HR: 1340h
AN: V53C-1432    [Abstracts]
TI: Liquidus tracking by vigorous convection in ascending magma
AU: * Winslow, N W
EM: nate.winslow@gmail.com
AF: Department of Earth & Planetary Sciences, Johns Hopkins University, 301 Olin Hall 3400 N. Charles Street, Baltimore, MD 21218, United States
AU: Marsh, B
EM: bmarsh@jhu.edu
AF: Department of Earth & Planetary Sciences, Johns Hopkins University, 301 Olin Hall 3400 N. Charles Street, Baltimore, MD 21218, United States
AB: Basaltic magmas commonly erupt at or near their liquidi and have never been observed to be superheated. In the light of the steep P-T slope of magma adiabats relative to liquidi, superheated magmas should be common. That they are not may reflect a fundamental feature of rapid convective heat transfer in ascending magmas, and that they seem to adhere to the liquidus may also reflect this process. Moreover, this may alleviate the well-known thermal entry length enigma pointed out by Delaney and Pollard that magma under laminar flow in dikes should solidify after a relatively short transit distance. (This is, in essence, because the flow velocity is normal to the thermal gradient and their vector product vanishes, leaving the sheet to progressively solidify by conduction regardless of flow rate.) Key insight on the meaning of the lack of superheat comes from thermal convection studies involving crystallizing fluids. In experiments intended to simulate thermal convection in magmas using analog crystallizing fluids (paraffin, isopropanol-water), a number of studies have found thermal convection to be vigorous only when the ‘magma' is superheated (Marsh, 89'; Brandeis & Marsh, 89'; 90'; Hort et al., 99'). Convection ceases once the superheat is evicted and further cooling is by conduction. Because of the relatively low viscosity and significant length scales of basaltic magmas, the governing Rayleigh number (Ra) for thermal convection is large for almost any appreciable superheat. All the physical features associated with convection can be related to Ra. The rate of convective heat transfer relative to conduction is measured by the Nusselt number (Nu) and, for example, Nu is proportional to Ra to the 1/3. We report here on analytical and numerical results that model this cooling process during magma ascent. The thermal history is a function of two dimensionless numbers: Rao based on the temperature difference between the liquidus at the initial depth and the surface, and the Fourier modulus, which is a dimensionless time measuring the rate of ascent or rate of superheat production. When superheat is available, thermal convection is rapid and cooling is rapid. With approach to the liquidus, convection and cooling wane, but continued ascent attempts to follow the adiabat, which initiates new superheat and the cycle repeats itself. Because the rise time for convection is short and heat transfer highly efficient, for a constant ascent velocity an equilibrium is established between the rates of superheat production and loss due to cooling. The pattern of cooling does not oscillate about the liquidus, but instead tracks the liquidus in a slightly superheated state. Because convection ceases at the liquidus, the cooling trajectory cannot reach or cross the liquidus unless conductive heat loss is also included. That the final temperature of erupting magmas is so often near the liquidus probably reflects the slow rate of conductive heat loss once the liquidus is crossed and the contribution of latent heat with the onset of nucleation, which acts as an internal heat source or an enhanced heat capacity. The faster the ascent rate, the more vigorous the rate of convection. The not uncommon presence in alkali basalts of anorthoclase megacrysts, which may be several cm in size, may reflect this process of large crystals growing and being suspended in vigorous convection, perhaps akin to hailstones, in rapidly ascending magmas.
DE: 8434 Magma migration and fragmentation
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting