Olivine xenocryst diffusion reveals rapid monogenetic basaltic magma ascent following complex storage at Pupuke Maar, Auckland Volcanic Field, New Zealand

Abstract

Abstract Monogenetic basalts are spatially and temporally unpredictable and are commonly interpreted to rise extremely rapidly and directly from their mantle source, increasing their potential hazard. The assumption of rapid ascent is commonly based upon the presence of xenocrysts and xenoliths as well as generally short OH and elemental diffusion profiles at the margins of xenocrystic material. We show that small-volume monogenetic basalts may also have complex multi-stage deep mantle magma storage prior to rapid ascent, using coupled diffusion modelling of major and trace elements and OH within olivine xenocrysts. The xenocrysts and crystal clusters were extracted from a tuff ring in Auckland City (New Zealand), within the Late Pleistocene-Holocene 100 km2 Auckland Volcanic Field. Forsterite-rich olivine xenocrysts (Fo# 89.5 - 91.7 ) have undergone Fe-Mg (Fo#), Ca, Ni and Mn element diffusion that extends up to ∼200 μm from their rims. Major and minor element diffusion at frozen melt-xenocryst interfaces was modelled using crystallographically oriented grains. These profiles show that the host basalt collected most of the olivine xenocrysts and xenoliths over approximately 1 month. The narrow OH diffusion profiles in the olivine suggests late-stage degassing over