IRON ISOTOPES IN TERRESTRIAL HEMATITE AND GOETHITE CONCRETIONS: IMPLICATIONS FOR DIAGENETIC HISTORY ON MARS

Iron oxide concretions on Earth (e.g., Utah “marbles”) can provide valuable models for understanding hematite spherules (Mars “blueberries”) and their diagenetic history at Meridiani Planum. Iron isotopes were examined from two end member concretion mineralogies, representative of contrasting, spatially distinct chemical reaction fronts in the Jurassic Navajo Sandstone near towns of Boulder and Moab, Utah. δ⁵⁶Fe values of iron oxide concretions range from -1.5 to +1.0, distinct from the near-zero δ⁵⁶Fe values that characterize iron oxide weathering products. The wide range in Fe isotope compositions indicate significant transport of Fe in solution, consistent with current published models for terrestrial iron oxide concretions. In a Boulder area spheroidal goethite concretion ~ 5 cm in diameter, a traverse across the hard outer rind and into the weakly layered interior shows a total range in δ⁵⁶Fe from -1.5 to -0.9. The negative values δ⁵⁶Fe are unusual for iron oxides, and suggests possible biological cycling or oxidation of low-δ⁵⁶Fe precursor minerals such as pyrite or siderite. In a Moab area sample, an elongate hematite concretion ~ 2 cm across shows well developed internal layering, where δ⁵⁶Fe regularly decreases inward from +0.9 to +0.4 towards the center. The overall positive δ⁵⁶Fe values are best explained through partial oxidation of aqueous Fe(II), where growth may have occurred from the outside towards the interior.

The significant Fe isotope variations indicate a complex diagenetic history where significant quantities of Fe were mobilized through fluid flow in an open groundwater system. Redox cycling of Fe is the most likely explanation for the large range in Fe isotope compositions, where reduced Fe components (Fe(II)aq, pyrite, siderite, etc.) intersected oxidizing zones to form the iron oxide concretions. Unlike surface weathering and oxidation, which produces little net Fe isotope fractionation, the large Fe isotope variations measured here indicate that oxidation of a significant reservoir of reduced Fe components was incomplete and likely required large fluid fluxes during concretion formation. If the Mars “blueberries” were formed through similar processes, it is possible that large fluid reservoirs were also required for their formation.