Geology Reference
In-Depth Information
8
Meteorite Misfits: Fuzzy Clues to Solar System Processes
Timothy J. McCoy
8.1. INTRODUCTION
that formed them (e.g., impact melting vs. asteroid
differentiation). In other cases, these meteorites can only
be fully understood in the context of their parent aster-
oids, and spacecraft missions are posing new challenges
and opportunities.
Since its inception in 1976, meteorites from the U.S.
Antarctic collection have revolutionized our under-
standing of the processes that occurred during the birth
and evolution of the solar system. Many of these advance-
ments are discussed in the preceding chapters of this
topic, ranging from understanding the full array of mate-
rials present in the solar nebula to gaining insights about
habitable environments on early Mars. In spite of these
advancements, our knowledge of the history of our solar
system remains woefully incomplete.
In this chapter, I examine meteorites that may offer
clues to these processes, but those clues remain unclear. A
partial list of misfit meteorites from the U.S. Antarctic
meteorite program is given in Table 8.1, along with brief
notes as to the nature or relationships of these meteorites.
The uncertain origin of these meteorites results from a
wide range of factors. For a few of the meteorites, no
detailed follow-up studies have ever been conducted,
probably as a result of the subsequent recovery of mete-
orites deemed more interesting. For others, detailed
initial studies lacked the analytical tools or paradigms
that exist today and prompt us to revisit earlier-recovered
older meteorites. As I show, some of these enigmatic
meteorites await recovery of related meteorites for the
emergence of a complete understanding of the chemical
and physical processes that formed and altered asteroids.
The origin of some meteorites, although discussed by one
or more authors, is often unclear even in the processes
8.2. MATERIALS AND PROCESSES IN THE
SOLAR NEBULA
Chondritic meteorites are essentially sedimentary rocks
composed of materials formed prior to their lithification
on an asteroid. The components themselves formed either
on the parent asteroid prior to final lithification or, as is
widely argued at least for chondrules and CAIs, in the
solar nebula. As such, the full range of materials found in
chondrites provides important clues to the range of mate-
rials present in the solar nebula. Much of the research in
this field has centered on either the abundant ferromag-
nesian chondrules dominated by the mafic silicates olivine
and pyroxene or on the ubiquitous, but volumetrically
minor (<10 vol.%), calcium-aluminum-rich inclusions
(CAIs) whose bulk composition, mineralogy, and isotopic
signatures suggest they were among the earliest formed
phases in the solar nebula. Far less attention has been
paid to the aluminum-rich chondrules [
Bischoff and Keil
,
1983]. The compositions of these objects suggest they are
intermediate between ferromagnesian chondrules and
CAIs, with bulk compositions controlled either by
nebular condensation or through remelting of mixed
components [
MacPherson and Huss
, 2005].
Within the Antarctic meteorite collection, two occur-
rences of Al-rich chondrules are of note.
Grossman et al
.
[1995] noted that Lewis Cliff (LEW) 87234 (paired with
Department of Mineral Sciences, National Museum of
Natural History, Smithsonian Institution
