MAGMATIC EVOLUTION OF THE ATIAMURI IGNIMBRITE SEQUENCE, TAUPŌ VOLCANIC ZONE: MAGMA STORAGE AND EXTRACTION THROUGH TIME

Eruption sequences evolve in a cyclical manner. Large-scale, caldera-forming eruptions empty enormous amounts of stored magma. Small-scale volcanic activity, such as lava domes, commonly occur within the caldera following initial collapse. Analysis of magma extraction pressures and pre-eruptive magma storage pressures help us understand the magmatic systems of calderas. The Atiamuri ignimbrite sequence (229 + 12 ka) is a sequence of low-volume (> 1 km³) pyroclastic deposits associated with rhyolitic lava dome eruptions within the Maroa Dome Complex of the Whakamaru caldera, located within the Taupō Volcanic Zone, New Zealand. Our study examines 27 individual pumice samples collected from 9 different horizons within the Atiamuri ignimbrite sequence to interpret changes in magmatic history through the stratigraphy. We collect whole-rock major element geochemistry through x-ray fluorescence spectrometry in tandem with matrix-glass composition analysis by electron-dispersive x-ray spectroscopy of individual pumice samples. From these compositions, we calculate the pre-eruptive storage pressures and pressures of magma extraction from the mush by rhyolite-MELTS geobarometry. These pressures are then used to calculate the depth of these magma bodies. Our results show that eruptible melt is extracted from partially crystalline mush at depths separate from melt storage prior to eruption; extraction occurs at pressures ranging from 200-400 MPa (8-15 km), while pre-eruptive storage occurs at constant shallow pressures of 42-164 MPa (2-6 km). The consistent difference between storage and extraction pressures implies physical separation between the mush and pre-eruptive magma bodies. Furthermore, samples at the deepest point in the stratigraphy are extracted at pressures of 200-265 MPa (8-10 km), while the samples shallowest in the stratigraphy are extracted at pressures of 296-400 MPa (11-15 km), implying that progressively deeper portions of the mush were tapped as the eruption sequence progressed. Our results demonstrate the fundamental separation of magma extraction and storage chambers and provide a better understanding of the organization of magmatic bodies and their evolution throughout an eruptive sequence.