GARNET PETROCHRONOLOGY: THE CONTEXT OF THE OTHER POINT(S) ON THE ISOCHRON (Invited Presentation)

Garnet, a common porphyroblastic rock-forming mineral, can be dated directly and linked directly to the P-T-X-d conditions spanning its growth. Unlike most accessory mineral petrochronometers, garnet petrochronology relies on the Sm-Nd or Lu-Hf system (rather than the U-Th-Pb system) system to access the unique age information it stores. Because of the slow decay rate of 147-Sm and 176-Lu (as compared to U), and the fact that garnet 147Sm/144Nd or 176Lu/177Hf are usually below about 10 (whereas U/Pb ratios in accessory phases can be much higher), knowledge of the initial daughter isotope ratio is a more significant factor in garnet age accuracy and precision. Generally, the initial daughter isotope composition is constrained by pairing one or more non-garnet points with one or more garnet points on an isochron. These other points may include the whole rock, the garnet-free matrix, other matrix minerals, or garnet mineral inclusions. The choice of which other point(s) to include in the isochron can potentially distort the age’s accuracy by many millions of years, or degrade the age’s precision. Factors to consider include the following: 1. Open system exchange or mobilization of parent or daughter, 2. Fractionation of parent from daughter due to garnet growth, 3. Heterogeneity of initial matrix and the lengthscales thereof, 4. Inheritance from older events. #1 is most significant if the open system event happened much later than the garnet growth. #2 is rarely a major concern, and can be easily evaluated by comparing whole rock and matrix values. #3 is a major consideration if the equilibrium lengthscale is much larger than the matrix heterogeneity, or if the garnet porphyroblast is much larger than the lengthscale of matrix heterogeneity. #4 is most problematic when the inherited mineral has a high parent/daughter ratio, which is rare for the Sm-Nd and Lu-Hf systems, unless the inherited mineral is garnet itself. In all cases, the far more important factor in successful garnet geochronology is a clean garnet with high parent/daughter ratio; high garnet parent/daughter ratio minimizes all of these potential effects of other points on the isochron. However, if garnet age precision and accuracy better than ±5-10 million years is required, careful consideration and evaluation of the other point(s) on the isochron is required.