09:45 〜 10:00
[BCG06-04] Thermodynamic simulations of Algoma-type banded iron formation deposition: implications for Archean seawater and hydrothermal fluid chemistry
キーワード：縞状鉄鉱層、太古代海水、アルカリ熱水、PHREEQC
Deciphering the ancient oceanic environment is crucial for understanding the coevolutionary history of life and Earth's surface conditions. Banded iron formations (BIFs), typical chemical sedimentary rocks from the Archean era, provide unique records of the hydrosphere during that period. It is classified into two types: Superior-type and Algoma-type. The latter is ubiquitously found in Archean greenstone belts in association with metabasalt and is believed to have formed under the influence of hydrothermal activity.
Traditionally, Fe-hydroxide has been considered the primary precipitate, with hydrothermal fluids thought to be acidic and Fe-rich, which is analogous to modern black smoker-type hydrothermal fluid. However, recent studies have provided alternative models. Archean high CO2 fluid is thermodynamically consistent with highly alkaline hydrothermal fluid (Shibuya et al., 2010) and petrographic observations suggest that Fe-silicate was the primary precipitate in both Superior-type and Algoma-type banded iron formations. (e.g. Ghosh, 2020; Nutman et al., 2017; Rasmussen et al., 2021). Furthermore, our recent investigations of Algoma-type BIFs from the Nuvvuagittuq supracrustal belt (>3.8 Ga) in Canada, which preserve distinct Fe-silicate and Fe-oxide bands, suggest that both silicate and hydroxide phases played major roles in Fe deposition.
Geochemical analyses indicate that Fe-silicate and silica precipitated in the absence of hydrothermal influence, forming non-hydrothermal ferruginous chert. Under weak hydrothermal conditions, enhanced Fe-silicate precipitation led to the formation of silicate-rich BIF, while stronger hydrothermal activity facilitated Fe-hydroxide precipitation, resulting in oxide-rich BIF.
The processes governing mineral precipitation are dependent on the chemical compositions of seawater and hydrothermal fluids. To elucidate these chemical properties, particularly the pH, of Archean seawater and hydrothermal fluids responsible for the formation of Algoma-type BIFs, we conducted thermodynamic simulations using the PHREEQC program. The study focused on three key components: (1) ferruginous and sulfate-free Archean seawater which is saturated in regard to Fe-silicate and silica, (2) high-temperature hydrothermal fluid resulting from interactions between seawater and basaltic materials, and (3) mineral precipitation processes triggered by the mixing of these fluids. Our results indicate that Fe-silicate and Fe-hydroxide precipitation occurs under weak and strong hydrothermal influence, respectively, only under conditions where (1) seawater is acidic and (2) hydrothermal fluid is alkaline. These findings suggest that Archean seawater was acidic and Fe-rich, with the deposition of Algoma-type BIFs occurring as a result of its mixing with alkaline hydrothermal fluids.
Traditionally, Fe-hydroxide has been considered the primary precipitate, with hydrothermal fluids thought to be acidic and Fe-rich, which is analogous to modern black smoker-type hydrothermal fluid. However, recent studies have provided alternative models. Archean high CO2 fluid is thermodynamically consistent with highly alkaline hydrothermal fluid (Shibuya et al., 2010) and petrographic observations suggest that Fe-silicate was the primary precipitate in both Superior-type and Algoma-type banded iron formations. (e.g. Ghosh, 2020; Nutman et al., 2017; Rasmussen et al., 2021). Furthermore, our recent investigations of Algoma-type BIFs from the Nuvvuagittuq supracrustal belt (>3.8 Ga) in Canada, which preserve distinct Fe-silicate and Fe-oxide bands, suggest that both silicate and hydroxide phases played major roles in Fe deposition.
Geochemical analyses indicate that Fe-silicate and silica precipitated in the absence of hydrothermal influence, forming non-hydrothermal ferruginous chert. Under weak hydrothermal conditions, enhanced Fe-silicate precipitation led to the formation of silicate-rich BIF, while stronger hydrothermal activity facilitated Fe-hydroxide precipitation, resulting in oxide-rich BIF.
The processes governing mineral precipitation are dependent on the chemical compositions of seawater and hydrothermal fluids. To elucidate these chemical properties, particularly the pH, of Archean seawater and hydrothermal fluids responsible for the formation of Algoma-type BIFs, we conducted thermodynamic simulations using the PHREEQC program. The study focused on three key components: (1) ferruginous and sulfate-free Archean seawater which is saturated in regard to Fe-silicate and silica, (2) high-temperature hydrothermal fluid resulting from interactions between seawater and basaltic materials, and (3) mineral precipitation processes triggered by the mixing of these fluids. Our results indicate that Fe-silicate and Fe-hydroxide precipitation occurs under weak and strong hydrothermal influence, respectively, only under conditions where (1) seawater is acidic and (2) hydrothermal fluid is alkaline. These findings suggest that Archean seawater was acidic and Fe-rich, with the deposition of Algoma-type BIFs occurring as a result of its mixing with alkaline hydrothermal fluids.