Eruption vs. storage: Key thermomechanical controls on the production of large silicic magma chambers
Abstract
The production of large-volume silicic magma chambers in the mid to upper crust is enigmatic: Why would buoyant and otherwise eruptible magma remain ponded at depth rather than drain to the surface roughly at the rate at which it is produced? One way that the rise and eruption of this magma can be checked is if the nucleation and/or propagation of dikes to the surface is suppressed. Additionally, if the average rate at which heat is carried in to the chamber by basaltic or silicic replenishments is insufficiently large relative to the rate of internal crystallization, the magma may become overly crystal rich and effectively "uneruptible". Bearing in mind these two mechanisms favoring chamber growth we will simple models to discuss three issues that ultimately govern whether buoyant magma becomes stored in a high-level magma chamber or erupts at the surface: 1) The long-term average supply of magma to the chamber; 2) the thermal structure, mechanical strength and background stress regime of the crust; and 3) the volume and shape of the magma chamber. For a given chamber volume, shape and cooling rate, the magma supply to a volcanic/plutonic system governs both the mean crystal content and the maximum average chamber overpressure available to propagate dikes to the surface. Whether such an overpressure can drive dike formation and propagation to the surface or lead to magma storage depends on the strength and thermal regime of surrounding crust, which depends, in turn, on their initial thermo-mechanical state and subsequent history of magmatism. In principle, even if a magmatic system is in a regime that favors eruption a very high magma supply (greater than the rates of eruption and crystallization) can ensure that magma accumulate in the crust. Thus, the most import parameter in the problem that must be constrained carefully is the magma supply. The long term magma supply is controlled primarily by the heat transfer properties of underlying mantle convection and the related rates of melt production and extraction. Isotopic studies, parameterized mantle convection calculations and varied geophysical observations suggest that mantle melt production rates in regions not associated with hot mantle plumes are order 10-4 to 10-2 km3 yr-1. Isotopic studies and heat balance considerations indicate that the supply to the mid crust can be increased by factors of 2-10 as a result of lower crustal melting. Applying these constraints on the average supply along with the corresponding thermal structure in the crust (also determined, in part, by the nature of mantle heat transfer) and reasonable crustal mechanical properties we identify conditions in which the storage of crystal-rich magma is favored. Additional effects and implications related to time-dependent magma supplies, varied chamber shapes, wall rock rheologies and background crustal stress regimes will also be discussed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V12A..08J
- Keywords:
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- 3618 Magma chamber processes (1036);
- 8439 Physics and chemistry of magma bodies