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Tracing Paleofluid Circulations Using Iron Isotopes: a Study of Hematite

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Tracing Paleofluid Circulations Using Iron Isotopes: a Study of Hematite Earth and Planetary Science Letters 254 (2007) 272–287 www.elsevier.com/locate/epsl Tracing paleofluid circulations using iron isotopes: A study of hematite and goethite concretions from the Navajo Sandstone (Utah, USA) ⁎ Vincent Busigny , Nicolas Dauphas Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA The Field Museum, 1400 South Lake Shore Drive, Chicago, IL 60605, USA Received 18 December 2005; received in revised form 17 November 2006; accepted 20 November 2006 Available online 4 January 2007 Editor: C.P. Jaupart Abstract 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 ∼50%, the bleached sandstones have Fe isotopic compositions similar to adjacent red sandstones (∼0‰/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‰/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. © 2006 Elsevier B.V. All rights reserved. Keywords: Fe isotopes; hematite; goethite; concretions; fluid circulations 1. Introduction Hematite (Fe2O3) is the most oxidized form of iron commonly found in natural environments. On Earth, he- ⁎ Corresponding author. Tel.: +1 33 14427 4799; fax: +1 33 14427 3752. matite is usually precipitated from aqueous fluids in lakes, E-mail address: [email protected] (V. Busigny). spring fluids or groundwater flows [1]. The formation of 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.11.038 V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287 273 hematite requires Fe to be dissolved, transported and the effects of dissolution, transport and precipitation on precipitated. Iron can be transported as ferrous or ferric the isotopic composition of Fe. This sandstone displays iron. Ferric iron, Fe(III), is dominant under very acidic clear color variations, from red to white (Fig. 2), reflect- (pHb2) and oxidizing conditions while ferrous iron, Fe ing the various amounts of Fe-oxide present as coatings (II), is dominant in more reducing conditions and over a on the detrital grains. It was suggested in previous larger range of pH (∼0 to 7). In terrestrial environments, studies that the light-colored rocks (hereafter referred to Fe is mostly mobilized as Fe(II)aq. When Fe-rich reducing as bleached) were derived from leaching of the red rocks fluids encounter oxidizing conditions, Fe can precipitate by reducing fluids such as hydrocarbons, methane, as hydrous ferric oxide (HFO). Over time, HFO can organic acids, or hydrogen sulfide [19–23]. Subsequent dehydrate to goethite (FeOOH), and then to hematite (e.g., mixing of Fe-rich reducing fluids with oxidizing [1,2]). Redox variations and exchanges between dissolved groundwater would have led to the precipitation of Fe(II)aq, Fe(III)aq, adsorbed Fe(II)ad, and precipitated hematite and goethite concretions [19,20,24]. In the past Fe(III)s can potentially be traced using Fe isotopes [3–5]. few years, the Navajo formation has been the focus of Within the last ten years, several experimental studies much attention because of similarities between the field have characterized Fe isotope fractionation associated occurrences of spherical iron concretions in Utah and the with biotic and abiotic dissolution, reduction, oxidation, so-called Martian Blueberries discovered at Meridiani adsorption, ligand-exchange, and precipitation [6–18]. Planum by the Mars Exploration Rover mission The Navajo Sandstone in south-eastern Utah, USA [20,24,25] (see also [26] for another terrestrial analogue (Fig. 1) represents a particularly interesting site to study of Martian Blueberries on Mauna Kea, Hawaii). Fig. 1. Schematic maps showing the location of the samples analyzed in this study. (A): Location in south-eastern Utah, USA. (B): Main geological unitsof the studied area. Samples were collected within the Navajo Sandstone Formation in the Capitol Reef National Park (CRNP) and the Grand Staircase- Escalante National Monument (GSENM). Capital letters in parentheses indicate the lithostratigraphic ages of the geological units (Q.: Quaternary,T.:Tertiary, M.J.: Middle Jurassic, L.J.: Lower Jurassic, U.T.: Upper Triassic, L.T.: Lower Triassic). (C): Zoom in the GSENM where most of the samples were collected. 274 V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287 Fig. 2. Photographs illustrating the various sample types found in the Navajo Sandstone. A: Dark spherical hematite/goethite concretions (diam. ∼2 cm) accumulated on red sandstone (GSENM; N37°41.342′, W111°22.949′). B: Massive tabular iron concretion (CRNP; N38°12.874′, W111°09.584′). C: Adjacent red and bleached sandstones (CRNP; N37°48.383′, W110°57.736′). D: Single spherical concretion (diam. ∼1.5 cm) embedded in sandstone (GSENM; N37°41.130′, W111°22.993′). E: Small spherical concretions (b5 mm; GSENM; N37°40.771′, W111°23.137′). F: Asymmetric oxidation pattern (“comet trail”), indicating the direction of fluid flow (GSENM; N37°41.097′, W111°22.944′). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) In the present contribution, we report Fe concentra- signatures recorded in hematite and goethite concretions tions and isotopic compositions of hematite and goethite can be used to trace subsurface paleofluid circulations. concretions as well as associated red and bleached host- rocks from south-eastern Utah. The data are used to gain 2. Sample descriptions new insights into the physico-chemical processes that governed the formation of Utah concretions. The main The samples were collected at two different sites in goal of this study is to evaluate whether Fe isotopic Utah (Fig. 1): the Capitol Reef National Park (CRNP) V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287 275 and the Grand Staircase-Escalante National Monument Their sizes vary from 3 to 68 mm in diameter (Fig. 2A, D (GSENM). Sampling sites from CRNP and GSENM are and E). Most of the samples display an egg-like structure located respectively on the west and east flanks of a with a hard outer cementation rind enclosing a weakly major anticline fold, the Circle Cliffs uplift, which is cemented inner part (Fig. 3A). The thickness of the rind oriented approximately in the direction N–S(Fig. 1B). varies from ∼0.5 to 7 mm. Only a few samples show Most of the samples were collected in a restricted area homogeneous distribution of hematite cement over the covering ∼20 km2 (5×4 km) in the GSENM (Fig. 1C). whole concretion (i.e., without any rind). Most of the samples display concentric “onion-skin” growth structures. 2.1. Host-rocks In Utah, hematite and goethite concretions are mostly found in the Jurassic Navajo Sandstone (deposited from ∼190 to 198 Ma; [27]). This sandstone and its related equivalents (the Aztec and Nugget sandstones) represent the largest eolian sand deposit in North America [28].The Navajo Sandstone is a well-sorted, fine- to medium-grain quartz arenite [19]. It shows high-angle, large-scale cross- bedding features. The detrital mineralogy is composed of quartz (∼90%), K-feldspar and accessory minerals such as zircon, garnet and tourmaline. Detrital minerals have been variably cemented by quartz, calcite, dolomite, kaolinite, goethite and/or hematite, reflecting a complex, multi-stage, diagenetic history [22]. In the Navajo Sandstone, the color variations, from red to white, were interpreted to reflect leaching of the red rocks by reducing fluids, associated with dissolution of Fe-oxide coatings [19–23]. The presence of small disseminated grains of pyrite in many
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