Geology of the Oceanic Crust asa Setting for Chemoautotrophy

Dr. Michael J. Mottl, Dept. of Oceanography, University of Hawaii

NASA Astrobiology InstituteWinter School, January, 2005

Interaction between seawater and oceanic basement is important for:

1. Heat removal from the solid Earth2. Evolution of the oceanic crust:

changes in mineralogy, chemistry, and physical propertieswith age

3. Geochemical mass balances4. Deposition of metalliferous deposits,

including ferromanganese nodules and crusts and polymetallic sulfides

5. Support of biological communities, including both microbes and macrofauna

6. Concentrations and distribution of minor and trace chemicalspecies in the oceans

7. Vertical mixing of mid-depth waters, and possibly deep-ocean circulation itself.

Geologic settings for chemoautotrophy in the seafloor:

1. Mid-ocean ridge axis: hot springs and hydrothermal plumes--represents ~80% of all magmatic activity in the oceans

2. Mid-ocean ridge flanks: warm springs3. Subduction zones:

a. Forearc: mud volcanoes, gas hydrate settingsb. Arc volcanoes (~10%): hot springs and plumesc. Backarc spreading: hot springs and plumes

4. Hot spot volcanoes (~10%; e.g., Hawaii, Iceland):hot springs and plumes

1. and 3. have both basalt and peridotite as source rocks.

Warm hydrothermal vents were discoveredin 1977 on the Galapagos Rift near 86oWusing bottom cameras and the Alvin manned submersible.

The vents ranged in temperature from 6-17oC, vs. an ambient T of 1.7oC.

[Note: Figures have been removed to reduce download time.]

Clams and mussels on pillow basalts

Enteropneusts. No biologist was onboard;the geologists called them “spaghetti”.

Serpulid (“featherduster”) worms

[Note: Figures have been removed to reduce download time.]

The most unusual and spectacular of the macrofauna discoveredat the Galapagos Rift in 1977 was the giant tubeworm Riftia pachyptila, of the class Pogonophora, up to 3 m in length.These animals survive via a symbiotic relationship with chemosynthetic bacteria that live within their tissues.

[Note: Figures have been removed to reduce download time.]

Two years later, in 1979,hot springs were foundon the East Pacific Risenear 21oN.

These hot (350oC) springswere immediately christened“black smoker chimneys”.

Both the smoke and the chimneysare composed of polymetallicsulfide minerals, chiefly pyrrhotite (FeS). pyrite (FeS2),chalcopyrite (CuFeS2) and sphalerite or wurtzite (ZnS).

Note the white Brachyurancrab, peculiar to the ventfields.

[Note: Figures have been removed to reduce download time.]

The crenulations on and withinthe chimneys are produced by deposition around polychaete worms of the family Alvinellidae that live within the sulfide chimneys.

The large rusty flocs in the plume are mainly bacterial mat,presumably broken free fromsurfaces beneath the seafloor.

[Note: Figures have been removed to reduce download time.]

The whitemineral in thechimney isanhydrite, CaSO4,which precipitateson mixing of thehot water withcold seawater.

The black mineral in the smoke is mainlypyrrhotite, FeS.

[Note: Figures have been removed to reduce download time.]

Interior of a chimney wallthat had been in contact with350oC water. The brassy mineral is chalcopyrite, CuFeS2.

(The hand is that of famousoceanographer and explorerDr. Robert Ballard, discovererof Titanic, Bismarck, and Yorktown.)

[Note: Figures have been removed to reduce download time.]

What is the nature of oceanic crust formed by seafloor spreading?1950’s: layered, as defined by seismic refraction:

Layer 1 = sedimentLayer 2 = “basaltic layer” (also 2A,B,C) = flows, dikesLayer 3 = “oceanic layer” = gabbro and metagabbro

Much of what we know in detailabout oceanic crust comes fromsequences of rocks calledOphiolites: slices of oceaniccrust that have been obducted onto land rather than subducted at a deep ocean trench.

Moho

Ophiolites such as that in Omanshow a characteristic crustal sequence:1) Pillow basalts are a typical

flow form under water.2) Dikes are fractures filled

with solidified magma.3) Gabbro represents a magma

chamber that has solidified.

Pillow basalt flows on the seafloor Sheet flow (less viscous)

Sequence of pillows in OmanDike intruding pillows

Sheeted dikes in Oman(hydrothermally altered)

Each near-vertical dike is intruded into previously intruded dikes—usually rightup the middle!

Sheeted dikes solidify from magma thatis feeding the flows above from the magma lens beneath.

Layered gabbro from the Oman ophiolite

Layers are alternatelyrich in plagioclase feldspar(white) or pyroxene (dark).

