[M-IS20] Aqua planetology

[MIS20-01] Origin of the size distribution of spherical iron concretions

*Sin-iti Sirono¹, Misaki Tamura¹ (1.Department of Earth and Planetary Sciences, Nagoya University)

Keywords:spherical iron concretion, calcium carbonate, spatial distribution , size distribution

A spherical concretion with a surface covered with iron oxide is discovered in Grand Staircase-Escalante National Monument in Utah, USA. Similar concretions were also found in the Gobi Desert of Mongolia. Yoshida et al. (2018: Sci. Adv. 4, eaau0872) proposed a model in which spherical calcium carbonate concretions were first formed as a precursor, and then acidic groundwater containing iron ions flowed to the precursor to form spherical iron concretions. Based on this model, the spatial distribution and size distribution of spherical iron concretions observed today are determined during the formation of calcium carbonate concretions. However, the formation process of calcium carbonate concretions is rarely discussed in Yoshida et al.(2018) Therefore, we attempted to constrain the formation environment based on the spatial distribution and size distribution of spherical iron concretions. By analyzing images taken in the field, we obtained the size distribution and nearest neighbor distance distribution of spherical iron concretions. Based on these data, we developed a formation scenario for spherical calcium carbonate concretions. Based on the nucleation rate of calcium carbonate, it is unlikely that calcium carbonate concretions of several centimeters in diameter were formed directly. The size of the first precipitates formed as a result of calcium carbonate supersaturation would have been much smaller and more numerous than those found today. On the other hand, the spacing between iron concretions is several times larger than the diameter of the iron concretions. If precipitation of calcium carbonate were the only one-way process, the narrow intervals of the initially formed precipitates would be quickly filled, and all the precipitates would be connected. Precipitation alone cannot explain the wide intervals, and dissolution is also necessary. These results indicate that spherical calcium carbonate concretions were formed by repeated dissolution and precipitation of calcium carbonate. Numerical calculations of the model of repeated dissolution and precipitation were performed to determine how the mean size and standard deviation of spherical calcium carbonate concretions evolve. The results show that as the mean size increases, the standard deviation also increases, which well explains the data observed in nature. It was also found that the pH must be less than 5 to form concretions of a few centimeters in diameter. These results suggest that simply obtaining images of the outcrop where spherical concretions are found can be expected to provide certain constraints on the environment in which they form.

Figure caption: Relationship between standard deviation of concretion diameter and mean diameter. Large: Diameter greater than 2 cm. Small: Diameter about 1 mm. Potter large and Potter small are from Potter and Chan (2011: Geofluids 111, 184).