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Phonolite pumice found floating offshore of Tristan da Cunha following intense seismic activity southeast of the island July 29–30, 2004 was analyzed for 238U- and 232Th-series nuclides to determine initial 230Th, 226Ra, 210Pb, 210Po, 228Ra, and 228Th activities. The initial (210Po/210Pb) value of 0.15 for the phonolite shows that, like most subaerial lavas, this subaqueous tephra degassed most of its 210Po upon eruption. The (230Th/232Th) and (238U/232Th) values for the phonolite are similar to those of the trachyandesites erupted in 1961 from Tristan da Cunha. However, the relative activities of 210Pb, 226Ra, and 230Th in the phonolite contrast with those of the trachyandesites, in that 210Pb and 230Th are both strongly enriched with respect to 226Ra. In addition, the phonolite had a small deficit in 228Ra with respect to 232Th. The Ra deficits likely resulted from partitioning into feldspars and hornblende in a time frame that extended over several decades to a century. These disequilibria can be explained by crystal fractionation at a decreasing rate through time at an average of 3–5 × 10−3 year−1. The calculated crystallization rate is about an order of magnitude faster than has been calculated for most other phonolites and trachytes, and about half that calculated for crystallization of the Makaopuhi lava lake. These data imply that the 2004 magma was not the differentiated cap of a much larger body that remained at depth. Instead, it was likely the residue of a relatively small body of more mafic magma that was injected into the crust southeast Tristan and underwent extensive and rapid crystal fractionation before it erupted.

Geochemical analysis is used to investigate fragmentation and physical fractionation in pyroclastic flows. Bulk analyses of the matrices (

The Snaefell volcanic centre is situated in central-east Iceland, at the northern end of a short flank volcanic zone. Its products represent a typical suite of Icelandic volcanics and comprise a bimodal suite of alkalic lavas: a series from basaltic to mugearitic compositions and a small cluster of peralkaline rhyolites. Compositional variations across the whole series can be broadly explained by fractional crystallisation of a family of related parental/primary magmas in a magma chamber at mid-crustal (∼13 km) levels, that has been subject to periodic replenishment and periodic tapping, with sufficient repose times for extreme differentiation. Interaction with the crust appears minimal, although some crustal input into the rhyolites is indicated by their isotopic characteristics. The Snaefell volcanics therefore represent largely new additions to the Icelandic crust. The apparent depth of the magma chamber appears significant and sets Snaefell apart from the axial rift zones at centres such as Krafla, where seismic studies (e.g. Brandsdottir, B., Menke, W., Einarsson, P., White, R.S., Staples, R.K., 1997. Faeroe–Iceland ridge experiment. 2. Crustal structure of Krafla central volcano. J. Geophys. Res. 102 (B4), 7867–7886) have detected the presence of magma bodies at depths of around 3 km beneath centres in the northern volcanic zone. Comparing Snaefell with volcanic centres in the propagating eastern volcanic zone suggests that, in terms of its stage of evolution, Snaefell is approximately equivalent to Torfajokull. Thus, the Oraefajokull–Snaefell volcanic zone may have represented a site of incipient rifting.

High-MgO liquids erupted in ocean-island settings and in some continental flood-basalt provinces commonly preserve a "depleted" composition, in terms of both highly incompatible trace elements and isotope ratios. These observations strongly imply that their source is also compositionally depleted. However, in at least one case (Iceland and the North Atlantic volcanic province), it can be shown that this depleted source is not the same as that feeding the present-day North Atlantic mid-ocean ridge. The depleted source must also have been much hotter than the mid-ocean ridge basalt (MORB) source to account for the volume of melt and primitive composition of some magmas that were generated. This depleted character, then, is an intrinsic component of mantle plumes, originating from the deep mantle. We propose that mantle plumes consist of a mixture of enriched or fusible streaks in a depleted, refractory matrix; preferential extraction of the enriched component occurs close to the plume axis. The depleted residue from this melting remains in the upper mantle and may therefore be a major contributor to the source region of MORB.