Melts from hydrous depleted back-arc mantle at a young boninitic volcano on the flank of Valu Fa Ridge (Lau basin, SW Pacific)

Volcano S3 is a young seamount located similar to 5 km east of the Valu Fa Ridge (VFR) in the Lau Basin, which has erupted boninitic lavas. We present new data for whole-rock compositions, minerals and olivine-hosted melt inclusions, and propose a new model for low-Si boninite (LSB) petrogenesis in a subduction-related setting. The whole-rock compositions reflect mixing of less primitive melt with variable amounts (similar to 30%) of minerals, whereas quenched glass compositions result from (similar to 60%) crystal fractionation of a primitive boninitic magma which is preserved as melt inclusions hosted in olivine crystals. The compositions of primitive olivine and spinel from S3 are more refractory than those from MORB and back-arc basin (BAB) lavas. Lower Fe8 and Na8 values of S3 than those of BAB lavas reflect melting of relatively depleted mantle at high temperature and low pressure. The mantle beneath S3, and the adjacent VFR and Ata is more depleted than those of BAB located further from the arc, and S3 and Ata lavas have similar slab input (similar to 1%). The geochemical differences between S3 and BAB lavas imply that the boundary between dry and wet mantle is located beneath VFR. We suggest that a vertical mantle flow beneath VFR brings depleted mantle containing slab components from deeper within the wedge. This depleted and wet mantle probably experienced a high degree of partial melting at low pressure to form LSB magmas beneath S3. The LSB lavas can be formed at back-arc spreading centers which are located close enough to an active arc.

Automated summary

This study examines the eruption rocks of Volcano S3 and proposes a model for the formation of low-Si boninite. The results show that the mineral and whole-rock compositions of S3 reflect the influences of mixing and crystallization processes. The differences in rock compositions between S3 and back-arc basin lavas imply the existence of a dry and wet mantle beneath the Valu Fa Ridge.