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• W57726814 abstract "Presolar S i c has been found in the most primitive members of all classes of chondritic meteorites 11-31. Isotopic studies of bulk S ic samples [4,5] and of individual S ic grains [6-141 have revealed a remarkable isotopic diversity indicating that meteoritic S ic was derived from a number of stellar sources. Important outstanding questions include: 1) is the same mixture of grains from different sources present in all meteorites; and 2) if the mixtures are different, were the differences inherited from the nebula or produced in the meteorite. Ne-E(H) and Xe-S abundances in acid residues suggest that all classes sampled the same mixture of Sic, although this mixture may have been modified during the subsequent history of the meteorites [3]. Single-grain work on Murchison K-series S ic [9,11,12] and S ic from Orgueil [7,13,14] indicates that the mixtures in these meteorites are the same, and an initial study of S ic in UOCs suggests these meteorites also contain the same S ic mixture [lo]. On the other hand, Murchison L-series S i c exhibits unique isotopic systematics [S], and S ic grains from Indarch (EH3-4) and Inman (LL3.3), meteorites that have been metarnorphozed, seem to have a more restricted range of isotopic compositions [7,10]. We have begun an ion-probe study of S i c in various classes of chondritic meteorites. In this abstract we report Si and C isotopic data for 33 individual Sic grains from Semarkona (LL3.0) and 23 grains from Bishunpur (LL3.1) and show that S ic from these meteorites has the same range of isotopic characteristics as Orgueil (CI) S ic and Murchison (CM2) K-series Sic. Silicon isotopic data are shown in Figure 1. All of the Semarkona grains and 20 of 23 Bishunpur grains fall along single arrays on Si 3-isotope plots. The arrays for these meteorites are indistinguishable from those exhibited by S ic from Murchison [11,12] and Orgueil [13,14]. (Figure 1 is a ratio plot so the slope of the array differs from that on a delta plot (-1.34) by the factor 0.050533110.033464, the ratio of the Si isotope ratios). No groupings are evident in the UOC data, consistent with observations from Murchison K-series and Orgueil [I 1-14]. Carbon isotopic data for S ic from Semarkona, Bishunpur, are compared with Orgueil S ic [14] in Figure 2a-c. S ic for all three meteorites shows essentially the same distribution of 12C/13C, which peaks between 40 and 75. Carbon isotopes are generally uncorrelated with Si isotopes. In Bishunpur, two grains were found which are highly enriched in 2*Si (Fig. lb, inset) and 12C (12C113C = 160,373) compared to most Sic. Such grains have been called 'X' grains and are believed to come from supernovae [9]. Another Bishunpur grain is depleted in 2% and enriched in 3O~i, falling well to the right of the main Si array and far below the terrestrial fractionation line (Fig. lb). Its carbon (12C/13C = 59) falls in the middle of the range for 'mainstream' grains. Such grains have been called 'Z' grains [lo] and their origin is not understood. One Semarkona S ic is enriched in 12C (12C113C = 122) and on Figure la falls below the terrestrial fractionation line and slightly to the right of the main Si array. Similar grains with 12c/13c > 150 have been called 'Y' grains [9,12]. Although the Semarkona grain has a lower 12c113c, it exhibits the isotopic signatures of He-burning characteristic of 'Y' grains. The Si and C isotopic compositions of S ic grains from Semarkona (LL3.0) and Bishunpur (LL3.1) display basically the same ranges and distributions observed in Murchison (CM2) Kseries grains and S ic from Orgueil (CI). Our S ic grains were 0.9 to 4 pm in diameter, with an average of -1.7 ym, most similar to Murchison KJE and KJF grains (mean grain sizes 1.14 and 1.86 pm) [ l l ] . (The smaller size of our grains is probably not a true size difference between meteorites but rather is due to the low abundance of Sic, and therefore of large Sic, relative to spinel, etc. in the residues.) Smaller grains tend to have a wider range of isotopic compositions and a somewhat higher percentage of highly anomalous grains than do larger grains [8, 11, 121. The number of grains with extreme carbon compositions and the relative abundances of 'X' (2 of" @default.
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• W57726814 date "1994-03-01" @default.
• W57726814 modified "2023-09-28" @default.
• W57726814 title "Silicon Carbide in Unequilibrated Ordinary Chondrites" @default.
• W57726814 hasPublicationYear "1994" @default.
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