Dynamically viable early

plate tectonics?Geoff Davies

Research School of Earth Sciences, Australian National University

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Convection with plates and tracers (present

mantle)

Tracers:

•Trace basaltic component ofmantle composition

•Upper melting zone:- defined by melting depth- move tracers into “crust”

•Store times of “sampling”

Crust thickness and tracer concentration profile

Tracer settling vs Temp

Thickness of oceanic crust

Presentcool mantle

Early hot mantle? ?

Effect of buoyant oceanic crust

1350 1400 1450 1500 1550 1600

Temperature ( ° C)

0

20

40

60

80

free plates

buoyancy-limited

1350 1400 1450 1500 1550 1600

Temperature ( ° C)

0

20

40

60

80

free plates

buoyancy-limited

Standard

thermal evoluti

on

Effect of thick oceanic crust:

Effect of buoyant crust

(reduced by 1.8)

Conclusions

•Strong early depletion of the upper mantle

•Thin oceanic crust from hot, refractory upper mantle

•Viable plate tectonics?•Bimodal thermal evolution?

Spacer

Thinning crust at high T:

Reduced effect of crust

(like eclogite effect)

To explain:apparent ages ~1.8Ga

(Pb-Pb),MORB-OIB differences

Results with heavy tracers:mean ages > 2 Gadense layer at bottomgradient through upper

mantle

0 1000 2000 3000 4000 50000

5

10

15

20

25

30

Age, Ma

MORB: mean age 2225 Ma 6096 tracers

OIB: mean age 2620 Ma 4373 tracers

0 0.5 1 1.5 2 2.50

0.2

0.4

0.6

0.8

1

Fraction

Fractionprocessed: 0.997

(a)(b)

0 1000 2000 3000 4000 50000

5

10

15

20

25

30

Age, Ma

MORB: mean age 2225 Ma 6096 tracers

OIB: mean age 2620 Ma 4373 tracers

0 0.5 1 1.5 2 2.50

0.2

0.4

0.6

0.8

1

Fraction

Fractionprocessed: 0.997

(a)(b)

Run at present mantle

temperature:

same number of overturns

0 1 2 3 4 50

4

8

12

16

20

Earth time (Ga)

Hot early mantle,1550°C

Hot mantle,1550°C

Thickness of oceanic crust