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 bleached rocks indicates that Fe was probably mobilized and transported as Fe(II) [22,23]. Mixing of Fe(II)-rich reducing fluids with oxygenated groundwater induced the precipitation of iron concretions [19,20,24]. Thus, the source of Fe in the concretions is believed to be the host-rock itself. The age of formation of the iron concretions is estimated to be ∼25–20 Ma [29].In order to estimate the isotopic composition of primary dissolved Fe, we measured the Fe concentrations and isotopic compositions in several pairs (n=6) of adjacent red and bleached sandstones (Fig. 2C). For each pair, the red and bleached rocks were sampled within less than 3 m from each other so as to avoid potential large-scale heterogeneity of the sandstone. A red rock (sample CR#5), located far from any bleached zone, was also analyzed to provide a reference, exempt from Fe mobilization.

2.2. Iron concretions

Iron concretions are made of subrounded quartz grains (100–600 μm in size) held together by a cement of Fig. 3. Cross-section analyses of five growing zones within a spherical hematite±goethite filling the porosity of the rock [19].Iron concretion showing four weakly cemented inner parts surrounded by a hard rind (sample B#20). (A) Photograph of the concretion (diam. oxides represent up to 35% of the concretions [19,20]. ∼4.3 cm). (B) Fe2O3totconcentrations and (C) Fe isotopic compositions Thirty-six bulk spherical hematite concretions were of the five growing zones. The dashed lines indicate the Fe2O3tot analyzed including 7 from CRNP and 29 from GSENM. concentration and Fe isotopic composition of the bulk concretion. 276 V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287

Two spherical concretions were analyzed in cross-sections ferricretes [19]. In this study, the Fe isotopic compositions along profiles running from cores to rims. One of these of 3 tabular concretions were measured (E#15-3, CR#40 samples (B#0) displays homogeneous hematite cementa- and CR#46). Sample E#15-3 is a cm-thick precipitate tion over the whole section (Fig. 4A) while the other (B#20) deposited along a fracture and is associated with a spherical is characterized by four distinct weakly cemented inner concretion (E#15-2) located less than 4 cm away from the parts surrounded by a hard rind (Fig. 3A). In the studied fracture. Sample CR#40 is a massive layer deposit (∼1m- area of GSENM (Fig. 1C), the direction of paleofluid thick) associated with a spherical concretion (CR#38-2). circulation is sometimes highlighted by the presence of “comet trails”. Comet trails are asymmetric structures that 3. Analytical procedure form when a fluid flowing through the sandstone encounters a less-permeable concretion and is deviated Iron concentrations and isotopic compositions were [19]. This results in a front of dissolution/precipitation at the determined after chemical separation by anion exchange contact between the fluid and the concretion, a shadow chromatography using multiple-collector inductively zone on the other side of the concretion, and a streaked coupled plasma mass spectrometry (MC-ICP-MS). hematite-stained sandstone indicating down flow fluid Although the technique has been thoroughly described movement (Fig. 2F). Comet trails observed in GSENM in previous contributions [5,30], details relevant to the point to a direction of fluid flow from N270E to N90E. present work are briefly recalled here. The samples were In the Navajo Sandstone, Fe-oxide deposits also occur powdered and homogenized in an agate mortar. They as massive, cm- to m-scale, tabular concretions (Fig. 2B). were then digested in acid mixtures (including HF, They are either deposited along fractures/faults or form HNO3, HCl, and HClO4) and were purified by anion- layers of hematite-cemented sandstone, referred to as exchange chromatography with AG1-X8 200–400 mesh resin (chloride form) following the protocol of Strelow [31] implemented at the Origins Laboratory of the University of Chicago. Iron concentrations and isotopic compositions were carried out on a Micromass IsoProbe MC-ICP-MS at the Isotope Geochemistry Laboratory of the Field Museum (Chicago). Iron isotopes were measured simultaneously at masses 54, 56, 57 and 58, while the contributions of Cr and Ni on masses 54 and 58 were monitored and corrected for by using ion intensities measured at masses 53 and 60, respectively. The samples were analyzed in 0.5 M HNO3 at a concentration of ∼2 ppm Fe in soft-extraction mode (a mode in which a small positive voltage is applied to the cones, which reduces the transmission of molecular ions). Isobaric interferences from Ar compounds were reduced using a desolvating nebulizer and a hexapole collision cell filled with Ar and H2. Instrumental mass discrimination was corrected for using the conventional sample-standard bracketing approach (e.g., [32–36]). The 56Fe/54Fe and 57Fe/54Fe ratios are expressed using the classical δ notation in per mil (‰) deviation relative to the IRMM- 014 reference material [37],

d56=54 ¼ ð56 =54 Þ =ð56 =54 Þ − Fe t Fe Fe sample Fe Fe IRMM014 1b 1000 d57=54 ¼ ð57 =54 Þ =ð57 =54 Þ − Fig. 4. Cross-section analyses of three growing zones within a Fe t Fe Fe sample Fe Fe IRMM014 1b spherical concretion showing homogeneous Fe-oxide cementation 1000 (sample B#0). (A) Mosaic SEM photograph of the concretion (diam.

∼1.8 cm). (B) Iron isotopic composition vs. Fe2O3 concentration of the three growing zones. The dashed line represents the Fe isotopic For convenience and because different laboratories use composition of the bulk concretion. different Fe isotope ratios, the Fe isotopic composition V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287 277

Table 1 Iron concentrations and isotopic compositions of two standard materials (IF-G and Payun) 56/54 57/54 Standard Analysis Fe2O3tot δ Fe δ Fe FFe (wt.%) (‰)(‰)(‰/amu) IF-G a #1 56.7 0.600±0.109 0.952±0.349 0.303±0.049 #2 63.0 0.648±0.087 0.997±0.177 0.327±0.035 #3 55.1 0.633±0.158 0.888±0.269 0.308±0.059 #4 49.0 0.625±0.045 0.915±0.094 0.310±0.018 #5 53.9 0.645±0.076 0.967±0.123 0.322±0.028 #6 50.9 0.704±0.101 1.155±0.303 0.359±0.045 #7 52.5 0.630±0.121 0.942±0.156 0.315±0.039 Average 54.5±9.1 0.641±0.040 0.974±0.086 0.320±0.016 Reference 55.85±0.22 b 0.631±0.019 c 0.945±0.041 c 0.316±0.010 c Payun d #1 104.2 0.068±0.247 −0.055±0.336 0.005±0.083 #2 107.1 0.011±0.087 0.086±0.176 0.014±0.035 #3 97.4 −0.045±0.208 −0.007±0.318 −0.013±0.074 #4 93.5 −0.002±0.075 0.085±0.149 0.010±0.030 Average 100.6±12.4 0.008±0.086 0.027±0.129 0.004±0.030 Reference 100 e –– 0.039±0.008 f 57 54 56 54 The reported FFe values are the uncertainty-weighted averages of the FFe values calculated for the two pairs Fe/ Fe and Fe/ Fe. Uncertainties are 95% confidence intervals. a IF-G (international standard): 3.8 Ga old banded iron formation from Isua (Greenland). b Mean value from [56]. c Value reported in previous studies (see compilation in [5]). d Payun (“home standard”): hematite from Payun Matru volcano (Argentina), pseudomorph after magnetite. e Fe2O3 content is expected to be 100 wt.% for pure hematite. f Mean FFe value of igneous rocks given here for comparison [5,34].

