LEAD ISOTOPE MEASUREMENTS ON LHERZOLITE INCLUSIONS AND HOST BASANITES FROM WESTERN VICTORIA AUSTRALIA

Lead samples extracted from a group of lherzolite inclusions and from their host basanites have been isotopically analyzed. The lead from the basanites varies only slightly in 206Pb/204Pb and 208Pb/204Pb and is similar to average modern terrestrial lead. The lherzolite lead is isotopically different, showing varying degrees of 206Pb and 208Pb deficiency. On a 206Pb/204Pb versus 207Pb/204Pb diagram, the data appear to follow the average regression line drawn through a world-wide selection of modern basaltic volcanics. The data require separate immediate source regions, differing in U, Th, Pb relationships, for the lherzolites and basanites and demonstrate that the lherzolites cannot have a cognate relationship to their host basanite. A more detailed analysis of the isotopic data suggests that an evolutionary model of at least two stages is required and calculations based on a two stage model suggest formation of the lherzolites from prior source material with higher μ and lower κ values at 2-2.5 billion years ago. © 1969.