Melting controls on the lutetium–hafnium evolution of Archaean crust

The lutetium–hafnium (Lu–Hf) isotope record, typically measured in zircon crystals, provides a major tool for the study of crustal growth and differentiation. Interpretations of Hf isotope datasets use an evolution array defined by source ¹⁷⁶Lu/¹⁷⁷Hf. However, the very process that drives crustal differentiation to produce such arrays – partial melting – is precisely that which may modify the trajectory of the array due to variable degrees of anatexis allied with the differing compatibilities of Lu and Hf in residual minerals. Further, Lu/Hf estimates derived from the composition of present-day continental crust may be inappropriate for modelling Archaean crustal evolution, where different geodynamic styles and magmatic sources prevailed.

Using an approach combining phase equilibria, and trace element and isotopic modelling, we quantify the effects of partial melting of both a modern (N-MORB) and Archaean (C-F2) mafic source on melt Lu/Hf. Melting N-MORB shows that the ¹⁷⁶Lu/¹⁷⁷Hf of the melt, which modelling predicts to be between 0.015 and 0.022, is sensitive to the degree of melting. This difference results in a variation of 4.5 epsilon units/billion years. By contrast, anatexis of C-F2 yields melts with ¹⁷⁶Lu/¹⁷⁷Hf ∼0.009 that are less affected by the degree of melting. Remelting TTG yields K-rich granitic melts (TTG + G) with ¹⁷⁶Lu/¹⁷⁷Hf ∼0.005. Thus, a partial melting event imposes a greater control on the resulting crustal reservoir Lu/Hf than the degree of melting. Archaean continental crust has a lower Lu/Hf than that of the average mid to upper continental crust, and therefore a lower ¹⁷⁶Lu/¹⁷⁷Hf (here 0.005–0.009) is appropriate to modelling its Hf isotopic evolution. There has been a secular change in average crustal Lu/Hf, with the median Lu/Hf of Proterozoic and Phanerozoic magmatic rocks being higher than that of Archaean TTG + G. We show that an enriched Archaean mafic source (C-F2) with a Lu/Hf ratio of ∼0.12 may produce TTG continental crust with a ¹⁷⁶Lu/¹⁷⁷Hf approaching that calculated in real rocks worldwide.