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100 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
be derived solely from the metal-rich and metalpoor
chondrule olivine with mean fayalite contents
of 3.0 and 14.1, respectively. Indeed, several abraded
chondrules with forsteritic olivine adjacent to
matrix material suggest that all silicates do reflect
primary compositions. Therefore, although silicates
in the matrix may include olivine fragments
derived from chondrules similar in composition to
those studied, it must contain a separate Fe-rich
olivine fraction. Secondly, olivine in metal-rich
chondrules is enriched in Ca and depleted in Mn
relative to olivine in metal-poor chondrules, possibly
suggesting two separate chondrule fractions.
The high-Ca olivine is Mg-rich (Fa, 0-10), occurs
Q_
1 .00,.
0.80..
0.60..
0.40..
0.20..
0.00
10 15 20 25 30
HOLE PCT FERROSILITE
predominantly in metal-rich chondrules, and trends
from a high of 0.67% CaO for nearly pure forsterite
to below 0.10% CaO at about Fa 10 (Figure 2).
This grouping corresponds to the proposed hightemperature
metal-rich chondrule fraction in
Sharps (Dodd, 1971). This fraction is depleted in
MnO but shows a positive rather than a negative
correlation with fayalite content, also to about
Fa 10 (Figure 3). Olivine with greater than about
8% Fa (Fa 10), which includes most metal-poor
chondrules and silicate fragments from the matrix,
shows no correlation of either CaO or MnO concentration
with fayalite content.
Most of the compositional zonation displayed in
A
• XTLS IN METflL-RICH CHOND
A XTLS IN METRL-POOR CHOND
+ MRTRIX CRYSTflLS
- ZONED CRYSTflLS
A
A A
FIGURE 4.—Plot of MnO vs ferrosilite content of low-Ca pyroxene (< 1% CaO) suggests an increase
in manganese with increasing iron (lines join compositions in zoned crystals).