Research ArticleGEOCHEMISTRY

Fe-oxide concretions formed by interacting carbonate and acidic waters on Earth and Mars

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Science Advances  05 Dec 2018:
Vol. 4, no. 12, eaau0872
DOI: 10.1126/sciadv.aau0872
  • Fig. 1 Similarity of concretions on Earth (Utah and Mongolia) and Mars (Meridiani Planum).

    Occurrences of calcite concretions and Fe-oxide concretions in the Jurassic Navajo Sandstone of Spencer Flat in Grand Staircase–Escalante National Monument (GSENM; 37°40′15″N, 111°23′0″W) and White Cliff (37°12′45″N, 112°23′24″W), Utah (A) and in the Cretaceous Djadohta Formation of the Tugrikiin Shiree area, southern Mongolia (44°13′54″N, 103°16′33″E) (B). These concretions are compared with similar concretions from the Meridiani Planum on Mars (C). In the localities on Earth, calcite concretions are preserved below bleaching fronts (A-1 and B-1; stage 1). Fe-oxide concretions formed during bleaching, and typically calcite is preserved inside (arrow; A-2 and B-2; stage 2), surrounded by a hard Fe-rich crust (A-3 and B-3; stage 3). Hematite spherules on Mars have similar occurrences and morphologies [C-1 and C-2: hematite spherules “blueberries” in Burns formation at Eagle crater, Meridiani Planum (image from the Opportunity sol 123 and sol 42) (3, 25); C-3: spherules with Fe-oxide crust texture “newberries” in Matijevic formation at Endeavour crater, Meridiani Planum (image from the Opportunity sol 3064) (12)]. Scale bars, 1 cm. All photos of Utah and Mongolia are taken by H. Yoshida.

  • Fig. 2 Elemental mapping of different development stages of concretions in Utah and Mongolia.

    SXAM profiles across buried calcite and Fe-oxide concretions from Utah and Mongolia show the different stages (stages 1 to 3) of concretion formation due to interaction between ferric iron mobilized in acidic water and precursor calcite concretions. Stage 1: Spherical calcite concretions formed in eolian sandstone. Stage 2: Start of Fe-rich crust formation by reaction with calcite in Utah and in Mongolia. Stage 3: Almost all the calcite has been dissolved out, and a thick Fe-oxide crust has developed around a “ghost” spherical calcite concretion. An Fe profile across an Fe-rich crust, developed around a calcite concretion, shows a high Fe peak at the reaction front on the concretion surface. This observation indicates that acidic water provided Fe from outside the concretion rather than the Fe being sourced inside the concretion. Scale bars, 1 cm.

  • Fig. 3 Formation process of Fe-oxide concretions on Earth.

    Generalized model for the formation of Fe-oxide concretions by acid water penetration through the eolian sandstone strata and subsequent reaction with spherical calcite concretions. Stage 1: Different sizes and shapes of calcite concretion were formed from calcite-saturated groundwater within buried eolian sandstone deposits during early diagenesis. Stage 2: Episodic acidic water penetration and formation of Fe-oxide concretion by pH buffering by calcite dissolution. The CO2-charged acidic groundwater flowed laterally under the influence of gravity, leading to its localized and heterogeneous bleaching. Stage 3: Variable shapes of Fe-oxide concretions that reflect the shapes of the precursor calcite concretions until the calcite has been completely dissolved. Subsequently, some of the Fe-oxide concretions could be dissolved by later acidic water penetration.

  • Fig. 4 Schematic illustration of Fe-oxide concretion formation on Earth and Mars.

    Fe-oxide concretion formation from spherical calcite concretion precursors during pH-buffering reactions that are interpreted to have occurred across the bleaching fronts observed in Utah and Mongolia and across an assumed bleaching front on Mars. The spatial distributions of Fe-oxide concretions at different stages of formation (e.g., rinds) and dissolution are considered to reflect the penetration of Fe-rich acidic waters. Acidic waters dissolved hematite in the reddish eolian sandstone to mobilize Fe, thereby bleaching the reddish-colored sandstone. When the acidic water encountered the calcite concretions, the calcite concretions were dissolved. At the same time, mobilized Fe precipitated on the preexisting calcite concretion surface due to the increased pH, thereby producing outer rinds consisting of Fe oxyhydroxide. When Fe-oxyhydroxide–encrusted calcite concretions were exposed, any remaining calcite dissolved. In the uppermost part of the succession, Fe-oxide concretions dissolved completely. Right column shows a model for the distributions of hematite spherules and expected relict carbonate spherules in underground strata at Meridiani Planum, Mars.

