Largest are 50 km across

1
Decarbonation, Serpentinization, Abiogenic
Methane, and Extreme pH beneath the Mariana
Forearc M.J. Mottl, C.G. Wheat, and P. Fryer
What is the nature of the source rocks and what
reactions are they undergoing?
ODP Leg 195, Site 1200, summit of South Chamorro
Seamount
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 CaCO3 chimneys Low CH4 High
CH4 pH 10.7 (11.1 at 2C) pH 12.5 (13.1 at
2C)
Distance from spring 7 m 20 m 80 m
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.
CO32- 4H2 CH4 H2O 2OH-
No S High S No microbial activity(?)
Microbial activity No macrofauna Macrofauna
vs. seawater 1.3 x
vs. seawater 0.94 x 1.3 x
1.9 x 8.0 x 7.8 x
0.94 x
1.9 x
8.0 x
7.8 x
CH4 SO42- 2OH- CO32- S 3H2O
Mariana Forearc Seamounts Serpentinite Mud
Volcanoes!
One-dimensional advection (upwelling)-diffusion.
Upwelling freshened fluid is enriched
in Na, K, Rb, B.
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!
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.
Cold springs with chimneys
300oC
150oC
Largest are 50 km across and 3 km high.
pH 12.5!
Deep upwelling water (at 2oC, from PHREEQC)
CH4 gt44 mmol/kg
DIC 7 /- 3
OH- 45 /- 8
The System Driver! Epidote sucks up the Ca!
pH 13.1
Anaerobic Oxidation of Methane by Archaea in the
upper 20 mbsf CH4 SO42- 2OH- CO32-
S 3H2O Methane is supplied by the deep
upwelling fluid.
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 H2 produced during
serpentinization CO32- 4H2 CH4 H2O
2OH- This reaction exchanges carbonate
alkalinity for hydroxyl alkalinity,
causing pH (at 25oC) to rise from 10.7 in
springs close to the trench to 12.5 in
springs gt70 km from the trench.
Summary
Composition of end-member deep-slab derived
fluid
Mariana pore waters
Mariana Forearc 10 sites on 9 seamounts
--can be used to estimate T!
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.
Prove it! Does carbonate suddenly dissolve
from the subducting crust?
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.
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
100-300C (?) --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. Why? Carbon is the
key dissolution of carbonate from the
subducting plate!
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.