Primitive Asteroids: Expanding the Range of Known Primitive Materials - 35 Seasons of U.S. Antarctic Meteorites - page 182

One of the many important characteristics of the CH
and CB chondrites is their zoned metal (Figure 4.3b),
initially described in ALH 85085 by Scott [1988]. The
preservation of zoning in metal grains attests to the
primitive character of the CH and CB chondrites; zoning
would have been erased under conditions of thermal meta-
morphism [ Righter et al ., 2005; Humayun , 2012]. The
range of compositions of P, Si, Cr, Co, and Ni as well as
Ru within individual zoned metal grains in CH and CB b
chondrites is generally consistent with calculated paths for
metal condensing from a solar gas [ Meibom et al ., 1999,
2000, 2001; Campbell et al ., 2001, 2005; Petaev et al ., 2001].
Analyses of trace siderophile elements of metal in Queen
Alexandra Range (QUE) 94411 helped verify that the
zoned metal grains in the CB b chondrites were indeed the
products of condensation [ Campbell et al ., 2001]. However,
it remains unclear whether the metal condensed from the
solar nebula gas or from a vapor plume produced during a
planetary-scale collision.

4.4. UNEQUILIBRATED ENSTATITE (E3)
CHONDRITES (PLATES 28 TO 31)

The enstatite (E) chondrites have important implications
for the evolution of the solar system and formation of the
inner planets (Mercury through Mars). They have
extremely different characteristics than those of other
chondrite groups. Their silicate, sulfide, and metal compo-
sitions indicate highly reducing conditions [e.g., Keil , 1968].
Their major silicate phase is near-pure endmember enstatite
(FeO < 1.0 wt.%). They contain higher amounts of FeNi
metal than ordinary or carbonaceous chondrites, and their
metal is Si-bearing with more than 2 wt.% Si in the EH
chondrites. Elements that are strictly lithophile in most
other chondrites (e.g., Mg, Ca, Mn, Na, K) in some cases
behave as chalcophiles in the E chondrites, occurring in a
wider variety of sulfide minerals than in any other chon-
drite group [e.g., El Goresy et al ., 1988]. E chondrites con-
tain the oxi-nitride and nitride phases sinoite (Si 2 N 2 O)
[ Andersen et al. , 1964; Keil and Anderson , 1964; Mason ,
1966; Rubin , 1997], Si 3 N 4 [ Alexander et al ., 1994], and
osbornite (TiN) [ Mason , 1966], which are also indicators
of highly reducing conditions. The reduced mineral assem-
blages of the primitive unequilibrated E3 chondrites
formed in a highly reducing solar nebula and/or asteroidal
environment, unlike that for other chondrite groups.
Another intriguing aspect of the E chondrites is that they
have the oxygen isotopic composition of the Earth and
Moon [ Clayton and Mayeda , 1984; Javoy , 1995; Javoy et al. ,
2010]. The significance of this is unclear, except that they
likely formed in the inner solar system and were possibly
among the building blocks of the inner planets.
The Antarctic meteorite collection has greatly contrib-
uted to the number of known primitive, unequilibrated
enstatite (E3) chondrites. The enstatite chondrites are
divided into two main groups, EH and EL. MAC 88136
[Figure 4.5a, Plate 31] was the first known primitive (EL3)
chondrite [ Lin et al ., 1991]. Prior to its discovery, only
highly metamorphosed examples of EL chondrites were
known. Other EL3 chondrites that have since been recog-
nized include ALH 85119, EET 90299, and MAC 02635
and their paired samples.
It has been suggested that asteroidal impact played a
major role in the formation of the EL3 chondrites and that
they are impact melt breccias that do not preserve their
primary textures [ Van Nierkirk and Keil , 2011]. This
interpretation is based on the occurrence of euhedral
enstatite in opaque mineral assemblages, the presence of
the mineral keilite (Fe, Mg)S, and possible evidence for
mobilization of metal. Rubin [1997] found euhedral, zoned
sinoite grains associated with euhedral enstatite and
graphite within impact-melted portions of QUE 94368,
the first known EL4 chondrite (Plate 32). He argued that
the sinoite crystallized from the impact melt. Arguments

4.3.1. New Type of Metal-Rich Chondrites

Finding new metal-rich meteorites with textural similar-
ities to CH or CB may provide a better understanding of
the origins of these unusual meteorites. GRO 95551
(Plate 23) is one of two meteorites that represent a new
type of highly reduced, metal-rich chondrite (Figures 4.4a
and 4.4b). The other is Northwest Africa (NWA) 5492.
GRO 95551 was initially thought to be a CB chondrite
because it contains large metal nodules and large barred
olivine chondrules (Figure 4.4b), but it has more reduced
mineral compositions and formed from a different oxygen
reservoir than the CB and CH chondrites [ Weisberg et al .,
2001, 2012].
GRO 95551 has similarities to CB chondrites with
~60 vol% metal. Some of the metal and silicate occur as
large >500 µm size chondrule-like objects. The silicates
include large barred olivine chondrules, like in CB
chondrites, but smaller chondrules showing a wider range
of textures, including porphyritic, are also present. The
compositions of the silicates are highly reduced, with an
average olivine composition of Fa 1.3 . Oxygen isotope ratios
for chondrules in GRO 95551 and NWA 5492 have unique
Δ 17 O values that are between values for the ordinary and
the enstatite chondrites (Figure 4.4c) [ Weisberg et al .,
2012] [Δ 17 O = δ 17 O - 0.52 (δ 18 O)].
The oxygen isotope ratios coupled with the reduced sili-
cate compositions suggest that GRO 95551 is a new type
of chondrite, related to NWA 5492. They may represent a
new primitive asteroid with characteristics that may be
transitional between ordinary and enstatite chondrites.
The discovery of additional chondrites like GRO 95551
will help characterize this new chondrite group and deci-
pher their origin.