GARNET GEOCHRONOLOGY: PROMISE AND PITFALLS

There has long been interest in using garnet as a geochronometer, dating back to early attempts with U/Pb (e.g., Mezger et al., J. Geol., 1991). As a major indicator mineral formed during metamorphism, direct dating of garnet formation can provide age constraints on the timing of metamorphism, as well as provide the “t” for P-T-t paths. Further, if the dated garnet can be placed in a textural context, ages can provide information on the timing of deformation. Recent technological advances have made Lu-Hf and Sm-Nd important geochronological tools and we can now reliably (if not routinely) date a wide variety of garnet compositions formed under diverse conditions. However, challenges, both analytical and geological, still remain, and can often result in complexities in the geochronological data and their interpretation.

This talk will review applications of garnet geochronology as well as some practical aspects of doing this work. Several examples of garnet geochronology will be used to illustrate the unique information this chronometer provides and the geological problems it can address. These examples will also illustrate some of the pitfalls for determining meaningful ages, as well as complexities in their interpretation. These include: 1) low concentrations of Nd and (particularly) Hf in garnet and the effect this has on sample size requirements; 2) presence of multiple growth zones in garnet and limitations of physical sampling; 3) inclusions of accessory minerals such as monazite, apatite, and zircon in garnet and their effect on the Sm-Nd and Lu-Hf isotopic budgets (e.g., Scherer et al., GCA, 2000); 4) differential partitioning of parent and daughter elements during garnet growth (e.g., Skora et al., CMP, 2006); 5) isobaric interferences during mass spectrometry; 6) differential diffusion of elements through the garnet matrix (e.g., Ganguly et al., GSA abstr., 2011; Bloch et al., Goldschmidt abstr., 2010); 7) effects of the duration of garnet growth on precision of Lu-Hf ages (Kohn, GCA, 2009); 8) effects of garnet resorption on Lu-Hf ages (Kelly et al., JMG, 2012); and 9) scatter in isochrons outside of analytical uncertainty and their interpretation.

Understanding these challenges, and overcoming the obstacles they present, will allow this technique to better realize its full potential.