LONGEVITY OF SUB-CONTINENTAL MANTLE LITHOSPHERE FROM OSMIUM ISOTOPE SYSTEMATICS IN OROGENIC PERIDOTITE MASSIFS

ATTEMPTS to understand the formation and evolution of the subcontinent al lithospheric mantle (SCLM) have been hampered by the absence of rel iable time constraints, reflecting a lack of appropriate isotopic dati ng techniques. The most commonly used methods, involving strontium, ne odymium and lead isotopes, yield ambiguous results in mantle rocks, an d show no relationship with magmatic processes, as the loa concentrati ons of these elements make them susceptible to later metasomatic distu rbance. Osmium, by contrast, is much more abundant in the mantle than in the crust(1), so that peridotite Os isotope ratios are largely immu ne to recent metasomatic imprints. This provides a way to date the mag matic processes that determine mantle major-element compositions(2). W e present here two examples of striking correlations between Os-187/Os -188 and Al2O3 concentration in orogenic peridotites, and argue that t hese 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 c ratons(2,6,7)-SCLM can remain isolated from the convecting mantle for more than a billion years.