Upwelling and melting of the Iceland plume from radial variation of super 238U- super 230Th disequilibria in postglacial volcanic rocks

• Published in: Earth and planetary science letters 2003-09, Vol.214 (1-2), p.167-186
• Main Authors: Kokfelt, T F, Hoernle, K, Hauff, F
• Format: Article
• Language: English
• ISSN: 0012-821X
• DOI: 10.1016/S0012-821X(03)00306-6

New super 238U- super 230Th disequilibria data by thermal ionisation mass spectrometry are presented for a comprehensive set of postglacial basaltic lavas from the neovolcanic zones in Iceland. The new data show a striking systematic decrease in super 230Th excess towards central Iceland and the presumed centre of the Iceland plume. This finding would appear paradoxical if source composition was the main factor responsible for generating the super 238U- super 230Th disequilibria, because generally main rift lavas erupted proximal to the plume should be generated from a melting column that initiates deeper in the garnet stability field, compared to the marginal rift zones. Preferential crustal interaction in central Iceland, where the crust is thickest, involving either old ( > 350 kyr) Icelandic crust or lower crustal melts, may provide a viable explanation for only part of the data variation, namely the moderately low super 238U- super 230Th disequilibria found in the more evolved SE rift lavas. Moreover, there is no variation of super 230Th excesses with degree of differentiation (Mg# or ppm Th) overall, or within individual rift systems, to indicate that crustal contamination causes the radial variation in super 230Th excess. The super 238U- super 230Th disequilibria variation is therefore ascribed to variable dynamic parameters in the melting regime induced by interaction of the Iceland plume with the rift systems. The higher super 230Th excesses in alkalic off-rift lavas (SnAEfellsnes Peninsula) (24+/-3 percent) compared to the main rift lavas (15+/-3 percent) is consistent with more garnet-rich lithologies dominating the bulk melt compositions away from the main rifts and indicates small-scale source heterogeneity beneath Iceland. The data are reconciled within a model in which mantle upwelling rates in the centre of the plume are significantly faster than at the margins, consistent with fluid dynamic predictions for a plume head. The radial variation observed in ( super 230Th/ super 238U) provides independent support that the centre of the Iceland plume is located beneath SE Iceland, as has been proposed from seismic tomographic studies. For a reasonable range of mantle porosities ( Phi =0.05-0.2 percent) we can explain the Iceland data with a dynamic melting model, by relatively fast mantle upwelling rates in the centre (~5-20 cm/yr), compared to those at the margins (~1-4 cm /yr). The radial variation is also shown to be consistent with, though no