HR: 1340h
AN: V23A-0686    [Abstracts]
TI: Sill Emplacement Dynamics: Experimental Textural Modeling of a Pulsing, Cooling, Particle-laden Magma as Applied to the Basement Sill, McMurdo Dry Valleys, Antarctica
AU: * Charrier, A D
EM: charrier@jhu.edu
AF: Morton K. Blaustein Department of Earth & Planetary Sciences, Johns Hopkins University, 3400 N. Charles Street Olin Hall, Baltimore, MD 21218 United States
AU: Marsh, B D
EM: bmarsh@jhu.edu
AF: Morton K. Blaustein Department of Earth & Planetary Sciences, Johns Hopkins University, 3400 N. Charles Street Olin Hall, Baltimore, MD 21218 United States
AB: Seminal observations of Baragar, Upton, Simkin, and others, has suggested that the sorting of phenocrysts in magmatic shear flows during ascent and emplacement of dikes and sills is a critical process in differentiation. Heavy phenocrysts in ascending crystal-laden magmatic flows migrate away from walls and concentrate in the flow center, forming a swarm of crystals, called a tongue, that lags behind the leading edge and is enveloped by crystal-poor magma. The tongue of phenocrysts often forms an ultramafic cumulate layer (in sills) distinct from any textural variation due to differential cooling of the phenocryst-free magma elsewhere. The geographic distribution and mass of the tongue along with the spatial variations in size, shape, and composition of the attendant phenocrysts together provide a record of the processes of ascent, emplacement, and solidification. A tongue of mostly large orthopyroxene crystals of a rough aerial extent of 10,000 km² occupies much of the ~330 m thick Basement Sill, which is the basal sill of the ~4 km thick Ferrar Dolerite sill sequence. The thickness of the tongue and the amount of the sill that it occupies locally is a direct indication of the location of the conduit that fed the Basement Sill during emplacement. The tongue is thickest near the zone of magma ascent (called the feeder zone), which is in the vicinity of Bull Pass where it fills nearly the entire sill. Away from the feeder zone the tongue thins, eventually disappearing, and also rises to maintain a vertically central position in the sill. Detailed vertical profiles of magnesia, a close proxy for Opx, show a progressive transition outward from Bull Pass as outward flow, settling, and sorting responded to inward advancing solidification fronts; the tongue rises and thins and the chilled margin compositions thicken. The magnesia profiles show a remarkable series of matching upper and lower symmetric steps that may reflect systematic hiatuses in flow, which may be similar to eruption repose times. We have previously suggested that the combined processes of heat loss through the boundaries and pulsing flow regimes are responsible for producing the step-like patterns (EOS Sp. '04). This explanation has its basis in fluid and particle dynamic models, and in heat-loss models. Here we attempt to experimentally recreate these modal variations in a laboratory setting using a solidifying, viscous, particle-laden flow whose parameters scale appropriately. The solidifying fluid component used in the lab is a paraffin wax, whose melting temperature is ~63^OC, and the particles used in the experiment are wax beads of another type, which have a higher melting temperature than the paraffin and so maintain their solid state throughout the experiment. The experiments are scaled to the actual process by matching as close as possible the following non-dimensional parameters: flow Reynolds number, particle Reynolds number, and Prandtl number. The experiments consist of forming a series of flows intruding into trough-like and sill-like structures using particle-laden paraffin. Upon solidification, the flows are dissected and the vertical and horizontal modal abundance of the particles is related to the rate, duration and periodicity of the flow. Although there is a certain amount of ambiguity in the results, the emerging patterns do support the initial hypothesis. Moreover, this technique promises to be of significant value in modeling magma-phenocryst flows in a host of emplacement regimes.
DE: 8414 Eruption mechanisms and flow emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005