Production of CO2 and H-2 by diking-eruptive events at mid-ocean ridges: Implications for abiotic organic synthesis and global geochemical cycling

We have calculated the amounts of CO2 and H-2 produced by complete degassin g 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 t o 2.2 x 10(12) mol CO2 per year for magma production at worldwide ocean rid ges. Observations indicate that many MORB magmas are emplaced in numerous s mall pulses of dikes and associated lava flows with very short emplacement times, which would result in release of relatively large amounts of CO2 ove r 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 wo uld degas 2.3 x 10(4) mol CO2 per m(2) surface area of dike, and produce an other 4.0 x 10(4) mol CO2 per m(2) on complete crystallization. Unlike CO2, which is not strictly governed by crystallization-alteration pr ocesses, H-2 is produced from MORB by the reduction of H2O by ferrous iron in the magma to form magnetite and H-2 as the magma cools and crystallizes. From published paired analyses of MORB glass and crystalline rock, we esti mate that the amount of H-2 produced from one cubic meter of rock averages 301 mel. We suggest that the oxidizing agent is H2O dissolved in the magma, which results in rapid generation of H-2. The amount of pre-alteration oxi dation may be limited by the amount of H2O dissolved in the magma; thus rel atively water-rich magmas will undergo greater oxidation. For the case of t he two-kilometer-high dike reaching the seafloor, the amount of H-2 release d is 6.2 x 10(5) moles H-2 per m(2) surface area of the dike. This is 10 ti mes greater than the total CO2 released by degassing and crystallization of the dike. Assuming that the H-2 generation rate for the entire basaltic la yer of the oceanic crust is the same as for MORB lavas (312 mol/m(3)), then the annual global H-2 production rate is 6.3 x 10(12) mol H-2 per year. Th is amount is about three times greater than our calculated annual CO2 produ ction 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, an d atmosphere, it is likely that the H-2 produced at mid-ocean ridges plays a significant role as a reducing agent in the global redox state of the Ear th's surface. In contrast to time-averaged global production rates, the rapid release of CO2 and H-2 in diking-eruptive events may locally result in formation of a separate gas phase containing H-2-CO2-H2O in that order of abundance. The a mounts of CO2 and H-2 produced could provide a significant energy source fo r autotrophic microorganisms. It has been demonstrated that such a CO2-H-2- H2O gas mixture yields methanol in magnetite-surface catalyzed reactions at seafloor hydrothermal conditions. Such abiotic synthesis reactions could h ave been important in early Earth processes.