is also expressed in FFe notation, in per mil per atomic shall only use the FFe notation. The external precision, mass unit deviation (‰/amu) relative to IRMM-014 [5]. estimated through replicate analyses of two standard For any pair of isotopes i and j, it can be written as, materials is better than ±0.04‰/amu (2σ; Table 1). i/j FFe =δ Fe/(i−j)(see[5,38] for the virtues and limita- Whole-rock major and trace element concentrations of tions of this notation). In the following discussion, we 19 concretions were determined by X-ray fluorescence at

Table 2 Iron concentrations and isotopic compositions of red and bleached rocks from the Navajo Sandstone (uncertainties are 95% confidence intervals) 56/54 57/54 a b Sample Coordinates Sample Fe2O3tot δ Fe δ Fe FFe Fe depletion FFe fluid # Latitude (N) Longitude (W) Type (wt.%) (‰)(‰)(‰/amu) (%) (‰/amu) Capitol Reef National Park (CRNP) CR#8-2 37°48.376′ 110°51.779′ Red 0.43 0.111±0.069 0.167±0.147 0.055±0.028 54.6 0.072±0.061 CR#8-1 37°48.376′ 110°51.779′ Bleached 0.20 0.111±0.076 0.159±0.163 0.055±0.031 CR#10-1 37°48.319′ 110°51.770′ Red 0.58 0.098±0.045 0.207±0.094 0.056±0.018 31.0 −0.020±0.103 CR#10-2 37°48.319′ 110°51.770′ Bleached 0.40 0.146±0.075 0.356±0.149 0.090±0.030 CR#25-1 37°52.832′ 111°01.873′ Red 0.45 0.143±0.075 0.231±0.149 0.073±0.030 54.1 0.056±0.058 CR#25-2 37°52.832′ 111°01.873′ Bleached 0.21 0.156±0.075 0.214±0.149 0.076±0.030 CR#5 37°48.239′ 110°58.218′ Red 0.19 0.087±0.098 0.109±0.081 0.038±0.024 ––

Grand Staircase-Escalante National Monument (GSENM) E#3-3 37°40.795′ 111°23.098′ Red 0.31 0.200±0.076 0.308±0.123 0.101±0.028 39.9 0.099±0.071 E#3-2 37°40.795′ 111°23.098′ Bleached 0.19 0.136±0.152 0.213±0.235 0.070±0.055 E#23-1 37°40.671′ 111°22.302′ Red 0.22 0.228±0.083 0.360±0.144 0.117±0.031 49.9 0.269±0.065 E#23-2 37°40.671′ 111°22.302′ Bleached 0.11 0.275±0.082 0.384±0.142 0.134±0.031 E#26-1 37°41.203′ 111°22.179′ Red 0.22 0.313±0.087 0.495±0.072 0.163±0.021 49.7 0.149±0.111 E#26-2 37°41.203′ 111°22.179′ Bleached 0.11 0.097±0.086 0.184±0.071 0.058±0.021 57 54 56 54 The reported FFe values are the uncertainty-weighted averages of the FFe values calculated for the two pairs Fe/ Fe and Fe/ Fe. a Fe depletion: calculated proportion of Fe loss due to bleaching. b FFe fluid: calculated Fe isotopic composition in the fluid for each pair of adjacent red and bleached rocks. See caption in Fig. 5. 278 V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287

the GeoAnalytical Laboratory of Washington State in GSENM. The Fe2O3 tot concentration and FFe value University (Pullman, USA). A detailed procedure, includ- of the red sandstone sampled away from any bleached ing the tabulated precisions and estimated accuracies, is zone (sample CR#5) are 0.19 wt.% and 0.087‰/amu, given in Johnson et al. [39]. respectively. These values are in the lower range but similar to those measured for the red sandstones 4. Results adjacent to bleached zones. The Fe isotopic compositions of red and bleached samples are close to 0‰/amu 4.1. Red and bleached sandstones and are thus very similar to the values measured in Phanerozoic detrital sediments and weathering products, The Fe concentrations and isotopic compositions of which average ∼0.04‰/amu relative to IRMM-014 the different pairs of red and bleached rocks from the (e.g. [4]). As shown in Fig. 5A, the bleached rocks are Navajo Sandstone are reported in Table 2 and shown in systematically depleted in Fe relative to the associated Fig. 5. The red and bleached rocks have Fe2O3 tot con- red rocks. In contrast, the Fe isotopic compositions centrations between 0.22 to 0.58 and 0.11 to 0.40 wt.%, show limited variability from red to bleached rocks respectively. The Fe isotopic compositions of the red and can be regarded as constant within analytical and bleached rocks range from 0.098 to 0.313 and 0.097 uncertainties. to 0.275‰/amu, respectively. The samples collected in CRNP tend to have higher Fe concentrations than those 4.2. Iron concretions

Major and trace element concentrations were measured in 19 spherical concretions (Supplementary online material) and show very limited variations. Silicon and iron always represent more than 91 wt.% of the bulk analysis. The SiO2 and Fe2O3totconcentrations show an inverse correlation (not shown), reflecting the comple- mentary proportions of the two main mineral phases, quartz and iron oxide. The K2OandAl2O3 concentrations

Fig. 5. (A) Fe2O3 tot concentrations and (B) Fe isotopic compositions of six pairs of adjacent red (red diamonds) and bleached (white diamonds) sandstones (CR#8-1 and -2; CR#10-1 and -2; CR#25-1 and -2; E#3-2 and -2; E#23-1 and -2; E#26-1 and -2). (C) Isotopic composition of the primary dissolved Fe, calculated for each pair of adjacent red and bleached rocks. The isotopic composition of Fe dissolved was calculated from a mass balance approach. The quantity

of Fe removed by reducing fluids (Fefluid) is, ¼½ −½ ð Þ Fefluid Fe red Mred Fe blch Mblch 1

where [Fe]red and [Fe]blch are the Fe concentrations in red and bleached rocks. Mred and Mblch are the masses of initial red (before leaching) and final bleached rocks (after leaching). The isotopic mass balance can be written as,.

