IRON ISOTOPE EVIDENCE FOR THE ORIGIN OF IRON OXIDE CONCRETIONS FROM THE NAVAJO SANDSTONE, UTAH (USA)

Previous workers (Chan et al., 2006; doi: 10.1130/GES00051.1; Busigny and Dauphas, 2007; DOI: 10.1016/j.epsl.2006.11.038) have shown that Fe oxides that form concretions in the Navajo Sandstone have very low δ⁵⁶Fe values. Although there is general agreement that the Fe that now resides in concretions was stripped from sand grains within the Navajo, δ⁵⁶Fe values obtained from these rocks have been considered to be enigmatic. We have collected Fe isotope data from Navajo concretions and related rocks. The δ⁵⁶Fe value of Navajo SS that we interpret to be unaltered by bleaching fluids is 0.21‰. This value is not sufficiently different from the δ⁵⁶Fe values of so-called comet tails (0.30‰) that apparent primary and secondary staining can be distinguished using Fe isotopes. Fe oxide cements, on the other hand, have δ⁵⁶Fe values as low as -0.86‰ (all values measured here are in good agreement with previous studies). We have proposed that these concretions are products of an evolving groundwater system. Mantle-derived CO₂ and small amounts of CH₄ migrated updip to traps sealed by the Carmel Formation. The CH₄ and CO₂ dissolved in formation waters, reducing the Fe on the sand grains and bleaching the sandstone. The CO₂-charged formation waters then sank by density-driven convection. There is little apparent evidence that bleaching was accompanied by Fe isotope fractionation; bleached sandstones have approximately the same δ⁵⁶Fe values as do red ones. There are, however, two Fe reservoirs in the Navajo. 1)A labile reservoir comprising the Fe oxide coating on sand grains and 2)a refractory reservoir comprising ferrous silicates. Bleaching results in near complete removal of Fe from the labile reservoir with preservation of Fe in the refractory reservoir. Although the δ⁵⁶Fe values of the bulk sandstone changed little during bleaching, the Fe²⁺ in the aqueous solution could have had δ⁵⁶Fe values as low as -0.9‰. Later fracturing facilitated degassing and precipitation of siderite. Siderite precipitated from this aqueous solution would have had δ⁵⁶Fe values that ranged upward from -1.4‰. Invasion of the Navajo by oxidizing groundwaters resulted in microbially-mediated oxidation of the siderite concretions. The strongly negative values of the Fe oxides result from the near-quantitative oxidation of the siderite in a closed system.