In Situ Argon Isotope Analyses of Chondrule-Forming Materials in the Allende Meteorite: A Preliminary Study for 40Ar/39Ar Dating Based on Cosmogenic 39Ar

The argon isotopic compositions of chondrule-forming minerals the Allende (CV3) meteorite were examined to evaluate possibility in situ 40Ar/39Ar dating planetary surface rocks based on cosmogenic 39Ar without neutron irradiation a reactor. investigated sample (ME-247H: 50 mm × 45 5 mm; 28.85 g) contains at least three textural types chondrules: barred olivine chondrule (BOC), porphyritic (POC), and unclassified (UC). Most chondrules contain olivine, low-Ca pyroxene, clinopyroxene, plagioclase as primary phases, with minor amounts nepheline sodalite formed during aqueous alteration CV3 parent body early solar system. In isotope analyses selected petrographic sections two BOCs, POCs, one UC using Nd:YAG pulse laser confirmed significant amount that by 39K (n, p) reaction an extraterrestrial environment. Laser step-heating five bulk irradiated reactor revealed plateau age (3.32 ± 0.06 Ga) from chondrules. spectra all show younger low-temperature fractions, resulting integrated ages 2.7 3.2 Ga. These results suggest experienced homogenization 3.3 Ga loss part after Apparent calculated J values nephelines BOC POCs do not any consistent relationship among (BOC, POC, UC). This might be attributed fact heterogeneity took place heterogeneous stage event.