Über die Geschichte der Meteorite aus Isotopenmessungen

Abstract

The results of radioactive age determinations on meteorites by various methods are summarized. The significance of these figures for the history of meteoritic matter is discussed. It is pointed out that indications for the presence of normal gases in meteorites are found by several investigators. The significance of measurements on these gases, especially on their isotopic composition, is outlined.

Special interest is given to the cosmic ray produced isotopes of gases, and the possibilities of obtaining “radiation ages” for meteorites are discussed. The only method used so far, which in principle gives precise radiation ages, consists in the measurement of the ³He/³H ratio. For the Norton County achondrite this method has given a radiation age of 260 m.y.⁴⁰, compared with an A/K age of 4400 m.y.³⁰. The influence of short-time variations of the cosmic ray flux on the ³He/³H age is discussed. The secular changes of the cosmic ray intensity may be investigated by the comparison of cosmic ray produced ⁴⁰K and for instance ³⁸A in iron meteorites.

³⁸A contents of stone meteorites can be used to estimate radiation ages with sufficient precision in order to draw conclusions about the meaning of these ages. It is derived that the ³⁸A contents imply radiation ages of chondrites of the order of 40 m.y. and of some achondrites of 10 to 20 m.y. It is shown that these radiation ages are determining the moment of the creation of the meteorite by the break-up of a larger body.