Constraints on the thermal history of the Allende (CV3) meteorite by gradual and stepwise pyrolyses of insoluble organic matter

To reveal the thermal history of the Allende (CV3) meteorite, we pyrolyzed insoluble organic matter (IOM) from the meteorite and examined the chemical and isotopic composition of the pyrolysates against the pyrolysis temperature. Major pyrolysates from the Allende IOM were sulfur (S)-bearing compounds (H2S, SO2, CS2, and OCS), oxygen (O)-bearing compounds (H2O, CO, and CO2), and hydrogen gas. S-bearing compounds mainly appeared in a pyrolysis temperature range of 250-300 degrees C, O-bearing compounds mainly appeared at all pyrolysis temperatures, and hydrogen gas mainly appeared in a range of 550-800 degrees C. The IOM scarcely released aliphatic and aromatic hydrocarbons and nitrogen (N)-bearing compounds, although they were major pyrolysates of the Murchison IOM at 450-550 degrees C. Regarding the calculated isotopic data of the Allende pyrolysates, the delta C-13 value was almost constant for all the pyrolysis temperatures. The delta N-15 value was constant up to 550 degrees C and then drastically decreased at 550-800 degrees C. The delta D values of the pyrolysates at all pyrolysis temperatures were lower than the delta D values of the starting IOM. If we assumed that the Allende IOM originated from a primitive IOM (such as the Murchison IOM), our results suggested that the Allende IOM suffered a two-stage thermal process: the first stage was a thermal event below 550-800 degrees C that caused the loss of primary materials (aliphatic and aromatic hydrocarbons, N-bearing compounds) enriched in C-13, D and N-15, and the second stage was a thermal event below 300 degrees C that led to the addition of secondary S- and O-bearing compounds depleted in D. Based on our data and previously reported data (the peak metamorphic temperature of the Allende IOM is 550-590 degrees C), it was proposed that the Allende meteorite experienced thermal metamorphism at 550-590 degrees C followed by an alteration below 300 degrees C.