HR: 0830h
AN: V51E-0325    [PDF]
TI: Rheological Cut-Off Temperatures in Nyiragongo lavas: Dynamic crystallisation experiments.
AU: Papale, P
EM:
AF: INGV-Pisa, Via della Faggiola, 32, Pisa, 56126 Italy
AU: * Dingwell, D B
EM: Dingwell@lmu.de
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Giordano, D
EM:
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AB: The lavas of the 2002 eruption of Nyiragongo were highly fluid at the vent. The ultimate extent of flow of such lavas is further constrained by the rapid transformation of portions of the lava into a homogeneous crust of constant thickness, composed of the fine-grained products of a multiphase paragenesis. We have sampled these lavas in the field. A selection of these materials have been subjected to high temperature rheological investigation in the rheological facilities of the Experimental Volcanology lab at Munich. Two types of investigation have been performed. Firstly, the equilibrium viscosities of the Nyiragongo liquids have been determined as a function of temperature, from superliquidus (and supereruptive) temperatures down to the glass transition. Combining the techniques of concentric cylinder and micropenetration viscometry yield viscosity curves spanning 10$^0$ - 10$^{12}$ Pa s. Secondly, dynamic cooling rheological experiments have been performed to investigate the influence of the kinetics of crystallisation of Nyiragongo lavas on their rheological response. The progress of crystallisation of Nyiragongo lavas leads to a transition in their rheological response from low viscosity (10$^1.5$ Pa s) Newtonian fluids to rigid solids. This we name the "rheological cut-off".The dynamic experiments were performed at constant cooling rates. Despite their kinetic/disequilibrium nature, they yield a highly reproducible dependence of the rheological cut-off temperature range as a function of constant cooling rate. The temperature range of the transition is very restricted (ca. 20K) for a given experiment. This is presumably due to the very rapid and efficient crystallisation in that temperature interval. Significant levels of F and Cl in these lavas likely facilitate mineralisation in these lavas. The rheological cut-off temperature ranges from 1050 to 1000$\deg$C for cooling rates in the range of 1 to several tens of K/min. These results thus provide quantitative temperature constraints for the development of lava crusts in Nyiragongo lavas. These will be incorporated in models for the simulation of conduit and surface flow of such lavas.
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting