Tracing paleofluid circulations using iron isotopes: A study of hematite and goethite concretions from the Navajo Sandstone (Utah, USA) | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  Tracing paleofluid circulations using iron isotopes: A study of hematite and goethite concretions from the Navajo Sandstone (Utah, USA)

Type de publication:

Journal Article

Source:

Earth and Planetary Science Letters, Volume 254, Ticket 3-4, p.272-287 (2007)

ISBN:

0012-821X

Mots-clés:

Physico-chimie des fluides géologiques ; Fe isotopes; hematite; goethite; concretions; fluid circulations, UMR 7154

Résumé:

<p>
Iron concentrations and isotopic compositions were measured in spherical hematite and goethite concretions, together with associated red (Fe-oxide coated) and white (bleached) sandstones from the Jurassic Navajo formation, Utah (USA). Earlier studies showed that, in the Navajo Sandstone, reducing fluids (presumably rich in hydrocarbons) mobilized Fe present as Fe-oxide coatings on detrital quartz grains. Dissolved Fe then precipitated as spherical concretions by interaction with oxidizing groundwater. Despite being depleted in Fe by similar to 50%, the bleached sandstones have Fe isotopic compositions similar to adjacent red sandstones (similar to 0 parts per thousand/amu relative to IRMM-014). This shows that dissolution of Fe-oxide did not produce significant isotope fractionation, in agreement with previous experimental studies of abiotic Fe-oxide dissolution. In contrast, the concretions are depleted in the heavy isotopes of iron by -0.07 to -0.68 parts per thousand/amu. This is opposite to the expected fractionation for partial Fe oxidation, which tends to enrich the precipitate in the heavy isotopes. Several scenarios are considered for explaining the measured Fe isotopic compositions. Although diffusion might be an important process in controlling the growth of spherical concretions, the associated isotopic fractionation is negligible compared to the observed variations. Kinetic isotope fractionation during precipitation can be ruled out as well because no isotopic zonation is seen within indurated concretions and Fe isotope evidence supports the occurrence of dissolution-reprecipitation reactions consistent with equilibrium growth conditions. The Fe isotopic compositions of the concretions are best explained by evolution of the fluid composition through precipitation and/or adsorption of isotopically heavy Fe during fluid flow through the sandstone. This scenario is supported by a regional trend in the isotopic composition of Fe, showing that this element was transported in fluids over several kilometres along major tectonic structures. These results demonstrate for the first time the virtue of Fe isotopes for tracing the directions and scales of paleofluid flows in porous media. (c) 2006 Elsevier B.V. All rights reserved.</p>

Notes:

Earth Planet. Sci. Lett.Busigny, Vincent Dauphas, Nicolas 57 ELSEVIER SCIENCE BV