super(238)U- and super(232)Th-decay series constraints on the timescales of crystal fractionation to produce the phonolite erupted in 2004 near Tristan da Cunha, South Atlantic Ocean

Phonolite pumice found floating offshore of Tristan da Cunha following intense seismic activity southeast of the island July 29-30, 2004 was analyzed for super(238)U- and super(232)Th-series nuclides to determine initial super(230)Th, super(226)Ra, super(210)Pb, super(210)Po, super(228)Ra, and super(228)Th activities. The initial ( super(210)Po/ super(210)Pb) value of 0.15 for the phonolite shows that, like most subaerial lavas, this subaqueous tephra degassed most of its super(210)Po upon eruption. The ( super(230)Th/ super(232)Th) and ( super(238)U/ super(232)Th) values for the phonolite are similar to those of the trachyandesites erupted in 1961 from Tristan da Cunha. However, the relative activities of super(210)Pb, super(226)Ra, and super(230)Th in the phonolite contrast with those of the trachyandesites, in that super(210)Pb and super(230)Th are both strongly enriched with respect to super(226)Ra. In addition, the phonolite had a small deficit in super(228)Ra with respect to super(232)Th. 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 x 10 super(-3) year super(-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.