Hf-Nd isotopic evolution of the lower crust

摘要

We report Hf isotopic data for over 50 well studied lower crustal samples from 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 Sm-Nd data, to constrain coupled Hf-Nd isotopic behavior and evolution within this reservoir. Although most of these samples have present-day parent/daughter (p/d) ratios consistent with Hf-Nd evolution within the terrestrial Hf-Nd array, some samples have divergent p/d ratios that would evolve out of the terrestrial array in 1 Ga or less. The present-day ~(176)Hf/~(177)Hf and ~(143)Nd/~(144)Nd isotopic compositions of all samples, with one lone exception, plot within the terrestrial array. This indicates that (1) some present-day 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 p/d variations exist on a small hand-size scale but not necessarily on a larger scale. The lower crust, from this initial data set, is broadly similar to the upper crust in terms of both its present-day p/d values and time-integrated Lu-Hf and Sm-Nd evolution. As a result, the lower crust appears to have a Hf and Nd isotopic composition similar to that of all other crust and mantle reservoirs so far characterized.