Eruptions of soft basalt volcanoes in the Kaikohe-Bay of Islands field

Snapshot

What crystals tell us about eruptions in the Kaikohe – Bay of Islands volcanic field

Volcanoes in the Kaikohe – Bay of Islands volcanic field have received very little scientific attention to date. The most recent eruption occurred 43,000 years ago, indicating the field is dormant but not extinct. A new eruption would impact regional agriculture, horticulture, tourism and aviation.

This research investigated whether magmas temporarily “stall” in the crust before eruption. If so, it may be easier to detect signals that point to volcanic unrest looming. Volcanoes in the field are basaltic and considered “monogenetic”, meaning they erupt only once and future eruptions will occur in new locations. Analysis of crystals erupted in previous events, indicate the magma was stored at shallow levels before eruption.

The characteristics of crystals in the basalt showed that they formed at depths around 15km and at 27km. The findings also showed that some crystals originated from an older magma and then continued to grow from new magma that invaded the older magma that had stalled in the crust. There was also evidence that some magma in the crust had not erupted. Analysing the volcanic crystals showed that this process of rising then stalling may happen several times without eruption at that location. This fits with geophysical data that indicates a low-velocity zone of partial melt under the field at 10 – 19km.

The effect of this “stalling” in the crust would prolong the unrest before an eruption, increasing the potential for detecting seismic or degassing signals. It also suggests that seismic activity could be at 15km and at 27km in the crust where the density of the rock changes and delays the buoyant rise of new hot magma.

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Technical Abstract

Late Quaternary basalts erupted in the Kaikohe-Bay of Islands area provide an opportunity to explore the ascent history of small volume magmas in an intra-continental monogenetic volcano field, and hence, improve our understanding of potential future precursor phenomena. To achieve this goal, we investigated the formation and growth history of phenocrysts (crystals) in the basalts. The plagioclase phenocrysts represent a diverse crystal cargo. Most of the crystals have a rim growth that is in equilibrium with the host basalt rock. The resorbed cores of the crystals variously formed in more differentiated or more primitive melts. The relic cores have 87Sr/86Sr ratios that are either mantle-like (~ 0.7030) or crustal-like (~ 0.7040 to 0.7060), indicating some are antecrysts formed in melts fractionated from plutonic basaltic forerunners, while others are true xenocrysts from greywacke basement and/or Miocene arc volcanics.

Clinopyroxene phenocrysts also record magma mixing and crystal entrainment in the crust. Like the accompanying plagioclase, many have a rim overgrowth which is in equilibrium with the host rock, but have a resorbed core that crystallised in a more silicic magma. These crystals record mafic recharge, presumably the trigger to eruption. Crystal-melt equilibria indicate that the clinopyroxene formed at a narrow temperature range (1095-1200 ºC), but wide pressure range (150-870 MPa). Most formed at 300-600 MPa (~11-23 km depth), and a subordinate population formed at 700-900 MPa (>27 km depth). These depths coincide with major seismic velocity contrasts at a zone of partial melt (10-19 km) and the Moho (~28 km) inferred from geophysical data. Thus, buoyancy or rheology contrasts in the crust temporarily slow magma ascent and promote periods of melt crystallisation and the assimilation of antecrystic and xenocrystic components.

It is envisaged that intrusive basaltic forerunners produced a zone where various degrees of crustal assimilation and fractional crystallization occurred. The erupted basalts represent subsequent mafic recharge of this system. This crystallization history contrasts with traditional concepts of low-flux basaltic systems where rapid ascent from the mantle is inferred. From a societal perspective, staged magma storage and crystallisation beneath some basalt intra-plate fields increases the likelihood of detecting pre-eruption geophysical phenomena that could act as signals to pending eruptions.