Abstract
The transformation of pyrite into pyrrhotite above 500 °C was observed in the Chelungpu fault zone, which formed as a result of the 1999 Chi-Chi earthquake in Taiwan. Similarly, pyrite transformation to pyrrhotite at approximately 640 °C was observed during the Tohoku earthquake in Japan. In this study, we investigated the high-temperature phase-transition of iron sulfide minerals (greigite) under anaerobic conditions. We simulated mineral phase transformations during fault movement with the aim of determining the temperature of fault slip. The techniques used in this study included thermogravimetry and differential thermal analysis (TG/DTA) and in situ X-ray diffraction (XRD). We found diversification between 520 °C and 630 °C in the TG/DTA curves that signifies the transformation of pyrite into pyrrhotite. Furthermore, the in situ XRD results confirmed the sequence in which greigite underwent phase transitions to gradually transform into pyrite and pyrrhotite at approximately 320 °C. Greigite completely changed into pyrite and pyrrhotite at 450 °C. Finally, pyrite was completely transformed into pyrrhotite at 580 °C. Our results reveal the temperature and sequence in which the phase transitions of greigite occur, and indicate that this may be used to constrain the temperature of fault-slip. This conclusion is supported by field observations made following the Tohoku and Chi-Chi earthquakes.
Highlights
Iron sulfide minerals, which include marcasite (FeS2), pyrite (FeS2), greigite (Fe3S4), and pyrrhotite (Fe7S8), are common in the Earth’s crust
Iron sulfide minerals can be formed by precipitation from hydrothermal fluids, forming submarine black smoker chimneys[15,16]
Magnetotactic bacteria, which are similar to greigite-producing bacteria, can produce greigite under anaerobic conditions[14]
Summary
Iron sulfide minerals, which include marcasite (FeS2), pyrite (FeS2), greigite (Fe3S4), and pyrrhotite (Fe7S8), are common in the Earth’s crust. Offshore drilling research found that pyrite was converted into pyrrhotite as a result of high-temperature friction in the fault-slip zone in the offshore sediments of northeastern Japan (Tohoku earthquake)[27]. A study of the 1999 Chi-Chi earthquake in Taiwan observed that high temperatures generated by the fault slip caused pyrite to transform into pyrrhotite[28]. The high-temperature phase transition of iron sulfide minerals such as greigite (Fe3S4) may be another method for measuring fault slip temperature. We investigated the high-temperature phase transition of pyrite in nature, caused by fault slip, by designing a series of experiments in which greigite undergoes a high-temperature phase transition in an anaerobic environment. Greigite is an interesting iron sulfide mineral, very few studies that discuss the greigite-pyrite-pyrrhotite transition under high-temperature conditions have been published. The second transformation in this sequence resembles the phase transition of pyrite as it occurs in nature when it is caused by friction due to fault slip
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