  • Table 1 Comparison of previous models (① and ②) and our proposed model (③).
    Formation mechanismPrecusorReaction fluid/liquidFe-oxide mineralizationFe
    state
    ① Chan et al.
    model
    Goethite precipitation by redox-oxic
    reaction
    NoneBuoyant, reducing fluid due to
    hydrocarbon
    Precipitation of goethite (not clear for crust
    formation)
    Fe2+
    ② Loope et al.
    model
    Siderite formation and microbial
    oxidation
    Siderite
    (not found)
    Reducing groundwater with methane
    dissolved
    Internal Fe source (siderite)Fe2+
    ③ Our proposed
    model
    Calcite-acid reaction by pH
    buffering
    Calcite
    (found)
    Penetration of CO2-charged
    acidic groundwater
    Inward precipitation of goethite by Fe-rich
    acidic water from outside
    Fe3+
  • Table 2 Carbon contents, δ13CVPDB, δ18OVPDB (CF-IRMS), and CaCO3 concentrations (XRF and estimated volume) of calcite concretions (stages 1 and 2) and host rock sandstone from Utah and Mongolia.

    VPDB, Vienna Pee Dee Belemnite; CF-IRMS, continuous flow isotope ratio mass spectrometry.

    Location and samplesC (wt %)CaCO3 (wt %)δ13CVPDB (‰)δ18OVPDB (‰)
    Utah (USA)Ca concretion (Escalante)3.46–4.4815.0–15.7−5.15 to −3.13−10.1 to −5.06
    Host rock sandstone0.01–0.510.13–0.14−5.96 to −3.36−10.3 to −8.56
    Ca concretion (White Cliff)1.80–2.6315.0–21.9*−4.63 to −4.21−12.0 to −11.5
    Host rock sandstone0.01<0.03–0.08*−7.44 to −6.81−12.9 to −2.02
    Gobi (Mongolia)Ca concretion3.85–6.2918.8–24.4−3.50 to −1.65−11.5 to −10.1
    Host rock sandstone0.01–1.120.01*–0.56−8.21 to −3.79−11.8 to −11.2

    *Estimated volume.

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/12/eaau0872/DC1

      Section S1. Geological setting

      Section S2. Mineralogical and geochemical analysis of concretions

      Section S3. Experimental formation of Fe-oxide concretions

      Section S4. Numerical simulation to estimate formation time scales

      Fig. S1. Landsat image, geologic map, CO2 reservoir and volcanic rock distribution, and conceptual model of CO2-charged groundwater in Utah.

      Fig. S2. Outcrop photographs, morphologies and microscopic textures of Fe-oxide, and calcite concretions in Utah.

      Fig. S3. Location map, outcrop photographs, morphologies and microscopic textures of Fe-oxide, and calcite concretions in Mongolia.

      Fig. S4. XRD pattern of concretions and Fe-oxide crusts.

      Fig. S5. Elemental distributions in and around the Fe-oxide concretions (stage 2) in Mongolia.

      Fig. S6. Elemental distributions in and around the Fe-oxide concretions (stages 2 and 3) in Utah.

      Fig. S7. Experimental study of Fe-oxide concretion formation.

    • Supplementary Materials

      This PDF file includes:

      • Section S1. Geological setting
      • Section S2. Mineralogical and geochemical analysis of concretions
      • Section S3. Experimental formation of Fe-oxide concretions
      • Section S4. Numerical simulation to estimate formation time scales
      • Fig. S1. Landsat image, geologic map, CO2 reservoir and volcanic rock distribution, and conceptual model of CO2-charged groundwater in Utah.
      • Fig. S2. Outcrop photographs, morphologies and microscopic textures of Fe-oxide, and calcite concretions in Utah.
      • Fig. S3. Location map, outcrop photographs, morphologies and microscopic textures of Fe-oxide, and calcite concretions in Mongolia.
      • Fig. S4. XRD pattern of concretions and Fe-oxide crusts.
      • Fig. S5. Elemental distributions in and around the Fe-oxide concretions (stage 2) in Mongolia.
      • Fig. S6. Elemental distributions in and around the Fe-oxide concretions (stages 2 and 3) in Utah.
      • Fig. S7. Experimental study of Fe-oxide concretion formation.

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