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Iron isotopes constrain the pathways and formation mechanisms of terrestrial oxide concretions: A tool for tracing iron cycling on Mars?

¹ Department of Geology & Geophysics, University of Utah, 719 WBB, 135 S. 1460 E., Salt Lake City, Utah 84112, USA
² Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., Madison, Wisconsin 53706, USA
³ Department of Geology & Geophysics, University of Utah, 719 WBB, 135 S. 1460 E., Salt Lake City, Utah 84112, USA

View larger version (149K): [in this window] [in a new window]   Figure 1. Example Navajo Sandstone samples analyzed with field context pictures. Whole-rock 56Fe values were measured from (A) red, original sandstones (locality 4, Fig. 2), and (B) white, bleached sandstones (locality 9, Fig. 2). (C–E) Row of terrestrial concretion images (concretion diameters typically 1 cm) shows in situ and weathered distributions as well as geometries (e.g., joined forms) and structures. (F–H) Row of Mars images shows hematite blueberries (averaging <0.5 cm in diameter, Squyres et al., 2004), and are comparable to terrestrial concretions. Mars images courtesy of NASA/JPL/Cornell.

View larger version (33K): [in this window] [in a new window]   Figure 2. Localities of Jurassic sampling for Fe isotope analyses: 1—Snow Canyon, 2—Sand Hollow, 3—Zion National Park, 4—Cedar City, 5–10—Grand Staircase Escalante National Monument, 11–13—Lake Powell area, and 14–18—Moab area. All samples are from the Lower Jurassic Navajo Sandstone (Ss) except for localities 12 and 17, which are from the Middle Jurassic Entrada Sandstone. Generalized stratigraphic column is at left (modified after Chan et al., 2005).

View larger version (27K): [in this window] [in a new window]   Figure 3. Summary of measured 56Fe values from different Jurassic sandstone samples relative to sample locality (Fig. 2). Whole-rock samples are shown in blue squares. Concretion samples are shown in red circles. I—inner portion of concretion sample, R—rim of concretion sample. Also shown is the range in 56Fe values for low-C and low-S clastic sedimentary rocks (light-green band) that reflects Fe(III)-rich weathering products (Beard et al., 2003b; Yamaguchi et al., 2005), which should represent the range in 56Fe values of the early diagenetic Fe(III) oxides in the Jurassic sandstones. Vertical dashed line is the average of igneous rocks (Beard et al., 2003a).

View larger version (21K): [in this window] [in a new window]   Figure 4. Comparison of <sup>56</sup> Fe values produced for oxides during oxidation of Fe(II)aq (A) and histograms for 56Fe values measured for oxide concretions (B), whole-rock sandstones (C), and reactive Fe(III) oxides and Fe(II)-bearing pore waters from modern marine sediments (D). In A, 56 Fe values were calculated using a Rayleigh model as a function of percent oxidation of Fe(II)aq for the reaction Fe(II)aq Fe(III)aq Fe(III) oxide; once oxidized, 100% of the Fe(III)aq is assumed to precipitate as oxides, which is appropriate for the circum-neutral pH conditions in which the concretions formed. Upper curves (warm colors) illustrate Fe isotope compositions produced for Fe(II)aq that had initial 56Fe = 0.0; lower curves (cool colors) were calculated for an initial 56 Fe value for Fe(II)aq of –2.0. Data for B and C are from Table DR1 (see text footnote 1). Data for D are from modern marine sediments from the California margin (Severmann et al., 2006); 56 Fe values for reactive Fe(III) oxides were estimated from acid extractions that produced Fe(III)/Fetotal > 0.3. Porewater compositions are shown for suboxic, low-S samples where dissimilatory Fe(III)-reducing bacteria (DIRB) play a major role in Fe cycling. Thick gray line at 56Fe = 0 notes average of igneous rocks and low-C, low-S clastic sedimentary rocks and weathering products. Thin gray lines were added for scale reference across the figures.

View larger version (39K): [in this window] [in a new window]   Figure 5. Conceptual grain-scale model of isotopic analyses for stages of redox reactions for Fe cycling in terrestrial examples of Navajo Sandstone concretion formation. (A) Early hematite (Fe3+) grain coatings in original red sandstone, where initial 56Fe = 0.0. (B) Influx of reducing fluids that "bleach" the buried sandstone during reduction of Fe3+ to Fe2+, where the Fe(II)-rich fluids have 56Fe values between –0.5 and –1.5, probably reflecting bacterial Fe reduction. (C) Bleached sandstone pores are saturated with waters containing reduced iron (Fe2+). (D) Influx of oxidizing groundwater creates redox front where concretions precipitate. (E) Concretions form along a reaction front with organized distribution and spherical shape, producing a range of 56Fe values that are generally negative, reflecting complete oxidation and precipitation of low 56 Fe Fe(II)aq.