Influence of host magma alkalinity on trachytic melts formed during incongruent orthopyroxene dissolution in mantle xenoliths

Spectacular reactions textures developed during the non-isochemical breakdown of orthopyroxene and spinel in peridotite mantle xenoliths in intraplate alkaline basalts from the Auckland Volcanic Field and the Dunedin Volcano in New Zealand, and Jeju Island in South Korea. Mantle orthopyroxene is replaced by symplectitic intergrowths of silica-rich glass and olivine (± clinopyroxene ± orthopyroxene) with individual minerals showing large diffusion gradients at their margins proximal to the host melt. The glasses are basaltic to trachytic and formed by incongruent melting of orthopyroxene and subsequent re-equilibration by diffusion and mixing during changing boundary conditions with the host magma. Although the glass composition is strongly dependent on the alkalinity of the host magma, the distance to the reacting contact and the respective phase assemblage that develops in the reaction zone (melt + ol ± clinopyroxene ± orthopyroxene), the most evolved melts converge at trachytic composition. Diffusion-driven Fe enrichment for mantle olivine indicates residence timescales within the host magma on the order of weeks to several months.