Highly Siderophile Elements and Coupled Fe-Os Isotope Signatures in the Temagami Iron Formation, Canada: Possible Signatures of Neoarchean Seawater Chemistry and Earth's Oxygenation History.

Abstract

Banded iron formations (BIFs) were deposited before and concurrent with the Great Oxidation Event at ∼2.33 Ga. They provide useful archives that document the transformation of the Precambrian hydrosphere from anoxic to progressively oxygenated conditions. Their formation involves removal of oceanic Fe by either inorganic or biologically promoted Fe2+ oxidation, or both. To evaluate depositional settings, elemental sources that affect seawater chemistry, and oxidation pathways, we present the first combined highly siderophile element (HSE) and Fe-Os isotope study for the ∼2.7 Ga Temagami BIF, Abitibi Greenstone Belt, Ontario (Canada). HSE abundances and 187Os/188Os ratios show no systematic variation between alternating magnetite and (meta)chert bands of the Temagami BIF. Whereas HSE concentrations mostly resemble modern crustal values, present-day 187Os/188Os ratios range from ∼0.17 to ∼10.8. Magnetite samples define a regression line corresponding to an age of 2661 ± 126 Ma. A chondrite-like 187Os/188Os initial value is in agreement with earlier studies on Neoarchean marine sediments and is thought to reflect seawater composition, which, unlike modern oceans, is dominated by mantle-like 187Os inventory most likely derived from deep-sea hydrothermal vents. Our δ56Fe data vary from about +0.6‰ to +0.9‰ and define a sawtooth-like pattern between alternating magnetite and (meta)chert layers. Partial oxidation of hydrothermally sourced Fe(II) and a lack of microbially mediated dissimilatory iron reduction provide the most plausible explanation for the positive δ56Fe values. Notably, our δ56Fe data for Temagami are in accord with trends defined by literature results for other Algoma-type BIFs that were deposited throughout the Archean.