M. D. Ballmer, J. van Hunen, G. Ito,
P. J. Tackley and T. A. Bianco
Intraplate volcano chains originating from small-scale sublithospheric
convection
Introduction
Results
Discussion
Conclusion
OUTLINE OUTLINE OUTLINE OUTLINE OUTLINE
MotivationSmall-scale ConvectionModel setup
T-dependent rheologyX-dependent rheology
Lateral heterogeneityApplication
Summary
INTRODUCTION INTRODUCTION INTRODUCTION INTR
Wessel (1997)
Distribution of volcanism
< 2.5 km
intermediate
> 3.5 km
sizes of seamounts
INTRODUCTION INTRODUCTION INTRODUCTION INTR
Wessel (1997)
Distribution of volcanism
< 2.5 km
intermediate
> 3.5 km
sizes of seamounts
Marshalls
Gilb
erts
Cook-Australs
Line Islands Pukapuka
INTRODUCTION INTRODUCTION INTRODUCTION INTRPukapuka
Small ridges aligning plate motion and gravity lineations violate hotspot age progressions.
(A) Pukapuka ridge (B) Hotu-Matua smts. (C) Sejourn ridge
INTRODUCTION INTRODUCTION INTRODUCTION INTR
van Hunen and Zhong (2005)
SSC is evolving in rolls aligning with plate mo-tion owing to instabilities of the thickened thermal boundary layer.
Small-scale convection (SSC)
0[k
m] 4
00
[km]
0 [km] from the ridge 4000
INTRODUCTION INTRODUCTION INTRODUCTION INTR
van Hunen and Zhong (2005)
SSC is evolving in rolls aligning with plate mo-tion owing to instabilities of the thickened thermal boundary layer.
Small-scale convection (SSC)
0[k
m] 4
00
[km]
0 [km] from the ridge 4000
fracture zone
Huang et al. (2003)
INTRODUCTION INTRODUCTION INTRODUCTION INTR
upwelling wet or hot, buoyant mantle
depletion and melt retention give addi- tional buoyancy
melting cell
decompression melting
furtherdecompression melting
buoyant decompression melting
Buoyant decompression melting is a self-sustaining process, which is driven by positive density changes due to depletion and melt retention.
INTRODUCTION INTRODUCTION INTRODUCTION INTRMelting model
6%
4%
2%
0%
approximation of melt extraction
depletion
melt fractioncritical porosity
INTRODUCTION INTRODUCTION INTRODUCTION INTR
1300 x [km] 2400
60
z[km]
300
0
y [km] 500
0% 2% melt retention
Numerical modeling: CITCOM
thermo-
-chemical
van Hunen et al. (2005)
log η
z
5.5 cm/a
6.5 cm/a
1 cm/a
INTRODUCTION INTRODUCTION INTRODUCTION INTRvelocity boundary conditions
Tm = 1380 °C ηeff = 1.6x1019 Pas H2Obulk=125 ppm φC =1%
RESULTS RESULTS RESULTS RESULTS RESULTSResults of 3D-simulations
RESULTS RESULTS RESULTS RESULTS RESULTS
melt depletion
20%
0% 1500
1300
1400
0
T/°C
6.5 cm/a
0
100
200
300
400 500
1500
2500
0
2000
1000
3000
250
500
750 920
0%
1%
y [km
]
z [k
m]
x [km]
1500
2500
2000
3000 0
250
500
750
920
x [km]
y [k
m]
0
2.7
height [k
Results of 3D-simulationsTm = 1380 °C ηeff = 1.6x1019 Pas H2Obulk=125 ppm φC =1%
for onset of small-scale convecion beneath rela-tively young and thin lithosphere (~25-50 Ma), partial melting emerges above the upwellings.
