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 age s. The continental lithosphere is typically weaker, although often much thi cker, than the oceanic lithosphere. Oceanic mantle lithosphere accretes new material at its base as it cools (thermal accretion), and so it is composi tionally similar to the underlying asthenosphere. Much of the continental m antle lithosphere, and particularly that beneath Archaean cratons, is relat ively depleted in major elements. Thus, continental mantle lithosphere is l ess dense than the oceanic lithosphere, and its thickness depends on the co mpositional difference between it and the underlying asthenosphere, as well as on the geothermal gradient. The errors and accuracy of the mineral ther mobarometry of peridotite samples are briefly reviewed, and both mineral th ermobarometry and heat-flow studies consistently suggest minimum thicknesse s of 150-200 km for the lithosphere beneath Archaean cratons. Os isotopes r eflect the age of major-element depletion, and, hence, the stabilization of the mantle lithosphere. They confirm that in many cases the continental cr ust and uppermost mantle stabilized at about the same time, and have remain ed 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.