2023 Volume 129 Issue 1 Pages 199-221
Spherical Fe-oxide concretions have been recognized in both terrestrial and Martian strata and are thought to record a variety of past environmental and alteration conditions. This paper presents a comprehensive review of the occurrence and elemental composition of spherical Fe-oxide concretions on Earth and Mars and their proposed mechanisms of formation. On the basis of geological evidence from Utah and Mongolia, Fe-oxide concretions are considered to form by pH-buffering reactions between Fe-rich acidic water and precursor calcite concretions. By comparing the characteristics of hematite spherules in Meridiani Planum and spherical nodules in Gale Crater, we propose that Martian concretions may also have been formed by interaction between pre-existing carbonates and sulfuric acidic water that infiltrated the rocks early in Martian history. The abundant hematite spherules in Meridiani Planum and spherical nodules in Gale Crater can be considered as relicts of the widespread deposition of carbonate that occurred during the late Noachian-early Hesperian (c. 3.8 to 3.7 Ga) and its dissolution during the late Hesperian (c. 3.5 to 3.2 Ga).