Noble gas study of Maralinga CK4 chondrite

Abstract

Maralinga is anomalous CK4 chondrite, which has abundant chondrules and Ca-Al-rich inclusions (CAIs) compared to other CK chondrites. Isotopic composition of He, Ne, and Ar of bulk Maralinga samples by total melting noble gas extraction have been reported [1, 2]. However, isotopic compositions of Kr and Xe have not been reported yet. We measured all noble gas isotopic compositions for a bulk sample from Maralinga by 13-stepwise heating from 400℃ to 1800℃ to investigate noble gas isotope characteristics. Noble gases of Maralinga are composed of three components; primordial gases trapped in Q-phase and in presolar grains, in-situ produced nuclides such as radiogenic, cosmogenic, and fissiogenic ones, and contaminants from Earth atmosphere. The atmospheric gases tend to desorbed at low temperatures (≤800℃). Release temperatures of radiogenic nuclides differ from each other representing different minerals containing K, I, U, and Th, i.e., 4He in 600－800℃, 40Ar in 800－1300℃, and 129Xe in 1200－1800℃. 20Ne/22Ne ratio ranging 1.2－0.83 indicates that Ne is mostly cosmogenic. 21Ne/22Ne ratio changes from 0.67 to 0.89, representing different minerals with different resistivity to heating and different target element compositions for production of cosmogenic isotopes. Trapped primordial heavy gases are released at high heating temperatures (≥1000℃). Q-136Xe/HL-136Xe ratio is 10, unlike the higher values of other carbonaceous chondrites such as CO and CV [3]. Much lower ratio of 2.6 was reported for Kobe CK4 chondrite, and the low value was explained by preferential loss of Q-Xe over HL-Xe caused by a shock event occurred at elevated temperature condition [4]. If similar case can be applied to Maralinga, the shock would have been less effective than Kobe. Cosmic-ray exposure age of 6.8 Ma obtained in this work is almost identical with 6.1 Ma reported in [1]. Gas retention ages of 3.5 Ga from and 4.4 Ga were calculated from U/Th-He and K-Ar dating methods, respectively. The different ages might have been resulted from preferential loss of He during an impact event on parent body of Maralinga. References: [1] Eugster et al. (1998) GCA 62, 2573-2582. [2] Scherer and Schultz (2000) MAPS 35, 145-153. [3] Huss et al. (1996) GCA 60, 3311-3340. [4] Matsumoto et al. (2002) Geochem. J. 36, 341-353.