Lead Isotope Evidence For Young Trace-element Enrichment In The Oceanic Upper Mantle

ISOTOPIC heterogeneity in ocean island basalts has generally been ascribed to processes related to the long-term cycling of mantle material1-6. A recent study of Cameroon line lavas reported higher Pb-208/Pb-204 and Pb-206/Pb-204 ratios towards the continent/ocean boundary (c.o.b.), but no corresponding increase in Pb-207/Pb-204, indicating large in situ fractionations of uranium and thorium relative to lead in the upper mantle approximately 10(8) years ago7. Here we present neodymium, strontium and lead isotope data for a variety of central Atlantic islands, and show that similar offsets in lead isotope ratios are found in lavas from the islands of Madeira and Trinidade. Like the Cameroon line c.o.b. lavas, these lavas are characterized by high U/Pb and Ce/Pb, low K/U and are located in areas of old oceanic lithosphere8. But in contrast to the Cameroon line7, the Madeira lavas are derived from an enriched MORB-type source with low Pb-207/Pb-204 and Sr-87/Sr-86 and high Nd-143/Nd-144. The lead isotope data can be explained if the U/Pb ratios in the sources are comparable to those observed for the lavas and the U/Pb fractionation occurred at the time of formation of the local oceanic lithosphere. Although we do not have a satisfactory explanation for the U/Pb fractionation, it must have occurred at shallow depths in the mantle, near a spreading ridge; and the resulting enriched source regions have since remained fixed relative to the migrating lithosphere.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62966/1/359623a0.pd

Trace-element Fractionation In Plumes And The Origin Of Himu Mantle Beneath The Cameroon Line

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62851/1/347523a0.pd

Helium and lead isotopes reveal the geochemical geometry of the Samoan plume

Hotspot lavas erupted at ocean islands exhibit tremendous isotopic variability, indicating that there are numerous mantle components hosted in upwelling mantle plumes that generate volcanism at hotspots like Hawaii and Samoa. However, it is not known how the surface expression of the various geochemical components observed in hotspot volcanoes relates to their spatial distribution within the plume4-10. Here we present a unique relationship between He and Pb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume. In Pb-isotopic space, the Samoan data form several distinct geochemical groups, each corresponding to a different geographic lineament of volcanoes. Each group has signatures associated with one of four mantle endmembers with low 3He/4He: EMII (enriched mantle 2), EMI (enriched mantle 1), HIMU (high μ=238U/204Pb) and DM (depleted mantle). Critically, the four isotopic-geographic groups converge on a common region of Pb-isotopic space with high 3He/4He. This observation is consistent with several low 3He/4He components in the plume mixing with a common high 3He/4He component, but not significantly with each other, otherwise the four isotopic groups would be obscured by mixing. The mixing relationships inferred from the new He and Pb isotopic data paint the clearest picture yet of the geochemical geometry of a mantle plume, and are best explained by a high 3He/4He plume matrix that hosts, and mixes with, several distinct low 3He/4He components