Effect of Thermal Metamorphism on Noble Gas of Carbonaceous Chondrites: Comparison of Vigarano (CV3) and Maralinga (CK4).

Abstract

Introduction: Noble gas isotopic and elemental compositions of meteorites are result of formation and evolution processes of solar system. Primitive chon-drites have various components of noble gases such as HL, G, and N from presolar grains, Q gas, solar gas, and in-situ components like radiogenic, fissiogenic, and cosmogenic noble gases. Otherwise, metamor-phosed chondrites lost their gases during thermal event, and most of noble gases including gases of presolar grains were disappeared at that time. Subsolar compo-nent is primordial noble gas that can be seen in some chondrites including metamorphosed ones [1-7]. It constrains formation process of parent body of meteorites affected by solar gases from Sun [4,5]. Example is enstatite chondrite whose noble gas component is represented as subsolar [6,7]. In carbonaceous chondrites, it is not well studied because it is not a major component unlike that in enstatite chondrites. We present noble gas compositions of Vigarano CV3 and Maralinga CK4 chondrites. Maralinga has 50:50 chondrules to matrix ratio similar with reduced CV3 chondrites [8]. In this study, changes of noble gas composition resulted from thermal metamorphism are discussed by comparing noble gases of the two carbo-naceous chondrites. Experimental method: Noble gas abundances and isotopic ratios of Vigarano and Maralinga were meas-ured with modified VG-5400 at Korea Polar Research Institute (KOPRI). Bulk samples weighing 22.34 mg of Vigarano and 313.5 mg of Maralinga were prepared for stepwise heating analyses from 400°C to 1800°C per increasing 100°C step. 500 and 1700°C steps of Maralinga were skipped. Released gases were purified by two Ti-Zr getters and two SAES getters before intro-ducing them into the mass spectrometer. All-noble gases were measured, and blank levels were lower than 1% of released noble gases of each heating step except 500 and 1800°C of Vigarano. 1800°C step of Vigarano is not presented as abundance of it is comparable with blank level in all noble gases. Results and discussion: Cosmogenic component of Xe in Vigarano and Maralinga is neglected in this study. Proportion of cosmogenic 130Xe and 132Xe is 0.3 and 0.04 percent of total 130Xe and 132Xe of Maralinga. In case of Vigarano, it is much smaller than that of Maralinga due to its high abundance of trapped com-ponents. Xe isotopic ratios and elemental ratios of trapped heavy noble gases. Fig. 1 is a plot of 130Xe/132Xe vs. 136Xe/132Xe of Vigarano and Maralinga. Xe of low heating steps of both samples is affected by Earth atmosphere, as clearly indicated in step 4 of Vigarano and steps 4-8 of Maralinga. Weathering of Maralinga was classified as Bx, indicating moderate degree of weathering with presence of carbonates derived from the local country rock [8]. Several steps of Maralinga showing atmospheric isotopic ratio coincides with ob-servation of terrestrial weathering. It is also can be seen in Fig. 2. Steps 4-10 of Maralinga show elementally fractionated air. Higher heating steps of Maralinga tend toward Q. Xe isotopic ratios of those steps are plotted between Q-HL-fission derived Xe with 136Xe/132Xe as 0.33-0.34 (Fig. 1). Xe isotope distribution of Maralinga (Fig. 1) represents that Xe-Q and Xe-HL are released together, and it is suggested that retrapping of HL gas on phase Q during thermal metamorphism [9]. Whereas 136Xe/132Xe of Vigarano ranges 0.32-0.39, and presolar diamond of Vigarano containing HL was separated by [15]. Heavy elements are less elementally fractionated from solar gases in steps 10-13 of Vigarano on Fig. 2, though Xe isotopic ratios of those steps are not different from Q for 11-13 steps, same with previous studies [5,7]. Release pattern of primordial 132Xe. Fig. 3 is re-leased pattern of primordial 132Xe of Vigarano and Maralinga. Primordial Xe is calculated by subtracting atmospheric contamination and fission derived Xe, and includes Q, subsolar, and HL.