ISOTOPIC COMPOSITION OF METEORITIC THALLIUM

Thallium contents have been determined as 2.5 ± 0.5 × 10-6 in Ivigtut, Greenland, galena; 7 ± 2 × 10-10 in Plainview chondrite; 1.5 ± 0.3 × 10-9 in Canyon Diablo troilite; and 2.5 ± 0.5 × 10-10 in Canyon Diablo metal. The 205Tl/203Tl ratio is identical within about 0.6% of that of terrestrial thallium reagent, thus yielding no indication of extinct natural radioactivity of 205Pb (half-life ∼24 m.y.). Using published lead contents of the meteoritic materials, of plot of 205Tl/203Tl versus 204Pb/203Tl has been made of our results and those of Anders and Stevens [1]. Cogenetic phases initially incorporating 205Pb should yield a straight-line isochron. Only limiting slopes can be indicated for various groups of data. The uncertainties of these data and the indicated variability of the Tl/Pb ratio in meteoritic materials suggest that more precise and extensive measurements may reveal radiogenic 205Tl. © 1969.