½ d ¼½ d þ d ð Þ Fe red Mred red Fe blch Mblch blch Fefluid fluid 2

δred, δblch and δfluid represent the Fe isotopic compositions of the red and bleached rocks, and the fluid. By combining Eqs. (1) and (2) and

considering that Mred ≈Mblch, the isotopic composition of Fe removed by reducing fluids is, ð½ d −½ d Þ d ¼ Fe red red Fe blch blch fluid ð½ −½ Þ Fe red Fe blch The calculated isotopic compositions are given in Table 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287 279

Table 3 Iron concentrations and isotopic compositions of spherical and tabular hematite/goethite concretions (uncertainties are 95% confidence intervals) 56/54 57/54 b Sample # Coordinates Sample Fe2O3tot δ Fe δ Fe FFe Fe/Mn Latitude (N) Longitude (W) Type⁎ (wt.%) (‰)(‰)(‰/amu) Capitol Reef National Park (CRNP) CR#21-1S 37°50.182′ 110°59.119′ Spherical (9) 34.2 −0.366±0.079 −0.521±0.265 −0.181±0.036 CR#24-1 37°50.309′ 110°59.312′ Spherical (7) 25.8 −0.868±0.076 −1.249±0.263 −0.431±0.035 CR#28-1 38°17.251′ 111°09.666′ Spherical (50) 8.0 −0.612±0.045 −0.901±0.062 −0.303±0.015 CR#36 38°17.251′ 111°09.666′ Spherical (68) 12.0 −0.500±0.063 −0.780±0.105 −0.255±0.023 CR#29-1 38°17.287′ 111°09.707′ Spherical (9) 0.17 −0.159±0.076 −0.161±0.168 −0.071±0.031 CR#29-5 38°17.287′ 111°09.707′ Spherical (8) 0.20 −0.188±0.065 −0.219±0.224 −0.091±0.030 CR#40 38°12.878′ 111°09.595′ Tabular 4.0 −0.555±0.073 −0.749±0.163 −0.269±0.030 CR#38-2 38°12.878′ 111°09.595′ Spherical (18) 15.6 −0.772±0.078 −1.115±0.264 −0.384±0.036 CR#46 38°12.847′ 111°09.512′ Tabular 9.0 −0.711±0.076 −1.022±0.168 −0.351±0.031

Grand Staircase-Escalante National Monument (GSENM) E#2-4-1 37°41.342′ 111°22.949′ Spherical (10) 21.5 −0.762±0.152 −1.161±0.234 −0.384±0.054 E#4 37°40.771′ 111°23.137′ Spherical (b0.3) 5.3 −0.505±0.088 −0.773±0.232 −0.254±0.038 E#12-1 37°40.913′ 111°22.598′ Spherical (17) 14.6 −0.653±0.145 −0.925±0.188 −0.316±0.047 14.7 −0.759±0.058 −1.105±0.129 −0.376±0.024 14.7 −0.706±0.078 −1.015±0.114 −0.346±0.027 E#12-2 37°40.913′ 111°22.598′ Spherical (34) 19.1 −0.708±0.143 −1.056±0.187 −0.353±0.047 19.1 −0.823±0.067 −1.291±0.160 −0.417±0.028 19.1 −0.766±0.079 −1.174±0.123 −0.385±0.027 E#12-3 37°40.913′ 111°22.598′ Spherical (31) 14.8 −0.785±0.145 −1.122±0.188 −0.382±0.047 17.4 −0.790±0.071 −1.161±0.122 −0.392±0.027 16.1 −0.788±0.081 −1.141±0.112 −0.387±0.027 E#12-4 37°40.913′ 111°22.598′ Spherical (28) 18.6 −0.826±0.152 −1.192±0.234 −0.405±0.054 17.1 −0.864±0.075 −1.263±0.210 −0.429±0.033 17.9 −0.845±0.085 −1.227±0.157 −0.417±0.032 E#12-5 37°40.913′ 111°22.598′ Spherical (40) 19.5 −0.815±0.172 −1.190±0.286 −0.403±0.064 16.4 −0.907±0.075 −1.368±0.210 −0.455±0.033 18.0 −0.861±0.094 −1.279±0.178 −0.429±0.036 E#12-6 37°40.913′ 111°22.598′ Spherical (20) 17.8 −0.785±0.152 −1.240±0.234 −0.403±0.054 14.8 −0.772±0.085 −1.151±0.147 −0.385±0.032 16.3 −0.779±0.087 −1.195±0.138 −0.394±0.032 E#15-2 37°40.885′ 111°22.583′ Spherical (33) 8.7 −0.658±0.058 −0.885±0.139 −0.320±0.025 E#15-3 37°40.885′ 111°22.583′ Tabular 32.6 0.033±0.055 −0.063±0.190 0.011±0.025 31.1 −0.047±0.088 −0.054±0.179 −0.021±0.035 31.9 −0.007±0.052 −0.059±0.130 −0.005±0.022 E#20-1 37°40.578′ 111°22.268′ Spherical (28) 15.5 −0.847±0.078 −1.326±0.128 −0.432±0.029 E#24-1 37°41.267′ 111°22.527′ Spherical (26) 28.9 −0.717±0.083 −1.020±0.267 −0.355±0.038 E#31-1 37°40.854′ 111°22.094′ Spherical (18) 11.4 −1.028±0.116 −1.607±0.200 −0.526±0.044 E#35-1 37°42.052′ 111°22.291′ Spherical (27) 16.7 −0.904±0.114 −1.235±0.197 −0.435±0.043 E#46-1 37°42.620′ 111°21.451′ Spherical (22) 15.7 −1.028±0.064 −1.478±0.107 −0.505±0.024 E#46-2 37°42.620′ 111°21.451′ Spherical (18) 18.0 −1.138±0.121 −1.647±0.156 −0.559±0.039 E#48-1 37°42.950′ 111°23.544′ Spherical (14) 7.1 −0.478±0.045 −0.712±0.061 −0.238±0.015 E#48-2 37°42.950′ 111°23.544′ Spherical (9) 7.1 −0.730±0.121 −1.101±0.156 −0.366±0.039 E#48-3 37°42.950′ 111°23.544′ Spherical (7) 7.1 −0.566±0.058 −0.850±0.151 −0.283±0.025 E#48-4 37°42.950′ 111°23.544′ Spherical (8) 9.4 −0.644±0.121 −0.986±0.156 −0.325±0.039 B#0 –– Spherical (18) 20.6 −1.338±0.247 −2.005±0.336 −0.669±0.083 37.5 18.4 −1.355±0.087 −2.080±0.176 −0.685±0.035 19.5 −1.347±0.131 −2.043±0.190 −0.677±0.045 B#0-S1 –– Cross-section (core) 7.8 −1.251±0.139 −1.841±0.211 −0.620±0.049 B#0-S2 –– Cross-section (int.) 12.9 −1.281±0.172 −1.992±0.285 −0.651±0.064 B#0-S3 –– Cross-section (rim) 22.5 −1.329±0.172 −2.005±0.285 −0.666±0.064 B#2 –– Spherical (19) 21.8 −1.309±0.150 −1.848±0.288 −0.640±0.059 48.7 22.4 −1.229±0.087 −1.842±0.177 −0.614±0.035 22.1 −1.269±0.087 −1.845±0.169 −0.627±0.034 (continued on next page) 280 V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287

