Hf-Nd isotopic evolution of the lower crust

We report Hf isotopic data for over 50 well studied lower crustal samples f rom three Proterozoic and Phanerozoic regions in southwest Europe, eastern Australia and southern Mexico. We use these data to characterize the Lu-Hf isotopic composition of the lower crust and, in combination with existing S m-Nd data, to constrain coupled Hf-Nd isotopic behavior and evolution withi n this reservoir. Although most of these samples have present-day parent/da ughter (pld) ratios consistent with Hf-Nd evolution within the terrestrial Hf-Nd array, some samples have divergent pld ratios that would evolve out o f the terrestrial array in 1 Ga or less. The present-day Hf-176/Hf-177 and Nd-143/Nd-144 isotopic compositions of all samples, with one lone exception , plot within the terrestrial array. This indicates that (1) some present-d ay p/d ratios may be a relatively recently acquired characteristic through magmatic or metamorphic processes not related to the time-integrated Sm/Nd and Lu/Hf ratios of their sources, and/or (2) the Lu/Hf and Sm/Nd pld varia tions exist on a small hand-size scale but not necessarily on a larger scal e. The lower crust, from this initial data set, is broadly similar to the u pper crust in terms of both its present-day pld values and time-integrated Lu-Hf and Sm-Nd evolution. As a result, the lower crust appears to have a H f and Nd isotopic composition similar to that of all other crust and mantle reservoirs so far characterized. (C) 2000 Elsevier Science B.V. All rights reserved.