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

Abstract

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 Fe 3+, and paramagnetic Fe 2+, whereas the greenish gray cherts contain paramagnetic Fe 3+ and paramagnetic Fe 2+ without hematite. The greenish gray parts contain larger amounts of paramagnetic Fe 2+ 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 Fe 2+-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.