EXPERIMENTAL INVESTIGATION OF FLUID PHASES IN THE SYSTEM HAPLOGRANITE-WATER-ALKALI CHLORIDE

In studies of genesis of pegmatite dikes, questions arise about properties of fluid phases and efficacy of fluids in material transport. Thermal modeling of zoned pegmatite-aplite dikes suggests that crystallization occurs under non-equilibrium conditions in the presence of a temperature gradient, while fluid inclusions indicate that alkali halides are significant components of the system. In a reconnaissance investigation to characterize fluid phases in the haplogranite system with water and alkali halides, we carried out a series of experiments in cold-seal pressure vessels at P of 200MPa, T from 680 to 710 C, and an imposed temperature gradient of 6 C between the hot and cold ends of platinum capsules. We used silicate gels or glasses as starting materials and added 2 molar Na-K chloride solution in amounts ranging from 50 to 80 wt%. Our objective is to assess the nature of silicate migration under vapor-saturated conditions in the presence of a saline brine and in response to a thermal gradient. After runs of up to 18 days duration, run products included euhedral crystals of quartz and alkali feldspar, watery saline brine, soft gel-like spheres, and hard isotropic glass in three distinct habits. The most unusual glass is a micro-botryoidal surface crust or adlayer which adheres to certain crystal faces and is absent from others. The hard glass also occurs as tiny spheroids attached to crystal faces and capsule walls, often displaying a planar surface at the point of attachment. To a lesser extent the glassy adlayer occurs as a smooth featureless sheet adhering to capsule walls. All hard glass is considered to be quenched melt which we analyzed by Auger electron spectroscopy. Of particular significance is discovery that glass at the hot lower end of capsules is enriched in sodium while glass at the cold upper end of capsules is enriched in potassium, replicating the chemical pattern found in zoned pegmatite-aplite dikes. Finally, physical distribution of glass suggests a possible new avenue for research investigating migration of the silicate melt phase in response to an imposed temperature gradient.