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High-precision magnesium isotope measurements of whole chondrules from the Allende carbonaceous chondrite meteorite show that some aluminum-rich Allende chondrules formed at or near the time of formation of calcium-aluminum-rich inclusions and that some others formed later and incorporated precursors previously enriched in magnesium-26. Chondrule magnesium-25/magnesium-24 correlates with [magnesium]/[aluminum] and size, the aluminum-rich, smaller chondrules being the most enriched in the heavy isotopes of magnesium. These relations imply that high gas pressures prevailed during chondrule formation in the solar nebula.

Abstract-The N and C abundances and isotopic compositions of acid-insoluble carbonaceous material in thirteen primitive chondrites (five unequilibrated ordinary chondrites, three CM chondrites, three enstatite chondrites, a CI chondrite and a CR chondrite) have been measured by stepped combustion. While the range of C isotopic compositions observed is only 413C = 30%0, the N isotopes range from 6I5N = 4 0 to 260%0.After correction for metamorphism, presolar nanodiamonds appear to have made up a fairly constant 3 -4 wt% of the insoluble C in all the chondrites studied. The apparently similar initial presolar nanodiamond to organic C ratios, and the correlations of elemental and isotopic compositions with metamorphic indicators in the ordinary and enstatite chondrites, suggest that the chondrites all accreted similar organic material. This original material probably most closely resembles that now found in Renazzo and Semarkona. These two meteorites have almost M-shaped N isotope release profiles that can be explained most simply by the superposition of two components, one with a composition between 615N = -20 and 40%0 and a narrow combustion interval, the other having a broader release profile and a composition of d15N = 260%0. Although isotopically more subdued, the CI and the three CM chondrites all appear to show vestiges of this M-shaped profile. How and where the components in the acid-insoluble organics formed remains poorly constrained. The small variation in nanodiamond to organic C ratio between the chondrite groups limits the local synthesis of organic matter in the various chondrite formation regions to at most 30%. The most IsN-rich material probably formed in the interstellar medium, and the fraction of organic N in Renazzo in this material ranges from 40 to 70%. The isotopically light component may have formed in the solar system, but the limited range in nanodiamond to total organic C ratios in the chondrite groups is consistent with most of the organic material being presolar. INTRODUCTIONThe majority of C in primitive chondritic meteorites is present as organic material. The synthesis of this material is poorly understood, and none of the formation models proposed thus far have been able to explain all of the observed chemical and isotopic features. The organic material in CI and CM chondrites, the most extensively studied of the chondrite groups, can be broadly divided into a soluble fraction and a much more abundant (70-95%) insoluble fraction (Becker and Epstein, 1982). The soluble fraction is a mixture that includes amino acids as well as aliphatic and aromatic hydrocarbons.The insoluble fraction is a complex macromolecular material, rich in polycyclic aromatic hydrocarbons (PAH), which has often been described as kerogen-like.For many years, the most popular hypothesis for the origin of organic material in CI and CM carbonaceous chondrites was via Fischer-Tropsch-type (FTT) synthesis in the solar nebula (e.g., Hayatsu and Anders, 1981). It was envisaged that, as the nebula cooled to 30WOO K ...