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

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 Ho, with delta Hg-196/202 values for the anomalous, thermal-release components from bulk samples ranging from -260 parts per thousand to +440 parts per thousand in Murchison and from -620 parts per thousand to +540 parts per thousand 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 parts per thousand. On-line thermal-release experiments were performed by coupling a programmable oven with the single-collector ICP-MS. Powdered aliquots of each meteorite were linearly heated from room temperature to 900 degreesC 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 interferences in previous NAA studies. The Ha-release patterns differ between Allende and Murchison. The Ha-release profile for Allende contains two distinct peaks, at 225 degrees and 343 degreesC, whereas the profile for Murchison has only one peak, at 344 degreesC. No isotopically anomalous Hg was detected in the thermal-release experiments at a precision level of 5 to 30 parts per thousand, depending on the isotope ratio. In both meteorites the Hg peak at similar to 340 degreesC 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 degreesC 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 Hg-203 and Se-75. However, the amount of Se released from both meteorites, relative to Hg, is insufficient to produce all of the observed anomalies. Copyright (C) 2001 Elsevier Science Ltd.