Variable ¹⁹⁶Hg/²⁰²Hg ratio in stone meteorites and in some of their carbon-rich residues

Abstract

The ¹⁹⁶Hg/²⁰²Hg ratio has been measured in Hg fractions released by stepwise heating (and in four cases in solvent extracts) from bulk samples of 17 different stone meteorites using the radiochemical neutron activation (RNA) method. Carbon-rich (CR) residues obtained from four stones (Allende, Ambapur Nagla, Plainview and Rangala) by the HF/1bHCI demineralisation process have been studied. About 50% of the Hg fractions of bulk samples (from 14 meteorites) have given both positive and negative types of anomalies (10-40%). The distillates of solvent extracts (Hg contents = 0.008-0.120 ppm) showed no clear anomaly. The residual meteoritic materials (SR-residues) after these extractions did show Hg isotopic anomalies in some distillates. In 17 out of 22 distillates from the CR-residues, large negative anomalies are noted. In some of these distillates the depletion of ¹⁹⁶Hg was > 90. Replicate measurements did not always give consistent results, presumably due to the heterogeneous dispersion of the anomalous components. The depletion of ¹⁹⁶Hg is more frequent in stone meteorites compared to irons, particularly in high-temperature distillates. Isotopic anomalies in Hg suggest either the presence of primitive grains with isotopic anomalies in stones or introduction of the anomalous components from other objects into the parent bodies of meteorites after their solidification.