Lead Isotopic Study of Young Volcanic Rocks from Mid-Ocean Ridges, Ocean Islands and Island Arcs

Lead isotopic compositions of young volcanic rocks from different tectonic environments have distinctive characteristics. Their differences are evaluated within the framework of global tectonics and mantle differentiation. Ocean island leads are in general more radiogenic than mid-ocean ridge basalt (m.o.r.b.) leads. They form linear trends on lead isotopic ratio plots. Many of the trends extend toward the field of m.o.r.b. On plots of $^{207}$Pb/$^{204}$Pb against $^{206}$Pb/$^{204}$Pb, their slopes are generally close to 0.1. Island arc leads in general are confined between sediment and m.o.r.b. type leads with slopes of ca. 0.30 on a plot of $^{207}$Pb/$^{204}$Pb against $^{206}$Pb/$^{204}$Pb. Pb, Sr and Nd isotopic data of Hawaiian volcanics are closely examined. Data from each island support a two-component mixing model. However, there is a lack of full range correlation between islands, indicating heterogeneity in the end members. This mixing model could also be extended to explain data from the Iceland-Reykjanes ridge, and from 45 degrees N on the Atlantic Ridge. The observed chemical and isotopic heterogeneity in young volcanic rocks is considered to be a result of long-term as well as short-term mantle differentiation and mixing. Lead isotopic data from ocean islands are interpreted in terms of mantle evolution models that involve long-term (more than 2 Ga) mantle chemical and isotopic heterogeneity. Incompatible element enriched 'plume' -type m.o.r.b. have Th/U ratios ca. 3.0 too low and Rb/Sr ratios ca. 0.04 too high to generate the observed $^{208}$Pb and $^{87}$Sr respectively for long periods of time. Elemental fractionation in the mantle must have occurred very recently. This conclusion also applies to mantle sources for ocean island alkali basalts and nephelinites. Depletion of incompatible elements in m.o.r.b. sources is most probably due to continuous extraction of silicate melt and/or fluid phase from the low-velocity zone throughout geological time. Data on Pb isotopes, Sr isotopes and trace elements on volcanic rocks from island arcs are evaluated in terms of mixing models involving three components derived from (1) sub-arc mantle wedge, (2) dehydration or partial melting of subducted ocean crust, and (3) continental crust contamination. In contrast to the relation between $^{87}$Sr/$^{86}$Sr and $^{143}$Nd/$^{144}$Nd ratios of ocean volcanics, there is a general lack of correlation between Pb and Sr isotopic ratios except that samples