Evidence for an early nitrogen isotopic evolution in the solar nebula from volatile analyses of a CAI from the CV3 chondrite NWA 8616. (15th March 2015)

• Record Type: Journal Article
• Authors: Füri, Evelyn; Chaussidon, Marc; Marty, Bernard
• Is Part Of: Geochimica et cosmochimica acta. Volume 153(2015:Mar. 15)
• Page Start: 183
• Page End: 201
• Publication Date: 2015-03-15
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.01.004
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Nitrogen and noble gas (Ne–Ar) abundances and isotope ratios, determined by CO2 laser extraction static mass spectrometry analysis, as well as Al–Mg and O isotope data from secondary ion mass spectrometry (SIMS) analyses, are reported for a type B calcium–aluminum-rich inclusion (CAI) from the CV3 chondrite NWA 8616. The high ( 26 Al/ 27 Al) i ratio of (5.06 ± 0.50) × 10 −5 dates the last melting event of the CAI at 39 - 99 + 109 ka after "time zero", limiting the period during which high-temperature exchanges between the CAI and the nebular gas could have occurred to a very short time interval. Partial isotopic exchange with a 16 O-poor reservoir resulted in Δ 17 O > −5‰ for melilite and anorthite, whereas spinel and Al–Ti-pyroxene retain the inferred original 16 O-rich signature of the solar nebula (Δ 17 O ⩽ −20‰). The low 20 Ne/ 22 Ne (⩽0.83) and 36 Ar/ 38 Ar (⩽0.75) ratios of the CAI rule out the presence of any trapped planetary or solar noble gases. Cosmogenic 21 Ne and 38 Ar abundances are consistent with a cosmic ray exposure (CRE) age of ∼14 to 20 Ma, assuming CR fluxes similar to modern ones, without any evidence for pre-irradiation of the CAI before incorporation into the meteorite parent body. Strikingly, the CAI contains 1.4–3.4 ppm N with a δ 15 N value of +8‰ to +30‰. Even after correcting the measured δ 15 N values for cosmogenic 15 N produced in situ, the CAI is highly enriched in 15 N compared to the protosolar nebula (δ 15 NPSN = −383 ± 8‰; Marty et al., 2011), implying that the CAI-forming region was contaminated by 15 N-rich material within the first 0.15 Ma of Solar System history, or, alternatively, that the CAI was ejected into the outer Solar System where it interacted with a 15 N-rich reservoir.