Nitrogen components in ureilites

Abstract

Abundances and isotopic compositions of nitrogen and argon have been investigated in bulk samples as well as in acid-resistant C-rich residues of a suite of ureilites consisting of six monomict (Haverö, Kenna, Lahrauli, ALH81101, ALH82130, LEW85328), three polymict (Nilpena, EET87720, EET83309), and the diamond-free ureilite ALH78019. Nitrogen in bulk ureilites varies from 6.3 ppm (in ALH 78019) to ∼55 ppm (in ALH82130), whereas C-rich acid residues have ∼65 to ∼530 ppm N, showing approximately an order of magnitude enrichment, compared with the bulk ureilites, somewhat less than trapped noble gases. Unlike trapped noble gases that show uniform isotopic composition, nitrogen shows a wide variation in δ 15N values within a given ureilite as well as among different ureilites. The variations observed in δ 15N among the ureilites studied here suggest the presence of at least five nitrogen components. The characteristics of these five N components and their carrier phases have been identified through their release temperature during pyrolysis and combustion, their association with trapped noble gases, and their carbon (monitored as CO + CO 2 generated during combustion). Carrier phases are as follows: 1) Amorphous C, as found in diamond-free ureilite ALH78019, combusting at ≤500°C, with δ 15N = –21‰ and accompanied by trapped noble gases. Amorphous C in all diamond-bearing ureilites has evolved from this primary component through almost complete loss of noble gases, but only partial N loss, leading to variable enrichments in 15N. 2) Amorphous C as found in EET83309, with similar release characteristics as component 1, δ 15N ≥ 50‰ and associated with trapped noble gases. 3) Graphite, as clearly seen in ALH78019, combusting at ≥700°C, δ 15N ≥ 19‰ and devoid of noble gases. 4) Diamond, combusting at 600–800°C, δ 15N ≤ –100‰ and accompanied by trapped noble gases. 5) Acid-soluble phases (silicates and metal) as inferred from mass balance are expected to contain a large proportion of nitrogen (18 to 75%) with δ 15N in the range –25‰ to 600‰. Each of the ureilites contains at least three N components carried by acid-resistant C phases (amorphous C of type 1 or 2, graphite, and diamond) and one acid-soluble phase in different proportions, resulting in the observed heterogeneity in δ 15N. In addition to these five widespread components, EET83309 needs an additional sixth N component carried by a C phase, combusting at <700°C, with δ 15N ≥ 153‰ and accompanied by noble gases. It could be either noble gas–bearing graphite or more likely cohenite. Some excursions in the δ 15N release patterns of polymict ureilites are suggestive of contributions from foreign clasts that might be present in them. Nitrogen isotopic systematics of EET83309 clearly confirm the absence of diamond in this polymict ureilite, whereas the presence of diamond is clearly indicated for ALH82130. Amorphous C in ALH78019 exhibits close similarities to phase Q of chondrites. The uniform δ 15N value of −113 ± 13 ‰ for diamond from both monomict and polymict ureilites and its independence from bulk ureilite δ 15N, Δ 17O, and %Fo clearly suggest that the occurrence of diamond in ureilites is not a consequence of parent body–related process. The large differences between the δ 15N of diamond and other C phases among ureilites do not favor in situ shock conversion of graphite or amorphous C into diamond. A nebular origin for diamond as well as the other C phases is most favored by these data. Also the preservation of the nitrogen isotopic heterogeneity among the carbon phases and the silicates will be more consistent with ureilite formation models akin to “nebular sedimentation” than to “magmatic” type.