Iron isotope evidence for siderite precursors to iron oxide concretions from the Navajo Sandstone, Utah (USA)

The discovery of spherical accumulations of iron oxide mineralization on Mars has prompted renewed interest in widespread but enigmatic accumulations of iron oxide on Earth. The rinded iron oxide concretions found in the Navajo Sandstone (Utah, USA) have been among the most extensively studied of these occurrences. At least four different models have been proposed for the formation of iron oxide concretions in the Navajo Sandstone. These include 1) mixing of reduced Fe(II)-bearing and oxygenated groundwaters, 2) neutralization of oxidized, acidic waters by calcite concretions, 3) neutralization of reduced, acidic waters by calcite concretions, and 4) oxidation of siderite by microbes during invasion of the Navajo aquifer by oxygenated groundwaters. We measured 856Fe values of iron-bearing minerals from whole-rock Navajo Sandstone, concretionary iron oxide cements, iron oxide stains that are oriented in the direction of paleo-groundwater flow, iron oxide stains on the interior of rinded concretions, and ferroan carbonate cements. We have then combined these data with published measurements to evaluate each of the four published models. Whole rock Navajo Sandstone from the Kolob Plateau has been interpreted to be unaltered by bleaching fluids and has a 856Fe value of 0.21%o. Iron in the Kolob Plateau sample comprises a mixture of hematite grain coatings (856Fe = 0.12%o) and a magnetic mineral fraction (interpreted to comprise largely detrital magnetite) that has a higher 856Fe value (0.45%o). Iron oxide cements have 856Fe values that are typically negative and as low as-0.74%o. In contrast, iron oxide-stained Navajo Sandstone in close proximity to a rinded iron oxide concretion has 856Fe values that are 0.9 to 1%o greater than the 856Fe value of the associated iron oxide concretion. Finally, ferroan carbonates from the Navajo Sandstone yield 856Fe values as low as-0.86%o. These values are in good agreement with measurements published previously for lithologies for which comparable measurements have been made. The presence of ferroan carbonates with negative 856Fe values is evidence that aqueous Fe(II) was transported through the Navajo Sandstone aquifer and produced carbonate mineralization with negative 856Fe values. The difference in 856Fe values between the iron oxide concretions and associated iron oxide staining is more consistent with equilibrium fractionation between aqueous Fe(II) and Fe(II) adsorbed on quartz grains than it is with equilibrium fractionation between aqueous Fe (II) and any reduced or oxidized iron oxide mineral species. The negative 856Fe values of the iron oxide con-cretions are best explained by oxidation of a reduced iron phase in a closed system from which no aqueous Fe(II) could escape. We interpret the iron isotope data as evidence that iron was stripped from Navajo sand grains by reducing fluids, reprecipitated in the Navajo aquifer as siderite, and oxidized by groundwaters under closed conditions during incision of the Colorado Plateau.

Automated summary

The discovery of spherical accumulations of iron oxide mineralization on Mars has led to renewed interest in similar accumulations on Earth. This study focuses on the rinded iron oxide concretions found in the Navajo Sandstone and evaluates four different models for their formation. The authors measured the iron isotope values of different iron-bearing minerals and combined their data with published measurements to assess each model. The results suggest a complex process involving the transport and oxidation of iron in a closed system.