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

Abstract

238U– 230Th– 226Ra disequilibria data are presented for dredged glasses from the slow-spreading, plume-influenced Reykjanes Ridge, and lavas from the Reykjanes Peninsula, Iceland. Southern Reykjanes Ridge samples (57.7°–58.8°N), identified as ‘zero-age’ (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 ‘enriched’ and ‘depleted’ 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.