VISCOSITIES OF HIGHLY FLUID MELTS: CARBONATITES AND ANALOGUES

Abstract:

Carbonatites are carbonate-rich and very fluid melts believed to form mainly from primary mantle melting. The mobility of these melts has attracted renewed interest as carbonatites are considered a major transport agent of carbon from the mantle to the crust and may be intimately linked to the generation of kimberlites.

The measurement of carbonatite viscosity is a priority in order to understand the carbonatite mobility and reaction rates. Obtaining accurate viscosity measurements of such low viscosity melts is however an experimental challenge due to volatility, very low torques and melt stability in the viscometer.

We present preliminary results of a rheological characterization study (Couette viscometry) on a series of very low viscosity natural melts and their analogues. Experiments have been performed using a Modular Compact Rheometer (Anton Paar MCR 502) equipped with 2 custom-made concentric-cylinder narrow gap geometries (steel and Pt-Au). The rheometer is characterized by an air-bearing-supported synchronous motor with torque ranging between 0.01 µNm and 230 mNm (resolution of 0.1 nNm).

The instrument has been calibrated in a temperature-viscosity-shear rate window comparable with carbonate melts. Initially, a certified silicone standard (980 and 39000 mPa sec in a temperature interval 100-25°C) at shear rate of 1 to 100 sec^-1 has been used. Then the viscosity of distilled water (0.08 – 0.05 mPa sec) at 30 and 60°C has been measured between 30 and 70 sec^-1. Finally, high temperature calibration measurements at 1000°C have been performed using a DGG standard glass.

Viscosity measurements on carbonate melts have been performed in the temperature range of 730-950°C. Measured values range between 3.4 and 15.2 mPa sec. Results show that in the investigated temperature range the melts exhibit, as expected, Newtonian viscosity. The results will be discussed in the context of all available data from the literature including high pressure determinations.