RESULTS RESULTS RESULTS RESULTS RESULTS
melt depletion
20%
0% 1500
1300
1400
0
T/°C
6.5 cm/a
0
100
200
300
400 500
1500
2500
0
2000
1000
3000
250
500
750 920
0%
1%
y [km
]
z [k
m]
x [km]
1500
2500
2000
3000 0
250
500
750
920
x [km]
y [k
m]
0
2.7
height [k
Removal of the depleted lid by SSC
0% 20% depletion
Melting due to SSC initiates afterremoval of the buoyant residue from previous ridge melting
Tm = 1380 °C ηeff = 1.6x1019 Pas H2Obulk=125 ppm φC =1%
melt depletion
20%
0% 1500
1300
1400
0
T/°C
6.5 cm/a
0
100
200
300
400 500
1500
2500
0
2000
1000
3000
250
500
750 920
0%
1%
y [km
]
z [k
m]
x [km]
1500
2500
2000
3000 0
250
500
750
920
x [km]
y [k
m]
0
2.7
height [k
RESULTS RESULTS RESULTS RESULTS RESULTS
0%
melt re
ten
tion
The partially molten zone is- elongated- aligned by plate-motion
melting zone is elongated
1%
Tm = 1380 °C ηeff = 1.6x1019 Pas H2Obulk=125 ppm φC =1%
melt depletion
20%
0% 1500
1300
1400
0
T/°C
6.5 cm/a
0
100
200
300
400 500
1500
2500
0
2000
1000
3000
250
500
750 920
0%
1%
y [km
]
z [k
m]
x [km]
1500
2500
2000
3000 0
250
500
750
920
x [km]
y [k
m]
0
2.7
height [k
RESULTS RESULTS RESULTS RESULTS RESULTSthickness of the harzburgite layer
Higher Tmantle increases the thickness of the buoyant harzburgite layer more stable stratification of the mantle late onset of SSC and related melting
melt depletion
20%
0% 1500
1300
1400
0
T/°C
6.5 cm/a
0
100
200
300
400 500
1500
2500
0
2000
1000
3000
250
500
750 920
0%
1%
y [km
]
z [k
m]
x [km]
1500
2500
2000
3000 0
250
500
750
920
x [km]
y [k
m]
0
2.7
height [k
RESULTS RESULTS RESULTS RESULTS RESULTS
1%
0%
Tm=1350 °C Tm=1380 °C Tm=1410 °C
50
km
60
km
70
km
80
km
90
km
10
0 k
m 1
10
km
melt re
ten
tion
Melting occurs deeper for higherTmantle, because of a thicker residue from previous ridge melting
Investigating temperature
RESULTS RESULTS RESULTS RESULTS RESULTSTemperature vs. viscosity
0
1
2
3
4
2.3∙1019ηeff [
Pa∙s
]
1350 °C1350 °C
1410 °C
1380 °C
1.3∙1019 Tm
melt volum
e flux per km
of plate [km³/M
yr/km]
age of the seafloor at the time of volcanism [Ma]
2.7(4.0).
1.3(2.4).
1.8(3.0).
0.8(1.7).
3.9(5.1).
3.2(4.5).
2.7(4.0).
2.2(3.4).
1.4(2.6). .5.0(5.9)
.4.2(5.3)
.3.3(4.5)
.2.1(3.3)
1.2(2.3).
20 30 40 50 60
The age of the seafloor, on which volcanism occurs, is mainly controlled by temperature, whereas its amount is predominantly dependent on viscosity
RESULTS RESULTS RESULTS RESULTS RESULTSBulk water content vs. viscosity
Similar to the affect of temperature, increasing water contents lead to delayed volcanism due to a thicker residue from previous ridge melting.
1.6∙1019
ηeff [Pa∙s]2.0∙1019
1.8∙1019
c(H2O)bulk
100 ppm 150 ppm 200 ppmTm=1380 °C
melt volum
e flux per km
of plate [km³/M
yr/km]
age of the seafloor at the time of volcanism [Ma]25 30 35 40 45 50 55
0
1
2
3
4
RESULTS RESULTS RESULTS RESULTS RESULTSdensity reduction due to depletion
A stronger reduction of density due to depletion (density of harzburgite vs. peridotite) delays the onset of SSC and therefore diminishes associated volcanism.
Tm = 1380 °CH2O = 125 ppm
20 25 30 35 40 45 50 550
1
2
3
4
5
6
1.6∙10 019
ηeff [Pa∙s] Δρdepl [kg/m³]
1.6∙10 15019
1.6∙10 72.619
1.6∙10 22519
RESULTS RESULTS RESULTS RESULTS RESULTScritical porosity
A larger critical porosity allows more melt retention and thus more vigorousbuoyant decompression melting. Whatsurever, less melt reaches the surface.
