ATTEMPTS to understand the formation and evolution of the subcontinental lithospheric mantle (SCLM) have been hampered by the absence of reliable time constraints, reflecting a lack of appropriate isotopic dating techniques. The most commonly used methods, involving strontium, neodymium and lead isotopes, yield ambiguous results in mantle rocks, and show no relationship with magmatic processes, as the loa concentrations of these elements make them susceptible to later metasomatic disturbance. Osmium, by contrast, is much more abundant in the mantle than in the crust(1), so that peridotite Os isotope ratios are largely immune to recent metasomatic imprints. This provides a way to date the magmatic processes that determine mantle major-element compositions(2). We present here two examples of striking correlations between Os-187/Os-188 and Al2O3 concentration in orogenic peridotites, and argue that these can be used to date the differentiation of the SCLM. The old ages obtained agree with associated lower-crustal Nd model ages(3-5), and indicate that-in these post-Archaean terrains as well as in Archaean cratons(2,6,7)-SCLM can remain isolated from the convecting mantle for more than a billion years.