Investigations of cosmic-ray-produced nuclides in iron meteorites, 4. Identification of noble gas abundance anomalies

Abstract

The literature on cosmic-ray-produced nuclides in iron meteorites occasionally reports unusual (“anomalous”) abundance proportions for the associated noble gases. The anomalies are in some cases ascribed to excesses of 4He, caused by the presence of primordial or radiogenic components; in other cases to abundance deficiencies of 3He, caused by partial loss of cosmogenic tritium. The arguments and data used previously for the recognition, identification and determination of anomalies are, however, imperfect or incorrect. New procedures are here proposed. They are based on more reliable data on the abundance patterns of the cosmogenic component and on a novel system of test correlations which describes these patterns. In most cases in which anomalies are recognised, the system allows an unequivocal identification of the nuclide which is the cause of the anomaly. This is a prerequisite for the quantitative determination of excesses and deficiencies. The procedures are applied to evaluate anomalous noble gas data reported in the literature for about 15 samples of various iron meteorites. In some cases, previous identifications of 3He deficiencies and of 4He excesses prove to be correct. However, guesses that 4He excesses were present in certain specimens from Arispe, Cranbourne, El Taco, Hoba, Pin˜on, Pitts and Sandia Mountains are invalidated by the present investigation. 4He excesses are more exceptional than heretofore believed.