[PPS09-16] Numerical Simulation of Iron Oxide Concretion Formation On Earth and Mars
Keywords:Iron Oxide Concretion, Blueberry, Mars
Introduction: Iron oxide concretions are found on Earth and Mars in sedimentary rocks. On Earth, in particular, concretions are commonly observed in eolian formations, such as Jurassic Navajo Sandstone, Utah, USA [1,2], and those found in the Gobi desert, Mongolia [3]. Although it is known that the formation conditions of the iron oxide concretions are affected by the paleoclimate of these regions, quantitative modeling providing thickness of an iron rim still requires further development. The key property or quantifying factor of an iron oxide concretion is the ratio between the thickness of the rim and the radius of the concretion. The comparison between the measured ratios of natural samples and the simulation results enables us to constrain the groundwater conditions when iron oxide concretions were formed.
Outline of the model: Inside a layer of a sandstone, a spherical region filled with calcite is formed by precipitation. The calcite is surrounded by groundwater containing Fe3+ and H+. Dissolution of calcite decreases [H+], inducing precipitation of Fe(OH)3 around the surface of the calcite sphere and a rim of Fe(OH)3 is formed. Both H+ and Fe3+ ions are transported from the outer region through diffusion.
Drying of Fe(OH)3 rim. In natural samples, iron oxide fills almost totally the pore spaces between sandgrains inside the iron rim. The remaining porosity is less than 10%. It should be noted that Fe(OH)3 precipitates as colloidal particles and cannot fill the pore spaces between sandgrains without drying. It has been well known that a colloidal particle migrate with the water-air interface as drying proceeds [4]. Drying proceeds from outside when the water surface is lower than the precipitation after draining. The high volume fraction of Fe(OH)3 around the calcite sphere delays the drying compared to the surrounding region. When drying proceeds from outside of the precipitation, the water surface moves toward the calcite sphere. As a result, Fe(OH)3 colloidal particles migrate to the surface of the calcite sphere.
Numerical results: At the beginning of the simulation, precipitation occurs outside the calcite sphere. Then precipitation of Fe(OH)3 proceeds inward as the calcite sphere dissolves. The volume fraction of Fe(OH)3 occupying pore space is at most 0.05, which cannot explain the typical volume fraction of iron oxide inside pore space 0.8-0.9 [3]. The dissolution of calcite sphere completes in 1.72 yr. If we assume that the migration of Fe(OH)3 proceeds completely through the drying, the thickness of the rim can be estimated as a function of time. Figure 1(left) displays the evolution of the concretion rim/radius ratio as a function of the normalized time.
From Fig. 1(left), the time when the dissolution is finished can be constrained from the natural samples. If the concentration of iron ion [Fe3+] is high, the time required to attain the observed ratio is short. If the iron concentration is low, the suitable time range for natural samples is limited only around t/t0=1, indicating that the probability is low and difficult to explain. Therefore, the iron concentration should be substantially high such that [Fe3+]0 > 10-2 mol L-1.
From the range of t/t0, the original size of the calcite sphere can be estimated from Fig.1(right). The gray regions are the estimated size range in which the natural samples are consistently explained (Fig. 1 left). It can be seen that the original size of the calcite sphere is larger than twice of that of the observed natural samples of concretions if the concentration of iron ion is low.
In the outcrops, the distance between iron oxide concretion is longer than ten concretion radii [1]. The distance between concretions should be longer than the ratio determined from Fig. 1(right), otherwise the initial calcite spheres overlap. Therefore, the iron concretions found in Utah can be formed through dissolution of calcite sphere followed by precipitation of Fe(OH)3 if the concentration of iron ion is higher than 10-2 mol L-1, taking account of the duration of the dissolution (Fig. 1 left).
Figure 1
Left: The ratio between the thickness of the rim and concretion radius as a function of the normalized time. Circles are the simulation results. Right: The ratio between the concretion radius and the initial radius of the calcite sphere as a function of the normalized time.
