Serpentinization, AbiogenicMethane, and ExtremophilicArchaea within the Seafloor

Michael J. MottlDepartment of OceanographyNAI 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 EARTHwt.%

Fe 36.0

O 28.7

Mg 14.8

Si 13.6

Subtotal: 93.1%

85 ± 4 % is in the core.

(Upper) Mantle:

(Mg,Fe)2SiO4 = olivine

(Mg,Fe)SiO3 = pyroxene

Ni 2.0 Ca 1.7 S 1.7 Al 1.3

TOTAL: 98-99%

Ordinary chondritic meteorite: 46% olivine 24% pyroxene (mantle) 11% plagioclase feldspar (Ca, Al) 8% metal 7% FeS (core)What happens when we add H2O?

Serpentinization: 0-500°CREACTANTS Mg:Si ratio(Mg end-members)

Forsterite: Mg2SiO4 2 : 1 (olivine) = HarzburgiteEnstatite: MgSiO3 1 : 1 (depleted) (orthopyroxene)

Diopside: CaMgSi2O6 (clinopyroxene)

PRODUCTS Serpentine Mg3Si2O5(OH)4 1.5 : 1

Brucite Mg(OH)2

Talc Mg3Si4O10(OH)2 0.75 : 1

Chlorite (Mg, Al, Fe)3(Si, Al)2O5(OH)4 (analog to serpentine)Smectite [(Ca,Na)(Al,Mg,Fe)]3(Al,Si)4O10(OH)2 (analog to talc)

Serpentinization (hydration) Reactions

Olivine to Serpentine ( + Brucite):

2Mg2SiO4 + 3H2O = Mg3Si2O5(OH)4 + Mg(OH)2

Enstatite to Serpentine ( + Talc):

6MgSiO3 + 3H2O = Mg3Si2O5(OH)4 + Mg3Si4O10(OH)2

Olivine + Enstatite to Serpentine:

Mg2SiO4 + MgSiO3 + 2H2O = Mg3Si2O5(OH)4

Addition of ~10 mol% Fe produces Magnetite and H2: Fayalite (Fe end-member olivine): (Fe3O4 = Magnetite = FeO . Fe2O3)

3Fe2SiO4 + 2H2O = 2Fe3O4 + 3SiO2(aq) + 2H2

Hydrocarbon production: 4H2 + CO2 = CH4 + 2H2O (Fischer-Tropsch rxn) (at high pH: 4H2 + CO3

2- = CH4 + H2O + 2OH-)

Origin of CH4 on Earth via:Biogenic processes:

--thermal breakdown of complex C compoundsin buried organic matter (“thermogenesis”)2 “CH2O” => CH4 + CO2

--microbial production (“methanogenesis”)CO2 + 4H2 = CH4 + 2H2OCH3COOH => CH4 + CO2

Abiogenic processes:--outgassing of juvenile CH4 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 fuelsfrom inorganic precursors.

--CO2 and CO reduced to CH4 by reaction with H2 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

“Abiogenic processesof CH4 production may be considerably more widespread thanpreviously anticipated.”

“Serpentinization and the hydration of ultra-mafic rocks are the proposed mechanismsfor CH4 and H2production . . .”

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 thestart 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):

Showed that Berndt et al. (1996) had misinterpreted their results,and that formate is formed rapidly rather than CH4 .

CO2 + olivine at 300°C, 350 bars

“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, productionof hydrocarbons was not observed in any of the experiments, except for traceamounts of methane, despite high concentrations of formate and strongly reducingconditions.”

Foustoukosand Seyfried(2004), Science304:1002-1005

Bonified Fischer-Tropsch synthesis of methane, ethane, and propanein aqueous solution, using an Fe-Cr catalyst (chromite).

CONCLUSION:There is no doubt that the Fischer-Tropsch reactions takes placein aqueous solution at elevated T and in the presence of metalliccatalysts, 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)

Largest are 50 km across and 2-3 km high.

Cold Springs with chimneys

Conical Seamount, 90 km behind the Mariana Trench axis at 19°32.5’N,drilled in 1989 on ODP Leg 125

Chimneys of CaCO3(calcite and aragonite)

(Chimneys at sites closer to the trench are brucite!)

Blocks of variably serpentinizedharzburgite suspended in serpentinite mud

Shinkai-6500 Dive 351, Nov. 1996, summit of S. Chamorro Seamount

Shinkai 351 Vent Field, summit of South Chamorro Seamount, 2003

vs. seawater: 1.3 x 1.9 x 8.0 x 7.8 x

Distance from spring: 7 m 20 m 80 m

0.94 x

One-dimensional advection (upwelling)-diffusion

ODP Leg 195, Site 1200, summit of South Chamorro Seamount

Distance from spring: 7 m 20 m 80 m

Anaerobic Methane Oxidation: CH4 + SO42- + 2OH- = CO3

2- + S= + 3H2O

pH 12.5!

Equilibrium of brucitewith serpentine at lowtemperature keepsdissolved Mg and SiO2 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 serpentinizationand methane generation at high pH:

4H2 + CO32- = CH4 + H2O + 2OH-

This reaction converts carbonate alkalinity to hydroxyl alkalinity, thus raising pH. It occurs where there is an external source of carbon.

2003: 75 gravity cores from 13 sites on 12 mud volcanoes

Mariana forearc:serpentinite mud volcanoes

1989: ODP drillingLeg 125, Site 780

1996: Shinkai-6500 dive

1997: 66 gravity coresfrom 13 siteson 10 mud volcanoes

2001: ODP drillingLeg 195, Site 1200

Na, Na/Cl, and K increase with distance from the trench.

0

10

20

30

40

50

60

70

80

50 55 60 65 70 75 80 85 90 95

Distance from Trench (km)

Alk

'y,

Su

lfa

te,

Ca

, K

(m

mo

l/k

g)

Rb

(µm

ol/

kg

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Na

/Cl,

B,

Sr

(mm

ol/

kg

)

Cs

(µm

ol/

kg

)

Alk'y

Sulfate

Ca

K

Rb

Na/Cl

B

Sr

Cs

Ca

SrK

Rb

B

SO4

Alk'y

Na/Cl

Blue Moon

Pacman Summit

Big Blue

S. Chamorro

Conical

Ca,Cs

Nip

Pacman

BB Mitt

Blip

Baby Blue

Quaker

Sr

Mariana Forearc, 2003: 10 sites on 9 seamounts

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) Far (70-90 km)

No C source: C source:Low alkalinity High alkalinityHigh Ca, Sr Low Ca, SrBrucite chimneys CaCO3 chimneysLow CH4 High CH4pH 10.7 (11.1 at 2°C) pH 12.5 (13.1 at 2°C)

CO32- + 4H2 = CH4 + H2O + 2OH-

No S= High S=

No microbial activity(?) Microbial activityNo macrofauna Macrofauna

CH4 + SO42- + 2OH- = CO3

2- + S= + 3H2O

1. Serpentinization: peridotite + water produces OH- and H2:

2Mg2SiO4 + 3H2O = Mg3Si2O5(OH)4 + Mg(OH)2

3Fe2SiO4 + 2H2O = 2Fe3O4 + 3SiO2(aq) + 2H2

Summary: reactions relevant to extraterrestrial environments:

2. Addition of C (preferably as a flux) produces methane:

CO32- + 4H2 = CH4 + H2O + 2OH-

3. Anaerobic oxidation of methane by Archaea produces sulfide:

CH4 + SO42- + 2OH- = CO3

2- + S= + 3H2O