Introduction Results Discussion Conclusion

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

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

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

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

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

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 Small-scale convection (SSC) 0 [km] 400 van Hunen and Zhong (2005) [km] SSC is evolving in rolls aligning with plate mo-tion owing to instabilities of the thickened thermal boundary layer. 0 [km] from the ridge 4000

10. fracture zone INTRODUCTION INTRODUCTION INTRODUCTION INTR Small-scale convection (SSC) 0 [km] 400 van Hunen and Zhong (2005) [km] Huang et al. (2003) SSC is evolving in rolls aligning with plate mo-tion owing to instabilities of the thickened thermal boundary layer. 0 [km] from the ridge 4000

11. melting cell depletion and melt retention give addi- tional buoyancy upwelling wet or hot, buoyant mantle furtherdecompression melting decompression melting INTRODUCTION INTRODUCTION INTRODUCTION INTR 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 Numerical modeling: CITCOM van Hunen et al. (2005) thermo- 60 z[km] 300 0 y [km] 500 -chemical 1300 x [km] 2400 0% 2% melt retention

14. INTRODUCTION INTRODUCTION INTRODUCTION INTR model setup

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

16. RESULTS RESULTS RESULTS RESULTS RESULTS Results of 3D-simulations Tm = 1380 °C ηeff = 1.6x1019 Pas H2Obulk=125 ppm φC =1%

17. RESULTS RESULTS RESULTS RESULTS RESULTS Results of 3D-simulations Tm = 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.

18. 0% 20% depletion RESULTS RESULTS RESULTS RESULTS RESULTS Removal of the depleted lid by SSC Tm = 1380 °C ηeff = 1.6x1019 Pas H2Obulk=125 ppm φC =1% Melting due to SSC initiates after removal of the buoyant residue from previous ridge melting

19. RESULTS RESULTS RESULTS RESULTS RESULTS melting zone is elongated Tm = 1380 °C ηeff = 1.6x1019 Pas H2Obulk=125 ppm φC =1% 1% • The partially molten zone is • elongated • aligned by plate-motion melt retention 0%

20. RESULTS RESULTS RESULTS RESULTS RESULTS thickness 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

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

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 Tm = 1380 °C H2O = 125 ppm A stronger reduction of density due to depletion (density of harzburgite vs. peridotite) delays the onset of SSC and therefore diminishes associated volcanism.

25. RESULTS RESULTS RESULTS RESULTS RESULTS critical porosity ηeff = 1.5∙1019 Pa∙s Tm = 1380 °C H2O = 125 ppm total melt generated total melt erupted A larger critical porosity allows more melt retention and thus more vigorousbuoyant decompression melting. Whatsurever, less melt reaches the surface.

26. RESULTS RESULTS RESULTS RESULTS RESULTS Compositional Rheology Tm = 1380 °C ηeff = 2.4x1018 Pas H2Obulk=125 ppm φC =2% ξ = 40

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. fracture zone DISCUSSION DISCUSSION DISCUSSION DISCUSSION Lateral heterogeneity Huang et al. (2003) Volcanism may be still possible for larger mantle viscosities, if the onset age of SSC is early due to small lateral density heterogeneity.

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 15°N 10°N 5°N Ralik smts. Anewetak smts. Magellan smts. Ratak smts. Ujlan smts. 160°E 170°E Koppers et al. (2004), modified

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 melting with and without SSC 0 100 200 0 °C 500°C no SSC km 5 10 20 30 40 50 60 70 Ma temperature below solidus 0 100 200 SSC km 5 10 20 30 40 50 60 70 Ma 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.

34. CONCLUSIONS CONCLUSIONS CONCLUSIONS CONCLU 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.