Kinetics of organic matter degradation in the Murchison meteorite for the evaluation of parent-body temperature history

Abstract

Abstract– To evaluate kinetic parameters for thermal degradation of organic matter, in situ heating experiments of insoluble organic matter (IOM) and bulk of Murchison (CM2) meteorite were conducted under Fourier transform infrared micro-spectroscopy combined with a heating stage. Decreases of aliphatic C–H band area under Ar flow were well fitted with Ginstling-Brounshtein three-dimensional diffusion model, and the rate constants for decreases of aliphatic C–H were determined. Activation energies Ea and frequency factors A obtained from these rate constants at different temperatures using the Arrhenius equation were Ea = 109 ± 3 kJ mol−1 and A = 8.7 × 104 s−1 for IOM, and Ea = 61 ± 6 kJ mol−1 and A = 3.8 s−1 for bulk, respectively. Activation energy values of aliphatic C–H decrease are larger for IOM than bulk. Hence, the mineral assemblage of the Murchison meteorite might have catalytic effects for the organic matter degradation. Using obtained kinetic expressions, the time scale for metamorphism can be estimated for a given temperature with aliphatic C–H band area, or the temperature of metamorphism can be estimated for a given time scale. For example, using the obtained kinetic parameters of IOM, aliphatic C–H is lost approximately within 200 years at 100 °C and 100 Myr at 0 °C. Assuming alteration period of 7.5 Myr, alteration temperatures could be calculated to be <15 ± 12 °C. Aliphatic C–H decrease profiles in a parent body can be estimated using time–temperature history model. The kinetic expression obtained by the infrared spectral band of aliphatic C–H could be used as an alternative method to evaluate thermal processes of organic matter in carbonaceous chondrites.