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 δ⁵⁶Fe 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 δ⁵⁶Fe value of 0.21‰. Iron in the Kolob Plateau sample comprises a mixture of hematite grain coatings (δ⁵⁶Fe = 0.12‰) and a magnetic mineral fraction (interpreted to comprise largely detrital magnetite) that has a higher δ⁵⁶Fe value (0.45‰). Iron oxide cements have δ⁵⁶Fe values that are typically negative and as low as −0.74‰. In contrast, iron oxide-stained Navajo Sandstone in close proximity to a rinded iron oxide concretion has δ⁵⁶Fe values that are 0.9 to 1‰ greater than the δ⁵⁶Fe value of the associated iron oxide concretion. Finally, ferroan carbonates from the Navajo Sandstone yield δ⁵⁶Fe values as low as −0.86‰. 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 δ⁵⁶Fe values is evidence that aqueous Fe(II) was transported through the Navajo Sandstone aquifer and produced carbonate mineralization with negative δ⁵⁶Fe values. The difference in δ⁵⁶Fe 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 δ⁵⁶Fe values of the iron oxide concretions 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.

火星上氧化铁矿化球状堆积物的发现，再次激发了人们对地球上广泛但神秘的氧化铁堆积物的兴趣。在纳瓦霍砂岩（美国犹他州）中发现的剥皮氧化铁结核是这些矿点中研究最广泛的之一。对于纳瓦霍砂岩中氧化铁结核的形成，至少提出了四种不同的模型。这些包括 1) 混合含还原铁 (II) 的地下水和含氧地下水，2) 方解石结核中和氧化的酸性水，3) 方解石结核中和还原的酸性水，以及 4) 菱铁矿被微生物氧化含氧地下水侵入纳瓦霍含水层。我们测量了 δ ⁵⁶来自全岩纳瓦霍砂岩、结核氧化铁胶结物、定向于古地下水流动方向的氧化铁斑、剥皮结核内部的氧化铁斑和铁碳酸盐胶结物的铁值。然后，我们将这些数据与已发布的测量结果相结合，以评估四个已发布模型中的每一个。来自 Kolob 高原的全岩纳瓦霍砂岩被解释为未受到漂白液的影响，其 δ ⁵⁶ Fe 值为 0.21‰。Kolob 高原样品中的铁包含赤铁矿晶粒涂层 (δ ⁵⁶ Fe = 0.12‰) 和具有较高 δ ⁵⁶ Fe 值 (0.45‰) 的磁性矿物部分（解释为主要由碎屑磁铁矿组成）的混合物。氧化铁水泥有δ⁵⁶ Fe 值通常为负值，低至 -0.74‰。相比之下，氧化铁染色的纳瓦霍砂岩靠近剥落的氧化铁结核的 δ ⁵⁶ Fe 值比相关氧化铁结核的 δ ⁵⁶ Fe 值高 0.9 至 1‰ 。最后，来自纳瓦霍砂岩的碳酸盐铁产生的 δ ⁵⁶ Fe 值低至 -0.86‰。这些值与之前公布的岩性测量值非常一致，已经进行了可比测量。^{具有负 δ 56} Fe 值的铁质碳酸盐的存在证明含水 Fe(II) 被输送通过纳瓦霍砂岩含水层并产生了具有负 δ ^{56的碳酸盐矿化}Fe 值。氧化铁结核和相关氧化铁染色之间的δ ⁵⁶ Fe 值差异与吸附在石英颗粒上的含水 Fe(II) 和 Fe(II) 之间的平衡分馏比与含水 Fe(II) 之间的平衡分馏更一致以及任何还原或氧化的氧化铁矿物物种。氧化铁结核的负 δ ^{56 Fe 值最好通过还原铁相在封闭系统中的氧化来解释，在该封闭系统中没有水性 Fe(II) 可以逸出。}我们将铁同位素数据解释为证据表明铁通过还原流体从纳瓦霍砂粒中剥离，在纳瓦霍含水层中重新沉淀为菱铁矿，并在科罗拉多高原切割过程中在封闭条件下被地下水氧化。