Table 3 (continued) 56/54 57/54 b Sample # Coordinates Sample Fe2O3tot δ Fe δ Fe FFe Fe/Mn Latitude (N) Longitude (W) Type⁎ (wt.%) (‰)(‰)(‰/amu) B#4 –– Spherical (23) 17.7 −0.975±0.064 −1.462±0.223 −0.488±0.030 40.8 B#7 –– Spherical (22) 15.4 −0.947±0.045 −1.389±0.094 −0.470±0.018 31.9 16.2 −0.915±0.152 −1.354±0.235 −0.455±0.055 15.8 −0.931±0.079 −1.372±0.127 −0.462±0.029 B#8 –– Spherical (24) 15.4 −1.083±0.055 −1.620±0.190 −0.541±0.025 22.2 B#10 –– Spherical (54) 12.2 −0.786±0.051 −1.062±0.146 −0.385±0.022 41.3 13.0 −0.878±0.087 −1.284±0.072 −0.430±0.021 12.6 −0.832±0.051 −1.173±0.082 −0.408±0.015 B#12 –– Spherical (18) 16.2 −1.225±0.065 −1.846±0.224 −0.613±0.030 77.1 B#20 –– Spherical (43) 17.3 −1.243±0.051 −1.786±0.147 −0.616±0.022 68.4 18.0 −1.039±0.212 −1.421±0.364 −0.500±0.080 19.2 −1.128±0.134 −1.706±0.229 −0.566±0.050 18.6 −1.137±0.085 −1.638±0.151 −0.561±0.032 B#20-S1 –– Cross-section (core) 0.67 −0.354±0.207 −0.420±0.321 −0.159±0.074 B#20-S2 –– Cross-section (int.) 0.19 −0.243±0.386 −0.451±0.441 −0.140±0.117 B#20-S3 –– Cross-section (int.) 0.41 −0.263±0.384 −0.527±0.439 −0.160±0.116 B#20-S4 –– Cross-section (int.) 0.56 −0.339±0.194 −0.456±0.221 −0.158±0.059 B#20-S5 –– Cross-section (rim) 28.1 −1.129±0.235 −1.687±0.360 −0.563±0.084 B#21 –– Spherical (21) 14.9 −1.174±0.045 −1.783±0.094 −0.589±0.018 37.4 15.4 −1.179±0.139 −1.823±0.211 −0.598±0.049 15.2 −1.176±0.073 −1.803±0.116 −0.594±0.026 B#23 –– Spherical (22) 20.9 −0.660±0.126 −0.879±0.286 −0.319±0.053 75.6 18.1 −0.637±0.138 −0.971±0.235 −0.321±0.052 19.5 −0.649±0.094 −0.925±0.185 −0.320±0.037 57 54 56 54 The reported FFe values are the uncertainty-weighted averages of the FFe values calculated for the two pairs Fe/ Fe and Fe/ Fe. ⁎For the spherical concretions, the number into brackets represents the diameter (in mm) of the concretion. Values in italic represent averages of two or three replicate analyses (including sample dissolution). bAtomic ratio calculated from data in Supplementary online material. are positively correlated reflecting the presence of detrital composition and the longitude of the samples (r2 =0.722; K-feldspars inherited from the host-rock. Fig. 7). The FFe values decrease from −0.238 to Spherical iron concretions display large variations of −0.559‰/amu from West to East. In order to assess both Fe concentrations and isotopic compositions (Table 3, whether Fe isotope variations are better correlated with Fig. 6). The Fe2O3totconcentrations and FFe values range another direction, the coordinates of the samples were from 0.17 to 34.2 wt.% and −0.071 to −0.677‰/amu, projected along lines oriented in all possible azimuths. The respectively (averages are 15 wt.% and −0.40‰/amu). best correlation was found for an azimuth of 84° The Fe concentration is not related to the size of the (r2 = 0.742), corresponding to a direction very close to concretion. It reflects the proportion of hematite/goethite West–East. This regional trend is not associated with cement relative to quartz and is mainly controlled by correlations of major and trace element ratios (e.g., Fe/Mn contribution from the Fe-rich outer rind. For instance, in Fig. 8). sample CR#21-1S shows the highest Fe concentration and The results obtained on the two samples analyzed in is indurated all the way through, from the rim to the core. cross-sections are reported in Table 3. Sample B#0 In contrast, samples CR#29-1 and CR#29-5 show the shows an increase of Fe2O3 tot concentration (from 7.8 lowest Fe2O3totconcentrations and have the thinnest rinds to 22.5 wt.%) from core to rim (Fig. 4). Its Fe isotopic (∼0.5 mm thick). These samples are quite unusual and composition is constant within analytical uncertainty their Fe2O3totconcentrations (∼0.2 wt.%) are as low as (∼−0.65‰/amu) but may display a slight decrease from those measured in the bleached rocks. Their FFe values are core to rim. In sample B#20 (Fig. 3), the four weakly however distinctly negative (∼−0.08‰/amu) relative to cemented inner parts have similar FFe values (−0.140 to those of the bleached rocks (∼+0.1‰/amu). No system- −0.160‰/amu) despite having different Fe2O3tot atic difference is seen between samples from CRNP and concentrations (0.19 to 0.67 wt.%). The inner parts of GSENM. In GSENM, where sampling extends over a this sample are very different from the hard hematite/ 2 relatively large area (20 km ; Fig. 1C), spherical iron goethite-rich rind, which has a Fe2O3 tot concentration of concretions display a correlation between the Fe isotopic 28.15 wt.% and an FFe value of −0.563‰/amu. V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287 281

Fig. 8. Fe isotopic compositions of spherical concretions from the GSENM as a function of Fe/Mn ratios (data from Supplementary online material).

