TRACING MELT PATHWAYS OF THE WHAKAMARU SUPERERUPTIONS WITH RHYOLITE-MELTS GEOBAROMETRY IN PUMICE WHOLE-ROCK AND GLASS

The Whakamaru Ignimbrite is the largest-volume eruption (>1,500 km.³) from the Taupo Volcanic Zone New Zealand. The traditional model of the Whakamaru magma system proposes a main vertically zoned magma chamber with upwardly evolving rhyolite types types (A, B, and C), and one additional rhyolite type (D) in a subjacent chamber. We test the validity of this model with Rhyolite-MELTS geobarometry calculations for two crucial processes in the Whakamaru magma’s evolution: (1) extraction of crystal-poor, eruptible magma from a crystal mush; and (2) storage of the extracted crystal-poor magma. Pressures are calculated from a single sample, granting a more robust view of magma plumbing by tracing individual melt pathways. Bulk rock compositions represent the extracted crystal-poor magma composition, while matrix glass compositions represent pre-eruptive magmatic storage.

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.