In Situ Determination of Viscosity and Structure of Carbonatitic to Carbonate-Silicate Melts as Function of Pressure and Temperature

Abstract:

Carbonatitic and carbonate-silicate magmas are representative of mantle-derived liquids that form by partial melting of carbonated peridotite and eclogite rocks at depths greater than 60 km in the Earth’s interior. Carbonatitic melts are expected to contain 1-5 wt% SiO₂, but at shallower conditions (about 100 km in depth), as a result of large melt fraction during decompression, SiO₂ content increases up to 10-20 wt%. This variation in composition is expected to cause significant changes in the physical properties (e.g. viscosity and melt structure) of these magmas.

The aim of this study was to determine the viscosity and structure of CO₂-rich melts with variable SiO₂ content representative of carbonatitic to carbonate-silicate natural melts. A mixture of CaCO₃, MgCO₃, SiO₂, FeO, and NaCl was used as starting material. Synthetic glasses with 5 wt% and 18wt% SiO₂ were quenched at high pressure using multi anvil presses. Viscosity measurements on CO₂-bearing liquids were conducted with the falling-sphere method using the Paris-Edinburg type large volume apparatus at pressures between 1.5-6 GPa and temperatures of 1100-1500 °C. Determinations of viscosity of these liquids were determined from radiographic images recorded with a high-speed camera installed at Sector 16-BM-B (APS, Argonne). Falling velocity of the platinum probing spheres was measured by ultrafast X-ray radiography using a high-speed camera with a 500 fps recording rate (exposure time of 2 ms). The viscosity was, then, calculated from the Stokes equation including the correction factors for the effect of the wall and the end effect (Kono et al. 2014). Structural measurements of the liquid at high temperature were also performed using multi-angle energy dispersive X-ray diffraction technique.

Preliminary results from this study will contribute to understand the variation of viscosity as function of pressure, temperature and degree of polymerization of CO₂- melts during up welling within the asthenospheric mantle.