Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.542m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 542
Computer Science
Noble Gases, Philippinite, Tektites
Scientific paper
Bubbles are common in tektites, but usually their sizes range up to only a few mm. They are most abundant in Muong Nong-type tektites. The gases contained in these bubbles are of terrestrial atmospheric composition, with pressures below 1 atm (e.g., [1]). The abundances of light noble gases (He, Ne) are controlled by diffusion from the atmosphere [2], and noble gases dissolved in tektite glass indicate that the glass solidified at atmospheric pressures equivalent to about 40 km altitude [3]. Large bubbles in splash-form tektites are rather rare. Thus, the finding that a philippinite (size: 6.0 x 4.5 cm; weight: 199.6 g) contains an unusually large bubble justified a detailed study. The volume of the bubble, which was confirmed by X-ray photography, was estimated at 5.4 cm^3, by comparing the density of this tektite (2.288 g/cm^3) to that of normal philippinites (2.438 g/cm^3). A device was specifically constructed for crushing the present sample under vacuum. The 10x10 cm cylindrical device has a piston that allows to gently crush the sample by turning a handle. Various disk spacers can be used to adjust the inner height to that of the sample. The device is made of stainless steel, yielding a low noble gas blank. The crushing device is connected to a purification line and a noble gas sector-type mass spectrometer (VG 5400) [4]. Before crushing, the complete tektite was wrapped in aluminum foil. A first crushing attempt, using stainless steel disk spacers, failed and resulted in damage to the steel spacers, indicating a high strength of the tektite. Using iron disk spacers resulted in an ambient pressure increase (probably due to hydrogen from the Fe) in the sample chamber. However, the noble gas blanks were negligible. The background pressure, at 2 x 10-4 Torr, increased to 3 x 10-4 Torr when the sample was crushed. From the volume of the crushing device and that of the bubble in the tektite, the total gas pressure in the bubble was estimated at about 1 x 10-4 atm. Part of the extracted gas was kept for total gas analysis, while the remainder has been purified for the noble gas measurements. Total amounts and isotopic ratios of all noble gases were measured. The amounts of Ar, Kr, and Xe close to the blank level, while those of He and Ne were about 3 to 4 orders of magnitude larger than the blank. The ^20Ne/^36Ar ratio in the bubble gas is more than 4 orders of magnitude higher than the atmospheric value, which is similar to the pattern previously observed in tektites [2,3,5]. The isotopic ratios of Ar, Kr and Xe were, within uncertainties, similar to those of the terrestrial atmosphere. However, the Ne isotopic ratios were significantly different from atmospheric values, and differ from the results reported in previous studies [2,5]. The Ne isotope data seem to lie on the mass fractionation line from the atmosphere in a ^20Ne/^22Ne vs. ^21Ne/^22Ne three isotope plot, suggesting that the Ne in the bubble has diffused in from the atmosphere. However, it is generally believed that the isotopic fractionation during a steady state is very small, and the observed Ne values are higher than those calculated from simple mass fractionation [6]. The high isotopic fractionation is likely to be associated with the non-equilibrium conditions prevailing during tektite formation. Acknowledgments: We are grateful to D. Heinlein for bringing the precious sample to our attention and for allowing its analysis. References: [1] Jessberger E. K. and Gentner W. (1972) EPSL, 14, 221-225. [2] Matsubara K. and Matsuda J. (1991) Meteoritics, 26, 217-220. [3] Matsuda J. et al. (1993) Meteoritics, 28, 586-599. [4] Maruoka T. and Matsuda J. (1995) J. Mass Spectrom. Soc. Jpn., 43, 1-8. [5] Hennecke et al. (1975) JGR, 80, 2931-2934. [6] Kaneoka I., EPSL, 48, 284-292.
Koeberl Christian
Maruoka Teruyuki
Matsuda J.
Pinti Daniele L.
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