ABSTRACT

Carbonaceous matter from the matrix of the Murchison (CM2) meteorite and Northwest Africa (NWA) 801 (CR2) meteorite that were extremely rich in deuterium (D) and ¹⁵N was studied using in situ isotope imaging. The association of carbon, hydrogen, and nitrogen suggests that the carbonaceous matter was organic in nature, and the maximum magnitudes of D- and ¹⁵N-enrichment in the organic matter were δD = 2,880‰ and δ¹⁵N = 2,590‰ in Murchison, and δD = 7,500‰ and δ¹⁵N = 2,200‰ in NWA 801. The organic matter did not display a simple correlation between the magnitudes of D- and ¹⁵N-enrichment, and the isotopically anomalous organic matter was classified into three types based on the H and N isotopic characteristics: extremely ¹⁵N-rich without large D-enrichment (¹⁵N-rich), extremely D-rich without large ¹⁵N-enrichment (D-rich), and highly D- and ¹⁵N-rich (D-¹⁵N-rich). The occurrence of isotopically anomalous organic matter was attributed to their origin in the molecular clouds and outer solar nebula, ion-molecule reactions at low temperatures, grain surface reactions at low temperatures, and self-shielding effects in gas phase molecules. However, the observed D- and ¹⁵N-enrichment of the organic matter is much smaller than predicted by ion-molecule and grain surface reactions. This suggests a secondary modification of the H and N isotopic compositions of the organic matter in the solar nebula and in the parent body. In Murchison, the proportion of D-rich organic matter among the isotopically anomalous organic matter is smaller than in NWA 801, which presumably indicates that the D-enrichments are easily modified by aqueous alteration processes on the parent body. Scanning electron microscopy revealed the morphology of the D- and ¹⁵N-rich organic matter as aggregates of globules/particles or non-aggregated globules, where the size of each globule was <1 μm. Although there is no obvious correlation between the morphology of the organic matter and the H and N isotopic compositions, the result that Murchison contains more abundant globule/particle aggregates might suggests formation of aggregated nature due to aqueous activity.