Huge variations in the isotopic ratio ¹⁹⁶Hg/²⁰²Hg in some acid-insoluble residues of Sikhote Alin and other iron meteorites

Abstract

The insoluble materials obtained on dissolution of iron meteorites in 2 M H₂SO₄ have been analysed by radiochemical neutron activation to measure the isotopic ratio ¹⁹⁶Hg/²⁰²Hg of Hg released at different temperatures. In many cases this ratio is found to be anomalous; its extreme values, relative to the reagent Hg, being: < 0.003 (2σ limit) and 1.26 ± 0.05. Often, in the same residue, both positive and negative deviations are found at different temperatures. The anomalous, normal, and isotopically diluted Hg distillates have been re-irradiated to eliminate the influence of neutron self-shielding and interfering reactions from matrix elements. The isotopic anomaly persists in the distillates, proving that any artifacts during irradiation are essentially absent. The presence of isotopically anomalous Hg in iron meteorites puts some severe constraints on the formation history of these objects. Either iron meteorites have been formed in a non-magmatic process entrapping presolar grains, or some foreign matter has been injected into their solidified parent bodies by collisions with objects of different isotopic composition.