RE-OS ISOTOPE EVIDENCE FOR THE COMPOSITION, FORMATION AND AGE OF THE LOWER CONTINENTAL-CRUST

Knowledge of the composition of the lower continental crust is importa nt for understanding the formation and evolution of the crust as a who le, and the petrogenesis of continental basalts. Here we present rheni um-osmium isotope data for two well characterized suites of lower-crus tal xenoliths from North Queensland, Australia(1-7), which have averag e major- and trace-element compositions similar to estimates of the bu lk lower continental crusts(8-9). Our data indicate that the lower cru st has 1 to 2 times as much osmium, about half as much rhenium, and is less radiogenic than the upper continental crust(10), We interpret th e rhenium-osmium isotope systematics to indicate that assimilation and fractional crystallization are important processes in the formation o f the lower crust, and lead to dramatic changes in the osmium isotopic composition of basalts that pond and fractionate there, A consequence of this is that the rhenium-osmium isotopic system should not be reli ed on to yield accurate mantle extraction ages for continental rocks.