High-resolution environmental record of bander iron formations through rock magnetic, crystallographic and geochemical properties

Abstract

Banded Iron Formations (BIFs) are chemical archives of Precambrian seawater chemistry and post-depositional iron cycling. Given that BIF accumulated on the seafloor for over two billion years of Earth's early history and that their deposition is coeval with the progressive oxygenation of the atmosphere and hydrosphere, changes in their chemical, mineralogical, magnetic properties and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving Earth. The Late Archean-Early Proterozoic Carajás and Late Proterozoic Urucum BIFs in North and Midwestern Brazil, respectively, are remarkable occurrences of these deposits. However, several key questions regarding their formation remain unsolved. No commonly accepted depositional model for the Fe (and Mn, in the case of Urucum) in these deposits exists and their depositional age is loosely constrained. Whether the glaciations that are commonly associated with the Urucum BIFs were important for their genesis and at which paleolatitude they formed are also not clear. In addition, the absence of glacial diamictites in the Carajás Supergroup is at odd with its stratigraphic equivalent in Australia, South Africa and North America, which all contain 2 to 4 glacial horizons, possibly of global extend (Snowball Earth). To better understand the formation of these deposits we aim at developing an integrated approach, which comprises mineralogical, crystallographic, magnetic and geochemical analyses. Geochemical analyses, including imaging and in-situ trace element analyses coupled with high-resolution crystallographic analyses of Fe-bearing phases will provide constraints on the environmental conditions of deposition of these BIFs. Together with C, S and Fe isotope analyses of organic matter, sulfides and Fe-oxides performed by other members of the FAPESP SPEC project 2015/16235-2, the results obtained will allow constraining the role of micro-organisms in the formation of these BIFs. Rock magnetic methods will provide information on the iron ores which are the main carriers of magnetization and principal components of BIFs. Recent advances in magnetic microscopy enable small-scale magnetic studies, which, when coupled with in-situ isotopic analysis, can provide a complex and detailed picture of the magnetic and geochemical properties of a sample, and better constrain their paleomagnetic signal and biological origin. The work will be conducted at the IAG/USP, Brazil, in close collaboration with the IPG Paris and Géoscience Montpellier, France, where a one-year stay is planned.