|Abstract or Summary
- High-precision instrumental neutron activation analysis has
been used to determine Al, Na, Mn, Sc, Cr, Fe, Co, and Ni in a
suite of more than 500 chondrules from 26 distinct meteorites. These
meteorite specimens represent the H, L, LL and C chemical classes
and most of the petrologic types. In addition radiochemical activation
analysis has been used to determine K, Rb and Cs in a suite of chondrules
from the LL chemical class. To a limited extent trace element
data on individual chondrules were correlated with petrographic observations
on the same chondrules.
I. In general it was found that the siderophilic elements Fe,
Co, Ni and Ir are depleted in chondrules compared to the whole chondrite.
The lithophilic elements Na, Mn, Sc, Cr and Al are generally
enriched in chondrules compared to the whole chondrite. In contrast
to the other lithophilic elements, Rb and Cs are generally depleted in
the chondrules. Both major and trace elements may exhibit multimodal
population distributions for the chondrules sets.
II. Petrographic observations of the same chondrules for
which trace element contents were determined by INAA suggest that
the trace element distribution may be consistent with the mineral
assemblages except for a positive Ir-Al and Al-Sc correlations which
occur in many chondrule sets. Correlations between chondrule mass
and Al, Na, Sc, Co, Lr and Cu contents were observed for certain
III. Chondrules from the H and LL groups appear to exhibit
consistent compositional variations in going from low petrographic
grades to high petrographic grades. The variations are observed most
readily for an increasing Al-Na correlation coefficients and decreasing
Mn-Na correlation coefficients with increasing petrologic types. A
decrease in the dispersion of Mn and Na was also observed with increasing
petrologic types. Na contents in the chondrules increase as a
function of petrologic type. These observations are interpreted as
indications of increasing equilibration of the chondrules with their
IV. It appears that there are slight chemical differences between
the Vigarano and Ornans subgroups as defined by Van Schmus (1969).
This work supports the conclusions of Van Schmus and Wood (1967)
and Van Schmus (1969) that the C2 and C3 groups are not generally
related to one another by thermal equilibration processes while
Karoonda may be a product of thermal equilibration of material similar
to the Ornans subgroup.
V. The Ni/Co ratio was found to be variable within chondrules
from the same meteorite; for example, the range in Ochansk chondrules
is from 10 to 60.
VI. Theories concerning the origin of chondrules are discussed
in the context of the elemental abundances and correlations observed
in this study.
a. Some chondrules may be produced by volcanism or impact
on a homogeneous magma but they are not believed to be the main
mechanism of production due to the chondrule inhomogeneity and the
Al-Sc and Al-Ir correlations.
b. The constrained equilibrium theory appears to be inconsistent
with the positive Al-Ir and Al-Sc correlations and with the mass element
correlations observed for Cu, Al, Sc and Ir.
c. Some chondrules may have been produced by impact onto solid
rock but this mechanism does not appear to be able to produce the
necessary chemical fractionation observed in some chondrules.
d. The chemical data is consistent with the remelting of preexisting
dust in the solar nebula. The remelting appears to have
occurred during terminal stages of the metal silicate fractionation or
by a preferential melting of silicate material. The chondrules produced
in any one event were apparently mixed with chondrules from
other events and finally incorporated into the parent body. Additional
chondrules may have been produced by impact during the terminal
stages of accretion. The chemical evidence then suggests that some
of the chondrules equilibrated with the matrix material of their parent