concretions, FFe values are −0.269 (CR#40) and −0.351 Fig. 6. Iron isotopic compositions versus Fe2O3 tot concentrations of (CR#46) ‰/amu, and are similar to most spherical red and bleached rocks and spherical and tabular Fe concretions. The concretions (Fig. 6). light-blue area represents the range of isotopic compositions of primary Fe dissolved by reducing fluids (also see Fig. 5). (For interpretation of the references to colour in this figure legend, the 5. Discussion reader is referred to the web version of this article.) Chan et al. [19,20,24] suggested that the source of Fe found in hematite/goethite concretions from Utah is The three tabular concretions have Fe2O3 tot con- dissolution by reducing fluids of primary Fe-oxide centrations and FFe values ranging from 4 to 32.6 wt.% coatings present on quartz grains. Extended bleached and −0.351 to +0.011‰/amu, respectively (Table 3; zones in some areas indicate that Fe was transported Fig. 6). Among them, sample E#15-3 is the only Fe over several kilometres before precipitation. The deposit with a positive FFe value (+0.011‰/amu). This regional distribution of bleaching within the sandstone tabular concretion has an isotopic composition very reveals that buoyant reducing fluids migrated upwards different from that of a spherical concretion sampled through major Laramide faults and were then trapped in b4 cm away (−0.320‰/amu). For the two other tabular large anticlinal folds, including the Circle Cliffs uplift [21]. Although the nature of the reducing fluids cannot be firmly established, several lines of evidence suggest that they were rich in hydrocarbons (e.g. presence of petroleum in southern Utah, tar sands in bleached sandstone [21]). After mobilization of Fe(II) by reducing fluids, mixing with oxygenated groundwater induced the precipitation of iron oxides within the Navajo Sandstone [19,20,22,24]. Several processes could have fractionated Fe isotopes during the formation of Fe concretions in Utah. These include dissolution, adsorption/chromatography, and oxidation/precipitation. Diffusion of Fe may have played a role in the growth of spherical concretions [40,41] and a previous experimental study demonstrated that diffusion of Fe and Zn ions in solution could produce isotopic fractionation [42]. However, this effect Fig. 7. Fe isotopic compositions of spherical concretions from is negligible compare to the variations of ∼−0.7‰/amu GSENM versus the longitude of the sample (in degree). Note that the slope (−6.99±2.06 at 95% confidence interval) is significantly observed in Utah concretions (a maximum fractionation different from 0. It supports the contention that Fe isotope variations of ∼−0.04‰/amu was estimated for growth of a reflect large-scale (km) fluid transport. spherical concretion in a diffusion-limited regime [5]). 282 V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287

