THE ROLE OF SUBDUCTED SLABS IN AN EVOLVING EARTH

The role of subducted oceanic lithospheric slabs in determining the composition and secular evolution of the Earth's mantle and continental crust is examined. In the Archean, it is argued that the behaviour of subducted slabs is largely determined by partial melting reactions at relatively shallow levels (< 200 km) in the upper mantle, a consequence of the younger mean age of subducted slabs. As a result, subducted Archean slabs are dehydrated and largely disaggregated in the uppermost 100-200 km of the mantle. This is likely to have had a dramatic effect on mantle convection due to the inability of subducted slabs to transport volatiles beyond this level and into the deep mantle. A low viscosity upper mantle 'lid' forms which is largely isolated from the lower mantle. It is proposed that the positive epsilon(Nd) values characteristic of many early Archean rocks are due to derivation of magmas from this highly depleted zone of uppermost mantle. In the post-Archean mantle, lower temperatures and reduced heat flow result in the formation of older, colder and stronger oceanic lithosphere which mainly underdoes dehydration rather than partial melting reactions during subduction. This allows slabs to penetrate through the relatively high viscosity transition zone (400-670 km deep) and into the lower mantle. Consequently, an influx of volatiles (predominantly H2O and CO2) and some relatively mobile elements such as U are carried into the deep mantle via post-Archean subduction. The post-Archean fluxing of U into the deep mantle can account for both the Pb paradox as well as the lower than anticipated Th/U ratios observed in modern mid-ocean ridge basalts. It is also proposed that the formation of the source region of chemically distinct ocean island basalts is a feature largely restricted to the post-Archean mantle. This is a consequence of the relatively short lifetime of recycled slabs in the Archean (< 10(8) yrs) compared to the post-Archean (> 10(9) yrs). Subducted slabs thus play a critical role in controlling the evolving styles of mantle convection, as well as the interaction between the mantle and the continental crust.