EVIDENCE SUPPORTING THE SIDERITE OXIDATION HYPOTHESIS FOR THE ORIGIN OF IRON OXIDE CONCRETIONS IN THE NAVAJO SANDSTONE

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 combined with oxidation, and 4)dissolution of siderite and oxidation of aqueous Fe(II) by microbes during invasion of the Navajo aquifer by oxygenated groundwaters. The siderite oxidation hypothesis has been questioned with the assertion that evidence for preexisting concretions consisting of siderite has never been found. We argue that there are at least 4 lines of evidence that support a siderite precursor. 1)Iron oxide mineralization that occurs in the cores of some concretions occurs as rhombohedral grains that we interpret to be pseudomorphs after siderite. 2)Ferroan carbonate also occurs in the Navajo Sandstone, evidence that the Navajo aquifer once contained reduced carbon dioxide-bearing waters. The median δ⁵⁶Fe value for the iron oxide concretions is the same as the δ⁵⁶Fe value of ferroan carbonate. 3)Sirono et al. (https://doi.org/10.1016/j.gca.2020.12.017) have shown that rind thickness increases with concretion diameter. We interpret this relationship as evidence that the iron was derived from the interior of the concretion. Only in the siderite oxidation hypothesis is the iron derived from the interior of the concretion. 4)Rinded and pipe-like concretions typically form in rocks in which siderite is a typical diagenetic phase.