Dissolution of Fe-oxides in nature can be controlled iron oxides was found to produce a fractionation of by biotic or abiotic reactions. Experimental studies ∼0.3‰/amu, very similar to the values determined in showed that microbial dissimilatory iron reduction [16]. Adsorption of Fe on the surface of quartz grains may (DIR, a form of respiration which consists in the also have been an important mechanism for the formation reductive dissolution of Fe(III) oxides/hydroxides), is of iron concretions in the Navajo Sandstone. Xu and Axe associated with negative Fe isotope fractionation [44] demonstrated experimentally that the degree of Fe- [6,14,16,43]. For instance Beard et al. [6] showed that oxide coating formed after adsorption or direct precipi- dissolved Fe(II) produced by microbial DIR was tation on silica is mainly controlled by the particle size of fractionated by ∼−0.6‰/amu relative to the ferrihydrite silica, the proportion of Fe-oxide coating increasing as substrate. Abiotic dissolution of iron oxides can proceed silica size decreases. The relatively small size of quartz along three different paths depending on the composi- grains in the Navajo Sandstone (100–600 μm) may have tion of the fluid: proton-promoted, ligand-controlled and facilitated Fe adsorption and subsequent oxidation– reductive dissolution [1]. Experiments of Fe-oxide precipitation reactions. However, no data of Fe isotopic dissolution in diluted HCl solutions showed that fractionation is available for Fe adsorption on quartz proton-promoted dissolution does not induce Fe isotope surface and only adsorption on Fe-oxides will be fractionation [11,18]. For ligand-controlled and reduc- considered in the following discussion. tive dissolution with b1% of the solid dissolved, kinetic Another process that could have affected Fe isotopes isotope effects dominate and tend to enrich the dissolved in Utah concretions is oxidation–precipitation. Indeed, fraction in the light isotopes of Fe, by ∼−1.3‰/amu oxidation of Fe(II)aq is known experimentally to enrich [18]. For ligand-controlled dissolution, when the Fe(III) in the heavy isotopes by ∼0.5 to 1.5‰/amu at fraction of solid dissolved exceeds 1%, the fractionation 25 °C, regardless of whether this oxidation is biologi- is opposite and dissolved Fe is enriched in the heavy cally mediated or not [9,10,12,17,45,46]. The precipita- isotopes by 0.25‰/amu [18]. This may correspond to tion of hematite from Fe(III)aq produces either (i) no equilibrium isotopic fractionation. In contrast to these fractionation if the precipitation is slow, or (ii) a kinetic results, Brantley et al. [7,8] found no significant isotope isotope fractionation, enriching hematite in the light fractionation for ligand-controlled goethite dissolution isotopes of Fe, if the reaction is rapid [11]. Johnson et al. in the presence of siderophore. For reductive dissolu- [43] and Balci et al. [17] found no fractionation at very tion, the fractionation may be negligible after ∼2% of high precipitation rate but this may be due to the fact that, dissolved solid [18]. Another aspect that must be taken in such cases, diffusion is not fast enough to replenish the into account in evaluating whether or not isotopic surface and the isotopic fractionation at the interface is fractionation can be expressed in bulk fluid/solid is the predominantly expressed in a boundary layer in the fact that in many cases, the scale of diffusion in minerals liquid at the surface of the growing solid [5]. The net at low temperature is smaller than the size of the grains isotopic fractionation during iron oxide formation from and at steady state, the flux of dissolved Fe will have the Fe(II)aq can thus be modulated by the rate of precipitation same isotopic composition as the solid source. but, in most cases, it produces a precipitate enriched in In Utah, adsorption of Fe on the surface of host-rock the heavy isotopes of Fe. A direct consequence is that minerals could have been associated with Fe isotope natural occurrences of iron oxide generally display fractionation. Recent experimental studies have demon- slightly negative to significantly positive FFe values strated that adsorption of Fe(II) onto the surface of Fe- depending on the composition of the source of Fe and the oxides enriches the sorbed fraction in the heavy isotopes fraction of Fe precipitated [9,36,47,48]. In contrast, Fe of Fe relative to the fluid [14–16]. Adsorption of Fe(II) on concretions in the Navajo Sandstone have distinctly goethite under anoxic conditions was found to produce a negative FFe values (average=−0.398±0.141‰/amu, fractionation between adsorbed and dissolved Fe of ∼1to 1σ). Only one tabular concretion shows a positive FFe 2‰/amu [14]. However, during experiments of microbial value of +0.011‰/amu (sample E#15-3). Ideally, one DIR, isotope fractionation between Fe(II) adsorbed on would like to build a quantitative transport model that minerals and Fe(II) dissolved in the fluid was estimated to would account for these unexpected isotopic composi- be 0.43±0.08 and 0.19±0.05‰/amu for goethite and tions as well as basic observations such as concretion hematite, respectively [16]. These fractionations were sizes, concentrations and chemical compositions. This is constant over the time of the experiment (280 days), a difficult task because there are far more uncertain suggesting that they represent true isotope equilibrium parameters than there are field observations to constrain fractionations. In an in-situ field experiment of oxidation them. However, as will be shown hereafter, several first of Fe(II)-rich groundwater [15],adsorptionofFe(II)aq on order observations on Fe isotope systematics can be used V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287 283 to constrain the mechanisms responsible for Fe isotope 5.2. Kinetic or equilibrium isotope fractionation during fractionation and refine the scenario of formation of iron precipitation: depth profiles in spherical concretions concretions in the Navajo Sandstone. Rapid precipitation of iron oxide from aqueous Fe(III) 5.1. Comparison between bleached and red sandstones: could potentially induce a kinetic isotope fractionation, no isotopic fractionation during dissolution enriching iron oxides in the light isotopes of Fe [11]. However such a kinetic effect is inconsistent with Fe Comparison of adjacent red and bleached rocks pro- isotope results obtained in concretion cross-sections. The vides a means of estimating (i) the isotopic fractionation concretion with homogeneous Fe-oxide cementation during Fe-oxide dissolution and (ii) the isotopic com- (sample B#0) shows the lowest FFe value analyzed in position of Fe removed by reducing fluids. The bleached this study. This sample is thus potentially the most rocks are systematically depleted in Fe relative to the red strongly affected by kinetic isotope fractionation. If Fe- rocks (Fig. 5A). This result is in good agreement with oxides precipitated with large kinetic isotope fraction- earlier results obtained on other red and bleached rocks ation, this fractionation is not expected to have been from the Navajo Sandstone [22,23]. The proportion of Fe constant during precipitation because the rate of concre- depletion ranges from 31 to 55% (Table 2). Therefore, tion growth is unlikely to have been perfectly constant (for approximately half of the initial Fe was lost from the red instance during diffusion-limited growth, r∝t0.5 and dm/ rocks. With the exception of one pair of samples (E#26-1 dt∝r, [40]). Concretions should therefore show varia- and E#26-2), the Fe isotopic compositions of bleached tions in cross-sections modulated by the rate of rocks are indistinguishable from their red counterparts precipitation. On the contrary, sample B#0 has constant (Fig. 5B). The large depletion in Fe content but near- FFe value from core to rim (Fig. 4), suggesting a slow constancy of Fe isotopic composition indicates that the precipitation rate under conditions close to isotopic dissolution of primary Fe-oxide coatings on quartz equilibrium. In addition, the constancy of FFe value grains was not associated with any isotopic fractionation. indicates that (i) the concretion precipitated from only one This conclusion does not depend on the assumption that fluid through a single episode and (ii) the amount of Fe in Fe was derived locally or was transported over large the fluid was large relative to the amount of Fe distances by fluid migration because all bleached and red precipitated. In sample B#20, the four weakly cemented sandstones have almost identical isotopic compositions, inner parts of the concretion have undistinguishable FFe regardless of the site of sampling. The lack of isotopic values despite having different Fe concentrations (Fig. 3). fractionation, while 50% of Fe was dissolved in the fluid, The Fe concentrations of the inner parts are close to those does not support the involvement of life since microbial of the surrounding sandstone. The low isotopic compo- DIR is associated with significant, negative, Fe isotopic sition measured in these inner parts may reflect mixing fractionation [6,14,16,43]. The results are best explained between the Fe isotopic composition of the fluid (as by abiotic dissolution and are compatible either with sampled by the indurated rind, which has an FFe value of proton-promoted or reductive dissolution [7,8,11,18]. −0.563±0.084‰/amu) and the composition of primary For the paired samples E#26-1 and -2, the bleached rock Fe already present as coating in the host rock (close to is isotopically lighter than the red rock, by ∼0.1‰/amu 0‰/amu). If this were strictly true, one would expect to (Table 2; Fig. 4B). This result can be explained by the see a correlation between the FFe values of the different presence of small (b1 mm), disseminated secondary layers and 1/[Fe]. Such a correlation is not observed for pyrite grains, precipitated from Fe(II). Indeed, pyrite the internal layers and the most likely interpretation is that may be enriched in the light isotopes of Fe relative to the Fe deriving from the fluid was mixed and equilibrated fluid from which it precipitated [47,49]. with Fe present as coating and was then reprecipitated to Using a mass balance approach, the isotopic form the different layers. The profile measured in B#20 composition of Fe removed by reducing fluids can be therefore suggests that dissolution–reprecipitation reac- calculated for each pair of adjacent red and bleached tions occurred, which would be consistent with equilib- sandstones (Table 2, see caption in Fig. 5 for details of rium growth conditions. the mass balance calculation). The calculated FFe values of the primary fluid range from −0.020 to 0.269‰/amu 5.3. Integrated scenario for the formation of iron and average 0.104±0.196‰/amu. These values are very concretions similar to those measured in red and bleached rocks, confirming that dissolution of primary hematite was not The development of a model for the formation of Utah associated with resolvable isotopic fractionation. iron concretions requires considering the processes of 284 V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287

Fig. 9. Cartoon illustrating the formation of Utah Fe concretions. Three major processes are involved: dissolution, adsorption and precipitation. In this model, Fe is dissolved in reducing fluids, presumably rich in hydrocarbons. Transport of Fe through the sandstone is associated with adsorption of Fe

