Three-dimensional study by synchrotron radiation computed tomography of melt distribution in samples doped to enhance contrast

The three-dimensional (3-D) distribution of melt in partially molten synthetic samples compositionally corresponding todiopside (90 wt%) - anorthite (10 wt%) and doped with PbO, WO₃, MoO₃, or Cs2O to enhance contrast was studied by X-ray computed tomography (CT) with synchrotron radiation at SPring-8. The heavy elements were strongly concentrated in the melt and contributed to an increase of the X-ray linear attenuation coefficient (LAC) of the melt. It was found that PbO is compatible with silicate melt (> 20 wt% in solution) and incompatible with diopside crystals. Other oxides WO₃ (∼10 wt%), MoO₃ (∼5 wt%), and Cs2O (< 5 wt%) are also soluble only in the melt.Such doping is useful not only for LAC control in X-ray CT measurement, but also for systematic control of the structure (wetting properties, distribution and connectivity) of partial melt. This technique gives basic information for discussion of 3-D distribution of partial melt having different wetting properties. Since PbO was the most effective in visualization of the diopside - anorthite partially molten system, the CT images of the PbO-bearing sample were used for further 3-D investigation of distribution. A distribution of dihedral angles at solid-melt-solid triple junctions ranging from 22° to 55° was observed with the 3-D data. This range in angle distribution was probably caused by anisotropy of crystals and the result supports the argument that there is some limitation in a theoretical framework 53 of stereology which estimates the 3-D structure based on 2-D observation. Investigators have begun to apply X-ray CT to the study of 3-D distribution of partial melt in rocks using synchrotron radiation facilities. Our study on the effect of doping is one approach for developing a technique to investigate 3-D melt distribution.