Transcript Document

LARGE IGNEOUS PROVINCES:
Results of Delamination?
Don L. Anderson
Caltech
1
A new GSA book
2
Delamination:
The Eclogite Engine
• Kay, R.W. & Kay, S.M., Delamination
and delamination magmatism,
Tectonophysics, 219, 177-189, 1993.
• mechanism can explain some longstanding geophysical problems, e.g.
– subsidence prior to LIP emplacement
– short duration
– bottoming of seismic tomography
anomalies beneath “hot spots”
• but what happens to this lower crust?
3
Summary of model
• When crust thickens to > 50 km:
– converts to dense eclogite
– delaminates
– sinks
– heats up
– rises
• eclogites have low Vs for their density may be confused with high T
4
Rocks and minerals arranged by
density:
crust
&
upper
mantle
Figure 3: Rocks and minerals arranged by density
SHEAR VELOCITY (P=0)
Rock type
STP
density 3
Vs (km/s)
4
5
6
km/s
(g/c c )
A'
CRUST
A"
granite
2.62
gabbro
dolerite
2.87
2.93
gneiss
2.98
eclogites &
3.45
arc eclogites
3.46
(arclogites,arcl)3.48
"
3.62
UPPER harzburgite
MANTLE dunite
pyrolite
peridotite
3.30
3.31
3.38
3.42
usual max. crustal thickness
50 km
unstable
root
eclogite
Vp= 8.1 km/s
Vp= 8.4 km/s
Vp= 8.3 Km/s
UPPER
MANTLE
eclogite: here used
as a general term
for garnet &
pyroxene-rich rock
5
Rocks and minerals arranged by
density:
crust
&
upper
mantle
Figure 3: Rocks and minerals arranged by density
SHEAR VELOCITY (P=0)
Rock type
STP
density 3
Vs (km/s)
4
5
6
km/s
(g/c c )
A'
CRUST
A"
granite
2.62
gabbro
dolerite
2.87
2.93
gneiss
2.98
eclogites &
3.45
arc eclogites
3.46
(arclogites,arcl)3.48
"
3.62
UPPER harzburgite
MANTLE dunite
pyrolite
peridotite
3.30
3.31
3.38
3.42
usual max. crustal thickness
50 km
unstable
root
eclogite
Vp= 8.1 km/s
Vp= 8.4 km/s
Vp= 8.3 Km/s
UPPER
MANTLE
• delaminates
when crust
> 50 km thick
• warmer than
MORB
6
Low-velocity
Zones
density
Vs
(g/cc)
(km/s)
granite
2.62
3.62
gabbro
dolerite
2.87
2.93
3.84
3.78
gneiss
2.98
4.03
eclogites &
arc eclogites
(arclogites)
"
3.45
3.46
3.48
3.62
4.60
4.77
4.68
4.80
3
4
5
6
km/s
Rocks and minerals arranged by
density: upper mantle
A'
CRUST
A"
UPPER
MANTLE
100
usual max. crustal thickness
50 km
unstable
root
eclogite
Vp= 8.1 km/s
harzburgite
3.30
4.90
Where does
delaminate
buoyancy?
dunite
3.31
4.84 reach
Vp= 8.4 neutral
km/s
UPPER
B
185
380
TZ
C'
480
580
C"
650
720
D'
pyrolite
peridotite
arcl(highMgO)
eclogite
Hawaii Lhz.
arcl(highMgO)
r
3.38
3.42
3.45
3.46
3.47
3.48
4.82
4.76
4.60
4.77
4.72
4.68
stable
magnesiowustite
pv
ca pv
3.59
3.60
"
"
3.60
3.63
3.67
3.70
3.68
3.70
3.74
3.75
3.75
4.04
4.11
4.13
LOWER MANTLE (STP ) 4.15
5.54
5.43
5.33
5.20
4.93
4.84
5.40
5.69
5.5+
4.91
4.93
4.93
5.6+
4.98
6.62
5.50
6.40
eclogite
Vp=8.1 km/s
4
8.1 km/s
3
-spinel(.1FeO)
(.12FeO)
pyrolite(410km)
majorite
arcl(lowMgO)
"
pyrolite(500km)
-spinel(.1FeO)
MORB(mj+coe)
archlogites
(low MgO)
"
MORB(mj+st)
MANTLE
Vp= 8.3 Km/s
Vs
4
5
6
km/s
X
410 km
9.3 km/s
8.3km/s
9.7 km/s
eclogite
500 km
8.6 km/s
ultra-stable
(when cold)
11 km/s
eclogite
oceanic crust
650 km
Ca,Al-poor, Si,Fe-rich
I
LOWER MANTLE
Repetti Discontinuity
7
Delaminated roots warm quickly
• will start to melt
before reaching
same T as
surrounding mantle
• already in TBL, so
starts off warm
• when 30% melt,
garnet mostly gone
& will start to rise
8
SHEAR VELOCITY (P=0)
Rock type
Vs (km/s)
STP
• pink eclogite is
only temporarily
stable at these
depths
• “arclogites” less
SiO2 than MORB
eclogite – do not
sink so far
• Vs of eclogite low
at depth
• low melting point
• as it warms, it
rises
density3
4
5
6
km/s
(g/cc)
A'
CRUST
A"
granite
2.62
gabbro
dolerite
2.87
2.93
gneiss
2.98
eclogites &
arc eclogites
(arclogites,arcl)
"
3.45
3.46
3.48
3.62
UPPER harzburgite
MANTLE dunite
pyrolite
peridotite
B
arcl(highMgO)
eclogite
Hawaii Lhz.
