RECALIBRATION OF THE SPHALERITE COSMOBAROMETER - EXPERIMENTAL AND THEORETICAL TREATMENT

Temperature dependence of the composition of sphalerite in equilibrium with troilite + metallic iron has been determined experimentally from 400 to 840 degrees C at 1 bar, The high-temperature runs (660-840 deg rees C) were conducted in evacuated silica tubes; a new version of the recrystallization in anhydrous halide flux technique was used for att aining equilibrium at 600-400 degrees C.The zero-pressure solvus of sp halerite proved to be at higher FeS contents than was located by Barto n and Toulmin (1966). Detailed calculations, based on updated thermoch emical appraisal of the sphalerite (Zn,Fe)S solution, have shown the n ew solvus to be in apparent consistency with the high-pressure experim ental data of Hutchison and Scott (1983). An improved calibration of t he cosmobarometer is presented, based on our experimental results and those of Hutchison and Scott (1983); the calibration takes into accoun t some inferences regarding thermodynamic properties of sphalerite sol ution and low-temperature polymorphism in FeS. Recently published meta llographic cooling rates of iron meteorites (Saikumar and Goldstein, 1 988) provide estimates of blocking temperatures for Fe diffusion in sp halerite, which fall in the range 205-217 degrees C Pressures of forma tion of these meteorites calculated from available sphalerite composit ions range from 0 for Landes to 1.8 kbar for Toluca. The most reliable of the pressure estimates exhibit a linear relationship with wt% Ni o f the meteorite.