DIFFERENTIAL DISSOLUTION AND RECRYSTALLIZATION: MICROSCALE CLUES TO PRECURSOR MINERAL SOLUBILITY

Petrographic insights from carbonate dissolution and recrystallization can be used as starting models to understand microscopic textures in sulfates in the Burns Formation, Mars. Most terrestrial marine carbonates contain aragonite, which is metastable with respect to calcite. During diagenesis, pore water chemistry determines the timing and rate of alteration of aragonite to calcite. If pore waters are saturated with respect to aragonite, as tropical marine waters are, they are supersaturated with respect to calcite. Calcite precipitates from the water, drawing down aragonite saturation, and aragonite starts to dissolve. If this process is slow, aragonite tends to recrystallize to calcite across a narrow dissolution-precipitation front, preserving petrographic textures of the primary aragonite. In contrast, if the dissolution of aragonite is rapid, calcite may not replace the aragonite, and porosity develops. This porosity can remain open or be filled with cements at a later date. The petrographic relationships among primary textures, recrystallized textures, and porosity provide temporal relationships that can be used to track mineral saturation states through time. The Burns Formation contains Mg and Ca sulfates among other minerals, and porosity developed along specific alteration fronts, e.g. the Whatanga Contact (McLennan et al. 2005). This zone is recrystallized with a loss of texture, suggesting pore fluids were significantly out of equilibrium with the mineral phases. In addition, the geometry of the vugs near this contact suggests that more soluble minerals were concentrated along depositional laminae, possibly due to: 1) different grain sizes being composed of different minerals; or 2) different layers having slightly different initial porosity and different early diagenetic pathways. In either case, dissolution occurred without the simultaneous precipitation of new minerals. Modern outcrops of the Burns Formation may be experiencing much slower recrystallization of sulfates. Outcrops are cracked, suggesting post-exposure water loss due to solar heating and desiccation, which may cause recrystallization of hydrous Mg sulfates to forms with less water. This process is sufficiently localized that primary structures are preserved.