HR: 09:15h
AN: V51D-06    [Abstracts]
TI: Viscosity of Anhydrous and Hydrous Basalt Melts at High Pressures
AU: * Tinker, D
EM: tinker@geology.ucdavis.edu
AF: Dept. of Geology, University of California, Davis, CA 95616
AU: Lesher, C E
AF: Dept. of Geology, University of California, Davis, CA 95616
AU: Baxter, G M
AF: Dept. of Geology, University of California, Davis, CA 95616
AU: Uchida, T
AF: Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637
AU: Wang, Y
AF: Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637
AU: Zhao, Y
AF: LANSCE, Los Alamos National Laboratory, Los Alamos, NM 87545
AB: We performed in situ falling-sphere experiments to determine the viscosity of anhydrous and hydrous basaltic (48 wt% SiO$_{2}$) melts from 1.5 to 5.3 GPa between 1600 and 1840 K, using the T-25 MA8 multianvil apparatus at the GSECARS 13-ID-D beamline at the Advanced Photon Source, Argonne National Lab. These falling-sphere experiments included monitoring the simultaneous settling of Pt and Mo spheres, an approach that provides redundant viscosity measurements for individual experiments and offers the opportunity to recover simultaneously melt density. Our results show that the viscosity of anhydrous basalt melt decreases with pressure up to 5.3 GPa, with an activation volume for viscous flow of -8.1 to -9.5 cm$^{3}$/mol between 2 and 5.3 GPa. The addition of a few wt. % water reduces melt viscosity by roughly 0.5 log units; however, there is no resolvable influence on activation volume. This negative pressure dependence is consistent with previous results for basaltic melts up to 3 GPa [1, 2], while the activation volume at low pressure is indistinguishable from the activation volume for O self-diffusion in the same bulk composition [3]. Application of the Eyring equation using O self-diffusion data for basaltic melt [3] predicts anhydrous melt viscosities that are 30-90% of the values determined in this study. This result is in stark contrast with our recent results for dacitic melt (68 wt% SiO$_{2}$) melt [4], in which the Eyring equation overestimates viscosity by as much as 40% at pressures $<$ 5 GPa. The limited utility of the Eyring equation for naturally-occurring silicate melts illustrates the difficulties in relating O self-diffusion to viscous flow in polymerized liquids. Adam-Gibbs theory [5] provides a means for addressing structural controls on these transport properties. The negative pressure dependence for anhydrous and hydrous basalt viscosity suggests that the extraction of partial melts from mantle source regions will be enhanced with pressure to 5.3 GPa. Future work will extend these observations to higher pressures in an effort to constrain melt transport properties approaching conditions for the top of the transition zone where [6] have postulated the accumulation of a dense hydrous melt layer as a result of dehydration melting of transition zone material. [1] Fujii, T, Kushiro, I (1977) CIW Yrbk., 76, 419-424; [2] Ando, R, et al. (2003) SPring-8 report No. 11, 45; [3] Lesher et al. (1996) GCA, 60, 405-413; [4] Tinker et al. (2004) Am. Min., in press; [5] Adam, G, Gibbs, JH (1965) J. Chem. Phys., 43, 139-146; [6] Bercovici, D, Karato, S-I (2003) Nature, 425, 39-44
DE: 8439 Physics and chemistry of magma bodies
DE: 3630 Experimental mineralogy and petrology
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting