Molecular distribution of monocarboxylic acids in Asuka carbonaceous chondrites from Antarctica

Molecular distribution of low-molecular-weight monocarboxylic acids was stu died in three CM2 Asuka carbonaceous chondrites (A-881280, A-881334 and A-8 81358), which were recovered from Antarctica by the 29th Japanese Antarctic Research Expedition in 1988. GC and GC/MS analyses identified more than 30 monocarboxylic acids in A-881458, including aliphatic and aromatic acids w ith various structural isomers, Isomeric phenolic compounds were also ident ified. The aliphatic carboxylic acids have straight-chain structures having 2 to 12 carbon atoms (C-2 to C-12), and branched-chain structures (C-4 to C-9). The quantities of straight-chain acids decrease logarithmically with increasing carbon number. At the same carbon number, a straight-chain isome r is always predominant compared to branched-chain isomers, All of the 14 p ossible C-4, C-5 and C-6 aliphatic monocarboxylic acids (not including opti cal isomers) have been identified, although all the isomers were not report ed in Murchison and Y-791198 meteorites. Of the 17 possible isomeric C-7 ac ids, at least 14 isomers were tentatively identified by mass spectra (EI an d CI mode). At C-8 or above, peaks of branched-chain isomers become obscure , probably due to the large number of isomers and small concentrations. Bra nched-chain monocarboxylic acids over C-6 have never been reported in Murch ison, Although occurrence of aliphatic acids are similar between A-881458 a nd Murchison at C-4, C-5 and C-6 acids, a major difference is that A-881458 as well as Y-791198 have straight-chain predominance among isomers in cont rast to Murchison bring branched-chain predominant. In the case of isomeric aromatic compounds such as toluic acids and cresols, m-toluic acid and p-c resol are more abundant among their isomers, respectively. The molecular di stribution may not reflect thermodynamic equilibrium but rather a kinetical ly controlled process for their formation mechanism. The other two CM2 chon drites (A-881280 and A-881334) were depleted in carboxylic acids in spire o f similar carbon contents. The depletion is not due to weathering on ice, b ecause the degrees of weathering are small and similar among the three chon drites. Probably those latter two chondrites may have been subjected to aqu eous alteration or metamorphism on their meteorite parent bodies.