TESTING THE PLUME HYPOTHESIS Ian Campbell The Australian National University

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

Parana at 120 Ma

Characteristics of Flood Basalts Equidimentional, typically 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

Testable Predictions of the Plume Hypothesis Plume tails (upper mantle) should be about 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 o C

Diameter of Plume Tail Decreases with  T Increases with plume flux For  T = o C and buoyancy flux = N/s, D = km

Depth (km)

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

Seismic tomography (Montelli et al.)

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

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

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

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

km Oahu Cross Section Picrites Basalts

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

Iso-thickness contour of the Maokou Fm

Biostratigraphic correlation of the Maokou Fm

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

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

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)

Zoom on one plume ISOSURFACE 140C

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

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