They have a cumulatetexture, indicating theyformed by crystal settling.

Steady-state formation of oceanic crust along the mid-ocean ridge:the “infinite onion” model (Cann, 1974)

Layer 2/3 boundary = dikes/gabbro?

Mohorivicicdiscontinuity:base of the crust

In this model most of the 6-7 km-thick crust is molten at the axis.

Crystal settlingforms layered gabbro.

Freezing formsisotropic gabbro.

Multichannel seismic reflection profile across the East Pacific Riseaxis at 9o30’N (Detrick et al., 1987)

Top of axial magma chamber (or lens)

Moho

Crust is already mature within a few km of the axis even at fast spreading rates! Magma chamber is small (and thin?)

A note on spreading rates:Slow-spreading ~1 cm/yr (2 cm/yr total opening rate)

Fast-spreading to 8 cm/yr (16 cm/yr total opening rate)

Dikes are the quantum event of seafloor spreading.Typical dikes are ~70 cm wide.

Slow ~ 1 dike every 35 years (only when magma is present)Fast ~ 1 dike every 4 years

At slow spreading rates, a magma chamber would cool toorapidly by conduction alone to be maintained at steady state.

Magma is supplied episodically but spreading is continuous,resulting in periods of amagmatic spreading.

Hydrothermal venting is less frequent at slow spreading rates.

Models of magma chamber based on multichannel seismics

Detrick et al. (1987)

Macdonald (1989)

Current models for oceanic crustal formation I

The Gabbro Glacier model: two versions:

a. Magma lens at the top; “g. glacier” flows downward

b. Magma lens at top and base of crust;“g. glacier” flows upward and downward

How do you form 6-7 km of crust, most of it gabbro, from a magma lens that is only a few hundred meters thick?

Current models for oceanic crustal formation II

c. Gabbro glacier combined with multiple sill intrusion

d. The Multiple Sill model (Some aspects are typical of slow-spreading crust,

except that such crust would lack a steady-state magma chamber.)

Note that all of these models can form both isotropic and layered gabbro.

Why is this relevant? Because hydrothermal cooling is an integral part of crustal formation!

In this typical cartoon,hydrothermal circulationoccurs only in shallowlevels of the crust, above the magma chamber.

Such circulation cools theflows and dikes of Layer 2,but can remove only thelatent heat of crystallizationfrom Layer 3 (which comprises most of the crust!)

Models for hydrothermal circulation at the MOR axis depend on:

1. Heat source:a. Dikes (too small to run whole system!)b. Magma chamber:

1) Latent heat2) Heat of cooling (= the largest fraction!)

2. Heat transfer mechanisma. Magma convectionb. Penetration of hot rock

3. Depth of circulation:Are the gabbros penetrated? On axis or off?

4. Scale and magnitude of crustal permeability

Stage 1. Magma chamber present,restricted circulation produces modest hydrothermal venting.

Stage 2. Magma chambercrystallized; penetration of hot rock produces vigoroushydrothermal venting.

Lister (1990)

I. Episodic magma chamber: --typical of slow-spreading ridges

such as the Mid-Atlantic Ridge

II. Steady-state magma chamber I: --predicts vigorous off-axis venting

(not observed to date)

III. Steady-state magma chamber II --probably typical of fast-spreading

ridges such as the E. Pacific Rise

Maclennan et al. (2005) Geology

Distance from ridge axis (0 to 20 km)

Spreading rate = 55 mm/yr as at EPR 9oN

Gabbro Glacier model Hybrid model:67% of crust formed as sills in the lower crust

Hydrothermal heat removal (W/m2)

Hydrothermal heat removal (W/m2)

Base of Crust Base of Crust>1200oC >1200oC

500oC500oC

Latest thermal model for a fast-spreading ridge matching heat flow, gravity, and bathymetry

Slow-spreading ridge (Mid-Atlantic Ridge)

Fast-spreading ridge (East Pacific Rise)

Lister (1976)

Jannasch and Mottl (1985)

Warm vents, as atthe Galapagos Rift,are simply hot ventsthat have mixed with seawater in theshallow subsurfacerather than at the seafloor.

-5

0

5

10

15

20

25

0 10 20 30 40 50 60

TAG

Snakepit

Linear (Snakepit)

Linear (TAG)

Mg (mmol/kg)

Seawater

High-T end-member

High-T end-member has no Mg.

