Decarbonation, Serpentinization, Abiogenic Methane, and Extreme pH beneath the Mariana Forearc

Low-chlorinity springs sampled from ten sites on nine serpentinite mud volcanoes show systematic chemical gradients across the outer Mariana forearc that result from progressive devolatilization of the subducting Pacific plate. Sites range from 50 to 90 km from the trench axis corresponding to depths to the top of the plate of ∼15 to 29 km. Dissolved sulfate, Na/Cl, K, Rb, Cs, and B in the springs all increase regularly with distance from the trench, leached from the subducting sediment and altered basalt in response to increasing temperature at depth from ∼100-300° C. Sites nearer the trench have high Ca (up to 75 mmol/kg) and Sr, low alkalinity, and pH 10.7, whereas sites farther from the trench have almost no Ca and Sr, alkalinity (some carbonate but mostly hydroxyl) as high as 69 meq/kg, and pH 12.5. Springs with high alkalinity also have high methane (>44 mmol/kg) that feeds sulfate-reducing microbial communities in the shallow subsurface and macrofauna at the seafloor. These distal springs form chimneys and crusts of CaCO₃, whereas the proximal springs form chimneys of brucite. High alkalinity at the distal sites apparently results from decarbonation at the top of the subducting plate; because serpentinization during ascent generates both high pH and H₂, the resulting dissolved carbonate is reduced to methane such that carbonate alkalinity is replaced by hydroxyl alkalinity: 4H₂ + CO₃⁼ = CH₄ + H₂O + 2OH^-. This reaction can account for the much higher pH of the distal springs. Chlorinity of the springs varies from 234-546 mmol/kg and is related more to latitude N-S than to distance from the trench. Distal springs have otherwise similar compositions over this entire range of chlorinity, implying that chloride derives from depth rather than from mixing with seawater within the seamounts themselves. The range in chlorinity can readily be explained by serpentinization at reasonable water/rock mass ratios of 0.2-1.0 if 30-40% of the spring water originates as residual pore water in subducted sediment and basalt rather than as H2O⁺ of dehydration. For convergence at 4.75 cm/y and per km of trench, the rate of subduction under the Mariana forearc is 24 km³/My or 3.4 x 10¹⁰ g/y of (dry) sediment 500 m thick, and 95 km³/My or 2.7 x 10¹¹ g/y of altered basalt 2 km thick. For loss from sediment of 1% H2O⁺ and 2% residual pore water, plus from altered basalt 2% H2O⁺ and 1% pore water, going from lawsonite-albite to epidote-blueschist facies as in the Catalina schist (Bebout, 1995), this amounts to a water flux of 9 x 10⁹ g/y per km of trench, upward into the mantle of the outer Mariana forearc. This water derives 90:10 from basalt:sediment and 63:37 from H2O⁺: residual pore water. Corresponding vertical fluxes of sulfate, C, Na, K, Rb, Cs, B, Ca, and Sr in the forearc springs represent 0.05-3% of the amounts subducted, consistent with continued supply at greater depths.