testing the plume hypothesis ian campbell the australian national university

TESTING THE PLUME HYPOTHESISTESTING THE PLUME HYPOTHESIS

Ian CampbellIan Campbell

The Australian National UniversityThe Australian National University

Testable Predictions of the Plume Testable Predictions of the Plume HypothesisHypothesis

New plumes consist of a large head New plumes consist of a large head followed by a small tailfollowed by a small tail

Parana at 120 Ma

Characteristics of Flood BasaltsCharacteristics of Flood Basalts

Equidimentional, typically 2000-2500 km Equidimentional, typically 2000-2500 km

Preceded by upliftPreceded by uplift

Large volumes of magmaLarge volumes of magma

Short eruption times, main phase 1 MyrShort eruption times, main phase 1 Myr

Rapid contraction of volcanism to narrow Rapid contraction of volcanism to narrow chain of volcanoes to current position of chain of volcanoes to current position of plumeplume


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

Diameter of Plume TailDiameter of Plume Tail

Decreases with Decreases with TT

Increases with plume fluxIncreases with plume flux

For For T = 200-300 T = 200-300 ooC and buoyancy flux = C and buoyancy flux = 101044-10-1055 N/s, D = 100-300 km N/s, D = 100-300 km

-100 k m 0 100k m

0

5

10

15

20

25

30 km

3.4

2.65

SE NW

Walvis Ridge0

5

10

15

20

25

30

2.95

3.4

from Goslin and Sibuet (1975)

g cm-3D

epth

(km

)


Plumes must originate from a hot Plumes must originate from a hot boundary layer – the core-mantle boundary layer – the core-mantle boundaryboundary

Seismic tomography (Montelli et al.)Tahiti/Cook

300

650

1000

1450

1900

2350

2800

HawaiiIceland


Flatten plume heads should be 2,000 to Flatten plume heads should be 2,000 to 2,500 km in diameter2,500 km in diameter


The hottest part of the head is at the The hottest part of the head is at the centre and the temperature tapers towards centre and the temperature tapers towards the marginthe margin


Both heads and tails should erupt high Both heads and tails should erupt high temperature picritestemperature picrites

However picrites are dense magmas that However picrites are dense magmas that often fail to reach the surfaceoften fail to reach the surface

Both heads and tails should erupt Both heads and tails should erupt high temperature picriteshigh temperature picrites

KarrooKarroo Deccan-ReunionDeccan-Reunion ParanaParana EmeishanEmeishan CaribbeanCaribbean HawaiiHawaii North Atlantic-IcelandNorth Atlantic-Iceland

16km

14

12

10

8

6

4

2

0

Basaltic Crust

Root

Zone

SL

SW NE

0 50 100 km

Oahu Cross Section

Picrites

Basalts


Flood volcanism should be preceded by 500 to Flood volcanism should be preceded by 500 to 1000 m of uplift1000 m of uplift

Uplift should be dome shaped and be greatest Uplift should be dome shaped and be greatest at the centre, tapering towards the marginsat the centre, tapering towards the margins

Plume hypothesis does not predict time-scale Plume hypothesis does not predict time-scale for uplift or volcanism both of which are for uplift or volcanism both of which are controlled by the viscosity at the top of the controlled by the viscosity at the top of the upper mantle upper mantle

Iso-thickness contour of the Maokou Fm

Biostratigraphic correlation of the Maokou Fm

Other Examples of Uplift Preceding Other Examples of Uplift Preceding VolcanismVolcanism

Natkusiak, in northwest Canada Natkusiak, in northwest Canada 520 Ma Antrim River flood-basalt in the 520 Ma Antrim River flood-basalt in the

northwest of Western Australianorthwest of Western Australia Ethiopia Ethiopia North Atlantic Igneous ProvinceNorth Atlantic Igneous Province Deccan TrapsDeccan Traps Siberian Traps????Siberian Traps????

The plume hypothesis does not The plume hypothesis does not predict the chemistry of plume predict the chemistry of plume

basaltsbasalts

Plumes sample whatever is at the CMB at Plumes sample whatever is at the CMB at the timethe time

The expectation is that it will be mainly The expectation is that it will be mainly “basalt”-rich mantle because basalt is “basalt”-rich mantle because basalt is dense component in the mantledense component in the mantle

However observations show that mantle at However observations show that mantle at CMB can be also depleted mantleCMB can be also depleted mantle

Headless PlumesHeadless Plumes

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

Zoom on one plume

ISOSURFACE 140C

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


Picrites should be most abundant near the Picrites should be most abundant near the centre of the plume head (flood basalt) centre of the plume head (flood basalt) and less abundant towards the marginand less abundant towards the margin

testing the plume hypothesis ian campbell the australian national university

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