Iron Mineralization and Trace Metal Uptake during Oxidative Precipitation: Insights into Paleo-Proxy partitioning during Banded Iron Formation (BIF) Deposition and Coupled Redox Reactions

Abstract:

BIFs are chemical sediments common in Precambrian marine settings. Traditionally the precursor phases to BIFs were thought to be ferric hydroxides precipitated from Fe2+aq in the overlying marine basin, thus the trace metal (TM) composition of the ocean body during BIF deposition has been calculated by partition coefficients (PCs) and speciation into these phases. However previous determination of TM PCs and speciation have analyzed variables with limited effects on iron oxidation pathways. Furthermore, new experiments and modeling of Precambrian seawater suggest the primary precursor of some BIFs may have been metastable partially oxidized green rust (GR), the mineralogical characteristics of which are determined by anion availability. In light of the need of further investigation of TM behavior during BIF precursor formation, we conducted closed system experiments with variable O2 fluxes into Fe2+aq solutions containing either SO42-, PO43-, or Cl- to test the effect of end member conditions on iron phase stability; and the partitioning and speciation changes of soluble Ni(II), Cr(VI), Mo(VI), and Cu(II) during progressive iron oxidation. Through analysis of synchrotron-based Fe and TM XAS, XRD, and sequential solid dissolution yields, our experiments have demonstrated that 1. intermediate phases formed during partial Fe oxidation typically have higher PCs of Cr, Cu, and Ni, but lower PCs of Mo than final ferric products in a given experiment, 2. the stabilization of GR in low O2 flux experiments affects the partitioning and speciation of Ni and Mo into ferric end products, and 3. the effects of anions on TM partitioning and speciation during all stages of precipitation are greater in low O2 flux experiments. These discrepancies in PCs suggest trends in elemental ratios of T.M to Fetot or Fe3+ in BIFs require corroboration with other geochemical proxies to reconstruct coeval seawater compositions. Furthermore, XANES analysis suggest novel TM redox changes accompanying Fe oxidation, including initial reduction followed by partial oxidation of precipitated Cu, and irreversible Mo reduction by GR. The latter of these phenomena may be used as a finger-print for the transient presence of GR in depositional processes while the former requires further consideration in the emerging use of Cu as a paleo-redox proxy.