INSIGHTS INTO SALINE PAN HALITE PARAGENESIS: THE BONNEVILLE SALT FLATS AS A CASE STUDY

Evaporite deposits in ephemeral saline pans are dynamic lacustrine mineral interfaces. Halite and gypsum saturation indices in saline pans greatly fluctuate in response to changing fresh water input and cycling temperatures during flooding, evaporation, and desiccation periods. Unique morphologies develop in response to brine movement and crystal dissolution, growth, and recrystallization. Experimental research in the past decade has shed insights into the processes governing morphology development under evaporation dominated efflorescent conditions and flooding dominated dissolution conditions. Little work, however, has been done to tie these morphologies to those seen in modern saline pan settings. This research uses thin sections, micro-CT imagery, and X-ray fluorescence analysis of Bonneville Salt Flats evaporite crust coupled with field-based observations to describe micron to mm-scale crust morphologies and associated environmental processes. This research yields insights into the formation of void filling halite and recrystallization features as well as the near-surface recrystallization of halite. Amorphous void filling halite distinctly differs from ~10-40 micron efflorescence crystals and larger 100-200 micron crystals formed by near-surface recrystallization of efflorescence. Dissolution features range from centimeter scale pipes to incipient micron scale dissolution features. Dissolution features create voids and interfaces that control brine flow and the sites of evaporite precipitation and dissolution. In addition to distinct environmental conditions on the surface, there are morphological regimes related to depth in the upper 10-20 cm of crust at the Bonneville Salt Flats. Thin section and micro-CT data indicate porosity domains change with depth from 1) high porosity surface halite, to 2) low porosity halite with void filling halite and incipient dissolution pipes, to 3) highly porous dissolution pipe dominated halite. This paragenetic sequence is a characteristic product of the fluctuating saturation indices at ephemeral saline pans. Collectively, these insights into halite microstructures enhance the models used to understand and describe the unique evaporite morphologies developed by changing saturation states and mineral surfaces in saline pans.