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NUMBER 19 93
the original presence of superheavy elements in the
Allende meteorite, is that the Tm excess may be a
previously unrecognized fission product.
Marvin, Wood, and Dickey (1970), who first described
the Ca/Al-rich inclusions in the Allende
meteorite, noted the general similarity between
their chemistry and mineralogy and the sequence
of compounds calculated to be among the highest
temperature condensates to form in a solar nebula.
Grossman (1972, 1973) has greatly extended this
work; he concluded (1973:1137) that the Ca/Alrich
inclusions in the Allende meteorite are enriched
in Sc, Ir, and the lanthanides by a factor of
22.8 ± 2.2 relative to Type I carbonaceous chondrites,
and that the large enrichments of these
refractory trace elements provided strong evidence
that the inclusions are high-temperature condensates
from the solar nebula. All but two of the
samples he analyzed belong to Group I, and his
conclusion appears well-founded for this group.
Can this conclusion be extended to Groups II and
III, or do they have a different origin? The trace
element distribution in the Group III samples is
not greatly different from Group I, except for the
negative Eu and Yb anomalies in the lanthanide
distribution, and these two groups may well be
genetically related. However, the strongly fractionated
lanthanide distribution in Group II samples,
the Eu, Tm, and Yb anomalies, and their relative
depletion in other refractory elements such as Zr,
Mo, and the platinum metals, strongly suggest extensive
chemical differentiation of their parent material.
One aspect still insufficiently explored is the
possible correlations between mineralogy, texture,
and chemistry of the Allende inclusions. Group I so
far comprises only melilite-rich chondrules; Group
II comprises four aggregates and one chondrule;
the two representatives of Group III are both aggregates;
Group IV comprises the common olivinerich
chondrules, but also includes some aggregates.
The term aggregate covers a variety of fine-grained
white, pink, or pale gray inclusions with irregular
form; if an aggregate approaches spherical form,
the distinction between aggregate and chondrule
may be arbitrary. The form of the aggregates and
their contacts with the enclosing matrix vary
greatly; some are angular fragments with sharp
contacts, many are irregular in form, with intricate
mossy-like margins against the matrix, others are
elongated shards or stringers. Subparallel elongation
of the latter type in a hand specimen often
indicates a vaguely oriented fabric, suggesting an
original flattening during deposition (as plastic glass
fragments?) or a secondary flattening produced by
later deformation. The overall impression is that
some aggregates may be devitrified glass fragments,
whereas others represent agglutinated dust particles.
The formation of chondrules, specifically the
melilite-rich chondrules of Group I, presents an additional
problem. Grossman's condensation curves
(1972) are for gas-solid equilibria, and if the Group
I material originated in this way, it should have
separated as a dust; the formation of chondrules
involved segregation of this dust and subsequent
melting, by such processes as lightning or shock effects.
Alternatively, these chondrules may have originated
in some nonequilibrium process, such as that
suggested by Blander and Katz (1967): metastable
nucleation of liquid droplets, formed by supersaturation
of the nebular gas, component by component,
as subcooling took place below the
temperatures where equilibrium solids should have
crystallized out.
TABLE 3.—Abundances (ppm) of Eu and Yb in Group H and Group m samples, and the Eu/Yb
ratios
Elements
Eu . . . .
•&,....
Eu/Yb • .
37
0.31
0.51
0.62
3598
0.18
0.84
0.21
Group II
3803
0.17
0.73
0.23
4691
0.62
1.6
0.39
4692
0.13
0.37
0.35
Group
3593
0.36
1.9
0.17
III
4698
0.25
0.74
0.34