UNTANGLING THE DEPTHS AND CONDITIONS OF MAGMA BODIES THROUGH TIME

• Main Author: Harmon, Lydia Jane
• Other Authors: Gualda, Guilherme A. R.
• Format: Thesis
• Language: English
• Published: 2022
• Subjects: Whakamaru group eruptions; rhyolite-MELTS; ignimbrites; pyroclastic fall deposits; New Zealand; Krafla; Iceland
• Online Access: http://hdl.handle.net/1803/17096

The magma that feeds volcanic eruptions can be stored in multiple different magma bodies and at different depths within the shallow crust. By reconstructing the configuration of these magma bodies and the timing of their eruption(s), we can better understand how, when, and where these magma bodies form and erupt. We combine field evidence, pumice composition data, and calculate mineral -melt equilibration pressures to determine the different magma bodies that sourced the compositionally distinct Whakamaru group eruptions, Taupō Volcanic Zone (TVZ), Aotearoa New Zealand. Storage pressures indicate that all Whakamaru magma bodies were stored at shallow storage pressures ~ 75-125 MPa (2.8-4.6 km). Compositions of whole pumice clasts determine that there must be two different magma subsystems, from which magma of Type A+D and Types B+C are extracted. Type A extraction is most likely with an extraction mineral assemblage of plagioclase+pyroxene, where extraction pressure depends on oxygen fugacity (fO2), but is likely ~ 250-350 MPa (9.3-13.0 km) with an fO2 equal to Δ NNO +1. Type B+C extraction is likely ~ 220-280 MPa (8.2-10.4 km) with an extraction mineral assemblage of quartz+feldspar. Using pumice clasts from pyroclastic fall deposits (PFDs) and ignimbrite deposits, we can understand how the different magma bodies erupted through time. The Rangitaiki ignimbrite contains only Type A pumice, so likely erupted earliest. The ignimbrites that contain Type C pumice clasts are the Manunui and Whakamaru ignimbrites, which correlate to the latest eruptions. There is no pumice information from the Te Whaiti ignimbrite. We extend our understanding of where magma can be stored by looking at confirmed ultra-shallow magma storage at Krafla, Iceland and geobarometry results indicating ultra-shallow storage in the flare-up eruptions in the TVZ, New Zealand. We find that ultra-shallow (<100 MPa; <4 km) magma storage is likely a global phenomenon, and requires petrologists to rethink the extensive potential storage ...