A novel organic-rich meteoritic clast from the outer solar system

The Zag meteorite which is a thermally-metamorphosed H ordinary chondrite contains a primitive xenolithic clast that was accreted to the parent asteroid after metamorphism. The cm-sized clast contains abundant large organic grains or aggregates up to 20 μm in phyllosilicate-rich matrix. Here we report organic and isotope analyses of a large (~10 μm) OM aggregate in the Zag clast. The X-ray micro-spectroscopic technique revealed that the OM aggregate has sp² dominated hydrocarbon networks with a lower abundance of heteroatoms than in IOM from primitive (CI,CM,CR) carbonaceous chondrites, and thus it is distinguished from most of the OM in carbonaceous meteorites. The OM aggregate has high D/H and ¹⁵N/¹⁴N ratios (δD = 2,370 ± 74‰ and δ¹⁵N = 696 ± 100‰), suggesting that it originated in a very cold environment such as the interstellar medium or outer region of the solar nebula, while the OM is embedded in carbonate-bearing matrix resulting from aqueous activities. Thus, the high D/H ratio must have been preserved during the extensive late-stage aqueous processing. It indicates that both the OM precursors and the water had high D/H ratios. Combined with ¹⁶O-poor nature of the clast, the OM aggregate and the clast are unique among known chondrite groups. We further propose that the clast possibly originated from D/P type asteroids or trans-Neptunian Objects.