Conductive Warming of Mantle Around a Plume: Evidence From the Spreading Axis North of Iceland

The abnormally thick crust under Iceland is attributed to the effects of a mantle plume whose higher temperature and/or lower solidus leads to excess melting. The spreading axes around Iceland also have abnormally thick crust and are anomalously shallow, implying that their mantle source is also producing anomalously large amounts of melt. As with Iceland, this could be due to the effects of either an exceptionally fertile or an exceptionally hot mantle. Directly north of Iceland the Mid-Atlantic Ridge is displaced some 50 km left-laterally over an along-axis distance of approx. 100 km in an area known as the Tjoernes Offset. Recent multibeam mapping of the area has shown this offset to consist of an en echelon arrangement of volcanic ridges (the Tjoernes Volcanic Ridges, TVR), observations and sampling of these ridges in summer 2005 with an ROV has provided evidence that many of them have experienced post-glacial volcanism, thus providing a quasi-instantaneous view into the mantle at the margin of this plume. The compositions of the samples from the TVR provide evidence that they are generated by increasingly higher degrees of melting of a depleted mantle source as the Icelandic coast is approached. Thus MgO rises, K/Ti and Na8 decrease towards Iceland. The decrease in indices of mantle enrichment such as K/Ti and the increase in indices of mantle temperature such as MgO suggest that the increasing degree of melting is the result of increasing mantle temperature. We interpret this as the thermal effect of approaching the Icelandic plume. Using observations on the width of the zone apparently affected by this thermal effect and estimates of the plume excess temperature and the thermal conductivity of peridotite, we can place limits on how long the mantle source for the Tjoernes volcanoes must have been in contact with the Iceland plume.