U-Pb GEOCHRONOLOGY OF GEM TSAVORITE AND DEMANTOID GARNET: IMPLICATIONS FOR HIGH-PRECISION GEOGRAPHIC ORIGIN DETERMINATION

Geographic origin determination of gem garnet is often hindered by a lack of discriminatory characteristics due to their sharing similar conditions of formation on a global scale. However, as precious garnet deposits are known to have formed at different times in Earth’s history, origin might be determined by obtaining individual crystal ages. Whilst most gemstones have low abundances of radiogenic isotopes commonly used in radiometric dating, gem quality garnet is an exception, proving amenable to dating by the U-Pb system. Here we demonstrate the discriminatory power of in situ LA-Q-ICP-MS U–Pb dating of gem quality tsavorite (V-bearing grossular) and demantoid (Cr-bearing andradite) for establishing the timing of crystallization.

We find that in most cases the geographic origin of both demantoid and tsavorite can be verified by comparison of the garnet’s U-Pb age with indirect ages obtained at the deposit by other mineral systems. Importantly, age-dating demantoid can help to separate gemstones from disparate geographic sources that otherwise share similar gemological properties (e.g., separating Iranian from Russian demantoid and Madagascar from Namibian demantoid). Hence, direct dating of garnet by U-Pb provides an innovation in high-precision origin determination for characterizing individual precious garnets, even in the absence of any other distinguishing criteria. This method can be applied worldwide to aid origin determination of gem quality garnet.