238U¯230Th constraints on mantle upwelling and plume¯ridge interaction along the Reykjanes Ridge

238U¯230Th¯226Ra disequilibria data are presented for dredged glasses from the slowspreading, plume-influenced Reykjanes Ridge, and lavas from the Reykjanes Peninsula, Iceland. Southern Reykjanes Ridge samples (57.7°¯58.8°N), identified as `zero-ageʹ (i.e. <8 ka) from measured 226Ra¯230Th disequilibrium, lie on a shallow trend on the (230Th/232Th) vs. (238U/232Th) equiline diagram. Within this 150 km ridge section, samples north of the weak segment boundary at 58.42°N have lower U/Th, higher Th contents and higher (230Th/238U) than those to the south. This local compositional control on the degree of 230Th¯238U disequilibrium is inferred to result from variable mixing between melts from `enrichedʹ and `depletedʹ components in the upwelling mantle. This is supported by a northward increase in 206Pb/204Pb and He isotope ratios within this region. A sample from the northern, shallow part of the Reykjanes Ridge (61.93°N) also lies within this trend, with relatively high U/Th. In contrast, post-glacial lavas on the Reykjanes Peninsula show broadly similar U/Th to the Reykjanes Ridge samples, but at significantly lower (230Th/232Th) (~1.15 vs. ~1.30). This distinction is most likely due to a difference in mantle upwelling rate ¯ a high upwelling rate associated with the mantle plume beneath Iceland will give rise to lower 230Th¯238U disequilibrium for a particular U/Th value due to the reduced opportunity for 230Th in-growth in the melt column ¯ although differences in initial melt productivity might also play a role. Active upwelling associated with the Iceland plume probably does not extend far along the Reykjanes Ridge.