Fe isotope and trace element geochemistry of the Neoproterozoic syn-glacial Rapitan iron formation
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.
References (87)
Iron stable isotopes: beyond biosignatures
Earth Planet. Sci. Lett.
(2004)- et al.
The photochemistry of manganese and the origin of banded iron formations
Geochim. Cosmochim. Acta
(1992) - et al.
Theoretical investigation of iron isotope fractionation between Fe(H2O)63+ and Fe(H2O)62+
Geochim. Cosmochim. Acta
(2005) - et al.
Iron isotope fractionation during microbially stimulated Fe(II) oxidation and Fe3+ precipitation
Geochim. Cosmochim. Acta
(2006) - et al.
Distribution of yttrium and rare-earth elements in the Penge and Kuruman iron-formations, Transvaal Supergroup, South Africa
Precambrian Res.
(1996) - et al.
Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling
Mar. Chem.
(1997) - et al.
Application of Fe isotopes to tracing the geochemical and biological cycling of Fe
Chem. Geol.
(2003) - et al.
Iron isotope fractionation by Fe(II)-oxidizing photautotrophic bacteria
Geochim. Cosmochim. Acta
(2004) - et al.
Iron and carbon isotope evidence for ecosystem and environmental diversity in the ~ 2.7 to 2.5 Ga Hamersley Province, Western Australia
Earth Planet. Sci. Lett.
(2010) - et al.
The chemical evolution of Precambrian seawater: evidence from REEs in banded iron formations
Geochim. Cosmochim. Acta
(1990)
Late Proterozoic rifting, glacial sedimentation, and sedimentary cycles in the light of Windermere deposition, Western Canada
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Fe, C, and O isotope compositions of banded iron formation carbonates demonstrate a major role for dissimilatory iron reduction in 2.5 Ga marine environments
Earth Planet. Sci. Lett.
Isotopic fractionation between Fe3+ and Fe(II) in aqueous solutions
Earth Planet. Sci. Lett.
Iron isotopes contain biological and abiological processes in banded iron formation genesis
Geochim. Cosmochim. Acta
Decoupling photochemical Fe(II) oxidation from shallow-water BIF deposition
Earth Planet. Sci. Lett.
Hydrothermal Fe fluxes during the Precambrian: effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers
Earth Planet. Sci. Lett.
The global variation in the iron isotope composition of marine hydrogenetic ferromanganese deposits: implications for seawater chemistry?
Earth Planet. Sci. Lett.
Origin and age of the Lake Nyos maar, Cameroon
J. Volcanol. Geotherm. Res.
Rare earth elements in hydrothermal fluids
Geochim. Cosmochim. Acta
Genetic modeling for banded iron-formation of the Hamersley Group, Pilbara Craton, Western Australia
Precambrian Res.
Rare Earth Element and yttrium composition of Archean and Paleoproterozoic Fe formations revisited: new insights on the significance and mechanisms of deposition
Geochim. Cosmochim. Acta
Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS
Chem. Geol.
Development of a sequential extraction procedure for iron: implications for iron partitioning in continentally derived particles
Chem. Geol.
The effect of early diagenesis on the Fe isotope composition of porewaters and authigenic minerals in continental margin sediments
Geochim. Cosmochim. Acta
Micro-scale tracing of Fe and Si isotope signatures in banded iron formation using femtosecond laser ablation
Geochim. Cosmochim. Acta
Deciphering formation processes of banded iron formations from the Transvaal and the Hamersley successions by combined Si and Fe isotope analysis using UV femtosecond laser ablation
Geochim. Cosmochim. Acta
Large iron isotope fractionation at the oxic–anoxic boundary in Lake Nyos
Earth Planet. Sci. Lett.
Iron isotopes constrain biogeochemical redox cycling of iron and manganese in a Palaeoproterozoic stratified basin
Earth Planet. Sci. Lett.
High precision Fe isotope measurements with high mass resolution
Int. J. Mass Spec.
Iron-formation and glaciogenic rocks of the Rapitan Group, Northwest Territories, Canada
Precambrian Res.
Precambrian of the Mackenzie fold belt—a stratigraphic and tectonic overview
Two Late Proterozoic glaciations, Mackenzie Mountains, northwestern Canada
Geology
Mackenzie tectonic arc—reflection of early basin configuration?
Geology
Metal stable isotopes in paleoceanography
Annu. Rev. Earth Planet. Sci.
Iron isotope constraints on Fe cycling and mass balance in oxygenated Earth oceans
Geology
Fe isotope variations in the modern and ancient Earth and other planetary bodies
Iron Formation: The Sedimentary production of complex interplay among mantle, tectonic, oceanic, and biospheric processes
Ec. Geol.
An hypothesis for an Australian–Canadian connection in the Late Proterozoic and the birth of the Pacific Ocean
Origin and palaeoenvironmental significance of major iron formations at the Archaean–Palaeoproterozoic boundary
Carbonate petrography, kerogen distribution, and carbon and oxygen isotope variations in an Early Proterozoic transition from limestone to iron-formation deposition, Transvaal Supergroup, South Africa
Precambrian Res.
Photo-oxidation of hydrated Fe2+ — significance for banded iron formations
Nature
Demonstration of significant abiotic iron isotope fractionation in nature
Geology
Precambrian solution photochemistry, inverse segregation, and banded iron formations
Nature
Cited by (0)
- 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.