Hg-196/Hg-202 ratio and Hg content in meteorites and terrestrial standard rocks: A RNAA study

Radiochemical neutron activation was applied for the determination of mercu ry isotopic ratios (Hg-196/Hg-202) for standard rock samples and meteorite samples. After neutron activation, Hg was released upon heating from 100 de greesC to 500 degreesC at 100 degreesC steps and the activity ratio of Hg-1 97/Hg-203 was measured for each released fraction. After the correction for the interfering gamma -rays of Se-75 to Hg-203, the ratios obtained from t he Dhajala meteorite (bulk and magnetically separated samples) were identic al to those of the Hg monitor as well as standard rock samples (JB-1, basal t, and JG-1, granodiorite). An apparently low ratio of Hg-196/Hg-202 was ob served at high temperature (500 degreesC) for the Allende meteorite referen ce sample. As such a low value was not reproduced in another run, the prese nce of isotopically anomalous Hg in the Allende reference sample can not be concluded. Hg/Se ratio was observed to be very high in the nonmagnetic fra ction of Dhajala compared with that in the magnetic fraction, suggesting th at Hg is less chalcophile than Se. Yamato 82050 (CO3) has an anomalously hi gh content of Hg with a normal Hg-196/Hg-202 ratio, being at least an order of magnitude higher than the expected value. This meteorite must have been contaminated with Hg, presumably on Antarctica.