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

U-238-Th-230-Ra-226 disequilibria data are presented for dredged glasses fr om the slow-spreading, plume-influenced Reykjanes Ridge, and lavas from the Reykjanes Peninsula, Iceland. Southern Reykjanes Ridge samples (57.7 degre es -58.8 degreesN), identified as 'zero-age' (i.e. <8 ka) from measured Ra- 226-Th-230 disequilibrium, lie on a shallow trend on the (Th-230/Th-232) VS . (U-238/Th-232) equiline diagram. Within this 150 km ridge section, sample s north of the weak segment boundary at 58.42 degreesN have lower U/Th, hig her Th contents and higher (Th-230/U-238) than those to the south. This loc al compositional control on the degree of Th-230-U-238 disequilibrium is in ferred to result from variable mixing between melts from 'enriched' and 'de pleted' components in the upwelling mantle. This is supported by a northwar d increase in Pb-206/Pb-204 and He isotope ratios within this region. A sam ple from the northern, shallow part of the Reykjanes Ridge (61.93 degreesN) 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 R eykjanes Ridge samples, but at significantly lower (Th-230/Th-232) (similar to1.15 vs. similar to 1.30). This distinction is most likely due to a diff erence in mantle upwelling rate - a high upwelling rate associated with the mantle plume beneath Iceland will give rise to lower Th-230-U-238 disequil ibrium for a particular U/Th value due to the reduced opportunity for Th-23 0 in-growth in the melt column - although differences in initial melt produ ctivity might also play a role. Active upwelling associated with the Icelan d plume probably does not extend far along the Reykjanes Ridge. (C) 2001 El sevier Science B.V. All rights reserved.