REMOVAL OF RADIOGENIC LEAD FROM POTASSIUM FELDSPARS BY VOLATILIZATION

The isotopic composition of lead was measured in potassium feldspars from granitic rocks and pegmatites having ages of 2720, 1850 and 400 m.y. Isotopic compositions were determined in three fractions for each sample: (a) the bulk material; (b) a fraction of lead, usually about one-third of the total amount, removed by volatilization; (c) the lead remaining in the sample after volatilization. In some cases the isotopic compositions of the lead were nearly identical in all three fractions, while in others, lead was more radiogenic in the volatile fraction and less radiogenic in the residual fraction than in the bulk sample. In most, though not in all cases, the isotopic composition of lead in the homogeneous samples agreed with the isotopic composition of lead in coeval galenas. In samples where radiogenic lead was heterogeneously distributed, the isotopic composition of lead in the residual samples agreed more closely with that in the galenas. In some samples the residual lead had an isotopic composition nearly identical to that of the galenas even though the lead in the bulk sample was more radiogenic. These experiments show that the radiogenic lead in potassium feldspars may be heterogeneously distributed in sites where it is less strongly bound than the remainder of the lead. © 1969.