Isotopic evolution of nitrogen and trapped xenon in the Acapulco parent body

Details Isotopic evolution of nitrogen and trapped xenon in the Acapulco parent body Isotopic evolution of nitrogen and trapped xenon in the Acapulco parent body

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.482k&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 482-483

Computer Science

5

Abundance, Crystallization, Inclusions, Meteoritic Composition, Nitrogen Isotopes, Pressure Effects, Rare Gases, Silicates, Troilite

Scientific paper

The Acapulco meteorite has two isotopically different N components. Most of the light N is carried by metal, while heavy N is observed in silicates and in troilite. Some exchange has taken place, as the metal surfaces reveal the presence of minor heavy N components. The micrometer-sized metal inclusion in orthopyroxene shows that N exchanged with the orthopyroxene host, but that the core regions retained the original light N signature. The N isotopic signature of chromite also reflects exchange and isotopic evolution of N during the early history within the Acapulco parent body. Therefore, metal and chromite represent useful monitors of early evolutionary histories of parent bodies. The carriers of N and of trapped heavy noble gas in Acapulco are distinct. Trapped gases in Acapulco are enriched in the 'dusty' orthopyroxene crystals, associated with metallic inclusions. We found that about half of the trapped noble gases are lost during crushing. This shows that in fact trapped gas is present in 'gas bubbles' that coexist with metal blebs in the 'dusty' silicates. The isotopic abundances agree within error limits with OC-Xe except for the enrichment of Xe-129. The evolved Xe-129 in the trapped Xe requires partial decay of I-129 before system closure of silicates. The ratio Xe-129/Xe-132 is observed in crushed and in uncrushed 'dusty' silicates, and the best ratio for trapped Xe is Xe-129/Xe-132 = 1.060 +/- 0.004. Material balance calculations show that trapped Xe in 'dusty' silicate accounts for 10% of the initial I-129 in Acapulco, while 30% is observed in phosphates and 60% is found in interstitial halogen-rich phase. We can calculate evolution times for trapped Xe of acapulcoites assuming a closed system and an initial OC-Xe signature. Using Xe-129/Xe-132 = 1.147 +/- 0.012 observed in bulk Acapulco samples to represent initial I-129 abundance, we obtain 4-m.y. and 15-m.y. evolution times for Acapulco and Yamato-74063 respectively.

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