Bestrahlungsalter und Herkunft der Eisenmeteorite

Abstract

Cosmic ray exposure ages of more than 60 iron meteorites have been determined using the 41K/40-K-method. Isotope fractionation processes and the reproducibility of age values have been studied. Iron meteorites with ages of 900, 700, 600, 400 × 106 y (and hexahedrites with ages of 10 to 100 × 106 y according to WÄNKE) are abundant, such with ages exceeding 1 × 109 y are rare. Correlations have been found between exposure ages and structural classes as well as Ga—Ge-groups. These prove that the remarkable groupings of iron meteorites according to some of their structural and compositional features are indeed due to the fact that it were only a few distinct parent iron masses which essentially contributed to the production of iron meteorites. Some of these parent masses supplied the meteorites mainly in one or two “catastrophic” collisions and break-up events, others in a quasi-continuous series of events over a period of many hundreds of million years. More specified, the results are as follows: Medium and fine octahedrites of the Ga—Ge-group III have, essentially, been produced in two major collisions 600 and 700 × 106 y ago, fine octahedrites of the Ga—Ge-group IV a in one collision about 400 × 106 y ago. These events alone supplied already about 50% of all iron meteorites. Hexahedrites, coarse octahedrites of the Ga—Ge-group I and nickel-rich ataxites reveal, on the other hand, age distributions which appear rather continuous. Mean interplanetary life times deduced from these distributions are about 100 × 106 y for hexahedrites (according to WANKE), 500 × 106 y for nickel-rich ataxites and 700 × 106 y for coarse octahedrites of the Ga — Ge-group I. The differences in the statistics of meteorite production events for different meteorite classes indicate that fundamental dissimilarities may exist in the parent iron masses and parent celestial bodies. Accordingly, one model is proposed for explaining the data on hexahedrites, coarse octahedrites of the Ga—Ge-group I, and ataxites, another one for explaining the data on the medium and fine octahedrites of the Ga—Ge-groups III and IV a: The hexahedrites, coarse octahedrites and ataxites are fragments of a multiplicity of nickel-iron inclusions (“pools” according to URET) within the surface layers of Moon, Mars and asteroids (Mars-asteroids), respectively. These fragments, together with stone meteorites, have been produced in a multiplicity of crater-forming impacts by comets on Moon, Mars or asteroids. Rather compact and large nickel-iron masses, perhaps fragments of asteroidal nickel-iron cores are considered as being the parent masses of the fine octahedrites and medium octahedrites of the Ga—Ge-groups III und IV a. The meteorites of each group have been produced either when such a nickel-iron mass as a projectile hit another asteroid or when a comet hit the nickel-iron mass. Mainly surface material is expected to survive the catastrophe in form of structurally undisturbed meteorites, of which a considerable portion may have been exposed to cosmic rays to a measurable degree prior to the collision. Possible conclusions from a comparison of 41K/40K-age values with 36Ar/36Cl- and similar ages are reconsidered.