HR: 11:20h
AN: V52A-05    [Abstracts]
TI: Rheology and Structure of Iron-Bearing Na$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$ Melts
AU: * Webb, S
EM: swebb@gwdg.de
AF: Mineralogy Department, Goldschmidtstr. 1, GZG, Georg-August University, Goettingen, 37077 Germany
AB: The viscosity of a series of Na$_{2}$O-FeO-Fe$_{2}$O$_{3}$-Al$_{2}$O$_{3}$-SiO$_{2}$ melts has been measured by micropenetration methods in the range 10$^{8.5}$-10$^{13}$ Pa s in argon. The viscosity of the Fe-bearing melts follows the same compositional trend as the Fe-free Na$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$ melts as a function of Na/Al at constant SiO$_{2}$ content: at a constant temperature, viscosity increases with decreasing peralkalinity and remains almost constant for all investigated compositions in the peraluminous field. The addition of 1 mol% Fe$_{2}$O$_{3}$ to the Fe-free peraluminous compositions causes ~1 log unit decrease in viscosity, while ~3 mol% Fe$_{2}$O$_{3}$ needs to be added to the peralkaline compositions to produce a similar decrease in viscosity. The need for more charge-balancing cations in the peraluminous melt structure appears to control the Fe$^{2+}$/Fe$^{total}$, with the peraluminous compositions having a large Fe$^{2+}$/Fe$^{total}$ (0.6) compared with that for the peralkaline compositions (0.3). There exist different structures and flow mechanisms in the peralkaline and peraluminous Na$_{2}$O-FeO-Fe$_{2}$O$_{3}$-Al$_{2}$O$_{3}$-SiO$_{2}$ melts. The structure in these melts changes when there are no longer enough charge-balancing Na$^{+}$ and Fe$^{2+}$ for the tetrahedrally co-ordinated Al$^{3+}$ and Fe$^{3+}$. As the melts become peraluminous the structure changes to probably include tri-clusters in which one oxygen is the apex of three tetrahedra. Although tri-clusters probably exist in peraluminous composition melts, the configurational entropy term at a viscosity of 10$^{12}$ Pa s [B$_{e}$/S$_{conf}$(T$_{g}$)] calculated for these melts suggests that the flow process is not initiated at a tri-cluster site. Flow appears to begin with the movement of an Al$^{Na}$-, Fe$^{Na}$- or Al$^{Fe}$ tetrahedron, with a tricluster being formed during the flow process. The general structure and flow mechanism of these Na$_{2}$O-FeO-Fe$_{2}$O$_{3}$-Al$_{2}$O$_{3}$-SiO$_{2}$ melts appear to be the same as those expected in peralkaline and peraluminous Na$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$ melts. It was not possible to vary the compositions of these melts sufficiently to determine the whether Fe$^{2+}$ and Na$^{+}$ have a preference for the type of tetrahedron (Al$^{3+}$ or Fe$^{3+}$) they charge-balance.
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting