Episodic Layering of the Early Mantle by the 'Basalt Barrier' Mechanism: Implications for Tectonic and Geochemical Evolution

Details Episodic Layering of the Early Mantle by the 'Basalt Barrier' Mechanism: Implications for Tectonic and Geochemical Evolution Episodic Layering of the Early Mantle by the 'Basalt Barrier' Mechanism: Implications for Tectonic and Geochemical Evolution

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.u21d..07d&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #U21D-07

Physics

1000 Geochemistry, 1500 Geomagnetism And Paleomagnetism, 3600 Mineralogy And Petrology, 8100 Tectonophysics

Scientific paper

Episodically layered mantle convection is induced during the first 1.5-2 Ga of evolving numerical models by the 'basalt barrier' mechanism, which is due to the buoyancy of subducted oceanic crust between depths of 660 km and about 750 km. During layered periods the upper mantle cools and the lower mantle warms, reaching temperature differences of up to 300°C. Also the oceanic crust is only a few kilometers thick, due to settling of the basaltic component through the upper mantle and consequent depletion of the uppermost mantle. This can explain the observed strong early depletion of the mantle, and the thin oceanic crust would facilitate subduction and plate tectonics. The layering inhibits heat loss, so that high mean mantle temperatures persist. The layering breaks down roughly every 100-150 Ma, at which time hot, fertile lower mantle floods the upper mantle and there is a dramatic burst of magmatism lasting a few million years and manifest in the models as oceanic crustal thicknesses of tens of kilometers. As radioactive heating declines and the mantle gradually cools, subducted plates eventually become thick and heavy enough to penetrate and disrupt the 'basalt barrier', and no further layering occurs. These results suggest causes for the apparently episodic growth of continental crust, for the lack of evidence for strongly episodic behaviour over the past billion years or so, and for the distinct 'vertical tectonics' of some Archean granite-greenstone terrains. The models yield mean residence times of mantle material consistent with the mantle lead-isotopic apparent age of about 1.8 Ga. Accumulations of (denser) basaltic material at the base of the models preserve a remarkably clear record of the early magmatic pulses. These accumulations persist strongly into the present. They are consistent with a recent proposal that helium isotopes record mantle melting events, and they would help to explain differences in Hf and Nd isotopes between the accessible mantle and chondritic meteorites. The accumulations, and 'piles' of more-basaltic material under mantle upwellings, are broadly consistent the seismic D" zone and with the large, seismically slow zones deep under Africa and the western Pacific.

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