Noble gas compositions of seven CV, CO, and CK chondrites from deserts

Abstract

Introduction: Carbonaceous chondrites have vari-ous noble gas components including trapped planetary and solar gases, and presolar noble gases [1]. These noble gas isotopic compositions could be preserved in carrier phases, or have been altered by loss of the gases and thermal processes. Cosmic-ray exposure (CRE) ages of CM and CI groups are similarly short (0.05？7 Ma), but those of CV, CO, and CK are longer CRE ages (0.15？63 Ma) [2, 3]. Scherer and Schultz [4] presented CRE ages ranging from 1 to 63 Ma calculated from cosmogenic 21Ne for CV, CO, CK, and other carbonaceous chondrites. In this study, we present noble gas compositions and CRE ages of two CO chondrites from Dhofar (Dho 1622 and 2005), and four CV and one CK chondrites from Northwest Africa (NWA 3304, 6743, 6746, 10670, and 7420). Experimental method: Abundances and isotopic ra-tios of noble gases of seven carbonaceous chondrites (Table. 1) were measured by modified VG-5400 noble gas mass spectrometer at KOPRI [5, 6]. About 20？31mg of fragments is wrapped in Al-foil. The samples were preheated for a day at 150ºC in ultrahigh vacuum condition to remove atmospheric contamination. Noble gases were extracted from the samples by heating at 1800ºC for 30 min in Mo crucible and purified by Ti-Zr getters and SAES getters. He, Ne, Ar, Kr, and Xe were introduced into the mass spectrometer separately using a cryogenic trap. Abundances of noble gases were calculated by using sensitivities determined by measuring artificial atmospheric standard gas. Isotopes of noble gases were measured in peak jumping mode. Results and discussion: Results are summarized in Table 1. The noble gas compositions indicate no pairing among the meteorites measured in this work. Helium and Neon. NWA 3304 has a 3He/4He ratio closed to that of solar wind [7]. Its Ne isotopic compo-sitions are distinguishable from those from other chon-drites that plotted near cosmogenic composition (Fig. 1). Ne isotopic composition of Dho 1622 is plotted on a mixing line between Ne-HL and cosmogenic Ne, which is consistent with that trapped Ne of CO meteor-ites is dominated by Ne-HL and Q [8]. NWA 7420 is plotted at lower end of cosmogenic area in Fig. 1, and its 21Ne/22Ne of 0.77 is lower than the range of 0.81？0.96 for CK meteorites [9]. It might indicate a presence of Ne-E in this chondrite. Argon, Krypton, and Xenon. Elemental ratios (36Ar/132Xe, 84Kr/132Xe) of trapped heavy noble gases are shown in Fig. 2. Chondrites studied are divided into three groups

(1) Dho 1622, Dho 2005, NWA 3304, and NWA 6743 plotted toward solar from Q, (2) NWA 6746 and NWA 10670 plotted close to Q, and (3) NWA 7420 plotted toward Earth's atmosphere, indicating an atmospheric contamination by weather-ing on desert [4, 10]. Although the NWA 7420 is plot-ted on the mixing line between Q and 244Pu-fission Xe (Fig. 3), it might be accidental and moved from Q-HL mixing because of relatively heavier contamination as shown in Fig. 2. Relatively high 40Ar/36Ar ratio of 1500 for NWA 7420 is due to its high petrologic type. Neutron-produced 80Kr and 82Kr from 79,81Br are exhib-ited for all chondrites studied except Dho 1622. Cosmic-ray exposure age. CRE ages of the seven chondrites in this study and those in [4] were calculat-ed from cosmogenic 21Ne using production rates and chemical compositions from [11, 12, 13, 14]. NWA 6743 was omitted from the calculation for its high proportion of solar wind Ne. CRE ages for chondrites studied are in the range reported so far (Fig. 4). An exception is NWA 6743 with 40 Ma, which is longer than the other CV chondrites, but in the wider ranges for CO and CK (Fig. 4). CRE ages of 9 Ma for CV and CK chondrites and 27 Ma for CO, CV, and CK chon-drites are considered as hypothetical clusters indicating common impact events [2, 4, 15]. It can be possible that CO, CV, and CK have another common impact event at around 40 Ma.