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92 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
is that of Grossman (1973) on 16 specimens of
Ca/Al-rich inclusions selected by Dr. E. A. King,
Jr., of the University of Houston. Grossman determined
Fe, Mn, Na, Sc, La, Sm, Eu, Yb, Co, Ir,
and Au in these specimens and on two samples of
the bulk meteorite. Through the kindness of Dr.
King, we received splits of specimens 1, 4, and 8
analyzed by Dr. Grossman. These had been selected
as covering a wide compositional range, and
our analyses show that sample 1 belongs to Group
I, sample 4 to Group III, and sample 8 to Group II.
Grossman's data for the remaining samples indicate
that they all belong to Group I.
Perhaps the most intriguing feature revealed by
the present research is the occurrence of at least
three different lanthanide distribution patterns in
the materials comprised under the description of
Ca/Al-rich inclusions. Although Groups I, II, and
III do show characteristic differences in major element
composition, these differences are relatively
much less than those for the trace elements. The
presence of marked Yb anomalies in Groups II and
III, and of a Tm anomaly in Group II appears to
be unique to these Allende materials; to our
knowledge Yb and Tm are closely coherent with
the neighboring lanthanides both in terrestrial and
in lunar materials.
It is remarkable at first glance that in Group II
a negative Eu anomaly is associated with a positive
Yb anomaly, whereas in Group III a negative
Eu anomaly is associated with a negative Yb anomaly.
However, when the Eu and Yb abundances in
individual samples in these two groups are compared,
one finds that the Eu/Yb ratio is relatively
constant (Table 3). Figure 1 shows that the Yb
anomaly is positive in Group II because it is
superimposed on a fractionated lanthanide pattern
with rapidly diminishing abundances of the heavier
elements, whereas it is negative in Group III because
it is superimposed on an unfractionated lanthanide
pattern in which all the other elements
(except Eu) are strongly enriched.
Thus in Groups II and III Eu and Yb show a
geochemical coherence unrelated to the other lanthanides.
After Eu, Yb is the lanthanide most
readily reduced to the divalent state, and this provides
a possible explanation for this coherence.
However, this explanation requires highly reducing
conditions at some stage in the origin of Group
II and III materials, more reducing, for example,
than for lunar rocks, which show Eu anomalies but
do not have Yb anomalies. In this connection, too,
it is puzzling to find no Yb anomalies either in
Group I materials or their constituent minerals
(Mason and Martin, 1974). Boynton (1975) comments
that it seems unlikely that divalent Yb can
explain the Yb anomalies, since Sm is nearly as
easily reduced as Yb, and there is no evidence of
an Sm anomaly. He points out that the condensation
of lanthanides from the solar nebula may be
controlled by thermodynamic equilibrium between
gas and condensed solids, and that highly fractionated
lanthanide patterns may result if condensates
are removed from the gas before condensation is
complete. Both Yb and Eu are predicted to be
extremely depleted in the early condensate without
the requirement of condensation in the divalent
state. According to Boynton's model, the
Group II inclusions may be a condensate from a
previously fractionated gas rather than from a gas of
solar composition. Thus the cosmochemical properties
of Eu and Yb (determined by gas-solid equilibria)
may be quite different from crystallochemical
properties (determined by liquid-solid and solidsolid
equilibria), and may allow an unambiguous
determination of which process is yielding a specific
lanthanide pattern.
The most remarkable and thought-provoking
anomaly is the positive Tm anomaly in Group II
samples. Group II samples 37 and 3598 were analyzed
in 1971, when no attempt was made to measure
Tm because of possible interference on the 169
mass number. In 1973 it was realized that the 169
mass number line in many Allende samples was
much too strong to be due to the multiple carbon
interference, and it was found that in Group I
samples Tm values were obtained that fell on the
smooth chondrite-normalized curve linking Er and
Yb. However, the Tm values for Group II samples
are anomalously high in comparison to Er and Yb
(Figure 1). A similar Tm anomaly in some Allende
aggregates has recently been reported by Conrad,
Schmitt, and Boynton (1975). The cause of this
Tm anomaly remains to be elucidated. Boynton
(1975) has discussed rare earth anomalies in terms
of fractional condensation in the solar nebula, and
this may be invoked as a possible cause. Another
possibility, suggested by the search for evidence for