Geology Reference
In-Depth Information
Table 10.4.
Examples of large pairing groups in the U.S. Antarctic Meteorite Collection.
Specimen
Mass (g)
Type
Number in Pairing Group
Total Mass (g)
ALH 83009
1.70
aubrite
20
3,587.09
CMS 04061
8,465.0
pallasite
19
170,545.9
LAP 02205
1,226.3
lunar—basalt
6
1,930.43
MIL 03346
715.2
nakhlite
4
1,871.0
ALH 76005
1,425.0
polymict eucrite
14
4,292.3
EET 87503
1,734.5
howardite
9
4,071.8
DRP 78001
15,200.0
IIAB iron
11
263,644.5
LAP 91900
786.87
diogenite
9
2,867.4
EET 87507
36.192
CK chondrite
48
2,315.4
ALH 83100
3,019.0
CM chondrite
21
5,380.5
EET 96005
1.34
CM chondrite
15
1,124.9
EET 87711
5.69
CR chondrite
50
1,341.07
MIL 07099
13.08
CO chondrite
169
8,946.04
DOM 08004
294.5
CO chondrite
7
1,251.7
MIL 07590
3.85
CV chondrite
34
539.7
QUE 90201
1,282.5
L/LL chondrite
>123
>24,874.6
ALH 77 011
291.5
L3.3 to L3.5
78
11,824.5
PCA 82518
21.878
EH3 chondrite
21
408.12
origin [
McCoy
, 2014 (this volume)]. Some of these
unusual meteorites, like the first-ever recognized lunar
meteorite ALH A81005 (Plate 64) and the ancient
hydrous phase-bearing martian meteorite ALH 84001
(Plate 69) (argued to contain evidence of past microbial
life) are even paradigm shifting. The chapters in this
volume on chondrites, achondrites, and lunar and mar-
tian meteorites are all testament to the strength that this
pivotal program has added to our understanding of the
origin and evolution of our solar system.
While inevitably a few terrestrial rocks are collected
along with the meteorites (the policy of bringing back
questionable rocks ensures that even potential meteorites
are at least examined in the lab) the number of “false pos-
itives” is remarkably low. There have been only 254 terres-
trial rocks collected in the 37 seasons since 1976 (the 1989
season was cancelled; Table 10.2). This number is just
1.23% of the total meteorites collected during those sea-
sons, a remarkable testament to the ease of recognition
of a meteorite relative to a terrestrial rock, and the power
of the human senses to be able to distinguish them.
Notably, almost 40% of these terrestrial rocks were gath-
ered at the Lewis Cliffs Ice Tongue, a site that is full of
small, dark-colored glacial moraine material scraped
from the walls of the surrounding Lewis Cliffs.
In a statistical evaluation of the U.S. Antarctic mete-
orite collection and, in particular, in comparison to other
collections (e.g., falls, non-Antarctic), pairing relation-
ships need to be considered. Two or more meteorites are
considered paired if they were originally part of the same
meteorite that broke apart upon entering the Earth's
atmosphere or hitting the ground; and statistical analysis of
the modern falls show that 1 in 10 falls consists of 10
individuals or more [
Harvey and Cassidy
, 1989]. As an
example, thousands of individual stones from the Allende,
Mexico, shower [
Clarke et al
., 1970] or the Chelyabinsk,
Russia, shower [
Popova et al
., 2013] are paired as single
meteorites. The U.S. Antarctic meteorite collection
contains a large number of pairing groups of a broad
range of meteorite types, including achondrites, chon-
drites, and irons (Table 10.4). The classification process
has identified a number of unequilibrated chondrite
pairing groups (e.g., ALH A77011 and PCA 82518;
Table 10.4), but many equilibrated chondrite groups have
no doubt gone unrecognized due to limited resources
for classification and low levels of interest among the
scientific community. One exception is the relatively well
characterized pairing group of QUE 90201 (Table 10.4;
Plate 9), with thousands of pairs of L/LL5 material
[
Welten et al
., 2011]. An interesting observation is that
many of these paired specimens are nearly whole fusion
crusted stones, strongly suggesting that showers produced
during atmospheric breakups are well preserved in
Antarctica, while fragmentation during ice movement
appears to be a far less common phenomenon. All of the
numbers presented in this chapter are without correction
for pairing groups.
10.2. OUTPUT BY SEASON AND SITE
ANSMET field seasons have yielded anywhere from
11 meteorites to a whopping 1353 meteorites, collected
during the 2003-2004 field season. Table 10.2 shows the
number of meteorites that have been collected during
