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TESTING THE PLUME HYPOTHESIS Ian Campbell The Australian National University
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Testable Predictions of the Plume Hypothesis New plumes consist of a large head followed by a small tail
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Parana at 120 Ma
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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
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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 o C
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Diameter of Plume Tail Decreases with T Increases with plume flux For T = 200-300 o C and buoyancy flux = 10 4 -10 5 N/s, D = 100-300 km
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Depth (km)
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Testable Predictions of the Plume Hypothesis Plumes must originate from a hot boundary layer – the core-mantle boundary
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Seismic tomography (Montelli et al.)
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Testable Predictions of the Plume Hypothesis Flatten plume heads should be 2,000 to 2,500 km in diameter
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Testable Predictions of the Plume Hypothesis The hottest part of the head is at the centre and the temperature tapers towards the margin
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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
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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
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0 50 100 km Oahu Cross Section Picrites Basalts
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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
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Iso-thickness contour of the Maokou Fm
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Biostratigraphic correlation of the Maokou Fm
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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????
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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
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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)
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Zoom on one plume ISOSURFACE 140C
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The head-tail structure? No, only 'hot fingers'
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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
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