Lu-Hf ratios of crustal rocks and their bearing on zircon Hf isotope model ages: The effects of accessories

Abstract

All other factors being equal, the calculation of zircon Hf two stage model ages (T DM Hf) depends on the particular Lu/Hf value assumed for the magmatic source, the effect being more pronounced as the age difference between zircon and magmatic source increases. It is generally considered that the Lu/Hf measured in the zircon-hosting rock does not represent the composition of the source because of potential garnet or zircon fractionation. Accordingly, most authors either assume a single fixed value for Lu/Hf source , often Lu/Hf≈0.079 to 0.108, or use two alternative models, one for felsic sources, often Lu/Hf≈0.09, and the other for mafic sources, often Lu/Hf≈0.165. In contrast with these opinions, however, here we show that partial melting of peraluminous sources causes little decoupling of Lu from Hf because of similar solubilities of zircon and monazite. Furthermore, the effects of residual garnet are largely compensated by the numerous zircon inclusions that garnet and other residual minerals almost always contain. Partial melting of metaluminous sources may significantly decouple Lu from Hf if allanite and/or titanite are not present in the source, but the effect decreases as the melt fraction increases. Similarly, fractional crystallization of metaluminous magmas may decouple Lu from Hf if amphibole or clinopyroxene begin to crystallize before zircon saturation. The Lu/Hf distribution in 4784 rocks from different regions and ages is lognomal rather than normal, and the calculated medians, i.e. the maximum of the probability density function for the logarithmically transformed Lu/Hf, are Lu/Hf mafic rocks ≈0.08, Lu/Hf felsic rocks ≈0.05, i.e. notably lower than the above-mentioned felsic and mafic magmatic source averages. Magmatic sources may be remarkably heterogeneous with respect to Lu/Hf. Our calculations show that fixed Lu/Hf source values translate the Lu/Hf heterogeneity of the source to the T DM Hf thus producing an artificial distribution of model ages that may be erroneously interpreted as different episodes of crustal growth. Therefore, we propose that the best strategy to calculate two stage Hf model ages of zircon is to use the analytically determined whole-rock Lu/Hf ratio as a proxy of the source. In the case of detrital or inherited zircons, for which no whole-rock information is available, it is advisable first to determine whether they come from a mafic or felsic rock by interpreting cathodoluminescence images, Th/U ratios and other chemical parameters, and then venture an estimate of the Lu/Hf source from the SiO2 average.