Author: Paul Michael Edwards

Publisher:

Published: 2019

Total Pages:

ISBN-13: 9781392212257

Falling sphere viscometry at high pressures and temperatures is used to constrain the viscosity of iron-rich silicate liquids (natural and synthetic CaFeSi2O6), approximating pyroxenitic melts in the Earth’s mantle. In-situ falling sphere viscometry experiments are conducted using synchrotron radiation and the multi-anvil apparatus to record in real-time the motion of a small Re sphere falling through the melt at high pressure and temperature. Terminal velocity is determined from sphere position versus time plots and used, together with constraints on sphere diameter, sphere and melt density and capsule geometry, to compute melt viscosity from the Stokes equation. Experiments were conducted at the GSECARS 13-ID-D beamline at the Advanced Photon Source, Argonne National Lab in the T-25 multi-anvil apparatus. For this work a new in-situ falling sphere assembly was developed with a double reservoir capsule permitting the drop of the Re sphere from the upper reservoir of refractory materials into superliquidus melt in the low reservoir. This configuration enables viscosity to be determined over a range of superliquidus temperatures at a given pressure. Hedenbergite melt viscosity is found to vary between 0.45 Pa∙s to 0.05 Pa∙s from 1 to 7 GPa and 1760 to 2130 K. There are no significant differences in superliquidus viscosities for the natural and synthetic compositions. These new data are combined with published data on enstatite, wollastonite, and diopside liquids to estimate the viscosity of pyroxenitic melts up to 7 GPa. These results show that pyroxenite liquids have lower viscosity and weaker pressure dependence than basaltic melts at upper mantle conditions. Considering the densities of pyroxenitic and basaltic melts in the upper mantle, we find that pyroxenite melts have higher mobility than basaltic melts and thus are expected to segregate more readily that basaltic melt from a heterogeneous mantle source during partial melting by adiabatic decompression of upwelling mantle.