Isotopic composition of noble gases in the dark and light lithologic constituents of the Efremovka carbonaceous chondrite

A refractory Ca- and Al-rich (CAI) and a dark (DI) inclusions in the Efremovka CV3 chondrite were determined not to have excess Xe-132 and Xe-131, which were previously found in a refractory inclusion from the Allende chondrite [11]. However, correlations between Xe isotopes in these two inclusions, two other refractory inclusions from the Efremovka meteorite, and the matrix led us to discover traces of anomalous Xe, which is qualitatively similar to the aforementioned Xe from CAI of the Allende chondrite. It was concluded that Xe in the Efremovka chondrite consists of three principal components: primary Xe (which is similar to Xe-P3), chemically fractionated fission Xe (CFF-Xe), and plutogenic Xe (or its residual after CFF-Xe migration). The examined inclusions from Efremovka were also determined to be high in excess Xe-129, which is the beta -decay product of the now-extinct I-129. A weak but statistically significant nonlinear correlation between apparently entrapped Xe-132 and radiogenic Xe-129 in our inclusions testifies that these inclusions entrapped not a mixture of primary Xe and radiogenic Xe-129 but rather a mixture of primary Xe and I, including the still-undecayed radioactive I-129, which gave rise to Xe-129 in situ, i.e., when already entrapped in the inclusions. The weak correlations between the neutron-deficient isotopes suggests that the samples contain small amounts of spallogenic Xe. A notable feature of the Kr isotopic composition of the inclusions is the correlated excesses of light isotopes with respect to Kr-84. These correlations were caused by the impact of the secondary thermal-neutron flux of cosmic radiation on Br-79 and Br-81 atomic nuclei, a process resulting in Kr-82 and Kr-80. One of the most characteristic features of the At isotopic composition of the inclusions is its isotopic heterogeneity, which can be definitely revealed during thermal annealing and is caused by the occurrence of two components of different provenance: cosmic and atmospheric. The dark inclusion additionally contains radiogenic Ar-40. Neon occurs in the inclusions as a number of distinct components, from cosmogenic neon to the nearly pure NeA component. Some temperature fractions of the inclusions contain noticeable concentrations of NeE with its typical strong Ne-20 deficit. In the inclusions, He consists of two main components: cosmogenic He with He-3/He-4 approximate to 10(-1) and a mixture of He-3 and He-4 in the proportion from 10(-3) to 10(-4). The Ar-36/Xe-130, Ne-20/Xe-130, and He-4/Xe-130 ratios of the inclusions suggest processes of the element fractionation of noble gases, perhaps, during the low-temperature sorption and desorption of gases or the loss of light gases during the thermal effect on solid materials. The average exposure age (which was calculated based on the concentrations of cosmogenic He, Ne, and Ar isotopes without allowance for the exposure age for Ar-38) is 11 +/- 3 Ma for the dark inclusion.