Petrographic and chemical features of Allende ferromagnesian chondrules previously analyzed for oxygen and silicon isotopes by Clayton et al. (1983a) provide additional information on chondrule origin. Allende, like other carbonaceous chondrites, contains two chondrule populations, but one of these is represented by only one chondrule in this isotopically characterized set. All Allende chondrules fall along an isotopic mixing line, probably defined by an16O-rich solid component and an isotopically heavier oxygen gaseous exchange component. Differences in the amounts of isotopic exchange for porphyritic and barred chondrules presumably resulted from varying degrees of melting. Those porphyritic chondrules containing abundant relict grains experienced the least isotopic exchange. Chondrules with high bulk FeO/(FeO + MgO) ratios apparently persisted longer as liquids and contain more of the exchange component. The distinct directions of oxygen isotopic exchange in chondrules from carbonaceous, ordinary, and enstatite chondrites indicate that each formed from different solid precursor materials. Silicon isotopic variations in Allende chondrules probably reflect evaporative loss of lighter isotopes; however, silicon loss is also controlled by chondrule sizes, which are unknown. Observed correlations point to the importance of kinetic factors in a gaseous nebula for chondrule genesis, and are not consistent with models that explain chondrules as mixtures of several solids with distinct oxygen and silicon isotopic compositions.
[1]
J. Wood.
On the formation of meteoritic chondrules by aerodynamic drag heating in the solar nebula
,
1984
.
[2]
E. Scott,et al.
Relict and other anomalous grains in chondrules: Implications for chondrule formation
,
1984
.
[3]
R. Clayton,et al.
Oxygen and Silicon Isotopic Composition of Dhajala Chondrules
,
1983
.
[4]
G. J. Taylor,et al.
Chondrules and other components in C, O, and E chondrites: Similarities in their properties and origins
,
1983
.
[5]
R. Clayton,et al.
Oxygen isotopic heterogeneities, their petrological correlations, and implications for melt origins of chondrules in unequilibrated ordinary chondrites
,
1983
.
[6]
H. McSween,et al.
Petrogenesis of light and dark portions of the Leighton gas-rich chondritic breccia
,
1982
.
[7]
E. Rambaldi.
Relict grains in chondrules
,
1981,
Nature.
[8]
H. Nagahara.
Evidence for secondary origin of chondrules
,
1981,
Nature.
[9]
H. McSween.
Chemical and petrographic constraints on the origin of chondrules and inclusions in carbonaceous chondrites
,
1977
.
[10]
H. McSween.
On the nature and origin of isolated olivine grains in carbonaceous chondrites
,
1977
.
[11]
R. Clayton,et al.
DISTRIBUTION OF THE PRE-SOLAR COMPONENT IN ALLENDE AND OTHER CARBONACEOUS CHONDRITES
,
1977
.