The effect of H2O and CO2 on the viscosity of sanidine liquid at high pressures

Abstract

As part of our continuing research on the nature of silicate liquids at high pressures and to provide insight into the flow properties of granitic magmas at depth, we determined the viscosity (η) of liquid KAlSi3O8, and the viscosities of liquids in the systems KAlSi3O8‐CO2 and KAlSi3O8‐H2O, as a function of pressure and temperature using the falling‐sphere method. At 1500°C, log η of KAlSi3O8 decreases from 5.0 at 1 bar to 4.36 and 3.96 at 15 and 20 kbar, respectively, consistent with the negative (∂η/∂P)T of other highly polymerized silicate liquids. However, log η isothermally increases to 4.23 at 25 kbar. Adding 0.50 wt % CO2 to the liquid produces an isothermal decrease in log η to 4.06, 3.79, and 3.41 at 15, 20, and 25 kbar and 1500°C. An equimolar amount of H2O (0.21 wt %) reduces log η to 3.45, 3.40, and 3.34 at the above conditions. At 25 kbar, 0.21 wt % H2O and 0.50 wt % CO2 produce nearly equal reductions in the viscosity of KAlSi3O8 at 1500° and 1600°C. These data indicate that H2O and CO2 disrupt the flow mechanism of KAlSi3O8 by breaking bridging‐oxygen bonds, either by reaction of the molecular volatile species with the tetrahedral network to form OH− and CO32−, or by CO2 reacting with a non‐bridging oxygen to form CO32−, which disrupts the network of the liquid by rearrangement of the local balance in charges. This is in accord with previous studies of the viscosities of H2O‐ and CO2‐bearing NaAlSi3O8 liquids. CO2 is more effective in reducing the viscosity of KAlSi3O8 relative to NaAlSi3O8 at 25 kbar, indicating that the former liquid may dissolve a larger percentage of total carbon as CO32−. This conclusion is consistent with our published feldspar‐CO2 phase equilibria. At 20 and 25 kbar and 1500°C, log η decreases to 2.30 as H2O increases to 2.00 wt %, with a highly positive (∂2(log η)/∂χ2)T,P. In contrast to KAlSi3O8‐H2O, KAlSi3O8‐CO2 exhibits a viscosity minimum between 0.50 and 1.00 wt % at 20 kbar and 1.00 and 1.50 wt % at 25 kbar, perhaps reflecting a concentration‐dependent speciation of the CO2 molecule similar to that for H2O, or a change toward non‐Newtonian, pseudoplastic behavior as the liquid becomes saturated with carbon.