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90 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
herence is illustrated in Figure 3. The wide range
of Zr/Nb ratios is somewhat surprising, since
Graham and Mason (1972) found a limited range
(10-32) for this ratio over a variety of meteoritic,
lunar, and terrestrial materials. A geochemical coherence
between Zr and Y has not previously been
remarked upon to our knowledge, yet Figure 3
shows that this coherence for the Allende samples
is comparable to that between Zr and Hf. The
cause may lie in similar condensation behavior for
Zr and Y. Another possibility is that under the
highly reducing conditions under which the Allende
materials were formed (evidenced by the occurrence
of Eu and possibly also Yb in the divalent
state), Zr was trivalent, in which case its ionic radius
is very close to that of Y and the two elements
would tend to be closely associated geochemically.
Taylor, et al. (1972) have provided evidence for the
presence of trivalent zirconium in lunar rocks.
Quantitative data on the platinum-group elements
(Ru, Rh, Pd, Os, Ir, Pt) could not be obtained
from the spark source mass spectrometer
records, because of the lack of suitable standards.
However, these elements were clearly present in
10 20 n. 30
AI2O3%
Group I and Group III samples and were not seen
in Group II and Group IV samples. Estimates
based on relative intensities would give the following
averages for Group I and Group III samples (in
ppm): Pt 10; Ir 7; Os 7; Ru 8; Rh 1; Pd 1; the
latter element appears to be depleted relative to
the other platinum-group metals, a feature probably
related to its having a considerably lower
boiling point and hence being less readily condensed.
Grossman (1973) has analyzed 16 Allende
inclusions for Ir; all except one (which belongs to
Group II from its Eu content) have Ir concentrations
at 5-11 ppm. Molybdenum, a comparable siderophile
element with a very high boiling point, is
present in all Group I and Group III samples at
a level of 4-14 ppm, is seen in two Group II samples
near the lower limit of detection (0.6 ppm),
and is present in Group IV samples at about 1
ppm.
In most samples (Table 1) uranium was close to
or below the limit of detectability (0.05 ppm); however,
at that level it is clearly considerably enriched
over the chondritic mean (0.01 ppm). Ura-
FIGURE 2.—The relationship between CaO and A12O3 (weight percent) in individual samples.
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