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

摘要

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, SE Pacific. Samples from the sea-mounts are fresh, highly vesicular olivine and plagioclase-phyric basanites. Their MgO contents lie between 7 and 4 wt. percent. Major element variation trends, especially decreasing SiO_2 with increasing MgO, cannot be explained by crystal fractionation, and suggest the influence of CO_2 during partial melting. Highly incompatible element ratios in both Friday and Domingo magmas are identical, with the exception 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 Al_2O_3. All these geochemical features can be explained if the Domingo magmas reacted with harzburgitic mantle materials during transit to the surface in a manner shown experimentally to occur during CO_2-dominated kimberlite magmatism. The metasomatism results in the stabilisation of clinopyroxene, rutile and zircon which withhold the elements depleted at Domingo, and the breakdown of garnet which releases HREE and Al into the magmas. Magmas erupting from the large, more mature Friday edifice have traversed a mantle region already metasomatised during earlier stages of volcanism and so are not significantly modified during passage. The Juan Fernandez trace element patterns are similar to the low ~(87)Sr/~(86)Sr, high ~(143)Nd/~(144)Nd components in many Pacific hotspots and to the pattern suggested for recycled, altered, dehydrated oceanic crust, implying that such recycled crust is a common component in many hotspots. Isotopically, the Juan Fernandez magmas lie between the composition of prevalent mantle (PREMA) and HIMU.