Assessing the Effect of Aqueous Alteration on Soluble Organic Matter in Primitive Carbonaceous Chondrites: The Case of Asuka 12236

Carbonaceous chondrites (CCs) are considered the closest solid material to the composition of the early Solar System and can provide information about the origin of organic matter (OM) in the system and on Earth. Despite being primitive, their parent bodies have undergone secondary processes shortly after their formation (hydrothermal or metamorphism alterations) that blur their initial organic or mineral composition. The most primitive CC members are hence very precious to study the original composition. Here, we report the characterization of the soluble organic matter (SOM) of Asuka 12236, classified among the most primitive members of the CM group in comparison with the SOM from other CM chondrites: Paris, Murchison, and Aguas Zarcas. Analysis of these SOM were performed using several analytical techniques, including elemental analysis, high-resolution mass spectrometry, and liquid chromatography coupled to different mass spectrometers. Our results show that Asuka 12236 has the lowest H content among the CMs studied here and presents heterogeneity in its C content, likely indicating variable degrees of aqueous alteration. High-resolution mass spectrometry analysis reveals that Asuka 12236 has a molecular diversity similar to the SOM of Murchison and Aguas Zarcas but with a higher abundance of sulfur compounds, more similar to Paris SOM. The content of organomagnesium compounds indicates no high-pressure or -temperature stress but rather mid-temperature aqueous alteration for Asuka 12236, like Murchison SOM, but unlike Paris or Aguas Zarcas SOM. From liquid chromatography coupled with mass spectrometry analysis, we observed the highest concentration of amino acids in Asuka 12236 compared to the other CM chondrites considered here. Amino acids are mainly of α forms, witnessing a Strecker formation or a formose-type reaction under low amounts of water, in agreement with the low H content recorded in this peculiar chondrite. This study of the SOM of chondrites with different degrees of alteration highlights that organic compounds may be transformed much more rapidly than the minerals during hydrothermal alteration. This OM evolution depends on the amount of water and/or the temperature and/or pressure, leading toward significant transformation from one chondrite to another, while the mineralogy can indicate a low degree of alteration. In addition, to better assess the initial OM in chondrites, the impact of aqueous alteration on organic matter calls for additional laboratory experimentation.