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

Abstract

New 238U– 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 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 238U– 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 238U– 230Th disequilibria found in the more evolved SE rift lavas. Moreover, there is no variation of 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 230Th excess. The 238U– 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 230Th excesses in alkalic off-rift lavas (Snæfellsnes Peninsula) (24±3%) compared to the main rift lavas (15±3%) 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 ( 230Th/ 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 ( Φ=0.05–0.2%) 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 not requiring, a model of deep dehydration melting [Ito et al., Earth Planet. Sci. Lett. 165 (1999) 81–96]. In such a scenario, the generation of (moderately) low 238U– 230Th disequilibria will be confined to the lowermost part of the melting column, which is characterised by fast upwelling and low porosity. For Φ values down to 0.05% in the lower part of the hydrous melting zone, moderately low 230Th excesses (5–10%) are likely to result, whereas higher 230Th excesses may arise for lower values of Φ.