5, 2005
and Cycle I October IASPE
arth’s Deep Water
E Hydration of Olivine (University of Colorado) Fabrizio Nestola
and
Financial support from
(Bayerisches Geoinstitut, Germany) US National Science Foundation
Olivine Hydration and J. R. Smyth, Dan Frost Alexander von Humboldt Foundation and Earth’s Deep Water Cycle e
f th o
71%
cover
planet’s surface.
ceans
O 71% of the surface
But only 0.025% of the mass e
O by weight in the top 2
s Deep Water Cycl ’ h
Eart 0.15 percent H 10 km of the descending slab is sufficient to recycle the entire ocean volume once over 4.5 billion years at current subduction rates. •
hydrous Phases
An
cycling: Role of -
H
Olivine Wadsleyite (Spinelloid III) Wadsleyite II (Spinelloid IV) Ringwoodite (Spinel) Pyroxene Protonation mechanisms Volume of Hydration – – – – – – – Synthesis Experiments Structure studies (X-ray, neutron): • •
Nominally
hydrous Phases
An
cycling: Role of -
H Brillouin Ultrasonic • • Isothermal Bulk Modulus P and S velocities – – Synthesis Experiments Effect of H on volume and density Effects of H on Transition Depths Effects of H on elastic properties: • • • •
Nominally Nominally Hydrous Nominally Anhydrous Brucite Periclase Phase A Olivine Chondrodite Clinoenstatite Clinohumite Stishovite 3.5 mm
5000 ton Press
Synthesis Experiments:
) (cm Absorbance -1
O 2
H : 12 GPa @ 1250°C
100
b E // c E //
Olivine Fo (With Clinoenstatite) ~8000 ppmw
a E // O (300 ppm) 2
Hydration of Olivine
Natural olivine contains less than ~0.03 wt % H H increases sharply with pressure 5000 to ~9000 ppm @ 12 GPa • • •
1600º 1000 O) 2
H 4400 1400º 3400
ppmw 8800 1250º 8000 5560 5770 1100º
FTIR results (
Si-XS Mg-XS
Hydration of Forsterite@ 12GPa
et al., 1996).
We observe roughly equivalent amounts in equilibrium with enstatite or clinohumite. We observe ~20% more H at 1100ºC than previous workers (Kohlstedt We observe ~ 2.5 x more at 1250ºC than previous workers at 1100ºC. • • • Hydration of Olivine @ 12GPa
2+
<=> Mg
+
2H »
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. • • Hydration of Olivine @ 12GPa
H content s v
Cell Volume
) e t e) t i ey oodi l ne) i gw v te i ads n e l i t i R W O y ( ( ( e oodi l e s d 5000 ngw near near near livin i 2 Li Li Li R Wa O O 20000 2 H
H content s v e m s lu o w) 15000 v m r v p p la
o O ( 2 m e H tiv 10000 la e R 5000 0 1
1.006 1.005 1.004 1.003 1.002 1.001 0.999
e m lu o v r la o m e v i t la e R
Cell Volume
O 2
* H
-5
O in olivine has same effect on 2 O 2 H H
) = 290.107 + 5.5 x 10 H) ∂ 3
V/ ∂ ( H = ppmw – – V (Å 8000 ppmw STP density as ~400ºC temperature rise. Hydration is the third state variable in the equation of state mantle minerals. • • • Hydration of Olivine @ 12GPa t
Diamond Anvil Cell
Compression Experimen
Single-crystal X-ray Diffraction a m µ 250
2 Olivines with quartz in DAC @ 4.4 GP Mg-Excess
8000 ppmw H2O Ko = 120 GPa Si-Excess K’ = 7.0 5000 ppmw H2O Ko = 122 GPa K’ = 5.8
Dry Olivine Ko = 129 GPa K’ = ~5
H content
vs
Isothermal Bulk Modulus due to melting
Hydration of Olivine Maximum H at ~1250°C decreases above 1250º Increases with pressure to 12 GPa (410km) Minimal effect if any – – – – Effect of Temperature: Effect of Pressure: Effect of Silica Activity: • • Hydration of Olivine
H becomes compatible at P > 10 GPa Hydration causes increase in cell volume. Hydration causes decreased bulk modulus and seismic velocities. • • • and Vs (no measurements on olivine)
Vp
Pressure strongly stabilizes H in cation vacancies. Decreases K and hence In ringwoodite, the effect of hydration on velocity is larger than effect of temperature! Hydration increases Vp/Vs – Earth’s Deep Water Cycle • • • • e
s Deep Water Cycl ’ h
Eart Below 200km H is captured by olivine in the overlying mantle wedge. H softens the olivine allowing it to lubricate the descent of cold slab H is thus effectively entrained in the descending slab. • • •
Nominally Anhydrous Minerals (NAMs) can incorporate up to 9 times the total Ocean mass if saturated. •
Interior Reservoir? Water:
Dynamics
The Third State
Variable in Mantle
thick and slow
A hydrous TZ should be
Red means Wet Blue means Dry
TZ May Reflect Hydration • • Lateral Velocity Variations in Shear Wave Velocity Structure
5.8 Hydration of
Wadsleyite 5.6 and ) s Ringwoodite / 5.4 m k (
is more y t i consistent c 5.2 lo
with model Ve e v 5 shear a W r velocity a e 4.8 structure Sh
4.6
4.4 300 350 400 450 500 550 600 650 Depth (km) MSA Short Course the Deep Earth: of Mantle Minerals”
Fall AGU, San Francisco
Verbania, Italy October, 2006
Upcoming Sessions on Water in “Water in Nominally Anhydrous Minerals” “Effect of Hydration on Physical Properties [email protected]
Reprints and preprints at
http://ruby.colorado.edu/~smyth/Home.html