Climate and the Development of Magma Chambers

Abstract

Whether magma accumulating in the crust develops into a persistent, eruptible magma body or an incrementally emplaced pluton depends on the energy balance between heat delivered to the bottom in the form of magma and heat lost out the top. The rate of heat loss to the surface depends critically on whether heat transfer is by conduction or convection. Convection is far more efficient at carrying heat than conduction, but requires both abundant water and sufficient permeability. Thus, all else being equal, both long-term aridity and self-sealing of fractures should promote development of persistent magma bodies and explosive silicic volcanism. This physical link between climate and magmatism may explain why many of the world’s great silicic ignimbrite provinces developed in arid environments, and why extension seems to suppress silicic caldera systems.

Thermal Constraints on the Longevity, Depth, and Vertical Extent of Magmatic Systems

“…The power needed to maintain a given mush volume once it has formed can be estimated by balancing thermal energy fed into the system with heat leaking out of it (Figure 3a). For a planar magma body fed by recharging magma and treated as a one‐dimensional system, this can be approximated by a magma accretion number where ρ is density, Λ is the heat released by recharging magma in cooling to the temperature of the magma body, including latent heat of crystallization, a is the rate at which new magma is accreted to the system, and the denominator is the difference between heat flow out the top of the system and background heat flow (Glazner, 2020). Here I assume that the recharging magma is intermediate (dacitic).…”

“…where ρ is density, Λ is the heat released by recharging magma in cooling to the temperature of the magma body, including latent heat of crystallization, a is the rate at which new magma is accreted to the system, and the denominator is the difference between heat flow out the top of the system and background heat flow (Glazner, 2020). Here I assume that the recharging magma is intermediate (dacitic).…”

“…If additional heat is extracted via hydrothermal convection, then the rate of magma accretion needed to maintain M = 1 goes up by a factor of the Nusselt number Nu, where Nu is the ratio of total heat flux to conductive heat flux. Nu can range GLAZNER 10.1029/2020GC009459 4 of 12 well above 10 in magmatic systems with vigorous hydrothermal convection, and thus magmatic systems undergoing cooling by convection will require much higher magma accretion rates to survive as persistent supersolidus regions (Glazner, 2020).…”

Constraints on the timescales and processes that led to high-SiO2 rhyolite production in the Searchlight pluton, Nevada, USA

“…Thermal modeling estimates upper-crustal residence times of silicic magmas to 10 4 -10 6 yr depending on the emplacement rate, the thermal maturation of the system (e.g., Annen, 2009;Gelman et al, 2013), and the local geotherm (Karakas et al, 2017). Hydrothermal circulation above magma reservoirs will cause much faster cooling (e.g., Glazner, 2020) but is frequently ignored in thermal models. Extensive evidence for hydrothermal activity above the Searchlight pluton exists in the form of mineralized veins and highly altered host rock (Callaghan, 1939), suggesting that this process likely played an important role in modulating melt residence within this system.…”

Exploring and Modeling the Magma–Hydrothermal Regime

“…Working in the opposite, descending direction, Glazner [12] presents the provocative idea that climate exerts a strong control on the development of magma bodies. He derives a dimensionless number, M, that shows as a function of volume rate of accretion of incremental intrusions and the difference between heat flow into the base of the plutonic box vs. out through the top, whether a magma body will grow or the plutonic accumulation will remain as separate, mostly solid units.…”