HR: 0800h
AN: C51B-0291    [Abstracts]
TI: Development of C-Axis Fabrics During Superplastic Flow of Ice
AU: * Goldsby, D L
EM: David_Goldsby@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook Street, Providence, RI 02912 United States
AU: Swainson, I
EM: ian.swainson@nrc.gc.ca
AF: Canadian Neutron Beam Centre, Chalk River Laboratories, Chalk River, ON KOJ 1JO Canada
AB: Recent laboratory studies strongly suggest that the grain-scale flow of glaciers and ice sheets over most of their depths occurs via so-called superplastic flow, in which deformation proceeds via basal dislocation slip acting in concert with grain boundary sliding (GBS), with the overall creep rate limited by GBS (Goldsby and Kohlstedt, 2001). Two fundamental questions concerning this creep mechanism arise: What c-axis fabrics, if any, form during superplastic flow of ice? Are those fabrics consistent with ones observed in the natural flow of, e.g., glaciers? To address these questions, uniaxial creep experiments were conducted on D₂O ice samples deformed within both the superplastic flow and the dislocation creep regimes, and the resulting c-axis fabrics were measured via neutron diffraction. In the superplastic flow regime, a sample with a starting grain size of 3-5 μm was deformed to a strain of 15% at a stress of 9 MPa and a temperature of -67 °C. In the dislocation creep regime, a sample of starting grain size 30 μm was deformed to a strain of 35% at 5 MPa and -20 °C. Two undeformed D₂O ice samples with grain sizes of 3-5 μm and 30 μm were also prepared. The orientations of the basal (002) planes were determined via neutron diffraction using neutrons of wavelength 0.183 nm from a Ge (331) monochromator. The sample deformed via superplastic flow and the sample deformed via dislocation creep both exhibit classic uniaxial compression fabrics, with c-axes clustered in a broad single maximum about the compression axis. The strength of the c-axis fabric for the sample deformed via superplastic flow is up to ~5X random, while that of the sample deformed via dislocation creep is up to ~7X random. Undeformed samples have random fabrics. The experiments demonstrate, as anticipated from theoretical considerations and experiments on other materials (Goldsby and Kohlstedt, 2003), that the development of c-axis fabrics is fully compatible with GBS when it accommodates dislocation slip. Our results thus lay to rest the recent controversy concerning the applicability to glacier and ice sheet mechanics of the GBS-rate-limited, superplastic flow law for ice, a controversy predicated on the assumption that this flow mechanism is inconsistent with the development of strong fabrics (Duval and Montagnat, 2003).
DE: 0720 Glaciers
DE: 0726 Ice sheets
DE: 0738 Ice (1863)
DE: 0774 Dynamics
DE: 3902 Creep and deformation
SC: Cryosphere [C]
MN: Fall Meeting 2005