Decarbonation Serpentinization Abiogenic Methane and Extreme p H

Decarbonation, Serpentinization, Abiogenic Methane, and Extreme p. H beneath the Mariana Forearc M. J. Mottl, C. G. Wheat, and P. Fryer ODP Leg 195, Site 1200, summit of South Chamorro Seamount Distance from spring: 7 m 20 m 80 m Character of springs with distance from trench: Near (50 -65 km) Far (70 -90 km) No C source: Low alkalinity High Ca, Sr Brucite chimneys Low CH 4 p. H 10. 7 (11. 1 at 2°C) What is the nature of the source rocks and what reactions are they undergoing? Calcite solubility increases with P. At 3 kbars and above, it also increases with T. (Caciagli and Manning, 2003) High alkalinity Low Ca, Sr Ca. CO 3 chimneys High CH 4 p. H 12. 5 (13. 1 at 2°C) Metabasic schists suspended in the serpentinite mud indicate metamorphism in the lawsonite-albite, lawsonite blueschist, and epidote blueschist facies: lawsonite or epidote + quartz + albite + white mica + Na-pyroxene + Na-amphibole + aragonite + chlorite +/- talc +/- tremolite. CO 32 - + 4 H 2 = CH 4 + H 2 O + 2 OHvs. seawater: 0. 94 x 1. 3 x Six cruises to the Mariana Forearc: -ODP Legs 125 (1989) and 195 (2001) -Alvin (1987) and Shinkai-6500 (1996) -Jason ROV and coring (1997, 2003) Pore water from 19 sites on 16 seamounts: -10 ODP holes (4 on Leg 125 and 6 on Leg 195) -54 push cores from manned submersibles or ROV -16 piston and 48 gravity cores (= 64 cores) Mariana Forearc Seamounts: 1. 9 xx 8. 0 xx 7. 8 x One-dimensional advection (upwelling)-diffusion. No S= No microbial activity(? ) No macrofauna High S= Microbial activity Macrofauna CH 4 + SO 42 - + 2 OH- = CO 32 - + S= + 3 H 2 O Equilibrium with Qtz + Ab at 5 kbar (Bowers et al. , 1984). Upwelling freshened fluid is enriched in Na, K, Rb, B. Serpentinite Mud Volcanoes! Cold springs with chimneys Largest are 50 km across and 3 km high. 300 o. C 150 o. C Solution composition is fixed by Qtz + Ab + Parag + Musc + Laws or Epid + Chlorite or Talc or Tremolite. p. H 12. 5! Ten sites on nine seamounts yielded upwelling pore water that is fresher than seawater! Deep upwelling water (at 2 o. C, from PHREEQC): Anaerobic Oxidation of Methane by Archaea in the upper 20 mbsf: CH 4 + SO 42 - + 2 OH- = CO 32 - + S= + 3 H 2 O Methane is supplied by the deep upwelling fluid. Summary: Composition of end-member deep-slab derived fluid: Mariana Forearc: 10 sites on 9 seamounts Shinkai-6500 Dive 351, Nov. 1996, summit of S. Chamorro Seamount: Mussels, whelks, crabs, tubeworms, and carbonate crusts on serpentinite Mariana Forearc: 10 sites on 9 seamounts: Sites closer to trench are in cool colors; sites farther from trench are in hot colors. p. H = 13. 1 OH- = 45 +/- 8 DIC = 7 +/- 3 CH 4 = >44 mmol/kg C comes off the subducting crust as carbonate, which is then reduced to methane by reaction with H 2 produced during serpentinization: CO 32 - --Depth to the top of the subducting Pacific Plate = 15 -29 km --P, T = 5 -9 kbars and ~100 -300°C (? ) --Alkalinity, sulfate, Na/Cl, K, Rb, Cs, B increase. --p. H (at 25 o. C) increases abruptly from 10. 7 to 12. 5. --Ca, Sr decrease as p. H and alkalinity increase. Na, and K are lower than in seawater near the trench but higher than in seawater farther from the trench. Mariana pore waters + 4 H 2 = CH 4 + H 2 O + This reaction exchanges carbonate alkalinity for hydroxyl alkalinity, causing p. H (at 25 o. C) to rise from 10. 7 in springs close to the trench to 12. 5 in springs >70 km from the trench. With distance from the trench of 50 -95 km: In the deep upwelling water: Epidote sucks up the Ca! Most of the alkalinity is OH-; most of the C is methane! 2 OH- Prove it! Non-accretionary subduction zones are optimal for sampling fluids from the subducting slab: -They lack an accretionary prism, which generates its own fluids and reacts with deeper, slab-derived fluids. -They provide a simple medium for ascent of fluids to the seafloor: depleted harzburgite of the overlying forearc mantle. The System Driver! Why? Carbon is the key: dissolution of carbonate from the subducting plate! Does carbonate suddenly dissolve from the subducting crust? --can be used to estimate T! Conclusions: Composition of low-chlorinity springs on serpentinite mud volcanoes across the Mariana Forearc varies systematically with distance from the trench: --Near the trench, springs have p. H 10. 7, low alkalinity, and high Ca and Sr. Farther from the trench, springs have p. H 12. 5, high alkalinity, and low Ca and Sr, because dissolution of carbonate has joined dehydration as a major process at the top of the subducting Pacific Plate. --Chemical trends are driven by replacement of lawsonite by epidote with increasing T. Uptake of Ca into epidote allows for extensive dissolution of Ca. CO 3 from the source rocks. --Pyrite dissolves over a similar T range, such that both CO 32 - and SO 42 - join chloride as major anions. -- CO 32 - is reduced to CH 4 during ascent through the mantle wedge, and traded for OH-. --This produces the observed trends of increasing p. H, alkalinity, sulfate, Na/Cl (and K, Rb, Cs, B) and decreasing Ca and Sr with distance from the trench and depth and T at the top of the subducting plate.