Primitive Asteroids: Expanding the Range of Known Primitive Materials - 35 Seasons of U.S. Antarctic Meteorites - page 180

discerned (Figures 4.2b and 4.2c). Many chondrules in
GRO 95577 have textures suggestive of the common por-
phyritic type I (silicates with Mg/(Mg + Fe) > 90) chon-
drules, observed in CR and other chondrites, but the
phenocrysts have undergone pseudomorphic replacement
by yellow to brownish serpentine-rich phyllosilicate, with
sharp original crystal outlines preserved. The chondrule
mesostasis is a green aluminous material, most likely a
hydration product of the feldspathic mesostasis com-
monly found in anhydrous type I chondrules in other
CRs. Some chondrules contain magnetite spheres that
most likely formed by oxidation of pre-existing metal.
GRO 95577 extends the range of asteroidal alteration
recorded in the CRs considerably, from CR chondrites that
contain unaltered glassy mesostasis in chondrule interiors
[e.g., Weisberg et al ., 1993; Ichikawa and Ikeda , 1995;
Noguchi , 1995] and olivine [ Zolensky et al ., 1993] and
amorphous material [ Abreu and Brearley , 2010] in the
matrix to heavily altered, like GRO 95577, in which no
anhydrous silicates have survived alteration [ Weisberg and
Huber , 2007]. MIL 090292 is another heavily hydrated CR
classified as a CR1 [ Satterwhite and Righter, 2012].
QUE 99177 and MET 00426 are the first CR chondrites
considered to be petrologic type 3 (CR3). They suffered
lower degrees of secondary aqueous alteration than all
other known CR chondrites [ Abreu and Brearley , 2010].
Unlike other CR chondrites, which are CR1 or CR2, the
matrix material in QUE 99177 and MET 00426 contains
little phyllosilicate and is dominated by amorphous sili-
cate, indicating the primitive, unaltered nature of these
meteorites [ Abreu and Brearley , 2010]. Another intriguing
characteristic of QUE 99177 and MET 00426 is that they
have high presolar silicate and oxide grain abundances,
and contain high abundances of phases with anomalous
C isotopic compositions [ Floss and Stadermann , 2011],
discussed below. With GRO 95577, the CR group is
the only carbonaceous chondrite group that spans the
range of asteroidal alteration from completely hydrated
(lacking anhydrous silicates) to almost completely anhy-
drous (lacking hydrous silicates).

aldehydes and ketones [ Martins et al ., 2007; Pizzarello
et al ., 2008; Pizzarello and Holmes , 2009]. Furthermore,
the amino acids exhibit chirality that is distinct from that
seen in terrestrial amino acids [ Glavin and Dworkin , 2009].
The presence of N-containing compounds, as well as a
great diversity of asymmetric compounds, shows that the
more complex molecules (amino acids) could have
inherited some of their unique asymmetry from synthesis
reactions involving the diversity of chiral prebiotic ingre-
dients such as ketones, aldehydes, and amines.
Insoluble organic matter in CR2 chondrites such as
EET 92042 is also known to contain isotopic anomalies in
H and N that are most likely acquired from the interstellar
medium, although formation in the protoplanetary disk
cannot be ruled out [ Busemann et al ., 2006; Alexander
et al ., 2007] (Plate 25). The bulk composition of insoluble
organic matter in EET 92042, on the other hand, is strik-
ingly similar to the CHON particles found in Comet
Halley, for example [ Alexander et al ., 2007]. The organic
matter in CR2 chondrites may provide constraints on the
sources for the prebiotic organic matter that helped pro-
mote the development of life. The continued discovery of
CR2 chondrites by ANSMET teams provides new material
with which to test many hypotheses for the origin of
organic matter in the early solar system and ultimately
better understand the origin of life.

4.3. METAL-RICH CH AND CB CHONDRITES

Ever since the first published studies of ALH 85085
[ Grossman et al ., 1988; Scott , 1988; Weisberg et al ., 1988;
Wasson and Kallemeyn , 1990] (Plate 24), metal-rich,
matrix-poor chondrites, such as the CH and CB chon-
drites, have been among the most perplexing and significant
meteorite discoveries. The initial description of ALH
85085 in the 1987 Antarctic Meteorite Newsletter was
exciting in that it was unlike any chondrite previously
described: a new type of asteroid was sampled. There are
now 23 CH chondrites, nine of which are Antarctic from
the ANSMET collection.
The CH chondrites are closely related to the CBs and
CRs ( Weisberg et al., 1995b); they may be among the most
primitive, unaltered chondritic materials, and planetesimal
collisions may have played major roles in their origin [e.g.,
Wasson and Kallemeyn , 1990; Krot et al ., 2005]. They are
characterized by small (<100 µm in diameter) chondrules,
which are mostly cryptocrystalline in texture. They have
high abundances of FeNi-metal (~20 vol%) with a large
range in composition, positive Ni versus Co trends, solar
Ni/Co ratios, and compositional zoning in many grains.
Interchondrule matrix is virtually absent, but hydrated
matrix-like clasts are commonly observed. Like the CR
and CB chondrites, CH chondrites have ~ CI (solar) bulk
refractory lithophile element relative abundances but are

4.2.1. Organic Compounds in CR2 Chondrites

Organic compounds in carbonaceous chondrites have
been the focus of organic geochemists for many years,
and studies of the CM2 Murchison in particular have
yielded a diversity of compounds. Antarctic CR2 chon-
drites, and in particular GRA 95229, LAP 02342, EET
92042, and QUE 99177, have also yielded a great diversity
of organic compounds, and quite different from those
found in other C chondrites, such as Murchison. For
example, the Antarctic CR2s contain several orders of
magnitude greater concentrations of amino acids (glycine,
isovaline, α-AIB, and alanine), ammonia, amines, and