Fe isotope and trace element geochemistry of the Neoproterozoic syn-glacial Rapitan iron formation

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Abstract

We have measured the iron isotope compositions and trace element concentrations of a suite of iron formation (IF) samples from the Neoproterozoic Rapitan Group, which was deposited during the older of two glacial episodes recorded in the Windermere Supergroup of the northern Canadian Cordillera. Like most other Neoproterozoic examples, iron in the Rapitan IF resides almost exclusively as hematite. This mineralogical simplicity compared to Archean and Paleoproterozoic banded iron formations is attributed to a limited supply of organic carbon to the Rapitan glacial ocean that inhibited diagenetic production of reduced iron phases. Sedimentological considerations indicate that the Rapitan IF was deposited during a rise in relative sea level related to a period of glacial advance and isostatic subsidence. Trace element data, including rare earth element plus yttrium (REE + Y) patterns, suggest an anoxic deep ocean dominated by low-temperature hydrothermal input and capped by a weakly oxic surface ocean. The iron isotope data show a trend of increasing δ57Fe (versus IRMM-14) up-section from ~−0.7‰ to 1.2‰, corresponding to a shift from a muddy IF facies to a dominantly jaspilitic IF facies. This distinct isotopic pattern likely records a steep isotopic gradient across the iron chemocline in Rapitan seawater.

Research Highlights

► We present trace element and iron isotope data on the Rapitan iron formation. ► The iron isotope data show a large rise in iron isotope values up-section. ► This isotopic trend is coupled to increasing water depth. ► The trend likely records a vertical iron isotope gradient in Rapitan seawater. ► We propose a new model for iron isotopic variability in ancient iron formations.

Introduction

The cycling of iron is of major interest in the study of the Earth system because of its abundance in rock-forming minerals, sensitivity to redox, role in microbial metabolism, and function as a micronutrient (Anbar, 2004, Beard et al., 2003a, Johnson et al., 2008a). Iron isotopes have become a mainstream tool in studying iron cycling due to the large isotopic fractionations attending redox transformations in the near surface environment. A natural application of iron isotope geochemistry is to ancient sedimentary banded-iron formation (BIF). A common theme among studies of Archean and Paleoproterozoic BIFs is the extraordinary variability in iron isotope compositions, from the stratigraphic (e.g. Beard et al., 2003a, Czaja et al., 2010, Heimann et al., 2010, Johnson et al., 2008b, Tsikos et al., 2010) to the mineral (Johnson et al., 2008b, Frost et al., 2007, Johnson et al., 2003) and micro scale (Steinhoefel et al., 2009, Steinhoefel et al., 2010). Although these variations are ultimately attributable to the large fractionation resulting from reduction and oxidation of iron and the isotopic differences between mineral phases, the models to account for the iron isotope patterns in ancient BIFs are diverse and controversial (Anbar and Rouxel, 2007, Johnson et al., 2008a).

Neoproterozoic iron formations (IF) present a unique opportunity to understand processes responsible for iron isotopic variations during deposition of iron-rich chemical precipitates. In contrast to some Archean–Paleoproterozoic BIFs in which iron occurs as both Fe2+ and Fe3+ in a range of different minerals, iron occurs almost exclusively in hematite in little-metamorphosed Neoproterozoic IFs (Klein and Beukes, 1993). Therefore, primary isotope signatures are easier to obtain and their interpretation is unhindered by complex mineralogy and diagenesis. Furthermore, the Neoproterozoic IFs are associated with episodes of global glaciation, and hence models to account for their iron isotope composition have implications for the chemistry of the glacial ocean. Here we report new iron isotope and trace element data from a section of the Rapitan IF in the Mackenzie Mountains, northwest Canada, and present a new model to account for large iron isotope variations in IFs.

Section snippets

Neoproterozoic iron formations

Whereas the precise mechanisms involved in the precipitation of iron minerals in BIFs are still debated (e.g. Konhauser et al., 2002), it is widely accepted that oxide facies BIFs record precipitation of precursor, poorly crystalline ferric oxyhydroxide (Fe-OH) minerals across a redoxcline, followed by deposition, diagenetic dewatering, and conversion to hematite (Beukes et al., 1990, Krapez et al., 2003). In turn, mixed valence and reduced iron phases in unmetamorphosed BIFs are dominantly

Methods

Twelve hand specimens spanning the 16.4 m-thick IF interval were collected from a stratigraphic section of the Sayunei and Shezal formations near Hayhook Lake in the central Rapitan basin (Fig. 1). All samples were slabbed for petrographic analysis, and ~ 50 g portions of unweathered bulk rock spanning multiple laminations were ground in an agate mortar to a fine powder, aliquots of which were used for X-ray diffraction (XRD), chemical, and isotopic analysis. XRD patterns were recorded on an Innel

XRD and trace elements

X-ray diffraction spectra of powdered bulk-rock samples confirm the preponderance of hematite and quartz in hematite–jaspilite samples, with a minor contribution of calcite (Table 1). In the hematitic mud/siltstone facies, illite, feldspar, and chlorite are also fairly common and calcite occurs in some instances (Table 1). Rare earth element and yttrium (REE + Y) data (Table 2) normalized to Post-Archean Australia Shale (PAAS) resemble data on the Rapitan produced by Klein and Beukes (1993) (

Iron isotope fractionation associated with the precipitation of oxide-facies BIF

Various mechanisms, both inorganic and biological, have been proposed as oxidation pathways for the precipitation of iron oxides from dissolved ferrous iron in seawater during BIF genesis. Although UV photo-oxidation of ferrous iron in the surface ocean (Braterman et al., 1983, Cairns-Smith, 1978) is a viable mechanism, Konhauser et al. (2007) argued that it was not a quantitatively significant pathway during Precambrian BIF genesis. This argument is supported by the observation that most BIFs

Conclusions

Sedimentological constraints and trace element data, including REE + Y patterns, from the Hayhook Lake section of the Rapitan IF in the northern Canadian Cordillera suggest that iron in the early Cryogenian Rapitan deep ocean was dominantly derived from low temperature alteration of oceanic crust. The Fe-OH precursors to the hematite in the IF were likely precipitated from a weak redoxcline, either abiotically or by chemolithotrophic iron oxidizing bacteria (cf. Beukes and Gutzmer, 2008). We

Acknowledgments

This research was supported by U.S. National Science Foundation grants OISE-0401772 to GPH and OPP-9817244 to PFH, a CNRS Eclipse Grant to AN, LMTG, and the University of Adelaide. Michel Thibaut is thanked for performing the XRD analyses. This manuscript was greatly improved by two anonymous reviews on a previous version and by reviews by R. Frei, H. Tsikos on the current draft. This work represents a contribution to IGCP project 512.

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    1

    Current address: Department of Earth and Planetary Sciences/GEOTOP, McGill University, 3450 University Street, Montréal, QC, Canada H3V 2A7.

    2

    Current address: Ecole Nationale Supérieure de Géologie, rue du Doyen Marcel Roubault – BP 40, 54501 Vandoeuvre-lès-Nancy cedex, France.

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