HR: 12:05h
AN: MR32A-08    [Abstracts]
TI: Pressure Effect on Hydrous Rhyolite Viscosity: a Model
AU: * Ardia, P
EM: paola.ardia@erdw.ethz.ch
AF: ETH Zurich, Institute for Mineralogy and Petrology; Clausiustrasse 25, Zurich, 8092, Switzerland
AU: Giordano, D
EM: dgiordan@uniroma3.it
AF: Department of Geological Sciences; Third University of Rome, Largo S. Leonardo Murialdo 1, Rome, 00154, Italy
AU: Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: ETH Zurich, Institute for Mineralogy and Petrology; Clausiustrasse 25, Zurich, 8092, Switzerland
AB: Viscosity is the single most important property governing the efficiency, rates and nature of melt transport. In geological environments viscosity controls eruption dynamics and rates of physicochemical processes (e.g., degassing, crystallization) in magmas. Composition, volatile content, temperature, pressure, as well as crystal and bubble contents are parameters influencing the viscosity of silicate liquids to various extents. The prediction of viscosity over the range of conditions encountered in nature and, in particular, at conditions relevant to eruptive events, is still challenging. The influence of H₂O-content on viscosity is dramatic (e.g., the addition of 1 wt% of water may decrease viscosity by 6 orders of magnitude). Rhyolitic melt at pressure conditions typical for magma chambers contain up to 10 wt% H₂O and in this study we experimentally determined the effect of pressure on the viscosity of hydrous rhyolitic melts. The Newtonian viscosity of synthetic rhyolitic liquid (HGG) containing 0-5.25 wt% dissolved water was measured at pressures from 4 to 26 kbar. The combined use of a high T concentric cylinder and the high-T, high-P falling sphere techniques allowed viscosity measurements in the interval from 10² to 10⁷ Pa s, pertaining to eruptive conditions. No previous experimental work has investigated such a wide range of viscosity. Concentric cylinder measurements were performed on dry melts up to 1650°C. The falling sphere technique on static and centrifuging piston-cylinders allowed to measure viscosities between 580 to 1350°C at different pressures. The increased acceleration field (50-1000 g), applied to the sinking sphere, enabled us to measure viscosities below the solidus temperature. As to be expected, viscosity decreases with water contents and with increasing temperature. The viscosity response of hydrous melt to pressure increase is different: as expected at high T the viscosity decreases, whereas at low T the viscosity increases. For dry compositions a decrease of viscosity is observed in the entire T-range, indicating that in the presence of H₂O, T and P- induced speciations and structural variations significantly affect melt rheology. A combination of our and previous studies on rhyolitic melts were employed to calculate an empirical model reproducing the experimental data and predicting the viscosity of rhyolitic melts in the full range of T-P-H₂O- X space for which data are available. The model is based on the Vogel-Fulcher-Tammann (VFT) equation. Our model reproduces the viscosity of about 500 viscosity data obtained on both synthetic and natural silicic melts from peraluminous to peralkaline with RMSE values of less than 0.25 log-units and less than 5% relative error on a logarithmic scale.
DE: 5139 Transport properties
DE: 8032 Rheology: general (8160)
DE: 8429 Lava rheology and morphology
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting