Preservation of Fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation

Abstract

We present the iron isotope composition of primary, diagenetic and metamorphic minerals in five samples from the contact metamorphosed Biwabik Iron Formation. These samples attained peak metamorphic temperatures of <200, <340,~500, <550, and <740°C respectively. d 56 Fe of bulk layers ranges from-0.8 to +0.8&; in some samples the layers may differ by >1& on the millimeter scale. Minerals in the lowest grade samples consistently show a sequence in which d 56 Fe of magnetite > silicate ‡ carbonate. The intermineral Fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperaturedependent fractionation factors. Textural evidence reveals that most, if not all, magnetite in the Biwabik Formation is diagenetic, not primary, and that there was tremendous element mobility during diagenesis. The short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine. Magnetite compositions therefore act as an isotope record of the environment in which these sediments formed. Larger-scale fluid flow and longer timescales may allow equilibration of Fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environments, but weakly regionally metamorphosed rocks may preserve small-scale Fe isotopic heterogeneities like those observed in the Biwabik Iron Formation. Importantly, Fe isotope compositions that are characteristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter-mineral variations, and at high grade in the form of unique bulk rock compositions. This observation confirms earlier work that has suggested that Fe isotopes can be used to identify sedimentary processes in the Precambrian rock record. Communicated by F. Poitrasson.

Micro-scale tracing of Fe and Si isotope signatures in banded iron formation using femtosecond laser ablation

“…In the Old Wanderer BIF, magnetite -siderite pairs show a relative difference of ~0.9‰. Very similar values have been reported for the Kuruman BIF in the Transvaal Basin, South Africa , and the Biwabik BIF (Frost et al, 2007) which have been interpreted as resulting from a coupled diagenetic formation process of siderite and magnetite due to DIR. Experimental work by Johnson et al (2005) has revealed an equilibrium fractionation factor ∆ siderite-magnetite of -1.3‰ for DIR, whereas Johnson et al (2008) prefer a fractionation factor of -1.8‰ (see Table 5).…”

“…The investigation of fractionation mechanisms of stable Fe isotopes is now well-advanced, and shows an overall range of ~5 ‰ in  56 Fe (for overview see Anbar, 2004;Beard and Johnson, 2004;Dauphas and Rouxel, 2006). The largest variability in the Fe isotope composition in any single type of sample has so far been measured in BIFs Dauphas et al, 2004Dauphas et al, , 2007Rouxel et al, 2005;Frost et al, 2007;Whitehouse and Fedo, 2007;Valaas Hyslop et al, 2008) and ranges from about -2.5 to 1.6‰ in  56 Fe. It has been shown that Fe isotope heterogeneities in BIFs can be preserved during diagenesis and metamorphism (Frost et al, 2007;Whitehouse and Fedo, 2007).…”

“…The largest variability in the Fe isotope composition in any single type of sample has so far been measured in BIFs Dauphas et al, 2004Dauphas et al, , 2007Rouxel et al, 2005;Frost et al, 2007;Whitehouse and Fedo, 2007;Valaas Hyslop et al, 2008) and ranges from about -2.5 to 1.6‰ in  56 Fe. It has been shown that Fe isotope heterogeneities in BIFs can be preserved during diagenesis and metamorphism (Frost et al, 2007;Whitehouse and Fedo, 2007). Magnetite, for example, appears to be of diagenetic origin and preserves its Fe isotope signature throughout all but the highest grades of prograde contact metamorphism (Frost et al, 2007).…”

“…It has been shown that Fe isotope heterogeneities in BIFs can be preserved during diagenesis and metamorphism (Frost et al, 2007;Whitehouse and Fedo, 2007). Magnetite, for example, appears to be of diagenetic origin and preserves its Fe isotope signature throughout all but the highest grades of prograde contact metamorphism (Frost et al, 2007). Therefore, Fe isotopes appear to be a powerful tool for the reconstruction of the early history of BIF deposition, and much effort is now being directed at deciphering the underlying processes that have caused their heterogeneous Fe isotope composition.…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…With that said, although petrographically most siderite (and ankerite) exhibit recrystallised and, therefore, apparently secondary textures, their geochemistry does indicate that some potentially "primary" signals can be retained in IF carbonates. Iron isotopes of extracted carbonate fraction remained unaffected by low-grade metamorphism (Frost et al, 2007) and carbonate-specific REE+Y patterns, both in situ (Alibert, 2016) and extracted as bulk (Oonk, 2017), mimic REE+Y aspects of present-day seawater.…”