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 <1 day (i.e., not extremely rapid ascent rates). Some olivine crystals have diffusion profiles requiring step function initial conditions; these indicate that magma resided in the mantle for up to one year and accumulated from multiple batches of mixed magmas. Our results show that primitive magmas in small volume monogenetic volcanoes may have complex lithospheric magmatic histories, but they may suddenly rise to eruption, with seismic detection providing less than a week of warning.
•Diffusion in lithospheric olivine gives magma ascent rates of Auckland alkaline basalt.•Ascent rates are 0.01–0.03 m s−1 based on Mg/Fe, Ca, Mn, Ni and Li diffusion, and apparent 1–10 m s−1 based on H2O diffusion.•Major and minor elements give ascent rates from lithospheric mantle depth.•Water diffusion gives timescales of shallow degassing.•Complex diffusion profiles indicate deep interaction of different magma batches.