H₂-rich fluids from serpentinization: Geochemical and biotic implications

Abstract

Metamorphic hydration and oxidation of ultramafic rocks produces serpentinites, composed of serpentine group minerals and varying amounts of brucite, magnetite, and͞or FeNi alloys. These minerals buffer metamorphic fluids to extremely reducing conditions that are capable of producing hydrogen gas. Awaruite, FeNi3, forms early in this process when the serpentinite minerals are Fe-rich. Olivine with the current mantle Fe͞Mg ratio was oxidized during serpentinization after the Moon-forming impact. This process formed some of the ferric iron in the Earth's mantle. For the rest of Earth's history, serpentinites covered only a small fraction of the Earth's surface but were an important prebiotic and biotic environment. Extant methanogens react H2 with CO2 to form methane. This is a likely habitable environment on large silicate planets. The catalytic properties of FeNi3 allow complex organic compounds to form within serpentinite and, when mixed with atmospherically produced complex organic matter and waters that circulated through basalts, constitutes an attractive prebiotic substrate. Conversely, inorganic catalysis of methane by FeNi3 competes with nascent and extant life.

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“…Considering the extent of ultramafic environments on the seafloor and the longevity of seawater circulation during serpentinization, the total biomass of the ultramafic-hosted subsurface biosphere may be substantial. Some researchers also propose that organic compounds produced abiotically in serpentinites may contribute to certain oil and gas reservoirs (Gold, 1979(Gold, , 1999Szatmari, 1989) and that similar reactions may have been the source of prebiotic organic compounds on early Earth (Holm and Andersson, 1998;Shock and Schulte, 1998;Sleep et al, 2004). Conditions of early Earth were likely similar to the hydrogen-rich anaerobic environment within the subsurface of Lost City, and the alkaline conditions are favorable for some aspects of prebiotic chemistry, such as formation of RNA-bearing vesicles (Russell, 2003).…”

The carbon cycle and associated redox processes through time

“…Owing to the postulated declines in J am (figure 4) and increases in [1K( J ad C J od )/J am ] (equation (6.2)), the required rate would rise only slowly from 0.5 Tmol yr K1 at 3500 Ma to 0.9 Tmol yr K1 at 2800 Ma. If, as is likely, conditions at the seafloor favoured serpentinization reactions (Sleep et al 2004), a flux of 0.7 Tmol H 2 yr K1 could be provided by alteration of less than 10 15 g Mg-rich (komatiitic) basalt per year. The present rate of generation of ocean crust is about 6!10 16 g yr K1 , so the requirement poses no problem.…”

Fuels for the future

“…The origins of the methane hydrates in seafloor sediments are under debate. Biogenic sources are likely and probably have at least a large share in methane hydrate formation (Fyke and Weaver 2006, Wagner et al 2007, Colwell et al 2008), but it may be that abiotic (thermogenic) processes have a substantial share in their occurrence too (Sleep et al 2004). There is a substantial biogenic addition to the stock of methane hydrates (Colwell et al 2008) and there are also substantial losses (Buffett 2000).…”