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Iron isotopes constrain the pathways and formation mechanisms of terrestrial oxide concretions: A tool for tracing iron cycling on Mars?

¹ Department of Geology & Geophysics, University of Utah, 719 WBB, 135 S. 1460 E., Salt Lake City, Utah 84112, USA
² Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., Madison, Wisconsin 53706, USA
³ Department of Geology & Geophysics, University of Utah, 719 WBB, 135 S. 1460 E., Salt Lake City, Utah 84112, USA

New iron isotope data document open-system formation of terrestrial iron oxide concretions and the potentially important role of iron-reducing bacteria in mobilizing iron. These terrestrial insights can provide valuable models for understanding extraterrestrial hematite spherules and their diagenetic history at Meridiani Planum, Mars. Whole-rock samples of Jurassic Navajo Sandstone host rock have ⁵⁶Fe values near 0 per mil (), whereas concretions typically have negative ⁵⁶Fe values. Negative ⁵⁶Fe values can be explained by complete oxidation and precipitation from aqueous fluids that had ⁵⁶Fe values of –0.5 to –1.5. The low ⁵⁶Fe values for the majority of concretions overlap those of Fe(II)_aq and reactive ferric oxides in modern marine sediments where iron-reducing bacteria are actively cycling Fe, suggesting that Fe mobilization in the Navajo Sandstone occurred through bacterial reduction of Fe oxides. Variations in ⁵⁶Fe values support an open-system model of concretion formation where Fe is recycled via different chemical reactions involving reduction, mobilization, and precipitation. If the Mars concretions formed in a similarly open system during Fe mobilization and precipitation, their ⁵⁶Fe values should also deviate from ⁵⁶Fe = 0, dependent upon the pathway, but positive ⁵⁶Fe values would be expected for oxides in the absence of a role for microbial redox cycling.

Keywords: iron isotopes • concretion • hematite • diagenesis • Mars

This article has been cited by other articles:

				W. T. Parry, M. A. Chan, and B. P. Nash Diagenetic characteristics of the Jurassic Navajo Sandstone in the Covenant oil field, central Utah thrust belt AAPG Bulletin, August 1, 2009; 93(8): 1039 - 1061. [Abstract] [Full Text] [PDF]