Transcript Document 7901885

Decarbonation, Serpentinization, Abiogenic Methane,
and Extreme pH 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:
C source:
Low alkalinity
High Ca, Sr
Brucite chimneys
Low CH4
pH 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
CaCO3 chimneys
High CH4
pH 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.
CO32- + 4H2 = CH4 + H2O + 2OHvs.
vs.seawater:
seawater:
0.94 x
1.3 x
7.8 x
CH4 + SO42- + 2OH- = CO32- + S= + 3H2O
One-dimensional advection (upwelling)-diffusion.
Mariana Forearc Seamounts:
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
140
80
Ten sites on nine seamounts yielded upwelling
pore water that is fresher than seawater!
Deep upwelling water (at 2oC, from PHREEQC):
In equilibrium with
Albite + Paragonite + Quartz
at 5 kbars
100
Clinozoisite: 223-500oC
80
60
Lawsonite: 0-222oC
+ 3H2O
C comes off the subducting crust as carbonate,
which is then reduced to methane by reaction with
H2 produced during serpentinization:
Methane is supplied by the deep upwelling fluid.
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.
Alk'y, Sulfate, Ca, K (mmol/kg)
Rb (mmol/kg)
Summary:
Nip
80
Pacman
BB Mitt
70
Blue Moon
60
Composition of end-member
deep-slab derived fluid:
CO32-
50
Alk'y
Ca
K
Rb
Ca
Na/Cl
40
B
Sr
30
Cs
Sr
20
10
0
50
55
4
Distance from Trench (km)
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.
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 25oC) increases abruptly from 10.7 to 12.5.
--Ca, Sr decrease as pH and alkalinity increase.
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.
2OH-
+ 4H2 = CH4 + H2O +
Mariana Forearc: 10 sites on 9 seamounts
This
reaction
exchanges
4.0
B
Pacman Summit
carbonate
alkalinity
S. Chamorro
Quaker
3.5
Baby Blue
Conical
for
hydroxyl alkalinity,
Big Blue
3.0
oC)
Alk'y
causing
pH
(at
25
2.5
SO
to
rise from 10.7 in
2.0
springs close to the
Na/Cl 1.5
1.0
trench to 12.5 in
K
0.5
springs
>70
km
from
Rb
Cs Ca, Sr
0.0
the
trench.
60
65
70
75
80
85
90
95
Sulfate
Blip
m
0
Most of the alkalinity is OH-; most of the C is methane!
Why? Carbon is the key:
dissolution of carbonate from the subducting plate!
Prove it!
Does carbonate
suddenly dissolve
from the
subducting crust?
50
100
150
200
250
300
350
Blip
50
Parag out
30
Quaker
20
Big Blue
Conical
100
150
200
SO4 = 30mM
SO4 high = 50 mM
CO3 = 10 mM
CO3 high = 100 mM
QAPMCaMgFeS
QAPMCaMgFeSC
QAPMCaMgFeSChi
8
Q.FeSC.Helgeson
Basalt w/r=1
Basalt w/r=10
6
Alt.MORB w/r=1
4
250
300
350
T (oC)
GEM Selektor: 5 kbars, 0.2M Cl
QAPMCaMg
1.5
QAPMCaMg
QAPMCaMgFe
Basalt w/r=1
Mariana springs
QAPMCa
10
Q.FeSC.Helgeson
Alt.MORB w/r=1
1.6
12
GEM Selektor: 5 kbars, 0.2M Cl
QAPMCaMgFeSChi
40
T (oC)
QAPMCa
QAPMCaMgFeSC
Basalt w/r=10
50
400
Na+/mCl- (mol/mol)
= CO3 +
S=
SO4 = 30mM
SO4 high = 50 mM
CO3 = 10 mM
CO3 high = 100 mM
60
0
m
2-
QAPMCaMgFe
10
Epidote sucks up the Ca!
0
QAPMCaMgFe
SO4 = 30mM
SO4 high = 50 mM
CO3 = 10 mM
CO3 high = 100 mM
QAPMCaMgFeS
1.4
QAPMCaMgFeSC
QAPMCaMgFeSChi
1.3
Mariana springs
Q.FeSC.Helgeson
Basalt w/r=1
1.2
Mariana
pore waters
Basalt w/r=10
Alt.MORB w/r=1 0.686
1.1
1.0
0.9
2
0.8
--can be used to estimate T!
0.7
0
50
100
150
200
250
300
50
350
+ Mt + Goeth
QAPMCaMg
40
+ Mt + Hem
SO4 = 30mM
SO4 high = 50 mM
CO3 = 10 mM
CO3 high = 100 mM
QAPMCa
QAPMCaMgFe
QAPMCaMgFeS
QAPMCaMgFeSC
30
QAPMCaMgFeSChi
20
+ Aragonite
10
+ Daphnite + Geoth + Pyrite
HS-
0
50
100
Pyrite out at 210-220oC
HS- + SO4=
150
150
200
250
300
350
120
All: Qtz + Ab + Parag + Musc + Epid + Talc
50
100
T (oC)
T (oC)
60
Total Sulfate sp. (mmol/kgw)
CH4 + SO4 +
2OH-
The System Driver!
20
K+ (mmol/kgw)
2-
DIC =
7 +/- 3
Total carbonate sp. (mmol/kgw)
Anaerobic Oxidation of Methane by Archaea in the upper 20 mbsf:
pH = 13.1
=
45 +/- 8
CH4 =
>44 mmol/kg
QAPMCaMg
QAPMCaMgFeS
40
OH-
QAPMCa
70
Ca2+ (mmol/kgw)
pH 12.5!
a2Na+/aCa2+
Largest are 50 km across
and 3 km high.
Nip
GEM Selektor: 5 kbars, 0.2M Cl
All data from Bowers et al. (1984)
120
Solution composition is fixed by
Qtz + Ab + Parag + Musc +
Laws or Epid +
Chlorite or Talc or Tremolite.
300oC
150oC
m
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)
8.0
8.0xx
High S=
Microbial activity
Macrofauna
Na/Cl, B, Sr (mmol/kg)
Cs (mmol/kg)
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)
1.9
1.9xx
No S=
No microbial activity(?)
No macrofauna
200
T (oC)
250
All: Qtz + Ab + Parag + Musc + Epid + Talc
100
350
SO4 = 30mM
SO4 high = 50 mM
CO3 = 10 mM
CO3 high = 100 mM
QAPMCa
QAPMCaMg
80
QAPMCaMgFe
QAPMCaMgFeS
Aragonite out at 250-300oC
QAPMCaMgFeSC
QAPMCaMgFeSChi
60
o
Aragonite out at 170-175 C
40
o
Pyrite out at 210-220 C
20
+ Aragonite
+ Daphnite + Geoth + Pyrite
0
300
+ Mt + Hem
+ Mt + Goeth
50
100
150
200
250
300
350
T (oC)
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 CaCO3 from the source rocks.
--Pyrite dissolves over a similar T range, such that both CO32- and SO42- join chloride as major anions.
-- CO32- is reduced to CH4 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.