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122 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
chondrites. Enstatite in enstatite chondrites contains
0.7% FeO (type i) or 0.2% FeO (type n),
according to Keil (1968). Their more reduced state
is also reflected in the presence of 1% to 3% Si in
the kamacite metal (Ringwood, 1961: Keil, 1968).
No Si was detected in the metal of the El Taco
silicate inclusions. It has also been shown by Olsen
and Fuchs (1967) that the coexistence of schreibersite
and phosphates in these inclusions allows
for an oxygen fugacity during their last hightemperature
transformation of 10~ 20 atm, which is
in the same range as that of normal chondrites. The
minor-element content of the orthopyroxene is
close to the values for this mineral from normal
chondrites but usually higher than that of enstatite
from enstatite chondrites. (Table 2).
FELDSPAR COMPOSITION
The anorthite content of 15 mole percent is close
to that of normal chondrites, but is even closer to
the feldspar of enstatite chondrites (Table 10).
Thus the El Taco oligoclase fits into this general
sequence of increasing An content with decreasing
Fe content of olivine and pyroxene. The Or content
of 3 mole percent is notably low in El Taco.
This is the reason that the addition of this feldspar
to the bulk silicate composition of the inclusions
gave a good match to chondritic values for Na, but
not for K (Figure 13). For the same reason the
analysis calculated from the mode of inclusion 1
is too low in potassium, whereas it agrees well with
the chemical analysis for sodium (Table 8). The
Na/K atomic ratio of the feldspar is 26.6, whereas
TABLE 10.—Feldspar composition (mole percentages) in El
Taco inclusions compared to the average compositions in L-,
H-, and enstatite chondrites
Sample
El Taco, range
L-chondrites
H-chordrites
average
Enstatite chandrites, type 2
Anorthite Qrthoclase
14.3-15.9
15.0
10
12
15
*Van Schraus and Ribbe, 1968, table 1,
f Keil, 1968, table 4
2.7-3.0
2.8
5
6
4
in the bulk analyses this ratio varies between 12
and 20 and is thus closer to the value of 13.5 for
the average chondrite (Table 7). This means that
an additional phase containing potassium must be
present. This was found as small K-feldspar grains,
which are only a few microns across and occur
usually at plagioclase-graphite grain boundaries.
The average composition of two grains large
enough for analysis is given in Table 2. Small Kfeldspar
grains associated with albite in the
Odessa iron meteorite were reported by El Goresy
(1967). Bunch, Keil, and Olsen (1970) observed Kenriched
zones in the feldspar of Odessa and
Toluca and thought them due to K-leaching by terrestrial
weathering. This explanation can be excluded
for El Taco by the microprobe analysis of
these grains. They can be explained by exsolution
from the albitic plagioclase because of the expanding
solvus at low temperatures. The temperature
of exsolution of the albite according to the solvus
determined by Goldsmith and Newton (1974)
should be below 600° C and lower than the last
equilibration temperature of chondritic feldspars.
Recently, a phase containing 50 mole percent Or
was observed as exsolution lamellae in the albitic
plagioclase of a silicate nodule in the San Cristobal
iron (Scott and Bild, 1974).
CLINOPYROXENE-ORTHOPYROXENETHERMOMETRY
The Wo content of the diopsides in El Taco of
44 mole percent would correspond to a temperature
of about 1000° C on the diopside solvus of
Davis and Boyd (1966). The KD value for the Fe/
Mg equilibrium between diopside and orthopyroxene
as used by Kretz (1963) and McCallum
(1968) gives a much higher temperature. For the
pyroxenes of inclusion PTS 8 KD = (Fe/Mg) orthopyroxene/(Fe/Mg)
clinopyroxene gives a value of
1.11. On McCallum's equilibrium curve this would
correspond to a temperature of about 1700° C. This
temperature is much too high, and if reduction of
FeO from the silicates took place the temperature
derived in this way is probably meaningless.
ORIGIN OF THE INCLUSIONS
The similarity between chondritic and the El
Taco silicate minerals favors a close relation between
the two. Whether this is a genetic one, so