Mercury abundances and isotopic compositions in the Murchison (CM) and Allende (CV) carbonaceous chondrites

摘要

The abundance and isotopic composition of Hg was determined in bulk samples of both the Murchison (CM) and Allende (CV) carbonaceous chondrites using single- and multi-collector inductively coupled plasma mass spectrometry (ICP-MS). The bulk abundances of Hg are 294 ± 15 ng/g in Murchison and 30.0 ± 1.5 ng/g in Allende. These values are within the range of previous measurements of bulk Hg abundances by neutron activation analysis (NAA). Prior studies suggested that both meteorites contain isotopically anomalous Hg, with δ~(196/202)Hg values for the anomalous, thermal-release components from bulk samples ranging from -260 ‰ to +440 ‰ in Murchison and from -620 ‰ to +540 ‰ in Allende (Jovanovic and Reed, 1976a; 1976b; Kumar and Goel, 1992). Our multi-collector ICP-MS measurements suggest that the relative abundances of all seven stable Hg isotopes in both meteorites are identical to terrestrial values within 0.2 to 0.5 ‰. On-line thermal-release experiments were performed by coupling a programmable oven with the single-collector ICP-MS. Powdered aliquots of each meteorites were linearly heated from room temperature to 900 ℃ over twenty-five minutes under an Ar atmosphere to measure the isotopic composition of Hg released from the meteorites as a function of temperature. In separate experiments, the release profiles of S and Se were determined simultaneously with Hg to constrain the Hg distribution within the meteorites and to evaluate the possibility of Se interference in previous NAA studies. The Hg-release patterns differ between Allende and Murchison. The Hg-release profile for Allende contains two distinct peaks, at 225°and 343 ℃, whereas the profile for Murchison has only one peak, at 344 ℃. No isotopically anomalous Hg was detected in the thermal-release experiments at a precision level of 5 to 30 ‰, depending on the isotope ratio. In both meteorites the Hg peak at ～340 ℃ correlates with a peak in the S-release profile. This correlation suggests that Hg is associated with S-bearing phases and, thus, that HgS is a major Hg-bearing phase in both meteorites. The Hg peak at 225 ℃ for Allende is similar to release patterns of physically adsorbed Hg on silicate and metal grains. Prior studies suggested that the isotopic anomalies reported from NAA resulted from interference between ~(203)Hg and ~(75)Se. However, the amount of Se released from both meteorites, relative to Hg, is insufficient to produce all of the observed anomalies.