K/Ar-Age Determinations of Iron Meteorites V

Abstract

K and ⁴⁰Ar were determined in 24 iron meteorites by neutron activation. We have also analyzed troilite of Hoba and Odessa, schreibersite of Odessa and Sikhote-Alin, and silicates from Odessa. The techniques for determining the K and ⁴⁰Ar content were improved. Measurements on Sikhote-Alin led to the result that schreibersite contains water-soluble K compounds.

The ⁴⁰Ar/K ratios previously measured are most likely increased by K losses during contact with water. Care was taken to avoid such losses. In samples containing troilite a much higher ⁴⁰Ar content was found by neutron activation than by mass spectrometry. This discrepancy was caused by the nuclide ⁸³Kr* built up from ⁸²Se, which decays with the same half-life as ⁴¹Ar. The two activities, ⁴¹Ar and ⁸³Kr*, were separated by a gas-chromatographic technique. The ⁸³Kr* activity in the metal phase, in schreibersite, and in schreibersite-containing samples, was of the same order of magnitude as ⁴¹Ar.

Iron meteorites must have a K content of at least 100 ppb to be suitable for K/Ar dating. The K/Arages of such iron meteorites were found to be 4.8 × 10⁹ years in Carthage, Chesterville, Odessa, andTombigbee River and 2.6 × 10⁹ years in Campo del Cielo. The silicates of Odessa have a K/Ar age of 4.6 × 10⁹ years. There exist, however, other iron meteorites with high K contents (Canon Diablo, Gundaring, Magura, Santa Rosa), which have much higher ⁴⁰Ar/K ratios. These meteorites show signs of weathering and might have suffered K losses. The small K content and the high spallogenic ⁴⁰Ar content prevented a correct K/Ar dating for most of the measured iron meteorites.