References: [1] Chan, M. A. et al. (2004) Nature, 429, 731–734. [2] Loope D. B. et al. (2010) Geology, 38, 999-1002. [3] Yoshida et al. (2018) Sci. Adv.4: eearu0872, [4] DeNovio et al. (2004) Vadose Zone J., 3 338–351. [5] Squyres et al. (2004) Science, 306 1698–1703.
Outline of the model: Inside a layer of a sandstone, a spherical region filled with calcite is formed by precipitation. The calcite is surrounded by groundwater containing Fe3+ and H+. Dissolution of calcite decreases [H+], inducing precipitation of Fe(OH)3 around the surface of the calcite sphere and a rim of Fe(OH)3 is formed. Both H+ and Fe3+ ions are transported from the outer region through diffusion.
Drying of Fe(OH)3 rim. In natural samples, iron oxide fills almost totally the pore spaces between sandgrains inside the iron rim. The remaining porosity is less than 10%. It should be noted that Fe(OH)3 precipitates as colloidal particles and cannot fill the pore spaces between sandgrains without drying. It has been well known that a colloidal particle migrate with the water-air interface as drying proceeds [4]. Drying proceeds from outside when the water surface is lower than the precipitation after draining. The high volume fraction of Fe(OH)3 around the calcite sphere delays the drying compared to the surrounding region. When drying proceeds from outside of the precipitation, the water surface moves toward the calcite sphere. As a result, Fe(OH)3 colloidal particles migrate to the surface of the calcite sphere.
Numerical results: At the beginning of the simulation, precipitation occurs outside the calcite sphere. Then precipitation of Fe(OH)3 proceeds inward as the calcite sphere dissolves. The volume fraction of Fe(OH)3 occupying pore space is at most 0.05, which cannot explain the typical volume fraction of iron oxide inside pore space 0.8-0.9 [3]. The dissolution of calcite sphere completes in 1.72 yr. If we assume that the migration of Fe(OH)3 proceeds completely through the drying, the thickness of the rim can be estimated as a function of time. Figure 1(left) displays the evolution of the concretion rim/radius ratio as a function of the normalized time.
From Fig. 1(left), the time when the dissolution is finished can be constrained from the natural samples. If the concentration of iron ion [Fe3+] is high, the time required to attain the observed ratio is short. If the iron concentration is low, the suitable time range for natural samples is limited only around t/t0=1, indicating that the probability is low and difficult to explain. Therefore, the iron concentration should be substantially high such that [Fe3+]0 > 10-2 mol L-1.
From the range of t/t0, the original size of the calcite sphere can be estimated from Fig.1(right). The gray regions are the estimated size range in which the natural samples are consistently explained (Fig. 1 left). It can be seen that the original size of the calcite sphere is larger than twice of that of the observed natural samples of concretions if the concentration of iron ion is low.
In the outcrops, the distance between iron oxide concretion is longer than ten concretion radii [1]. The distance between concretions should be longer than the ratio determined from Fig. 1(right), otherwise the initial calcite spheres overlap. Therefore, the iron concretions found in Utah can be formed through dissolution of calcite sphere followed by precipitation of Fe(OH)3 if the concentration of iron ion is higher than 10-2 mol L-1, taking account of the duration of the dissolution (Fig. 1 left).
Figure 1
Left: The ratio between the thickness of the rim and concretion radius as a function of the normalized time. Circles are the simulation results. Right: The ratio between the concretion radius and the initial radius of the calcite sphere as a function of the normalized time.
References: [1] Chan, M. A. et al. (2004) Nature, 429, 731–734. [2] Loope D. B. et al. (2010) Geology, 38, 999-1002. [3] Yoshida et al. (2018) Sci. Adv.4: eearu0872, [4] DeNovio et al. (2004) Vadose Zone J., 3 338–351. [5] Squyres et al. (2004) Science, 306 1698–1703.