Noble gas composition, cosmic‐ray exposure age, 39Ar‐40Ar, and I‐Xe analyses of ungrouped achondrite NWA 7325. (12th March 2018)

• Record Type: Journal Article
• Authors: Hopp, Jens; Schröter, Natalie; Pravdivtseva, Olga; Meyer, Hans‐Peter; Trieloff, Mario; Ott, Ulrich
• Is Part Of: Meteoritics & planetary science. Volume 53:Number 6(2018)
• Page Start: 1150
• Page End: 1163
• Publication Date: 2018-03-12
• Subjects: Meteorites -- Periodicals; Planetology -- Periodicals; 523.4
• DOI: 10.1111/maps.13062
• Languages: English
• ISSNs: 1086-9379

Abstract

Abstract: Northwest Africa (NWA) 7325 is an anomalous achondrite that experienced episodes of large‐degree melt extraction and interaction with melt under reducing conditions. Its composition led to speculations about a Mercurian origin and provoked a series of studies of this meteorite. We present the noble gas composition, and results of 40 Ar/ 39 Ar and 129 I‐ 129 Xe studies of whole rock splits of NWA 7325. The light noble gases are dominated by cosmogenic isotopes. 21 Ne and 38 Ar cosmic‐ray exposure ages are 25.6 and 18.9 Ma, respectively, when calculated with a nominal whole rock composition. This 38 Ar age is in reasonable agreement with a cosmic‐ray exposure age of 17.5 Ma derived in our 40 Ar/ 39 Ar dating study. Due to the low K‐content of 19 ± 1 ppm and high Ca‐content of approximately 12.40 ± 0.15 wt%, no reliable 40 Ar/ 39 Ar age could be determined. The integrated age strongly depends on the choice of an initial 40 Ar/ 36 Ar ratio. An air‐like component is dominant in lower temperature extractions and assuming air 40 Ar/ 36 Ar for the trapped component results in a calculated integrated age of 3200 ± 260 (1σ) Ma. This may represent the upper age limit for a major reheating event affecting the K‐Ar system. Results of 129 I‐ 129 Xe dating give no useful chronological information, i.e., no isochron is observed. Considering the highest 129 Xe*/ 128 XeI ratio as equivalent to a lower age limit, we calculate an I‐Xe age of about 4536 Ma. In addition, elevated 129 Xe/ 132 Xe ratios of up to 1.65 ± 0.18 in higher temperature extractions indicate an early formation of NWA 7325, with subsequent disturbance of the I‐Xe system.