Noble gases in Muong Nong-type tektites and their implications

We have measured the elemental abundances and isotopic compositions of noble gases in Muong Nong-type tektites from the Australasian strewn field by crushing and by total fusion of the samples. We found that the abundances of the heavy noble gases are significantly enriched in Muong Nong-type tektites compared to those in normal splash-form tektites from the same strewn field. Neon enrichments were also observed in the Muong Nong-type tektites, but the Ne/Ar ratios were lower than those in splash-form tektites because of the higher Ar contents in the former. The absolute concentrations of the heavy noble gases in Muong Nong-type tektites are similar to those in impact glasses. The isotopic ratios of the noble gases in Muong Nong-type tektites are mostly identical to those in air, except for the presence of radiogenic Ar-40. The obtained K-Ar ages for Muong Nong-type tektites were about 0.7 Myr, similar to ages of other Australasian tektites. The crushing experiments suggest that the noble gases in the Muong Nong-type tektites reside mostly in vesicles, although Xe was largely affected by adsorbed atmosphere after crushing. We used the partial pressure of the heavy noble gases in vesicles to estimate the barometric pressure in the vesicles of the Muong Nong-type tektites. Likely, Muong Nong-type tektites solidified at the altitude (between the surface and a maximum height of 8-30 km) lower than that for splash-form tektites.