HR: 08:35h
AN: V21B-03 INVITED     [PDF]
TI: Viscous Fingering of Miscible Fluids in Laboratory Experiments and the Oceanic Mantle Asthenosphere
AU: * Weeraratne, D S
EM: Dayanthie_Weeraratne@brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02906 United States
AU: Parmentier, E M
EM: EM_Parmentier@brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02906 United States
AU: Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02906 United States
AB: Oceanic intraplate volcanism at a range of spatial scales is not simply explained by the fixed hotspot paradigm. The Pukapuka ridge, spanning 2000 km of the Pacific plate, forms from a geochemically enriched OIB source but propagates toward the EPR at rates greatly exceeding plate motion. A rapid localized flow of mantle material eastward to the EPR from the Pacific Superswell region is suggested. A similar mechanism could also explain the regularly spaced gravity lineations aligned with plate motion observed over large areas of the Pacific seafloor. At a smaller scale, seamounts between oceanic spreading ridges and nearby volcanic hotspots often form in uniformly spaced parallel ridges or chains, perhaps associated with material transport toward the spreading axis. The Sojourn, Hotu-Matua, and Rano Rahi seamount fields as well as the Musician seamounts are other examples of these parallel volcanic chains. Viscous fingering that develops as low viscosity mantle displaces higher viscosity mantle in the asthenosphere may be one explanation for these regular patterns in material transport. To better understand the dynamics facilitating such viscous fingering instability, we perform laboratory fluid experiments with two miscible fluids consisting of high viscosity sugar solutions. Fluid is injected into the gap between two horizontal plates initially filled with the higher viscosity fluid. The radial pattern of fluid flow that develops is observed for varying viscosity ratios, plate spacing, injection rate, and injection hole diameter. Fingering behavior is observed for viscosity ratios greater than about 10. Both injection rate and plate spacing influence the fingering wavelength and morphology. The fingering wavelength to spacing ratio of ${\sim}$13 is nearly constant and a factor of 3 higher than previously reported results. A complex interaction between the fluids near the bounding surfaces may change the effective fluid layer thickness. Our preliminary results suggest that if viscous fingering is responsible for the dominant 200 km wavelength gravity lineations observed for the south Pacific seafloor, the thickness of the layer guiding mantle flow should be less than 20 km. Could this be a thin, melt rich region at the top of the asthenosphere? Ongoing work is considering the potentially important effect of a moving plate at one boundary of the fluid layer.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8155 Plate motions--general
DE: 8159 Rheology--crust and lithosphere
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting