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NUMBER 19 121
The match with chondrites for the main elements
extends also to the minor ones, such as Ti and Mn.
Notable exceptions, however, are Cr and P, which
are highly variable, and K, which is usually too low.
In chondrites, Cr is mainly contained in
chromite. If FeO is reduced to iron metal, the next
element to be reduced would be Cr, according to its
redox potential. If chromite is consumed by reduction,
first Fe is reduced, so that the residual chromite
should become very high in Cr. This is found
in the El Taco chromites (Bunch, Keil, and Olsen,
1970, table 7). Upon further reduction, Cr also
leaves this mineral and enters troilite. The troilite
of the El Taco inclusions contains 0.3% Cr (Bunch,
Keil, and Olsen, 1970, table 8). Thus, Cr can be
lost from these inclusions in variable amounts
according to its degree of reduction and the fate of
the troilite of a specific inclusion. Inclusion 1,
which is lowest in FeS (Table 1), is also lowest in
Cr (Table 9).
In the case of P, both reduction and partial
melting may be responsible for its variations.
Apatite may be reduced to schreibersite, as their
coexistence close to one another shows. It could
also be removed or redistributed by partial melting.
In the thin sections small apatite grains are found
throughout the silicates, usually in association with
troilite grains, but larger ones occur only at the
border to the metal host or to metal /graphite
veins.
VARIATIONS IN MINERAL COMPOSITION
Besides the variations in bulk composition, slight
but distinct differences exist in mineral composition,
as shown in Table 2 and Figure 8. In several
of the FeO-histograms of Figure 8 measurements
of different areas of the same inclusion are plotted.
These show that in a given inclusion the distribution
of FeO values for pyroxenes and olivines and
the average are constant, but that different inclusions
can have differences outside the analytical
error. Also, the ratio of Fe/Mg in olivine to this
ratio in pyroxene varies, as low FeO contents in
olivine may coexist with high FeO content of
pyroxene, and vice versa.
In general, in the El Taco inclusions and in the
silicate inclusions of other IAB irons (Bunch,
Keil, and Olsen, 1970), the Fe/(Fe + Mg) atom
ratios are higher in pyroxene than in olivine,
whereas in normal chondrites and terrestrial rocks,
olivine in equilibrium with orthpyroxene is more
iron-rich (Keil and Fredriksson, 1964; Medaris,
1969). This led Olsen and Fuchs (1967) to the conclusion
that equilibrium was not reached in these
silicate inclusions. Later, however, Olsen and
Bunch (1970) argued that equilibrium should have
been reached during the long cooling times of iron
meteorites (1° to 10° C per 10 6 years, according to
Goldstein and Short, 1967:1733). As they did not
find any signs of disequilibruim, such as variable
Fe contents or zoning, they concluded that an
equilibrium with a reverse trend occurs at low FeO/
(FeO + MgO) percentages, so that the pyroxene
becomes more iron-rich than the coexisting olivine.
The measurements on El Taco show that there are
clear indications of disequilibrium between different
inclusions. The most striking indication is the
variation in the ratio of Fe/Mg in olivine over
Fe/Mg in pyroxene (Table 4). The other is the
slight zoning of the FeO content in the mafic minerals.
Table 6 and Figure 9 show that the rims of
olivine, pyroxene, and diopside grains have lower
iron contents than the central part. The crystallization
zoning should be just the opposite, as Fe-rich
olivines and pyroxenes melt at lower temperatures
than their Mg-rich counterparts.
It should be emphasized that we are speaking of
three kinds of disequilibrium: (1) disequilibrium
between different inclusions, expressed as different
Fe/Mg ratios in olivine compared to Fe/Mg in
pyroxene; (2) disequilibrium within single olivine
and pyroxene grains of one inclusion, the rim
being depleted in iron compared to the center; and
(3) "equilibrium" within one inclusion, that is a
narrow range of Fe/Mg ratios in olivines and
pyroxene. In a given inclusion, the large olivine
and pyroxene grains which are close to the metal
border usually have the highest FeO contents. On
the other hand, there is no simple relationship
between the average grain size of an inclusion and
the average FeO content of its mafic minerals.
Contrary to the observation on single grains mentioned
above, inclusion 5 with the highest average
FeO content in the olivine has one of the smallest
grain-size textures.
The composition of the main minerals is very
close to that of chondrites except for the lower FeO
content of the mafic minerals. This implies a
slightly more reduced condition of this material,
but not nearly as reduced as that of enstatite