Iron (oxyhydr)oxide concretion formation: New insights from southern Utah, USA

Iron (oxyhydr)oxide concretions are cemented mineral masses formed via authigenic cements in sedimentary rocks at any time during diagenesis (syndepositional, burial, and late-stage). These features are common in porous and permeable sandstone and even present on Mars in at least two different locations and formations. One notable study area for the spherules is within the Navajo Sandstone in the Grand Staircase Escalante National Monument (GSENM) in southern Utah. 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. These two fluids could be a reducing fluid that mobilizes iron as Fe²⁺ and then mixes with an oxidizing fluid to precipitate iron oxyhydr(oxide), but an acidic fluid could mobilize iron as Fe³⁺ and then interact with a neutral fluid for the same result.  Another proposed model for iron (oxyhydr)oxide concretions calls for calcium carbonate precursor concretions and mobilization of iron by acidic fluids. The acidic, iron-bearing fluid then dissolves the carbonate concretion, which buffers the solution enough to precipitate iron oxyhydr(oxide) in the same morphology as the original calcite concretion. Our research shows that in GSENM, iron concretions and calcite concretions are present within the same stratigraphic horizon and in close proximity. Another field observation is the presence of calcite concretions in clusters along paleo water tables, rather than dispersed in a self-organized spacing within three dimensions like the iron features. Also present within the region are concretions with manganese oxide phases and the iron concretions tend to include manganese oxide, but not calcite. Calcite concretions do commonly contain some iron (oxyhydr)oxides, particularly as rims around grains. In the Entrada Sandstone, also in southern Utah, iron concretions are precipitated from fluid brought in with an igneous intrusion that mobilized the iron within the host rock. Structures in the area acted as baffles keeping the fluid stagnant and iron (oxyhydr)oxide concretions are only present between the igneous dike and the nearest baffle. Calcite concretions in the area are dispersed throughout the host rock (both within and outside of the baffles), suggested that mineral precipitation rates control concretion formation and that iron (oxyhydr)oxide concretions need a longer period of fluid stagnation for formation than do calcite concretions. Understanding the complex formation mechanisms can help to unravel the history of diagenetic fluids of varying chemistries and therefore, the habitability of subsurface environments on both Earth and Mars.