Timescales of magmatic processes at Ruapehu volcano from diffusion chronometry and their comparison to monitoring data

•Diffusion chronometry has been completed on pyroxenes from five historical eruptions at Mt. Ruapehu•All diffusion timescales record a period of magma injection 3-5 months before each eruption•Recent phreatic eruptions were preceded by seismic swarms 5 months prior•This agreement suggests that deep seismicity may be a useful medium-term precursor to phreatic/magmatic eruptions

Recent eruptions from Mt. Ruapehu have been difficult to predict, despite the presence of a multi-parametric monitoring network. As a result, it is necessary to assess precursory signals prior to an eruption and align those to magmatic processes at depth. Fortuitously, scoria from all historical Ruapehu eruptions contains pyroxene crystals that are strongly reversely zoned in the form of a thin (2 to 3 μm), outermost rim. These crystals therefore preserved changes in the magmatic system soon before their eruption. We used experimentally determined diffusion coefficients to assess the timescales of magma–magma interaction, and compared those to the monitoring record. Four of the five eruptions analysed (1969, 1971, 1977, 1995) gave diffusion timescales ~ 3 to 5 months before their eruption, with an increased number of crystals recording timescales within 1 month of eruption. Pyroxene crystals from the 1996 eruption record events that occurred prior to and during the 1995 eruption suggesting that the bulk of the 1996 crystals was derived from the 1995 magma. These diffusion timescales do not compare well to a change in any monitoring signal before historical eruptions. However, an examination of recent seismicity (2005–2013) since a significant upgrade (both in number of stations and type of seismometers) showed that two phreatic eruptions in 2006 and 2007 were preceded by a seismic swarm from ~ 5 to 15 km depth, ~ 3 to 5 months before each eruption — consistent with the diffusion timescales. Based on this correlation, deep seismic swarms likely indicate a period of pressurisation in the magmatic system, which may lead to gas-rich, phreatic eruptions.