THERMODYNAMIC MODELLING OF CRUST-MAGMA HEAT INTERACTIONS IN THE TVZ

Understanding how heat is transferred away from a cooling magma chamber in an active silicic volcanic setting, such as the Taupo Volcanic Zone (TVZ), is crucial for improving modern volcano monitoring systems as well as magma storage time estimates for large caldera-forming eruptions. Heat can be transferred conductively or convectively away from a cooling chamber, but both depend on physical properties of the crust, and convection in particular depends heavily on the efficiency of any overlying hydrothermal systems. This study focuses on the well-characterized magma chamber that produced the ~240 ka Mamaku ignimbrite and formed the Rotorua caldera. There are estimates for volume (~150 km³), chemical composition (79% wt. SiO₂), depth (~4 km), storage pressure (~150 MPa), crystal weight percent at time of eruption (~10 wt. %), and storage time prior to eruption (decades to centuries). We input these constraints into Rhyolite-MELTS and estimate the total amount of heat dissipated during magma storage to be 1.02 x 10¹⁶ kJ. We then model heat flow at the Earth’s surface above a 1 km thick sill at 4 km depth, with the stated constraints, average crustal conductivity, and no magma recharge, under both conductive and convective conditions. With vertical conduction only, it would take ~500 ka for heat to reach the surface of the Earth which is inconsistent with timescale evidence. We then consider completely efficient vertical convection and discover that for decadal to centennial storage times, the surface heat flow would be 100 to 10 W/m², which is as much as two orders of magnitude higher than the modern heat flow of 0.8 W/m². More realistic convection rates that are based on how much meteoric water penetrates the shallow crust in the TVZ show a maximum of 350 years to transfer the heat away from the magma chamber. Allowing for more water to penetrate the crust could bring the timescale for heat transfer down to ~70 years, which is within the decadal to centennial timescales estimated. We argue here that the modern Rotorua caldera heat flow is inconsistent with a Mamaku-sized eruptible magma body at depth, and that monitoring heat flow at calderas with actively convecting hydrothermal systems could provide clues for increased quantities of stored magma. For magma storage timescales of a decade, this could be a powerful monitoring tool.