Radioactivation determination of palladium in basaltic and ultrabasic rocks

Abstract

Neutron activation analysis has been used to determine the Pd content of 23 basaltic rocks, 4 ultrabasic rocks and a sample of Sudbury quartz diorite. A Pd content of 15.8 ± 1.4 ppb (parts per billion) was obtained for 10 analyses of W-1 and compares favourably with 18.5 ppb (average of duplicate analyses) found by Vincent and Smales (1956). It was found experimentally that a U Pd ratio of 180 caused a 1 per cent interference due to production of Pd isotopes by U 235 fission. This potential source of interference is not serious for Pd determination in basalts and ultrabasics by neutron activation but could cause serious interference in analyses of granitic rocks for Pd. The basaltic rocks analysed were largely representatives of the nonorogenic intra-oceanic basalts and the continental plateau-building basalts. The oceanic rocks were characterized by less variation in Pd than the continental basalts and averaged 1.9 ± 1.2 ppb (range 0.25 to 3.7 ppb). The latter group was characterized by regional variations in Pd content such that the variation shown by any single province (e.g. Deccan or Columbia River) was considerably less than that shown by the group as a whole. The range of Pd values found for continental basalts was 0.45 to 29.4 ppb. A few basaltic rocks from orogenic regions were also analysed. Three ultrabasic rocks, an associated chromite and a quartz diorite from Sudbury, Ontario had a range of 1.3 to 22.2 ppb Pd. Although crystal fractionation during magma crystallisation is probably an important factor in producing Pd variations in basalts, the regional trends found in the plateau-building rocks suggest that the Pd content of the source material is also an important parameter. The Pd data are briefly discussed in terms of recent hypotheses on the petrogenesis of basaltic rocks.