Presentation on theme: "Largest are 50 km across and 3 km high. Shinkai-6500 Dive 351, Nov. 1996, summit of S. Chamorro Seamount: Mussels, whelks, crabs, tubeworms, and carbonate."— Presentation transcript:
Largest are 50 km across and 3 km high. Shinkai-6500 Dive 351, Nov. 1996, summit of S. Chamorro Seamount: Mussels, whelks, crabs, tubeworms, and carbonate crusts on serpentinite Decarbonation, Serpentinization, Abiogenic Methane, and Extreme pH beneath the Mariana Forearc M.J. Mottl, C.G. Wheat, and P. Fryer 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) Ten sites on nine seamounts yielded upwelling pore water that is fresher than seawater! 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. vs. seawater: 1.3 x 0.94 x1.9 x8.0 x7.8 x Distance from spring: 7 m 20 m 80 m ODP Leg 195, Site 1200, summit of South Chamorro Seamount Anaerobic Oxidation of Methane by Archaea in the upper 20 mbsf: CH 4 + SO 4 2- + 2OH - = CO 3 2- + S = + 3H 2 O Methane is supplied by the deep upwelling fluid. One-dimensional advection (upwelling)-diffusion. Upwelling freshened fluid is enriched in Na, K, Rb, B. Mariana Forearc: 10 sites on 9 seamounts vs. seawater: 0.94 x 1.3 x 1.9 x 8.0 x 7.8 x pH 12.5! Sites closer to trench are in cool colors; sites farther from trench are in hot colors. In the deep upwelling water: Na, and K are lower than in seawater near the trench but higher than in seawater farther from the trench. Summary: Composition of end-member deep-slab derived fluid: 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 pH 10.7, low alkalinity, and high Ca and Sr. Farther from the trench, springs have pH 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 CaCO 3 from the source rocks. --Pyrite dissolves over a similar T range, such that both CO 3 2- and SO 4 2- join chloride as major anions. -- CO 3 2- is reduced to CH 4 during ascent through the mantle wedge, and traded for OH -. --This produces the observed trends of increasing pH, 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. Cold springs with chimneys With distance from the trench of 50-95 km: --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. --pH (at 25 o C) increases abruptly from 10.7 to 12.5. --Ca, Sr decrease as pH and alkalinity increase. Why? Carbon is the key: dissolution of carbonate from the subducting plate! pH = 13.1 DIC = 7 +/- 3 CH 4 = >44 mmol/kg OH - = 45 +/- 8 Most of the alkalinity is OH - ; most of the C is methane! C comes off the subducting crust as carbonate, which is then reduced to methane by reaction with H 2 produced during serpentinization: CO 3 2- + 4H 2 = CH 4 + H 2 O + 2OH - This reaction exchanges carbonate alkalinity for hydroxyl alkalinity, causing pH (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. Mariana Forearc Seamounts: Serpentinite Mud Volcanoes! Mariana Forearc: 10 sites on 9 seamounts: Character of springs with distance from trench: Near (50-65 km)Far (70-90 km) No C source:C source: Low alkalinity High alkalinity High Ca, Sr Low Ca, Sr Brucite chimneys CaCO 3 chimneys Low CH 4 High CH 4 pH 10.7 (11.1 at 2°C) pH 12.5 (13.1 at 2°C) CO 3 2- + 4H 2 = CH 4 + H 2 O + 2OH - No S = High S = No microbial activity(?) Microbial activity No macrofauna Macrofauna CH 4 + SO 4 2- + 2OH - = CO 3 2- + S = + 3H 2 O Deep upwelling water (at 2 o C, from PHREEQC): Prove it! Does carbonate suddenly dissolve from the subducting crust? 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) 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. Equilibrium with Qtz + Ab at 5 kbar (Bowers et al., 1984). Solution composition is fixed by Qtz + Ab + Parag + Musc + Laws or Epid + Chlorite or Talc or Tremolite. 150 o C 300 o C The System Driver! Epidote sucks up the Ca! Mariana pore waters --can be used to estimate T!