Cosmic-ray-produced 53Mn in thirty-one meteorites

Abstract

Cosmic-ray-produced 53Mn (t ½ = 3.7 × 10 6years) has been determined by neutron activation analysis in twenty-two chondrites including three Antarctic meteorites: Yamato-7301 (j), -7305 (k) and -7304 (m). 53Mn was also measured in four mesosiderites, three iron meteorites, Bencubbin (unique) and Udei Station (iron with silicate inclusions). In addition, preliminary results for 10Be (1.6 × 10 6 years) were obtained in the Yamato meteorites using a low-background needle GM counter. Based on published values of rare gas ages, corrections were made for undersaturation; the average specific saturation activities of 53Mn were found in the range 450 ± 90dpm 53Mn/kg Fe in most of the chondrites and 490 ± 75dpm 53Mn/kg Fe in the mesosiderites. Two meteorites had extremely low contents of 53Mn: 102 ± 6 in Yamato-7301 and 48 ± 3dpm 53Mn/kg Fe in Bondoc. The Bondoc mesosiderite was already known to have a low concentration of cosmogenic radionuclides due to its large pre-atmospheric size. Several possible mechanisms are discussed to explain the low 53Mn activity in Yamato-7301: (1) long terrestrial age of about 7 m.y.; (2) low production rate of 53Mn due to heavy pre-atmospheric shielding (>70cm); (3) multi-stage irradiation history resulting in an undersaturation of 53Mn; and (4) a mechanism in which two or three of the above factors are combined. The ratio of 53Mn production rate in Ni to that in Fe has been estimated to be 1/3, based on the measurements of 53Mn in the metallic and silicate phases of St. Séverin meteorite, as well as on published results of some high-energy bombardment experiments.