Thermal and dynamical regimes of single‐ and two‐phase magmatic flow in dikes

Abstract

Finite element calculations of magma flow in dikelike channels with length‐to‐width ratios of 1000:1 or more have been used to investigate the coupling between thermal and dynamical regimes due to temperature‐dependent viscosity and dissipation. Steady state solutions with realistic thermal and dynamical parameter values have been obtained. The models show that the onset of solidification on the boundaries of a basaltic or andesitic dike, as predicted by idealized laminar flow models, can be prevented or significantly delayed by a small amount of transverse flow induced by rising bubbles, boundary roughness, or turbulence. This effect will reduce the critical initial widths of dikes estimated by Bruce and Huppert (1989). In the absence of transverse flow, the bulk temperature of the magma may actually increase slightly with distance along the dike as a result of viscous dissipation even while solidification is occurring on the walls of the dike. With converging or necking dike walls it is found that boundary temperatures fall to a minimum and then increase with distance along a dike even if viscous heating is neglected. For viscous heating to offset significant rates of heat loss (2 kW/m2) in a plane‐parallel 1‐m‐wide basaltic dike, an average flow velocity of 2.7 m/s driven by a pressure gradient of 1.7 MPa/km is required. A 7% void fraction caused by exsolution of volatiles or chamber gas in the magma will produce this pressure gradient. The ease of producing such a gradient by reducing the density of the magmatic column with addition of a gas phase makes it likely that flows of basaltic magma could be maintained in dikes tens of kilometers long. Furthermore, a gas phase may be important for the propagation of the fracture prior to the initial injection of magma during dike emplacement. Rapid transport of more silicic magmas through dikes is inhibited by the requirement of large driving pressure gradients exceeding several hundred megapascals per kilometer. However, the pressure gradient can be substantially reduced if the magma is a heterogenous mixture of a predominantly silicic component with a more mafic component. Pipe flow experiments involving molten polymers that exhibit dynamical similarity to magmas strongly suggest that unmixing occurs when a two‐component magma mixture, in which the components have different viscosities, rises within a dike. Within a few dike widths of the inlet the less viscous mafic component encapsulates the more viscous silicic part, effectively lubricating the passage of the more viscous component. Compositional variations in the Obsidian Dome volcano support the occurrence of this pressure reduction or self‐lubrication mechanism. Such a self‐lubrication process often may be necessary to permit very viscous magmas to reach Earth's surface. If so, chemical and lithologic zoning would be anticipated as a common feature of near‐surface intrusions or lava flows that are characterized by a high silica and/or high crystal content such as at Deadman Dome in Long Valley, California.

Thermal weakening of the carbonate basement under Mt. Etna volcano (Italy): Implications for volcano instability

“…Etna volcano. The maximum experimental temperature of 800 °C represents our predicted (based on standard heat conduction theory, see Carslaw and Jaeger, 1986; see also Carrigan et al, 1992;Bonaccorso et al, 2010) upper boundary for the carbonates of the Hyblean Plateau. In practice, this may only represent the volume at the margins of magmatic bodies and dykes (although this volume may be significant).…”

“…Etna will be exposed to high temperatures as a result of their proximity to large, long-lived magmatic bodies (Murru et al, 1999;Chiarabba et al, 2000;Bonaccorso et al, 2010), and lateral growth of the reservoir via eccentric dyking and ponding (Carrigan et al, 1992;Behncke and Neri, 2003;Andronico et al, 2005;Carbone et al, 2009;Bonforte et al, 2009), as well as circulating hot fluids (Siniscalchi et al, 2010): "proximity without intimacy". Indeed, accelerated flank movements are commonly observed to follow magmatic events within the sedimentary substratum (Walter et al, 2005), highlighting the intimate link between basement deformation and flank instability.…”

Formation of lunar mare domes along crustal fractures: Rheologic conditions, dimensions of feeder dikes, and the role of magma evolution

“…In the general case, the thermal behaviour of the magma is complex and governed by the relative importance of factors such as cooling due to conduction of heat into the host rock, heat transport within the dike by transversal flow of magma, and heating by viscous dissipation (Carrigan, 2000). The simulations by Carrigan et al (1992) reveal that the temperature differences across the dike inferred for stationary magma are an upper limit to those obtained for vertically ascending magma. A mechanism that also decreases the value of ΔT , compared to the stationary case, is lateral motion of magma in the dike towards the walls, leading to a more uniform temperature distribution across the magma especially near the dike walls and thus preventing solidification of magma in the boundary layer.…”

“…According to Carrigan et al (1992), a preheated crust is a realistic assumption if the region has previously undergone an extended phase of active volcanism. Numerical modelling of high-viscosity magma with a considerable crystalline fraction ascending through dikes of large width at low velocities is necessary to examine in more detail the relative importance of cooling by heat conduction, heating by lateral magma motion and viscous dissipation, and the possible necessity of the host rock being preheated to prevent magma from solidification.…”

Granitoid pluton formation by spreading of continental crust: the Wiley Glacier complex, northwest Palmer Land, Antarctica

“…Escape is enhanced and the rate of transfer of granitic magma s.l. is substantially increased by the addition of a mingled component of lower viscosity mafic magma (Carrigan et al, 1992). Abundant enclaves of mingled mafic magma (Vaughan et al, 1995, Vaughan and provide circumstantial evidence that magma-mingling assisted the ascent of Wiley Glacier complex pluton magmas.…”