论文部分内容阅读
To understand the influence of fluid CO_2 on ultramafic rock-hosted seafloor hydrothermal systems on the early Earth,we monitored the reaction between San Carlos olivine and a CO_2-rich NaCl fluid at 300 C and 500 bars.During the experiments,the total carbonic acid concentration(∑XO_2) in the fluid decreased from approximately 65 to 9 mmol/kg.Carbonate minerals,magnesite,and subordinate amount of dolomite were formed via the water-rock interaction.The H_2 concentration in the fluid reached approximately 39 mmol/kg within 2736 h,which is relatively lower than the concentration generated by the reaction between olivine and a CO2-free NaCl solution at the same temperature.As seen in previous hydrothermal experiments using komatiite,ferrous iron incorporation into Mg-bearing carbonate minerals likely limited iron oxidation in the fluids and the resulting H_2 generation during the olivine alteration.Considering carbonate mineralogy over the temperature range of natural hydrothermal fields,H_2 generation is likely suppressed at temperatures below approximately 300℃ due to the formation of the Mg-bearing carbonates.Nevertheless,H_2 concentration in fluid at 300℃ could be still high due to the temperature dependency of magnetite stability in ultramafic systems.Moreover,the Mg-bearing carbonates may play a key role in the ocean-atmosphere system on the early Earth.Recent studies suggest that the subduction of carbonated ultramafic rocks may transport surface CO_2 species into the deep mantle.This process may have reduced the huge initial amount of CO_2 on the surface of the early Earth.Our approximate calculations demonstrate that the subduction of the Mg-bearing carbonates formed in komatiite likely played a crucial role as one of the CO_2 carriers from the surface to the deep mantle,even in hot subduction zones.
To understand the influence of fluid CO_2 on ultramafic rock-hosted seafloor hydrothermal systems on the early Earth, we monitored the reaction between San Carlos olivine and a CO_2-rich NaCl fluid at 300 C and 500 bars. During the experiments, the total carbonic acid concentration (ΣXO_2) in the fluid decreased from approximately 65 to 9 mmol / kg. Carbonate minerals, magnesite, and subordinate amount of dolomite were formed through the water-rock interaction. H_2 concentration in the fluid reached approximately 39 mmol / kg within 2736 h, which is relatively lower than the concentration generated by the reaction between olivine and a CO2-free NaCl solution at the same temperature. As seen in previous hydrothermal experiments using komatiite, ferrous iron incorporation into Mg-bearing carbonate minerals likely limited iron oxidation in the fluids and the resulting H_2 generation during the olivine alteration. C onidering carbonate mineralogy over the temperature range of natural hydrothermal fields H 2 generation is likely suppressed at temperatures below about 300 ° C. due to the formation of the Mg-bearing carbonates. However, H 2 concentration in fluid at 300 ° C could be still high due to the temperature dependency of magnetite stability in ultramafic systems. Moreover, the Mg-bearing carbonates may play a key role in the ocean-atmosphere system on the early Earth. Published studies suggest that the subduction of carbonated ultramafic rocks may transport surface CO_2 species into the deep mantle. This process may have reduced the huge initial amount of CO 2 on the surface of the early Earth. Our approximate calculations demonstrate that the subduction of the Mg-bearing carbonates formed in komatiite likely played a crucial role as one of the CO_2 carriers from the surface to the deep mantle, even in hot subduction zones .