57FE MöSSBAUER SPECTROSCOPIC ANALYSIS OF DEEP-SEA PELAGIC CHERT: EFFECT OF SECONDARY ALTERATION TO PALEO-REDOX INDICATORS

The chemical states of iron in accreted deep-sea pelagic cherts have been utilized as redox indicators for lost paleo-ocean prior to 200 Ma, e.g. in the Permian-Triassic boundary case. In general, red chert contains hematite suggesting an oxidizing condition, while gray/black chert contains pyrite suggesting a reducing one. The oxidizing and reducing conditions here are relatively defined on the basis of the redox potential of Fe(III)/Fe(II) transition. However, red chert sometimes has been secondarily altered into green one even in the same single chert bed on the outcrop. In order to assign the primary paleo-redox in chert properly, we performed quantitative chemical analysis of the secondary alteration in a single chert bed by ⁵⁷Fe Mössbauer spectroscopy.

We analyzed the iron species in 13 samples of Upper Triassic-Lower Jurassic pelagic cherts at Hisuikyo in central Japan. Red cherts contain hematite (28 % of total iron, in average) and paramagnetic Fe³⁺ (22 %) without pyrite, whereas green parts of the same beds contain lesser Fe³⁺ (17 %) without hematite and pyrite. The rest of iron components are unidentified paramagnetic Fe²⁺ that are likely hosted in clay minerals and/or in siderite. The results suggest that hematite has changed into Fe²⁺-bearing minerals through secondary alteration. Despite the secondary alteration, green chert can be clearly distinguished from the primary gray/black chert with pyrite that was deposited under a reducing condition.

The present results indicate that detailed analyses on redox indicators, such as ⁵⁷Fe Mössbauer spectroscopy, are effective in quantitative evaluation of paleo-redox and also of secondary alteration.

Our previous studies on the chemical states of iron in pelagic cherts suggest that the oxidizing deep ocean has already appeared in the latest Neoproterozoic and persisted throughout the Phanerozoic, except for some unique anoxic episodes, e.g. the P-T boundary Superanoxia and the Toarcian OAE.