arcl(highMgO)
usual max. crustal thickness
50 km
unstable
root
Vp= 8.1 km/s
3 .3 0
3 .3 1
3 .3 8
3 .4 2
3.45
3.46
3.47
3.48
C'
C"
-spinel(.1FeO)
(.12FeO)
pyrolite(4 1 0 km)
majorite
arcl(lowMgO)
"
pyrolite(5 0 0 km)
-spinel(.1FeO)
MORB(mj+coe)
archlogites
(low MgO)
"
MORB(mj+st)
3.59
3.60
"
"
3.60
3.63
3 .6 7
3.70
3.68
3.70
3.74
3.75
3.75
magnes iowus tite 4 .0 4
perovs kite(pv)
4 .1 1
c alc ium pv
4 .1 3
LOWER MANTLE
4 .1 5
D'
UPPER
MANTLE
Vp= 8.4 km/s
Vp= 8.3 Km/s
stable
eclogite
Vp=8.1 km/s
8.1 km/s
3
TZ
eclogite
4
5
6
km/s
X
410 km
9.3 km/s
8.3km/s
9.7 km/s
eclogite
500 km
8.6 km/s
ultra-stable
(when cold)
eclogite
oceanic crust
650 km
11 km/s
Ca,Al-poor, Si,Fe-rich
I
1 0 0 0 km
Repetti Discontinuity
9
Mantle stratification
• irregular chemical
discontinuities
expected
• difficult to see in
tomography
• can be seen in
receiver functions
10
Underside reflections 0 – 1,000 km
depth
• 410 & 660-km
discontinuities
clear
• ~ 10 others
• may be chemical
11
ROOT FORMA TION
Delamination
cycle
• dense roots
– fall off
– warm up in
ambient mantle
– rise
• possible mechanism
for Atlantic & Indian
ocean plateaus &
DUPAL anomaly
1
D ELA MINA TION
ridge
2
SPREA D ING
3
heating
UPWELLING
4
SPREA D ING
5
12
Many ways for eclogite to get into
the mantle
• collision belts, arcs
• can fuel melting anomalies at normal T
13
LIPs are associated with
continental breakup
• reconstruction at
~ 30 Ma
• dual volcanism
– on breakup
– ~ 30-40 Myr later
• oceanic plateaus form
~ 1,000 km offshore
• = rising of
delaminated root?
14
Eclogite 70% molten before
peridotite starts to melt
• eclogite sinkers
warmed by
conduction
• rise before T has
risen to that of
ambient mantle
• eclogite 70%
molten at
peridotite solidus
15
• delamination
controls crustal
thickness
• very sharp cutoff at 50 km
• interpreted as
eclogite phase
change
from Mooney et al., 1998
16
Example 1: Rio Grande rift
Are LVZs delaminated roots?
hot?
from Gao et al., 2004
17
Example 2: Sierra Nevada
garnet
peridotite
garnet
pyroxenite
P-wave slowness
attenuation
Vp/Vs
anisotropy
from Boyd et al., 2004
18
Example 3: Iceland
•
•
•
•
Restricted LVZ
possibly Caledonian
arc roots delaminated
on breakup
Cold, dense, sinking
eclogite can be LVZ
warmed, melted,
rising eclogite can
also be buoyant if ~
1/2 garnet eliminated
Ritsema et al., 1999
19
Summary
• Dense, mafic cumulates may be twice the
thickness of arc crust
• Delamination accompanied by upwelling &
adiabatic decompression of the asthenosphere; a
whole cycle may take 30-40 Myr
• The global recycling flux of arcologite is ~ 10%
that of oceanic crust, i.e. ~ hotspot volume rate
• It starts out hotter & by-passes normal
subduction zone processing
• Delaminated arclogites preferentially melt & form
a unique component of hotspot & ridge magmas
(e.g. suggested DUPAL = Gondwana crust).
20
Resources
Please visit:
www.mantleplumes.org/Eclogite.html
www.mantleplumes.org/LowerCrust.html
End
21