Elemental and isotope behavior of macromolecular organic matter from CM chondrites during hydrous pyrolysis

Abstract

A new insight into carbon and hydrogen isotope variations of insoluble organic matter (IOM) is provided from seven CM chondrites, including Murchison and six Antarctic meteorites (Y-791198, Y-793321, A-881280, A-881334, A-881458 and B-7904) as well as Murchison IOM residues after hydrous pyrolysis at 270-330 °C for 72 h. Isotopic compositions of bulk carbon (δ 13Cbulk) and hydrogen (δD) of the seven IOMs vary widely, ranging from -15.1 to -7.6%o and +133 to+986%o, respectively. Intramolecular carboxyl carbon (δ13CCOOH) is more enriched in 13C by 7.5 -11%o than bulk carbon. After hydrous pyrolysis of Murchison IOM at 330 °C, H/C ratio, δ13C bulk, δ13CCOOH, and δD values decrease by up to 0.31,3.5%o, 5.5%o, and 961 %o, respectively. The O/C ratio increases from 0.22 to 0.46 at 270 °C and to 0.25 at 300 °C, and decreases to 0.10 at 330 °C. δ13Cbulk- δD cross plot of Murchison IOM and its pyrolysis residues shows an isotopic sequence. Of the six Antarctic IOMs, A-881280, A-881458, Y-791198 and B-7904 lie on or near the isotopic sequence depending on the degree of hydrous and/or thermal alteration, while A-881334 and Y-793321 consist of another distinct isotope group. A δ13Cbulk-δ13C COOH cross-plot of IOMs, including Murchison pyrolysis residues, has a positive correlation between them, implying that the oxidation process to produce carboxyls is similar among all IOMs. These isotope distributions reflect various degree of alteration on the meteorite parent bodies and/or difference in original isotopic compositions before the parent body processes. © The Meteoritical Society, 2009.