05 Classification of.. - Department of Earth and Planetary Sciences

Classification of Chondritic Meteorites 97
Figure 11 Backscattered electron images (a, b) and elemental maps in Ni Ka X-rays (c, d) of the: (a) CR chondrite
PCA91082, (b) CH chondrite PCA91467, (c) CB a chondrite Gujba, and (d) CB b chondrite Hammadah al Hamra 237.
(a) In the CR chondrite PCA91467, FeNi-metal occurs as nodules in chondrules, igneous rims around chondrules
and in the matrix. (b) In the CH chondrite PAT91546, FeNi-metal occurs largely as irregularly shaped grains outside
chondrules. (c) In the CB a chondrite Gujba, most FeNi-metal grains are texturally similar to metal-sulfide aggregates
in the CB b chondrite Hammadah al Hamra 237. Although there are compositional variations between metal grains,
each grain is compositionally uniform. (d) In Hammadah al Hamra 237, FeNi-metal occurs exclusively outside
chondrules; ,20% of all metal grains are compositionally zoned, with cores enriched in Ni relative to edges. Some
of the zoned metal grains contain silicate inclusions. Metal-sulfide aggregates are compositionally uniform.
of these unusual characteritics. Wasson and
Kallemeyn (1990) suggested that it is a
modified “subchondritic” meteorite formed as
a result of a highly energetic asteroidal
collision. More recently, it has been argued
that the refractory inclusions, chondrules, and
zoned metal grains in these meteorites are in
fact pristine nebular products (Meibom et al.,
1999; Krot et al., 2000c; McKeegan et al.,
2003).
CB (Bencubbin-like) chondrites. The CB chondrites
(Figures 10(c), (d), and 11(c), (d)) are a
group of very metal rich (60–70 vol.%) meteorites
including Bencubbin, Gujba, Weatherford,
Hammadah al Hamra (HH) 237, and Queen
Alexandra Range (QUE) 94411 which is
paired with QUE94627 (Weisberg et al., 2001).
Characteristics of the CB chondrites include:
(i) very high metal/silicate ratios (Table 2);
(ii) extreme depletion in moderately volatile
lithophile elements (Figure 2); (iii) extreme
enrichment in d 15 N(Figure 6); (iv) bulk oxygen
isotopic compositions plotting near CR chondrites
(Figure 4); (v) bulk refractory lithophile element
abundances of (1.1–1.5) £ CI (Figure 2(a));
(vi) FeNi-metal with a positive nickel versus
cobalt trend and a solar Ni/Co ratio;
(vii) magnesium-rich (Fa #4 ,Fs #4 ), FeNi-metalfree
chondrules with only nonporphyritic
(cryptocrystalline and skeletal-olivine) textures;