MAGMA IN MOTION: DETERMINING VERTICAL MAGMA POSITIONING IN THE MAGMATIC SYSTEM THAT FED THE OHAKURI ERUPTION, TAUPO VOLCANIC ZONE, NEW ZEALAND

The location and behavior of magma prior to large volcanic eruptions is of paramount importance, but elucidating the physical character of magmatic systems remains a persistent petrologic challenge.

In this study, we consider the physical magmatic system that led to the massive 290 ka Ohakuri eruption in New Zealand’s Taupo Volcanic Zone. We determine the distance and character of pre-eruptive vertical magma migration by comparing the pressure at which melt-rich magma was extracted from a crystal mush (‘extraction pressure’) with the pressure at which this magma was stored in a reservoir that fed the eruption (‘storage pressure’).

We applied the rhyolite-MELTS geobarometer to glass compositions to determine storage pressures and to whole-pumice compositions to determine extraction pressures (n = 22). We tested both qtz-plag and plag-opx sources for extraction. Although we cannot definitively determine the source assemblage, mineral saturation curves from rhyolite-MELTS suggest this source was compositionally heterogeneous.

We determine that fO₂ of extraction conditions was likely between ∆NNO = -0.5 and ∆NNO = -0.7. In this range, saturation curves of the inferred minerals (plag, opx, and mt) intersect.

Estimated storage pressures are 60 - 120 MPa. Extraction pressures depend on the assemblage and fO₂; extraction pressures are 150 – 460 MPa for a qtz-plag assemblage, 100 – 290 MPa for a plag-opx assemblage at ∆NNO = -0.5, and 80 – 240 MPa for a plag-opx assemblage at ∆NNO = -0.7.

Comparing storage and extraction pressures for each sample allows us to estimate the migration distance of each parcel of magma. For a qtz-plag assemblage, migration distances range from 2-13 km, while for a plag-opx assemblage at ∆NNO = -0.6, estimated migration ranges 0-7 km.

The inferred pressures and assemblages suggest that Ohakuri magma could have been extracted from variable depths and source assemblages, painting a different picture than a classic “mush model” conceptualization. This suggests a vertically extensive crystal mush, with extraction and storage occurring at different depths.

This study is the first to compare storage and extraction pressures from the same samples, and therefore the first to estimate migration distance for each sample. This approach provides a promising new perspective on pre-eruptive magmatic behavior.