Iron isotopic variations of the Cryogenian banded iron formations: a new model

Highlights
• A systematic Fe isotope investigation was carried out for Cryogenian banded iron formation from Xinyu, South China.

• Almost all Cryogenian banded iron formations worldwide have an increasing trend for δ56Fe values upwards stratigraphically.

• Fe isotope variation in Cryogenian banded iron formations is controlled by varying degrees of Fe precipitation in seawater, which can be controlled by changes of Eh and/or pH condition.

• Ocean acidification (pH decrease) is a possible mechanism resulting in Fe isotope variation in Cryogenian banded iron formations.

Abstract
The Fe isotope composition of banded iron formation (BIF) is regarded as a powerful tracer in paleoceanography, and the Cryogenian banded iron formation associated with “Snowball Earth Events” provides a precious record of environmental change in the Neoproterozoic. However, Fe isotope studies on Cryogenian BIFs are rare and previous interpretations of Cryogenian Fe isotope data are problematic. Here we present a systematic investigation of the Fe isotope composition of the Cryogenian Xinyu BIF sections from four localities in the Yangtze region, South China. These BIF sections comprise banded magnetite quartzite, banded chlorite-magnetite quartzite, and magnetite-bearing chlorite phyllite, in stratigraphically ascending order. The δ56FeIRMM-014 values of the Xinyu BIF vary significantly and show an overall increase upsection, from ca. 0‰ to ca. 1.5‰. This stratigraphic trend in Fe isotope compositions is similar to those reported previously for Cryogenian BIFs in North America and Australia, and thus seems to be a common phenomenon. We interpret Fe isotope variation in Cryogenian BIF to be essentially controlled by varying degrees of Fe precipitation in seawater, rather than resulted from “a water column Fe isotope gradient” as proposed previously. The variation in the degree of Fe precipitation can be controlled by changes of Eh and/or pH conditions in seawater resulting from transgression or ocean acidification.

Zhu X.-K., Sun J. & Li Z.-H., in press. Iron isotopic variations of the Cryogenian banded iron formations: a new model. Precambrian Research. Article (subscription required).

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