Earth’s Deep Water Cycle
description
Transcript of Earth’s Deep Water Cycle
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Earth’s Deep Water Cycle
Suzan van der Lee
Northwestern University
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Fen
g et
al.
(200
7); V
an d
er L
ee a
nd W
iens
(E
DW
C)
mantle wedge seismically slow and wet, dry below?
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Mantle outgassing 90% efficient; at least 10% stays in mantle, and likely more in slab
Rupke et al. (EDWC)
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Son
g an
d H
elm
berg
er (
ED
WC
)
lvz on 410 (red o) right next to “normal” mantle (white o)
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Kar
ato
et a
l. (E
DW
C)
melt on 410 from upwelling saturated wadselyite, but…
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Hirschmann et al. (EDWC)
< 7 kmPredicted melt layer thinner than observed lvz
wd-->ol transition thickens with increasing H2O at undersaturated conditions
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0 ppm
200 ppm
500 ppm
1000 ppm
35
40
45
30
25
20
15
10
5
0
Phase transition
interval (km)
velocity
(after Wood, 1995)
>25 km
Calculations at D=20 suggest that ol-->wd transition can be very thick at only 0.1 wt % water
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VSL, Italy PAB, Spain KEG, Egypt
410
660
dep
th (k
m)
f = 0
.75 H
zf =
0.6
2 H
zf =
0.5
Hz
f = 0
.4 H
zf =
0.3
5 H
zf =
0.3
Hz
f = 0
.25 H
zf =
0.2
Hz
f = 0
.15 H
z
f = 0
.75 H
zf =
0.6
2 H
zf =
0.5
Hz
f = 0
.4 H
zf =
0.3
5 H
zf =
0.3
Hz
f = 0
.25 H
zf =
0.2
Hz
f = 0
.15 H
z
410
660
f = 0
.75 H
zf =
0.6
2 H
zf =
0.5
Hz
f = 0
.4 H
zf =
0.3
5 H
zf =
0.3
Hz
f = 0
.25 H
zf =
0.2
Hz
f = 0
.15 H
z
Water would thus be an explanation for puzzling receiver functions, but…
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Hirs
chm
ann
et a
l. (E
DW
C)
<15 km
With D=5 and 0.1 wt % H2O transition would be less than 15 km thick
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Conder and Wiens (2006); Roth et al. (1999); Van der Lee and Wiens (EDWC)
V low and Q very low in mantle wedge --> water
V low, but Q high below 200 km --> warm
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velocity attenuation
temperature
water
major element chemistry
partial melting
grain-size
large*
modest*
small1
potentially large2
very small
large
large
very small
small*
small
Shito et al. (EDWC)
Separate effects of water from other effects
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Shi
to e
t al.
(ED
WC
)
Water above 400 km, from upwelling TZ or from slab
Still some trade-off btw w and T
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Use other seismic measurements to evaluate the relative role of w and T, such as transition-zone discontinuity properties:
Sm
yth
and
Jaco
bsen
(20
06)
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Bra
unm
iller
et a
l. (E
DW
C)
TZ thickens but ol-->wd does not: deep SAm mantle dry (or saturated)
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Suetsugu et al. (EDWC)
Vp and 660: >1 wt % water near slab
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Courtier and Revenaugh (EDWC)
410, 520, and Vs: >0.2 wt % water
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low Vs: < 1 wt% water
low Vs above slab in top of lower mantle
VdLee & Frederiksen (2005)
Grand (2002)
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Inoue et al. (EDWC)
Experiments show that Shy-B is stable in TZ and cool slab
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Komabayashi (EDWC)
Calculations show that abc phases are stable throughout upper mantle in cool slab; breakdown occurs in top of lower mantle.
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Hydrous TZ likely less densethan dry TZ, and:
Water lowers the viscosity by 4-5 orders of magnitude, at least above 300 km.
Hydrous mantle can well up and hydrate lithosphere.
Karato and Jung (2003);
Karato (EDWC)
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Deformation model Temperature and water-sensitive yield and thermal-mechanical feedback
Ocean continent Boundary
Sediment loading
Seafloor age turned into temperatures70 km thick Lithosphere cross section shown
Mid Atlantic
ridge
Regenauer-Lieb et al. (2001)
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lithosphere
wet rheology dry rheology time0 km --
100 km --
Regenauer-Lieb et al. (2001)
lithosphere breaks only in wet conditions; subduction of dense lithosphere enabled.
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Connecting past and future episodes of subduction
200-300 m.y.
subducting plate
(Farallon)
continent(N America)
subducti
ng plate
(Atla
ntic?)
continent(N America)
-- 660 km --
-- 0 km --
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Connecting past and future episodes of subduction
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Connecting past and future episodes of subduction
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Connecting past and future episodes of subduction
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Connecting past and future episodes of subduction
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Connecting past and future episodes of subduction
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Present
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Conclusions
1. Deep water cycle may sustain plate tectonics over many Gy.
2. Water in mantle is detectable in various ways
seismic V from tomography or triplication branches
Q/attenuation
discontinuity depths and properties
3. More work is needed
mineral physics: elasticity at p, T, and C
seismology: benchmarking, denser data (USArray!)