Combined Hf and Nd isotope microanalysis of co-existing zircon and REE-rich accessory minerals: High resolution insights into crustal processes

Abstract

The Sm-Nd and Lu-Hf isotope tracer systems, applied to whole rocks or mineral separates, have provided powerful insights into the formation, differentiation, and evolution of the Earth's continental crust. However, some key questions remain, such as how certain igneous rocks form, and how reliable radiogenic isotope tracers are for tracking melt sources. Here, the potential for combining data from the two isotope tracers, Nd and Hf, obtained on a sub-mineral-scale is explored to further understand how crustal rocks receive their sometimes-puzzling geochemical fingerprints. New data, in combination with results from previous studies, reveal one of the key strengths of combining the two independent isotope systems at a micrometre scale, namely identifying open versus closed system metamorphic and igneous processes. Such knowledge is key for understanding how the continental crust formed and stabilized, and for elucidating the role of mantle-derived magmas in the production of granitic rocks, a long-standing issue that is still highly debated. We show how measurement of Nd and Hf isotope ratios in accessory minerals from the same sample has helped to evaluate the fundamentally different models (e.g., magma mixing; crustal assimilation; reactive melt transport vs. incomplete geochemical homogenization of melt at its source) invoked to explain heterogeneous isotope signatures in igneous rocks. Lastly, we discuss how the dual in situ Nd and Hf isotope approach can be used to to evaluate the extent to which metamorphism obscured the primary isotope signature of Archean gneisses derived from felsic igneous protoliths, which has profound implications for our interpretation of early crust formation processes.