Publication Date:
2020-10-29
Description:
In-situ falling-sphere viscometry using shadow radiography in a multianvil apparatus was conducted on a series of samples
along the NaAlSi3O8–H2O join up to 2.8 wt.% H2O at the Spring-8 synchrotron radiation facility (Hyogo, Japan). This allowed us
to determine viscosities normally too low to be measured at ambient pressure for hydrous silicate melts at high temperatures due to
rapid devolatilization. Pressure was fixed at 2.5 GPa for all experiments allowing us to gauge the effect of chemical composition on
viscosity. In particular, the series of samples allowed us to vary the melt's degree of polymerization while maintaining a constant Al
to Si ratio. Our results show that, for all samples, viscosity decreases as a function of pressure between 1 atm and 2.5 GPa at
1550 °C, indicating that the pressure anomaly can still be observed as depolymerization of the melt increases from nominally 0 (dry
albite liquid) to NBO/T=0.8 (assuming water speciation entirely as hydroxyl groups at experimental conditions). We also find that
the magnitude of the decrease in viscosity over this pressure interval does not appear to be dependent on the amount of water in the
melt (i.e., NBO/T). An explanation for this behavior might be that the molar volume, at least over this limited compositional range, is
nearly constant and the effects of compression of these melts, though different in degree of polymerization, are similar.
Description:
Published
Description:
2-9
Description:
JCR Journal
Description:
reserved
Keywords:
Viscosity
;
Silicate melts
;
High pressure
;
04. Solid Earth::04.01. Earth Interior::04.01.05. Rheology
;
04. Solid Earth::04.08. Volcanology::04.08.03. Magmas
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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