Lydia Harmon , Guilherme Augusto Rosa Gualda , Blake Wallrich, Calvin F Miller

Identifying the storage depths of melt-dominated magma bodies prior to eruption is critical for understanding magma transport, eruption hazards, and magma body longevity. Rhyolite-MELTS has been used effectively to calculate pre-eruptive storage pressures for silicic magma bodies in the upper crust (~100-350 MPa), but its precision and accuracy in very low-pressure systems (<100 MPa) has not been sufficiently investigated. During the recent Krafla IDDP-1 drilling project, magma was surprisingly intersected at 2.1 km depth. Here, we test the use of rhyolite-MELTS geobarometry for this very low-pressure system, using natural Krafla IDDP-1 compositions that were stored at a known depth. We input the composition of the melt (preserved as glass) and search in pressure and temperature space at a range of oxygen fugacity (fO2) to model the storage conditions of the Krafla magma. For the average composition of the drilled melt, rhyolite-MELTS yields reasonable storage pressures (~40-50 MPa). After converting calculated pressure to depth, the calculated depths are 1.6-1.9 km. These estimates are only 0.2-0.5 km different from that of the intersected magma, showing that rhyolite-MELTS provides excellent estimates for very shallow magma storage, further strengthened by results from a Monte Carlo analysis. The agreement between rhyolite-MELTS pressures and the drilled depth of the Krafla magma supports the previously calculated very shallow storage pressures in other locations, like the Taupō Volcanic Zone (TVZ), Aotearoa New Zealand. This shallowest storage zone of melt-dominated magmas has significant implications for modeling volcanic unrest and evaluating geothermal and economic resource potential.