Deep Sea Axial Hot Springs

0

50

100

150

200

250

300

350

400

450

500

550

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200T (

oC

) a

nd

lo

we

st

Mg

me

as

ure

d (

mm

ol/

kg

x10)

BSW

GSC

EPR21N

EPR 13N

EPR 11N

EPR 10N

EPR 9N

EPR 17S

JFR End

JFR CoAx

JFR Ax

JFR NCt

JFR SCt

MAR MG

MAR LS

MAR BS

MAR TAG

MARK

SedR Gu

SedR MV

SedR ET

BAB Lau

BAB MarT

BAB OkT

BAB NFB

BAB Man

Suiyo SM

MAR RB

min.Mg

JFR Axial:

ISCA

28oC

EPR 18S

>210oC

ET 217oC

Lowest Mg measured

(mmol/kg x10)

Temperature (oC)

n = 195

from 148 different vents

Seawater Mg

T ranges smoothly from 260-370oC, and overall from 217-403oC.

Based on their high Mg content, vents <217oC deliver water thathas mixed with seawater in the shallow subsurface; their low T’s mainly reflect this mixing.

#

Wolery and Sleep (1976)

Heat flow on young crust is1) much lower than predicted2) highly scattered3) eventually reaches the predicted value.

Sclater et al. (1976)

The extent to which the crust reheatsfollowing its cooling at the axis depends on how fast and uniformly it is sedimented.

Warm hydrothermal circulation canpersist for tens of millions of years,even after the seafloor is sealed.

Warm to cool water will continue to vent until the seafloor is sealed by a thick (>160m) continuous blanket of sediment.

Most (70%) of hydrothermal heat is lost through mid-ocean ridgeflanks rather than at the axis. Because flank T’s are lower, the seawater flux is ~30 to 300 times larger through the flanks than through the axis.

Axis: ~3 TW

Flanks: ~7 TW

DSDP Hole 504B:S. flank of the Costa Rica Rift (Alt et al., 1994)

Cold oxidative

Alteration zones:

Warm reducing

High-T reducing:greenschist facies

(complement to axial hot springs)

~600 m

~300 m

into basement

in 6-Ma crust

Primary igneous phases: minerals and glass

Secondary (alteration) minerals

As seawater penetrates young, hot oceanic crust, the entire chemical system is initially wildly out of equilibrium.The primary igneous phases are unstable and tend to dissolve.Reaction ensues, the primary rock recrystallizes to a secondary(alteration) mineral assemblage, and the aqueous solution tendstoward equilibrium with this secondary assemblage.

The degree of equilibration achieved depends on:1. the reaction rate for: a) primary phases (dissolution)

b) secondary (alteration) phases2. the flow rate of the solution through the system.

Aq. Solution

Water-Rock Reaction

Chemical changes: hot spring waters relative to seawater:

Mg is almost completely taken up by the rock.Ca, K, Si, Fe, Mn, and CO2 are leached from the rock.Na can go either way.

Sulfate is precipitated as anhydrite (CaSO4) and reduced to H2S by reaction with Fe2+.

Seawater is converted to a Na-K-Ca-Cl solution that is saturated with quartz (SiO2) and Fe-sulfide minerals.

Warm springs along mid-ocean ridge flanks:

Major element changes are similar to those above,but generally less extreme.

Few have been discovered and sampled.

Typical seafloor hot springsfall in the supercritical field,at least for P if not T.

Where magmatic activity ispresent at shallow levels the supercritical solution canseparate into vapor andbrine phases.

This phase separation willlook more like condensationof a brine phase than boiling.

System NaCl-H2O at 350-600oC

Where separation has occurred,individual vents tend to emiteither vapor or brine for periodsof months to years.

Summary1. 80% of the magmatic and hydrothermal activity on the seafloor

takes place along the global mid-ocean ridge system.

2. Hydrothermal cooling with accompanying hot springs is anintegral part of formation of the oceanic crust.

3. It is likely that the crust is penetrated by seawater and cooled over most of its 6-7 km depth even at fast-spreading ridges with steady-state magma lenses.

4. Warm springs along the ridge axis result from mixing of hot, altered seawater with cold seawater in shallow crustal levels.

5. Only 30% of hydrothermal power output occurs along the ridge axis; the other 70% occurs on the flanks at much lower T.

Summary (cont.)6. Hydrothermally driven seawater flux through ridge flanks

is 30-300 times larger than that through the ridge axis.

7. Warm springs along ridge flanks have not been widely sampled; their chemical changes appear to be similar to those of axial hot springs only less extreme.

8. Hydrothermal circulation through ridge flanks is probably mainly restricted to the upper few hundred meters of basement and produces an upper oxidized zone and a lower reduced zone.

9. Phase separation into vapor and brine can occur where magmatic activity is present along the axis. Individual vents in these settings emit either vapor or brine for many months to years.