CAVEATS IN THE USE OF HF MODEL AGES IN PROVENANCE (AND OTHER) STUDIES

U-Pb ages of detrital zircons have proven to be valuable indicators of sedimentary provenance in countless studies over the past two decades. Recently, Hf isotope compositions of the zircons have been used to provide additional resolution in the detrital zircon record. This approach is very powerful because it can integrate the crystallization history of the zircon with its Hf isotopic composition, which can provide important information on its past history and thereby help distinguish between potential sedimentary sources. Hf model “ages” of these zircons are often reported as part of many studies. These can also provide useful qualitative information about the source of the zircons. It is important to keep in mind, however, that Hf model “ages” are not true ages and should not be interpreted as such. Depleted mantle (DM) model ages are suggested to estimate the timing of derivation of the continental crust from the depleted mantle. Implicit in this model is the assumption that this crust is derived from a known reservoir that evolves with a discrete isotopic composition. Most models assume that DM evolution starts at ~4.5 Ga with a CHUR composition and evolves approximately linearly to a composition of the most depleted MORB mantle today (epsilon Hf ~ +16 to +18). Hf model ages based on these parameters assume that continental crust is only derived from the most depleted mantle and that this reservoir is of a known composition. Neither assumption is true in the strict sense. Based on the current Hf isotope data for mantle-derived rock through time, for example, the most appropriate depleted mantle curve for calculating model ages does not appear to diverge from CHUR until ~ 3.5 Ga and should probably be calculated using average DMM or juvenile oceanic island arc composition (i.e., epsilon Hf < +16). Further, there is additional uncertainty in the CHUR model for the Earth (i.e., whether the chondritic model is appropriate for the bulk silicate Earth as has been long assumed). In summary, Hf model ages are useful qualitative indicators of the integrated history of zircons and the materials they have been derived from but should not be interpreted as providing unambiguous chronological information.