(II)aq on Fe-oxides and precipitation of Fe(II)aq into Fe(III)s from mixing with oxygenated circum-neutral pH groundwater. As a result, the residual fluid evolves towards more negative FFe values. Concretions formed from such fluid are expected to show light FFe values. Fe(II)aq, Fe(II)ad, Fe(III)aq and Fe(III)s represent ferrous iron dissolved and adsorbed on Fe-oxides, and ferric iron dissolved and precipitated as Fe-oxide, respectively. Grey areas around quartz grains represent Fe-oxide coatings. The Δ notation corresponds to Fe isotopic fractionations related to dissolution (this study, [7,8,11,18]), adsorption [14–16] and precipitation [9,10,12,17,45,46]. dissolution, adsorption and oxidation–precipitation (see oxides in the order Fe2+ NCu2+ NZn2+ NNi2+ NMn2+ Fig. 9). As stated above, dissolution of Fe was not [1,50–53]. This means that Fe2+ is more easily adsorbed associated with Fe isotope fractionation and cannot than Mn2+. Accordingly, adsorption of metals ions explain the negative FFe values measured in the during fluid migration is expected to induce correlated concretions. The correlation observed between the Fe variations of Fe isotopic ratios and Fe/Mn ratios. Such a isotopic composition and the longitude of the spherical correlation is not observed (Fig. 8) but other processes, concretions (Fig. 7) suggests that Fe isotope variations such as element fractionation during precipitation of are related to km-scale fluid migration. Two processes Fe-oxides, were likely involved and one cannot rule out that could have operated together (i and ii, described the possibility that chromatography played a role in hereafter) can account for this observation. (i) Succes- producing the very negative FFe values measured in sive precipitation of isotopically heavy Fe-oxides (as Utah concretions. Thus, iron with positive FFe values individual Fe-concretions or dispersed Fe-coatings) must have been precipitated and/or adsorbed prior to the could have induced a decrease in the FFe value of the precipitation of concretions with negative FFe values. remaining fluid (Fig. 9). Subsequent precipitation from In a preliminary study of Utah Fe concretions, this evolved fluid would have led to the formation of Chan et al. [54] measured FFe values between −0.75 concretions characterized by negative FFe values. (ii) and +0.5‰/amu. This range overlaps with the range Migration of Fe in reducing fluids through the sandstone reported here and shows that positive FFe values are may have been associated with adsorption of dissolved indeed present. Fe(II) on host-rock Fe-oxide coatings (Fig. 9). Because Regardless of whether the Fe isotope signature of heavy Fe isotopes are preferentially adsorbed on the Utah concretions is inherited from (i) evolution of the surface of iron oxides [14–16], the FFe value of the composition of the fluid through successive precipitation dissolved fraction would have decreased farther away of Fe-oxides and/or (ii) adsorption of Fe during fluid 54 from the source (i.e. the elution peak of Fe moves faster flow through the Navajo Sandstone, the FFe values of than that of 56Fe). Successive adsorption during fluid concretions precipitated during a same event should transfer acts like chromatography and could potentially show a progressive evolution along the main direction of lead to very negative FFe values. Adsorption of dissolved fluid circulation. The decrease in the Fe isotopic Fe would presumably be accompanied with adsorption composition as a function of longitude (Fig. 7) implies of other dissolved transition metals such as Mn, Cu, Zn that Fe(II)-rich fluids migrated over several (N5) kilo- and Ni. These elements have different affinities for Fe- metres from West to East, since progressive adsorption V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287 285

This scenario is supported by regional variations of FFe values coupled with field observations. This study illustrates for the first time the potential of Fe isotopes for tracing paleofluid circulations and reconstructing hydrocarbon migration in porous media. Quantitative reactive transport modelling, integrating Fe dissolution, adsorption and precipitation, is not developed in this Fig. 10. W–E schematic cross-section showing the position of the Navajo case because many parameters are unknown and cannot Sandstone relative to major structural and topographic features (GSENM be constrained from field observations (e.g. T, pH, eH, and CRNP areas; modified after [55]). The direction of paleofluid flow (blue arrow), inferred from Fe isotope measurements in the GSENM, is composition of the fluid). The study of active natural controlled by the presence of the Circle Cliffs anticlinorium (see text for systems, where both solids and solutions can be sampled details). (For interpretation of the references to colour in this figure and analyzed, are crucial to evaluate the potential of Fe legend, the reader is referred to the web version of this article.) isotopes for tracing paleofluid circulations. and precipitation both tend to lower FFe values in the Acknowledgments residual pool of dissolved Fe. This conclusion is also supported by several field observations. For instance, the Meenakshi Wadhwa, Philip Janney, Frank Richter, comet trails point to a direction of fluid flow from N270E Andrew Davis, and Reika Yokochi are thanked for their to N90E (Fig. 2F), i.e. from West to East. Fluid flow is help and discussions. This study was made possible controlled by rock permeabilities and tectonic structures. through Scientific Research and Collecting Permits Iron-rich fluids were likely buoyant, migrated upwards from the Grand Staircase-Escalante National Monument through faults and were then trapped in anticlinal folds (#UT-05-033-07-G) and the Capitol Reef National Park [21]. One can reasonably assume that these fluids (#CARE-2005-SCI-0009). We would like to address travelled from the external parts of the folds towards special thanks to Thomas O. Clark from the National the inner parts. This scenario agrees well with the present Park Service and Marietta Eaton from the Bureau of data because samples from GSENM were collected on Land Management for their help with this project. This the west flank of the Circle Cliffs uplift, which is oriented work was supported by the National Aeronautics and N–S(Figs. 1 and 10). Tectonic structures are thus Space Administration through grant NNG06GG75G (to compatible with fluid migration from West to East. This ND). Claude Jaupart is acknowledged for handling the suggests that Fe isotopes can be used to trace directions manuscript. We thank Franck Poitrasson and an of paleofluid flows. From the trend observed in Fig. 7 anonymous reviewer for their very careful reviews, and the model proposed in Fig. 9, it is expected that iron which greatly improved the quality of this paper. with positive FFe values was adsorbed and/or precipitat- Correspondence and requests for materials should be ed (as concretions or coatings disseminated within the addressed to [email protected]. sandstone) at the west of the sampling zone in GSENM. Appendix A. Supplementary online material 6. Conclusions Supplementary data associated with this article can Iron isotopes in Utah concretions and associated red be found, in the online version, at doi:10.1016/j. and bleached sandstones are used to gain insights into epsl.2006.11.038. the processes that governed the formation of the concretions, and the scale and direction of fluid References circulations. Comparison between red and bleached sandstones shows that dissolution of Fe-oxides by [1] R.M. Cornell, U. Schwertmann, The Iron Oxides: Structure, Properties, Reactions, Occurrence and Uses, VCH Verlagsge- reducing fluids was not associated with any isotopic sellschaft mbH, Weinheim, 1996. fractionation. Analyses of Fe isotopic compositions [2] R.A. Berner, Goethite stability and the origin of red beds, along depth profiles in two concretions support the view Geochim. Cosmochim. Acta 33 (1969) 267–273. that precipitation was not associated with any kinetic [3] A.D. Anbar, Iron stable isotopes: beyond biosignatures, Earth – isotope fractionation. The negative F values measured Planet. Sci. Lett. 217 (2004) 223 236. Fe [4] B.L. Beard, C.M. Johnson, Fe isotope variations in the modern in the concretions can be explained by evolution of the and ancient Earth and other planetary bodies, in: C.M. Johnson, fluid composition through km-scale flow, associated B.L. Beard, F. Albarède (Eds.), Geochemistry of Non-Traditional with successive precipitation and/or adsorption of iron. Stable Isotopes, Reviews in Mineralogy and Geochemistry, vol. 286 V. Busigny, N. Dauphas / Earth and Planetary Science Letters 254 (2007) 272–287

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Vincent Busigny and Nicolas Dauphas

