BEHAVIOR OF HAFNIUM AND NEODYMIUM ISOTOPES IN THE CRUST - CONSTRAINTSFROM PRECAMBRIAN CRUSTALLY DERIVED GRANITES

Due to the large partition coefficients of garnet for Lu compared to S m, Nd, and Hf, garnet residual from crustal melting events has a large potential for retaining Lu and, over time, producing high Hf-176/Hf-1 77 reservoirs in the lower crust. Therefore, melts derived from such g arnet-bearing residual assemblages may be high in Hf-176/Hf-177 relati ve to Nd-143/Nd-144. In order to determine the extent of potential haf nium isotopic heterogeneities in the lower crust from residual garnet, and to detail coupled Hf-Nd isotopic behavior in older crustal rocks in general, we have determined the hafnium and neodymium isotopic comp ositions of thirty-two Precambrian granites and rhyolites from diverse suites of crustally derived magmatic suites. The majority of these ro cks are known, from major-element and neodymium isotopic evidence, to have been derived from older (>300 m.y.) crust. Modeling of Lu/Hf and Sm/Nd partitioning during melting events in the lower crust indicates that anomalously high Hf-176/Hf-177 compositions should develop within 300-400 m.y. in residual assemblages provided melting was at least 25 -30% and 10% or more garnet was left in the residue. In spite of the d iverse older crustal sources for the granites and rhyolites in this st udy, and the potential for garnet to be present in their source region s, none of the granitoids have anomalously high initial Hf-176/Hf-177 compositions: all samples (with one exception) have initial Hf and Nd compositions that plot within a +/-8 epsilon(Hf) unit wide band of the reference line (epsilon(Hf) = 2 epsilon(Nd) + 2) for juvenile crust. The absence of high initial Hf-176/Hf-177 compositions in these granit es and rhyolites most likely implies that either (1) garnet is not a c ommon residual phase in the lower crust or (2) garnet-bearing restite is not easily incorporated into later melts or (3) not enough time was available to develop anomalous Hf compositions.