How long have we got to act? Magma ascent rates in the Auckland volcanic field

Abstract

The city of Auckland faces the possibility of a volcanic eruption occurring within its bounds. Forecasting when that might occur is still beyond our capabilities, but we are dealing with matters of “when” rather than “if”. Preparedness is therefore the only tool at our disposal to minimize impact. It is therefore crucial that we know how much time might elapse between the onset of seismic unrest, indicative of magma making its way to the surface, and the ensuing eruption.

To our aid, come tiny olivine crystals that were originally residing in the mantle (between 30-80 km depth). They witnessed past volcanic events as passengers in the magma that fed some of the eruptions in the Auckland Volcanic Field. During this journey to the surface, the crystals exchange atoms with their host magma by diffusion, and acquire a specific chemical signature along their rims. The width of these diffusion rims was employed to estimate the time that crystals spent travelling from their residence site in the mantle to the surface.

The model results indicate that magma can rise from the mantle in as little as one month to a few weeks, after protracted periods of deep storage of months to years. This might sound like ample enough time to arrange an emergency response. However, seismometers can only detect small cracks opening in the crust once the magma has reached about 15 km depth, meaning that it has already travelled 4/5 of the way to the surface. As a consequence, our warning time might be only one fifth of the time employed by the magma to reach the surface from the mantle; a few days at best.

The investigation of diffusion rims in mantle olivine crystals carried to the surface by Auckland magmas has highlighted the complexity of deep plumbing processes, as well as the rapid ascent of magma batches that ultimately generate eruptions. The existence of long lived deep magma storage and rapid ascent are a further reminder that we can not complacently remain under the impression that nothing has happened for the past 600 years, and hence we can sit back and relax. Awareness and preparedness are our best tools.

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Technical 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 have shown 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 (AVF). 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 microns 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.