Geology Reference
In-Depth Information
Table 10.5.
Antarctic meteorites compared to falls (global) and the hot deserts of Africa.
Meteorite Class
U.S. Antarctic Program
Falls—Global
Oman/North African Deserts
Ordinary chondrites (OC)
17,293
852
6,790
Carbonaceous chondrites (CC)
798
44
381
CM
303
15
23
Enstatite chondrites
128
17
106
R chondrites
27
1
81
Primitive achondrites (PA)
96
9
199
How/Euc/Dio (HED)
314
61
432
Other achondrites
54
11
75
Lunar
24
0
112
Martian
15
5
73
Irons
111
49
67
Total
18,931
1,049
8,316
Total non-OC
1,638
197
1,526
% OC/All Else
91.3
81.2
81.6
% HED/All Else
1.7
5.8
5.2
% CC/non-OC
48.7
22.3
25.0
% CM/CC
38.0
34.1
6.0
% R chon/non-OC
1.6
0.5
5.3
% PA/non-OC
5.9
4.6
13.0
% Martian/non-OC
0.9
2.5
4.8
% Lunar/non-OC
1.5
0.0
7.3
% Irons/non-OC
6.8
24.9
4.4
Note:
Primitive achondrites = acapulcoites, lodranites, brachinites, ureilites and winonaites.
Sources: Grady
, 2000;
Welzenbach and McCoy
, 2006;
McBride and Righter
, 2010.
population coupled with the ease of relatively complete
recovery of small stones from the ice.
Perhaps the most striking mismatch between modern
falls and the U.S. Antarctic population is in the abun-
dance of iron meteorites. Iron meteorites, with only 111
samples, only represent 0.6% of the entire U.S. Antarctic
population (Table 10.5; again, without pairing). This
number is surprisingly low when compared with the
number of iron meteorites that are collected as falls or
finds around the world. Approximately 5% of meteorite
falls worldwide are iron meteorites, and 40% of the non-
Antarctic finds worldwide are irons [
Grady
, 2000].
Interestingly, the number of irons collected in the hot
desert meteorite collections of the world is also low: only
0.8% of the meteorites collected from the hot deserts are
irons. These percentages would rise somewhat if the dom-
inant ordinary chondrites were subjected to rigorous
pairing, but a factor of 2-3 decrease in the total number
of Antarctic meteorites would not increase the percentage
of irons at levels comparable to either modern falls or
non-Antarctic finds. Also of note are the average masses
of irons from both hot and cold deserts. No mass over
25 kg has been returned from the Antarctic ice (though 15
Derrick Peak iron meteorites, the largest of which is
138 kg, were collected from a nunatak, which is solid
ground that rises above the surface of the ice; Plate 78).
So, are Antarctic irons anomalously low in abundance?
Within the ice flows of Antarctica, the relatively high
density of the iron meteorites coupled with their high
heat capacity might prevent them from reaching the sur-
face of the moving ice flows and being recovered. But
Nagata
[1978] argues that a specimen would have to be
~0.8 m for this mechanism to apply, which seems
inconsistent with the lack of sizes smaller than this as
well. Instead, perhaps iron meteorite recoveries are
enhanced among the finds and falls in the populated
parts of the world (where metallic specimens are distinct
from the background terrestrial rock) rather than
depleted in Antarctica (where meteorites of any kind
are distinctive).
Despite the discrepancies in the numbers and masses of
iron meteorites, it is interesting to note that the Antarctic
seems to be producing more interesting irons than hot
deserts and falls. 40% of the U.S. Antarctic iron meteor-
ites are ungrouped. Ungrouped irons make up 15% of
all nondesert irons (and less than 5% of North African
irons collected.) The small size and high percentage of
ungrouped irons likely go hand in hand, as small irons
might be more readily perturbed from their source aster-
oids in zones of the asteroid belt far from meteorite-
delivering resonances [see
Mittlefehldt and McCoy
, 2014
(this volume)]. Aside from the high abundance of ordinary
chondrites, Antarctica seems to sample an underabundance
of HED achondrites relative to falls and hot deserts.
