NORMAL ALKANES IN METEORITES: MOLECULAR δ13C VALUES INDICATE AN ORIGIN BY TERRESTRIAL CONTAMINATION

Abstract

In the time period since 1961, n-alkanes (straight-chain hydrocarbons) have been detected in varying amounts in many meteorites. Proposed origins for these compounds have included extraterrestrial biotic, extraterrestrial abiotic and terrestrial contamination processes. To help establish the source of these compounds, we determined the carbon isotopic compositions of individual n-alkanes in meteorites of several different classes and terrestrial histories: Orgueil (CI1), Cold Bokkeveld (CM2), Murchison (CM2), Vigarano (CV3), Allende (CV3), Ornans (CO3), and Bishunpur (LL3). The efficiency of supercritical fluid extraction (SFE) was exploited to provide extractable non-polar organic matter from the meteorites. The n-alkanes in these extracts were then identified with gas chromatography-mass spectrometry (GC-MS) and the carbon isotopic composition of individual molecules was determined using isotope ratio monitoring-gas chromatography-mass spectrometry (irm-GC-MS). n-Alkanes were found in all but one of the meteorites analysed (Allende), but carbon number distributions varied between samples. Pristane and Phytane were also detected in five of the seven meteorites. δ13C values for the individual n-alkanes occupied a range from -25.3 to -38.7%%. The δ13C values for the meteoritic n-alkanes have a similar range as those for n-alkanes measured from petroleum in the literature. Therefore, the n-alkanes in meteorites appear to be terrestrial contaminants which may have originated from fossil hydrocarbons or petroleum products. This type of contamination is analogous to that which will be threatening future meteorite falls and the samples returned from spaceflight missions over the next two decades and beyond. For falls already contaminated, irm-GC-MS appears useful in discriminating between indigenous compounds and those introduced by terrestrial contamination.