ηeff = 1.5∙1019 Pa∙s Tm = 1380 °CH2O = 125 ppm
0 0.05 0.1 0.15 0.2 0.250
0.2
0.4
0.6
0.8
1
critical porosity [%]
cum
ulat
ive
mel
tcol
umn
heig
ht [k
m]
total melt g
enerated
total melt erupted
Tm = 1380 °C ηeff = 2.4x1018 Pas H2Obulk=125 ppm φC =2% ξ = 40
RESULTS RESULTS RESULTS RESULTS RESULTSCompositional Rheology
RESULTS RESULTS RESULTS RESULTS RESULTSwater exhaustion stiffening factor
ξ
For taking into account stiffening due to water exhaustion, volcanism is predicted to emerge earlier and to span a wider range of seafloor ages.
1.5-2.7∙10 4018
ηeff [Pa∙s] ξ1.5-1.7∙10 119
3.0-4.8∙10 1018
Tm=1380 °C
melt volum
e flux per km
of plate [km³/M
yr/km]
age of the seafloor at the time of volcanism [Ma]25 30 35 40 45 50
0
1
2
3
4
DISCUSSION DISCUSSION DISCUSSION DISCUSSION Lateral heterogeneity
Volcanism may be still possible for larger mantle viscosities, if the onset age of SSC is early due to small lateral density heterogeneity.
fracture zone
Huang et al. (2003)
Hot anomaly (10°C) in the middle of the box
400
[k
m]
0
0 [
km]
920450
[km from the ridge] 3150T/°C
1230 1530
depletion
20%
0%melt
0%
1%
Tm=14
00 °C
, ηeff =
0.9
e20
P
DISCUSSION DISCUSSION DISCUSSION DISCUSSION Linear ridges in the southern pacific
At Pukapuka, ages of the edifices relative to the underlying seafloor are not constant, violating the implications by the hotspot hypothesis. These may rather be due to the Pacific plate moving over an elongate anomaly.
0 500 1000 1500
distance from Pukapuka2000 2500 3000
10
15
20
25
30
age
of th
e se
amou
nts [M
yr]
seamounts sampled
DISCUSSION DISCUSSION DISCUSSION DISCUSSION Seamount-trails in the NW-Pacific
Koppers et al. (2004), modified
160°E 170°E
15°N
10°N
5°N
Magella
n sm
ts.
Rata
k sm
ts.
Ralik
smts.
Ujla
n sm
ts.
Anew
eta
k sm
ts.
Cook-Austals Bonneville et al. 2006
Mid
-Pac
ific
Mts
.
Tuam
otus
1000
km
King
man
Reef
Nec
ker
Ridg
e
88
86 81
84
79
76
8593
72 7036 59
72 7170698486 82
8373
70-7
2
68
82
71 69
Davis et al. 2003
Line Islands
43
DISCUSSION DISCUSSION DISCUSSION DISCUSSION Cook-Austral and Line Islands
DISCUSSION DISCUSSION DISCUSSION DISCUSSION
0
100
200
0
100
200
no SSC
SSC
km
km
5 10 20 30 40 50 60 70 Ma
5 10 20 30 40 50 60 70 Ma
melting with and without SSC
Temperature anomalies of >100 K are needed to obtain intraplate volcanism without SSC.Effective mantle Viscosities of about 1019 Pas are required to activate SSC already beneath 25 to 55 Ma old lithosphere triggering melting.
tem
pera
ture
belo
w
solid
us
0 °C
500°C
Conclusions• Melting is triggered by small-scale convection and promoted by melt retention and depletion buoyancy.
• Melting due to small-scale convection occurs along elonga- ted anomalies (~1000 km) and works for average mantle temperatures (Tm) and realistic viscosities.
• The associated volcanic chains are predicted to display irregular age-distance relationships.
• The age of the seafloor, over which volcanism occurs is predominantly correlated with Tm, whereas the amount of volcanism is mainly dependent on effective viscosity.
• The onset of volcanism may be earlier and its duration longer if accounting accounting for compositional rheology.
• Lateral heterogeneity reduces the onset age of small- scale convection and increases the viscosity required for volcanism.
CONCLUSIONS CONCLUSIONS CONCLUSIONS CONCLU