Oceanic island Pb: Two-stage histories and mantle evolution

Leads in basaltic suites from seven oceanic islands form linear arrays on206Pb/204Pb versus207Pb/204Pb diagrams. These arrays are more reasonably interpreted as secondary isochrons than as mixing lines, because of their systematic relationship. Separate two-stage histories calculated for the leads from each island indicate that the source materials for the magmas were derived from a single primary reservoir with present238U/204Pb of 7.91 ± 0.04 by secondary enrichment in U/Pb at different times from 2.5 to 1 Ga ago. This is confirmed by a plot of isochron slope versus intercept, on which the points describing each island's Pb-Pb array all lie very near a single straight line. The isochrons for the Canary Islands and Hawaii, at least, are significantly different. The208Pb/204Pb versus206Pb/204Pb relationships are less coherent. The lead isotopic characteristics are consistent with a model in which lead in the oceanic island magmas is derived from ancient subducted oceanic crust. In particular, this explains the close relationship between lead in mid-ocean ridge and oceanic island basalts without invoking mixing.