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 degrees C for 72 h. Isotopic compositions of bulk carbon (delta-13Cbulk) and hydrogen (delta-D) of the seven IOMs vary widely, ranging from 15.1 to 7.6 and +133 to +986 per mil, respectively. Intramolecular carboxyl carbon (delta-13CCOOH) is more enriched in 13C by 7.5 -11 per mil than bulk carbon. After hydrous pyrolysis of Murchison IOM at 330 degrees degrees C, H/C ratio, delta-13Cbulk, delta-13CCOOH, and delta-D values decrease by up to 0.31, 3.5 per mil, 5.5 per mil, and 961 per mil, respectively. The O/C ratio increases from 0.22 to 0.46 at 270 degrees C and to 0.25 at 300 degrees degrees C, and decreases to 0.10 at 330 degrees C. delta-13Cbulk-delta-D 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 delta-13Cbulk-delta-13CCOOH 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.