Melt Inclusions in Olivine Phenocrysts From Primitive Submarine Subduction-Related Lavas From the Hunter Ridge (North Fiji Backarc Basin, SW Pacific)

The southern end of the North Fiji backarc basin is a volcanically active submarine boundary between the backarc basin spreading centre, the Vanuatu trench and the Hunter Ridge. This area has been mapped and sampled during the SS10/2004 and SS08/2006 voyages of the R/V Southern Surveyor. The processes occurring during southward propagation of the backarc basin involve sequential formation of 50-100 km long rift zones that split the crust of the Hunter Ridge along its strike. Initially such rift zones form as narrow (2-3 km wide) well-defined grabens with incipient magmatic activity. Then the graben widens to form a robust magmatic rift with clear volcanic centres and incipient spreading. Finally, the rift undergoes rotation and evolves into a N-S aligned robust spreading segment. Volcanic rocks erupted during rifting within each segment include a large spectrum of subduction-related magma compositions which vary from incompatible element-depleted to strongly enriched, with boninitic, tholeiitic and adakitic affinities. The rift lavas often display clear petrographic and geochemical evidence for extensive mixing between high-Mg adakitic and backarc basin magmas. Many of the samples are very primitive high-MgO olivine-phyric lavas representing the entire spectrum of the geochemical magma types. Olivine phenocrysts in most samples have a range of compositions from Fo> 92 to Fo < 85 and contain abundant primary melt inclusions. Olivine grains with melt inclusions have been experimentally reheated in a Vernadsky-type heating stage under visual control and inclusions quenched to glass and exposed for chemical analysis. Inclusions and host olivines were analysed by electron microprobe, FTIR spectroscopy and LA-ICPMS for major elements, H2O and trace elements including base metals. The compositions obtained will be used to constrain magma generation and evolution process in this complex tectonic setting.