- The abundances of 12 elements, Fe, A1, Na, Ni, Cr, Mn,
Co, Ca, Sc, Ir, Sm and Eu have been determined by INAA in 61
individual chondrules (0.1-100 mg) separated from five Allende (C3)
meteoritic fragments ( 25g). Ti and V have been determined in 18
of these chondrules, and Au in several others.
Average abundances of Al, Na, Sc, Sm and Eu are decidedly
higher in chondrules compared to the matrix. The enrichments are
(average % in chondrules/average % in bulk): A1, 2.6-3.4/1.6;
Na, 0.61-0.74/0.33; Sc, 20.2-21.5/11.0; and Sm, 0.50-0.61/
0.29. Determinations of Eu abundances are incomplete at this time.
Abundances for Sc and Sm are in ppm. Cr and Ir abundances
appear to be about the same in both chondrules and matrix; the corresponding
abundances are: Cr, 0.43-0.44/0.38; and Ir, 0.59-
0.60 ppm/0.71 ppm. Mn, 0.11-0.14/0.17 is slightly depleted and
Fe, (9.9-10.3/26.6); Ni, (0.64-0.82)/1.53 and Co, (260-330)
ppm/640 ppm are markedly depleted. The low Fe content of
chondrules is consistent with a much lower Fe²⁺ content of the
olivine in chondrules relative to the matrix. The range of averages
for chondrules given above represents average abundances by the
normal average value and by mass weighted average value; in each
case the mass weighted average was the largest. Since there is a
definite mass-abundance correlation in the selected chondrules and
larger chondrules were preferred in the selection process, the
averages calculated by both methods represent reasonable upper and
lower limits on the abundances found in chondrules.
The above averages conceal the tremendous variation in abundances
among the individual chondrules. Ranges observed are:
% A1, 0.44-17.3; % Fe, 0.37-17.7; % Na, 0.11-2.04; % Cr,
0.02 -1.15; % Ni, 0.065-2.24; % Ca, 0.45-19.7; % Ti, 0.07-1.25;
ppm V, 81-974; ppm Co, 7.6-877; ppm Ir, 0.035-5.8; ppm Mn,
240-3820; ppm Sc, 5.6-133; ppm Sm, .04-3.4. Abundances of
Ni, Ir and Sm in some chondrules were depleted beyond detection
and below the lower values indicated above. The ranges for Ti
and V are for only 18 samples.
Despite a bewildering array of compositions and a large variation
in abundances, the elements Ni and Co exhibit a remarkable
correlation of abundances, yielding a correlation coefficient of about unity. The resulting Ni/Co abundance ratio is 22.6, fairly close to
the ratio of 21 found in fine octahedrites.
In 7 of the 8 chondrules for which Eu has been determined at
this time, the Sm/Eu ratios are closely scattered, within statistical
error, about a mean of 2.6 (one exhibits a ratio of 5.0 ± 1.8). The
average ratio of 2.7 agrees with the ratio found for the bulk Allende,
as well as that for ordinary chondrites, both 2.7. Two explanations
of the constancy of the Sm/Eu ratio in the face of such drastic elemental
fractionations among the chondrules are possible. Either a
magmatic differentiation process could have occurred under oxidizing
conditions, and all Eu was present as trivalent Eu or the accretion
of the chondrules was very rapid, volatile constituents were lost,
and no magmatic differentiation occurred.
Only slight correlations among Na and A1, as well as Na and
Ca are found, indicating a much more complex relationship among
these elements than would be encountered with a simple plagioclase
mineral system. Much of the sodium in chondrules is known to occur
in the mineral nephiline, as well as sodalite. The albite content of
the plagioclase in these chondrules is generally very small (Ab₅ being
typical). Calcium is present largely in gehlenite, anorthite, and
fassaite (augite with high Ti).