A chemical sequence of macromolecular organic matter in the CM chondrites

A new organic parameter is proposed to show a chemical sequence of organic matter in carbonaceous chondrites, using carbon, hydrogen, and nitrogen concentrations of solvent-insoluble and high-molecular weight organic matter (macromolecules) and the molecular abundance of solvent-extractable organic compounds. The H/C atomic ratio of the macromolecule purified from nine CM chondrites including the Murchison, Sayama, and seven Antarctic meteorites varies widely from 0.11 to 0.72. During the H/C change of ∼0.7 to ∼0.3, the N/C atomic ratio remains at ∼0.04, followed by a sharp decline from ∼0.040 to ∼0.017 between H/C ratios from ∼0.3 to ∼0.1. The H/C-N/C sequence shows different degrees of organic matter thermal alteration among these chondrites in which the smaller H/C-N/C value implies higher alteration levels on the meteorite parent body. In addition, solvent-extractable organic compounds such as amino acids, carboxylic acids, and polycyclic aromatic hydrocarbons are abundant only in chondrites with macromolecular H/C values >∼0.5. These organic compounds were extremely depleted in the chondrites with a macromolecular H/C value of <∼0.5. Possibly, most solvent-extractable organic compounds could have been lost during the thermal alteration event that caused the H/C ratio of the macromolecule to fall below 0.4.