The thermal release of Hg from chondrites and their thermal histories

Abstract

A quantitative treatment and implications of isothermal and linear heating data on Hg in meteorites are given as a sequel to a more qualitative analysis of meteorite thermal histories (Reed and Jovanovic, 1968). Studies of Hg in terrestrial metamorphic rocks establish that thermal events to which meteorites were subjected fall in the same temperature range, ~400–900°C, as exists during terrestrial metamorphism. Hg diffusion parameters based on data from the linear and isothermal heating experiments are calculated. The conclusions are: 1.(1) Meteorites experienced thermal events of the same magnitude as those measured by primarily mineralogical metamorphic indicators reviewed by Dodd(1969).2.(2) No correspondence with mineralogical-petrological metamorphic grade is evident.3.(3) Hg data for some chondrites correlate with shock facies (non-thermal) indicators (Dodd and Jarosewich, 1979).4.(4) Small Hg activation energies (6–14 kcal/mole) require that the meteorites must have been stored in closed systems until low temperatures were attained. Hg must be present as an involatile mineral(s) or as a substituent in a host phase at temperatures below ~100°C. Consistent with this interpretation is the fact that despite diffusion times of 102–106 years at 200 K, Hg was retained in small objects over cosmic ray exposure periods of 107 years.