Graduate Thesis Or Dissertation
 

The chemical composition of 61 individual Allende chondrules

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  • 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).
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