Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta meteorite

Abstract

This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon‐rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2–2.3 wt% carbon; most showed the major carbon release at temperatures of 600–700 °C with peak values of δ13C from −7.3 to +0.4‰, similar to literature values for unbrecciated (“monomict”) ureilites. They also contained a minor low temperature (≤500 °C) component (δ13C = ca −25‰). Bulk nitrogen contents (9.4–27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600–750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release δ15N values of −53 to −94‰ fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen‐rich and isotopically heavier. Thus, although the parent asteroid 2008TC3 was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ13C and δ15N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources.

Organic matter in carbonaceous chondrite lithologies of Almahata Sitta: Incorporation of previously unsampled carbonaceous chondrite lithologies into ureilitic regolith

“…OM. The elemental and isotopic compositions of both AhS 671 and AhS 91A are clearly distinguished from those ureilitic lithologies of AhS (Downes et al 2015) (Fig. 1), consistent with the fact that AhS 671 and AhS 91A are xenoliths and the amount of ureilitic material in these lithologies is small (Goodrich et al 2019).…”

Nitrogen and carbon fractionation during core–mantle differentiation at shallow depth

“…An alternative scenario explored by is that a fraction of the late veneer consisted of differentiated bodies with high C:N ratios. Examples of such objects include CO and CV chondrites from the compilation of Kerridge (1985) (C/N=129±53 and 136±54), H ordinary chondrites compiled by Bergin et al 2015 (C/N=390±250), and ureilite achondrites (C/N=1000±600; Downes et al 2015). As a demonstration of the potential influence of a C-rich late veneer, we calculate the effect of accreting a mixture consisting of a material with a C:N ratio similar to H ordinary chondrites (C/N = 390; white diamonds on Fig.…”

“…7). Two compositions of the "C-rich" late veneer are calculated: the red diamonds show the results for late veneer composed of a mixture of 30% of a material with a C:N ratio (23) similar to CI and 70% of a material with a C:N ratio (1000) similar to ureilite (Downes et al 2015) with 30,000 ppm C and 30 ppm N; the white diamonds show the results for a late veneer made of a material with a C:N ratio equal to 390, similar to the H ordinary chondrites (CH).…”

Origin and Early Differentiation of Carbon and Associated Life-Essential Volatile Elements on Earth

“…Among all stony meteorites, ureilites are the only group of primitive achondrites that are rich in C and depleted in N, leading to higher C/N ratios. [101][102][103] Carbon in ureilites is also present in relatively non-labile phases (i.e. as graphites, nanodiamonds, and lonsdaleite) and hence is likely to survive high-temperature accretional processes.…”