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that El Taco silicate inclusions were derived by
transformation of chondrite material, cannot be
answered with certainty. Furthermore, there are
many ways in which a chondrite could be transformed
to give the El Taco silicates. Three features
have to be explained: the lower FeO content of the
mafic minerals, the apparent differences in bulk
composition, and the disequilibrium between different
inclusions.
One simple model that would explain most of the
El Taco silicate inclusion features would be the
incorporation of chondritic matter into a hot iron
metal, at or near to the nickel-iron melting point
of about 1500° C. Heating of a chondrite with Htype
olivine and pyroxene in an atmosphere of CO
and CO2 in equilibrium with one another will
lead to a reduction of iron silicates above temperatures
of about 1200° C. It has been shown by
Mueller (1963) and Speidel and Nafziger (1968)
that an olivine-silica-metal assemblage will change
towards lower fayalite contents in the olivine when
heated from 1100° to 1300° C. A reduction of the
silicates to about 6 mole percent FeO, which is
found in most of the silicates of iron meteorites
with silicate inclusions (Bunch, Neil, and Olsen,
1970), may then reflect the equilibrium at the temperature
of molten nickel-iron. At this temperature
albite and diopside will melt also, but not
olivine and enstatite, a condition which could explain
the depletion of the former minerals discussed
above. The apparent disequilibrium between
different inclusions was probably not established at
these high temperatures, but later during cooling,
and may be the last change produced by a continuing
reduction of iron oxide from the silicates. The
lower FeO mole percent of olivine may then simply
reflect the fact that olivine is easier to reduce
than pyroxene. That this is the case was shown
in the ureilite Havero, where olivine next to
carbon-rich veins is reduced to lower FeO contents,
whereas pyroxene is not (Wlotzka, 1972).
The question remains, why did the hightemperature
reduced assemblage not change back to
its original, more oxidized form upon slow cooling?
This difficulty can be overcome by changing
from a closed to an open system, which would be
the result in a change from rapid heating to slow
cooling. A rapid heating process, as by an introduction
of silicates into a molten metal pool or
invasion of a chondritic rock by a metal melt,
would essentially result in a closed system with respect
to C/CO/CO2 because the time for escape
of the gases was too short. Thus the equilibrium
between CO and CO2 would establish the low Fa
and Fs content of the mafic minerals at this high
temperature. Slow cooling, which took place during
the y-a transformation between 850° and 600° C
over some 100 million years (Goldstein and Short,
1967), will result in an open system, where oxygen
removed by the reduction of Fe-silicates by carbon
may escape and provide for the continuing reduction.
The differences from inclusion to inclusion can
then be explained by local differences in carbon
available per inclusion or weight-unit of silicates.
These differences are mainly due to a different FeO
content of the olivines, since the pyroxene compositions
are more uniform. As olivine is more
easily reduced than pyroxene, its composition will
follow these changes better than that of pyroxene.
Thus the low FeO content of olivines from the
small, veinlike inclusion 6 may be caused by the
higher amounts of carbon available from the metal
in this area.
The proportionality of Fe and Ca in orthopyroxene
(Figure 10) is not in contradiction to a loss of
FeO by reduction. Both FeO and CaO will be
leaving the pyroxene grains by diffusion—FeO because
it is reduced to Fe metal, and CaO because
at lower temperatures the solubility of CaSiO3 in
the enstatite is lowered (Davis and Boyd, 1966).
Another possibility is a genetic relationship of
the material of the El Taco inclusions to chondrites
like Kakangari (Graham and Hutchison, 1974),
which already contain olivines with a low fayalite
and pyroxene with a low ferrosilite content, so that
a reduction is not necessary. Only the late reduction
leading to the disequilibrium between olivine
and pyroxene and between different inclusions
has to be operative here too.
Summary and Conclusions
It has been shown that (1) the silicate minerals
of the El Taco inclusions have a composition close
to the minerals of chondrites with the exception
of a lower iron content of olivine and pyroxene;
(2) the iron content of olivine and pyroxene shows
small but distinct variations from inclusion to inclusion;
and (3) the bulk chemical composition is