Effects of mantle flow on the chemistry of Coriolis Troughs backarc magmas

Abstract

Ridge subduction along convergent plate margins can cause a tear to develop in the underlying slab, which allows the influx of hot sub-slab mantle materials into the mantle wedge. Such a process has a profound influence on the chemistry of Vanuatu arc magmas, but its effect on the genesis of Vanuatu backarc magmas remains to be assessed. Here we present new geochemical data for samples from the Coriolis Troughs (part of the Vanuatu backarc) and combine our new data with published analyses of samples from the entire Vanuatu arc. Many samples from the Coriolis Troughs have major element systematics, chalcophile element systematics and trace element ratios (i.e., Ba/Nb, Th/Nb) that are more similar to the global MORB array than Vanuatu arc samples. These systematics can be attributed to slab rollback prior to ridge subduction, which caused a progressive decline in the proportion of slab fluxes to the mantle source of the Coriolis Trough magmas. As rollback progressed, deep upwelling of enriched mantle components beneath the North Fiji Basin influenced the compositions of the erupted backarc basin magmas. Melting of this enriched mantle source generated the most recent Nifonea Caldera Floor magmas. In contrast to the other recent Coriolis Trough magmas, the Young Lava Plain samples have geochemical characteristics that are comparable to Vanuatu arc magmas. The Young Lava Plain magmas are situated to the southeast of a slab tear. We therefore suggest that influx of hot sub-slab mantle materials through a slab tear caused southeast flow (i.e., mantle flow away from the trench) of sub-arc mantle (i.e., mantle that was enriched by hydrous and oxidising components released from the slab at sub-arc depths) and that this sub-arc mantle source generated the Young Lava Plain magmas. In particular, we suggest that this oxidised source can account for the anomalous chalcophile element signatures of the Young Lava Plain samples compared to the other Coriolis Trough samples. Because all of the Coriolis Trough samples have similar depth-to-slab, we propose that changes in mantle flow patterns caused by ridge subduction and the development of a slab tear can account for the genesis of proximal magmas with comparable ages but distinct compositions.