Testing the Plume Hypothesis for Flood Basalts

1. TESTING THE PLUME HYPOTHESIS Ian Campbell The Australian National University

8. Testable Predictions of the Plume Hypothesis • New plumes consist of a large head followed by a small tail

9. Parana at 120 Ma

11. Characteristics of Flood Basalts • Equidimentional, typically 2000-2500 km • Preceded by uplift • Large volumes of magma • Short eruption times, main phase 1 Myr • Rapid contraction of volcanism to narrow chain of volcanoes to current position of plume

12. Testable Predictions of the Plume Hypothesis • Plume tails (upper mantle) should be about 100-300 km across and have higher temperature that the adjacent mantle • However, plume theory does not predict the temperature of plumes. This must be obtained from observation which suggests a temperature excess of 200 to 300 oC

13. Diameter of Plume Tail • Decreases with DT • Increases with plume flux • For DT = 200-300 oC and buoyancy flux = 104-105 N/s, D = 100-300 km

14. Depth (km)

15. Testable Predictions of the Plume Hypothesis • Plumes must originate from a hot boundary layer – the core-mantle boundary

16. Seismic tomography (Montelli et al.)

17. Testable Predictions of the Plume Hypothesis • Flatten plume heads should be 2,000 to 2,500 km in diameter

19. Testable Predictions of the Plume Hypothesis • The hottest part of the head is at the centre and the temperature tapers towards the margin

22. Testable Predictions of the Plume Hypothesis • Both heads and tails should erupt high temperature picrites • However picrites are dense magmas that often fail to reach the surface

23. Both heads and tails should erupt high temperature picrites • Karroo • Deccan-Reunion • Parana • Emeishan • Caribbean • Hawaii • North Atlantic-Iceland

24. Oahu Cross Section Basalts Picrites 0 50 100 km

25. Testable Predictions of the Plume Hypothesis • Flood volcanism should be preceded by 500 to 1000 m of uplift • Uplift should be dome shaped and be greatest at the centre, tapering towards the margins • Plume hypothesis does not predict time-scale for uplift or volcanism both of which are controlled by the viscosity at the top of the upper mantle

27. Iso-thickness contour of the Maokou Fm

28. Biostratigraphic correlation of the Maokou Fm

32. Other Examples of Uplift Preceding Volcanism • Natkusiak, in northwest Canada • 520 Ma Antrim River flood-basalt in the northwest of Western Australia • Ethiopia • North Atlantic Igneous Province • Deccan Traps • Siberian Traps????

33. The plume hypothesis does not predict the chemistry of plume basalts • Plumes sample whatever is at the CMB at the time • The expectation is that it will be mainly “basalt”-rich mantle because basalt is dense component in the mantle • However observations show that mantle at CMB can be also depleted mantle

35. Headless Plumes • A recent study by Farnetani of thermo-compositional plumes suggests that the heads of weak plumes cannot penetrate the 670 km discontinuity • However, the light component can separate from the dense component and form a new plume that originates from 670 km • The new plume has a small head because it rises only 500 km (D = 200 km)

36. ISOSURFACE 140C Zoom on one plume

37. The head-tail structure? No, only 'hot fingers'

38. Testable Predictions of the Plume Hypothesis • Picrites should be most abundant near the centre of the plume head (flood basalt) and less abundant towards the margin