Hematitic concretions at Meridiani Planum, Mars: Their growth timescale and possible relationship with iron sulfates

Using diffusion-based models for concretion growth, we calculate growth times of hematitic concretions that have been found in the Burns formation at Meridiani Planum, Mars, by NASA's Opportunity Mars Exploration Rover. Growth times of ~ 350–1900 terrestrial years are obtained for the observed size range of the concretions over a range of parameters representing likely diagenetic conditions and allowing for an iron source from diagenetic redistribution. This time scale is consistent with radiometric age constraints for the growth time of iron oxide concretions in sandy sediments of the acid-saline Lake Brown in Western Australia (< 3000 yr) reported elsewhere. We consider the source of the iron for Meridiani concretions by calculating the constraints on the supply of Fe3+ to growing concretions from the dissolution and oxidation rates of iron minerals on early Mars. Mass balance arguments suggest that acid dissolution of jarosite ((H3O,K)(Fe3+3(OH)6(SO4)2) and minor ferric sulfates is probably the most plausible dominant contributor to Fe3+ in the concretions. Ferrous iron released from melanterite (Fe2+SO4·7H2O) that is subsequently oxidized could also have been an important iron source if melanterite existed prior to diagenesis. Our conclusion that the iron is sourced from iron sulfates may explain the global observation from orbiters that grey crystalline hematite occurs in association with sulfate deposits.