1. M. D. Ballmer, J. van Hunen, G. Ito, P. J. Tackley and T. A. Bianco Intraplate volcano chains originating from small-scale sublithospheric convection

2. Introduction Results Discussion Conclusion OUTLINE OUTLINE OUTLINE OUTLINE OUTLINE Motivation Small-scale Convection Model setup T-dependent rheology X-dependent rheology Lateral heterogeneity Application Summary

3. INTRODUCTION INTRODUCTION INTRODUCTION INTR Wessel (1997) Distribution of volcanism < 2.5 km intermediate > 3.5 km sizes of seamounts

4. INTRODUCTION INTRODUCTION INTRODUCTION INTR Wessel (1997) Distribution of volcanism < 2.5 km intermediate > 3.5 km sizes of seamounts Marshalls Gilberts Cook-Australs Line Islands Pukapuka

5. INTRODUCTION INTRODUCTION INTRODUCTION INTR Pukapuka Small ridges aligning plate motion and gravity lineations violate hotspot age progressions. (A) Pukapuka ridge (B) Hotu-Matua smts. (C) Sejourn ridge

6. INTRODUCTION INTRODUCTION INTRODUCTION INTR models put forward Harmon et al. (2006, 2007)

7. INTRODUCTION INTRODUCTION INTRODUCTION INTR models put forward Harmon et al. (2006, 2007)

8. INTRODUCTION INTRODUCTION INTRODUCTION INTR models put forward Harmon et al. (2006, 2007)

9. 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[km] 400 [km] 0[km] from the ridge 4000

10. 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[km] 400 [km] 0[km] from the ridge 4000 fracture zone Huang et al. (2003)

11. INTRODUCTION INTRODUCTION INTRODUCTION INTR upwelling wet or hot, buoyant mantle depletion and melt retention give addi- tional buoyancy melting cell decompression melting further decompression 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.

12. INTRODUCTION INTRODUCTION INTRODUCTION INTR Melting model 6% 4% 2% 0% approximation of melt extraction depletion melt fraction critical porosity

13. 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)

14. INTRODUCTION INTRODUCTION INTRODUCTION INTR model setup

15. log η z 5.5 cm/a 6.5 cm/a 1 cm/a INTRODUCTION INTRODUCTION INTRODUCTION INTR velocity boundary conditions

16. T m = 1380 °C η eff = 1.6x10 19 Pas H 2 O bulk =125 ppm φ C =1% RESULTS RESULTS RESULTS RESULTS RESULTS Results of 3D-simulations

17. RESULTS RESULTS RESULTS RESULTS RESULTS Results of 3D-simulations T m = 1380 °C η eff = 1.6x10 19 Pas H 2 O bulk =125 ppm φ C =1% for onset of small-scale convecion beneath rela- tively young and thin lithosphere (~25-50 Ma), partial melting emerges above the upwellings.

18. RESULTS RESULTS RESULTS RESULTS RESULTS Removal of the depleted lid by SSC 0% 20% depletion Melting due to SSC initiates after removal of the buoyant residue from previous ridge melting T m = 1380 °C η eff = 1.6x10 19 Pas H 2 O bulk =125 ppm φ C =1%

19. RESULTS RESULTS RESULTS RESULTS RESULTS 0% melt retention The partially molten zone is - elongated - aligned by plate-motion melting zone is elongated 1% T m = 1380 °C η eff = 1.6x10 19 Pas H 2 O bulk =125 ppm φ C =1%

20. RESULTS RESULTS RESULTS RESULTS RESULTS thickness of the harzburgite layer Higher T mantle increases the thickness of the buoyant harzburgite layer  more stable stratification of the mantle  late onset of SSC and related melting

21. RESULTS RESULTS RESULTS RESULTS RESULTS 1% 0% T m =1350 °C T m =1380 °C T m =1410 °C 50 km 60 km 70 km 80 km 90 km 100 km 110 km melt retention Melting occurs deeper for higher T mantle, because of a thicker residue from previous ridge melting Investigating temperature

22. RESULTS RESULTS RESULTS RESULTS RESULTS Temperature vs. viscosity The age of the seafloor, on which volcanism occurs, is mainly controlled by temperature, whereas its amount is predominantly dependent on viscosity

23. RESULTS RESULTS RESULTS RESULTS RESULTS Bulk 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.

24. RESULTS RESULTS RESULTS RESULTS RESULTS density 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. T m = 1380 °C H 2 O = 125 ppm

25. RESULTS RESULTS RESULTS RESULTS RESULTS critical porosity A larger critical porosity allows more melt retention and thus more vigorous buoyant decompression melting. Whatsurever, less melt reaches the surface. η eff = 1.5∙10 19 Pa ∙ s T m = 1380 °C H 2 O = 125 ppm total melt generated total melt erupted

26. T m = 1380 °C η eff = 2.4x10 18 Pas H 2 O bulk =125 ppm φ C =2% ξ = 40 RESULTS RESULTS RESULTS RESULTS RESULTS Compositional Rheology

27. RESULTS RESULTS RESULTS RESULTS RESULTS water 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.

28. 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)

29. 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.

30. 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 Magellan smts. Ratak smts. Ralik smts. Ujlan smts. Anewetak smts.

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

32. DISCUSSION DISCUSSION DISCUSSION DISCUSSION Cook-Austral and Line Islands

33. DISCUSSION DISCUSSION DISCUSSION DISCUSSION 0 100 200 0 100 200 no SSC SSC km 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 10 19 Pas are required to activate SSC already beneath 25 to 55 Ma old lithosphere triggering melting. temperature below solidus 0 °C 500°C

34. 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 (T m ) 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 T m, 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