Anomalous Ar⁴⁰contents of iron meteorites

Abstract

The Ar40, Ar36, and K41 contents of several iron meteorites have been measured by neutron activation analysis. The data are in agreement with previous results which were interpreted as measuring a K40‐Ar40 age. The possible effects of cosmic rays, primordial rare gases, and atmospheric gas diffusion were also investigated. These results are not in disagreement with an ‘age’ interpretation; however, the nonconstancy of the K40/Ar40 ratio within the Santa Rosa meteorite argues against this interpretation. The origin of the Ar40 in iron meteorites is not presently resolved.

Potassium: Argon problem in iron meteorites

“…The difficulties involved in show next that shock emplacement of terresreconciling such high apparent ages with the trial Ar is also unimportant. We had thought large body of conflicting astrophysical, geo-that the meteoritic-terrestrial impact might logic, and nuclear data have been previously shock terrestrial Ar into the iron lattice; we discussed [Fisher, 1965; Burnett and Wasset-point out that only about 10 •' atoms/g are burg, 1967]. Rancitelli et al [1967] concluded necessary and that most meteoritic measurethat the Weekeroo Station data, in which the ments have been made on specimens close to metal phase gave K:Ar ages of ,•10 X 109 the surface, owing to the mechanical and psyyears and the silicate phase gave a Rb :Sr chol.…”

“…The meteorite was then placed in another 10-ml portion of distilled water and the procedure repeated, and finally It is natural to suppose that an excess of Ar 4ø over that provided by decay of K 4ø is responsible for the observed high ages. Stringent arguments against the presence of sufficient amounts of terrestrial, primoridal, cosmogenic, and cosmoradiogenic Ar (Ar •ø formed from the decay of cosmogenic K •ø) have been previously presented [Fisher, 1965;Miiller and Zgihringer, 1966] reached. This last argument shows that surface adsorption of Ar is not important; we years are common.…”

“…shocked and unshocked specimens of Canyon Inclusions in the Toluca and Odessa meteorites Diablo, generously provided by E. Anders. In have been dated by the Rb-Sr technique [Bur-Table 1, samples 6, 7, and 8 showed no erinerr and Wasserburg, 1967]; the metal phase dence of shock effects, whereas samples 10 and of Toluca has been previously dated by K:Ar 11 were heavily shocked (Anders, private com- [Fisher, 1965;Stoenner and Zgihringer, 1958], munication). All these samples show apparent and data on the metal phase of Odessa is re-K:Ar ages of 11-13 X 109 years.…”

“….on of about 20% of the mass of the meteorite removes 65-85% of the total K. It is therefore clear that earlier experiments[Fisher, 1965; Stoenner and Z•ihringer, 1958], in which the sample was vigorously etched in acid to remove surface contamination, gave K results that were too 1.ow because of such dissolution during the etching process. If the sample is not etched at all, however, terrestrial K from sawing, sanding, and handling will introduce a large error.…”