SEM and TEM characterization of framboidal pyrite with jagged-like structure  in the Lower Cambrian Shuijingtuo Formation shale and its indication to early Cambrian dynamic ocean oxygenation

Abstract

Pyrite is widespread in modern and ancient shales, among them, framboidal pyrite has been paid more attention because its formation and oxidation are not only related of circulation of some elements such as S, Fe and O, but may reflect the evolution characteristics of the paleo-environment during the synsedimentary-early diagenetic stage. In this study, a continuous core well in the Lower Cambrian Shuijingtuo Formation, the Yangtze Plate, China was selected, and pyrite elaborate morphology at micro-nanometer scale was intensively investigated by using Ar ion milling-scanning electron micrography (SEM) technologies, and aberration corrected transmission electron microscope (AC-TEM). The results showed that the some pyrite framboids grains with jagged-like structure appeared in the upper part of the Shuijingtuo Formation. The oxygen content of framboids with jagged-like structure is higher than that of framboids with complete crystal grain based on EDS elemental data. These jagged-like pyrite grains displayed a series of oxidation byproducts as precipitated phases which are interpreted as marcasite, ferrous sulfate, and iron sulfate hydrate by means of HRTEM calibration. In some cases, about 5nm thick amorphous passivation layer occurred on the edge of pyrite grains. All of our observations indicate that the jagged-like structure of these pyrite framboids is likely caused by oxidation and dissolution during the synsedimentary-early diagenesis, and two scenarios are further suggested: In scenario 1, a passivation layer on the surface of pyrite framboids was formed at the beginning of pyrite oxidation, and in scenario 2, along with increasing oxidation extent and elevated Eh in pore water, the jagged-like structure was formed from the partial dissolution of pyrite framboids grains and the precipitation of oxidation byproducts resulted in an increased impurity of pyrite crystal pattern. Our observations are consistent with geochemical paleoenvironmental parameters. UEF , U/Th and V/Cr ratios record that there is intermittent oxygenation during the lower Cambrian organic-rich shale deposition. The discrepancy of morphology of framboidal pyrite microcrystalline is coincided with the diversification of redox indicators, turning into a kind of mineral indicator of seafloor and superficial sediment pore water oxidation.  Key words: In-situ TEM; Framboidal pyrite; Pyrite oxidation; Jagged-like structure; Lower Cambrian