Two submarine volcanoes (named Friday and Domingo) have been mapped and sampled to the west of the youngest island (Alexander Selkirk) in the Juan Fernandez chain. Samples from the seamounts are fresh, highly vesicular olivine and
plagioclase-phyric basanites. Their MgO contents lie between 7 and 4 wt. %. Major element variation trends, especially decreasing SiO2 with increasing MgO, cannot be explained by crystal fractionation, and suggest the influence of a partial melting factor. From published experimental work on the genesis of basanites, we propose that this factor is the presence of CO2 during melting. Contents of most highly incompatible element in Friday and Domingo magmas are similar, but differences between the volcanoes occur in the less incompatible heavy rare-earth elements. These differences can be modelled as resulting from variations in the amounts of garnet in the mantle source of the two volcanoes. The slope of the rare-earth element patterns can best be modelled by 2-4% fractional melting of a source containing 5-10% garnet whose REE content reflects a 50:50 mixture of primitive mantle and N-MORB. Although most highly incompatible element ratios in both Friday and Domingo magmas are identical, Nb and especially Th are enriched relative to the other elements. The enrichment of Nb relative to Ta is particularly unusual, as under most magmatic conditions these elements should behave identically. Examination of literature data for carbonatites (to investigate the possible influence of CO2 on the trace element behaviour during melting) shows that such magmas often have extreme enrichment in Nb and/or Th, but never in Ta, and so are comparable to the Juan Fernandez magmas at least in the style if not the magnitude of their trace element enrichment. The calcio-carbonatites, whose enrichment of Th relative to Nb best mirrors that seen at Juan Fernandez, have been suggested to be generated from depleted harzburgitic or lherzolitic source regions. Such a depleted source for the Juan Fernandez magmas is in agreement with the REE modelling results outlined above. Isotopically, the Juan Fernandez magmas lie between the composition of prevalent mantle (PREMA) and HIMU. The latter has also been suggested, based on ion microprobe studies of xenoliths, to be associated with CO2-rich melting conditions. The presence of CO2 in the mantle during melting was a major factor influencing the petrography, major and trace element chemistry of the Juan Fernandez magmas.