Abstract
A novel method has been developed for compound-specific nitrogen isotope compositions with an achiral column which was previously shown to offer high precision for nitrogen isotopic analysis. We applied the method to determine the amino acid contents and stable nitrogen isotopic compositions of individual amino acids from the thermally metamorphosed (above 500 °C) Antarctic carbonaceous chondrites Ivuna-like (CI)1 (or CI-like) Yamato (Y) 980115 and Ornans-like (CO)3.5 Allan Hills (ALH) A77003 with the use of gas chromatography/combustion/isotope ratio mass spectrometry. ALHA77003 was deprived of amino acids due to its extended thermal alteration history. Amino acids were unambiguously identified in Y-980115, and the δ15N values of selected amino acids (glycine +144.8 ‰; α-alanine +121.2 ‰) are clearly extraterrestrial. Y-980115 has experienced an extended period of aqueous alteration as indicated by the presence of hydrous mineral phases. It has also been exposed to at least one post-hydration short-lived thermal metamorphism. Glycine and alanine were possibly produced shortly after the accretion event of the asteroid parent body during the course of an extensive aqueous alteration event and have abstained from the short-term post-aqueous alteration heating due to the heterogeneity of the parent body composition and porosity. These carbonaceous chondrite samples are good analogs that offer important insights into the target asteroid Ryugu of the Hayabusa-2 mission, which is a C-type asteroid likely composed of heterogeneous materials including hydrated and dehydrated minerals.
Highlights
Carbonaceous chondrites are primitive solar nebular aggregates that did not experience extensive planetary formation processes
We have compared the δ15N values for amino acid standards determined by GC/C/IRMS to elemental analyzer (EA)/IRMS independently (Fig. 2b)
The linearity of the observed δ15N values supports a high precision and repeatability of the isotopic analysis we present in this study (R2 = 0.9995)
Summary
Carbonaceous chondrites are primitive solar nebular aggregates that did not experience extensive planetary formation processes. They contain organic carbon materials that are soluble in common organic solvents. Carbonaceous chondrites may provide clues to the abiotic chemical evolution processes that predate and promote the onset of life and offer important insights into the target asteroids of the sample return missions Hayabusa-2 and OSIRIS-REx, which aim to collect and return to Earth samples from the surfaces of the C-type asteroids. Among the wealth of organic materials in carbonaceous chondrites, amino acids demonstrate a crucial significance on chemical evolution as they are the monomers of protein and enzymes that are indispensable to life on Earth. Over 80 amino acids have been found in the Mighei-like (CM) chondrites Murchison and Murray, and a significant portion of these amino acids are rarely found to occur in biological context, such as α-aminoisobutyric acid (α-AIB) and isovaline
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