Modelling the Composition of Melts Modelling the Composition of Melts Formed During the Continental Formed During the Continental Break-up of the North AtlanticBreak-up of the North Atlantic

J. ArmitageJ. Armitage, T. Henstock, T. Minshull and , T. Henstock, T. Minshull and J. HopperJ. Hopper

Motivation

• Recreate the volume of melt generated during the opening of the North Atlantic.

• Understand what the effect would be on the composition of primary melts.

Outline

• Introduction

• Model set-up and evolution

• Steady state composition

• Constant spreading rates

• Fast spreading upon opening

• Summary

Composition Parameterisation

• Melting occurs when upwelling mantle crosses the solidus.

• We use the parameterisation of Watson & McKenzie (1991).

• Relates major element composition to instantaneous melt depletion.

• Integrate over the whole melt region

Model Set-up

• Top is driven to the right at a chosen half spreading rate.

• No flow of material or heat across the boundaries.• Optional hot layer with temperature increases of 100

and 200 ºC.

120

km50

km

Mantle potential temperature of 1325 ºC. Half spreading rate of 10 mm/yr

Mantle potential temperature of 1325 ºC. Half spreading rate of 10 mm/yr

Steady State Composition

• For models run at mantle potential temperatures of 1300 to 1325ºC and half spreading rates of 10 to 20 mm/yr.

Southeast Greenland

Two Scenarios

• Constant spreading– variation in hot layer temperature

• Fast spreading upon opening– Variation in hot layer temperature

Mantle potential temperature of 1325 ºC. Half spreading rate of 10 mm/yr

Primary melt estimates from Thy et al 1998 and Larsen et al 1999

Primary melt estimates from Thy et al 1998 and Larsen et al 1999

Igneous thickness from Holbrook et al 2001

Primary melt estimates from Thy et al 1998 and Larsen et al 1999

Igneous thickness from Holbrook et al 2001

Second Scenario

• The S.E. Greenland margin opened with a pulse of elevated spreading rates.

• Hopper et al (2003) estimate 33 mm/yr, Larsen and Saunders (1998) give 44 mm/yr abating to 12 mm/yr.

• We include this with a pulse of fast spreading of 40 mm/yr for 4 Myrs that abates to 10 mm/yr.

Primary melt estimates from Thy et al 1998 and Larsen et al 1999

Igneous thickness from Holbrook et al 2001

Primary melt estimates from Thy et al 1998 and Larsen et al 1999

Igneous thickness from Holbrook et al 2001

Primary melt estimates from Thy et al 1998 and Larsen et al 1999

Igneous thickness from Holbrook et al 2001

Summary

• We can re-create the volume of melt generated.

• The effect is a temporal variation in primary melt composition.

• Is this prediction backed up by geochemical evidence?

More Evolved Rocks

Peak Production

Steady State

Composition Parameterisation

• Also tested Niu and Batiza (1991) parameterisation.

• Relates melt composition to the incremental melt production.

• Then integrate at over the whole melt region.