USE AND ABUSE OF HF MODEL AGES

Hf model ages have been reported with increased frequency in recent years, especially due to the explosion of Hf isotope data from zircons by LA-MC-ICPMS. The attraction of this technique is the integration of the crystallization history of the zircon known from the U-Pb chronometer with its Hf isotopic composition, which can provide important information on its prior history—for example, whether the zircons have been derived from juvenile crust, or from crust with a history predating the crystallization of the zircons. This approach has been powerful in provenance studies of zircons in sediments by helping to decipher different source regions, but also has been used in addressing fundamental questions such as the growth of continents through time. Along with understanding the utility of this technique, however, it is also important to recognize its limitations. This contribution follows in the spirit of similarly titled papers in the past (i.e., Use and Abuse of Crust Formation Ages, Arndt and Goldstein, Geology, 1987; Use and Abuse of Crustal Accretion Calculations, Pallister et al., Geology, 1990) and examines the application of Hf model ages but also highlights the limitations of this technique and how it can potentially be misused. The most significant of the limitations is the uncertainty in the depleted mantle or CHUR reservoir model itself. Hf model ages have two fundamental assumptions. The first is that continental crust is derived from a specific reservoir such as the depleted mantle (or alternatively an undifferentiated mantle, or CHUR). The second is that it is assumed that we know the composition of that reservoir through time. In truth, however, there is considerable uncertainty inherent in both of these assumptions and we do not know quantitatively the composition of these mantle reservoirs, how variable they are through time, and how they contribute to the formation of the continents (i.e., do they derive exclusively from the depleted mantle?). In summary, while Hf model ages are useful quantities that can provide important information on the isotopic history of rocks and zircons, it is essential to keep in mind that they are qualitative, model-based estimates that should not be interpreted strictly as ages with unambiguous chronological information.