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NUMBER 19
having a relatively high Ca/Al ratio, the latter two,
relatively low ones. Mineralogically this is expressed
by the dominance of melilite in Group I
samples, and its absence or presence in only minor
amounts in other samples (except 4691).
Some other features of major element chemistry
deserve notice (Table 2). Group IV is consistently
higher in SiO2 and MgO and lower in TiO2 than
the other groups. In Group I, Na2O and FeO are
notably low; in fact, some of these melilite chondrules
are essentially iron- and alkali-free, and the
ferrous iron and alkali in others may have been a
later introduction, since microprobe analysis shows
that these elements are concentrated at the chondrule
margins.
Returning now to the trace element data, we
find characteristic differences between the different
groups. Group I samples have notably high Sr and
low Rb, giving unusually high Sr/Rb ratios. Gray,
Papanastassiou, and Wasserburg (1973) and Wetherill,
Mark, and Lee-Hu (1973) have analyzed sim-
ilar samples with concordant results, and they have
also found that the 87 Sr/ 86 Sr ratios are the lowest
yet recorded, i.e., the Sr is "primordial" and has
had essentially no radiogenic 87 Sr added to it. Other
Allende samples analyzed by them have much
higher Rb, lower Sr, and higher 87 Sr/ 86 Sr ratios, and
probably belong to Group II or Group III as these
are denned here.
The neighboring elements Y, Zr, and Nb show
rather coherent geochemical behavior, with a distinctive
pattern in each of the four groups. They
are most highly enriched in Group III (however,
only two samples were analyzed, so the statistical
basis is weak), notably enriched (at about 10 times
chondritic level) in Group I, and relatively low (at
about average chondritic level) in Groups II and
IV. On examining the data for the individual
samples (Table 1), an interesting relationship is apparent;
the ratio Zr/Nb varies considerably from
1.1 to 53, whereas the ratio Zr/Y is remarkably constant,
ranging only from 2.1 to 2.9. The Zr/Y co-
FIGURE 1.—Chondrite-normalized lanthanide abundance patterns of the average of each group
and of a bulk sample of the Allende meteorite: Group i: Ca, Al-rich chondrules; Groups n and
m: Ca, Al-rich aggregates; Group iv: Mg-rich chondrules and aggregates. Note the Eu and Yb
anomalies in Groups n and m and the Tm anomaly in Group n.
Yb
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