Isotopic composition of lead from oceanic islands

Lead isotopic compositions have been determined for recent volcanic rocks from the islands of Tristan da Cunha and St. Helena in the Atlantic Ocean by an analytical method that allows correction of observed ratios to absolute ratios. Lead and uranium concentrations were also measured for all samples. The phonolitic rocks on St. Helena have 207/204 ratios that are similar within the group, but are very different from the 207/204 ratios of basaltic rocks on St. Helena. This difference in Pb²⁰⁷/Pb²⁰⁴ appears to be due to original heterogeneity in the mantle U/Pb ratio. Within the phonolites, a clear linear trend with a slope corresponding to 1.8×10⁹ years was found. The highly radiogenic leads from St. Helena phonolites are interpreted by a two-stage growth model, the second stage starting 1.8×10⁹ years ago. There is an indication among the basalt samples of a similar two-stage history, the second stage starting at approximately the same time as for the phonolites. A three-stage model with fractionation events at 4.1×10⁹ and 1.8×10⁹ years is also considered for the St. Helena samples. Trachyandesites from the 1961 flow on Tristan da Cunha show considerable variation in their lead isotopic composition. This variation indicates the presence of two lead components, one of which belonged to the original magma. The second component is inferred to have come from the walls of a magma chamber via the incorporation of xenocrysts into the magma. The distribution of lead isotopic compositions from oceanic islands is discussed on a global scale. There is evidence for original heterogeneity in U/Pb and Th/U ratios amounting to about 5%. The implications of several models that attempt to explain the observed distribution pattern are discussed.