ASTROBIOLOGICAL CONSIDERATIONS IN CONCRETION FORMATION

Diagenetic concretions are cemented mineral masses found in both clastic and chemical sedimentary rocks and can form in a range of morphologies including spheroidal. Both the MER and MSL rovers have also observed diagenetic spherules in the sedimentary rocks on Mars. Diagenetic concretions form via two end-member models: radial or pervasive growth, though concretions can grow via a combination of both mechanisms. Spheroidal concretions form via diffusion of reactants in a homogenous medium that controls the diffusion rate so that it is equal from all directions. Elongate forms form via advecting fluid. Diagenetic concretions, particularly iron (oxyhydr)oxide mineralogies are thought to form via a two-fluid mixing model where one fluid has the reactants in solution, then another fluid meets and mixes with the reactant-bearing fluid, and the concretions precipitate. Of course, in the subsurface, reservoir fluids can change from oxidizing to reducing by simply lowering and raising the water table. Because of the complexities of formation mechanisms, interactions with subsurface biota and preserved biological material, and interactions of evolving subsurface fluids over geologic time, diagenetic concretion formation mechanisms are difficult to understand, yet they preserve a record of interactions that can be deduced via petrographic and other laboratory methods utilized to interpret the mineralogy and chemistry of these forms. Life is almost certainly involved in some capacity during precipitation of these spherules as subsurface life is concentrated around redox reaction fronts; however, to what degree these forms preserve biological molecules is not known and has not been systematically studied to date. This research presents relations of different morphologies, mineralogies, and stratigraphic locations to infer formation mechanisms and link formation mechanisms to clues about biological involvement during precipitation.