Hydration of Olivine and Earth’s Deep Water

Cycle

IASPEI October 5, 2005

Olivine Hydration and Earth’s Deep Water Cycle

J. R. Smyth, (University of Colorado)

Dan Frost and Fabrizio Nestola (Bayerisches Geoinstitut, Germany)

Financial support from

Alexander von Humboldt Foundation and US National Science Foundation

Oceans cover 71% of the planet’s surface.

71% of the surface

But only 0.025% of the mass

Earth’s Deep Water Cycle

• 0.15 percent H2O by weight in the top 10 km of the descending slab is sufficient to recycle the entire ocean volume once over 4.5 billion years at current subduction rates.

H-cycling: Role of Nominally Anhydrous Phases• Synthesis Experiments

– Olivine – Wadsleyite (Spinelloid III)– Wadsleyite II (Spinelloid IV)– Ringwoodite (Spinel)– Pyroxene

• Structure studies (X-ray, neutron): – Protonation mechanisms – Volume of Hydration

H-cycling: Role of Nominally Anhydrous Phases

• Synthesis Experiments• Effect of H on volume and density• Effects of H on Transition Depths• Effects of H on elastic properties:

– Isothermal Bulk Modulus– P and S velocities

• Brillouin• Ultrasonic

Nominally Hydrous

Brucite

Phase A

Chondrodite

Clinohumite

Nominally Anhydrous

Periclase

Olivine

Clinoenstatite

Stishovite

Synthesis Experiments: 5000 ton Press

3.5 mm

Olivine Fo100 : 12 GPa @ 1250°C (With Clinoenstatite)

~8000 ppmw H2O

Ab

sorb

an

ce (

cm-1)

E // a

E // c

E // b

Hydration of Olivine

• Natural olivine contains less than ~0.03 wt % H2O (300 ppm)

• H increases sharply with pressure • 5000 to ~9000 ppm @ 12 GPa • Silica activity has minimal effect on Fe-

free forsterite. May have some effect on Fo95 compositions.

Hydration of Forsterite@ 12GPa FTIR results (ppmw H2O)

1100º 1250º 1400º 1600º

Si-XS 5770 8000 3400 1000

Mg-XS 5560 8800 4400

Hydration of Olivine @ 12GPa

• We observe roughly equivalent amounts in equilibrium with enstatite or clinohumite.

• We observe ~20% more H at 1100ºC than previous workers (Kohlstedt et al., 1996).

• We observe ~ 2.5 x more at 1250ºC than

previous workers at 1100ºC.

Hydration of Olivine @ 12GPa

• We observe a measurable effect of hydration on unit cell volume.

• Structure refinements from X-ray single crystal data and FTIR spectra are consistent with M1 site vacancy as the principal hydration mechanism.

»2H+ <=> Mg2+

Cell Volume vs H content

Hydration of Olivine @ 12GPa

• V (Å3) = 290.107 + 5.5 x 10-5 * H2O

• 8000 ppmw H2O in olivine has same effect on density as ~400ºC temperature rise.

• Hydration is the third state variable in the equation of state of mantle minerals.

2 Olivines with quartz in DAC @ 4.4 GPa

250m

Si-Excess

5000 ppmw H2O

Ko = 122 GPa

K’ = 5.8

Dry Olivine Ko = 129 GPa K’ = ~5

Mg-Excess

8000 ppmw H2O

Ko = 120 GPa

K’ = 7.0

Hydration of Olivine

Effect of Temperature:– Maximum H at ~1250°C – decreases above 1250º due to melting

• Effect of Pressure:– Increases with pressure to 12 GPa (410km)

• Effect of Silica Activity:– Minimal effect if any

Hydration of Olivine

• H becomes compatible at P > 10 GPa

• Hydration causes increase in cell volume.

• Hydration causes decreased bulk modulus and seismic velocities.

Earth’s Deep Water Cycle

• Pressure strongly stabilizes H in cation vacancies.

• Decreases K and hence – Vp and Vs (no measurements on olivine)

• In ringwoodite, the effect of hydration on velocity is larger than effect of temperature!

• H in nominally anhydrous high pressure silicates may constitute the largest reservoir of water in the planet.

• This ‘water’ may have profoundly affected the geology of the planet.

• Dehydration melting of olivine may control melt generation in the upper mantle.

Earth’s Deep Water Cycle

Earth’s Deep Water Cycle

• Below 200km H is captured by olivine in the overlying mantle wedge.

• H softens the olivine allowing it to lubricate the descent of the cold slab

• H is thus effectively entrained in the descending slab.

Interior Reservoir?

• Nominally Anhydrous Minerals (NAMs) can incorporate up to 9 times the total Ocean mass if saturated.

Water:The Third State

Variable in Mantle Dynamics

Reprints and preprints at

http://ruby.colorado.edu/~smyth/Home.html

smyth@colorado.edu

Hydration of Olivine

Nominally Anhydrous Minerals

• Water in the mantle appears to be controlled by H solubility in the nominally anhydrous minerals– Olivine– Wadsleyite – Ringwoodite – Garnet – Pyroxene (Cpx, Opx, Clen)– Perovskite

Olivine : 12 GPa @ 1250°C (With Clinohumite)

~8800 ppmw H2O

Ab

sorb

an

ce (

cm-1)

Compression ExperimentDiamond Anvil Cell

Single-crystal X-ray Diffraction

Nominally Hydrous

Brucite

Phase A

Norbergite

Chondrodite

Humite

Clinohumite

ShyB

Phase B

Nominally Anhydrous

Periclase

Anhydrous B

Olivine

Wadsleyite

Wadsleyite II

Ringwoodite

Clinoenstatite

Stishovite

Ringwoodite (spinel) (-Mg2SiO4):

Radiative Heat Transfer

• -Mg1.63Fe0.22

H0.4 Si0.95O4)• ~10 % of Fe

present as ferric (Mössbauer)

• Dark blue color

Ringwoodite 1 bar

0

50

100

150

200

250

300

350

400

450

500

0 5000 10000 15000 20000 25000 30000

Wavenumber (1/cm)

Abs

orpt

ion

Coe

ffic

ient

(1/c

m)

Ringwoodite

Ringwoodite absorbance vs. Blackbody emission

Ringwoodite @ P

Radiative Heat Transfer

• IR / Visible spectra of ringwoodite show that radiative term cannot be ignored in thermal conduction.

• The transparency of ringwoodite in the radiative transfer region increases with pressure.

Reprints and preprints at

http://ruby.colorado.edu/~smyth/Home.html

smyth@colorado.edu

Water:The Third State

Variable in Mantle Dynamics

CU September 19, 2005

Shear Wave Velocity Structure

4.4

4.6

4.8

5

5.2

5.4

5.6

5.8

300 350 400 450 500 550 600 650

Depth (km)

Sh

ea

r W

ave

Ve

loci

ty (

km/s

)

Hydration of Wadsleyite and Ringwooditeis more consistentwith model shearvelocity structure

Lateral Velocity Variations in TZ May Reflect Hydration

• Red means Wet• Blue means Dry

A hydrous TZ should be thick and slow

• There has been running water (implying oceans) as far back as we can see in Earth history.

• Water dominates the geology and biology of the planet.

• Water also controls the interior processes.

Earth’s Deep Water Cycle

Earth’s Deep Water Cycle

• Water fluxes melting at ridges.– MORBs contain 0.1 to 0.3 wt % H2O

– OIBs contain 0.6 to 1.5 wt % H2O

• Plates hydrate on ocean floor.• Plates dehydrate on subduction.

– But do they dehydrate completely?

Are the oceans just the tip of the iceberg?