Post on 11-Jan-2016
description
Earth’s Deep Water Cycle
Suzan van der Lee
Northwestern University
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?
Mantle outgassing 90% efficient; at least 10% stays in mantle, and likely more in slab
Rupke et al. (EDWC)
Son
g an
d H
elm
berg
er (
ED
WC
)
lvz on 410 (red o) right next to “normal” mantle (white o)
Kar
ato
et a
l. (E
DW
C)
melt on 410 from upwelling saturated wadselyite, but…
Hirschmann et al. (EDWC)
< 7 kmPredicted melt layer thinner than observed lvz
wd-->ol transition thickens with increasing H2O at undersaturated conditions
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
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…
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
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
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
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
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)
Bra
unm
iller
et a
l. (E
DW
C)
TZ thickens but ol-->wd does not: deep SAm mantle dry (or saturated)
Suetsugu et al. (EDWC)
Vp and 660: >1 wt % water near slab
Courtier and Revenaugh (EDWC)
410, 520, and Vs: >0.2 wt % water
low Vs: < 1 wt% water
low Vs above slab in top of lower mantle
VdLee & Frederiksen (2005)
Grand (2002)
Inoue et al. (EDWC)
Experiments show that Shy-B is stable in TZ and cool slab
Komabayashi (EDWC)
Calculations show that abc phases are stable throughout upper mantle in cool slab; breakdown occurs in top of lower mantle.
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)
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)
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.
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 --
Connecting past and future episodes of subduction
Connecting past and future episodes of subduction
Connecting past and future episodes of subduction
Connecting past and future episodes of subduction
Connecting past and future episodes of subduction
Present
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!)