Noble Gas Compositions in Muong Nong-type Tektites

Tektites are natural silica-rich glasses and are thought to be produced during meteorite collisions with the Earth, similar to impact glasses. They occur in four geographically restricted areas and can be divided in three subgroups: normal or splash-form tektites, aerodynamically shaped tektites, and Muong Nong-type tektites [1]. Splash-form and aerodynamically shaped tektites are several grams in weight and are generally homogeneous in chemistry. On the other hand, Muong Nong-type tektites are up to several kilograms in weight, are irregular in shape, and show layered structure. They are inhomogeneous in chemistry and are enriched in volatile elements such as halogens, boron, zinc, etc., compared to splash-form tektites [1]. Muong Nong-type tektites have larger vesicles than splash-form and aerodynamically shaped tektites. This shows that Muong Nong-type tektites are different from splash-form and aerodynamically shaped tektites in several aspects. We measured noble gas compositions in splash-form tektites [2] and impact glasses [3,4]. Although Ne concentrations in tektites and impact glasses were similar to each other, heavy noble gas (Ar, Kr, and Xe) concentrations in tektites were about 2 orders of magnitude lower than those in impact glasses. In this study, we studied noble gas compositions in some Muong Nong-type tektites in order to compare them with splash-form tektites. Muong Nong-type tektite samples used in this study originated from Ubon Ratchatani in East Thailand, near the border of Laos. Geochemical studies of the samples were made by Koeberl [5]. We measured noble gas concentrations and Ne and Ar isotopic compositions in four Muong Nong-type tektites using mass spectrometry. Noble gases were extracted by three methods: laser probe, crushing, and stepwise heating. Chipped samples of two Muong Nong-type tektites were used in laser probe analysis. We used 160-380 mg of samples for noble gas analysis by crushing and stepwise-heating methods. Noble gas concentrations in tektites, impact glasses, and Muong Nong-type tektites are shown in Fig. 1. Tektites shown are splash-form types collected from three strewn fields [2,6]. Impact glasses shown are Aouelloul, Zhamanshin, Libyan Desert glasses [4], and Darwin glass [3]. Heavy noble gas concentrations in Muong Nong-type tektites are higher than those in splash- form tektites and are similar to those in impact glasses. From the results of laser probe and crushing analysis of noble gases in Muong Nong-type tektites, it seems that vesicles are unevenly distributed in these samples and that large amounts of noble gases exist in theses vesicles. Neon isotopic compositions in vesicles in Muong Nong-type tektites agree well with a terrestrial atmosphere. 40Ar/36Ar ratios in vesicles are higher than that in air, suggesting that radiogenic 40Ar exists in vesicles and/or that radiogenic 40Ar in glass may degas by crushing. Fig. 1, which appears here in the hard copy, shows noble gas concentrations in splash-form tektites (open circles), impact glasses (open triangles), and Muong Nong-type tektites (filled cirles). References: [1] Koeberl C. (1988) Proc. NIPR Symp. Antarct. Meteorites, 1, 261-290. [2] Matsubara K. and Matsuda J. (1991) Meteoritics, 26, 217-220. [3] Matsuda J. et al. (1989) GCA, 53, 3025-3033. [4] Matsubara K. et al. (1991) GCA, 55, 2951-2955. [5] Koeberl C. (1992) GCA, 56, 1033-1064. [6] Hennecke E. W. et al. (1975) JGR, 80, 2931-2934.