Melting controls on the lutetium-hafnium evolution of Archaean crust

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.