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 Efremo vka CV3 chondrite were determined not to have excess Xe-132 and Xe-131, whi ch were previously found in a refractory inclusion from the Allende chondri te [11]. However, correlations between Xe isotopes in these two inclusions, two other refractory inclusions from the Efremovka meteorite, and the matr ix led us to discover traces of anomalous Xe, which is qualitatively simila r to the aforementioned Xe from CAI of the Allende chondrite. It was conclu ded that Xe in the Efremovka chondrite consists of three principal componen ts: primary Xe (which is similar to Xe-P3), chemically fractionated fission Xe (CFF-Xe), and plutogenic Xe (or its residual after CFF-Xe migration). T he examined inclusions from Efremovka were also determined to be high in ex cess Xe-129, which is the beta -decay product of the now-extinct I-129. A w eak but statistically significant nonlinear correlation between apparently entrapped Xe-132 and radiogenic Xe-129 in our inclusions testifies that the se inclusions entrapped not a mixture of primary Xe and radiogenic Xe-129 b ut rather a mixture of primary Xe and I, including the still-undecayed radi oactive I-129, which gave rise to Xe-129 in situ, i.e., when already entrap ped 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 correlat ions were caused by the impact of the secondary thermal-neutron flux of cos mic radiation on Br-79 and Br-81 atomic nuclei, a process resulting in Kr-8 2 and Kr-80. One of the most characteristic features of the At isotopic com position of the inclusions is its isotopic heterogeneity, which can be defi nitely revealed during thermal annealing and is caused by the occurrence of two components of different provenance: cosmic and atmospheric. The dark i nclusion additionally contains radiogenic Ar-40. Neon occurs in the inclusi ons as a number of distinct components, from cosmogenic neon to the nearly pure NeA component. Some temperature fractions of the inclusions contain no ticeable concentrations of NeE with its typical strong Ne-20 deficit. In th e inclusions, He consists of two main components: cosmogenic He with He-3/H e-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 rati os of the inclusions suggest processes of the element fractionation of nobl e gases, perhaps, during the low-temperature sorption and desorption of gas es 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 a ge for Ar-38) is 11 +/- 3 Ma for the dark inclusion.