CAN DIFFUSION IN A THERMAL GRADIENT PRODUCE CRYSTALLOGRAPHIC PREFERRED ORIENTATION?

Can a preferred orientation of crystals develop in a rock when subjected to a thermal gradient under hydrostatic conditions? We have carried out a preliminary study to address this question using electron backscatter diffraction (EBSD) on samples that were produced during diffusion-reaction experiments in a piston cylinder apparatus at 0.5 GPa. Samples contained powders of basaltic andesite overlying a leucocratic gabbro, with the andesite end held at at 1200°C and the gabbro extending down to the cold end which was held at temperatures of between 800°C and 500°C. Some experiments were nominally anhydrous, whereas some contained 5 wt % saline water. Experiments lasted between 13 and 26 days, and were run in NaCl-pyrex assemblies which should provide a near hydrostatic pressure environment. EBSD mapping analysis of plagioclase, clinopyroxene, and orthopyroxene in the gabbroic areas of both experiments yielded an isotropic fabric. However, analysis of olivine, a new phase formed during the experiment (not present in the initial powders), displays a well-developed fabric, with the [010] axes of the olivine grains clustering at a moderately inclined angle to the assumed plane of the thermal gradient. These intial results suggest that the presence of a rock fabric in intrusive rocks might be at least partially due to the existance of a thermal gradient during crystallization, and thus not all preferred orientation neccessarily reflects reorientation by magmatic flow, gravitational settling, or tectonic shear. The effect we have observed might also apply to development of preferred mineral orientation in the wall rock of thermal aureoles adjacent to plutons. Ongoing experiments examining diffusion-reaction processes occurring in a horizontally oriented temperature gradient will allow us to distinguish gravitational effects from the temperature gradient effects.