The Earthquake Flat and Rotoiti Subsurface Relationship (Taupo Volcanic Zone, New Zealand): A Cryptic Connection at Depth?

The assembly and storage conditions of large magma bodies that lead to caldera-forming eruptions are poorly understood. Study of the eruptive products exposed at the surface suggests that crystal-poor rhyolites are separated from a cogenetic yet less-evolved mush pile at depth. The Rotoiti eruption from the Okataina Volcanic Center in the Taupo Volcanic Zone, New Zealand, exhibits 15-25% crystallinity and shows changes in mineralogy that are consistent with the eruption having tapped progressively deeper regions of the magma body. The Earthquake Flat (EQF) vent is located ~20 km SW of the Rotoiti vent and erupted days to weeks after the Rotoiti event. EQF has a crystallinity of 40% and has been interpreted to represent a rejuvenated portion of a crystalline mush. If the magma system is laterally continuous across the two vent zones, it is possible that the EQF eruption could represent a deeper, more crystalline part of the same magma body as Rotoiti; however, the mineral geochemistry between the two eruptions does not necessarily illustrate a clear relationship linking the two bodies. We present new SHRIMP-RG trace element data from zircon surfaces and interiors from the EQF and Rotoiti eruptions. While similar in some geochemical indices, the EQF and Rotoiti zircon populations show a distinct geochemical difference in HREE/U and Eu/Eu*. Geochemically, the Rotoiti interiors and surfaces are broadly similar, the EQF interiors are slightly different from the Rotoiti zircon, and the EQF surfaces are substantially different from the Rotoiti zircon. If the two systems are connected at depth, it is possible that lateral transport of a unique parcel of magma provided both the mechanism for rejuvenation of the EQF crystalline mush and the geochemical variability reflected by the zircon surfaces. This suggests that the subsurface storage conditions for caldera-forming magmas are complex and that the relationship between coeval eruptions may not be obvious.