FORMATION OF IRON-RIMMED SANDSTONE NODULES; MECHANISM OF FORMATION AND TERRESTRIAL ANALOGUE FOR MARTIAN BLUEBERRIES?

NASA's twin Spirit and Opportunity Rovers have identified numerous geologic features that hint at a watery past on Mars. Prominent among these features are the "Martian Blueberries" which occur as spherical hematite (Fe₂O₃) nodules and often accumulate as a weathering residue in topographic depressions. Hematite formation is facilitated by the presence of water, although presently there are no known sources of liquid water on Mars. These "blueberries" also possess similarities to nodules found in the Jurassic aged Navajo Sandstone of Utah (Chan et al., Nature 2004) and other regions. The Utah nodules, locally referred to as Moqui Marbles, have Fe- and Mn-oxide cemented rims surrounding a central core of quartz cemented sandstone. An understanding of the processes controlling the formation of the Utah nodules may thus be key in determining the processes of nodule formation on Mars.

Static batch tests were performed to manipulate different types of geochemical environments and see which ones will precipitate iron. A ferrous ammonium sulphate leachant solution (pH = 4.5) was reacted with various mineral phases in a static batch test to induce pH and/or Eh changes that could cause a decrease in iron solubility. A pH rise following reaction with calcite readily induced iron precipitation. Glass tubes packed with loose quartz sand and isolated crystals of calcite were also exposed to a solution containing ferrous ammonium sulphate at a slow flow rate. The reaction with the calcite induced the precipitation of reaction rims of ferric iron that mimic the original geometries about the calcite grains. Two flow tests were also conducted with mechanically rounded samples of calcite cemented Dakota Sandstone, one with relatively friable and the other with tightly cemented sandstone grains. A ring of iron-oxide precipitate was generated, in which the friable sandstone produced a thin ring near the outer edge of the Dakota sandstone samples, while the well-cemented sandstone produced a ring that was spaced further away from the Dakota sample. Based on these test results, it appears plausible that the reaction between upward migrating basinal fluids enriched in Fe²⁺ and sedimentary calcite nodules may be responsible for the formation of the Utah nodules. Could a similar process have occurred on Mars?