Dynamically viable early plate tectonics? Geoff Davies Research School of Earth Sciences, Australian National University

Convection with plates and tracers (present mantle)

Tracers : Trace basaltic component of mantle 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

Present cool mantle Early hot mantle? ?

Effect of buoyant oceanic crust

Standard thermal evolution

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 Ga dense layer at bottom gradient through upper mantle

Run at present mantle temperature: same number of overturns

Hot early mantle, 1550°C

Hot mantle, 1550°C

Thickness of oceanic crust