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

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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°C, O-bearing compounds mainly appeared at all pyrolysis temperatures, and hydrogen gas mainly appeared in a range of 550–800°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°C. Regarding the calculated isotopic data of the Allende pyrolysates, the δ¹³C value was almost constant for all the pyrolysis temperatures. The δ¹⁵N value was constant up to 550°C and then drastically decreased at 550–800°C. The δD values of the pyrolysates at all pyrolysis temperatures were lower than the δD 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°C that caused the loss of primary materials (aliphatic and aromatic hydrocarbons, N-bearing compounds) enriched in ¹³C, D and ¹⁵N, and the second stage was a thermal event below 300°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°C), it was proposed that the Allende meteorite experienced thermal metamorphism at 550–590°C followed by an alteration below 300°C.