Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta meteorite. (22nd January 2015)

• Record Type: Journal Article
• Authors: Downes, H.; Abernethy, F. A. J.; Smith, C. L.; Ross, A. J.; Verchovsky, A. B.; Grady, M. M.; Jenniskens, P.; Shaddad, M. H.
• Is Part Of: Meteoritics & planetary science. Volume 50:Number 2(2015:Feb.)
• Page Start: 255
• Page End: 272
• Publication Date: 2015-01-22
• Subjects: Meteorites -- Periodicals; Planetology -- Periodicals; 523.4
• DOI: 10.1111/maps.12413
• Languages: English
• ISSNs: 1086-9379

Abstract

<abstract abstract-type="main" id="maps12413-abs-0001"> <title>Abstract</title> <p>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 δ<sup>13</sup>C from −7.3 to +0.4‰, similar to literature values for unbrecciated ("monomict") ureilites. They also contained a minor low temperature (≤500 °C) component (δ<sup>13</sup>C = 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 δ<sup>15</sup>N 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 2008TC<sub>3</sub> was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ<sup>13</sup>C and δ<sup>15</sup>N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources.</p> </abstract>