The cosmochemical behavior of mercury

We review long-standing reports of both the 'Hg paradox' that Hg abundances in meteorites are inconsistent with its volatile behavior and the anomalou s Hg-196/Hg-202 ratios in all types of extraterrestrial material. A careful study of the literature suggests that both results may be due to overestim ation of Hg-202 abundances from neutron activation analysis. We reevaluate Hg condensation in the solar nebula using both volatility trends fbr chondr itic meteorites and thermodynamic calculations. The extent of Hg depletion in Allende indicates a 50% condensation temperature of 350 K. Thermodynamic calculations show that Hg condenses into various metal alloys at 300 K or lower. At lower temperatures, Hg can be incorporated into Fe,Ni sulfides as a trace element. We investigate chemisorption as a method for incorporatin g Hg into solid material in the solar nebula. A large fraction of Ha can ch emisorb on metal surfaces at temperatures as high as 515 K, depending on th e grain size. (C) 1999 Elsevier Science B.V. All rights reserved.