Supplementary Online Material

Table 1. Major and trace element concentrations of spherical hematite/goethite concretions. Sample # E#2-4-1 E#4 E#12-1 E#12-2 E#12-3 E#12-4 E#12-5 E#12-6 E#15-2 E#20-1

Major (wt%)

SiO2 69.59 84.24 77.54 72.22 72.56 73.80 73.28 74.32 83.32 78.48

TiO2 0.06 0.10 0.04 0.05 0.07 0.09 0.03 0.05 0.07 0.07

Al2O3 2.17 4.05 1.87 2.32 2.48 2.70 1.99 2.29 2.89 2.55

Fe2O3tot 22.12 6.27 15.48 19.06 18.97 17.46 19.06 17.79 9.10 13.77

Mn2O3to 0.41 0.36 0.49 0.96 0.64 0.77 0.87 0.69 0.53 0.15 MgO 0.11 0.21 0.07 0.10 0.10 0.11 0.07 0.09 0.09 0.08 CaO 0.16 0.13 0.26 0.13 0.12 0.10 0.07 0.09 0.08 0.09

Na2O 0.08 0.08 0.04 0.05 0.06 0.06 0.05 0.06 0.05 0.08

K2O 1.05 1.73 0.98 1.22 1.29 1.43 1.11 1.20 1.51 1.41

P2O5 0.16 0.12 0.10 0.10 0.11 0.10 0.07 0.10 0.05 0.07 LOI 3.93 2.56 2.97 3.55 3.39 3.15 3.21 3.12 2.16 3.12 Total 99.84 99.84 99.84 99.76 99.78 99.76 99.80 99.79 99.86 99.88 Trace (ppm) Ni 116.0 41.7 113.4 125.4 111.8 114.5 107.2 124.9 48.5 74.6 Cr 31.5 17.3 6.3 8.6 9.1 11.9 3.9 6.6 8.8 21.8 Sc 17.2 8.8 14.1 9.5 10.8 10.9 8.5 10.7 4.6 10.9 V 112.4 96.5 155.8 153.8 138.0 135.2 98.6 162.4 59.1 73.7 Ba 233.7 550.6 391.0 1008.7 767.6 928.1 815.6 695.8 574.7 279.2 Rb 28.5 46.0 26.5 32.0 32.5 36.3 26.9 31.5 39.8 37.3 Sr 41.2 48.3 59.5 64.1 63.0 73.6 45.4 63.2 75.0 43.8 Zr 81.6 122.2 39.1 50.5 103.7 110.0 26.9 43.4 70.9 97.2 Y 19.8 21.3 6.7 13.8 11.5 14.3 12.6 22.7 11.4 14.9 Nb 5.3 1.6 2.2 1.7 3.4 2.7 2.4 3.3 1.7 6.4 Ga 0.9 5.3 3.3 3.5 2.6 3.2 2.4 2.6 4.3 3.8 Cu 6.3 26.9 36.9 13.0 8.9 15.3 9.4 21.2 10.2 2.3 Zn 550.0 202.8 380.8 452.6 453.6 442.9 453.5 451.0 192.6 298.6 Pb 12.0 30.4 21.7 28.2 26.1 27.7 30.8 29.5 28.0 16.5 La 8.8 11.6 5.4 9.7 8.2 8.8 3.2 12.5 4.4 8.2 Ce 19.4 52.6 17.1 20.3 17.7 18.3 14.8 32.9 17.9 12.4 Th 0.0 13.1 5.4 0.0 0.0 0.0 0.0 0.0 1.7 0.5 Nd 2.1 15.1 3.9 10.6 9.3 9.2 8.4 24.3 10.6 2.0 Table 1. (Continued). Sample # E#24-1 E#31-1 E#35-1 E#46-1 E#46-2 E#48-1 E#48-2 E#48-3 E#48-4

Major (wt%)

SiO2 76.84 77.31 74.93 72.01 72.86 84.59 85.93 86.90 82.45

TiO2 0.05 0.08 0.05 0.07 0.05 0.06 0.07 0.05 0.06

Al2O3 1.97 2.39 2.19 2.45 2.26 2.70 2.40 2.15 2.71

Fe2O3tot 16.39 13.46 17.21 19.63 18.95 8.50 7.84 7.57 10.52

Mn2O3to 0.84 1.89 0.90 0.29 0.58 0.10 0.08 0.07 0.11 MgO 0.06 0.09 0.09 0.11 0.10 0.09 0.07 0.06 0.07 CaO 0.09 0.07 0.08 0.10 0.10 0.11 0.08 0.07 0.07

Na2O 0.05 0.05 0.05 0.06 0.05 0.07 0.04 0.07 0.07

K2O 1.06 1.31 1.23 1.31 1.19 1.40 1.05 0.92 1.37

P2O5 0.08 0.07 0.06 0.13 0.12 0.08 0.07 0.07 0.09 LOI 2.32 2.97 3.00 3.64 3.53 2.19 2.26 1.97 2.35 Total 99.75 99.70 99.79 99.81 99.79 99.89 99.88 99.89 99.87 Trace (ppm) Ni 163.7 71.5 100.6 148.8 143.3 44.5 54.0 53.0 68.3 Cr 13.5 8.8 7.2 12.4 7.1 27.4 22.6 31.8 28.1 Sc 12.2 23.1 11.1 15.4 11.7 13.4 16.7 14.2 22.9 V 97.4 157.4 91.3 198.2 195.9 123.5 114.4 105.0 129.3 Ba 975.7 1305.5 897.3 290.7 524.2 269.9 203.9 191.1 273.0 Rb 26.7 34.0 31.4 32.5 30.6 38.1 28.9 27.3 36.2 Sr 59.8 99.0 54.9 37.5 36.9 35.9 27.2 25.7 35.9 Zr 71.4 96.4 47.4 84.4 57.6 58.2 113.8 79.3 65.1 Y 12.9 13.0 16.0 28.6 35.2 5.8 7.0 15.0 14.7 Nb 2.0 2.0 2.4 2.8 2.0 1.3 2.1 4.1 2.5 Ga 1.7 5.7 4.3 8.7 5.5 4.5 7.3 0.0 8.1 Cu 9.5 37.0 13.9 22.4 26.6 36.8 45.5 47.6 40.5 Zn 556.1 619.7 431.0 595.7 575.2 192.8 225.7 223.7 273.3 Pb 19.7 21.7 13.7 20.4 23.2 22.5 22.6 18.7 17.9 La 9.0 9.1 7.2 4.7 13.4 7.7 3.5 8.7 0.0 Ce 14.6 8.9 17.2 18.7 11.6 12.4 11.6 18.9 27.4 Th 5.9 0.0 0.0 0.0 2.4 21.3 19.9 17.0 10.7 Nd 15.1 7.3 17.8 19.4 20.1 5.4 5.3 0.0 5.1