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Chapter 7. Volcanic outgassing and mantle redox evolution Outline 7.1 Historical context: strongly and weakly reduced atmospheres 7.2 Volcanic outgassing and metamorphic degassing of major volatile species 7.2.1 Mechanisms of volcanic outgassing 7.2.2 Outgassing and metamorphic degassing of CO2 7.2.3 Subaerial outgassing of H2O, SO2, H2S, and N2 7.3 Oxidation state of the upper mantle 7.3.1 Oxidation state of the present mantle 7.3.2 How the mantle became oxidized 7.4 Release of reduced gases from subaerial volcanism 7.5 Reduced gases released from submarine volcanism 7.5.1 H2S and H2 7.5.2 CH4 7.6 Past rates of volcanic outgassing 7.7 Summary --------------------------------------------------------------------------------------------------------In the last chapter we discussed how Earth’s atmosphere formed. Here, we begin the discussion of how Earth’s atmosphere has evolved over time. Volcanic outgassing is one of the most important processes influencing atmospheric evolution because it affects both atmospheric composition and redox state, that is, the balance between reduced and oxidized gases, e.g., H2 and O2. The redox state of volcanic gases is, in turn, influenced by the redox state of Earth’s mantle—a topic that will require us to delve into some aspects of petrology. To motivate our discussion of volcanic outgassing, we briefly review what previous authors have said about the composition of the prebiotic atmosphere. This is a crucial part of understanding the difference between inhabited and uninhabited worlds. Our own, more detailed, analysis will be saved for Ch. 9, but the discussion here will show why volcanic outgassing rates and mantle redox state are important. Furthermore, some of these older ideas about the primitive atmosphere are so deeply entrenched in popular and scientific thinking that it is difficult to discuss any of these concepts without understanding how they originated. 7.1 Historical context: strongly and weakly reduced atmospheres Past theories of the prebiotic atmosphere have been heavily influenced by the perceived requirement that it provide a suitable environment for the origin of life. As we discuss further in Ch. 9, this constraint may be more apparent than real, because there are other pathways to make complex organic compounds besides atmospheric chemistry. However, serious thinking about the earliest atmosphere began with the origin of life (e.g., see Chang et al. (1983)), so this is where we start. A modern scientific theory for the origin of life was first developed by the Russian astrochemist Alexander Oparin and published in Russian (Oparin, 1924) and later, several times, in English (Oparin, 1938, 1957, 1968). Similar but less detailed views were published several years later by the British geneticist and evolutionist John Haldane (Haldane, 1929). The Oparin-Haldane hypothesis, as it later came to be called,

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suggested that life was formed from complex organic compounds that were synthesized by photochemical or thermochemical processes (e.g., lightning) in a highly reduced atmosphere containing methane, ammonia, and other compounds rich in hydrogen. The process of assembling non-living compounds into organic precursors to biological entities was termed chemical evolution. Oparin’s model for the early atmosphere superceded that of the American paleontologist Henry Osborn (1917), cited by Oparin (1938). Osborn had proposed that the early atmosphere consisted primarily of CO2 and H2O. The pendulum eventually swung back in favor of Osborn’s idea, but it took nearly a century for it to do so. Fig. 7.1 here. Oparin’s theory of the prebiotic atmosphere received a tremendous boost in the early 1950s when the American geochemist Harold Urey and his graduate student Stanley Miller performed a famous experiment in which they synthesized possible prebiotic compounds from plausible early atmospheric gases (Miller, 1953, 1955). A diagram of Miller’s experimental apparatus is shown in Fig. 7.1. Urey was aware that methane and ammonia had been discovered in the atmospheres of Jupiter and Saturn, and, through a chain of logic, argued for their presence in Earth’s earliest atmosphere. He reasoned that these gases were present on the giant planets because their gravitational attraction was strong enough to prevent hydrogen from escaping. Deep convective mixing also plays a role, as it allows carbon, nitrogen, and hydrogen to be mixed downward to regions where temperatures and pressures are high enough for thermodynamic equilibrium. At high pressures in the presence of large amounts of H2, the stable form of carbon is methane, CH4, and that of nitrogen is ammonia, NH3. Urey further reasoned, perhaps not quite correctly, that if one went back to a time before hydrogen had had a chance to escape, Earth’s primitive atmosphere would have resembled those of the giant planets. The logic here is suspect because, as we saw in Ch. 5, the time scale for hydrogen loss is short enough that Earth’s atmospheric hydrogen has probably always been more or less in steady state, with production of hydrogen from volcanoes and impacts balanced by escape of hydrogen to space and reduction of oxidized volatiles to reduced ones (e.g., CO2  CH2O). One should bear in mind, however, that Urey was working well before astronomers had deduced the time scales for solar nebula evolution and planetary formation. If the fully formed Earth had been embedded in a dense solar nebula from which it captured large amounts of H2, then Urey’s picture of the initial conditions for life might be reasonable. We argued in Sec. 6.4.2, though, that any primordial captured atmosphere was probably dissipated during the accretion process itself and replaced with one derived from impact degassing. Urey’s ideas about early atmospheric composition became firmly ingrained in scientific thought when Stanley Miller performed his historic experiment, which is recounted in detail by Miller’s student Jeffrey Bada (Wills and Bada, 2000). Miller set up a pair of electrodes within a flask containing CH4, NH3, and H2O. Hydrogen sulfide (H2S) was also included in some experiments. Over periods of hours to days, the electrodes provided spark discharges to the mixture, simulating lightning in the early atmosphere. Then, Miller analyzed reddish-brown material that accumulated in the water and on the walls of the flask. This residue contained all sorts of complex hydrocarbons, including amino acids, the building blocks of proteins. Both the scientific community and

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the general public were encouraged that we were well on the way to understanding the origin of life. Carried along by this sense of optimism, the CH4/NH3-rich gas mixture used in the experiment became a common model of primitive atmosphere composition. At about this same time, unnoticed by nearly everyone except his fellow geochemists, William Rubey of the U. S. Geological Survey was developing his own ideas of early atmospheric composition (Rubey, 1951, 1955). Rubey tabulated data on the gases emanating from modern volcanoes. These gases are dominated by CO2 and H2O, rather than CH4 and NH3. Indeed, NH3 concentrations are too low to be detected in gases released from surface volcanism (Holland, 1984), and CH4/CO2 ratios are