although pairing of equilibrated ordinary chondrites was
typically suggested in the first decade of the program, the
limited classification approach typically assignment of
chemical group from oil immersion techniques and petro-
logic type from visual examination of a chip [
Lunning
et al
., 2012], is insufficient for robust pairing. Pairing of
the enormous number of ordinary chondrites that come
through the classification pipeline each year is far beyond
the capabilities of the existing curatorial system, requiring
far more time than this simple statistic is probably worth.
A few in-depth efforts have been made to assess the
relative contributions of pairing to the Antarctic
population.
Scott
[1989] conducted a study of pairing of
the meteorites found in Victoria Land and the Thiel
Mountains (TIL) Regions in Antarctica, concluding that
most equilibrated ordinary chondrites are paired with
1-3 other meteorites.
Lindstrom and Score
[1995] examined
10.3. STATISTICS OF ANTARCTIC METEORITE
COLLECTION SITES
One of the most important questions surrounding the
collection of meteorites in Antarctica is whether or not
we are collecting a representative sampling of what is
actually there. In addition, it is perfectly reasonable to
wonder if there are mechanisms at play that are resulting
in our selectively collecting certain types of meteorites as
opposed to others.
Prior to considering the Antarctic meteorite collection
as a whole, it is worth considering the variability in the
collection between field sites. Table 10.3 and Figure 10.2
show the number of meteorites collected from each field
site the U.S. teams have visited in Antarctica. When an
individual field site is coarsely subdivided into meteorite
types (e.g., carbonaceous chondrites, achondrites, H, L,
LL) (Figure 10.4), ordinary chondrites comprise the vast
majority in every case, with carbonaceous and achon-
drites sampling the largest remaining percentages.
However, collection sites with fewer meteorites exhibit
wider variation in abundance. For example, 10% of the
352 total Graves Nunataks meteorites are achondrites, a
percentage far higher than other field areas.
Interestingly, field sites with at least 1000 total meteor-
ites converge at ~90% ordinary chondrites with the
remaining ~10% including other types of chondrites,
achondrites, stony-iron and irons. These ratios exhibit
minimal changes between 1000 meteorites (MAC, MET)
