15:45 〜 16:00
[BBC03-08] Origins of volcanic SO2 on Earth
キーワード：volcanic SO2, evolution of the atmospheric oxygen, arc magmatism, origin of MIF-S
SO2 is a principal volcanic gas today. The two important questions to be addressed are: (i). the origin of S atoms in volcanic SO2; and (ii) whether or not SO2 was an important volcanic gas of early Earth. I have approached these questions from: (a) thermodynamic- and kinetic analyses of various reactions involving S-bearing species in silicate melts, fluids, rocks, minerals, and aqueous solutions at T = 0-1500 °C and P = 1-10 kb, and (b) examinations of the chemical and S-isotopic characteristics of volcanic gases and rocks in various tectonic settings on modern Earth. Here are some of the important suggestions from these investigations:
1. All of the S-bearing species from submarine volcanic eruptions have been nearly completely trapped in oceans through reactions such as: H2S(g)= H2S(aq); 4SO2(g)+ 4H2O = 3HSO4-+ H2S(aq)+ 3H+; S8(g)= 8S(s); and 7H2S(aq)+ HSO4-+ 4Fe2+= 4FeS2(py)+ 7H++ 4H2O. S-bearing volcanic gases have only been emitted into the atmospheres by subaerial eruptions. Therefore, on the ocean-covered planets, possibly including the pre-3.0 Ga Earth, S-bearing gases have not been emitted into the atmospheres.
2. The fluids that originated from normal-mantle-derived-magmas (i.e., logfO2= FMQ-2 to FMQ+0.5) were initially H2S-rich and SO2-poor, whereas those from subduction-related arc magmas (i.e., logfO2= FMQ+0.5 to FMQ+3) were initially SO2- and/or H2S-rich.
3. H2S-rich magmatic fluids may become SO2-rich gas, if: (i) the fluids were derived from high fO2magmas (i.e., logfO2> FMQ) and ascended slowly through conduits (i.e., diffusive eruptions) and re-equilibrated with wall-rocks at PH2O<10 bars, or (ii) the fluids are oxidized by reactions with an O2-rich atmosphere, -groundwater and/or high-fO2wall-rocks during and/or after the ascent through conduits. In the absence of (i) and (ii), such as the Earth prior to the oxygenation of the atmosphere, H2S-rich magmatic fluids were not transformed to SO2 during eruptions.
4. SO2-rich fluids from arc magmas would remain SO2-rich during explosive or diffusive eruptions. However, if the fluids cool down to <~700°C, SO2 transforms to H2S.
5. Volcanic SO2 from arc magmas was transformed from seawater sulfate through the following processes: (i) formation of pyrite, hematite and anhydrite by reactions between SO42--rich seawater and hot basalts on MORs; (ii) devolatilization of H2O- and SO2-rich fluids from the subducting oceanic crust; (iii) formation of oxidized- and SO42--rich magmas by partial melting of peridotite in the mantle wedges, and (iv) degassing of SO2-rich fluids from these magmas. If the oceans were SO42--poor, volcanic gases from arc magmas would be SO2-poor.
6. Oceanic SO42- has been produced mostly by the oxidative weathering of pyrite on land. Therefore, subaerial volcanic gases on planets with O2-poor atmospheres would be SO2-poor and H2S-rich. Then, the current paradigm concerning the origin of mass-independent-fractionations of sulfur isotopes (MIF-S) in Archean-aged sedimentary rocks (i.e., the UV photolysis of volcanic SO2 in an O2-poor atmosphere) becomes invalid. Consequently, the MIF-S record in sedimentary rocks is not supportive evidence for the “Great Oxidation Event” at ~2.5 Ga ago. Furthermore, the current atmospheric O2-evolution models, which are based on the premise of SO2-rich volcanic gases throughout geologic time, become invalid.
(7). The MIF-S in the Archean- and younger sedimentary rocks may have been created by the chemisorption isotope effects during thermochemical reductions of seawater sulfates by hydrothermally-generated, very reactive organic matter.
1. All of the S-bearing species from submarine volcanic eruptions have been nearly completely trapped in oceans through reactions such as: H2S(g)= H2S(aq); 4SO2(g)+ 4H2O = 3HSO4-+ H2S(aq)+ 3H+; S8(g)= 8S(s); and 7H2S(aq)+ HSO4-+ 4Fe2+= 4FeS2(py)+ 7H++ 4H2O. S-bearing volcanic gases have only been emitted into the atmospheres by subaerial eruptions. Therefore, on the ocean-covered planets, possibly including the pre-3.0 Ga Earth, S-bearing gases have not been emitted into the atmospheres.
2. The fluids that originated from normal-mantle-derived-magmas (i.e., logfO2= FMQ-2 to FMQ+0.5) were initially H2S-rich and SO2-poor, whereas those from subduction-related arc magmas (i.e., logfO2= FMQ+0.5 to FMQ+3) were initially SO2- and/or H2S-rich.
3. H2S-rich magmatic fluids may become SO2-rich gas, if: (i) the fluids were derived from high fO2magmas (i.e., logfO2> FMQ) and ascended slowly through conduits (i.e., diffusive eruptions) and re-equilibrated with wall-rocks at PH2O<10 bars, or (ii) the fluids are oxidized by reactions with an O2-rich atmosphere, -groundwater and/or high-fO2wall-rocks during and/or after the ascent through conduits. In the absence of (i) and (ii), such as the Earth prior to the oxygenation of the atmosphere, H2S-rich magmatic fluids were not transformed to SO2 during eruptions.
4. SO2-rich fluids from arc magmas would remain SO2-rich during explosive or diffusive eruptions. However, if the fluids cool down to <~700°C, SO2 transforms to H2S.
5. Volcanic SO2 from arc magmas was transformed from seawater sulfate through the following processes: (i) formation of pyrite, hematite and anhydrite by reactions between SO42--rich seawater and hot basalts on MORs; (ii) devolatilization of H2O- and SO2-rich fluids from the subducting oceanic crust; (iii) formation of oxidized- and SO42--rich magmas by partial melting of peridotite in the mantle wedges, and (iv) degassing of SO2-rich fluids from these magmas. If the oceans were SO42--poor, volcanic gases from arc magmas would be SO2-poor.
6. Oceanic SO42- has been produced mostly by the oxidative weathering of pyrite on land. Therefore, subaerial volcanic gases on planets with O2-poor atmospheres would be SO2-poor and H2S-rich. Then, the current paradigm concerning the origin of mass-independent-fractionations of sulfur isotopes (MIF-S) in Archean-aged sedimentary rocks (i.e., the UV photolysis of volcanic SO2 in an O2-poor atmosphere) becomes invalid. Consequently, the MIF-S record in sedimentary rocks is not supportive evidence for the “Great Oxidation Event” at ~2.5 Ga ago. Furthermore, the current atmospheric O2-evolution models, which are based on the premise of SO2-rich volcanic gases throughout geologic time, become invalid.
(7). The MIF-S in the Archean- and younger sedimentary rocks may have been created by the chemisorption isotope effects during thermochemical reductions of seawater sulfates by hydrothermally-generated, very reactive organic matter.