Geology Reference
In-Depth Information
4
Primitive Asteroids: Expanding the Range of Known
Primitive Materials
Michael K. Weisberg 1,2,3 and Kevin Righter 4
4.1. INTRODUCTION
Nearly all known primitive (unequilibrated) EL chon-
drites are from Antarctica (Plates 30 and 31) and the CK
chondrite group was essentially defined by Antarctic mete-
orites (Figures  4.1a and 4.1b; Plate  20). An anomalous
unequilibrated enstatite chondrite, Lewis Cliff (LEW)
87223, has also been recovered. The first meteorite from
the metal-rich CH chondrite group, Allan Hills (ALH)
85085 (Plate 24), is Antarctic and the CB b chondrite sub-
group was not well established until Queen Alexandra
Range (QUE 94411; Plate  21) was studied. Grosvenor
Mountains (GRO) 95551 (Plate 23) led to the discovery of
another type of metal-rich chondrite, not directly related
to CH or CB. One of the earliest recognized R3 chondrites
is ALH 85151, and one of the two known K chondrites,
LEW 87232 (Plate 37), is from Antarctica. The majority of
CM (Plates 11 to 14) and a large number of CK chondrites
are from Antarctica (Figure  4.1a). The Antarctic mete-
orite collection has provided ordinary chondrite (impact-)
melt rocks, low petrologic types, and rare recrystallized
(type 7) examples (Figure 4.1d; Plates 1, 3, 4, and 8). The
collection has also provided materials for studies of preso-
lar circumstellar grains and possibly interstellar organic
components. With regard to studies of primitive solar
system materials, the Antarctic meteorite collection has
clearly had a major influence. Here we explore some
examples.
Chondrites are ancient rocks derived from primitive
asteroids that escaped the processes of melting and
differentiation. Their major constituents (chondrules, cal-
cium-aluminum-rich inclusions, metal, and matrix) are
interpreted to have formed in the early disk (solar nebula).
Thus, chondrites are records of the physical and chemical
processes that were active in the early solar system and of
the accretion and impact histories of early-formed plane-
tesimals. Chondrites also contain presolar materials that
can provide information on stellar and interstellar processes.
Additionally, many chondrites record secondary processes
of aqueous and thermal alteration that early-formed plan-
etesimals were subjected to.
Most chondrite groups are represented in the Antarctic
meteorite collection and some varieties are known only
from the Antarctic collection (Figures 4.1a, 4.1b, 4.1c, and
4.1d). The Antarctic meteorites have also provided more
pristine varieties of chondrite groups, enabling the study
of primary nebular features, thereby better constraining
conditions and processes in the early solar nebula. They
have given us more heavily altered examples of existing
chondrite groups, allowing us to study the range of alter-
ation conditions on primitive asteroids.
The most pristine CR chondrites (QUE 99177 and MET
00426; Plate  26) are Antarctic, as are the most heavily
altered CRs (GRO 95577 and MIL 090292; Plate  27).
4.2. CR CHONDRITES
The CR chondrites are widely considered to be among
the most primitive chondrites and, based on Antarctic
CRs, exhibit a wide range in degree of alteration from
heavily altered (petrologic type 1) to essentially pristine
(petrologic type 3). In general, many of the CR chondrites
are Antarctic (e.g., EET 87711, EET 96259, GRO 03115,
GRO 95577, MIL 07513, MIL 090001, MIL 090292,
1 Department of Physical Sciences, Kingsborough Community
College, City University New York
2 Earth and Environmental Sciences, Graduate Center,
City University New York
3 Department of Earth Planetary Sciences, American
Museum of Natural History, New York
4 NASA Johnson Space Center
,
 
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