Serpentinization, Abiogenic Methane, and Extremophilic Archaea within the Seafloor Michael J. Mottl Department of Oceanography NAI Winter School, Jan. 2005
Earth s mantle is 68% of its mass and 83% of its volume. It is made of ultramafic rock: peridotite.
BULK COMPOSITION OF THE EARTH wt.% Fe 36.0 85 ± 4 % is in the core. O 28.7 Mg 14.8 Si 13.6 Subtotal: 93.1% Ni 2.0 Ca 1.7 S 1.7 Al 1.3 TOTAL: 98-99% (Upper) Mantle: (Mg,Fe) 2 SiO 4 = olivine (Mg,Fe)SiO 3 = pyroxene Ordinary chondritic meteorite: 46% olivine 24% pyroxene (mantle) 11% plagioclase feldspar (Ca, Al) 8% metal 7% FeS (core) What happens when we add H 2 O?
Serpentinization: 0-500 C REACTANTS Mg:Si ratio (Mg end-members) Forsterite: Mg 2 SiO 4 2 : 1 (olivine) = Harzburgite Enstatite: MgSiO 3 1 : 1 (depleted) (orthopyroxene) Diopside: CaMgSi 2 O 6 (clinopyroxene) PRODUCTS Serpentine Mg 3 Si 2 O 5 (OH) 4 1.5 : 1 Brucite Mg(OH) 2 Talc Mg 3 Si 4 O 10 (OH) 2 0.75 : 1 Chlorite (Mg, Al, Fe) 3 (Si, Al) 2 O 5 (OH) 4 (analog to serpentine) Smectite [(Ca,Na)(Al,Mg,Fe)] 3 (Al,Si) 4 O 10 (OH) 2 (analog to talc)
Serpentinization (hydration) Reactions Olivine to Serpentine ( + Brucite): 2Mg 2 SiO 4 + 3H 2 O = Mg 3 Si 2 O 5 (OH) 4 + Mg(OH) 2 Enstatite to Serpentine ( + Talc): 6MgSiO 3 + 3H 2 O = Mg 3 Si 2 O 5 (OH) 4 + Mg 3 Si 4 O 10 (OH) 2 Olivine + Enstatite to Serpentine: Mg 2 SiO 4 + MgSiO 3 + 2H 2 O = Mg 3 Si 2 O 5 (OH) 4 Addition of ~10 mol% Fe produces Magnetite and H 2 : Fayalite (Fe end-member olivine): (Fe 3 O 4 = Magnetite = FeO. Fe 2 O 3 ) 3Fe 2 SiO 4 + 2H 2 O = 2Fe 3 O 4 + 3SiO 2 (aq) + 2H 2 Hydrocarbon production: 4H 2 + CO 2 = CH 4 + 2H 2 O (Fischer-Tropsch rxn) (at high ph: 4H 2 + CO 3 2- = CH 4 + H 2 O + 2OH - )
Origin of CH 4 on Earth via: Biogenic processes: --thermal breakdown of complex C compounds in buried organic matter ( thermogenesis ) 2 CH 2 O => CH 4 + CO 2 --microbial production ( methanogenesis ) Abiogenic processes: CO 2 + 4H 2 = CH 4 + 2H 2 O CH 3 COOH => CH 4 + CO 2 --outgassing of juvenile CH 4 from the mantle --inorganic synthesis at high T (Fischer-Tropsch rxn)
Fischer-Tropsch process: Fischer F. (1935) Die synthese der Treibstoffe (Kogaisin) und Schmierole aus Kohlenoxyd und Wasserstoff bei Gewohnlichem Druck. Brennstoff-Chemie 16:1-11. --developed by the Germans in ~1940 to produce hydrocarbon fuels from inorganic precursors. --CO 2 and CO reduced to CH 4 by reaction with H 2 at 200-250 C and high pressure in the gaseous phase --Became one of three leading hypotheses for the origin of organic matter in meteorites. The other two are: --origin during accretion --Miller-Urey process (electric discharges and UV).
Sherwood Lollar et al. (1993) Rocks from the Canadian and Fennoscandian shields Serpentinization and the hydration of ultramafic rocks are the proposed mechanisms for CH 4 and H 2 production... Abiogenic processes of CH 4 production may be considerably more widespread than previously anticipated.
Berndt et al. (1996) Geology 24:351-354 First report of experimental Fischer-Tropsch synthesis of methane, ethane, and propane in aqueous solution (rather than in the gas phase). Shown to be bogus by McCollum and Seewald (2001): dissolved hydrocarbons were already present in olivine at the start of the experiments!
Horita and Berndt (1999) Science 285:1055-1057 Bonified Fischer-Tropsch synthesis of CH4 (but no C-2 or C-3 HC s) in aqueous solution, using a Ni-Fe catalyst. Abiogenic methane may be more widespread than previously thought.
