Meteorites 257
FIGURE 6 Cross section of Antarctic ice sheet and subice
topography: meteorites fall (1), are collected by the ice sheet and
buried (i.e., preserved), transported, and concentrated near a
barrier to the ice sheet (2), and are exposed by strong South
Polar winds that ablate the stagnant ice (3). [Reprinted from
“Workshop on Antarctic Glaciology and Meteorites,” C. Bull and
M. E. Lipschutz (eds.), LPI Tech. Rept. 82-03. Copyright 1982
with kind permission from the Lunar and Planetary Institute,
3600 Bay Area Boulevard, Houston, TX 77058-1113.)
Expedition) in Queen Maud Land; 13,907 by ANSMET
(Antarctic Search for Meteorites), the US-led team up-
stream of the Trans-Antarctic Mountain Range; and> 677
by a European consortium, which is now an Italian-led ef-
fort]. Hot desert-clusters in Australia, North Africa (mainly
Algeria and Libya), China, and the United States have
yielded>5000 more to date. (These discoveries are pos-
sible in these areas because dark meteorites can be readily
distinguished from the local, light-colored terrestrial rocks,
“meteorwrongs.”) The 14-million-km^2 ancient Antarctic ice
sheet is a meteorite trove because of the continent’s unique
topography and its effect on ice motion, which promotes
the meteorites’ collection, preservation, transportation and
concentration (Fig. 6). Assuming four fragments per mete-
oroid, Antarctic meteorites recovered thus far correspond
to about 7500 different impact events; no one has estimated
the number of fragments produced in a hot-desert mete-
orite fall. Desert meteorites are named for the nearest to-
pographic feature, usually abbreviated by a one- or three-
letter code, and number: the first two digits of Antarctic
meteorites denote the expedition year.
To complicate matters, expeditions have taken two paths
in characterizing their meteorite recoveries. ANSMET
chooses to characterize each fragment by type. Other ex-
peditions scan their collection to identify meteorites of rare
type, which are of intrinsic interest for more complete study
(see Table 1). The “pairing” of even these samples, let alone
the more common meteorites in these other collections, has
not yet been addressed.2. Meteorite Classification
2.1 General
Meteorites, like all solar system matter, ultimately derive
from primitive materials that condensed and accreted from
the gas- and dust-containing presolar disk. Most primitive
materials were altered by postaccretionary processes—as
in lunar, terrestrial, and martian samples—but some sur-
vived essentially intact, as specific chondrites or inclusions
in them. Some primitive materials are recognizable unam-
biguously (albeit with considerable effort), usually from
isotopic abundance peculiarities; others are conjectured
as unaltered primary materials. Postaccretionary processes
produced obvious characteristics that permit classification
of the thousands of known meteorites into a much smaller
number of types. Many classification criteria contain genetic
implications, which we now summarize.
At the coarsest level, we class meteorites as irons, stones,
or stony-irons from their predominant constituent (Figs. 7a
and 8): each can then be classified by a scheme with ge-
netic implications (Fig. 7b). Stones include the numerous,
more-or-less primitive chondrites (Table 1; Figs. 8a and
8b) and the achondrites (Fig. 8d) of igneous origin. Irons
(Fig. 8e), stony-irons (Fig. 8c), and achondrites are dif-
ferentiated meteorites, presumably formed from melted
chondritic precursors by secondary processes in parent
bodies (Fig. 2). During melting, physical (and chemical)
separation occurred, with high-density iron sinking to form
pools or a core below the lower density achondritic par-
ent magma. Ultimately, these liquids crystallized as par-
ents of the differentiated meteorites, the irons forming
parent body cores or, perhaps, dispersed “raisins” within
their parent. Stony-iron meteorites are taken to represent
metal-silicate interface regions. Pallasites (Fig. 8c), which
have large (centimeter-sized) rounded olivines embedded
in well-crystallized metal, resemble an “equilibrium” as-
semblage that may have solidified within a few years but
that cooled slowly at iron meteorite formation-rates, a few
degrees per million years (Ma). Mesosiderite structures
suggest more rapid and violent metal and silicate mixing,
possibly by impacts.
During differentiation, siderophilic elements are eas-
ily reduced to metal; they follow metallic iron geochemi-
cally and are extracted into metallic melts. Such elements
(e.g., Ga, Ge, Ni, or Ir) are thus depleted in silicates and
enriched in metal to concentrations well above those in pre-
cursor chondrites. Conversely, magmas become enriched
in lithophilic elements—like rare earth elements (REE),
Ca, Cr, Al, or Mg—above chondritic levels: concentra-
tions of such elements approach zero in metallic iron.