Models of iron oxide concretion formation: field, numerical, and laboratory comparisons

Well-exposed Jurassic Navajo Sandstone iron oxide concretions preserve important diagenetic records of groundwater flow and water–rock interactions. Field relationships, precipitation patterns, and geometries of the Navajo concretions provide the basis for input parameters in numerical computer simulations and laboratory chemical bench tests. Although field geometries are very difficult to replicate, numerical simulations and laboratory experiments examine end results such as nucleation and growth of iron oxide concretions, produced from known input parameters. Three numerical simulations show the development of periodic self-organized nucleation centers through Liesegang-type double-diffusion of iron and oxygen. This numerical model simulates a scenario where oxygen is provided by shallow fresh water and iron is sourced from deeper reduced formation water. Concretions form in the region where the two waters interact with each other. Model sensitivities show that advection of water is an important mechanism for supplying the iron, and that acidic conditions in the iron-charged water can cause iron to stay in solution longer to produce nucleation centers that are farther from the input source. Laboratory bench tests with reactions of FeSO4 or Fe(NO3)3 with KOH show how the precipitation of hydrated iron sulfates or iron-hydroxides may form rinds around an initial, spherical source of iron (i.e. nucleation center). These rinds may show inward growth depending on the concentration of the iron source in relation to the surrounding fluid. A number of complex factors such as concentration and flux, time, and multiple events can create banded patterns during rind growth. Comparisons of the terrestrial examples with numerical and laboratory models have strong implications for understanding similar hematite concretions on Mars.