McCollum and Seewald (2001): CO 2 + olivine at 300 C, 350 bars Showed that Berndt et al. (1996) had misinterpreted their results, and that formate is formed rapidly rather than CH 4. Abiotic formation of hydrocarbons during serpentinization may be much more limited than previously believed...
McCollum and Seewald (2003) Although formate has been suggested to be a reaction intermediate in the formation of abiotic hydrocarbons from reduction of aqueous CO2, production of hydrocarbons was not observed in any of the experiments, except for trace amounts of methane, despite high concentrations of formate and strongly reducing conditions.
Foustoukos and Seyfried (2004), Science 304:1002-1005 Bonified Fischer-Tropsch synthesis of methane, ethane, and propane in aqueous solution, using an Fe-Cr catalyst (chromite). CONCLUSION: There is no doubt that the Fischer-Tropsch reactions takes place in aqueous solution at elevated T and in the presence of metallic catalysts, which are common in peridotite.
Mariana subduction zone
Mariana subduction system, Western Pacific Ocean: remnant arc, backarc basin, active arc, forearc, trench, Pacific Plate
e.g., Mariana
Outer half of Mariana forearc (Fryer et al., 1999, Geology 27:103-106)
Cold Springs with chimneys Largest are 50 km across and 2-3 km high.
Chimneys of CaCO 3 (calcite and aragonite) (Chimneys at sites closer to the trench are brucite!) Blocks of variably serpentinized harzburgite suspended in serpentinite mud Conical Seamount, 90 km behind the Mariana Trench axis at 19 32.5 N, drilled in 1989 on ODP Leg 125
Shinkai-6500 Dive 351, Nov. 1996, summit of S. Chamorro Seamount
Shinkai 351 Vent Field, summit of South Chamorro Seamount, 2003
ODP Leg 195, Site 1200, summit of South Chamorro Seamount Distance from spring: 7 m 20 m 80 m vs. seawater: 0.94 x 1.3 x 1.9 x 8.0 x 7.8 x One-dimensional advection (upwelling)-diffusion
Distance from spring: 7 m 20 m 80 m ph 12.5! Anaerobic Methane Oxidation: CH 4 + SO 4 2- + 2OH - = CO 3 2- + S = + 3H 2 O
Equilibrium of brucite with serpentine at low temperature keeps dissolved Mg and SiO 2 low and ph high: ~10.7 (11.1 at 2 C), as observed in springs near the trench.
The higher ph of 12.5 (13.1 at 2 C) for springs farther (>70 km) from the trench results from serpentinization and methane generation at high ph: 4H 2 + CO 3 2- = CH 4 + H 2 O + 2OH - This reaction converts carbonate alkalinity to hydroxyl alkalinity, thus raising ph. It occurs where there is an external source of carbon.
Mariana forearc: serpentinite mud volcanoes 1989: ODP drilling Leg 125, Site 780 1996: Shinkai-6500 dive 1997: 66 gravity cores from 13 sites on 10 mud volcanoes 2001: ODP drilling Leg 195, Site 1200 2003: 75 gravity cores from 13 sites on 12 mud volcanoes
Na, Na/Cl, and K increase with distance from the trench.
Alk'y, Sulfate, Ca, K (mmol/kg) Rb (µmol/kg) Nip 80 Pacman BB Mitt 70 Blue Moon 60 50 40 30 20 10 0 Ca Sr Mariana Forearc, 2003: 10 sites on 9 seamounts Blip Pacman Summit Quaker Baby Blue A lk'y Sulfate Ca K Rb Na/Cl B Sr Cs Big Blue 50 55 60 65 70 75 80 85 90 95 Distance from Trench (km) S. Chamorro Conical Cs B Alk'y SO 4 Na/Cl K Rb Ca, Sr 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Na/Cl, B, Sr (mmol/kg) Cs (µmol/kg) 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 --Ca, Sr decrease Why? Carbon!
Character of springs with distance from trench: Near (50-65 km) No C source: Far (70-90 km) 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
Summary: reactions relevant to extraterrestrial environments: 1. Serpentinization: peridotite + water produces OH - and H 2 : 2Mg 2 SiO 4 + 3H 2 O = Mg 3 Si 2 O 5 (OH) 4 + Mg(OH) 2 3Fe 2 SiO 4 + 2H 2 O = 2Fe 3 O 4 + 3SiO 2 (aq) + 2H 2 2. Addition of C (preferably as a flux) produces methane: CO 3 2- + 4H 2 = CH 4 + H 2 O + 2OH - 3. Anaerobic oxidation of methane by Archaea produces sulfide: CH 4 + SO 4 2- + 2OH - = CO 3 2- + S = + 3H 2 O