We attempt to investigate noble gas components and their carrier phases by melting a small portion of meteorites employing a laser microprobe and by measuring all noble gas isotopes. As a preliminary study, noble gases were extracted from various materials of Allende and Murchison carbonaceous chondrites using a pulse Nd-YAG laser. The laser shot makes a pit 100-300 micrometers in diameter. The weight of melted material was estimated as 20-200 micrograms depending on the laser output power and the focusing condition. Evolved noble gases were purified by Ti-Zr getter and separated three fractions He-Ne, Ar, and Kr-Xe for mass spectrometry. Blank levels of the system were 10, 5, 1, 300, 0.03 and 0.003 in unit of 10^-12 cm^3STP for ^4He, ^20Ne, ^36Ar, ^40Ar, ^84Kr, and ^132Xe, respectively. Recently, the noble gas mass spectrometer has been equipped with ion counting system, resulting in improved detection limit and S/N ratio. New data are now being acquired with this system. Noble gas concentrations and isotopic ratios vary widely in Allende. The noble gas concentrations are very heterogeneous in chondrules. Whereas medium concentrations of noble gases were found in matrixes, of which the isotopic ratios were similar to those of bulk sample. CAIs have various noble gas compositions. Radiogenic isotopes, ^4He, ^40Ar, and ^129Xe, are greatly concentrated in some type of CAIs, indicating high concentrations of U, Th, K, and I. The highest ^129Xe/^132Xe ratio of 44 was found in a coarse-grained CAI, for which the ^129Xe concentration was 3.9x10^-8 cm^3STP/g. The I and K contents in the CAI were calculated as high as 2 ppm and 0.7 wt%, respectively, assuming ^129I/^127I ratio of 1x10^-4 at the beginning of Xe retention and 4.55 Ga for Ar retention. High Br content in the CAI was also suggested by excess ^80Kr and ^82Kr isotopes produced by n-capture reaction on ^79Br and ^81Br. Low ^20Ne/^22Ne ratios of the CAIs might be produced by Na spallation. These results imply the presence of both volatile elements such as alkali metal and halogen and refractory ones such as U and Th. The high I concentrations in CAIs confirm the previous work by others (1,2). A single chondrule of Allende, which has a round core and irregular shaped mantle, showed complex distribution of noble gases; core is well degassed, while the concentration is higher in mantle. A core-mantle boundary has exceptionally high concentrations of ^40Ar and ^129Xe, suggesting the concentrated K and I around the boundary. In this chondrule, some mechanism such as zone refining might work at the solidification of mantle material from its surface. Ne isotopic ratios of Allende show no systematic differences between chondrules and matrix materials, and their trapped ^20Ne/^22Ne ratio is 10.3 which is similar to that of Ne-C. Murchison has Ne-A in matrix, whereas its chondrule has Ne similar to solar Ne-B. In Murchison, ^4He, ^40Ar, and ^129Xe concentrations are positively correlated with each other, and they are high in chondrule and low in matrixes. References: (1) Reynolds J. H. and Lumpkin G. R. (1980) Naturforsch. 35a, 257266. (2) Kirschbaum C. (1988) Geochim. Cosmochim. Acta 52, 679-699.
- Pub Date:
- July 1992