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 models put forward

Harmon et al. (2006, 2007)

INTRODUCTION INTRODUCTION INTRODUCTION INTR models put forward

Harmon et al. (2006, 2007)

INTRODUCTION INTRODUCTION INTRODUCTION INTR models put forward

Harmon et al. (2006, 2007)

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)

INTRODUCTION INTRODUCTION INTRODUCTION INTRmodel setup

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.

DISCUSSION DISCUSSION DISCUSSION DISCUSSION Seamount-trails in the NW-Pacific

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