Argon-39 and tritium in meteorites.

Abstract

The radioactive isotopes A39 (26o-year half- life) and tritium (I 2.4-year half-life) were measured in seven iron and in two stony meteorites. A39 was detected and measured in the iron meteorites Sikhote-Alin (1947), Treysa (1916), Pitts (1921), and in the stony meteorites Norton County (1948) and St. Michel (1910). The detection and measurement of A39 in these meteorites makes it possible to determine their argon exposure ages (Fireman, E. L. Nature, i8i, 1613, 1958). On the basis of the same assumptions used in that article, the ages are 4.9 X I0~ years for the Sikhote-Alin meteorite, 5.7 X 108 years for the Treysa meteorite, 6.~ X I0~ years for the Norton County meteorite, 1.7 X 108 years for the St. Michel meteorite, and, for the Pitts meteorite, an unusually low figure in the range of I0~ years. The short argon exposure ages may be interpreted in two ways: they may result from the loss of shielding material, worn off the meteorite in space by interplanetary dust and gas (suggested in private discussion with Professor Fred L. Whipple); or they may represent the times since the breakup of planets. From the lack of A39 in the Carbo and Grant meteorites and the small amount of A39 in the Washington County meteorite whose dates of fall are unknown, one can conclude that Carbo and Grant fell more than 1500 years ago and that the Washington County meteorite fell about 1000 years ago. In the case of the Caflon Diablo meteorite the He3 content is so low that the measured absence of A39 and tritium does not necessarily mean that it fell long ago. The ratio of tritium to A39 radioactivities (decays g~i min-i) in the Norton County meteorite is 230 to I at the time of fall. This ratio in the Sikhote-Alin meteorite is anomalously low being less than I to I at the time of fall. The work was supported in part by a grant from the Atomic Energy Commission. Smithsonian Astrophysical Observatory, Cambridge, Mass.