Organic compounds in meteorites—I. Aliphatic hydrocarbons

Abstract

The benzene-methanol extracts of thirty meteorites (carbonaceous chondrites, noncarbonaceous chondrites and graphite-troilite or troilite nodules from iron meteorites) were fractionated by silica gel chromatography and the aliphatic (n-heptane eluate) hydrocarbon fractions were analyzed by gas chromatography and gas chromatography-mass spectrometry. A study of the chromatograms from the n-heptane eluates permits the following conclusions: (1) Aliphatic hydrocarbons (predominantly paraffinic) were found in varying amounts in all the meteorites studied. The hydrocarbons ranged from 0.1 to 25.8 ppm for Type I, 8.3 to 150 ppm for Type II, 8.2 to 415 ppm for Type III, 0.1 to 3.2 ppm for the non-carbonaceous chondrites, and 2.8 to 15.5 ppm for the graphitic nodules. (2) Peaks corresponding to norpristane, pristane and phytane (in addition to normal and isomeric alkanes) were observed in practically all of the meteorites. Mass spectrometric evidence for the latter two isoprenoids was obtained in at least five different carbonaceous chondrites and one graphitic-troilitic nodule. Relatively low levels of pristane and phytane were found in the Type I carbonaceous chondrites and the non-carbonaceous chondrites. (3) No general predominance of odd over even carbon-number alkanes in the range analyzed was observed except for one sample of Orgueil in the n-C 22-n-C 27 range. An examination of the lower molecular weight alkanes in a few meteorite samples (Orgueil and Mokoia) showed a predominance of normal pentadecane over normal hexadecane. The hydrocarbons in three meteorites (Ivuna, Alais and Warrenton) had bimodal distributions indicating two syntheses or sources for the hydrocarbons. (4) Analysis of inside samples and samples from different sources showed the distribution of aliphatic hydrocarbons in Orgueil to be heterogeneous qualitatively and quantitatively. (5) Identical patterns were not observed within, or between, Wiik Types I and II. However, most of the meteorites of Type II were similar qualitatively in the n-C 15-n-C 21 range. In addition, a few Type III meteorites were similar qualitatively, in this range, to the meteorites of Type II. (6) The majority of the hydrocarbon patterns of the meteorites analyzed were similar, in the low molecular weight range, to chromatographic patterns of ancient sediments, crude oil, terrestrial graphite and other terrestrial samples. (7) The chromatographic patterns of the aliphatic hydrocarbons from the graphitic-troilitic nodules of iron meteorites were similar to those of Type II and some of Type I (Orgueil) carbonaceous chondrites. The external part of the nodules was found to contain 5 times more hydrocarbons than the inside suggesting either an external source of the hydrocarbons or that the hydrocarbons originally inside were partially destroyed or consumed.