The cosmic ray exposure ages obtained from the Ar/sup 39/ and Ar/sup 38/ are: 4.2 x 10/sup 8/ years for the Siknote-Alin Fe meteorite, 4.5 x 10/sup 8/ years for the Treysa Fe meteorite, 0.3 x 10/sup 8/ years for the Pitts Fe meteorite, 5 x 10/sup 8/ years for the Norton County achondrite, and 0.8 x 10/sup 8/ years for the St. The stable isotope He/sup 3/ is also measured and the Ar/sup 38/ content is estimated from the He/sup 3/ content. The radioactive isotopes Ar/sup 39/ and tritium are measured in a number of Fe and stone meteorites. Quantitative data on radioactive concentration are presented. In the other 4 old siderites, with a limit of detection of about 10 gammas/min/kg, no gamma-ray activity in the energy range 0.2 to 2 Mev was detectable. In Aroos, measured 120 days after fall, Mn/sup 54/ was the predominant radioactivity Co/sup 60/ was the only radioactivity measured in the 4 samples of Siknote-Alin. Of the 6 siderites studied, only Aroos and more » Siknote-Alin showed detectable gamma radioactivity. Furthermore, thorium was detected and measured in 5 of the 9 achondrites. More unusual gamma emitters measured were 291-day Mn/ sup 54/, 2.6-year Na/sup 22/, and probably a mixture of Sc/sup 46/ and Co/sup 56+58/ (half lives approximately 80 days) these nuclides were detected in relatively recent falls (the youngest, the Harleton, Texas, chondrite, was measured 21 days after fall). Potassium and Al were the predominant radioactivities measured. The gamma spectra of 32 samples of 25 chondrites, 4 carbonaceous chondrites, and 9 achondrites were investigated using a large NaI (Tl) scintillation spectrometer. Investigation of the nearby sequence of volcanic rocks has been undertaken as an initial step in an effort to obtain definitive age data on the environment of the uranium occurrence. Metamorphosed Todilto limestone (Jurassic), found in the Section 33 mine beneath the lava on the mesa, contains uranium which has been dated on a single unconfirmed determination at 78 plus or minus 5 million years by Miller and Kulp. The porosity and high water content of the perlite, which give it the commercially valuable property of expanding when heated, are believed to have resulted from magmatic or hydrothermal hydration. Thus, preliminary observations suggest thnt meteoric water, which would be expected to introduce air argon, is not responsible for the hydration of the perlite. The perlite, a porous, hydrous glass, contains no more air argon than the massive vitreous obsidian. These are considered doubtful because of more » the low potassium content. Two determinations made on the plagioclase give 4 and 9 million years. Plagioclase from an andesitic plug which Pierces the mesa contains only 0.43% potassium. Perlite now being mined in an open pit along the northeast edge of the mesa corresponds with the obsidian in age. Obsidian masses along the north side of the mesa are dated at3.3 plus or minus 0.5 million years. Mex., yield age determinations which date one of the several recognized periods of volcanic activity. Six potassium-bearing specimens from Grants Mesa, a youthful volcanic complex just north of Grants, N. The lead-207 excess in the meteorite is 2.2 x 10 -9/ which would require 10/sup 10/ years for this excess to result from 1.5 x 10 -13 g/g of uranium-235. The experimental method used is described. The assumption was that both contain lead isotopes of the same isotopic composition when the bodies were formed. Thus, the uranium and lead contents of the meteorite are inconsistent with the common assumption made in determining lead ages for earth and meteorites. This amount is more than a factor of one hundred too small to account for the excess lead-207 in the Siknote- Alin meteorite in 5 x 100 more » years. Results showed less than 1.4 x 10 -14 g/g of uranium-235, corresponding to less than 2 x 10 -11 g/g of normal uranium. To test the assumption that the excess of lead-206 is due to the radioactive decay of the isotope uranium-238, of lead-207 to that of uranium-235, and of lead-208 to that of thorium-232, a search was made for uranium-235 in the Siknote-Alin meteorite, which is a meteorite that clearly has a lead-207 excess.
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