Metasomatic reactions between carbonated plume melts and mantle harzburgite: the evidence from Friday and Domingo Seamounts (Juan Fernandez chain, SEPacific)

Two submarine volcanoes (named Friday and Domingo) have been mapped and sam pled to the west of the youngest island (Alexander Selkirk) in the Juan Fer nandez chain, SE Pacific. Samples from the seamounts are fresh, highly vesi cular olivine and plagioclase-phyric basanites. Their MgO contents lie betw een 7 and 4 wt.%. Major element variation trends, especially decreasing SiO 2 with increasing MgO, cannot be explained by crystal fractionation, and su ggest the influence of CO2 during partial melting. Highly incompatible elem ent ratios in both Friday and Domingo magmas are identical, with the except ion of ratios involving Th and Nb for which Domingo shows depletions. These depletions are coupled with depletions in Zr, Hf and Ca and enrichments in the heavy rare-earth elements and Al2O3. All these geochemical features ca n be explained if the Domingo magmas reacted with harzburgitic mantle mater ials during transit to the surface in a manner shown experimentally to occu r during CO2-dominated kimberlite magmatism. The metasomatism results in th e stabilisation of clinopyroxene, rutile and zircon which withhold the elem ents depleted at Domingo, and the breakdown of garnet which releases HREE a nd Al into the magmas. Magmas erupting from the large, more mature Friday e difice have traversed a mantle region already metasomatised during earlier stages of volcanism and so are not significantly modified during passage. T he Juan Fernandez trace clement patterns are similar to the low Sr-87/Sr-86 , high Nd-143/Nd-144 components in many Pacific hotspots and to the pattern suggested for recycled, altered, dehydrated oceanic crust, implying that s uch recycled crust is a common component in many hotspots. Isotopically, th e Juan Fernandez magmas lie between the composition of prevalent mantle (PR EMA) and HIMU.