Chemical and temporal variations in the Earth's lithosphere

The lithosphere is the rigid outer layer of the Earth, but its composition, and by implication its origins, are different in terrains of different ages. The continental lithosphere is typically weaker, although often much thicker, than the oceanic lithosphere. Oceanic mantle lithosphere accretes new material at its base as it cools (thermal accretion), and so it is compositionally similar to the underlying asthenosphere. Much of the continental mantle lithosphere, and particularly that beneath Archaean cratons, is relatively depleted in major elements. Thus, continental mantle lithosphere is less dense than the oceanic lithosphere, and its thickness depends on the compositional difference between it and the underlying asthenosphere, as well as on the geothermal gradient. The errors and accuracy of the mineral thermobarometry of peridotite samples are briefly reviewed, and both mineral thermobarometry and heat-flow studies consistently suggest minimum thicknesses of 150-200 km for the lithosphere beneath Archaean cratons. Os isotopes reflect the age of major-element depletion, and, hence, the stabilization of the mantle lithosphere. They confirm that in many cases the continental crust and uppermost mantle stabilized at about the same time, and have remained as a coherent unit ever since. In general, areas of Archaean lithosphere appear not to have thickened significantly since they were formed, whereas the lithosphere beneath Proterozoic and younger areas has been more prone to subsequent magmatic and tectonic events.