57Fe Mössbauer spectroscopic analysis of deep-sea pelagic chert: Effect of secondary alteration with respect to paleo-redox evaluation

Chemical states of iron in the pre-Jurassic pelagic deep-sea cherts are used as one of redox indicators of lost deep-oceans, e.g. the Permian–Triassic boundary case. Primarily red hematite-bearing cherts were often altered secondarily into greenish gray cherts. We examined the pattern of secondary change in chemical state of iron, associated with color change, in the Middle Triassic chert beds at Hisuikyo in central Japan by 57Fe Mössbauer spectroscopy. Three sets of chert bed with the lateral color change from red to greenish gray were analyzed. The analyses confirmed that the red cherts contain hematite, paramagnetic Fe3+, and paramagnetic Fe2+, whereas the greenish gray cherts contain paramagnetic Fe3+ and paramagnetic Fe2+ without hematite. The greenish gray parts contain larger amounts of paramagnetic Fe2+ component of amorphous siderite-like mineral, in contrast to the red parts have lesser amounts. These results confirmed that the primary hematite has changed into paramagnetic Fe2+-bearing minerals, in accordance with the color change, by the secondary alteration. Further comparison with the pyrite-bearing primary black to dark-gray cherts was discussed with respect to the evaluation for primary redox change in the past oceans.

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► Chemical states of Fe in the Triassic cherts were checked by Mössbauer analysis.
► Red chert with hematite changes laterally into hematite-free greenish gray one.
► Secondary alteration changed primary hematite likely into Fe2+-bearing minerals.
► Analysis of Fe can discriminate altered chert from the primary anoxic black chert.