TRACING MELT PATHWAYS OF THE WHAKAMARU SUPERERUPTIONS WITH RHYOLITE-MELTS GEOBAROMETRY IN PUMICE WHOLE-ROCK AND GLASS
Thirteen pumice samples are categorized into the 4 magma types, then thermodynamically modeled using Rhyolite-MELTS to calculate the pressure at which liquid melt last equilibrated with crystals. All simulations are run over the temperature range 1300-700 °C with 1 °C intervals, and the pressure range 400-25 MPa with 25 MPa intervals.
Ten samples are identified as type A, two are type B, and one is type D. Storage ranges in pressure from 95 to 220 MPa, with a phenocryst assemblage of qtz+plag. Extraction results are dependent on oxygen fugacity for a simulated assemblage of plag+opx in the crystal mush, so simulations are run at -0.5, 0, and +0.5 dNNO. This shows a range of possible extraction pressures for each sample, which are considered based on their relationship with the calculated storage pressure.
Results indicate that oxidizing conditions (+0.5 dNNO) are probable in the extraction regime. For most samples extraction is contiguous with storage at +0.5 dNNO, but is up to 80 MPa greater in one type A and type B sample. Storage pressures are overlapping for all rhyolite types but on average type B storage occurs beneath type A storage. Overlapping storage pressures and mostly contiguous extraction indicate a magma plumbing system with different magma compositions stored in separate melt lenses at the same depth.