Modelling isotopic responses to disequilibrium melting in granitic systems

Abstract

Granite petrogenesis is an important component of crustal growth and evolution; however, the isotope systems commonly applied to investigations of such processes (Sr, Nd, Pb and Hf) may behave in a more complex manner during partial melting and crystallization than is often assumed in petrogenetic models. Using a range of experimentally determined melting reactions and accessory mineral dissolution equations, the Sr-, Nd- and Hf-isotopic compositions of melt, source (protolith), and restite (residual source material including peritectic phases) have been calculated for a variety of hypothetical melting scenarios, in which the protoliths have acquired an isotopically heterogeneous mineral assemblage by aging in the crust prior to melting. It is shown that the disequilibrium amphibole dehydration melting of meta-igneous protoliths that have resided in the crust for 1·0 Gyr can generate differences between protolith and melt compositions of -4·2 to +7·2 εHf units. This implies that bulk-rock (particularly mafic, restite-rich) samples may have Hf isotope compositions significantly different from the melt-precipitated zircons within them. The modelling also predicts differences between protolith and melt Sr and Nd isotope compositions and decoupling between these systems. Furthermore, we demonstrate that simple restite separation from a single protolith can produce magmas exhibiting a range of Sr-Nd-Hf isotope compositions; that is, producing within-suite isotopic heterogeneity independent of source variation. The results imply that great care should be taken in the interpretation of the isotopic compositions of zircons in granites, and that bulk-rock compositions of mafic samples from granitic suites, not zircons, may provide the most reliable constraint on the protolith isotopic composition.