Numerical simulation of iron oxide concretions on Earth and Mars through calcite dissolution

Abstract

Iron oxide concretions are found in sedimentary rocks on both Earth and Mars. On Earth, concretions are common in eolian formations, such as the Jurassic Navajo Sandstone in Utah, USA and those found in the Cretaceous Djadokhta Formation, Gobi Desert, Mongolia. Although it is known that the formation conditions of the iron oxide concretions were affected by the paleoclimate of these regions, quantitative modeling of such formations still requires development, especially concerning initial and diagenetic conditions. A 1-D diffusion–reaction simulation was conducted by assuming that a calcite concretion was initially located in a homogeneous layer of sandgrains. Favorable conditions for the formation of iron oxide concretions have been found to be 4.5⩽pH⩽6, and 10-7⩽[Fe2+]fO2⩽10-5, where [Fe2+] and fO2 are the concentration of ferrous Fe2+ ions and dissolved oxygen relative to the atmospheric value, respectively. An iron-rinded concretion from ferric Fe3+ ions is not possible. For the case of Fe2+ ions, the flow speed of the groundwater should be faster than 2×10-5mms-1. The formation timescale is determined by the diffusion flux of the hydrogen ion, and varies between 2.7×102 and 1.5×104 years for a calcite concretion with an initial radius of 15 mm. Formation conditions of iron-rinded concretions on Earth and Mars are discussed.