Production of CO2 and H2 by Diking-Eruptive Events at Mid-Ocean Ridges: Implications for Abiotic Organic Synthesis and Global Geochemical Cycling

Abstract

We have calculated the amounts of CO2 and H2 produced by complete degassing of mid-ocean ridge basalt (MORB) magma, and by degassing during transient diking-eruptive events. Our CO2 calculations are based on a model estimate of an initial CO2 content of 1800 ppm in MORB magma, which is equivalent to 2.2 × 1012 mol CO2 per year for magma production at worldwide ocean ridges. Observations indicate that many MORB magmas are emplaced in numerous small pulses of dikes and associated lava flows with very short emplacement times, which would result in release of relatively large amounts of CO2 over short intervals. For example, a dike injected into the oceanic crust that extends from the top of its magma chamber at 2 km depth to the seafloor would degas 2.3 × 104 mol CO2 per m2 surface area of dike, and produce another 4.0 × 104 mol CO2 per m2 on complete crystallization. Unlike CO2, which is not strictly governed by crystallization-alteration processes, H2 is produced from MORB by the reduction of H2O by ferrous iron in the magma to form magnetite and H2 as the magma cools and crystallizes. From published paired analyses of MORB glass and crystalline rock, we estimate that the amount of H2 produced from one cubic meter of rock averages 301 mol. We suggest that the oxidizing agent is H2O dissolved in the magma, which results in rapid generation of H2. The amount of pre-alteration oxidation may be limited by the amount of H2O dissolved in the magma; thus relatively water-rich magmas will undergo greater oxidation. For the case of the two-kilometer-high dike reaching the seafloor, the amount of H2 released is 6.2 × 105 moles H2 per m2 surface area of the dike. This is 10 times greater than the total CO2 released by degassing and crystallization of the dike. Assuming that the H2 generation rate for the entire basaltic layer of the oceanic crust is the same as for MORB lavas (312 mol/m3), then the annual global H2 production rate is 6.3 × 1012 mol H2 per year. This amount is about three times greater than our calculated annual CO2 production from MORBs. Given that the annual CO2 production rate from MORBs over 3.3 Ga can account for all CO2 found in the Earth's crust, hydrosphere, and atmosphere, it is likely that the H2 produced at mid-ocean ridges plays a significant role as a reducing agent in the global redox state of the Earth's surface. In contrast to time-averaged global production rates, the rapid release of CO2 and H2 in diking-eruptive events may locally result in formation of a separate gas phase containing H2-CO2-H2O in that order of abundance. The amounts of CO2 and H2 produced could provide a significant energy source for autotrophic microorganisms. It has been demonstrated that such a CO2-H2-H2O gas mixture yields methanol in magnetite-surface catalyzed reactions at seafloor hydrothermal conditions. Such abiotic synthesis reactions could have been important in early Earth processes.