Ion Microprobe Confirmation of Pb Isotope Ratios and Search for Isomer Precursors in Polonium Radiohaloes

RADIOHALOES associated with decay of several Po α emitters 1,2 have been studied by optical microscopic techniques and more recently by mass spectrometric examination of the halo inclusion using ion microprobe techniques 3,4 . In such cases a large excess of 206 Pb compared with 207 Pb was found to be incompatible with the radiogenic decay of 238 U and 235 U, yet was explainable on the basis of polonium decay independent of uranium 3 . A straightforward attempt to account for the origin of these Po haloes by assuming that Po was incorporated into the halo inclusion at the time of host mineral crystallization meets with severe geological problems: the half-lives of the polonium isotopes ( t 1/2 =3 min for 218 Po) are too short to permit anything but a rapid mineral crystallization, contrary to accepted theories of magmatic cooling rates.